comparison of satellite measurements of the low-latitude nighttime upper ionosphere with iri

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Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 2107–2118

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Comparison of satellite measurements of the low-latitudenighttime upper ionosphere with IRI

Heejun Kima,�, Kyoungwook Mina, Jaeheung Parka, Jaejin Leeb, Ensang Leeb,Hyosub Kilc, Vitaly P. Kimd, Sunmie Parke

aDepartment of Physics, Korea Advanced Institute of Science and Technology, 373-1, Guseong-dong,

Yuseong-gu, Daejeon 305-701, Republic of KoreabSpace Science Laboratory, University of California, Berkeley, CA, USA

cApplied Physics Laboratory, Johns Hopkins University, MD, USAdPushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, Troitsk, Russia

eInstitute for Gifted Students, Korea Advanced Institute of Science and Technology, Daejeon, Republic of Korea

Received 13 December 2005; received in revised form 1 August 2006; accepted 11 August 2006

Available online 29 September 2006

Abstract

In this paper, we report the results of our comparison study between satellite measurements and the International

Reference Ionosphere (IRI) model on the seasonal and longitudinal changes of the low-latitude nighttime topside

ionosphere during the period of solar maximum from June 2000 to July 2001. Satellite measurements were made by

KOMPSAT-1 and DMSP F15 at 685 km altitude and 840 km altitude, respectively. The results show that the IRI2001

model gives reasonable density estimations for the summer hemisphere and the March equinox at both altitudes. However,

the observed wintertime densities are smaller than the predictions of the IRI2001 model, especially at a higher (840 km)

altitude, manifesting strong hemispheric asymmetries. The observed electron temperatures generally reside between the

two estimations of IRI2001, one based on the Aeros–ISIS data and the other based on Intercosmos, and the latter

estimation better represents the observations. With more or less monotonic increase with latitude, the temperature profiles

of the IRI2001 model do not predict the enhancement seen around 151 magnetic latitude of the winter hemisphere.

Longitudinal variation, probably caused by the zonal winds, is seen in all seasons at both altitudes, while the IRI2001

model does not show a large variation. The observed density and temperature show significant changes according to the

F10.7 values in the whole low-latitude region from 401S to 401N geomagnetic latitude. The effect is manifested as increases

in the density and temperature, but not in the hemispheric asymmetry or in the longitudinal variation.

r 2006 Elsevier Ltd. All rights reserved.

Keywords: Ionosphere; Solar activity; Plasma temperature and density; Seasonal and longitudinal variation; IRI model

e front matter r 2006 Elsevier Ltd. All rights reserved

stp.2006.08.006

ing author. Tel.:+82 42 869 2565;

5525.

esses: [email protected] (H. Kim),

c.kr (K. Min), [email protected]

1. Introduction

In situ measurements of the topside ionosphere ataltitudes above 300 km have been made with rocketsand satellites since the early years of space explora-tion. For example, the ISIS 1 satellite measured the

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ARTICLE IN PRESSH. Kim et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 2107–21182108

electron density and temperature for the altitudesbetween 600 and 3600 km over the period betweenJanuary 1969 and June 1971. ISIS 2 was launched in1971 and collected data at 1400 km altitude until1973. Atmospheric Explorer-C was launched in1973 into an eccentric orbit with apogee and perigeealtitudes of 4300 and 150 km, respectively, but laterits orbit was circularized and the measurementswere carried out at altitudes between 200 and400 km. The data obtained from these missions, aswell as from other numerous measurements, havebecome the basis of empirical ionospheric models(Bent et al., 1972; Brace and Theis, 1981; Bilitzaet al., 1993) and the references of theoretical models(Anderson et al., 1989; Huba et al., 2000).

While primary objectives of empirical modelshave been to obtain vertical profiles of the iono-sphere, the seasonal and longitudinal variations ofthe topside ionosphere have also been proven to besignificant, as the data from Hinotori and DefenseMeteorological Satellite Program (DMSP) haveindicated. For example, Watanabe et al. (1995)noted the asymmetry in electron temperature withenhancement in the winter hemisphere in theirHinotori data obtained at 600 km altitude andattributed its physical origin to the plasma transportalong the magnetic field line caused by neutralwinds. Recently, measurements at an altitude of830 km by DMSP F10 confirmed the seasonaland longitudinal variation of the nighttime tempera-ture troughs (Venkatraman and Heelis, 1999) aswell as its relationship with the interhemisphericplasma transport (Venkatraman and Heelis, 2000).In addition, strong ion temperature peaks were

Fig. 1. Effects of the meridional and zonal winds when their componen

equator. As the zonal wind is eastward in the evening, it occurs in the re

solstice and in the region of negative magnetic declination angles dur

(geographic longitude) and the vertical axis (geographic latitude) are diff

for illustration purpose only and do not represent their relative magnit

found in the low-latitude winter hemisphere, ofwhich the origin was ascribed to adiabatic heatinginfluenced by the downward neutral wind.

The observed nighttime plasma temperatures ofthe equatorial topside ionosphere have also beenmodeled (Bailey et al., 2000b). The model confirmedthat the temperature crests/troughs are due toadiabatic heating/cooling as plasma is transportedalong the magnetic field lines from the summerhemisphere to the winter hemisphere, and that theinterhemispheric plasma transport needed to pro-duce the required adiabatic heating/cooling can beinduced by F-region neutral winds. It was alsoshown that the longitudinal variations in theobserved troughs and crests arise mainly from thelongitudinal variations in the magnetic meridionalwind. For example, as Fig. 1 indicates, the net effectof the meridional and the zonal winds is to make theneutral wind become more or less parallel to themagnetic field line for the region of positivemagnetic declination angle during the Decembersolstice, generating a favorable condition for plasmatransport along the field line. During the Junesolstice, with the southward meridional wind, thefavorable condition for plasma transport occurs inthe region of negative magnetic declination angle.

Hence, it seems desirable to compare the observedseasonal and longitudinal variations of the topsideionosphere with the present ionospheric models sothat the models can be improved in the future.Recently, there have been several comparativestudies of the models with observations. Rich andSultan (2000), using the comprehensive DMSPion density data set, tested the validity of various

ts along the magnetic field lines are constructive near the magnetic

gion of positive magnetic declination angles during the December

ing the June solstice. Note that the scales of the horizontal axis

erent. Also, the lengths of the arrows for the two winds are chosen

udes.

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models by studying the dependence on magneticlatitudes and solar cycle variations. Venkatraman etal. (2005) compared the another model of iono-sphere (SAMI2) with DMSP measurements at840 km altitude as well as the ground-basedobservation at Jicamarca. Hinotori data were alsoused for the comparison of �600 km altitudeionosphere with the International Reference Iono-sphere (IRI) model (Bhuyan and Chamua, 2002).The SROSS C2 satellite observed the �500 kmaltitude ionosphere of the Indian region during thesolar minimum, and the results were compared withthe IRI (Bhuyan et al., 2002, 2003, 2004). Appar-ently, all these studies focused mainly on thediurnal, latitudinal, and solar cycle variations anddid not take into account the longitudinal variation;thus, the comparison was not global.

In this paper, we present the results of ourcomparative study for the topside pre-midnightionosphere at low latitudes during the solar max-imum period of 2000–2001. We employ theKOMPSAT-1 data obtained at 685 km as well asthe DMSP F15 data at 840 km altitude. With sun-synchronous polar orbits at constant altitudes, thesesatellites cover most of the longitudes and latitudesat fixed local times. We compare the observedplasma density and temperature data with themodel estimations based on IRI2001.

2. Observation

KOMPSAT-1 was launched on December 21,1999 into a sun-synchronous polar orbit with analtitude of 685 km and an orbital inclination angleof 981. The Ionospheric Measurement Sensor (IMS)on board the KOMPSAT-1 monitored electrondensity and temperature with a Langmuir Probe(LP) and an Electron Temperature Probe (ETP).One of the most critical problems associated withLPs is the effect of contamination. Hence, weemployed pulsed sweep voltages for the KOMPSATLP to reduce the contamination effect (Holmes andSzuszczewicz, 1975). The resulting current (I)–vol-tage (V) curves of the KOMPSAT LP did not showsignificant hysteresis, confirming that the effect ofcontamination was negligible. The ETP is amodified LP, in which the I– V characteristic curveis deformed by superposing the rf voltage on thebiased probe. Electron temperature is obtainedfrom the deviation of the I– V curve according tothe variation of the amplitude of the rf voltage(Hirao and Oyama, 1970). The LP measured

electron density and temperature with a timeresolution of 4 s, and the ETP measured the electrontemperature and floating potential with a 1 s timeresolution. The measurement ranges of the densityand temperature were 104–106 cm�3 and 0–104K,respectively. We processed the data as follows.First, we obtained full current-voltage curves with232 data points per sweeping period of 4 s, withoutfurther onboard data processing. Fitting of theseI– V curves to obtain electron density and tempera-ture was carried out on the ground after examiningthe raw data. This was possible because KOMP-SAT-1 was equipped with a large memory system aswell as a high-speed down link. The electron densityand temperature were derived in electron saturationand retarding regions, respectively, with traditionalLP equations. We verified the data of the LP againstthose obtained from a separate and independentinstrument on the same satellite, an ETP. In thispaper, we analyze the electron density and tempera-ture data obtained from the LP.

The IMS measured the topside nighttime iono-sphere during the solar maximum period from June2000 to July 2001 with a fixed equatorial crossing at22:50 (Lee et al., 2002; Park et al., 2003). DMSPF15 was operated during the same period at analtitude of 840 km. DMSP F15 is a polar, sun-synchronous satellite with an equatorial crossing at21:30 LT. We employ the ion density and theelectron/ion temperature data of DMSP F15 tocompare them with the model estimations at 840 kmaltitude. During the period of KOMPSAT-1 opera-tion solar activity was quite high and variedsubstantially. To remove this dependence, sincethe IRI model does not respond well to the changesof F10.7, we limit our analysis to the data for F10.7between 140 and 170 and Kpo5- in the compara-tive study with the models. The observed solaractivity dependence will be discussed independentlyof the IRI model. In addition, the current analysis isrestricted to the low-latitude regions, with geo-graphic or geomagnetic latitudes between 401N and401S. The KOMPSAT-1 data for the Septemberequinox are insufficient for statistical analysis.Hence, we analyze here only the data of the Marchequinox and the December and June solstices.

3. Results

Fig. 2 shows the electron density and temperaturemeasured by KOMPSAT-1 at 685 km altitude(upper panel) as well as the ion density and

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Fig. 2. Global maps of electron density and temperature measured by KOMPSAT-1 (upper panels) and ion density and temperature

measured by DMSP F15 (lower panels) are shown in geographic coordinates for the December solstice, March equinox, and the June

solstice. The black solid line represents the magnetic dip equator. Note that the scales are adjusted for KOMPSAT-1 observation, which

results in saturation in the color code for the plot of DMSP F15.

H. Kim et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 2107–21182110

temperature measured by DMSP F15 at 840 kmaltitude (lower panel). The data are averaged with a11 interval in latitude and a 31 interval in longitude.Seasonal variation is obtained by selecting twomonths around the solstices and the Marchequinox: December solstice is from November 21,2000 to January 21, 2001, March equinox is fromFebruary 21, 2001 to April 21, 2001, and Junesolstice is from May 21, 2001 to July 21, 2001. InFig. 2, both data sets of KOMPSAT-1 and DMSPF15 show asymmetry in the density with higherdensity in the summer hemisphere. Broad enhanceddensity regions are seen around the magnetic

equator, and these regions are shifted toward thesummer hemisphere by 10–201. The Decemberplasma density is on the average slightly higherthan that of June. Though the longitudinal varia-tion of the density is not clear in this map, thehighest density is seen in the longitude sector240–3301 of the southern hemisphere during theDecember solstice. More on the longitudinal de-pendence will be discussed later with plots made forindividual longitude sectors.

The temperature maps of both KOMPSAT-1 andDMSP F15 show temperature troughs and enhance-ments around the magnetic equator. In fact, these


are the dominant features in the low-latitude region.The seasonal and longitudinal variation of theplasma temperature is seen in both data sets ofKOMPSAT-1 and DMSP F15, though the variationis much more significant in the ion temperature dataof DMSP F15. The variation is well in accord withprevious reports: strong troughs and crests in150–2701 geographic longitudes during the Decem-ber solstice and in 300–3601 geographic longitudesduring the June solstice (Venkatraman and Heelis,1999). These variations are clearly due to the effectsof meridional neutral winds combined with thezonal winds that form different angles with themagnetic field line according to the magneticdeclination at the magnetic equator (Venkatramanand Heelis, 1999).

The temperature troughs are also seen during theMarch equinox in both data sets of KOMPSAT-1and DMSP F15: lower temperature south of themagnetic equator for 150–2701 geographic long-itudes and north of the magnetic equator for300–3601 geographic longitudes. The same figuresalso show the local temperature increases in theopposite regions to the above with respect to themagnetic equator, indicating the temperature crests.The generation of temperature troughs and crests

Fig. 3. Electron density profiles at 685 km altitude against geomagne

320–3501, respectively. Solid lines are for the KOMPSAT-1 data, with e

the IRI2001 model.

during the equinoctial seasons is due to the eastwardzonal winds, since the southern region of themagnetic equator in the longitude sector 150–2701and the northern region in the longitude sector300–3601, in which temperature troughs are seen,are all located west of the magnetic equator wherethe field-aligned component of the eastward zonalwind is upward. Likewise, the temperature increasein the eastern regions of the magnetic equator is dueto the downward field-aligned component of thezonal wind.

As the dominant features for variations withrespect to the longitudinal changes are thesedependences on the magnetic declination angles,especially in the electron temperature, we selectgeographic longitude sectors 60–1201, 180–2401,and 320–3501 for a closer examination and compar-ison with the IRI model. These three longitudesectors represent zero, positive, and negative mag-netic declination angles, respectively. We plot theaveraged plasma density and electron temperatureprofiles against magnetic latitudes for given seasons.In Fig. 3, the electron density measured byKOMPSAT-1 is compared with the model esti-mation of IRI2001 at 685 km altitude. It is seen thatthe IRI2001 predictions, generally following the

tic latitudes for the longitudinal sectors 60–1201, 180–2401, and

rror bars representing the standard deviation and dotted lines for


KOMPSAT-1 trend, show hemispheric asymmetrieswith enhanced density in the summer hemisphere.The IRI2001 model shows higher density than theobservation in the mid-latitude region around 201 ofthe winter hemisphere by a factor of two or more,except in the longitude sector 320–3501 during theDecember solstice. Hence, though the summer towinter meridional wind effect appears in the IRImodel as the hemispheric asymmetry represents it,the effect is more significant in the observed data,except in the longitude sector 320–3501 during theDecember solstice where the zonal wind maystrongly reduce the effect of the meridional wind.In fact, the zonal wind effect is seen in most of theplots of Fig. 3. For example, during the Junesolstice, the density depression in the winter hemi-sphere is especially significant in the longitude sector320–3501 in the KOMPSAT-1 data, while signifi-cant density depression is seen in the longitudesector 180–2401 in the December data. The Marchequinox data also shows the effect of the zonalwind, with higher density in southern (western)region of the magnetic equator in the 180–2401longitude sector and in the northern (western)region of the magnetic equator in the 320–3501longitude sector. The IRI model does show such an

Fig. 4. Electron temperature profiles at 685 km altitude against geomag

320–3501, respectively. Solid lines are for the KOMPSAT-1 data, with er

and the dashed lines are for the IRI2001 models with Aeros–ISIS and

indication, but it is more prominent in the observeddata.

As the IRI2001 provides two options for thetopside electron temperature model, one based onthe Aeros–ISIS data and the other based on theIntercosmos data, we compare our data with bothestimations. The observed temperatures by KOMP-SAT-1, shown in Fig. 4, generally reside between thetwo values. The estimations with the Intercosmosdata seem to agree better with the observations,though they give slightly higher temperature pre-dictions, especially at high latitudes, while thosewith the Aeros–ISIS data yield significantly lowertemperatures than the observations in most of thecases. Neither of the two model profiles showssignificant temperature troughs seen around themagnetic equator and shifted slightly toward thesummer hemisphere. In addition, the longitudinaldependence is not clear in the models.

The discrepancies between observations and themodels are even more significant at higher altitudes.In Fig. 5, the ion density measured by DMSP F15 iscompared with that of IRI2001 modeled at 840 kmaltitude. The IRI model generally predicts higherdensity than the observations, and it being lessvariable, the difference is especially large for the

netic latitudes for the longitudinal sectors 60–1201, 180–2401, and

ror bars representing the standard deviation, while the dotted lines

Intercosmos topside electron temperature models, respectively.

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Fig. 5. Ion density profiles at 840 km altitude against geomagnetic latitudes for the longitudinal sectors 60–1201, 180–2401, and 320–3501,

respectively. Solid lines are for the DMSP F15 data, with error bars representing the standard deviation and dotted lines for the IRI2001

model.

H. Kim et al. / Journal of Atmospheric and Solar-Terrestrial Physics 68 (2006) 2107–2118 2113

winter hemisphere where the IRI value can even bean order of magnitude higher than that of theDMSP observation. The broad density maximum,seen in the KOMPSAT-1 data around the magneticequator, is now more localized in the DMSP data.The significant density difference between the modeland the observations that appeared in the southernhemisphere of the 320–3501 longitude sector duringthe March equinox is clearly due to the eastwardzonal wind, which lowers the upper ionosphereeastward (southward) of the magnetic equator.A similar zonal wind effect is seen in the 180–2401longitude sector of the March equinox, where thedensity is higher in the southern hemisphere becausethe ionosphere is raised by the eastward zonal windin this western region of the magnetic equator.

Fig. 6 shows the electron temperatures measuredby DMSP F15 and the IRI2001 model estimationswith Aeros–ISIS and Intercosmos options corre-sponding to the DMSP F15 altitude. Again, theobserved values generally reside between the twomodel estimations, while the profiles based on theIntercosmos data are much closer to the observa-tions. The DMSP F15 observations show largevariations along the magnetic latitudes with deeptroughs around the magnetic equator, enhance-

ments around 151 latitude in the winter hemisphere,and high temperatures at higher latitudes. The twoIRI temperature models show more or less mono-tonic increase toward the high latitudes withoutdeep localized temperature troughs and enhance-ments. The zonal wind effect is seen in all seasons inthe data, with deep troughs and enhanced tempera-tures in the 180–2401 longitude sector during theDecember solstice and in the 320–3501 longitudesector during the June solstice. In addition, duringthe March equinox, the electron temperature isenhanced in the 180–2401 longitude sector of thenorthern hemisphere and in the 320–3501 longitudesector of the southern hemisphere; these sectors areeastward of the magnetic equator, with respectivetemperature peaks around the 101 magnetic lati-tude. No such longitudinal dependences are clear inthe IRI models.

4. Discussion

As Hinotori observed the low-latitude topsideionosphere at 600 km during the solar maximumperiod 1981–1982, it should be interesting tocompare the present KOMPSAT-1 data with thoseof Hinotori. According to Fig. 2a of Bailey et al.

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Fig. 6. Electron temperature profiles at 840 km altitude against geomagnetic latitudes for the longitudinal sectors 60–1201, 180–2401, and

320–3501, respectively. Solid lines are for the DMSP F15 data, with error bars representing the standard deviation, while the dotted lines

and the dashed lines are for the IRI2001 models with Aeros–ISIS and Intercosmos topside electron temperature models, respectively.


(2000a), in which yearly variation of the nighttimeHinotori measurements is described, the peakdensity at the March equinox is (3–3.5)� 105 cm�3

at 600 km altitude, while the current data shows it is(2–3)� 105 and (1.5–2)� 105 cm�3 at 685 and840 km, respectively. The KOMPSAT-1 electrondensity, measured at heights between the altitudesof Hinotori and DMSP F15, nicely fits between thetwo values measured below and above it. Theelectron temperature measured by KOMPSAT-1 isaround 1200K for the March equinox at themagnetic equator, which is close to that of theHinotori observation, 1200–1250K, as can be seenfrom Fig. 2b of Bailey et al. (2000a).

There have been some recent comparative studiesof the ionospheric models with the satellite observa-tions for the upper ionosphere. Bhuyan andChamua (2002) compared the Hinotori data at600 km altitude obtained during the solar maximumperiod of February 1981–June 1982 with IRIpredictions for low-latitude regions between 251Nand 251S along 751E meridian. They concluded thatthe IRI model underestimated the daytime electrondensity, while it matched well with observations forthe nighttime ionosphere. Bhuyan et al. (2002, 2003,2004) compared the data of SROSS C2 obtained at

500 km altitude during the low solar activity periodof 1995–1997 with the IRI predictions for the Indianzone ionosphere. The results showed that the IRImodel overestimated the electron density at all localtimes in all seasons. The electron temperatures weresimilar to those of IRI during nighttime but higherthan the IRI values at other local times, while theion temperatures were overestimated by the IRImodel. Rich and Sultan (2000) compared variousmodels with the DMSP observations for a full solarcycle and concluded that none of the models wereable to predict the entire variation of the iono-sphere. Venkatraman et al. (2005) compared thedata from the ground-based Jicamarca radar andthe DMSP satellites with the SAMI2 model for theperiod 11–13 June 2002 and concluded that the totalion density was well reproduced by the modelduring daytime and nighttime, while the ion andelectron temperatures agreed well at night.

The current observation shows IRI predictselectron density reasonably well for the summerhemisphere and during the March equinox. How-ever, being less latitude-dependent, it predicts higherdensity in the winter hemisphere, and the differenceis more significant at higher altitudes. The zonalwind effect seems significant in the observed data as


it is seen in the March equinox data at bothaltitudes. The observed electron temperature variessignificantly across the magnetic latitudes, and thevariation is larger at higher altitudes. The newIRI2001 model, with Intercosmos data for topsideelectron temperature estimation, generally agreeswith the observations, though it slightly over-estimates the electron temperature, especially at685 km altitude. Being more or less monotonic, theIRI temperatures do not show significant tempera-ture troughs or enhancements, which are caused bythe effects of neutral winds.

Since the electron densities at the two altitudes of685 and 840 km are available, it should be interest-ing to see how the IRI density profiles fit to theobserved densities. For example, we can computethe scale heights assuming exponential decay ofthe density with increasing altitude. Fig. 7 shows theresult in comparison with the IRI model. Theobserved scale heights are smaller than thoseestimated from IRI by more than a factor of twoin most cases, demonstrating that the densitydecreases more rapidly than the IRI model predicts.In most of the cases, the discrepancy comes fromthe high density of the IRI model at 840 kmaltitude, and it is more significant in the winterhemisphere. The observed scale heights are gener-

Fig. 7. Scale heights estimated from the observed data at two altitudes, 8

altitude. The result is compared with the scale heights obtained from t

ally less than 100 km without significant variationswith seasonal, longitudinal, and latitudinal changes,though they increase slightly around the magneticequator.

The electron temperatures of the trough regionsat 840 km are generally lower than those measuredat 685 km at solstices, except in the longitude sector320–3501 during the December solstice and in thelongitude sector 180–2401 during the June solstice.It is also seen that during March equinox the troughtemperature at 840 km is lower than that of 685 kmin the longitude sector 320–3501, where the mag-netic declination angle deviates most from zero,making the effect of the zonal wind more important.Hence, whenever the meridional wind and the zonalwind are constructive to generate a favorablecondition for field-aligned plasma transports, thereis a negative temperature gradient in the troughregion. For example, when we compare the troughtemperatures in the 320–3501 longitude sector forthe December solstice, we find they are 1100and 700K at 685 and 840 km, respectively. Whileplasma temperatures of the troughs are not expectedto fall below the expected neutral exospherictemperatures (Venkatraman and Heelis, 1999),the ion and electron temperatures observed by theOGO 6 satellite were occasionally seen below them.

40 and 685 km, assuming exponential decrease of the density with

he IRI profiles.

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Fig. 8. Latitudinal profiles of the plasma density and electron temperature at 840 km altitude for F10.7 values greater than 200. These

profiles are compared with the corresponding average profiles (dotted lines) for the same season in the same longitudinal bin as in Figs. 5

and 6.


This phenomenon was termed ‘plasma supercooling’and was conjectured to be the result of adiabaticcooling of the plasma caused by interhemispherictransport of plasma along the magnetic field lines(Hanson et al., 1973). The present data show how

this adiabatic cooling can be effective with thecombination of meridional and zonal winds.

We have limited our analysis of the comparativestudy with the IRI model to the data obtained forF10.7 values between 140 and 170, since IRI does


not respond to the daily variation of solar activity.However, it should be interesting to see how thetopside ionosphere behaved according to the solaractivity changes during the solar maximum periodof 2000–2001. Relations between the solar activityand ionospheric plasma parameters have beenstudied for a long time (Brace and Theis, 1981;Brace et al., 1987). For example, the plasma densityand temperature observed by DMSP satellitesvaried with a period of 27 days in response toF10.7 changes (Rich and Sultan, 2000; Rich et al.,2003). Hinotori data showed that maximal ioniza-tion increased by about 40% from moderate(F10.7—164) to high (F10.7—238) solar activity(Bhuyan and Chamua, 2002). Ground observationsat mid-latitude stations showed the peak electrondensity of the F2 region (NmF2) increased withF10.7 until it reached 175 over a half solar cycle,while there was little relationship between F10.7 andNmF2 at solar maximum (Richards, 2001). It hasalso been reported that dependence existed up to acertain level of F10.7, above which saturation wasseen (Bilitza et al., 1993; Balan et al., 1993). We plotin Fig. 8 the latitudinal profiles of the plasmadensity and electron temperature at 840 km altitudefor F10.7 values greater than 200, in comparisonwith the corresponding average profiles for the sameseason in the same longitudinal bin, as shown inFigs. 5 and 6. As can be seen in Fig. 8, the densityincreases by a factor of two and the temperatureincreases by 100–200K in all low-latitude regions,regardless of the longitude, latitude, or the seasons.

5. Summary

In this paper, we have made a comparison studybetween satellite observations and the IRI2001model for the nighttime upper ionosphere usingthe data obtained during the solar maximum period2000–2001 for the altitudes at 685 and 840 km. Theresults show that the IRI2001 model gives reason-able density estimations for the summer hemisphereand the March equinox at both altitudes. However,being less variable, it predicts higher densities forthe winter hemisphere, and the difference is moresignificant at a higher altitude. The electrontemperature of the new IRI2001 model, with theoption of Intercosmos data for the topside estima-tion, agrees quite well with the observed values,though it slightly overestimates. However, signifi-cant temperature troughs and enhancements are notpredicted by the model. One of the main differences

between the model and the observations is the effectof the zonal wind, which is observed as significant inall seasons and causes large longitudinal variations,while IRI2001 does not show significant changesaccording to the longitudes. We also investigatedthe solar activity dependence of the density andtemperature profiles. The results show that both theplasma density and the temperature increase in theentire low-latitude region with increasing F10.7values, but the longitude variations do not seem tochange much.

Acknowledgments

We acknowledge the partial support given for thepresent work by the Korea Science and EngineeringFoundation and the Satellite Technology ResearchCenter. The authors gratefully acknowledge theCenter for Space Sciences at the University of Texasat Dallas and the US Air Force for providing theDMSP thermal plasma data.

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