Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272

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Journal of Atmospheric and Solar-Terrestrial Physics

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Properties of auroral radio absorption patches observed in themorning sector using imaging riometer and incoherent-scatter radar

M.J. Birch a,b,n, J.K. Hargreaves a,c, B.J.I. Bromage a

a Jeremiah Horrocks Institute for Mathematics, Physics and Astronomy, University of Central Lancashire, UKb John Tyndall Institute, University of Central Lancashire, UKc Department of Physics, Lancaster University, UK

a r t i c l e i n f o

Article history:

Received 1 February 2012

Received in revised form

4 December 2012

Accepted 9 December 2012Available online 3 January 2013

Keywords:

Incoherent scatter radar

Auroral radio absorption

Imaging riometer

F-region drift

26/$ - see front matter & 2013 Elsevier Ltd. A

x.doi.org/10.1016/j.jastp.2012.12.004

esponding author at: Jeremiah Horrocks Instit

ronomy, University of Central Lancashire, UK

ail address: mjbirch@uclan.ac.uk (M.J. Birch).

a b s t r a c t

The investigation concerns the size and motion of spatial fine structure in auroral radio absorption as

observed by imaging riometer. Electron density profiles in the D and E regions determined by

simultaneous incoherent-scatter radar were inverted to provide an estimate of the energy spectrum

of the incoming electrons, and it is shown that the gradient-curvature drift at these energies is not

consistent with the observed motion of the absorption fine structure, being too large by a factor of 5 or

6. However, a comparison with F-region drifts determined by the SuperDARN radar network supports a

hypothesis that the cause lies in moving structures within the magnetosphere.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

The absorption of radio waves at high latitude during auroralactivity was first observed by Appleton et al. (1933), but detailedstudies of the phenomenon began with the development of theriometer (Little and Leinbach, 1959), a sensitive and stablereceiver based on the techniques of radio astronomy, and withthe identification by Reid and Collins (1959) of ‘‘auroral absorp-tion’’ as a distinct phenomenon. Most of the early work usedrelatively simple antennae having broad beams, such as Yagis orpairs of dipoles, thereby covering some 200–250 km of the lowerionosphere. Even when multiple systems were deployed, there-fore, the spatial information was limited to relatively broadstructures. Results from this early work were summarised byHargreaves (1969).

The dynamic character of auroral radio absorption was firstappreciated in the mid-1960s with the observation that absorp-tion near noon tended to follow a burst of energetic electrons inthe magnetotail (Parthasarathy and Reid, 1967), and with themeasurement of time differences between absorption eventsdetected by riometers widely separated in longitude(Hargreaves, 1967, 1968). The obvious explanation (first statedby Parthasarathy and Reid, 1967) was based on the time taken for

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ute for Mathematics, Physics

.

trapped electrons to drift eastward by the gradient-curvature driftmechanism from a night-time injection region. It soon becameclear, however, that this could not be the whole explanationbecause during the hours before midnight the motion wasgenerally westward. It has been suggested that protons may beresponsible in that sector but, alternatively, Pudovkin et al. (1968)pointed out the likely influence of the circulation of the magneto-sphere, which reverses direction at midnight. The likely signifi-cance of gradient-curvature drift has been supported by furtherstudies across wide baselines (Jelly, 1970; Berkey et al., 1974).

Measurements across narrower baselines, such as 300 km(Hargreaves, 1970; Hargreaves and Berry, 1976), gave resultssimilar to the widely-spaced studies with respect to direction,but with considerably lower speeds. This has also been supportedby later work, particularly when riometer systems using multiplenarrow beams began to be deployed (Kikuchi et al., 1990), sincethese enabled smaller spatial structures to be identified withinthe absorption events. In particular, with the advent of the‘‘imaging riometer’’ (Detrick and Rosenberg, 1990) it has beenpossible to construct images of the radiowave absorption overdistances of 200–300 km and thus to observe structure andmovement directly. The spatial size, shape, and motion of struc-tures smaller than 100 km could then be observed and a numberof investigations have considered various aspects. Regarding thedynamics of auroral absorption there is no overall consensus,though it appears that the dominant cause of motion seen atrelatively short distances (for example, less than 100 km) is ExBdrift, equivalent to magnetospheric circulation (Hargreaves and

Table 1Studies relevant to the structure and dynamics of auroral absorption in the morning sector (G-C: gradient-curvature drift; ExB: magnetospheric circulation mechanism).

Author Separation Method Absorption features studied Suggested mechanism

Hargreaves (1967) Wide Widebeam Onsets and events G-C, and a faster one

Hargreaves (1968) Wide Widebeam Onsets and events Magneto-acoustic wave

Pudovkin et al. (1968) Wide Widebeam Bays G-C and ExB

Jelly (1970) Wide Widebeam Substorms G-C, some faster

Hargreaves (1970) Narrow Widebeam Peaks and dips ExB (magnetosphere convection)

Berkey et al. (1974) Wide Widebeam Substorms G-C and other

Hargreaves and Berry (1976) Narrow Widebeam Peaks ExB

Kikuchi et al. (1990) Narrow Scanning narrowbeam Patches ExB

Nielsen and Honary (2000) Narrow IRIS Patches following SC ExB

Kainuma et al. (2001) Narrow 256-beam IR Patches ExB

Makarevitch et al. (2004) Narrow IRIS Patches ExB

M.J. Birch et al. / Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272 263

Berry, 1976; Kikuchi et al., 1990; Nielsen and Honary, 2000;Kainuma et al., 2001; Makarevitch et al., 2004). Some of the moresignificant papers relevant to the above brief review are listed inTable 1, in which a comparison is drawn between separation ofantennae, method of observation, type of features studied, andthe suggested mechanism.

The present study makes use of the 49-beam ImagingRiometer for Ionospheric Studies (IRIS) at Kilpisjarvi (69.051N,20.791E, L¼5.9), Finland (Browne et al., 1995), operating at38.2 MHz, in conjunction with the European Incoherent Scatter(EISCAT) VHF radar at Tromso (69.591N, 19.231E, L¼6.0).The study is restricted to the morning sector, and considers thespatial size and dynamics of absorption patches and the corre-sponding electron density profiles in the D-region (from whichthe spectrum of the incoming electron flux may be estimated),and makes a comparison with the dynamics of the F-region.

Fig. 1. Observations during the period 0200–0820 UT on March 9, 2008:

(a) EISCAT VHF vertical electron density distribution between 70 and 120 km

altitude (units: log(Ne electrons/m3)), and (b) the measured (red) and calculated

(black) 38.2 MHz radio absorption in the vicinity of the EISCAT radar site.

2. Observations

D- and E-region observations were made with the EISCAT VHFradar on 7–9 March, 2008. These observations were scheduled inrelation to the approach of a low-latitude coronal hole towardsthe solar meridian. Such coronal holes are the source of the fastsolar wind. The coronal hole had been rotating with the Sun forseveral months enabling forward planning of the EISCAT observa-tions. It was not until the end of the second day that activitybegan, but by the time the allotted observation period ended at0820 UT on March 9 there had been several hours of significantdisturbance. Throughout this period the auroral radio absorptionwas monitored with IRIS and significant absorption was observed.Fig. 1(a) shows contours of the electron density distributionbetween 70 and 120 km for the period 0200–0820 UT on March9, and Fig. 1(b) shows absorption covering the same period, boththe 38.2 MHz radio absorption in the vicinity of the EISCAT radarsite as measured by beams 9 and 16 of the imaging riometer, andthe absorption estimated from the electron density measure-ments. (IRIS beams 9 and 16, which are adjacent, intersect theD-region most nearly over the EISCAT radar site, the VHF beambeing vertical throughout.) A comparison between the measuredand computed absorption confirms that the same phenomenon isbeing observed by both techniques and also illustrates thevariable character of the activity.

3. Results

3.1. Dynamics and dimensions of absorption patches

An example of the morning sector structures and dynamicsobserved from Kilpisjarvi between 0300 and 0404 UT on March 9

is given in Fig. 2. These samples, each derived by interpolating thedata from the 49 IRIS beams, are integrated over one-minuteintervals, and the distribution covers an area 240 km squarecentred over Kilpisjarvi, viewed as if from above. A height of90 km is assumed. Drifting patches of absorption are clearlyvisible in each square, the direction of motion being mainly W–E.

In order to quantify the size and motion of each structure, aprocedure is employed which selects the largest absorption valuein each patch; adjacent values are then applied to estimate themagnitude and location of the maximum, and also the width ofthe patch in the N/S and E/W directions, assuming a Gaussianform. The width given is the distance between the location of themaximum and a value 0.61 of the maximum. The total widthacross the patch between points 0.61 of the maximum will betwice this distance. The results for the period illustrated in Fig. 2are shown in Fig. 3.

Using this method, 12 moving features may be identified onMarch 9 between 0200 and 0900 UT (Table 2). Before 0800 UT themovements are predominantly eastward, though with somenorth-south component. After 0800 UT the movements are slowerand are directed westward and southward. Some of the featuresappear in sequence, a new one entering the field of view at about

Fig. 2. IRIS absorption images (plan view) over Kilpisjarvi, Finland (69.051N, 20.791E) from 0300 to 0404 UT on March 9, 2008, at one-minute resolution (each panel

represents a 240�240 km square at the height of the absorption layer, assumed to be at 90 km altitude).

M.J. Birch et al. / Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272264

the time the previous one leaves it. The E–W separation betweensuccessive features has been estimated using the time differencebetween their overhead crossings and the mean speed of the twofeatures concerned. Sequences of this kind may imply wave-likestructure as suggested by Kainuma et al. (2001).

The same procedure has been repeated for the activity onseveral other days of 2008, namely March 27, June 17 and June 18and, as illustrated in Fig. 4, the results for features movingeastward between midnight and 0700 UT are generally similar.Over the four periods the median (M) and quartiles (Q) of theparameters illustrated in Fig. 4 are summarised in Table 3.The median quasi-period, as seen by a stationary observer, istherefore about 12 min. The speed does not depend on the time ofday after 0200 UT, but there may be some variation before then.

Fig. 5 and Table 4 summarise the observed patch widths(as defined above) for each of the days studied (March 9, 27;June 17, 18). In Fig. 5 the small symbols are the averages forindividual patches and the large ones are the averages over each

day. There is no evidence for any seasonal change between springand summer, nor any systematic variation with the time of day.Although individual patches may be elongated N–S or E–W, theaverage patch is round, again with no apparent systematicvariation. The overall mean size is 49.576.0 km E–W and47.072.6 km N–S.

3.2. Analysis of incoherent scatter radar observations, giving

electron density profiles, spectra of ionising electrons, and height

of radio absorption

3.2.1. Typical electron density peaks

Specimen electron-density profiles corresponding to fourselected absorption peaks at different times of day (0253, 0424,0659, and 0813 UT) on March 9 are illustrated in Fig. 6, whileFig. 7 shows the corresponding electron spectra as derived fromthe profiles by the method of Hargreaves and Devlin (1990).(The technique is outlined in the Appendix to that paper.

Fig. 3. Value, position and width of absorption maxima from 0200 to 0500 UT on March 9, 2008.

Table 2Dynamics of absorption patches observed on March 9, 2008, derived from plots

such as those shown in Fig. 3.

Overhead(UT)

Separation(mins)

Speed(km/min)

Angle (deg.)with respectto N

SeparationE-W (km)

W to E S to N

0229 45.1 No value

0308 26.1 1.2 87

19 399

0327 15.9 0.0 90

20 328

0347 16.9 0.0 90

12 343

0359 17.4 �1.9 96

23

0422 25.6 �17.9 124

0546 23.1 15.3 56

17 362

0603 19.5 8.3 67

13 280

0616 23.6 0.0 90

0805 �4.2 �6.9 �149

29 219

0834 �10.9 �3.6 �108

31 230

0905 �3.9 No value

M.J. Birch et al. / Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272 265

The variation of production rate with height and the initial energyof the precipitating electrons were based on Rees (1964). The

recombination and specific absorption coefficients used in ourprofile inversion are given in the Appendix A to this paper.)Variations in the shape of the profile imply correspondingchanges in the spectrum, with increases at the lower heightsindicating a gradual hardening of the spectrum after 0630 UT. Thegeneral tendency for the day precipitation to be harder than thatat night has been recognised for many years (e.g. Pudovkin et al.,1968), and was originally based on the observation that luminousaurorae generally maximise at night whereas radio absorptionmaximises by day. Hargreaves and Devlin (1990) recognised asoftening during the decay of an absorption event in the earlymorning hours (0300–0500 UT), whereas Hargreaves et al. (2007)observed a hardening between 0300 and 0600 UT, with a distinctpeak in the spectrum at 30–50 keV. Burns et al. (1990) notedspectral variations in the evening sector, the spectra beinggenerally softer than those observed during the morning.The present observations appear to be the first to show aprogressive hardening between 0630 UT and 0830 UT (0750–0850 LT), that time period being within the daytime statisticalmaximum of auroral absorption.

3.2.2. Comparison of peaks and dips

The activity observed here comprises two extended events (0230–0440 and 0600–0900), each not atypical of the morning sector, andmost of the peaks are part of the structure within the overall event. Toinvestigate the cause of the fluctuations, within the first burst ofactivity the average electron densities over four peaks (0254, 0329,0358, 0418 UT) and five dips (0256, 0320, 0334, 0351, 0414 UT) weretaken. Fig. 8 compares the average electron density profiles for thesepeaks and dips, and Fig. 9(a) gives the derived spectra. The fluctua-tions are seen to be due to the variation at energies above 10 keV, the

Fig. 4. Dynamics of absorption patches: (a) W-to-E speeds; (b) S-to-N speeds; (c) direction of movement; and (d) estimated separations between maxima. (M ¼ median;

Q ¼ quartiles.).

Table 3Statistics of absorption patch dynamics illustrated in Fig. 4.

Parameter Median Upper quartile Lower quartile

W to E speed (km/min) 23.1 26.1 17.5

S to N speed (km/min) 1.2 11.5 0.0

Azimuth (deg.) 87 90 61

W to E separation (km) 266 328 206

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greatest relative change being at 65–80 keV (Fig. 9b). There is nodiscernible variation below 10 keV, and the fluctuations in electrondensity are mainly between 80 and 110 km altitude. The variationsobserved in the present study have a periodicity of about 12 min, andthey are not therefore ‘‘pulsations’’ in the normal sense. However, it isworth noting that pulsations with periodicity of about 1 min alsoshow a spectral hardening at the peaks of absorption (Hargreaves andDevlin, 1990; Kavanagh et al., 2002).

Fig. 5. N-S and E-W sizes for each patch on March 9, March 27, June 17 and June 18 (Key: large symbols mark the average patch size for each day; dotted lines indicate

patch symmetry; dashed lines represent 20% departure from symmetry).

M.J. Birch et al. / Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272 267

We have seen that the 38.2 MHz radio absorption calculated fromthe measured electron density profile is very similar to that observedby riometer (Fig. 1b). The same type of computation can provideestimates of the altitude and thickness of the absorption layer. Fig. 10shows (a) the incremental absorption (given by the electron densitymultiplied by the specific absorption coefficient, defined as theabsorption in dB per km of path for an electron density of 1/cm3, asa function of height), and (b) the total absorption (between 60 kmand the height indicated) against height, for the averages of the aboveselection of peaks and dips. The middle of the absorption layer isbetween 85 and 90 km, and the layer is about 10 km thick – i.e. 50%of the absorption occurs within this depth. The factor of 2 inabsorption magnitude between peak and dip corresponds to a factorof 4 in ionising particle flux (Fig. 9), but the height difference is only

about 1 km. No more than 5% of the absorption occurs below 75 kmor above 105 km.

4. Mechanisms

4.1. Gradient-curvature drift

The first study of the dynamics of auroral radio absorptionevents (Hargreaves, 1967) was made by comparing the timing ofabsorption features of similar appearance as detected bywide-beam riometers separated by 80–901 of longitude, and thisclearly showed eastward movement in the morning sector. Thededuced speeds were quite variable, but averaged to about 41 of

Fig. 6. Electron density profiles at absorption enhancements at 0253, 0424, 0659,

and 0813 UT.

Fig. 7. Electron spectra derived from the electron density profiles in Fig. 6.

Fig. 8. Comparison of average electron density profiles at peaks and dips in the

absorption pattern.

Fig. 9. (a) Spectra and (b) peak/dip ratios derived from the electron density

profiles in Fig. 8.

Table 4Statistics of patch widths (km) on the four days illustrated in Fig. 5.

Event East-West North-South

LQ M UQ IQ range LQ M UQ IQ range

March 9th 39.0 48.5 61.5 22.5 38.0 45.0 58.0 20.0

March 27th 47.0 53.0 68.5 21.5 33.0 47.0 54.5 21.5

June 17th 30.0 36.0 48.0 18.0 31.0 39.0 49.0 18.0

June 18th 36.0 44.0 62.0 26.0 40.0 45.0 57.0 17.0

M.J. Birch et al. / Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272268

longitude per minute, equivalent to 170 km/min. Comparing thetimes of the beginning of events without consideration of formgave the typical eastward speed of about 1.41/min, equivalent to60 km/min, though with even greater variability. If these motionsare assumed to be due to the gradient-curvature drift of trappedelectrons mirroring at the equator, speeds of 60 and 170 km/minwould imply electron energies of 25 and 70 keV (respectively) atthe latitude concerned. (The drift period (P) in hours is given byP¼733/ER0 where the energy E is measured in keV, and R0 is thegeocentric distance to the nose of the field line in earth radii(Hargreaves, 1992)). The gradient-curvature drift mechanismappears to be satisfactory for movements over a wide baseline.This conclusion has been supported by several independent

observations over the years, for instance those by Berkey et al.(1974): see Table 1.

Fig. 10. (a) Incremental absorption (dB/km) and (b) total absorption (dB) at

38.2 MHz.

Fig. 11. Medians and quartiles of absorption height distribution for the structures

observed on March 9, 2008.

Fig. 12. SuperDARN eastward velocity in the F-region in the vicinity of Kilpisjarvi,

with spot measurements of speed of absorption features shown as *.

M.J. Birch et al. / Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272 269

Speeds deduced from more closely spaced riometers (E300 km),however, came out smaller (Hargreaves, 1970; Hargreaves and Berry,1976), with a median speed of 38 km/min and quartiles at 20 and55 km/min, and lower values of speed over narrow baselines con-sistent with these measurements have also been reported by Kikuchiet al. (1990), Nielsen and Honary (2000), Kainuma et al. (2001), andMakarevitch et al. (2004). The present results from the imagingriometer, which of course provides greater spatial detail, are ifanything somewhat lower still (quartiles 18 and 29 km/min).

From the electron density profiles observed by the EISCAT VHFradar between 70 and 120 km altitude (e.g. Fig. 1), we may (as inSection 3.2.2) derive the median and quartiles of the absorptiondistribution with respect to height and time (Fig. 11). During thefirst burst of activity (0230–0430 UT) the median of the absorp-tion distribution was between 85 and 90 km. During the secondburst (0630–0800 UT) it declined steadily from 90 to 80 km,implying a gradual increase of particle energy.

Precipitating electrons producing maximum absorption at 90,85 and 80 km would require initial energies of 42, 71 and 120 keVand, according to the theory of gradient-curvature drift, wouldtravel eastward at 93, 157 and 265 km/min. These speeds farexceed those observed in the present study. Our observed driftspeeds (18–29 km/min) would correspond to electrons withenergy between 7.6 and 11.7 keV which would not penetratebelow 100 km (Rees, 1964).

We conclude that in the present case the electron densitydistributions observed using the radar and the horizontal speedsof the absorption patches observed using the imaging riometerare incompatible if gradient-curvature drift is taken to be thecausal mechanism.

4.2. F-region drift

That movements observed from the ground may be influencedby the circulation pattern of the magnetosphere is another long-established idea, attractive because it can accommodate thereversal of direction in the midnight sector (Pudovkin et al.,1968). Fig. 12 compares the eastward velocity of the F region inthe vicinity of Kilpisjarvi, as determined by the SuperDARNsystem (Ruohoniemi and Baker, 1998), with the spot measure-ments taken from the absorption features on March 9. Thecomparison looks reasonable before about 0400 UT, but after thatthe speed of the absorption patch is two to three times that of theF region.

In the SuperDARN system the F-region velocity at a givenlocation is obtained from a model fitted to radar observations atfixed sites. As illustrated in Fig. 13, which shows the circulationpatterns at the times of four measurements on absorptionfeatures, the observations vary from plentiful (a) to scarce(b and d) and even to none (c). The velocity ascribed to theionospheric medium over Kilpisjarvi must therefore be subject toa varying degree of uncertainty. Out of 24 comparisons between0000 and 0500 UT we have therefore rejected three cases whenthe SuperDARN speed was reported as passing through a markedminimum or reversed direction, and further seven cases for whichthere were fewer than three velocity vectors on the SuperDARNconvection maps in the vicinity of Kilpisjarvi. Over the remaining

Fig. 13. SuperDARN plots of estimated F-region velocity in 2008: K denotes Kilpisjarvi; (a) March 27, 00:14 UT (midnight, E flow over K, plentiful data); (b) June 18, 01:26

UT (post-midnight, E flow over K, sparse data); (c) June 18, 01:42 UT (post-midnight, E flow over K, no data); (d) March 9, 08:30 UT (morning sector, W flow over K, sparse

data). Superimposed arrows indicate direction of flow.

M.J. Birch et al. / Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272270

14 points of comparison, the ratio between the speed determinedby SuperDARN and that measured by IRIS has a median value of0.76 (Fig. 14). The correlation coefficient is 0.66, which (accordingto Fisher’s z-transformation) is significant at the 0.4% level.

Recent work by Gillies et al. (2010), in which SuperDARNvelocity measurements were calibrated against F-region driftspeeds determined by the EISCAT radar, has indicated that Super-DARN speeds are too low by a factor of 0.78, a discrepancy whichis attributed to small-scale irregularities in the ionosphericmedium. Applying this correction to our result implies that theobserved speed of absorption patches and the inferred F-regionspeed agree to 3% on the average. The implication is that in themorning sector the modulation of electron precipitation of tens ofkeV, which is responsible for structures within the pattern ofauroral absorption, is generally caused by features or processeswithin and drifting with the magnetosphere, though there may beexceptions.

The movements of features observed after 0700 UT aredifferent in character from those tracked during the earlier period.The speeds are smaller, and the directions are westward and

southward instead of consistently eastward (Table 2). By 0830 UTKilpisjarvi has moved to the other cell of the F-region flowpattern, where the circulation is clockwise.

5. Summary and conclusions

The object of this investigation has been to elucidate the proper-ties and behaviour of the finer structures of auroral radio absorptionobserved during the morning sector. It has been recognised since theearliest observations that the temporal behaviour of the auroralabsorption changes diurnally, the morning activity typically beingdescribed as ‘‘slowly varying’’ though with the possibility of super-imposed pulsations with periods of several minutes. By means of animaging riometer and an incoherent scatter radar with overlappingfields of view it has been possible to specify more clearly thespatial and dynamic character of the precipitation as well as theenergy spectrum of the incoming electrons. These dual obser-vations relate to the time period 0200–0900 UT (equivalent toapproximately 0330–1030 LT at the observing sites in Norway and

Fig. 14. Comparison between the W-E velocities of D-region patches from IRIS,

and F-region drift velocities from SuperDARN selected according to the criteria in

Section 4.2 (dotted line – median for this study; dashed line – result anticipated

using the SuperDARN calibration by Gillies et al., 2010).

Table A1Recombination coefficient (cm3/s) and specific absorption coefficient (dB/km for

1 electron/cm3 at 38.2 MHz) for a range of heights (km).The derivation of the

recombination coefficient is described in Hargreaves et al. (2007).

Height Recombination coefficient Specific absorption coefficientkm cm3 s�1

120 2.40E-07 2.70E-09

116 2.64E-07 5.65E-09

112 2.88E-07 1.18E-08

108 3.15E-07 2.47E-08

104 3.48E-07 5.03E-08

100 3.80E-07 9.40E-08

96 4.88E-07 1.58E-07

92 6.27E-07 2.77E-07

88 8.08E-07 4.58E-07

84 1.09E-06 7.28E-07

80 1.70E-06 1.30E-06

76 3.37E-06 2.40E-06

72 6.68E-06 4.47E-06

68 1.19E-05 8.50E-06

64 2.01E-05 1.60E-05

60 3.96E-05 2.70E-05

M.J. Birch et al. / Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272 271

Finland), supplemented by imaging riometer observations between0000 and 0500 UT on three other days. The main results are asfollows:

1.

The variations of absorption are due to localised patches,varying in shape though circular on the average, moving acrossthe field of view of the imaging riometer at speeds around20 km/min. The average diameter of the patches is 80–110 km,and they sometimes occur in sequence with a typical separa-tion of 200–350 km, suggesting wave-like behaviour. Themovements were essentially eastward during most of theperiod, though slower westward and southward motions wereseen later.

2.

The variations should be regarded as modulations of theabsorption activity which did not, in general, fall to zerobetween the patches. There is some hardening of the spec-trum, greatest at 50–70 keV, at a peak of absorption. Theheight of the absorption is reduced at the peak, but onlyslightly.

3.

The west-to-east speeds are similar to the F-region drift speedsdetermined by the SuperDARN system, as adjusted by theGillies et al. (2010) correction. This supports the idea that thevariations of precipitation into the D-region on the fine scaleare caused by processes or structures within and drifting withthe magnetosphere, rather than by the gradient-curvature driftof trapped particles. A change of character after 0700 UT onMarch 9 is also seen by both techniques: when the patchmotion turns westward the observing site is then in thevicinity of the F-region cell where the motion is also tothe west.

Acknowledgements

We appreciate the assistance of the EISCAT group at theRutherford Appleton Laboratory, Chilton, in setting up the experi-ment and in supplying us with the data. The imaging riometer atKilpisjarvi is a joint operation between the University of Lancasterand the Geophysical Observatory, Sodankyla, Finland. We thankDr. A. Grocott for providing the SuperDARN data and advisingon its use.

Appendix A

(Table A1).

References

Appleton, E.V., Naismith, R., Builder, G., 1933. Ionospheric investigations in highlatitudes. Nature 132, 340.

Berkey, F.T., Driatskiy, V.M., Henriksen, K., Hultqvist, B., Jelly, D.H., Shchuka, T.I.,Theander, A., Yliniemi, J., 1974. A synopticinvestigation of particle precipita-tion dynamics for 60 substorms in IQSY (1964–1965) and IASY (1969).Planetary and Space Science 22, 255–307.

Browne, S., Hargreaves, J.K., Honary, B., 1995. An imaging riometer for ionosphericstudies. Electronics and Communication Engineering Journal 7, 209.

Burns, C.J., Howarth, W.G., Hargreaves, J.K., 1990. High resolution incoherentscatter radar measurements during electron precipitation events. Journal ofAtmospheric and Terrestrial Physics 52, 205.

Detrick, D.L., Rosenberg, T.J., 1990. A phased-array radiowave imager for studies ofcosmic noise absorption. Radio Science 25, 325.

Gillies, R.G., Hussey, G.C., Sofko, G.J., Wright, D.M., Davies, J.A., 2010. A comparisonof EISCAT and SuperDARN F-region measurements with consideration of therefractive index in the scattering volume. Journal of Geophysical Research 115,A06319.

Hargreaves, J.K., 1967. Auroral motions observed with riometers: movementsbetween stations widely separated in longitude. Journal of Atmospheric andTerrestrial Physics 29, 1159.

Hargreaves, J.K., 1968. Auroral motions observed with riometers: latitudinalmovements and a median global pattern. Journal of Atmospheric and Terres-trial Physics 30, 1461.

Hargreaves, J.K., 1969. Auroral absorption of HF radio waves in the ionosphere: areview of results from the first decade of riometers. Proceedings of the IEEE 57,1348–1373.

Hargreaves, J.K., 1970. Conjugate and closely-spaced observations of auroral radioabsorption: (IV) The movement of simple features. Planetary and SpaceScience 18, 1691.

Hargreaves, J.K., Berry, M.A., 1976. The eastward movement of the structure ofauroral radio absorption events in the morning sector. Annales Geophysicae32, 401.

Hargreaves, J.K., Devlin, T., 1990. Morning sector precipitation events observed byincoherent scatter radar. Journal of Atmospheric and Terrestrial Physics 52,193.

Hargreaves, J.K., 1992. The Solar Terrestrial Environment. Cambridge UniversityPress.

M.J. Birch et al. / Journal of Atmospheric and Solar-Terrestrial Physics 105-106 (2013) 262–272272

Hargreaves, J.K., Birch, M.J., Bromage, B.J.I.B., 2007. D- and E-region effects in theauroral zone during a moderately active 24-hour period in July 2005. AnnalesGeophysicae 25, 1–13.

Jelly, D., 1970. On the morphology of auroral absorption during substorms.Canadian Journal of Physics 48 (3), 335–345 10.1139/p70-045.

Kainuma, S., Ishii, M., Murayama, Y., Kikuchi, T., Mori, H., Igarashi, K., 2001. Driftmotion of ionospheric arc-like absorption regions observed with a 256-beam

imaging riometer in Alaska. Earth, Planets and Space 53, 753–760.Kavanagh, A.J., Honory, F., McCrea, I.W., Donovan, E., Woodfield, E.E., Manninen, J.,

Anderson, P.C., 2002. Substorm related changes in precipitation in the dayside

auroral zone—a multi instrument case study. Annales Geophysicae 20,1321–1334.

Kikuchi, T., Yamagishi, H., Lester, M., 1990. Drift of auroral absorption due to themagnetospheric convection observed with the scanning narrow beam

riometer during SUNDIAL-86. Annales Geophysicae 8, 431–440.Little, G.G., Leinbach, H., 1959. The riometer—a device for the continuous

measurement of ionospheric absorption. Proceedings of the IRE 47, 315–320.

Makarevitch, R.A., Honory, F., McCrea, I,W., Howells, V.S.C., 2004. Imaging riometerobservations of drifting absorption patches in the morning sector. AnnalesGeophysicae 22, 3461–3478.

Nielsen, E., Honary, F., 2000. Observations of ionospheric flows andparticleprecipitation following a sudden commencement. Annales Geophysicae 18,908–917.

Parthasarathy, R., Reid, G.C., 1967. Magnetospheric activity and its consequencesin the auroral zone. Planetary and Space Science 15, 917.

Rees, M.H., 1964. Note on the penetration of energetic electrons into the Earth’satmosphere. Planetary and Space Science 12 (7), 722–725.

Pudovkin, M.I., Shumilov, O.I., Zaitzeva, S.A., 1968. Dynamics of the zone ofcorpuscular precipitations. Planetary and Space Science 16, 881.

Reid, G.C., Collins, C., 1959. Observations of abnormal VHF radio wave absorptionat medium and high latitudes. Journal of Atmospheric and Terrestrial Physics14, 68.

Ruohoniemi, J.M., Baker, K.B., 1998. Large-scale imaging of high-latitude convec-tion with Super Dual Auroral Radar Network HF radar observations. Journal ofGeophysical Research 103, 20797.