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Advances in Space Research 36 (2005) 2201–2206

GPS ionospheric imaging of the north polar ionosphereon 30 October 2003

C. Stolle a,*, S. Schluter b, S. Heise c, Ch. Jacobi a, N. Jakowski b, S. Friedel d,D. Kurschner e, H. Luhr c

a University of Leipzig, Institute for Meteorology, Stephanstr. 3, 04103 Leipzig, Germanyb DLR, Institute of Communications and Navigation, Kalkhorstweg 53, 17235 Neustrelitz, Germany

c GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germanyd ETH, Institute of Geotechnical Engineering, Honggerberg HIL C 34.3, 8093 Zurich, Switzerland

e University of Leipzig, Institute of Geophysics and Geology, Collm Observatory, 04779 Wermsdorf, Germany

Received 30 September 2004; received in revised form 22 August 2005; accepted 26 August 2005

Abstract

Maps of electron content retrieved from a 3-dimensional assimilation process of GPS TEC measurements are applied to observe thenorth polar ionosphere in the evening of 30 October 2003. Their reliability for large-scale imaging is discussed in comparison with GPSdata coverage and radar observations. The electron content results are interpreted together with the behaviour of the InterplanetaryMagnetic Field (IMF). The images show a region of ionisation expanding across the polar cap from the day to the night side between20:00 and 21:30 UT. A patch-like structure is observed at successive times with locations approaching the night side ionosphere. At cor-responding time the IMF Bz was steadily negative. To explain the observations, an anti-sunward plasma transport is proposed. Consec-utively to 21:30 UT a low electron content is imaged polward of 70�N coinciding with an increasing and then positive IMF Bz. During afurther phase of negative IMF Bz an enhanced electron content appears at night side mid-latitudes beyond 50�N. The assimilation imagesare a useful observational tool of the large-scale distribution of electron content, especially together with further data of ionosphericparameters.� 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: GPS ionospheric remote sensing; 3D ionospheric imaging; October storm 2003

1. Introduction

The assimilation of line-of-sight Total Electron Content(TEC) measurements along rays of the Global PositioningSystem (GPS) has become a rapidly developing tool for 3-or 4-dimensional ionospheric electron density imaging.Therefore, different reconstruction methods have been inuse, such as stochastic inversion (Ruffini et al., 1999), direct

0273-1177/$30 � 2005 COSPAR. Published by Elsevier Ltd. All rights reserv

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* Corresponding author. Present address: Now at GeoForschungsZen-trum Potsdam, Telegrafenberg, 14473 Potsdam, Germany. Tel.: +49 3312881882; fax: +49 331 2881884.

E-mail address: stolle@gfz-potsdam.de (C. Stolle).

inversion by single value decomposition (Mitchell andSpencer, 2003), Kalman filtering (Schunk et al., 2004;Wang et al., 2004), the linear unbiased estimation (Anglingand Cannon, 2004), the 3-dimensional variational dataassimilation technique (Bust et al., 2004), and the multipli-cative algebraic reconstruction technique (MART) (Schlu-ter et al., 2003; Cornley, 2003). Most of the developedprocesses are designed for the assimilation of multi-sensordata, such as GPS TEC, ionosonde soundings, or in situmeasurements of electron density. Such analyses aim toprovide near real-time monitors or forecasters for spaceweather. In parallel to their development, the analysesare used as case studies to observe ionospheric processesand, by this, to evaluate their actual potential as iono-

ed.

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spheric monitors (e.g., Mitchell, 2003; Hajj et al., 2004;Stolle et al., 2005a). Beside some discussion on the electrondensity results, these studies mainly apply the vertical TECre-integrated from the 3-dimensional (3D) ionosphericspecifications. This results from the restricted availabilityof vertical sounders such as ionospheric radio occultation(IRO) experiments, or ionosonde stations. However,Meegs et al. (2004) showed for the European sector thatvertical TEC re-integrated from a 3D imaging analysisgives results superior to the 2D TEC mapping functiontechnique, which justifies the useful application of TECmaps retrieved from 3D analyses.

Major magnetic storms occurred in October/Novemberof 2003 accompanied by observations of the solar-terres-trial system with extreme magnitudes (e.g., Gopalswamyet al., 2005). Intensive ionospheric storms happened on29 and 30 October when the Bz component of the Inter-planetary Magnetic Field (IMF) was negative. The obser-vation of the low- and mid-latitude day side ionosphereof North America during 30 October was subject of manypublications so far, where vertical TEC with over100 TECU (1 TEC Unit = 1016 electrons/m2) were re-ported (e.g., Foster and Ridout, 2005; Chi et al., 2005;Zhao et al., 2005). As the cause for the positive ionosphericstorm over North America on 30 October 2003 around20:00 UT Foster and Ridout (2005) proposed the occur-rence of storm enhanced density (SED) (Foster, 1993).SED means low-latitude thermal plasma transported sun-ward (toward noon) by the convection electric field. Foster(1993) further reported the streaming of low latitude SEDF region plasma into the polar cap to be a source for polartongues of ionisation. In correspondence, Foster and Rid-out (2005) observed the enhanced TEC over North Amer-ica to reach high latitudes in the evening of 30 October2003. Mitchell et al. (2005) observed the TEC over thenorth polar region and proposed a stream of polar anti-sunward ionisation at 21:00 UT. They compared the TECmapping results with patterns from convection modelsand found both in good agreement to each other. Polarplasma patches may form in anti-sunward polar drift pat-terns during negative IMF Bz. They have been observedby local instrumentation many times (e.g., Weber et al.,1984; Valladares et al., 1994) and have been subject tomodel studies (e.g., Sojka et al., 1993, 1994). The patch sizewas observed to be 800–1000 km (Weber et al., 1984), andit was modelled as hundreds to 2000 km (Sojka et al.,1994). Weber et al. (1984) gave the patch velocity to rangebetween 500 and 1000 m s�1 coinciding with a dawn–duskelectric field of 25–50 mV m�1.

In this paper, we present north polar TEC maps re-inte-grated from 3D assimilation results of electron density for30 October 2003 between 20:00 and 23:40 UT. We want toimage the large-scale distribution of electron content of thenorth polar ionosphere and compare these observationswith the behaviour of the IMF. The reliability of the recon-structed TEC maps based on the actual GPS ray availabil-ity is discussed.

2. GPS ionospheric imaging process

The applied ionospheric imaging is based on GPS dataassimilation using iterative MART. The process is initia-lised with a combination of the International ReferenceIonosphere 2000 (Bilitza, 2001) and the plasmaspheric glo-bal core plasma model (Gallagher, 2000). The data baseconsists of slant TEC measurements from ground-basedreceivers of GPS signals. When available, the analysisincorporates IROs retrieved from TEC observations onboard the CHAMP (CHAllanging Minisatellite Payload)satellite (Reigber et al., 2002), which have been calibratedbeforehand (Heise et al., 2005). The process is also de-signed to include ionosonde soundings, or in situ electrondensity measurements. Before starting the analysis, thecontribution of the plasmaspheric electron density to theGPS TEC values is estimated based on a model-assistedtechnique and it is subtracted from the respective TEC va-lue. The assimilation area is principally variable. For thenorth polar region, it is defined from 50� northward andbetween 80 and 1000 km altitude. Geodetic grid coordi-nates are applied with a resolution of Dlon = 10�,Dlat = 4�, and Dalt = 20 km giving 16,560 voxels. TECfrom ground-based networks like the International GPSService (IGS) provide up to 300 GPS rays simultaneouslyfor the north polar region, and one occultation includes�360 rays. This results in �6000 intersected voxels. Thedata coverage can be increased using an extended intervalof data collection. Here, we do not apply any temporaldata integration (except of the IROs which take �6 min)in favour for the best possible temporal resolution espe-cially during very disturbed times. The accuracy of theassimilation results, of course, depends on the location ofavailable data. Due to the large grid size, the integratednature of the input data, and the smoothing technique ap-plied during the iterative assimilation, the imaging resultscan show only large-scale distribution of electron content.The reliability of our TEC images for 30 October 2003 isdiscussed in the next paragraph and in Section 3. A detaileddescription of the assimilation process including extendedvalidation work is given in Stolle (2004) and Stolle et al.(2005b).

To image the north polar ionosphere on 30 October2003, ground-based GPS TEC from the IGS and theScripps Orbit and Permanent Array Center (SOPAC) areused in 10-min time steps. When available, CHAMP IROsare also incorporated. Fig. 1 shows one example of theGPS rays applied in the assimilation process for 30 October2003. At that time no occultation data were available forthe north polar region. A rather dense network of GPSreceivers is given over Europe and Northern Canada. Thegap between the two continents is sparsely filled by somereceivers at Greenland, Iceland and Svalbard. A poor datacoverage is provided over Asia. Therefore, during the even-ing hours, the results are more reliable on the dusk sidethan on the dawn side. From receivers lying at the bound-ary of the assimilation area, only northward looking rays

Fig. 1. GPS ray distribution available for the 21:00 UT assimilationprocess.

C. Stolle et al. / Advances in Space Research 36 (2005) 2201–2206 2203

are considered since it is required that the entire GPS ray(from 80 to 1000 km altitude) is located in the assimilationarea. Therefore, ionospheric observations at 50�N might bemissed here. TEC results at higher latitudes are more reli-able. Fig. 2 shows the time series of TEC re-integrated fromelectron density profiles obtained by the EISCAT SvalbardRadar (ESR42) from 100 to 1000 km. These data are avail-able at http://www.eiscat.com and were first presented byBarthelemy et al. (2003). The accuracy of the EISCAT elec-tron density data for this period is given within 10–20%(M. Rietveld, personal communication). The elevation ofthe radar beam was 81.6� with an azimuth of 181� duringthe whole evening. Fig. 2 gives the TEC along the radarbeam. Additionally, vertical TEC above the radar stationbut retrieved from the assimilation results are plotted withasterisks. The good agreement between both ionosphericspecifications is evident for their long term evolution, de-spite the lower temporal and spatial resolution of the imag-ing compared to EISCAT. In particular, a simultaneouslylarge increase of electron content is observed at�20:45 UT. The highest ESR42 TEC values with 46 TECUare measured after 21:00 UT. Here, significant differencesto the assimilated TEC appear, which cannot result from

Fig. 2. TEC time series retrieved from the EISCAT Svalbard Radar (solidline) and the assimilation results at the EISCAT site (asterisks).

missing data in the assimilation map. The Svalbard receiverwas continuously operating during these times. The obser-vation of high TEC might rather be attributed to local,small-scale events which are actually not covered by the3D assimilation.

3. Ionospheric imaging on 30 October 2003

Maps of vertical TEC re-integrated from assimilationresults of north polar 3D electron density fields are givenin Fig. 3. They show the electron content on 30 October2003 from 20:00 to 23:40 UT in 20-min time steps. (The fullTEC series in 10-min resolution is available in Stolle(2004).) Fig. 3(a) images a region of enhanced TEC of�35 TECU above North Canada/North-West Greenland.Fig. 3(b) shows a tongue of ionisation between the day sideand Western Europe. Inside the ionisation stream, a patch-like region of high electron content reappears in Fig. 3(c)above Northern Greenland. Such a patch appears equallyin Fig. 3(d) and (e) but further toward the night side nearSvalbard/Northern Europe with TEC values up to40 TECU. The electron content at high northern latitudesis imaged to deplete below 10 TECU from 21:40 UTonward. Until 23:00 UT TEC values up to 20 TECU appearonly at lower latitudes. Consecutively, an elevated TEC isobserved on the night side (Fig. 3(k) and (l)) reaching valuesof up to 30 TECU. The ionisation band expands over severallongitudes and it is only observed at latitudes below 70�N.

The patch-like regions of enhanced TEC are a promi-nent feature on the polar TEC maps. The reliability ofthe assimilation results to image this structure shall be dis-cussed for the example shown in Fig. 3(d) at 21:00 UT.There is an ionised region over Svalbard with steep electroncontent gradients on its east side and smoother gradientson its west side. The corresponding GPS ray distributionavailable for this assimilation has been given in Fig. 1. Agood coverage with GPS rays exists at the eastern side ofthe imaged patch. Therefore, the sharp electron contentgradient should be reliably captured by the data. The wes-tern side of the patch is only sparsely covered by some raysfrom Iceland, southern Greenland and Northern Canada.Hence, the details of the west side boundary are uncertainon the image. Nevertheless, the few rays with lower TECvalues provide at least some constraint to the west-sideboundary of the patch. Note, only the Svalbard and North-ern European rays produced elevated TEC values of40 TECU. Additionally, discrete boundaries of such plas-ma blobs are difficult to define with the GPS assimilationdue to the finite voxel size and to the integrative characterof the measurements over long distance. The large-scalelocations of structures at high latitudes at the dusk sidein Fig. 3 should be sufficiently resolved by the imagingresults. However, their exact positions are difficult todetermine with the available data set.

It was checked that the enhancement of the night sideionisation after 23:00 UT is not purely induced by chang-ing data availability (not shown here). At all times after

b ca

e fd

h ig

k lj

Fig. 3. Maps of vertical TEC re-integrated from north polar 3D electron density fields obtained by the assimilation of IGS and SOPAC ground data for 30October 2003. If a black cross is given right next to the map, also CHAMP IRO data are assimilated. TEC contours are labelled in TECU and the givencoordinates are geographic.

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23:00 UT, a number of European stations locating between0 and 45�E was receiving, and either the Iceland (�339�E/64�N) or the southern Greenland station (�314�E/60�N)

received at least one GPS ray with eastern azimuth. There-fore, the night side was not in a total data gap for anyassimilation time after 23:00 UT.

C. Stolle et al. / Advances in Space Research 36 (2005) 2201–2206 2205

4. Discussion

During times of negative IMF Bz enhanced dawn–duskelectric field is known to establish in the polar cap due toreconnection processes between the IMF and the geomag-netic field (Dungey, 1961). The electric field is responsiblefor driving anti-sunward plasma drift patterns which maylead to the formation of tongues of ionisation. InFig. 3(a)–(e) high electron content extending from theday side toward the night side are observed. Fig. 4 showsthe IMF Bz during this evening which is pronounced nega-tive in coincidence with the imaged anti-sunward directedplasma. The observation of a localised region of enhancedelectron content at successive locations toward the nightside suggests the formation of a polar plasma patch. Sojkaet al. (1993) proposed a time varying convection pattern asone cause for the patch formation. For a largely changingIMF By, as it occurred during the period of a persistentlynegative IMF Bz (see Fig. 4) a varying convection patternis expected (Heppner and Maynard, 1987). The formationof a patch by that mechanism is at least possible for ourobservations. The size of the imaged patch is between�1500 and 2500 km which is large but compares to formerpatch modelling (Sojka et al., 1994). When defining a cen-tre of the patch, its drift velocity can be estimated betweenthe consecutive images. By that a patch velocity of�1600 m s�1 is inferred from the images between 20:30and 20:50 UT and a velocity of �800 m s�1 is estimatedfrom successive images until 21:20 UT. The derived veloc-ity of the ionisation structure is plausible in comparisonwith former observations (Weber et al., 1984), especiallywhen expecting a high dawn–dusk electric field at theobservation time, e.g., due to the very low IMF Bz.

The high latitude TEC values diminish in Fig. 3(f) andare low afterwards. The fast electron content depletion heresuggests a high ionospheric recombination rate. The coinci-dence with increasing IMF Bz after 21:30 UT further pro-poses decreasing cross polar plasma transport. For thesake of completeness it cannot be excluded, that the ob-served ionisation anomaly still propagated some way east-ward before it disappeared since the GPS stations density

Fig. 4. Time series of IMF By (grey) and Bz (black) as 4-min averageddata. Data are available from http://www.srl.caltech.edu/ACE/ASC/level2/lvl2DATA_MAG.html.

thins out beyond Eastern Europe. The extremely fast deple-tion of the patch can only be speculated from these TECreconstruction maps. However, a quick decrease of plasmadensity is also confirmed by EISCAT radar observations(see Fig. 2). At following times, enhanced night side elec-tron content in the sub-auroral latitudes is imaged inFig. 3(k) and (l). At this stage, we can only conclude thatthis observation was made during another negative phaseof the IMF Bz and shortly after large oscillations of it(see Fig. 4), while the description of a possible mechanismis out of the scope of this paper. Our analysis, however,does not propose a transport of accumulated day side plas-ma across the polar cap to be responsible for the observednight side TEC enhancement. The polar cap shows contin-uously low electron content after 21:30 UT. The period ofpositive IMF Bz during this time is theoretically not in fa-vour of increased anti-sunward plasma transport, too.

5. Summary

Electron content maps resulting from 3D GPS TECassimilations were applied to observe the north polar iono-sphere in the evening of 30 October 2003. By comparisonwith a TEC time series from the Svalbard EISCAT radar,we showed that the large-scale distribution of electron con-tent, as well as the absolute TEC values, can be well esti-mated by the assimilation analysis provided that datacoverage is adequate. On the maps, an anti-sunward direc-ted ionisation stream and a patch-like structure was ob-served at successive times between 20:00 and 21:30 UT.This period is accompanied by lasting negative IMF Bz.From that we suggest the observations of day side plasmato drift toward the night side. The derived velocity and thesize of the structure were put in context with findings in theliterature and their plausibility could be stated. The patchand the anti-sunward stream disappeared on the maps withincreasing IMF Bz after 21:30 UT. Afterwards, low elec-tron content was imaged inside the polar cap. During a fur-ther negative phase of the IMF Bz, at 23:00 UT, the nightside electron content at sub-auroral was imaged toenhance.

The use of the electron content maps alone allows, ofcourse, only limited conclusions about the ionosphericbehaviour. Here, we presented these information and com-pared them with the behaviour of the IMF. A detaileddescription of the ionosphere–magnetospheric systemwould require further observations of additional parame-ters, such as the plasma drift, plasma temperatures, or geo-magnetic field measurements. The reconstructed maps ofelectron content seem to be a promising method within thisset of observations because it combines several virtues, e.g.,providing instantaneous images of the ionosphere on ahemispheric scale at successive times.

The TEC images retrieved from GPS 3D assimilationtools are well suitable to observe the large-scale distribu-tion of electron content. It should be kept in mind that lo-cal observations may be uncertain due to the actually

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uneven distribution of GPS ground stations in different re-gions. An increasing global coverage can be expected in thefuture from ionospheric sounding satellite missions, such asCOSMIC (Constellation Observing System for Meteorol-ogy, Ionosphere and Climate) enhancing the assimilationaccuracy also in polar regions.

Acknowledgements

This study was supported by DFG under Grant RA569/5-2. Many thanks to Volker Wilken for pre-processing theSOPAC GPS data. Thanks are due to the referees for valu-able suggestions. We acknowledge the use of the ACE datasupplied from the ACE Science Center at Caltech. EISCATis a scientific association of national funding agencies ofFrance, Finland, Germany, Japan, Norway, Sweden andthe United Kingdom.

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