Low to middle-latitude X-ray emission from Jupiter

Low- to middle-latitude X-ray emission from Jupiter

Anil Bhardwaj,1 Ronald F. Elsner,2 G. Randall Gladstone,3 J. Hunter Waite Jr.,3

Graziella Branduardi-Raymont,4 Thomas E. Cravens,5 and Peter G. Ford6

Received 18 April 2006; revised 24 July 2006; accepted 31 August 2006; published 22 November 2006.

[1] The Chandra X-ray Observatory (CXO) observed Jupiter during the period24–26 February 2003 for �40 hours (4 Jupiter rotations), using both the spectroscopyarray of the Advanced CCD Imaging Spectrometer (ACIS-S) and the imaging array of theHigh-Resolution Camera (HRC-I). Two ACIS-S exposures, each �8.5 hours long, wereseparated by an HRC-I exposure of �20 hours. The low- to middle-latitude nonauroraldisk X-ray emission is much more spatially uniform than the auroral emission. However,the low- to middle-latitude X-ray count rate shows a small but statistically significanthour angle dependence and depends on surface magnetic field strength. In addition, theX-ray spectra from regions corresponding to 3–5 gauss and 5–7 gauss surface fields showsignificant differences in the energy band 1.26–1.38 keV, perhaps partly due to lineemission occurring in the 3–5 gauss region but not the 5–7 gauss region. A similarcorrelation of surface magnetic field strength with count rate is found for the 18 December2000 HRC-I data, at a time when solar activity was high. The low- to middle-latitude diskX-ray count rate observed by the HRC-I in the February 2003 observation is about 50% ofthat observed in December 2000, roughly consistent with a decrease in the solar activityindex (F10.7 cm flux) by a similar amount over the same time period. The low- tomiddle-latitude X-ray emission does not show any oscillations similar to the �45 minoscillations sometimes seen from the northern auroral zone. The temporal variation inJupiter’s nonauroral X-ray emission exhibits similarities to variations in solar X-ray fluxobserved by GOES and TIMED/SEE. The two ACIS-S 0.3–2.0 keV low- tomiddle-latitude X-ray spectra are harder than the auroral spectrum and are different fromeach other at energies above 0.7 keV, showing variability in Jupiter’s nonauroral X-rayemission on a timescale of a day. The 0.3–2.0 keV X-ray power emitted at low tomiddle latitudes is 0.21 GW and 0.39 GW for the first and second ACIS-S exposures,respectively. We suggest that X-ray emission from Jupiter’s disk may be largelygenerated by the scattering and fluorescence of solar X rays in its upper atmosphere,especially at times of high incident solar X-ray flux. However, the dependence of countrate on surface magnetic-field strength may indicate the presence of some secondarycomponent, possibly ion precipitation from radiation belts close to the planet.

Citation: Bhardwaj, A., R. F. Elsner, G. R. Gladstone, J. H. Waite Jr., G. Branduardi-Raymont, T. E. Cravens, and P. G. Ford (2006),

Low- to middle-latitude X-ray emission from Jupiter, J. Geophys. Res., 111, A11225, doi:10.1029/2006JA011792.

1. Introduction

[2] X-ray emission from Jupiter was first unambiguouslyobserved nearly 2 1/2 decades ago by the Earth-orbiting

Einstein observatory [Metzger et al., 1983] (see Bhardwajand Gladstone [2000] for a review of earlier searches forX-ray emission from Jupiter). These initial observationswere followed about a decade later by a series of ROSATobservations spanning a period of about 6 years [Waite et al.,1994, 1995, 1997; Gladstone et al., 1998]. More recently,both X-ray cameras on the Chandra X-ray Observatory(CXO), the spectroscopy array of the Advanced CCDImaging Spectrometer (ACIS-S) and the imaging array ofthe High-Resolution Camera (HRC-I), have observed Jupiter[Gladstone et al., 2002; Elsner et al., 2002, 2005a, 2005b,2005c], as has the XMM-Newton X-ray Observatory[Branduardi-Raymont et al., 2004, 2006a, 2006b; Bhardwajet al., 2005a]. These observations have stimulated theoret-ical studies of X-ray emission from Jupiter [Metzger et al.,1983; Barbosa, 1990; Waite, 1991; Singhal et al., 1992;

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1Space Physics Laboratory, Vikram Sarabhai Space Centre, TrivandrumKerala, India.

2Space Science Branch, NASA Marshall Space Flight Center,Huntsville, Alabama, USA.

3Southwest Research Institute, San Antonio, Texas, USA.4Mullard Space Science Laboratory, University College London,

Surrey, UK.5Department of Physics and Astronomy, University of Kansas,

Lawrence, Kansas, USA.6Kavli Institute for Astrophysics and Space Research, Massachusetts

Institute of Technology, Cambridge, Massachusetts, USA.

Copyright 2006 by the American Geophysical Union.0148-0227/06/2006JA011792$09.00

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http://dx.doi.org/10.1029/2006JA011792

Cravens et al., 1995, 2003, 2006; Kharchenko et al., 1998;Liu and Schultz, 1999; Maurellis et al., 2000; Bhardwaj etal., 2002; Bhardwaj, 2003, 2006; Bunce et al., 2004].[3] X-ray emission from Jupiter separates spatially into

two categories: (1) the high-latitude auroral (or polar)emission, and (2) the low- to middle-latitude disk (non-auroral) emission. As we show in this paper, these twocategories have different morphology, temporal behavior,and spectra.[4] Previous papers on the Chandra and XMM-Newton

X-ray observations of Jupiter have concentrated on theauroral emissions [Gladstone et al., 2002; Elsner et al.,2005a; Branduardi-Raymont et al., 2004, 2006a, 2006b;Bhardwaj et al., 2005a]. The Chandra observations haveshown that most of Jupiter’s northern auroral X-rays comefrom a ‘‘hot spot’’ that is fixed in System III latitude andlongitude and located significantly poleward (on field linesconnecting to equatorial regions in excess of 30 RJ from theplanet, where RJ is Jupiter’s equatorial radius) of thelatitudes connected to the inner magnetosphere, and thatthe auroral X-rays sometimes, but not always, vary with a45-min quasi-periodicity, similar to that reported for high-latitude radio and energetic electron bursts observed bynear-Jupiter spacecraft. The CXO/ACIS-S [Elsner et al.,2005a] and XMM-Newton [Branduardi-Raymont et al.,2004, 2006a, 2006b] observations provided soft (0.2–2.0 keV) X-ray spectra of Jupiter’s aurora, which areconsistent with high-charge states of precipitating heavy(C, O, S) ions. Such a spectral interpretation suggests ener-getic ion precipitation from the outer magnetosphere or thesolar wind or a mixture of both, the ions then undergoing alarge acceleration to attain energies of >1 MeV/nucleonbefore impacting Jupiter’s upper atmosphere [Cravens etal., 2003; Elsner et al., 2005a; Branduardi-Raymont et al.,2004, 2006a, 2006b; Bunce et al., 2004] (see Bhardwaj et al.[2006] for review).

[5] Using data from the ROSAT/HRI, Waite et al. [1996,1997] reported low-latitude soft X-ray emission with abrightness of about 0.01–0.2 R and raised the possibilitythat this equatorial X-ray emission might be due to theprecipitation of energetic sulfur or oxygen ions into theatmosphere from Jupiter’s inner radiation belts. Evidence

Table 1. Details of the CXO Jupiter Observations in 2003

February

Instrument/Parameter Start Timea/Valueb Stop Timea/Valueb

Chandra ACIS-SOBSID 3726

24 February 20031559:13

25 February 20030019:00

Chandra HRC-IOBSID 2519

25 February 20030030:23

25 February 20032017:56

Chandra ACIS-SOBSID 4418

25 February 20032326:15c

26 February 20030808:23

R.A., hhmm:ss 0851:51.67 0851:07.71Dec, deg:mm:ss +18:31:24.1 +18:34:23.3Sun distance, AU 5.3198 5.3203Earth distance, AU 4.4084 4.4204Diameter,darcsec 44.71 44.60Elongation,edegree 154.67 152.81Phase,fdegree 4.56 4.87Apparent visible magnitude �2.53 �2.52

a2003 February date hhmm:ss in UT.bStart value of parameters on 24 February 2003 at 1559 and corresponding

stop value on 26 February at 0808.cAfter removing the first 11.296 ks, during which the planet overlapped

the bump in the bias frame.dProjected equatorial diameter of Jupiter. Jupiter’s equatorial radius is

71,492 km.eSolar elongation = Sun-Earth-Jupiter angle.fPhase = Sun-Jupiter-Earth angle.

Figure 1. Color-coded two-dimensional histograms ofChandra events from observations on 18 December 2000(HRC-I upper right panel) and 24–26 February 2003(HRC-I upper left panel, ACIS-S first exposure lower left,second exposure lower right; see Table 1) as seen in aframe moving across the sky with Jupiter. The histogramswere smoothed with two-dimensional Gaussians withsigmas of 0.79100 for the HRC-I data and 0.73800 for theACIS-S data. The scale bar in the lower right of eachpanel represents 500, and the small circles near the centerrepresent the sub-Earth and subsolar points. The super-imposed graticules show latitude and longitude atintervals of 30�. Note that Jupiter’s angular diameterwas 7% larger in December 2000 than in February 2003.The color scale is clipped at 1 Rayleigh (R) in bothpanels in order to emphasize the disk emissions. Theauroral emissions are ‘‘overexposed’’ (maximum brightnessgoes to about 6 R in auroral regions) in all panels; the disk andauroral emissions are easily separated spatially. The low- tomiddle-latitudeX-ray spectrumwas extracted from the regioninside both the white rectangle and the white circle (withradius 1.05 RJ). Auroral spectra (north and south) wereextracted from the regions inside the white circle but outsidethewhite rectangle. The conversions to Rayleighs was carriedout using effective areas of 54.5 cm2 for the HRC-I data and76.3 cm2 for the ACIS-S data. These effective areas werecalculated as averages over the nominal energies of Jupiterevents for the February 2003 ACIS-S data.

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Figure 2. Distribution of counts in latitude measured from the east-west line through the middle of theplanet as seen from Chandra (not in System III). The bin size is 10� and the distribution is counts perlatitudinal bin; hence the expected distribution for a uniform disk varies as the square of the cosine of theangle from the equator. The data points are summed over all longitudes measured from the north-southline through the middle of the planet as seen from Chandra. There is no need to correct for exposure timeas this distribution is over the visible disk. (top) Distribution for all on-planet 2003 ACIS-S (0.3–2.0 keV)and HRC-I events. The black curve shows the best fit cos2q distribution over the latitude interval(�45�,+45�). The value of c2 for this fit is 9.6 for 9 degrees of freedom, with probability of chanceoccurrence of 38%, indicating an acceptable fit. Deviations from cos2q at higher latitudes are due to auroralemission. (bottom) Distributions for 2003 ACIS-S events (black), 2003 HRC-I events (blue), and2000 HRC-I events (red). The corresponding curves show the best fit cos2q distributions over thelatitude interval (�45�,+45�). The values of c2 for these fits are (2003 ACIS-S 4.7, 2003 HRC-I14.4, 2000 HRC-I 28.5), with probabilities of chance occurrence (86, 11, 0.08)%. The fits from 2003are acceptable, that for the 2000 HRC-I observation is not.


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for a correlation between low-latitude regions of lowmagnetic field strength and enhanced X-ray emission[Gladstone et al., 1998] lent additional support to thismechanism, since the loss cone for precipitating particles

ought to be larger over regions with weaker surface mag-netic field strength. However, Maurellis et al. [2000]showed that elastic scattering of solar X rays by atmosphericneutrals and fluorescent scattering of carbon K-shell X rays

Figure 3. (top) Distribution of rates (c/ks in each 0.5 hour bin) versus HA as defined by equation (1)and the best fit to equation (2). The data points include the HRC-I and ACIS-S observations on 24–26February 2003. Latitudes are restricted to the interval (�45�, +45�), as measured from the System IIIequator. (bottom) Residuals to the best fit.


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from methane molecules located below Jupiter’s homopauseare also potential sources. This model predicts an X-raybrightness that agrees within a factor of two with the bulk ofthe low-latitude ROSAT measurements, suggesting thatscattering and fluorescence of solar X rays may accountfor a significant fraction of Jupiter’s nonauroral X-rayemission. This solar X-ray scattering mechanism is alsosupported by correlations of Jupiter’s nonauroral X-rayemission with the F10.7 cm solar flux and of the X-ray limbwith the bright visible limb [Gladstone et al., 1998].[6] Bhardwaj et al. [2005a] studied the variability of

Jupiter’s nonauroral X-ray emission, comparing it to varia-tions in the solar X-ray flux. They noted a solar X-ray flarethat matched a feature in Jupiter’s low- to middle-latitudeX-ray light curve and suggested that the X-ray emissionfrom Jupiter’s disk is primarily due to scattered solar X rays.A recent study [Bhardwaj et al., 2005b] of X-ray emissionfrom Saturn, the planet that has an atmospheric compositionmost similar to Jupiter’s, provided convincing evidence forthe strong influence of a solar X-ray flare on the X-rayemission from that planet’s disk. Cravens et al. [2006]demonstrated that the spectrum measured by CXO at lowlatitudes is consistent with scattering and fluorescence ofsolar photons, at least for photon energies below about900 eV. The modeled intensities were lower than themeasured intensities by about a factor of 2 for higherenergies, which Cravens et al. [2006] attributed to problemswith the solar irradiance model used for these energies and/orthe presence of another source intrinsic to the Jupiter.[7] In this paper we present an analysis of the 24–26

February 2003 high spatial resolution (�0.300 half-powerradius for the energies of interest for Jupiter) CXO obser-vation of Jupiter focusing on the nonauroral emission.Elsner et al. [2005a] have previously described the high-latitude auroral X-ray emission observed at that time. Wedescribe the morphology and temporal behavior of Jupiter’slow- to middle-latitude X-ray emission and compare with theprevious CXO/HRC-I 10-hour observation of 18 December2000.We find that the spectrum of Jupiter’s nonauroral X-rayemission is distinctly different from that of its X-ray auroraand compare the nonauroral spectrum with a thermal emis-sion model spectrum.

2. Chandra Observations and Data Reduction

[8] The CXO observed Jupiter during 24–26 February2003 for four rotations of the planet, using both the ACIS-Sand HRC-I X-ray cameras. Table 1 provides details of theseobservations. During the two ACIS-S exposures, separatedby the HRC-I exposure, the telescope focus and the planetwere placed on the back-illuminated CCD, designated S3, inorder to take advantage of this CCD’s sensitivity to low-energy X-rays. Each CCD has four readout nodes, 256columns of pixels per node. Since the response varies

somewhat from node to node, the spacecraft was orientedto allow the planet to move along a single CCD node. TheACIS data were taken using the standard ACIS frame time of3.241 s. No repointings were necessary during any of theindividual CXO exposures, although the pointings for eachexposure were different. The response of the ACIS opticalblocking filter has an interference peak in its transmissionnear �9000 A (�1.4 eV) that can affect ACIS S3 observa-tions of optically bright solar system objects [Elsner et al.,2002]. The steps taken to minimize the effects on the X-raydata from Jupiter are described by Elsner et al. [2005a].Owing to this necessary procedure, we employ a low-energycutoff at 0.3 keVand use the standard response matrix for ourspectral analyses, keeping in mind that there may still be atendency to undercount the higher grades events at the lowend of our band. Higher-grade events occur when theabsorption of a single photon produces charge in more thanone pixel. Charge-transfer-inefficiency (CTI) effects areminimal for the back-illuminated S3 CCD, and no CTIcorrections were made. For spectral modeling it is necessaryto take into account the time-dependent contamination layeron the ACIS optical blocking filter [Plucinsky et al., 2003].We do this by multiplying the ACIS S3 effective area by anenergy-dependent correction factor calculated using theCIAO tool acisabs. As a check, we also carried out thespectral analysis correcting for contamination by usingthe energy-dependent effective area correction factor calcu-lated using the contamarf tool (H. L. Marshall, privatecommunications, 2003). The spectral modeling results wereindistinguishable for these two correction methods.[9] The Very Faint mode used in the ACIS-S observations

effectively suppresses the background (see Elsner et al.[2005a] for more details). Therefore we do not subtractbackground for the ACIS-S exposures. However, for theHRC-I exposures the background was higher and must betaken into account for determination of the planet’s countrate. Cosmic rays and radioactive decay within the micro-channel plate of HRC-I are the principal sources of HRC-Ibackground and thus are not blocked by the planet. For thiswe assumed a circular region of 1.2 RJ to 5 RJ aroundJupiter and calculated the count rate (which is 14 counts perks, c/ks, for the same area as the planet’s disk) per unit area.This was then subtracted from the count rate of Jupiter’sdisk (including the auroral zones) to derive the backgroundsubtracted count rate of 45 c/ks. The HRC-I count rate inthe nonauroral disk region (see Figure 1) was 28 c/ks. On18 December 2000 the disk rate was 68 c/ks (82 c/ks for thewhole planet including the auroral zones), while the equiv-alent background rate was 13 c/ks. Thus the backgroundrates are very similar for the two HRC-I observations, butJupiter’s disk was significantly brighter in soft X rays inDecember 2000 than in February 2003.[10] The ACIS S3 and HRC-I data were transformed into

a frame of reference centered on Jupiter using appropriateephemeris data obtained from the JPL HORIZONS ephem-eris generator and Chandra orbit ancillary data provided inthe data products from the Chandra X-ray Center (CXC).

3. Morphology

[11] Figure 1 shows the Chandra HRC-I and ACIS-SX-ray images of Jupiter from 24–26 February 2003, and,

Table 2. Goodness of Fit of Equation (2) to Hour Angle

Distribution

Data Set d.o.f. c2 Confidence, %

2003 All 23 75.90 1.4e–52003 ACIS 23 39.82 1.62003 HRC-I 22 43.89 0.372000 HRC-I 21 45.59 0.078


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Figure 4. (top) Rate map for the 24–26 February 2003 Chandra data, summed over both ACIS-Sexposures and the HRC-I exposure, in SIII coordinates, convolved with a two-dimensional Gaussian withs = 10�. The white-line contours display the surface magnetic field strength, and black lines crossing theplot from 360� to 0� in the northern and southern hemispheres denote the magnetic footprints of the Ioflux tube (RJ) = 5.9), as defined by the VIP4 model of Connerney et al. [1998]. The color bar of thefigure is in counts per kilosecond per square degree. Since the assignment of System III longitude is mostuncertain near the limb, only events more than 30� longitude from the limb are included. In addition theintensity scale has been clipped at 0.15 times the maximum auroral rate in order to emphasize anyvariations at low to middle latitudes. (bottom) Map of Jupiter’s surface magnetic field strength inSystem III coordinates using the VIP4 model of Connerney et al. [1998]. Again the white-line contoursdisplay the surface magnetic field strength, and black lines crossing the plot from 360� to 0� in the Northernand Southern Hemispheres denote the magnetic footprints of the Io flux tube (RJ = 5.9).


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on the same scale, the corresponding HRC-I image on18 December 2000. All images show pronounced X-raysfrom the north and south auroral regions. After account-ing for the 7% decrease in Jupiter’s angular diameter, the

low- to middle-latitude X-ray emission on 25 February 2003is dimmer by about 50% compared to that on 18 December2000. The solar F10.7 cm flux (indicator of solar activity) was192 on 18 December 2000 and 96 on 25 February 2003. Thus

Figure 5. Distribution of count rates (c/ks-sq.deg-gauss) versus surface magnetic field strength (gauss;in half gauss bins) for (top) the 24–26 February 2003 data (includes both HRC-I and ACIS-S) and(bottom) the 18 December 2000 data. The black points are for the full disk, blue points for System IIIlatitudes in the range (�45�, +45�), and red points for System III latitudes in the range (�30�, +30�). Thegreen points are for uniform disk emission (with no aurora) assuming the same number of counts. Onlyevents more than 30� in longitude from the limb are included in this analysis.


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the decrease in Jupiter’s low- to middle-latitude disk X-rayemission fromDecember 2000 to February 2003 is consistentwith a corresponding decrease in solar activity.[12] The low- to middle-latitude X-ray emission from

Jupiter appears relatively uniform to the eye. Figure 2shows the distributions of counts versus latitude as mea-sured from the east-west line running through the middle ofthe planet. The low- to middle-latitude points are consistentwith the cosine-squared dependence expected from a disk ofuniform surface brightness. Departures from the cosine-squared law at higher latitudes are due to the auroralX-ray emission.[13] Gladstone et al. [1998] found a dependence of X-ray

intensity on hour angle in ROSAT data. The hour angle (HA)of a point on the projected disk is defined by

HA ¼ lIII sunð Þ � lIII pointð Þ½ �=15:0þ 12:0; ð1Þ

where lIII is system III longitude. If Jupiter’s rotation axiswere not tilted with respect to the sky plane and the subsolarpoint were identical with the subobserver point, then thedistribution in HA would be identical with the azimuthaldistribution as measured from the north-south line throughthe center of the planet, even though the planet is rotating.For a disk of uniform surface brightness, the expectedazimuthal distribution varies as the cosine of azimuth.Because of the small tilt of Jupiter’s rotation axis withrespect to the sky plane (with an inclination to the ecliptic of1.305 deg) and the small offset of the subsolar point fromthe subobserver point (�0.08 RJ), the hour angle distribu-tion is not quite the same as such an azimuthal distribution;however, these effects are small enough that for uniformsurface brightness the hour angle distribution should befairly close to that distribution

f HAð Þ ¼ cos 15 HA� 12ð Þ½ �: ð2Þ

[14] Table 2 and Figure 3 show, for the combined HRC-Iand ACIS-S data from 24–26 February 2003, the results offitting the observed HA distribution for latitudes in the range(�45�, +45�) to equation (2), as measured from theSystem III equator. The distribution is in count rate space,as it is necessary to correct for the variation of exposure timewith HA. The confidence levels listed in Table 2 indicate theprobability (in percent) of a good fit, based on statistical errorsonly. None of the listed fits are particularly good, but this maybe due the geometrical effects discussed above.[15] The Chandra data are time-tagged and hence each

X-ray photon can be mapped into System III coordinates(latitude and longitude). The top of Figure 4 shows the ratemap for the 24–26 February 2003 data, including bothHRC-I and ACIS-S data, in System III coordinates. In thisrepresentation, the location of the X-ray aurora near thenorth and south magnetic poles is readily apparent. Thebottom of Figure 4 shows the map of Jupiter’s surfacemagnetic field strength in System III coordinates predictedby the VIP4 model [Connerney et al., 1998]. Comparisonof these two maps suggests to us the possibility of a relationbetween X-ray intensity and surface magnetic field strengthat low to middle latitudes. Also, using ROSAT data,Gladstone et al. [1998] found that the low-latitude X-rayintensity from Jupiter was higher in regions of low surfacemagnetic field strength.[16] In order to explore this possibility further, we bin the

data in surface magnetic field strength bins according to themagnetic field magnitude predicted by the VIP4 model atthe System III longitude and latitude corresponding to eachevent. After corrections for exposure time and projectedarea on the sky, Figure 5 and Table 3 show the results of thisprocedure for both the 24–26 February 2003 data and the18 December 2000 data. Both distributions show significantstructure at magnetic field strengths less than 8 gauss, whichaccording to the magnetic field strength map (bottom panelof Figure 4) are not associated with auroral emissions. Theoverall envelope for these distributions is roughly a rise in

Table 3. Count Rates, c/ks-sq.deg-gauss, Versus Surface Magnetic Field Strength, gauss

Field Strength Gauss All Latitudes Latitudes in Range (�45�, +45�) Latitudes in Range (�30�, +30�)

24–26 February 2003 HRC-I ACIS-S Data3.0–4.5 0.0654 ± 0.0023 0.067 ± 0.0023 0.0737 ± 0.00274.5–6.0 0.0727 ± 0.0032 0.0835 ± 0.0038 0.1399 ± 0.00786.0–6.5 0.0327 ± 0.0020 0.0438 ± 0.0028 0.112 ± 0.00996.5–7.0 0.0366 ± 0.0028 0.0669 ± 0.0059 0.1087 ± 0.01137.0–7.5 0.0508 ± 0.0048 0.129 ± 0.0155 0.3266 ± 0.05167.5–8.0 0.0569 ± 0.0055 0.2083 ± 0.0311 0.4229 ± 0.090211.0–11.5 0.1697 ± 0.0175 0.9316 ± 0.2196 1.6824 ± 0.449611.5–12.0 0.2002 ± 0.0181 0.3961 ± 0.1771 0.1900 ± 0.190012.0–12.5 0.3171 ± 0.0253 1.005 ± 0.2686 1.4239 ± 0.538212.5–13.0 0.7540 ± 0.0469 0.7089 ± 0.2506 1.3703 ± 0.6852

18 December 2000 HRC-I Data3.0–4.5 0.1500 ± 0.0046 0.1534 ± 0.0048 0.1703 ± 0.00554.5–6.0 0.1952 ± 0.0071 0.2259 ± 0.0086 0.3972 ± 0.01786.0–6.5 0.0824 ± 0.0043 0.112 ± 0.0062 0.2763 ± 0.02116.5–7.0 0.1200 ± 0.0070 0.2221 ± 0.0147 0.3631 ± 0.02827.0–7.5 0.1286 ± 0.0104 0.3376 ± 0.0341 0.9274 ± 0.11977.5–8.0 0.1464 ± 0.0120 0.6728 ± 0.0752 1.9731 ± 0.268511.0–11.5 0.1853 ± 0.0262 1.3880 ± 0.3710 2.3215 ± 0.734111.5–12.0 0.2625 ± 0.0351 2.1256 ± 0.5681 2.9176 ± 1.031512.0–12.5 0.3106 ± 0.0371 0.8282 ± 0.3381 2.3643 ± 0.965212.5–13.0 0.7011 ± 0.0601 4.7589 ± 0.8993 9.2974 ± 2.4848


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X-ray intensity with magnetic field strength, which is ofcourse to be associated with the auroral X-ray emission.However, both distributions show a striking drop at fieldstrengths �6 gauss, with a much narrower dip at �12 gauss.The depth and statistical significance of these dips dependon the chosen latitude range. While the numerology maysuggest some sort of resonant or harmonic phenomena, thephysical origin of these dips is not presently understood byus. These results indicate that the low- to middle-latitudeX-ray intensity does correlate with surface magnetic fieldstrength but in a more complicated manner than thesimple anticorrelation found in the ROSAT data byGladstone et al. [1998].[17] Additional clues might be provided by examining

and comparing high-quality X-ray spectra for each mag-netic field interval. Unfortunately, there are not enough

X-ray events in the two ACIS-S observations to allowthis more extensive analysis. We therefore compare spec-tra in the magnetic field strength intervals 3–5 gauss and5–7 gauss, as shown in Figure 6. Allowing for amultiplicative scale factor, the two spectra are not con-sistent with each other with 99.9% confidence. Adding agaussian line at 1.32 keV produces a somewhat better fit,acceptable with 8.18% confidence. The existence of sucha line (or blend) at the lowest magnetic field strengths isnot unambiguously established by these data. Assuming itis real, examination of the X-ray spectral line databasesCHIANTI V4.2 [Dere et al., 1997; Young et al., 2003]and ISIS [Houck and DeNicola, 2000] reveals the mostcommon lines within ±90 eV of 1.32 keV to be those ofhighly ionized Cr, Fe, Ni, and Co. If we may excludelines from these heavy atoms, then other possibilities arelines from Ne X, Na XI, and Mg X and XI. Thesedatabases do not list any lines of O, C, or S within ±90 eVof1.32 keV. More detailed analysis of these spectral differencesrequires additional, longer ACIS-S observations of Jupiter.

4. Temporal Variability and Relation to SolarX-Ray Flux

[18] In view of the surprising finding of 40–45 minquasi-periodic oscillations in the 18 December 2000 ChandraHRC-I data for the northern auroral zone [Gladstone et al.,2002], we searched the 24–26 February 2003 ACIS-Sand HRC-I data for time variability in the low- to middle-latitude X-ray emission. For this analysis we imposed noconstraints on System III longitude. The upper left panelof Figure 7 shows the combined ACIS-S and HRC-IX-ray light curve, using 4-min bins, for 24–26 February2003 for latitudes in the range (�60�, +60�. There is anoticeable rise and fall near the beginning of the HRC-Idata. The same behavior is seen in the upper right panel ofFigure 7, showing the light curve for the HRC-I back-ground, computed as described in section 2. Presumably,this rise and fall in HRC-I background is due to adisturbance in the solar wind passing by Chandra at thistime. We elected to excise the first �197 min of the HRC-Idata in order to avoid introducing effects due to this riseand fall into the power spectral density (PSD). The lowerleft panel of Figure 7 shows the X-ray light curve with theexcised HRC-I data removed.[19] Fourier analysis of the unsmoothed time series then

produced the PSD shown in the lower right panel ofFigure 7. There are no statistically significant peaks forperiods from 100 min down to 8 min. In order tounderstand the structure appearing in the PSD at periodslonger than 100 min, we fit a linear trend to the observedlight curve as shown in the upper left panel of Figure 8.We carried out 100 Monte Carlo simulations addingPoisson noise about this trend and removing data in thetwo gaps we have introduced in the X-ray time series,leading to the average simulation light curve shown inthe lower left panel of Figure 8. The only time depend-ences in the simulated light curves arise from the lineartrend, the presence of gaps, and Poisson noise. For eachsimulated light curve, we calculated the correspondingPSD. The lower right panel of Figure 8 shows theaverage of these 100 PSDs, which can be compared with

Figure 6. (top) X-ray spectra (c/ks-sq deg-kev), summedover the two ACIS-S observations taken over 24–26February 2003, for the magnetic field intervals 3–5 gauss(blue) and 5–7 gauss (red), with events restricted tolatitudes in the range (�45�, +45�) and more than 30� inlongitude from the limb. For both spectra, each energy bincontains at least 10 events. Testing whether the two spectraare consistent with each other, except for a multiplicativescale factor, leads to a value for c2 = 42.43 for 18 degrees offreedom. Thus we can reject this hypothesis with 99.90%confidence. (middle) The difference spectrum divided by itserrors (chi). Note the relatively large excursions (in twocases greater than 3s) between 1.0 and 1.5 keV. (bottom)The difference spectrum divided by its errors (chi), but witha Gaussian line added to the 5–7 gauss spectrum. The lineenergy determined by the new fit is �1.3 keV with linewidth comparable to the energy resolution (�120 eV) of theACIS-S. Now we find an improved value for c2 = 23.11 for15 degrees of freedom, acceptable with 8.18% confidence.


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Figure 7. Timing results for 24–26 February 2003 X-ray events in the latitude range (�60�, +60�). Forall light curves the time origin corresponds to UT 1558:06 on 24 February 2003. The light curves havebeen created by 12-min boxcar smoothing of a 4-min binning of the data. The black vertical lines fromtop to bottom mark the boundaries of excised data (see text). (upper left) X-ray light curve for ACIS-Sand HRC-I data. (upper right) Off-planet HRC-I background rescaled to the size of Jupiter, showing thesharp rise and fall near the beginning of the HRC-I exposure. (lower left) Same as upper left but with theanomalous HRC-I interval removed. (lower right) Power spectral density (PSD) versus period (in min),computed from the unsmoothed 4-min binning of the data. The horizontal solid line shows theexpectation value for a steady source with Poisson statistics. The dotted lines show the single periodprobabilities of chance occurrence as labeled on the right.

Figure 8. Comparison of observed and simulated light curves and PSDs. (upper left) Observed X-raylight curve with best-fit linear trend. (upper right) PSD for the observed light curve. (lower left) Theaverage simulated light curve (100 trials). (lower right) The average simulated PSD (100 trials).


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the observed PSD repeated in the upper right panel. Weconclude that the structure in the observed PSD for periodslonger than 100min is due to the rising linear trend interactingwith the two gaps we introduced into the data.[20] A similar procedure applied to the 18 December

2000 HRC-I data also found no evidence for periodicvariability in the range 10–100 min. These null resultswould certainly be expected for emission due to scatteringand fluorescence of solar X-rays.[21] According to Bhardwaj et al. [2005a], longer time-

scale variability in the X-ray emission measured fromJupiter’s low to middle latitudes during the 28–29 November2003 XMM-Newton observations was similar to that in

the solar X-ray flux incident on the planet. In Figure 9 wedisplay the solar X-ray/EUV flux measured by several Sun-observing satellites during the 24–26 February 2003 CXOobservations, along with the X-ray light curve fromJupiter’s low to middle latitudes. These near-Earth satelliteswere facing the same solar hemisphere as Jupiter within�22�; the Sun-Earth-Jupiter angle was �153� with Jupitertrailing. The solar X-ray flux clearly rises on averagethroughout the period 24–26 February 2003. The trendfor the CXO observations is less obvious, in part becausethe middle 20 hours of data are from the HRC-I and the firstand last 8.5 hours are from the ACIS-S, and the twocameras have different responses. However, it is clear thatthe X-ray flux in the second ACIS-S exposure is strongerthan in the first ACIS-S exposure, showing an increase�60% This is similar to the increases in the GOES 10 data(�80%) and the TIMED/SEE data (�40%). However, theSOHO/SEM 1–500 A and 260–340 A fluxes showincreases of only �5%. On the other hand, in these bandsthe solar X-ray flux is dominated by emission at lowerenergies where variability is less. Solar activity was at a lowlevel during 24–26 February 2003, and no flare in the C classor greater was observed. As then expected for scattering andfluorescence of solar X-rays, no large variation in Jupiter’slow- to middle-latitude X-ray emission was observed. Suchvariations in response to solar X-ray flares have been reportedat other times for Jupiter [Bhardwaj et al., 2005a] and forSaturn [Bhardwaj et al., 2005b].[22] Figure 10 compares light curves at 60 min binning

for Jupiter’s low- to middle-latitude X-ray emission on18 December 2000 with the corresponding GOES 10 solarflux data in the 1–8 A and 0.5–4 A bands. At this time, theGOES 10 satellite was viewing the same hemisphere of theSun as Jupiter within an angle of 19� (the Sun-Earth-Jupiterangle was 156.5� with the Jupiter trailing, and the Sun-

Figure 9. Time evolution of solar X-ray–EUV fluxesduring 24–26 February 2003 at 1 AU compared to theJupiter’s low- to middle-latitude X-ray flux measured byChandra: (a) solar 1–8 A (1.55–12.4 keV) flux measuredby GOES 10 with 30 min time bins; (b) solar 1–500 A(0.248–12.4 keV) and 260–340 A (0.03–0.477 keV) fluxmeasured by SOHO/SEM with 5 min bins (note the break inthe y-axis); (c) solar 5–65 A (0.19–2.48 keV) and 5–25 A(0.19–0.50 keV) measured by TIMED/SEE with 3-minobservation-averaged flux obtained every orbit for �15measurements per day (note the break in the y-axis); and(d) Jupiter’s low- to middle-latitude X-ray measured byChandra with 30 min bins. The Chandra time series isshifted by �4353 s (0.050382 in Earth day) in order tocorrect for light travel time differences from the Sun toEarth and from the Sun to Jupiter to Earth. Also shownin Figure 9d is the daily averaged solar flux in 1–70 A(0.18–12.4 keV) measured by TIMED/SEE (vertical scaleon the right). The solid vertical line at �0.95 day marksthe time of exposure transition from ACIS-S to HRC-Iand that at�1.8 day the transition from HRC-I to ACIS-S. Agap �1.8–1.9 day appears because we expunge 11.296 ksdata taken at the beginning of the second ACIS-S exposurewhen Jupiter overlapped its location in the second bias frame.

Figure 10. Light curves (60-min binned) for theChandra HRC-I observation of Jupiter disk X-rays and forGOES 1–8 A and 0.5–4 A solar fluxes on 18 December2000. TheGOES data are shown at themidpoint of the 1-hourbin. The time of Jovian X-rays is shifted by �4083 s toaccount for light travel time delay between Sun-Jupiter-Earthand Sun-Earth. Note that the GOES 0.5–4 A solar flux isplotted after multiplying by a factor of 10.


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Jupiter-Earth angle was 5.3�). The errors for the JupiterX-ray light curve are large, but it may show a similartrend to that in the solar data.

5. Jupiter’s Low- to Middle-Latitude X-RaySpectrum

[23] Figure 11 shows Jupiter’s low- tomiddle-latitudeX-rayspectra obtained during each of the two ACIS-S exposures(separated by about a day and each of �8.5 hour duration),together with the spectra from the north and south auroralzones. The ACIS-S background rate derived from the 0.3–2.0 keVevents located outside 1.2 Jupiter radii and scaled tothe area of the planetary disk is less than 3% of the emissionfrom the total disk. In addition, the background contributionfrom true X-rays from beyond Jupiter’s orbit is blocked bythe planet. We therefore neglect the background in ourspectral analysis of the ACIS data.[24] Jupiter’s low- to middle-latitude X-ray spectra peak

at and extend to higher energies than the auroral spectra,

raising the possibility that different physical mechanismsare responsible for the X-ray emission in each case. Inaddition, the two low- to middle-latitude X-ray spectra,separated by about 1 day, are different from each other,with increased X-ray emission above 0.6 keV in the secondexposure. From Figure 9 we note that the TIMED/SEE5–25 A and 25–65 A solar fluxes increased by �40%and �20%, respectively, between 24 and 26 February2003. The increase in the hardness of the nonauroral X-rayemission could thus be due to an increase in the hardness ofthe incident solar X-ray flux, as suggested by Figure 9, wherethe degree of flux increase between beginning and end of theobservations is larger at higher energies. Because of thischange in spectral shape, we fit model spectra to the twoexposures separately. In order to obtain reasonable fits with-out invoking overly complicated models, with questionablephysical applicability, we reduced the energy range forspectral analysis, covering �0.5 to �1.5 keV for singletemperature fits and �0.3 to �1.5 keV for two temperaturefits. There is very little fluxmeasured above 1.5 keV, althoughthere is measurable flux at least down to 0.3 keV, belowwhich the data are affected by the procedures describedin section 2. We use the X-ray astronomy spectral fittingcode XSPEC [Arnaud, 1996] to fit model X-ray spectrato the spectral data.[25] A simple thermal bremsstrahlung model in XSPEC

gives a very poor fit to the 0.5–1.5 keV nonauroral X-rayspectra, with values of cred

2 (c2 per degree of freedom)�10 for each exposure. We therefore turned to theMEKAL model (http://xspec.gsfc.nasa.gov/docs/xanadu/xspec/manual/XSmodelMekal.html) based on model cal-culations of X-ray emission from a hot, optically thinplasma in ionization equilibrium [Mewe et al., 1985;Mewe et al., 1986; Kaastra, 1992; Liedahl et al., 1995](see also Arnaud and Rothenflug [1985] and Arnaud andRaymond [1992] for the adopted ionization balance). TheMEKAL model, which allows some choices for abundan-ces and cross sections, is widely used in X-ray astronomyto fit the X-ray spectra from stellar coronae, and weexpect it to provide a reasonably close approximation tothe solar X-ray spectrum incident on Jupiter. To theextent that Jupiter’s low- to middle-latitude X-ray spectrumarises from scattering and fluorescence of solar X rays in itsatmosphere and to the extent that X-ray interactions inJupiter’s atmosphere preserve the incident spectral shape,the MEKAL model would be a reasonable choice for fittingJupiter’s low- to middle-latitude X-ray spectrum (but seeCravens et al. [2006] for discussion of X-ray albedos for theouter planets). In our case, using default (solar) abundancesand cross sections, theMEKALmodel does provide fitsmuchimproved over those using a simple thermal bremsstrahlungmodel. Including a gaussian line with line center fixed at1.35 keV and zero intrinsic width gives even betterresults. Table 4 lists the parameters of the best-fit models,while Figure 12 compares the best-fit models (includingthe line) with the spectral data. For comparison, Table 4includes the best-fit parameters for the 28–29 April 2003XMM-Newton data [Branduardi-Raymont et al., 2004].[26] The 0.5–1.5 keV low- to middle-latitude energy

fluxes implied by these fits are �3.7 10�14 and �7 10�14 erg/s cm2 for the first and second exposures,respectively. The corresponding X-ray luminosities of

Figure 11. Jupiter’s auroral X-ray spectra compared to thelow- to middle-latitude X-ray spectrum observed during thetwo Chandra ACIS-S exposures on 24 February 2003 and25–26 February 2003: (a) north auroral spectrum; (b) southauroral spectrum; and (c) low- to middle-latitude spectrum.Note the different scales on the vertical axes. The threeregions for spectral analysis are shown in the bottom panelsof Figure 1. The two ACIS-S exposures were of nearly thesame duration and taken �1 day apart (see Table 1). For thedisplayed spectra, ACIS-S energy channels were groupedtogether so that each point represents at least 10 events.Jupiter’s low- to middle-latitude X-ray spectrum isnoticeably harder than the auroral spectra, extending tohigher energies.


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Figure 12. Fit of model X-ray spectrum (MEKAL plus gaussian line at 1.35 keV, see text), withresiduals, to Jupiter’s low- to middle-latitude X-ray spectra measured by Chandra ACIS-S on 24 February2003 (top) and 25–26 February 2003 (bottom). Each spectral point represents at least 20 events. Best-fitparameters are given in Table 4.


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0.21 and 0.39 GW, respectively, show a factor of twoincrease in just 1 day. For comparison, the X-ray lumi-nosity for the northern auroral zone was 0.68 GW duringthe first ACIS-S exposure [Elsner et al., 2005a]. Addinga second narrow Gaussian line at 1.2 keV slightlyimproves the fit for the second exposure (cred

2 = 1.2 for23 degrees of freedom). For the MEKAL plus one linefits, the best-fit temperature for the second exposure is23% higher than for the first exposure. As alreadypointed out, this harder spectrum appears to correlatewith increased hardness of the incident solar X-ray flux.[27] According to the X-ray spectral line databases CHI-

ANTI V4.2 [Dere et al., 1997; Young et al., 2003] and ISIS[Houck and DeNicola, 2000], the most common lineswithin ±90 eV of 1.35 keV are those of highly ionized Cr,Fe, Ni, and Co. Excluding lines from these heavy atoms,other possibilities are lines from Ne X, Mg X, and Mg XI.The line due to Mg XI (1S�1P0) emission is a strong line inthe solar X-ray spectrum and so scattering of photons in thisline may account for the possible line emission. Thesedatabases do not list any lines of O, C, or S within±90 eV of 1.35 keV.[28] Recently, Cravens et al. [2006] compared the low- to

middle-latitude ACIS-S spectra to model spectra for scat-tering and fluorescence of solar X-rays from Jupiter’s upperatmosphere, finding reasonable agreement in the energyband 0.4–0.8 keV. However, their models fall below thedata by a factor of about 2 in the 0.8–1.3 keV band. Onereason could be the known variability of the incident solarX-ray spectrum. A true test of their model would involvecomparing the measured spectrum with the solar spectrumaveraged over the same time interval as the X-ray observa-tions (taking light travel time into account). The solar X-rayflux can change by factors of a few to a few tens over the

solar cycle and it can increase by factors of hundreds tothousands during an X-ray flare [cf. Peres et al., 2000].

6. Summary

[29] The low surface brightness of Jupiter’s nonauroralX-ray emission hampers our ability to determine thephysical processes which are responsible for it. Ourresults point to scattering (and fluorescent emission fromcarbon in methane molecules [Cravens et al., 2006]) ofthe incident solar X-ray flux as an important mechanism.The nonauroral X-ray spectra in the 0.5–1.5 keV energyband can be fitted by thermal emission from a hotoptically thin plasma in ionization equilibrium, a modelrepresentative of the incident solar X-ray spectrum. TheX-ray flux from low to middle latitudes on Jupiter varieson timescales from days to years in a manner similar tovariations in the solar X-ray flux as measured by near-Earthsatellites and by the F10.7 cm proxy for solar activity. On theother hand, Cravens et al. [2006] found that applying acalculated albedo function to an assumed incident solarX-ray spectrum provides a bad fit at energies above 0.8 keV.We find evidence for temporal correlation between Jupiter’sobserved nonauroral X-ray emission and solar fluxes mea-sured at Earth, although this result could be put on firmerground by, for example, the observation of a large flare (suchas was seen at Saturn [Bhardwaj et al., 2005b]).[30] Because of Chandra’s superb spatial resolution and

time-tagging of events, we were able to construct relation-ships between the X-ray intensity from Jupiter in System IIIcoordinates and the surface strength of Jupiter’s magneticfield. As shown in Figure 5, the correlation is morecomplicated than the simple anticorrelation reported fromROSAT observations [Gladstone et al., 1998]. Thereappears to be no reason that scattering and fluorescence of

Table 4. Best Fit Parameters for Jupiter’s Low- to Middle-Latitude X-Ray Spectra

Modela c2red

b dof c kT1, keVd Norm1

e kT2, keVd Norm2

e Line Strengthf

OBSID 3726g

M 3.28 15 0.50�0.25+0.27 15�2

+3

M + L 2.70 14 0.48�0.18+0.09 15�2

+2 1.1�0.9+0.7

M + M 3.49 15 0.61�0.09+0.09 14�2

+3 0.08+0.01h 49�44+26

M + M + L 3.13 14 0.60�0.06+0.06 14�2

+2 0.08+0.01h 50�44+24 0.9�0.9

+0.9

OBSID 4418i

M 1.79 24 0.63�0.04+0.04 28�2

+2

M + L 1.34 23 0.62�0.04+0.03 27�2

+3 1.9�0.4+1.4

M + M 2.04 24 0.64�0.04+0.04 28�3

+3 0.08+0.01h 50�31+22

M + M + L 1.54 23 0.63�0.04+0.04 23�2

+2 0.08+0.01h 51�26+19 1.5�0.8

+0.8

XMMj

M + L + Lk 0.76 57 0.51�0.04+0.04 15.8�1.2

+1.0 0.48�0.36+0.46

aSpectral models fitted to the data: M = MEKAL; M + L = MEKAL + gaussian line; M + M = MEKAL + MEKAL; and M + M + L = MEKAL +MEKAL + gaussian line. The energy range for fits to single and double MEKAL models were 0.5–1.5 keV and 0.3–1.5 keV, respectively. Thus thenumber of spectral bins differs for single and double MEKAL model fits.

bReduced c2 equal to the best-fit value for c2 divided by the number of degrees of freedom.cNumber of degrees of freedom equal to the number of spectral bins minus the number of free parameters.dTemperature in keV of the MEKAL model.eXSPEC normalization of the MEKAL model in 10�6 photons/s cm2.fTotal line photon flux in 10�6 photon/s cm2 for a gaussian line with line center fixed at 1.35 keV and with zero intrinsic width.gThe first ACIS-S exposure on 24 February 2003.hSince this temperature lies well below our low energy cutoff of 0.3 keV, XSPEC was unable to set a lower error bar for this parameter.iThe second ACIS-S exposure on 25–26 February 2003.jThe XMM observation on 28–29 April 2003 [Branduardi-Raymont et al., 2006b].kThe second line in the XMM spectral model is Si XIII at 1.86 keV.


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solar X-rays should lead to such a correlation. Such behav-ior could result from increased particle precipitation fromradiation belts closer to the planet than in other regions.This component of the low- to middle-latitude X-ray emis-sion would then map out regions similar to the SouthAtlantic Anomaly in the Earth’s radiation belt. There isalso an apparent difference in the spectra for regions withsurface fields in the 3–5 gauss range and those with surfacefields in the 5–7 gauss range. The lower field regions showexcess emission for several tens of eV around 1.32 keV.However, particle precipitation seems unlikely to accountfor all, or even most, of Jupiter’s nonauroral X-ray emissionas we would not expect it to correlate temporally with theincident solar X-ray flux.[31] In conclusion, we have found evidence for two

mechanisms possibly being responsible for Jupiter’s low-to middle-latitude nonauroral X-ray emission. We suggestthat scattering and fluorescence of the incident solar X-rayflux in Jupiter’s atmosphere normally predominates, pro-ducing the observed hard spectrum and temporal correla-tions; in addition, we propose the existence of a componentfrom particle precipitation in regions of low surface mag-netic field strength.[32] The ideal way to investigate Jupiter’s nonauroral

X-ray emission in more detail and resolve the outstandingquestions about its origin would be simultaneous highstatistical quality, spatially resolved spectral measurementsof both the emission from the planet, and the solar X-rayflux incident on it which would allow an effective scatteringalbedo to be determined and compared with theoreticallycalculated albedos [i.e., Cravens et al., 2006].

[33] Acknowledgments. We thank Tom Wood for help in providingthe TIMED/SEE Version 7 Data Products. The solar SEM data were takenwith the CELIAS/SEM experiment on the SOHO spacecraft which is a jointESA and NASA mission. The GOES 10 X-ray data were obtained from theSpace Physics Interactive Data Resource sitehttp://spidr.ngdc.noaa.gov/spidr/logoff.do. The Solar Radiation and Climate Experiment (SORCE)Solar Spectral Irradiance (SSI) data were taken fromhttp://lasp.colorado.edu/sorce/ssi\data.html. This research was supported in part by guestobserver grants from the Chandra X-ray Center. Part of this research wasconducted when A. Bhardwaj was a National Research Council SeniorResident Research Associateship at the NASA Marshall Space FlightCenter. Finally, we thank one of the referees for suggesting the analysisleading to Figure 5 and associated discussion.[34] Wolfgang Baumjohann thanks Vasili Kharchenko and another

reviewer for their assistance in evaluating this paper.

ReferencesArnaud, K. A. (1996), XSPEC: The first ten years, in Astronomical DataAnalysis Software and Systems V, ASP Conf. Ser., vol. 101, edited byG. Jacoby and J. Barnes, pp. 17–20, Astron. Soc. of the Pac., SanFrancisco, Calif. (Available at http://heasarc.gsfc.nasa.gov/docs/xanadu/xspec/)

Arnaud, M., and J. Raymond (1992), Iron ionization and recombinationrates and ionization equilibrium, Astrophys. J., 398, 394–406.

Arnaud, M., and M. Rothenflug (1985), An updated evaluation of recom-bination and ionization rates, Astron. Astrophys. Suppl., 60, 425–457.

Barbosa, D. D. (1990), Bremsstrahlung X-rays from Jovian auroral elec-trons, J. Geophys. Res., 95, 14,969–14,976.

Bhardwaj, A. (2003), X-ray emissions from the Jovian system, Bull. Astron.Soc. India, 31, 159–166.

Bhardwaj, A. (2006), X-ray emission from Jupiter, Saturn, and Earth: A shortreview, in Advances in Geosciences, edited by A. Bhardwaj et al., vol. 3,pp. 215–230, World Sci., Tokyo.

Bhardwaj, A., and G. R. Gladstone (2000), Auroral emissions of the giantplanets, Rev. Geophys., 38, 295–353.

Bhardwaj, A., et al. (2002), Soft X-ray emissions from planets, moons, andcomets, Eur. Space Agency Spec. Publ., ESA-SP-514, 215–226.

Bhardwaj, A., G. Branduardi-Raymont, R. F. Elsner, G. R. Gladstone,G. Ramsay, P. Rodriguez, R. Soria, J. H. Waite Jr., and T. E. Cravens(2005a), Solar control on Jupiter’s equatorial X-ray emissions: 26–29 November 2003 XMM-Newton observation, Geophys. Res. Lett.,32, L03S08, doi:10.1029/2004GL021497.

Bhardwaj, A., R. F. Elsner, J. H. Waite Jr., G. R. Gladstone, T. E. Cravens,and P. G. Ford (2005b), Chandra observation of an X-ray flare at Saturn:Evidence for direct solar control on Saturn’s disk X-ray emissions, Astro-phys. J. Lett., 624, L121–L124.

Bhardwaj, A., et al. (2006), X-rays from solar system bodies, Planet. SpaceSci., in press.

Branduardi-Raymont, G., R. F. Elsner, G. R. Gladstone, G. Ramsay,P. Rodriguez, R. Soria, and J. H. Waite Jr. (2004), First observation ofJupiter by XMM-Newton, Astron. Astrophys., 424, 331–337.

Branduardi-Raymont, G., A. Bhardwaj, R. F. Elsner, G. R. Gladstone,G. Ramsay, P. Rodriguez, R. Soria, J. H. Waite, and T. E. Cravens(2006a), XMM-Newton observations of X-ray emission from Jupiter,Eur. Space Agency Spec. Publ., ESA SP-604, 15–20.

Branduardi-Raymont, G., A. Bhardwaj, R. F. Elsner, G. R. Gladstone,G. Ramsay, P. Rodriguez, R. Soria, J. H. Waite, and T. E. Cravens(2006b), Latest results on Jovian disk X-rays from XMM-Newton,Planet. Space Sci., in press.

Bunce, E. J., S. W. H. Cowley, and T. K. Yeoman (2004), Jovian cuspprocesses: Implications for the polar aurora, J. Geophys. Res., 109,A09S13, doi:10.1029/2003JA010280.

Connerney, J. E. P., M. H. Acuna, N. F. Ness, and T. Satoh (1998), Newmodels of Jupiter’s magnetic field constrained by the Io flux tube foot-print, J. Geophys. Res., 103, 11,929–11,939.

Cravens, T. E., E. Howell, J. H. Waite Jr., and G. R. Gladstone (1995),Auroral oxygen precipitation at Jupiter, J. Geophys. Res., 100, 17,153–17,161.

Cravens, T. E., J. H. Waite, T. I. Gombosi, N. Lugaz, G. R. Gladstone, B. H.Mauk, and R. J. MacDowall (2003), Implications of Jovian X-ray emis-sion for magnetosphere-ionosphere coupling, J. Geophys. Res.,108(A12), 1465, doi:10.1029/2003JA010050.

Cravens, T. E., J. Clark, A. Bhardwaj, R. F. Elsner, J. H. Waite Jr., A. N.Maurellis, G. R. Gladstone, and G. Branduardi-Raymont (2006), X-rayemission from the outer planets: Albedo for scattering and fluorescenceof solar X-rays, J. Geophys. Res., 111, A07308, doi:10.1029/2005JA011413.

Dere, K. P., E. Landi, H. E. Mason, B. C. Monsignori Fossi, and P. R.Young (1997), CHIANTI: An atomic database for emission lines, Astron.Astrophys. Suppl., 125, 149–173.

Elsner, R. F., et al. (2002), Discovery of soft X-ray emission from Io,Europa, and the Io plasma torus, Astrophys. J., 572, 1077–1082.

Elsner, R. F., et al. (2005a), Simultaneous Chandra X-ray, HST Ultraviolet,and Ulysses radio observations of Jupiter’s aurora, J. Geophys. Res., 110,A01207, doi:10.1029/2004JA010717.

Elsner, R. F., B. D. Ramsey, J. H. Waite Jr., P. Rehak, R. E. Johnson, J. F.Cooper, and D. A. Swartz (2005b), X-ray probes of magnetosphericinteractions with Jupiter’s auroral zones, the Galilean satellites, and theIo plasma torus, Icarus, 178, 417–428.

Elsner, R. F., B. D. Ramsey, D. A. Swartz, J. A. Gaskin, P. Rehak, J. H.Waite Jr., J. F. Cooper, and R. E. Johnson (2005c), X-ray probes ofJupiter’s auroral zones, Galilean moons, and the Io plasma torus, SPIEProc., 5906, 59061B-1–59061B12.

Gladstone, G. R., J. H. Waite Jr., and W. S. Lewis (1998), Secular and localtime dependence of Jovian X ray emissions, J. Geophys. Res., 103,20,083–20,088.

Gladstone, G. R., et al. (2002), A pulsating auroral X-ray hot spot onJupiter, Nature, 415, 1000–1003.

Houck, J. C., and L. A. DeNicola (2000), ISIS: An interactive spectralinterpretation system for high-resolution X-ray spectroscopy, in Astro-nomical Data Analysis Software and Systems IX, ASP Conf. Ser., vol.216, edited by N. Manset, C. Veillet, and D. Crabtree, p. 591, Astron.Soc. of the Pac., San Francisco, Calif.

Kaastra, J. S. (1992), An X-ray spectral code for optically thin plasmas,internal report, updated version 2.0, Netherlands Inst. for Space Res.,Leiden, Netherlands.

Kharchenko, V., W. Liu, and A. Dalgarno (1998), X-ray and EUVemissionspectra of oxygen ions precipitating into the Jovian atmosphere, J. Geo-phys. Res., 103, 26,687–26,698.

Liedahl, D. A., A. L. Osterheld, and W. H. Goldstein (1995), New calcula-tions of Fe L-shell X-ray spectra in high-temperature plasmas, Astrophys.J. Lett., 438, L115–L118.

Liu, W., and D. R. Schultz (1999), Jovian X-ray aurora and energeticoxygen ion precipitation, Astrophys. J., 526, 538–543.

Maurellis, A. N., T. E. Cravens, G. R. Gladstone, J. H. Waite, and L. W.Acton (2000), Jovian X-ray emission from solar X-ray scattering,Geophys. Res. Lett., 27, 1339–1342.


15 of 16

A11225

Metzger, A. E., D. A. Gilman, J. L. Luthey, K. C. Hurley, H. W. Schnopper,F. D. Seward, and J. D. Sullivan (1983), The detection of X-rays fromJupiter, J. Geophys. Res., 88, 7731–7741.

Mewe, R., E. H. B. M. Gronenschild, and G. H. J. van den Oord (1985),Calculated X-radiation from optically thin plasmas. V., Astron. Astrophys.Suppl., 62, 197–254.

Mewe, R., J. R. Lemen, and G. H. J. van den Oord (1986), CalculatedX-radiation from optically thin plasmas. VI - Improved calculations forcontinuum emission and approximation formulae for nonrelativisticaverage Gaunt factors, Astron. Astrophys. Suppl., 65, 511–536.

Peres, G., S. Orlando, F. Reale, R. Rosner, and H. Hudson (2000), The Sunas an X-ray star. II. Using the Yohkoh soft X-ray telescope-derived solaremission measure versus temperature to interpret stellar X-ray observa-tions, Astrophys. J., 528, 537–551.

Plucinsky, P. P., et al. (2003), Flight spectral response of the ACIS instru-ment, in X-Ray and Gamma Ray Telescopes and Instruments for Astron-omy, edited by E. Truemper, and H. D. Tananbaum, Proc. SPIE, 4851,89–100.

Singhal, R. P., S. C. Chakravarty, A. Bhardwaj, and B. Prasad (1992),Energetic electron precipitation in Jupiter’s upper atmosphere, J. Geo-phys. Res., 97, 18,245–18,256.

Waite, J. H., Jr. (1991), Comment on ‘‘Bremsstrahlung X rays from Jovianauroral electrons’’ byD.D. Barbosa, J. Geophys. Res., 96, 19,529–19,532.

Waite, J. H., F. Bagenal, F. Seward, C. Na, G. R. Gladstone, T. E. Cravens,K. C. Hurley, J. T. Clarke, R. Elsner, and S. A. Stern (1994), ROSATobservations of the Jupiter aurora, J. Geophys. Res., 99(A8), 14,799–14,809.

Waite, J. H., Jr., et al. (1995), ROSAT observations of X-ray emissions fromJupiter during the impact of Comet Shoemaker-Levy 9, Science, 268,1598–1601.

Waite, J. H., Jr., W. S. Lewis, G. R. Gladstone, A. C. Fabian, and W. N.Brandt (1996), Jovian X-ray emissions, in Rontgenstrahlung from the

Universe: International Confrence on X-ray Astronomy and Astrophysics,MPE Rep. 263, edited by H. U. Zimmermann, J. Trumper, and H. Yorke,pp. 641–644, Max Planck Institute for Extraterrestrial Physics, Garching,Germany.

Waite, J. H., G. R. Gladstone, W. S. Lewis, P. Drossart, T. E. Cravens, A. N.Maurellis, B. H. Mauk, and S. Miller (1997), Equatorial X-ray emissions:Implications for Jupiter’s high exospheric temperatures, Science, 276,104–108.

Young, P. R., G. Del Zanna, E. Landi, K. P. Dere, H. E. Mason, andM. M. Landini (2003), CHIANTI: An atomic database for emissionlines. VI. Proton rates and other improvements, Astrophys. J. Suppl.,144, 135–152.

��A. Bhardwaj, Space Physics Laboratory, Vikram Sarabhai Space Centre,

Trivandrum 695022, Kerala, India. ([email protected])G. Branduardi-Raymont, Mullard Space Science Laboratory, University

College London, Holmbury St. Mary, Dorking, Surrey RH5 6NT, UK.([email protected])T. E. Cravens, Department of Physics and Astronomy, University of

Kansas, Lawrence, KS 66045, USA. ([email protected])R. F. Elsner, Space Science Branch, NASA Marshall Space Flight Center,

NSSTC/VP62, 320 Sparkman Drive, Huntsville, AL 35805, USA.([email protected])P. G. Ford, Kavli Institute for Astrophysics and Space Research,

Massachusetts Institute of Technology, 70 Vassar Street, Cambridge, MA02139, USA. ([email protected])G. R. Gladstone and J. H. Waite Jr., Southwest Research Institute, 6220

Culebra Road, San Antonio, TX 78228-0510, USA. ([email protected])


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Low to middle-latitude X-ray emission from Jupiter