Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 219–224

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Journal of Atmospheric and Solar-Terrestrial Physics

1364-68

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jastp

Global characteristics of the correlation and time lag between solar andionospheric parameters in the 27-day period

Choon-Ki Lee a,n, Shin-Chan Han b,c, Dieter Bilitza d,e, Ki-Weon Seo a

a Division of Polar Earth-System Sciences, Korea Polar Research Institute, 12 Gaetbeol-ro, Yeonsu-gu, Incheon 406-840, Koreab Planetary Geodynamics Laboratory, Code 698, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USAc University of Maryland at Baltimore County, 1000 Hilltop Circle, Baltimore, MD 21250, USAd Heliophysics Laboratory, Code 672, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USAe Space Weather Laboratory, George Mason University, 4400 University Drive, Fairfax, VA 22030, USA

a r t i c l e i n f o

Article history:

Received 24 October 2011

Received in revised form

19 January 2012

Accepted 22 January 2012Available online 2 February 2012

Keywords:

Ionosphere

Electron density

GRACE

CHAMP

26/$ - see front matter & 2012 Elsevier Ltd. A

016/j.jastp.2012.01.010

esponding author. Tel.: þ82 10 3635 9798.

ail address: cklee92@kopri.re.kr (C.-K. Lee).

a b s t r a c t

The 27-day variations of topside ionosphere are investigated using the in situ electron density

measurements from the CHAMP planar Langmuir probe and GRACE K-band ranging system. As the

two satellite systems orbit at the altitudes of �370 km and �480 km, respectively, the satellite data

sets are greatly valuable for examining the electron density variations in the vicinity of F2-peak. In a

27-day period, the electron density measurements from the satellites are in good agreements with the

solar flux, except during the solar minimum period. The time delays are mostly 1–2 day and represent

the hemispherical asymmetry. The globally-estimated spatial patterns of the correlation between solar

flux and in situ satellite measurements show poor correlations in the (magnetic) equatorial region,

which are not found from the ground measurements of vertically-integrated electron content. We

suggest that the most plausible cause for the poor correlation is the vertical movement of ionization

due to atmospheric dynamic process that is not controlled by the solar extreme ultraviolet radiation.

& 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The Earth’s ionosphere is strongly controlled by solar extremeultraviolet irradiance. It is therefore not surprising that the27-day periodicity observed in solar flux recordings, that is dueto the solar rotation period, can also be observed in ionosphericparameter recordings like the F peak plasma frequency, foF2, andthe total ionospheric electron content, TEC. Nevertheless, someearlier studies have found only a weak correlation between theseparameter’s 27-day periodicities (Doherty et al., 2000; Richards,2001). It was suggested that, besides the solar extreme ultraviolet(EUV) forcing, other factors could affect the local ionosphereparameters and at times even more strongly than the solar EUVflux. They include neutral winds, plasma vertical drift, verticalcoupling in the atmosphere-ionosphere system through atmo-spheric waves (tidal and planetary waves), and geomagneticactivity (Rich et al., 2003).

Unlike the local ionospheric parameters, the global mean TECmeasured by GPS and as compiled in the Global Ionospheric Map(GIM) shows a correlation with the solar activity proxy such asthe radio flux at 10.7 cm (F10.7) or the Mg II index (Afraimovich

ll rights reserved.

et al., 2008; Hocke, 2008). The Defense Meteorological SatelliteProgram (DMSP) satellite also observed a 27-day variation inplasma density with an amplitude of up to 40–50% at the altitudeof �840 km (Rich et al., 2003). A similar result was found fromthe analysis of KOMPSAT-1 measurements at�675 km (Min et al.,2009).

In this study, we investigate the 27-day ionospheric modula-tion at the altitudes of �370 km and �480 km close to the F2peak by analyzing the electron density measured by the CHAllen-ging Minisatellite Payload (CHAMP) planar Langmuir probe (PLP)and the Gravity Recovery and Climate Experiment (GRACE)K-band ranging system. We study the spatial dependence ofcross-correlations between the electron density measurementsand the solar flux measurements globally. We also discuss thealtitude dependence of the 27-day ionospheric variations byanalyzing the in situ satellite measurement and the ground-basedTEC from GPS.

2. Data and method

The CHAMP satellite measures the electron density with theon-board PLP instrument every 15 seconds in a near-circular orbitwith an inclination of 87.31 (Reigber et al., 2002). Its meanaltitude decreased from �450 km in 2000 to �320 km in 2009.

0

4

8

Ne

(x 1

05 cm

−3)

CHAMP

−50050

% C

HA

NG

E0

4

8

Ne

(x 1

05 cm

−3)

GRACE

−50050

% C

HA

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E

0

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X (x

102

sfu)

F10.7

−50050

% C

HA

NG

E

2003 2004 2005 2006 2007 2008 2009 20100

2

4

6

FLU

X (x

1010

cm

−2 s

−1)

YEAR

SOHO/SEM

−50050

% C

HA

NG

E

Fig. 1. Time-series of the daily averaged electron density from CHAMP and GRACE

and of the F10.7 index and the solar EUV radiation in the 0.1–50 nm wavelength

(SOHO/SEM). They are depicted in black. The percentage changes of the daily

values are shown in gray.

C.-K. Lee et al. / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 219–224220

The orbital configuration is such that CHAMP sweeps through alllocal times every 131 day allowing the satellite to monitor theglobal ionospheric climatology. The two GRACE satellites measureinter-satellite distance changes every 5 seconds from the dual-frequency K-band ranging system at an orbit altitude of �480 kmand an inclination of 891 (Tapley et al., 2004). The phasedifference between the K- and Ka-band signals can be used todeduce the (biased) electron content between the two GRACEsatellites. The electron density at the intermediate positionbetween the two satellites is approximated by the electroncontent divided by the baseline length of �220 km (Lee et al.,2011).

We use two solar flux measurements, F10.7 index and SOHO/SEM, to derive the correlation with the electron density in iono-sphere. The F10.7 indicates the solar radiation flux on thewavelength of 10.7 cm in s.f.u. units (10�22 Wm�2 Hz�1)(Tapping and Charrois, 1994), and has been most widely usedfor ionospheric studies and monitored over 60 years. Recently, thesolar EUV radiation in two bands (0.1–50 nm and 26–34 nm) hasbeen measured by the Solar EUV Monitor (SEM) spectrometeraboard the Solar Heliospheric Observatory (SOHO) (Judge et al.,1998). We use the SOHO/SEM daily averages in the 0.1–50 nmband since the two bands are strongly correlated. Although it isapparently nonlinear with F10.7, the solar EUV radiation fromSOHO/SEM is more linearly correlated with a solar activity factorP, (F10.7þF10.7A)/2 (Liu et al., 2006). Thus a few extremely largeor small records in SOHO/SEM apart from the linear relation withP are removed and then interpolated using their neighbors.

The 27-day periodic variation in the satellite measurements ismodulated by long-period trends. As discussed in Rich et al.(2003) and Oinats et al. (2008), the percentage change is moreuseful than the original time-series of measurements for comput-ing the cross-correlation with the solar flux recordings. We takethe following steps to compute the cross-correlation of thepercentage change: (i) All electron density measurements withina day are averaged (denoted by Nd) to be comparable to the dailyF10.7; (ii) By using a finite impulse response low-pass filter, weremove short-period (o13 day) variations, such as the strongperiodicities of 7 and 9 day that might be associated withvariations in solar wind high-speed streams and geomagneticactivity and that are generally not observed in the F10.7 data(Thayer et al., 2008); (iii) Long-period trends are removed bysubtracting the 27-day running average Nd from the low-passfiltered daily electron density Nd and then a percentage changedNd is calculated by dNd ¼ ðNd�NdÞ=Nd; (iv) The percentagechanges of the F10.7 and the SOHO/SEM are also calculatedfollowing the same steps (ii) and (iii); (v) The time-series ofelectron density measurements and SOHO/SEM have an effectivetime tag of 12 h UT because we have averaged the data over aday; conversely, the F10.7 has a time tag of 20 h UT since it ismeasured at noon (local time) at Penticton, Canada (Tapping andCharrois, 1994). In order to match the time tags of F10.7 andothers and improve the resolution of time delay, all time-seriesare resampled every 8 h at time tags of 4 h, 12 h, and 20 h UT by acubic spline interpolation; (vi) Finally, the correlation functionsbetween the two time-series of percentage changes in theelectron density and the solar flux are calculated with a timelag varying from �5 to 5 day. The correlation coefficient (C) andtime delay (D) are defined as the maximum value of the correla-tion function and the corresponding time lag, respectively (Oinatset al., 2008).

The ground-based TEC data in our study are from the CODE(Center for Orbit Determination in Europe) GIMs (http://aiuws.unibe.ch/ionosphere). The GIM TEC (CHAMP) and GIM TEC(GRACE) represent the TEC interpolated at the time and locationof the CHAMP and GRACE measurements, respectively.

3. Results

The time-series of daily-averaged electron densities (Nd) fromCHAMP and GRACE are characterized by annual, semiannual, and27-day variations with a decreasing trend due to the 11-year solarcycle (Fig. 1). Our low and high-pass filtering removes most ofthese variations and the percentage change shown in the lowerpart of the panels in Fig. 1 is dominated by the 27-day variation.The percentage change of solar flux is visibly correlated with thepercentage change of the satellite measurements. To study thetime-dependence of the correlation, we divide the time-seriesinto the overlapped subsets and then calculate the correlationcoefficient in each subset. The length of subset segments is 81 dayand the interval between two consecutive subsets is 27 day. Foran 81-day sample length, the correlation coefficient correspond-ing to the 0.01 significance level (99% confidence level) is 0.285 inStrudent’s t distribution. In general, the correlation varies from0.7 to 0.9 depending on time, as shown in Fig. 2. The correlationcoefficient is decreasing with decreasing solar activity becausewith diminishing solar influence other effects like the meteor-ological forcing from below become more influential. The magni-tude of the meteorological forcing does not depend on solaractivity and therefore during low solar activity it is responsiblefor a larger percentage of the ionospheric variability than duringhigh solar activity. Another possible cause of the de-correlationwith diminishing solar influence is the random noise in the solarflux measurements because, for example, the F10.7 variation atthe solar minimum is only in the range of 70.7 S.F.U. close to theaccuracy of F10.7 index. The SOHO/SEM is more highly correlatedwith the electron densities than the F10.7 is (C¼0.84 for F10.7-CHAMP, C¼0.85 for F10.7-GRACE, C¼0.90 for SOHO/SEM-CHAMP,and C¼0.89 for SOHO/SEM-GRACE, averaged from 2003 to 2007),

0

0.2

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0.6

0.8

1

CO

RR

. CO

EF.

CHAMP

GRACE

2003 2005 2007 2009

−4

−2

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Y (

day)

YEAR

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GRACE

0 20 40

POPULATION

0

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GRACE

2003 2005 2007 2009

−4

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Y (

day)

YEAR

CHAMP

GRACE

0 20 40

POPULATION

Fig. 2. Correlation coefficients and time delays (a) between the electron density and the F10.7 and (b) between the electron density and the SOHO/SEM. The dashed lines in

the left plots indicate the correlation coefficient of 0.01 significance level. The right plots show the histograms of the respective time-series. The gray symbols in the time

delay are only from the sub-set data with low correlations (0.6 or below), which are excluded for the histogram.

C.-K. Lee et al. / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 219–224 221

representing that the SOHO/SEM is a better proxy for ionosphericparameters than F10.7.

The temporal distribution of the time delay is also shown inFig. 2. The electron density dominantly correlates best with thesolar flux at a time lag of 1–2 day. This means that it takes 1–2day for changes in solar activity to document themselves in theionosphere. Similar time lags have been reported based on radiobeacon and GPS-TEC observations (Jakowski et al., 1991, 2002).Such a time lag may be caused by the long lifetime of atomicoxygen, a major source for ionospheric electrons and Oþ ions. O iscreated at lower altitudes (�100 km) by photo-dissociation, andthen diffuses upward where neutral densities are much lower andchances for the required 3-body collisions are very small resultingin Oþ lifetimes of one to several days. From 2003 to 2007, the

time delay for F10.7 (1.771.2 day) is larger than that for SOHO/SEM (1.171.0 day), as locally observed (Maruyama, 2010). Largedelays (43 day) are observed from F10.7 in year 2004 and every2 years, which are not found in the SOHO/SEM. They induce suchaveraged difference in the time delay estimates (�0.6 day).

The spatial patterns of correlation and time delay are com-puted by locally averaging the electron density measurements inthe period from 2003 to 2007. In a specific area, the ascending anddescending track represent the observations with a 12-hour localtime difference. Therefore, we grouped the satellite measure-ments into ascending and descending track sets and then calcu-late the correlation and time delay. In each set, the data within aday are binned and averaged on an 18�16 grid (i.e., 101 inlatitude and 22.51 in longitude) that corresponds to the satellites’

−90

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eg)

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LONGITUDE (deg)

0

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3

4day

Fig. 3. Spatial distributions of (a–d) correlation coefficient and (e–h) time delay between the F10.7 and four data sets: (a, e) CHAMP, (b, f) GRACE, (c, g) GIM TEC (CHAMP),

and (d, h) GIM TEC (GRACE). The white line and white cross indicate the magnetic equator and the magnetic pole, respectively.

C.-K. Lee et al. / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 219–224222

sampling interval of 16 revolutions per day. The time-series ofaveraged electron density in each grid are processed following theprocedure described in the previous section. No significantdifference is found between the results from ascending anddescending track sets, which allows us to look into only one set,for example, ascending track data. The time-series in each gridcell may include the local time variation as well as the seasonalvariation, since the CHAMP and GRACE sweep all local time every131 day and 160 day, respectively. However, significant changesin the correlation with respect to either local time or season arenot observed.

The spatial patterns of the correlation between F10.7 andelectron density on a 101 (latitude)�22.51 (longitude) grid areshown in Fig. 3 (The ones for SOHO/SEM are similar and notshown). The highest correlation is found at mid-latitudes. Thecorrelation coefficients are relatively low in high-latitudes andgenerally higher in the northern hemisphere than in the southernhemisphere. In the southern hemisphere, the low correlationextends from the South Pole to the mid-latitude over the Indianand Atlantic sectors. At high latitudes, the direct impact of thesolar wind through open magnetic field lines and the downpourof precipitating particles at auroral latitudes compete with thesolar EUV influence on the ionosphere. Most interestingly, at lowlatitudes around the magnetic equator the in situ satellite datashow a low correlation while the TEC data exhibit a highcorrelation. At low latitudes, the atmospheric dynamic processessuch as the equatorial ionization fountain (often called EquatorialIonization Anomaly, EIA) are the dominant forces shaping theionosphere plasma (Stening, 1992). Those distinct effects at thehigh and low latitudes may be responsible for the spatial patternin the correlation of the electron density with the solar flux.

When the correlation maps from in situ satellite data near theF2-peak altitudes and the TEC from GIM are compared, we findsystematic differences, particularly along the magnetic equator.Within the latitude band of 101 from the magnetic equator, thecorrelation coefficients from in situ satellite data are low, i.e. lessthan 0.5 for CHAMP, whereas those of GIM TEC (CHAMP) are 0.7 orhigher. A similar pattern is found for the GRACE in situ and TEC dataas well, although to a lesser extent. Rich et al. (2003), on the otherhand, using the DMSP satellite data, found that the correlationcoefficient in the equatorial region is higher than in the mid-latituderegion, similar to our results with the GIM TEC data. Recalling the

altitudes of these satellites (i.e., �370 km for CHAMP, �480 km forGRACE, and �840 km for DMSP), our analysis suggests that thelower correlation in the equatorial region may be confined to the F2peak altitude and its vicinity.

For the time delay, all four data sets (Fig. 3e–h) show a consistentspatial distribution. The dominant pattern is a hemispheric asym-metry with a time delay of predominantly one day in the northernhemisphere and two days or longer in the southern hemisphere.

4. Discussion

Studying the correlation between electron density and solarflux, we find that while many of the global correlation featuresare similar for F-region in situ data and corresponding GPS-TECdata there is a distinct difference in correlation behavior at andnear the magnetic equator. TEC data are well correlated with solarflux by means of the 27-day solar rotation, the GRACE data,however, exhibit only a marginal correlation with solar flux, andthe CHAMP data not at all. Fig. 4 shows the variations ofcorrelation coefficients and time delays along geomagnetic lati-tude (for a 21 resolution). A substantially decreased correlationcoefficient is found around the magnetic equator (7301) for theCHAMP satellite data. Across all four data sets and both solarfluxes, the hemispherical asymmetry of time delay is apparent.The time delay in the southern hemisphere (especially 101S–701S)is a day longer than in the northern hemisphere.

Fig. 5a indicates the altitude ranges of CHAMP and GRACE from2003 to 2007 as well as the samples of the F2-peak heights(hmF2) predicted by the IRI-2007 model (Bilitza and Reinisch,2008) for the time and location of the CHAMP data. The CHAMPorbits are closer to the hmF2 than the GRACE orbits and aresometimes even below hmF2 in the equatorial region. The map ofmean distance from the height of CHAMP satellite to the hmF2,depicted in Fig. 5b, shows a pattern similar to the correlation mapin Fig. 3a. The distance is smallest in the equatorial region and thedistances over the American and Eurasian continents are largerthan those over the Pacific Ocean in the northern hemisphere.These coherent spatial patterns support the idea that thedecreased correlation along the magnetic equator is likely dueto the proximity of the satellite orbit to the F2 peak.

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)

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LAY

(day

)

GEOMAGNETIC LATITUDE (°)

Fig. 4. The correlation coefficient and time delay between four data sets (two

in situ measurements from CHAMP PLP and GRACE KBR and two TECs along

CHAMP and GRACE satellite orbits) and the solar fluxes, (a) F10.7 and (b) SOHO/

SEM, along geomagnetic latitude. The black dashed lines indicate the correlation

coefficients between the global mean TEC and the solar fluxes.

GEOMAGNETIC LATITUDE (deg)

ALT

ITU

DE

(km

)

CHAMP

GRACE

HmF2

−60 −30 0 30 60200250300350400450500550

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LATI

TUD

E (d

eg)

0 50 100 150 200 250 300 350

−50

0

50

40

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80

100

120(km)

Fig. 5. (a) Sample density of the hmF2 values from IRI-2007 model, corresponding

to the CHAMP data. The blue and red lines indicate the altitude range of CHAMP

and GRACE, respectively. (b) Average distance from the CHAMP height and

the hmF2.

C.-K. Lee et al. / Journal of Atmospheric and Solar-Terrestrial Physics 77 (2012) 219–224 223

So what is disturbing the correlation between the electrondensity and the solar EUV radiation in the equatorial F2 layer? The(vertically-integrated) TEC maintains a strong correlation in theequatorial region unlike the satellite measurements nearthe F-region, even though the F-region ionization dominantlycontributes to the TEC. This fact indicates that the ionosphereas a whole is less affected by the re-distribution of ionizationcausing the low correlation near the F2-peak. These distinct beha-viors of electron contents near the F2-peak and in the wholeionosphere suggest that the vertical movement of ionization (e.g.ion drift constituting the EIA) near the F2-peak plays a leading rolein the short-term variation of electron density near the F2-peak.

5. Conclusion

We have studied the correlation and time delay between the27-day solar rotation period as seen in the solar flux and asdocumented in in situ electron density measurements by theCHAMP and GRACE satellites and TEC measurements from GPS.We find good correlation except at high latitudes where particleprecipitation competes with the solar influence. At low latitudesthe correlation also breaks down for electron density data close tothe F peak because of the dynamic process that is not under directsolar control. As expected, we find that the correlation decreases

towards the solar cycle minimum because of the increasingimportance of influences other than solar, e.g. meteorologicalforcing from below. The time delay between the solar rotation asmanifested in solar flux and the satellite data shows a consistentbehavior across all four data sets. We find a predominantly 1-daydifference of time delay between the northern and southernhemisphere. Further study is needed to investigate the causes ofthis hemispherical asymmetry.

The results of this study are important for an improvedrepresentation of the solar dependence in data-based ionosphericmodels like the International Reference Ionosphere (IRI). IRI iswidely used for many space weather applications but currentlyonly use solar dependence at a 12-month average level. With theresults of this study it might be possible to introduce solardependence at a daily F10.7 level keeping in mind the globaldifferences in correlation and time delay.

Acknowledgments

This work was supported by NASA Earth Surface and Interiorprogram and GRACE projects and Korea Polar Research Institute(KOPRI) projects (PE11070). We thank DLR for providing theGRACE telemetry data and JPL for producing the high-qualityLevel-1B products.

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