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poseidonmeasurements
\ PERGAMON
Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 170Ð187
TEC climatology derived from TOPEX:POSEIDON measurements M[V[ Codrescua\ \ S[E[ Palob\ X[ Zhangb\ T[J[ Fuller!Rowella\ C[ Poppea a
Cooperative Institute for Research in Environmental Sciences\ University of Colorado\ and NOAA\ Space Environment Center\ Boulder\ Colorado\ U[S[A[ b Department of Aerospace En`ineerin`\ University of Colorado\ Boulder\ Colorado\ U[S[A[ Received 2 March 0887^ accepted 11 June 0887
Abstract We have used the TOPEX data sets available from JPL to create a customized data base of the TOPEX TEC measurements that contain data from 0881 through part of 0885[ The data base includes time\ geographic and geo! magnetic coordinates of the measurement\ geomagnetic indices "Kp\ previous Kp\ Hemispheric Power\ and integral of hemispheric power over the previous 25 h#\ solar index "F 09[6#\ and International Reference Ionosphere "IRI# results corresponding to the TOPEX measurements[ In this paper we present global maps of TEC for low solar activity conditions "F 09[6 ¾ 019# for quiet "integral of hemispheric power less than 799 GWh\ roughly corresponding to Kp 1#\ moderately disturbed "integral of hemispheric power greater than 799 GWh but less than 0199 GWh\ roughly corresponding to Kp 2#\ and disturbed "integral of hemispheric power greater than 0199 GWh\ roughly corresponding to Kp 3# geomagnetic conditions\ derived by binning all appropriate TOPEX TEC data from 0881 to 0885[ The analysis is performed in a Magnetic!Local!Time\ Magnetic!Latitude coordinate system[ The most prominent feature of the global TEC maps is the feature corresponding to the equatorial anomaly[ The feature becomes wider in magnetic latitude and more pronounced in amplitude as the activity level increases[ The equatorward shift of the crests\ with increased magnetic activity\ can produce apparent decreases in TEC at their quiet time location for individual storms as evident in the con~icting conclusions of TEC geomagnetic dependence studies of the 0859s[ For the same activity level\ TEC values in the equatorial anomaly are higher during equinox compared to solstice[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
0[ Introduction In an e}ort to provide accurate\ high quality measure! ments of the global sea surface height\ a joint e}ort was undertaken by the National Aeronautics and Space Administration "NASA# in the United States and the Centre National d|Etudies Spatiales "CNES# in France[ Through this joint e}ort the TOPEX:POSEIDON sat! ellite mission "Fu et al[\ 0883# was developed and then deployed in August of 0881[ The TOPEX:POSEIDON platform contains six scien! ti_c instruments including a dual frequency altimeter\ a
Corresponding author[ Tel[] 990 292 386 5652^ fax] 990 292 386 2534[ E!mail address] codrescuÝsec[noaa[gov "M[V[ Codrescu#
single frequency altimeter\ a three channel radiometer\ and three tracking systems designed to provide precision orbit information[ To provide the needed accuracy in the sea surface height measurements "approx[ 03 cm# both the position of the satellite and any propagation path delays between the sea surface and the satellite must be known accurately[ The satellite position can be deter! mined to within 09 cm using the three independent track! ing systems located on the TOPEX:POSEIDON plat! form[ The path delays between the sea surface and the sat! ellite can exceed 14 cm and are due to changes in the atmospheric index of refraction[ These changes in the atmospheric index of refraction are primarily due to free electrons in the ionosphere but also arise as a result of tropospheric and stratospheric water vapor[ Because the ionosphere is a dispersive medium\ the path delay can be
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estimated to _rst order by transmitting at two frequenc! ies[ With the path delay known\ it is a simple linear transformation "Monaldo\ 0882^ Robinson and Beard\ 0884# to obtain the total electron content "TEC# in a column extending from the satellite\ orbiting at 0225 km\ to the subsatellite re~ection point on the surface of the ocean[ Through the use of a dual frequency altimeter\ operating at 4[2 GHz "C!band# and 02[5 GHz "Ku!band#\ the TOPEX:POSEIDON spacecraft can reduce the path delay errors to less than 2 cm[ The measurements of TEC are a byproduct of the sea surface height measurements\ and are computed for each sea surface measurement[ Operating since August of 0881\ there are currently more than 4 years of TEC data available in the public domain through the NASA EOS physical oceanography distributed active archive center "PO!DAAC# located at the Jet Propulsion Laboratory "JPL#[ Additionally\ the TOPEX:POSEIDON mission has recently been extended to six years while the spa! cecraft was designed with enough fuel to operate for 09 years[ The TOPEX:POSEIDON measurements of TEC over the past few years provide a tremendous and virtually untapped database of ionospheric measurements[ Most empirical ionospheric models and ionospheric data bases have been constructed using ground based measurements and therefore\ large regions over the oceans are not well represented[ The TEC measurements from TOPEX\ obtained entirely over the oceans\ o}er ionospheric data over previously poorly represented regions\ and are complementary to the historical ground!based TEC measurements[ The goal of this study is to utilize the TOP! EX:POSEIDON TEC data to analyze the average spatial structure of the ionosphere[ In particular\ we are inter! ested in global TEC climatology and in TEC mani! festations of the response of the middle and low latitude ionosphere to geomagnetic disturbances[ We will analyze speci_c events and their departure from the average behavior presented here in a future paper[ We also plan to repeat the climatological analysis to include more TOPEX data as they become available[ For this study we have created a customized data base of the TOPEX TEC measurements that contains data from launch through 0885[ The data base includes time\ geographic and geomagnetic coordinates of the measure! ment\ geomagnetic indices "Kp\ previous Kp\ Hemispheric Power\ and integral of hemispheric power over the pre! vious 25 h#\ a solar activity index "F 09[6#\ and empirical model "IRI# results corresponding to the TOPEX measurements[ Results from our data analysis\ as well as the data base itself\ are available from the authors\ on request[ The data provide an excellent database for model validation purposes[ This is especially important for model cali! bration over the large ocean areas where ionosonde\ GPS
TEC\ or other ground based data are not available[ In fact\ of the limited analysis which has been performed on the TOPEX:POSEIDON TEC data\ a majority of these "Robinson and Beard\ 0884^ Bilitza et al[\ 0884\ 0885# have been comparisons of the TEC data with the Inter! national Reference Ionosphere "IRI# "Bilitza\ 0889#[
1[ Data analysis The TOPEX:POSEIDON satellite orbits the Earth at an altitude of 0225 km with an inclination of 55>[ With this orbital geometry the TOPEX:POSEIDON altimeter provides data between ¦55> and −55> geographic lati! tude over all of the Earth|s oceans[ The precession of this orbit is 1> per day\ which corresponds to 7 min of local time[ Utilizing data from both the ascending and descending orbital nodes\ it takes nearly 89 days to pro! vide full local time coverage[ The original TEC data were obtained from the NASA EOS physical oceanography distributed active archive center "PO!DAAC# located at the Jet Propulsion Lab! oratory "JPL#[ Measurements located in 1×3> bins\ in geographic latitude and longitude respectively\ were _rst averaged along each orbit[ The number of points that contribute to an average varies from 0 to more than 39\ depending on the way any particular orbit crosses the bin boundaries[ The averages were tagged by measurement time and by the latitude and longitude of the center of the bin where the measurements were taken[ The geo! magnetic latitude and longitude associated with each bin center "record# along each orbit were computed using quasi dipole coordinates Richmond "0884#[ The number of points that contribute to the average\ the standard deviation\ and the median value associated with each bin average are also written to each record in the data base[ Geomagnetic indices "2!h Kp\ previous 2 h Kp value\ Hemispheric Power\ and integral of hemispheric power over the previous 25 h#\ solar index "F 09[6#\ and empirical model "IRI# results corresponding to the TOPEX measurements were added to each record to create a customized data base[ The Hemispheric Power is an esti! mate of the energy deposition in a single hemisphere by auroral precipitating particles "Fuller!Rowell and Evans\ 0876#[ Particle precipitation measurements from the NOAA:TIROS satellite series over individual half orbits are compared with statistical patterns of particle pre! cipitation[ The matching pattern\ appropriately scaled for any remaining di}erences\ is then considered to rep! resent the hemispheric energy input carried into the high! latitudes by precipitating particles\ for that particular time[ The index o}ers a good representation of the mag! netospheric forcing acting on the thermosphere: ionosphere system "see Foster et al[\ 0875#[ The integral of hemispheric power was added in an e}ort to account for the long!lived thermospheric features that produce
M[V[ Codrescu et al[:Journal of Atmospheric and Solar Terrestrial Physics 50 "0888# 170Ð187
signi_cant ionospheric storm e}ects "Fuller!Rowell et al[\ 0883\ 0885#[ The data base contains over 449\999 records per full year\ and covers 0882Ð84\ and part of 0881 and 0885[ Most of the measurements were taken during the late declining phase of solar cycle 11 or during solar minimum between cycles 11 and 12 and\ therefore\ the data base is representative of low solar activity conditions "low F 09[6#[ In the present analysis we have used only the measurements that correspond to a daily value of F 09[6 ¾ 019[ Using the data base\ we have constructed global pat! terns of TEC vs magnetic local time "MLT# and magnetic latitude for several sets of geomagnetic:seasonal conditions\ by binning the appropriate measurements[
2[ Global TEC maps Plate 0 shows the global\ averaged TEC maps for low\ medium\ and high geomagnetic activity bins\ and the associated variance "s1#[ The coordinate system is mag! netic local time "259> correspond to 13 h MLT# and Apex latitude[ The color scale ranges from 2 to 24 TECU "0 TECU 0905 el:m1# for the TEC averages\ and from 2 to 025 TECU1 for the variance s1[ The TEC maps and the variance patterns have been smoothed using a 4!point running average[ The activity bins are de_ned based on the integral of the power carried into the atmosphere by precipitating particles during the 25 h preceding the measurement[ The limit values for the power integral "799 and 0199 GWh# were chosen so as to distribute the data approximately equally between the three geo! magnetic activity bins[ The bins correspond to 25!h per! iods of quiet "steady Kp of 1#\ moderately disturbed "ste! ady Kp of 2#\ and disturbed "steady Kp of 3# conditions[ A truly great storm lasting close to 13 h would result in a value for the integral power in excess of 1499 GWh[ Obviously\ the bin labeled high in our analysis is not representative of such storms[ This choice of geomagnetic activity dependence is motivated by our interest in clima! tology\ our desire to emphasize global\ long lasting e}ects of geomagnetic activity\ and the hypothesis that geo! magnetic e}ects persist longer in TEC than in NmF1 due to the longer time constants for the dominant processes on the top side compared to the peak[ The number of averages that contribute to a bin "0> MLat[ and 0> MLT# is not uniform due to the nature of the satellite orbit[ The best coverage occurs at mid! latitudes in the southern hemisphere during low geo! magnetic activity conditions when more than 19 averages are recorded in each bin[ Generally the counts decrease slowly toward northern high latitudes[ The coverage is slightly better for the low activity bin compared to the high\ and somewhat better for the high compared to the medium geomagnetic activity bin[
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The most prominent feature of the TEC maps is the equatorial anomaly[ The feature becomes wider in MLat[ and more pronounced in amplitude as the activity level increases[ The two crests seem to move closer and the hint of a local minimum at the magnetic equator\ visible in the low and medium activity patterns\ seems to dis! appear in the high activity pattern[ The equatorward shift of the crests can produce apparent decreases in TEC at their quiet time location for individual storms as evident in the con~icting conclusions of geomagnetic dependence studies of the 0859s "Hunter\ 0869#\ when both positive and negative\ as well as non correlations between TEC and geomagnetic activity were reported by di}erent authors based on data from various sites[ This lack of agreement was probably due to the nature of the geo! magnetic indices "actual values and not integrals over time# used in the correlation studies[ Between plus and minus 19> MLat[\ TEC values reach a minimum "close to 4 TECU# between 9399 "59># and 9599 "89># MLT before rising sharply to a peak of over 29 TECU[ The rate of increase in electron content in the morning is remarkably constant and is a good indication of ionization production "Garriott and Smith\ 0854#[ The morning peak occurs earlier at lower magnetic latitudes and increases with geomagnetic activity[ This is con! sistent with the fountain e}ect as plasma in the fountain is _rst seen at the equator\ before it di}uses down to form the crests[ The standard deviation over the bins in the feature is of the order of 00 TECU or about 29) of the associated statistical average[ The in~uence of the aurora on TEC is visible in the southern hemisphere where the latitudinal coverage extends to −79> MLat[\ but is not noticeable in the northern hemisphere where coverage ends between 59 and 69> MLat[ The auroral contribution is larger for low activity conditions than for medium and high activity conditions[ This can be explained\ at least in part\ by the formation of the composition bulge "Fuller!Rowell et al[\ 0883\ 0885# which more than compensates for the additional ionization produced by particle precipitation\ through increased ionization loss[ The patchiness and the large standard deviation in the auroral zone "almost 49) of the mean TEC value# are due to the variability of the aurora\ the integral nature of our geomagnetic activity binning\ and the dynamics of the neutral composition bulge[ Figure 0 shows cuts through the TEC maps presented in Plate 0\ at four MLTs] 9299 "34>#\ 9899 "024>#\ 0499 "114># and 1099 "204>#[ The thin line represents the num! ber of points contributing to each 0> magnetic latitude bin[ The drop to zero TEC values at high!latitudes is an artifact of the lack of coverage and does not re~ect reality[ The increase in total electron content due to the aurora is noticeable at all MLTs in the Southern Hemisphere but is absent in the North due to lack of coverage[ The auroral contribution decreases\ generally\ with increasing
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activity levels[ This can be explained by the height integral nature of TEC combined with the long "25 h# integration of the geomagnetic activity index as the upwelling of air parcels rich in molecular species "neutral bulge# accelerate the loss of ionization faster then production is increased by additional precipitating particles[ Exceptions are vis! ible at 0499 "114># and 1099 "204># MLT\ when TEC values in the auroral zone are slightly larger for high activity[ One could argue that the neutral composition bulge does not penetrate as far equatorward on the day! side but we would like to note the rather low number of measurements that contribute at high latitudes[ An equatorward movement of the auroral trace is also visible but the low counts again warrant caution in interpret! ation[ In the southern hemisphere where coverage is better\ mid!latitude TEC values generally increase from low to medium activity[ This is expected as an e}ect of downwel! ling of air parcels rich in atomic oxygen\ during the initial phases of storms and for small storms[ For high activity conditions\ however\ the downwelling zone moves equ! atorward and the arrival of the neutral density bulge produces a decrease in TEC values below quiet time levels[ In the 9299 "34># MLT sector\ TEC values in the aur! oral zone are larger than at any other latitude for quiet conditions[ For the medium and high activity bins the depressed auroral peak is comparable to the increased low latitude values[ In the 9899 "024># MLT sector\ the single low!latitude peak situated at the magnetic equator dominates the pro_le[ The peak increases with magnetic activity from 14 TECU for the low to 17 TECU for the medium and 20 TECU for the high activity bin[ At mid! latitudes\ a depletion is visible in the southern hemisphere during high activity conditions and a small increase in TEC for medium conditions[ The development of the equatorial anomaly after 9899 "024># MLT produces dou! ble peaks in TEC for later MLTs as shown by the 0499 and 1099 MLT pro_les[ Plate 0 was constructed using all the data regardless of season[ The seasonal di}erence in mean values and the di}erent ionospheric e}ects produced at mid!latitudes by geomagnetic activity "increases in NmF1 in the winter and decreases in the summer hemisphere# result in the relatively small TEC di}erences between the di}erent activity levels and large standard deviations[ Note that the non!uniform coverage "in the sense that some MLT\ MLat[ area may be dominated by data taken during a particular season# may also introduce a slight seasonal bias[
3[ Seasonal characteristics We have binned the TOPEX TEC measurements as before\ but using season as an additional category[ Each
season contains three month of data per year of measure! ment\ with November\ December\ and January con! tributing to winter\ February\ March and April to spring and so on[ Season names refer to conditions in the north! ern hemisphere[ Due to poor coverage in the set of pat! terns so constructed\ we combined spring and autumn together in one global pattern "plots labeled Spring#\ and also winter in the northern hemisphere with winter in the southern hemisphere and summer in the southern hemisphere with summer in the northern hemisphere "plots labeled Winter# to produce the average seasonal patterns of Plate 1[ The coordinate system for the plots in Plate 1 is ident! ical to that used in Plate 0[ The logarithmic color scale ranges from 2 to approximately 24 TECU[ The TEC patterns are smoothed using a 6!point running average[ The additional smoothing compared to the all!data TEC maps was needed due to the lower counts resulting from the extra binning[ Note that the combination of slow orbit precession "1> per day#\ varying geomagnetic activity levels\ and changing seasons results in highly nonuniform coverage even for this large data set[ The magnetic midnight "9># MLT e}ects in Plate 1 are arti! _cial\ produced by the smoothing routine\ and should be ignored[ The low!latitude enhancement in TEC remains the most prominent feature for both equinox and solstice[ The feature is extended poleward and starts at earlier magnetic local times in the summer hemisphere "as illus! trated by the extent of green#\ but this is only an apparent e}ect due to the generally higher TEC levels in the summer[ The variation with MLT\ at low latitudes\ is similar to that in the all seasons averages\ with a mini! mum before sunrise\ a steep increase toward a peak in the afternoon\ and a slow decay through the night[ The maximum value over the feature decreases with decreas! ing activity levels for both seasons[ For the same activity level\ TEC values in the equa! torial anomaly are higher during equinox compared to solstice "Plate 1#[ This seasonal dependence of TEC has been described before "e[g[ Hunter\ 0869#[ One possible explanation for the seasonal variations is the di}erence in neutral winds^ the symmetric\ equatorward neutral winds at anomaly latitudes during equinox versus the prevalant summer to winter winds during solstice[ Equ! atorward winds lift plasma up the ~ux tubes into regions of decreased ionization loss^ in contrast poleward winds tend to transport plasma down into regions of increased loss[ Another contributing factor is the di}erence in neu! tral composition between equinox and solstice due to di}erent eddy mixing rates "Fuller!Rowell\ 0887#[ A third possibility is the stronger pre!reversal enhancement in vertical plasma drifts at equinox compared to solstice "Fejer\ 0886#[ The perturbation neutral winds produced by the additional energy input at high!latitudes during geo!
)
"a#
"b# Plate 0[ Global patterns of Total Electron Content "TECU# and associated variances "TECU1#[ In Magnetic Local Time "degrees# vs Magnetic Latitude "degrees# coordinates for solar minimum conditions^ "a# TEC for quiet\ "b# TEC for moderately disturbed\ "c# TEC for disturbed geomagnetic activity conditions\ "d# variance for quiet\ "e# variance for moderately disturbed\ and "f# variance for disturbed geomagnetic activity conditions[ The patterns were constructed using the TOPEX TEC data\ regardless of season[
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"a#
"b# Plate 1[ Seasonal global patterns of Total Electron Content "TECU# in Magnetic Local Time "degrees# vs Magnetic Latitude "degrees# coordinates for solar minimum conditions^ "a# Winter quiet\ "b# Spring quiet\ "c# Winter moderately disturbed\ "d# Spring moderately disturbed\ "e# Winter disturbed\ "f# Spring disturbed\ geomagnetic conditions[ Winter refers to the northern hemisphere[ TOPEX winter and summer data from both hemispheres were appropriately combined to construct the winter patterns[ Spring and fall data were averaged in the patterns labeled Spring[
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"d# Plate 1 "continued#
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Fig[ 0[ Cuts through the TEC patterns presented in Plate 0\ at four MLTs] 9299\ 9899\ 0499 and 1099[ The thin line represents the number of points contributing to each 0> by 0> magnetic local time\ magnetic latitude bin[
magnetic storms are similar to equinox winds and are expected to produce symmetric increases in low latitude TEC values[ However\ hemispheric asymmetries in the energy input and di}erent arrival times for traveling iono! spheric disturbances launched from the two hemispheres can produce asymmetric e}ects during particular storms[ The lowest TEC values at mid!latitudes\ for our aver! aged seasonal patterns\ occur in the winter hemisphere
during night and measure between 4 and 09 TECU[ The corresponding minimum values for the summer hemi! sphere are in the 09 to 04 TECU range[ This represents a 29Ð49) seasonal variation in minimum values[ The daytime peak TEC values at mid!latitudes show a marked seasonal dependence[ Winter shows generally lower TEC averages compared to summer at mid!latitudes[ Noon values in the winter hemisphere do not exhibit a seasonal
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Fig[ 0 "continued#
anomaly similar to the seasonal anomaly in NmF1 "larger values in winter# probably due to the averaging of the winter data from the two hemispheres\ and to the 2! month length of our seasonal bins[ The neutral com! position mechanism for the semiannual anomaly "Mill! ward et al[\ 0885# however\ should produce e}ects in TEC also[ Such e}ects in TEC have been reported before "e[g[ "Buonsanto et al[\ 0868#[ The proper seasonal patterns "not presented because of poor coverage# point to an
annual anomaly with noon peaks in winter in the north! ern hemisphere and a semiannual anomaly with noon peaks during equinoxes in the southern hemisphere*but poor coverage again\ warrants caution[ 4[ Geomagnetic activity effects Figure 1 illustrates the equinox and solstice disturbed to quiet TEC ratios for the high geomagnetic activity
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Fig[ 1[ High to Low TEC Ratio for the winter and spring conditions de_ned in Plate 1\ at four MLTs] 9299\ 9899\ 0499 and 1099[ The thin continuous line represents the number of points contributing to each 0> by 0> magnetic local time\ magnetic latitude bin for the disturbed case\ while the dash dot line represents the corresponding number of points for the quiet reference[
bins for the same MLTs as in Fig[ 0[ The ratios "thick line# are plotted on a constant scale "9Ð1[4#[ The thin lines represent the number\ divided by 09\ of the points contributing to the average^ high activity bin "solid# and
the quiet level used in the division "dash dot#[ The pat! terns were smoothed using a 4!point running average before the ratios were calculated[ The ratios were then similarly smoothed before plotting[ The additional
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Fig[ 1 "continued#
smoothing was necessary due to the nonuniform spatial coverage between di}erent activity levels[ Please note that the slow precession of the satellite orbit and the nature of our seasonal binning may not produce average storm responses in these plots[ It is poss! ible that most points in close bins are from a single storm
and therefore no average behavior can be claimed[ How! ever\ some general storm characteristics can be inferred[ The low latitude ratios show only increases in TEC during geomagnetic activity for both equinox and solstice[ This has been reported before "e[g[ Lanzerotti et al[\ 0864#\ and is not surprising[ The disturbance electric
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Fig[ 1 "continued#
_elds and neutral winds cause upward plasma drifts that are expected to produce increases in TEC[ Initial neutral composition changes are dominated by downwelling that also have positive e}ects for electron density[ The com! position bulge "Fuller!Rowell et al[\ 0883\ 0885# which arrives more than 01 h later is weak and does not com!
pensate for the e}ects of winds\ electric _elds\ and initial downwelling[ The rise in peak TEC values\ in the high activity bin compared to low\ increases with MLT from about 19) at 9299 MLT to close to 49) at 1099 MLT during both spring and winter conditions "Fig[ 1#[ Smaller increases with MLT are seen in the medium activity pat!
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Fig[ 1 "continued#
terns "not shown#[ The shape of the ratios and the pos! ition of the peak\ for low latitudes\ varies with season and MLT[ However the combination of uneven coverage and interhemispheric averaging would make any con! clusions about the shapes rather di.cult to defend[ The midlatitudes exhibit mostly negative TEC e}ects
but show pronounced structure in response to geo! magnetic activity[ This is to be expected in view of the TEC response to geomagnetic activity described by Men! dillo et al[\ "0861#[ According to their study of 17 storms at one midlatitude station "Hamilton\ MA\ 28>N\ 69>W#\ the average behavior of TEC in response to a geo!
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magnetic storm consists of an initial increase of over 04) lasting some 01 h\ followed by a 09Ð19) decrease that lasts up to 2 days[ While our geomagnetic binning "the 25 hour integration of the activity index# favors the decrease phase\ it does not eliminate the initial increase phase from the bin average[ The di}erences between MLTs in Fig[ 1 are most likely produced by the characteristics of individual storms "start time\ energy dissipation vs time and space\ and initial state of the system#[
5[ Conclusions The TOPEX:POSEIDON measurements of TEC over the past few years provide a tremendous and virtually untapped database of ionospheric measurements[ The measurements\ obtained entirely over the oceans\ o}er ionospheric data over previously poorly represented regions\ and are complementary to the historical ground! based TEC measurements[ The data can be used for both the study of TEC morphology and the validation of ionospheric models[ We have constructed global patterns of TEC vs mag! netic local time "MLT# and magnetic latitude for several sets of geomagnetic:seasonal conditions\ by binning the appropriate TOPEX TEC measurements[ The patterns correspond to low solar activity conditions "daily F 09[6 ¾ 019#[ The most prominent feature of the global TEC maps is the feature corresponding to the equatorial anomaly[ The feature becomes wider in MLat[ and more pro! nounced in amplitude as the activity level increases[ The two anomaly crests move closer to the magnetic equator with increasing geomagnetic activity[ The equatorward shift of the crests can produce apparent decreases in TEC at their quiet time location during individual storms[ For the same activity level\ TEC values in the equatorial anomaly are higher during equinox compared to solstice[ Mostly decreases in total electron content are observed at mid!latitudes when geomagnetic activity increases[ These changes can be explained by neutral composition and neutral dynamics changes occurring in response to increased high!latitude energy input[ The variance associated with the average global TEC maps illustrates the large variability that needs to be taken into account when comparing speci_c measure! ments with the averages presented in this paper[
Acknowledgements Support of M[ V[ Codrescu\ S[ E[ Palo\ and C[ Poppe for this work was by NSF grant ATM!8502752 "National Space Weather Program# to the University of Colorado[
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References Bilitza\ D[ "Ed[#\ International Reference Ionosphere 0889\ National Space Science Data Center\ NSSDC89Ð19\ Green! belt\ Maryland[ December 0889[ Bilitza\ D[\ Koblinsky\ C[\ Beckley\ B[\ Zia\ S[\ Williamson\ R[\ 0884[ Using IRI for the computation of ionospheric cor! rections for altimeter data analysis[ Adv[ Space Res[ 04\ 002Ð 008[ Bilitza\ D[\ Koblinsky\ C[\ Zia\ S[\ Williamson\ R[\ Beckley\ B[\ 0885[ The equator anomaly region as seen by the TOP! EX:POSEIDON satellite[ Adv[ Space Res[ 07\ "5#12Ð"5#21[ Buonsanto\ M[J[\ Mendillo\ M[\ Klobuchar\ J[A[\ 0868[ The ionosphere at L 3] average behavior and the response to geomagnetic storms[ Ann[ Geophys[ 24\ 04Ð15[ Codrescu\ M[V[\ Fuller!Rowell\ T[J[\ Kutiev\ I[S[\ 0886[ Mode! ling the F!layer during speci_c geomagnetic storms[ J[ Geophys[ Res[ 091\ 03\204Ð03\218[ Codrescu\ M[V[\ Fuller!Rowell\ T[J[\ Foster\ J[C[\ 0884[ On the importance of E!_eld variability for Joule heating in the high! latitude thermosphere[ Geophys[ Res[ Lett[ 11\ 1281Ð1285[ Evans\ D[S[\ Fuller!Rowell\ T[J[\ Maeda\ S[\ Foster\ J[\ 0877[ Speci_cation of the heat input to the thermosphere from mag! netospheric processes using TIROS:NOAA auroral particle observations[ Adv[ Astronaut[ Sci[ 54\ 0538Ð0556[ Fejer\ B[G[\ 0886[ The electrodynamics of the low latitude iono! sphere] recent results and future challenges[ J[ Atmos[ Terr[ Phys[ 48\ 0354Ð0371[ Forbes\ J[M[\ Revelle\ D[\ Zhang\ X[\ Markin\ R[E[\ 0885a[ Longitude structure of the ionosphere F!region from TOP! EX:POSEIDON and ground based data during January 19Ð 29\ 0882\ including the quasi!two!day oscillation[ J[ Geophys[ Res[ in review[ Foster\ J[C[\ Holt\ J[M[\ Musgrove\ R[G[\ Evans\ D[S[\ 0875[ Ionospheric convection associated with discrete levels of par! ticle precipitation[ Geophys[ Res[ Lett[ 02\ 545Ð548[ Fu\ L[L[\ Christensen\ E[J[\ Yamarone Jr[\ C[A[\ Lefebvre\ M[\ Menard\ Y[\ Dorrer\ M[\ Escudier\ P[\ 0883[ TOP! EX:POSEIDON mission overview[ J[ Geophy[ Res[ 88\ 13\258Ð13\270[ Fuller!Rowell\ T[J[\ Evans\ D[S[\ 0876[ Height integrated Ped! ersen and Hall conductivity patterns inferred from the TIROS!NOAA satellite data[ J[ Geophys[ Res[ 81\ 6595Ð6507[ Fuller!Rowell\ T[J[\ Codrescu\ M[V[\ Rishbeth\ H[\ Mo}ett\ R[J[\ Quegan\ S[\ 0885[ Response of the thermosphere and ionosphere to geomagnetic storms[ J[ Geophys[ Res[ 090\ 1232Ð1242[ Fuller!Rowell\ T[J[\ Codrescu\ M[V[\ Mo}ett\ R[J[\ Quegan\ S[\ 0883[ Response of the thermosphere and ionosphere to geomagnetic storms[ J[ Geophys[ Res[ 88\ 2782Ð2803[ Fuller!Rowell\ T[J[\ 0887[ The Semiannual variation in neutral composition <<<<< J[ Geophys[ Res[ 092\ ZZZZÐXXXX[ Garriott\ O[K[\ III Smith\ F[L[\ 0854[ The rate of production of electrons in the ionosphere[ Planet Space Sci[ 02\ 728Ð738[ Hunter\ A[N[\ 0869[ Total electron content studies in the equa! torial regions[ Radio Sci[ 4\ 758Ð779[ Lanzerotti et al[\ 0864[ Mendillo\ M[\ Papagiannis\ M[D[\ Klobuchar\ J[\ 0861[ Average behavior of the midlatitude F!region parameters NT\ Nmax and t during geomagnetic storms[ J[ Geophys[ Res[ 66\ 3780Ð3784[ Millward\ G[H[\ Rishbeth\ H[\ Fuller!Rowell\ T[J[\ Aylward\
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A[D[\ Quegan\ S[\ Mo}ett\ R[J[\ 0885[ Ionospheric F1 layer seasonal and semiannual variations[ J[ Geophys[ Res[ 090\ 4038Ð4045[ Monaldo\ F[\ 0882[ TOPEX ionospheric height correction pre! cision estimated from prelaunch test results\ IEEE Trans[ Geosci[ Remote Sens[ 20\ 260Ð264[ Richmond\ A[D[\ 0884[ Ionospheric electrodynamics using mag! netic apex coordinates[ J[ Geomagn[ Geoelectr[ 36\ 080Ð101[
Robinson\ T[R[\ Beard\ R[\ 0884[ A comparison between elec! tron content deduced from the IRI and that measured by the TOPEX dual frequency altimeter[ Adv[ Space Res[ 05\ 044Ð 047[ Rodger\ A[S[\ Wrenn\ G[L[\ Rishbeth\ H[\ 0878[ Geomagnetic storms in the Antarctic F!region\ II\ Physical interpretation[ J[ Atmos[ Terr[ Phys[ 40\ 740Ð755[
Journal of Atmospheric and Solar!Terrestrial Physics 50 "0888# 170Ð187
TEC climatology derived from TOPEX:POSEIDON measurements M[V[ Codrescua\ \ S[E[ Palob\ X[ Zhangb\ T[J[ Fuller!Rowella\ C[ Poppea a
Cooperative Institute for Research in Environmental Sciences\ University of Colorado\ and NOAA\ Space Environment Center\ Boulder\ Colorado\ U[S[A[ b Department of Aerospace En`ineerin`\ University of Colorado\ Boulder\ Colorado\ U[S[A[ Received 2 March 0887^ accepted 11 June 0887
Abstract We have used the TOPEX data sets available from JPL to create a customized data base of the TOPEX TEC measurements that contain data from 0881 through part of 0885[ The data base includes time\ geographic and geo! magnetic coordinates of the measurement\ geomagnetic indices "Kp\ previous Kp\ Hemispheric Power\ and integral of hemispheric power over the previous 25 h#\ solar index "F 09[6#\ and International Reference Ionosphere "IRI# results corresponding to the TOPEX measurements[ In this paper we present global maps of TEC for low solar activity conditions "F 09[6 ¾ 019# for quiet "integral of hemispheric power less than 799 GWh\ roughly corresponding to Kp 1#\ moderately disturbed "integral of hemispheric power greater than 799 GWh but less than 0199 GWh\ roughly corresponding to Kp 2#\ and disturbed "integral of hemispheric power greater than 0199 GWh\ roughly corresponding to Kp 3# geomagnetic conditions\ derived by binning all appropriate TOPEX TEC data from 0881 to 0885[ The analysis is performed in a Magnetic!Local!Time\ Magnetic!Latitude coordinate system[ The most prominent feature of the global TEC maps is the feature corresponding to the equatorial anomaly[ The feature becomes wider in magnetic latitude and more pronounced in amplitude as the activity level increases[ The equatorward shift of the crests\ with increased magnetic activity\ can produce apparent decreases in TEC at their quiet time location for individual storms as evident in the con~icting conclusions of TEC geomagnetic dependence studies of the 0859s[ For the same activity level\ TEC values in the equatorial anomaly are higher during equinox compared to solstice[ Þ 0888 Elsevier Science Ltd[ All rights reserved[
0[ Introduction In an e}ort to provide accurate\ high quality measure! ments of the global sea surface height\ a joint e}ort was undertaken by the National Aeronautics and Space Administration "NASA# in the United States and the Centre National d|Etudies Spatiales "CNES# in France[ Through this joint e}ort the TOPEX:POSEIDON sat! ellite mission "Fu et al[\ 0883# was developed and then deployed in August of 0881[ The TOPEX:POSEIDON platform contains six scien! ti_c instruments including a dual frequency altimeter\ a
Corresponding author[ Tel[] 990 292 386 5652^ fax] 990 292 386 2534[ E!mail address] codrescuÝsec[noaa[gov "M[V[ Codrescu#
single frequency altimeter\ a three channel radiometer\ and three tracking systems designed to provide precision orbit information[ To provide the needed accuracy in the sea surface height measurements "approx[ 03 cm# both the position of the satellite and any propagation path delays between the sea surface and the satellite must be known accurately[ The satellite position can be deter! mined to within 09 cm using the three independent track! ing systems located on the TOPEX:POSEIDON plat! form[ The path delays between the sea surface and the sat! ellite can exceed 14 cm and are due to changes in the atmospheric index of refraction[ These changes in the atmospheric index of refraction are primarily due to free electrons in the ionosphere but also arise as a result of tropospheric and stratospheric water vapor[ Because the ionosphere is a dispersive medium\ the path delay can be
0253Ð5715:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved PII] S 0 2 5 3 Ð 5 7 1 5 " 8 7 # 9 9 0 2 1 Ð 0
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estimated to _rst order by transmitting at two frequenc! ies[ With the path delay known\ it is a simple linear transformation "Monaldo\ 0882^ Robinson and Beard\ 0884# to obtain the total electron content "TEC# in a column extending from the satellite\ orbiting at 0225 km\ to the subsatellite re~ection point on the surface of the ocean[ Through the use of a dual frequency altimeter\ operating at 4[2 GHz "C!band# and 02[5 GHz "Ku!band#\ the TOPEX:POSEIDON spacecraft can reduce the path delay errors to less than 2 cm[ The measurements of TEC are a byproduct of the sea surface height measurements\ and are computed for each sea surface measurement[ Operating since August of 0881\ there are currently more than 4 years of TEC data available in the public domain through the NASA EOS physical oceanography distributed active archive center "PO!DAAC# located at the Jet Propulsion Laboratory "JPL#[ Additionally\ the TOPEX:POSEIDON mission has recently been extended to six years while the spa! cecraft was designed with enough fuel to operate for 09 years[ The TOPEX:POSEIDON measurements of TEC over the past few years provide a tremendous and virtually untapped database of ionospheric measurements[ Most empirical ionospheric models and ionospheric data bases have been constructed using ground based measurements and therefore\ large regions over the oceans are not well represented[ The TEC measurements from TOPEX\ obtained entirely over the oceans\ o}er ionospheric data over previously poorly represented regions\ and are complementary to the historical ground!based TEC measurements[ The goal of this study is to utilize the TOP! EX:POSEIDON TEC data to analyze the average spatial structure of the ionosphere[ In particular\ we are inter! ested in global TEC climatology and in TEC mani! festations of the response of the middle and low latitude ionosphere to geomagnetic disturbances[ We will analyze speci_c events and their departure from the average behavior presented here in a future paper[ We also plan to repeat the climatological analysis to include more TOPEX data as they become available[ For this study we have created a customized data base of the TOPEX TEC measurements that contains data from launch through 0885[ The data base includes time\ geographic and geomagnetic coordinates of the measure! ment\ geomagnetic indices "Kp\ previous Kp\ Hemispheric Power\ and integral of hemispheric power over the pre! vious 25 h#\ a solar activity index "F 09[6#\ and empirical model "IRI# results corresponding to the TOPEX measurements[ Results from our data analysis\ as well as the data base itself\ are available from the authors\ on request[ The data provide an excellent database for model validation purposes[ This is especially important for model cali! bration over the large ocean areas where ionosonde\ GPS
TEC\ or other ground based data are not available[ In fact\ of the limited analysis which has been performed on the TOPEX:POSEIDON TEC data\ a majority of these "Robinson and Beard\ 0884^ Bilitza et al[\ 0884\ 0885# have been comparisons of the TEC data with the Inter! national Reference Ionosphere "IRI# "Bilitza\ 0889#[
1[ Data analysis The TOPEX:POSEIDON satellite orbits the Earth at an altitude of 0225 km with an inclination of 55>[ With this orbital geometry the TOPEX:POSEIDON altimeter provides data between ¦55> and −55> geographic lati! tude over all of the Earth|s oceans[ The precession of this orbit is 1> per day\ which corresponds to 7 min of local time[ Utilizing data from both the ascending and descending orbital nodes\ it takes nearly 89 days to pro! vide full local time coverage[ The original TEC data were obtained from the NASA EOS physical oceanography distributed active archive center "PO!DAAC# located at the Jet Propulsion Lab! oratory "JPL#[ Measurements located in 1×3> bins\ in geographic latitude and longitude respectively\ were _rst averaged along each orbit[ The number of points that contribute to an average varies from 0 to more than 39\ depending on the way any particular orbit crosses the bin boundaries[ The averages were tagged by measurement time and by the latitude and longitude of the center of the bin where the measurements were taken[ The geo! magnetic latitude and longitude associated with each bin center "record# along each orbit were computed using quasi dipole coordinates Richmond "0884#[ The number of points that contribute to the average\ the standard deviation\ and the median value associated with each bin average are also written to each record in the data base[ Geomagnetic indices "2!h Kp\ previous 2 h Kp value\ Hemispheric Power\ and integral of hemispheric power over the previous 25 h#\ solar index "F 09[6#\ and empirical model "IRI# results corresponding to the TOPEX measurements were added to each record to create a customized data base[ The Hemispheric Power is an esti! mate of the energy deposition in a single hemisphere by auroral precipitating particles "Fuller!Rowell and Evans\ 0876#[ Particle precipitation measurements from the NOAA:TIROS satellite series over individual half orbits are compared with statistical patterns of particle pre! cipitation[ The matching pattern\ appropriately scaled for any remaining di}erences\ is then considered to rep! resent the hemispheric energy input carried into the high! latitudes by precipitating particles\ for that particular time[ The index o}ers a good representation of the mag! netospheric forcing acting on the thermosphere: ionosphere system "see Foster et al[\ 0875#[ The integral of hemispheric power was added in an e}ort to account for the long!lived thermospheric features that produce
M[V[ Codrescu et al[:Journal of Atmospheric and Solar Terrestrial Physics 50 "0888# 170Ð187
signi_cant ionospheric storm e}ects "Fuller!Rowell et al[\ 0883\ 0885#[ The data base contains over 449\999 records per full year\ and covers 0882Ð84\ and part of 0881 and 0885[ Most of the measurements were taken during the late declining phase of solar cycle 11 or during solar minimum between cycles 11 and 12 and\ therefore\ the data base is representative of low solar activity conditions "low F 09[6#[ In the present analysis we have used only the measurements that correspond to a daily value of F 09[6 ¾ 019[ Using the data base\ we have constructed global pat! terns of TEC vs magnetic local time "MLT# and magnetic latitude for several sets of geomagnetic:seasonal conditions\ by binning the appropriate measurements[
2[ Global TEC maps Plate 0 shows the global\ averaged TEC maps for low\ medium\ and high geomagnetic activity bins\ and the associated variance "s1#[ The coordinate system is mag! netic local time "259> correspond to 13 h MLT# and Apex latitude[ The color scale ranges from 2 to 24 TECU "0 TECU 0905 el:m1# for the TEC averages\ and from 2 to 025 TECU1 for the variance s1[ The TEC maps and the variance patterns have been smoothed using a 4!point running average[ The activity bins are de_ned based on the integral of the power carried into the atmosphere by precipitating particles during the 25 h preceding the measurement[ The limit values for the power integral "799 and 0199 GWh# were chosen so as to distribute the data approximately equally between the three geo! magnetic activity bins[ The bins correspond to 25!h per! iods of quiet "steady Kp of 1#\ moderately disturbed "ste! ady Kp of 2#\ and disturbed "steady Kp of 3# conditions[ A truly great storm lasting close to 13 h would result in a value for the integral power in excess of 1499 GWh[ Obviously\ the bin labeled high in our analysis is not representative of such storms[ This choice of geomagnetic activity dependence is motivated by our interest in clima! tology\ our desire to emphasize global\ long lasting e}ects of geomagnetic activity\ and the hypothesis that geo! magnetic e}ects persist longer in TEC than in NmF1 due to the longer time constants for the dominant processes on the top side compared to the peak[ The number of averages that contribute to a bin "0> MLat[ and 0> MLT# is not uniform due to the nature of the satellite orbit[ The best coverage occurs at mid! latitudes in the southern hemisphere during low geo! magnetic activity conditions when more than 19 averages are recorded in each bin[ Generally the counts decrease slowly toward northern high latitudes[ The coverage is slightly better for the low activity bin compared to the high\ and somewhat better for the high compared to the medium geomagnetic activity bin[
172
The most prominent feature of the TEC maps is the equatorial anomaly[ The feature becomes wider in MLat[ and more pronounced in amplitude as the activity level increases[ The two crests seem to move closer and the hint of a local minimum at the magnetic equator\ visible in the low and medium activity patterns\ seems to dis! appear in the high activity pattern[ The equatorward shift of the crests can produce apparent decreases in TEC at their quiet time location for individual storms as evident in the con~icting conclusions of geomagnetic dependence studies of the 0859s "Hunter\ 0869#\ when both positive and negative\ as well as non correlations between TEC and geomagnetic activity were reported by di}erent authors based on data from various sites[ This lack of agreement was probably due to the nature of the geo! magnetic indices "actual values and not integrals over time# used in the correlation studies[ Between plus and minus 19> MLat[\ TEC values reach a minimum "close to 4 TECU# between 9399 "59># and 9599 "89># MLT before rising sharply to a peak of over 29 TECU[ The rate of increase in electron content in the morning is remarkably constant and is a good indication of ionization production "Garriott and Smith\ 0854#[ The morning peak occurs earlier at lower magnetic latitudes and increases with geomagnetic activity[ This is con! sistent with the fountain e}ect as plasma in the fountain is _rst seen at the equator\ before it di}uses down to form the crests[ The standard deviation over the bins in the feature is of the order of 00 TECU or about 29) of the associated statistical average[ The in~uence of the aurora on TEC is visible in the southern hemisphere where the latitudinal coverage extends to −79> MLat[\ but is not noticeable in the northern hemisphere where coverage ends between 59 and 69> MLat[ The auroral contribution is larger for low activity conditions than for medium and high activity conditions[ This can be explained\ at least in part\ by the formation of the composition bulge "Fuller!Rowell et al[\ 0883\ 0885# which more than compensates for the additional ionization produced by particle precipitation\ through increased ionization loss[ The patchiness and the large standard deviation in the auroral zone "almost 49) of the mean TEC value# are due to the variability of the aurora\ the integral nature of our geomagnetic activity binning\ and the dynamics of the neutral composition bulge[ Figure 0 shows cuts through the TEC maps presented in Plate 0\ at four MLTs] 9299 "34>#\ 9899 "024>#\ 0499 "114># and 1099 "204>#[ The thin line represents the num! ber of points contributing to each 0> magnetic latitude bin[ The drop to zero TEC values at high!latitudes is an artifact of the lack of coverage and does not re~ect reality[ The increase in total electron content due to the aurora is noticeable at all MLTs in the Southern Hemisphere but is absent in the North due to lack of coverage[ The auroral contribution decreases\ generally\ with increasing
173
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activity levels[ This can be explained by the height integral nature of TEC combined with the long "25 h# integration of the geomagnetic activity index as the upwelling of air parcels rich in molecular species "neutral bulge# accelerate the loss of ionization faster then production is increased by additional precipitating particles[ Exceptions are vis! ible at 0499 "114># and 1099 "204># MLT\ when TEC values in the auroral zone are slightly larger for high activity[ One could argue that the neutral composition bulge does not penetrate as far equatorward on the day! side but we would like to note the rather low number of measurements that contribute at high latitudes[ An equatorward movement of the auroral trace is also visible but the low counts again warrant caution in interpret! ation[ In the southern hemisphere where coverage is better\ mid!latitude TEC values generally increase from low to medium activity[ This is expected as an e}ect of downwel! ling of air parcels rich in atomic oxygen\ during the initial phases of storms and for small storms[ For high activity conditions\ however\ the downwelling zone moves equ! atorward and the arrival of the neutral density bulge produces a decrease in TEC values below quiet time levels[ In the 9299 "34># MLT sector\ TEC values in the aur! oral zone are larger than at any other latitude for quiet conditions[ For the medium and high activity bins the depressed auroral peak is comparable to the increased low latitude values[ In the 9899 "024># MLT sector\ the single low!latitude peak situated at the magnetic equator dominates the pro_le[ The peak increases with magnetic activity from 14 TECU for the low to 17 TECU for the medium and 20 TECU for the high activity bin[ At mid! latitudes\ a depletion is visible in the southern hemisphere during high activity conditions and a small increase in TEC for medium conditions[ The development of the equatorial anomaly after 9899 "024># MLT produces dou! ble peaks in TEC for later MLTs as shown by the 0499 and 1099 MLT pro_les[ Plate 0 was constructed using all the data regardless of season[ The seasonal di}erence in mean values and the di}erent ionospheric e}ects produced at mid!latitudes by geomagnetic activity "increases in NmF1 in the winter and decreases in the summer hemisphere# result in the relatively small TEC di}erences between the di}erent activity levels and large standard deviations[ Note that the non!uniform coverage "in the sense that some MLT\ MLat[ area may be dominated by data taken during a particular season# may also introduce a slight seasonal bias[
3[ Seasonal characteristics We have binned the TOPEX TEC measurements as before\ but using season as an additional category[ Each
season contains three month of data per year of measure! ment\ with November\ December\ and January con! tributing to winter\ February\ March and April to spring and so on[ Season names refer to conditions in the north! ern hemisphere[ Due to poor coverage in the set of pat! terns so constructed\ we combined spring and autumn together in one global pattern "plots labeled Spring#\ and also winter in the northern hemisphere with winter in the southern hemisphere and summer in the southern hemisphere with summer in the northern hemisphere "plots labeled Winter# to produce the average seasonal patterns of Plate 1[ The coordinate system for the plots in Plate 1 is ident! ical to that used in Plate 0[ The logarithmic color scale ranges from 2 to approximately 24 TECU[ The TEC patterns are smoothed using a 6!point running average[ The additional smoothing compared to the all!data TEC maps was needed due to the lower counts resulting from the extra binning[ Note that the combination of slow orbit precession "1> per day#\ varying geomagnetic activity levels\ and changing seasons results in highly nonuniform coverage even for this large data set[ The magnetic midnight "9># MLT e}ects in Plate 1 are arti! _cial\ produced by the smoothing routine\ and should be ignored[ The low!latitude enhancement in TEC remains the most prominent feature for both equinox and solstice[ The feature is extended poleward and starts at earlier magnetic local times in the summer hemisphere "as illus! trated by the extent of green#\ but this is only an apparent e}ect due to the generally higher TEC levels in the summer[ The variation with MLT\ at low latitudes\ is similar to that in the all seasons averages\ with a mini! mum before sunrise\ a steep increase toward a peak in the afternoon\ and a slow decay through the night[ The maximum value over the feature decreases with decreas! ing activity levels for both seasons[ For the same activity level\ TEC values in the equa! torial anomaly are higher during equinox compared to solstice "Plate 1#[ This seasonal dependence of TEC has been described before "e[g[ Hunter\ 0869#[ One possible explanation for the seasonal variations is the di}erence in neutral winds^ the symmetric\ equatorward neutral winds at anomaly latitudes during equinox versus the prevalant summer to winter winds during solstice[ Equ! atorward winds lift plasma up the ~ux tubes into regions of decreased ionization loss^ in contrast poleward winds tend to transport plasma down into regions of increased loss[ Another contributing factor is the di}erence in neu! tral composition between equinox and solstice due to di}erent eddy mixing rates "Fuller!Rowell\ 0887#[ A third possibility is the stronger pre!reversal enhancement in vertical plasma drifts at equinox compared to solstice "Fejer\ 0886#[ The perturbation neutral winds produced by the additional energy input at high!latitudes during geo!
)
"a#
"b# Plate 0[ Global patterns of Total Electron Content "TECU# and associated variances "TECU1#[ In Magnetic Local Time "degrees# vs Magnetic Latitude "degrees# coordinates for solar minimum conditions^ "a# TEC for quiet\ "b# TEC for moderately disturbed\ "c# TEC for disturbed geomagnetic activity conditions\ "d# variance for quiet\ "e# variance for moderately disturbed\ and "f# variance for disturbed geomagnetic activity conditions[ The patterns were constructed using the TOPEX TEC data\ regardless of season[
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"d# Plate 0 "continued#
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"f# Plate 0 "continued#
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"a#
"b# Plate 1[ Seasonal global patterns of Total Electron Content "TECU# in Magnetic Local Time "degrees# vs Magnetic Latitude "degrees# coordinates for solar minimum conditions^ "a# Winter quiet\ "b# Spring quiet\ "c# Winter moderately disturbed\ "d# Spring moderately disturbed\ "e# Winter disturbed\ "f# Spring disturbed\ geomagnetic conditions[ Winter refers to the northern hemisphere[ TOPEX winter and summer data from both hemispheres were appropriately combined to construct the winter patterns[ Spring and fall data were averaged in the patterns labeled Spring[
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"d# Plate 1 "continued#
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"f# Plate 1 "continued#
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Fig[ 0[ Cuts through the TEC patterns presented in Plate 0\ at four MLTs] 9299\ 9899\ 0499 and 1099[ The thin line represents the number of points contributing to each 0> by 0> magnetic local time\ magnetic latitude bin[
magnetic storms are similar to equinox winds and are expected to produce symmetric increases in low latitude TEC values[ However\ hemispheric asymmetries in the energy input and di}erent arrival times for traveling iono! spheric disturbances launched from the two hemispheres can produce asymmetric e}ects during particular storms[ The lowest TEC values at mid!latitudes\ for our aver! aged seasonal patterns\ occur in the winter hemisphere
during night and measure between 4 and 09 TECU[ The corresponding minimum values for the summer hemi! sphere are in the 09 to 04 TECU range[ This represents a 29Ð49) seasonal variation in minimum values[ The daytime peak TEC values at mid!latitudes show a marked seasonal dependence[ Winter shows generally lower TEC averages compared to summer at mid!latitudes[ Noon values in the winter hemisphere do not exhibit a seasonal
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Fig[ 0 "continued#
anomaly similar to the seasonal anomaly in NmF1 "larger values in winter# probably due to the averaging of the winter data from the two hemispheres\ and to the 2! month length of our seasonal bins[ The neutral com! position mechanism for the semiannual anomaly "Mill! ward et al[\ 0885# however\ should produce e}ects in TEC also[ Such e}ects in TEC have been reported before "e[g[ "Buonsanto et al[\ 0868#[ The proper seasonal patterns "not presented because of poor coverage# point to an
annual anomaly with noon peaks in winter in the north! ern hemisphere and a semiannual anomaly with noon peaks during equinoxes in the southern hemisphere*but poor coverage again\ warrants caution[ 4[ Geomagnetic activity effects Figure 1 illustrates the equinox and solstice disturbed to quiet TEC ratios for the high geomagnetic activity
M[V[ Codrescu et al[:Journal of Atmospheric and Solar Terrestrial Physics 50 "0888# 170Ð187
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Fig[ 1[ High to Low TEC Ratio for the winter and spring conditions de_ned in Plate 1\ at four MLTs] 9299\ 9899\ 0499 and 1099[ The thin continuous line represents the number of points contributing to each 0> by 0> magnetic local time\ magnetic latitude bin for the disturbed case\ while the dash dot line represents the corresponding number of points for the quiet reference[
bins for the same MLTs as in Fig[ 0[ The ratios "thick line# are plotted on a constant scale "9Ð1[4#[ The thin lines represent the number\ divided by 09\ of the points contributing to the average^ high activity bin "solid# and
the quiet level used in the division "dash dot#[ The pat! terns were smoothed using a 4!point running average before the ratios were calculated[ The ratios were then similarly smoothed before plotting[ The additional
183
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Fig[ 1 "continued#
smoothing was necessary due to the nonuniform spatial coverage between di}erent activity levels[ Please note that the slow precession of the satellite orbit and the nature of our seasonal binning may not produce average storm responses in these plots[ It is poss! ible that most points in close bins are from a single storm
and therefore no average behavior can be claimed[ How! ever\ some general storm characteristics can be inferred[ The low latitude ratios show only increases in TEC during geomagnetic activity for both equinox and solstice[ This has been reported before "e[g[ Lanzerotti et al[\ 0864#\ and is not surprising[ The disturbance electric
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Fig[ 1 "continued#
_elds and neutral winds cause upward plasma drifts that are expected to produce increases in TEC[ Initial neutral composition changes are dominated by downwelling that also have positive e}ects for electron density[ The com! position bulge "Fuller!Rowell et al[\ 0883\ 0885# which arrives more than 01 h later is weak and does not com!
pensate for the e}ects of winds\ electric _elds\ and initial downwelling[ The rise in peak TEC values\ in the high activity bin compared to low\ increases with MLT from about 19) at 9299 MLT to close to 49) at 1099 MLT during both spring and winter conditions "Fig[ 1#[ Smaller increases with MLT are seen in the medium activity pat!
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Fig[ 1 "continued#
terns "not shown#[ The shape of the ratios and the pos! ition of the peak\ for low latitudes\ varies with season and MLT[ However the combination of uneven coverage and interhemispheric averaging would make any con! clusions about the shapes rather di.cult to defend[ The midlatitudes exhibit mostly negative TEC e}ects
but show pronounced structure in response to geo! magnetic activity[ This is to be expected in view of the TEC response to geomagnetic activity described by Men! dillo et al[\ "0861#[ According to their study of 17 storms at one midlatitude station "Hamilton\ MA\ 28>N\ 69>W#\ the average behavior of TEC in response to a geo!
M[V[ Codrescu et al[:Journal of Atmospheric and Solar Terrestrial Physics 50 "0888# 170Ð187
magnetic storm consists of an initial increase of over 04) lasting some 01 h\ followed by a 09Ð19) decrease that lasts up to 2 days[ While our geomagnetic binning "the 25 hour integration of the activity index# favors the decrease phase\ it does not eliminate the initial increase phase from the bin average[ The di}erences between MLTs in Fig[ 1 are most likely produced by the characteristics of individual storms "start time\ energy dissipation vs time and space\ and initial state of the system#[
5[ Conclusions The TOPEX:POSEIDON measurements of TEC over the past few years provide a tremendous and virtually untapped database of ionospheric measurements[ The measurements\ obtained entirely over the oceans\ o}er ionospheric data over previously poorly represented regions\ and are complementary to the historical ground! based TEC measurements[ The data can be used for both the study of TEC morphology and the validation of ionospheric models[ We have constructed global patterns of TEC vs mag! netic local time "MLT# and magnetic latitude for several sets of geomagnetic:seasonal conditions\ by binning the appropriate TOPEX TEC measurements[ The patterns correspond to low solar activity conditions "daily F 09[6 ¾ 019#[ The most prominent feature of the global TEC maps is the feature corresponding to the equatorial anomaly[ The feature becomes wider in MLat[ and more pro! nounced in amplitude as the activity level increases[ The two anomaly crests move closer to the magnetic equator with increasing geomagnetic activity[ The equatorward shift of the crests can produce apparent decreases in TEC at their quiet time location during individual storms[ For the same activity level\ TEC values in the equatorial anomaly are higher during equinox compared to solstice[ Mostly decreases in total electron content are observed at mid!latitudes when geomagnetic activity increases[ These changes can be explained by neutral composition and neutral dynamics changes occurring in response to increased high!latitude energy input[ The variance associated with the average global TEC maps illustrates the large variability that needs to be taken into account when comparing speci_c measure! ments with the averages presented in this paper[
Acknowledgements Support of M[ V[ Codrescu\ S[ E[ Palo\ and C[ Poppe for this work was by NSF grant ATM!8502752 "National Space Weather Program# to the University of Colorado[
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