Low-Altitude Trapped Protons at the Geomagnetic Equator

Transcription

1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 94, NO. AI, PAGES , JANUARY 1, 1989 Lw-Altitude Trapped Prtns at the Gemagnetic Equatr T. G. GUZIK, M. A. MIAH, J. W. MITCHELL, AND J.P. WEFEL Department f Physics and Astrnmy, Luisiana State University, Batn Ruble Gemagnetically trapped prtns in the 0.6- t 9-MeV energy range were measured at latitudes near the gemagnetic equatr by the Phenix 1 experiment n bard the S81-1 missin frm May t Nvember The prtn shw a distributin in latitude alng the line f minimum magnetic field strength with a full width at half maximum f --, 10 ø but with n appreciable lngitudinal variatin. Between 170 and 290 km the peak prtn flux shws a fifth-pwer altitude dependence, in cntrast t previus measurements at higher altitudes, pssibly demnstrating surce attenuatin. The efficiency f the telescpe is calculated as a functin f particle pitch angle and used t investigate the time dependence ( ) f the intensity. INTRODUCTION The existence f a lw-altitude trapped prtn ppulatin in the vicinity f the gemagnetic equatr was established by Ht;estadt et al. [1972] and Mritz [1972], wh reprted bservatins f prtns with energies between 0.25 and 1.65 MeV at altitudes between 400 and 1000 km frm tw different instruments n the Azur satellite. In the 400- t 470-km altitude range these bservatins were extended t energies between 10 kev and 0.25 MeV by Mizera and Blake [1973] frm measurements n the OVl-17 satellite. At these altitudes the prtn lifetimes are f the rder f a few bunce perids and are shrt in cmparisn t the drift perid. Thus the particles must be regarded as being quasi-trapped since they are unable t cmplete a full lngitudinal drift. The intense, cntinuus surce necessary t sustain this ppulatin has been identified as arising frm the lss f ring current prtns by charge exchange interactins with exspheric hydrgen [Mritz, 1972]. The resulting neutrals leave the surce regin in a directin which depends nly n the velcity vectr f the prtn at the time f neutralizatin and is unaffected by the Earth's magnetic field. A fractin f the neutrals are directed tward the Earth and, at lw altitude, are stripped by cllisins with the atmsphere. The resulting energetic prtns are trapped, temprarily, in the Earth's magnetic field, frming the bserved ppulatin. The gemetry f this surce reflects the prtn distributin in the uter radiatin belt and results in a lw-altitude prtn ppulatin with an equatrial pitch angle distributin which is strngly peaked tward 90 ø and, cnsequently, has a narrw latitudinal distributin. This is cnsistent with previus bservatins as well as with thse reprted here. In this surce mdel, the prtn flux at a particular altitude is directly prprtinal t the neutral hydrgen flux at that altitude. Mritz [1972] estimates that the energetic hydrgen flux is nt appreciably attenuated dwn t 400 km, resulting in a prtn flux which is independent f altitude, cnsistent with the data available within the range cvered by the Azur measurements. In the present paper we reprt measurements f the quasitrapped prtn ppulatin at,- 1 MeV made in 1982 with a slid-state detectr telescpe n the plar-rbiting S81-1 satel- Cpyright 1989 by the American Gephysical Unin. Paper number 88JA /89/88JA lite. In the 170- t 290-km altitude range the measurements shw a marked altitude dependence. SATELLITE AND INSTRUMENTATION Our investigatin f the lw-altitude equatrial regin was perfrmed with the Phenix 1 instrument n the S81-1 missin frm May thrugh Nvember This three-axisstabilized satellite was in lw-altitude ( km) nearly circular plar rbit, inclinatin 85.5 ø, with a Sun-synchrnized rbital plane frm 1030 t 2230 lcal time. The Phenix! experiment included tw telescpes: the mnitr telescpe and the main telescpe. The main telescpe prvided infrmatin fi' the determinatin f the istpic cmpsitin f slar energetic particles and is nt used fr the analysis presented here. The mnitr telescpe, shwn schematically in Figure 1, was a passively shielded unit with a single, 40-pm-thick, fully depleted silicn detectr. It had an pening angle f 75 and a gemetrical factr f 0.5 cm ' sr and was munted n the spacecraft with the telescpe axis tilted at an angle f 2.35 ø t the lcal vertical in a plane perpendicular t the rbital plane. At the diple equatr, where the rbital inclinatin and the tilt f the telescpe axis have little effect, the mnitr received particles f pitch angle.,. 55c_129. The mnitr telescpe returned three cunting rates, ML, MM, and MH, all with an accumulatin time f s, crrespnding t three different threshld settings fr the pulse height frm the detectr. Rate ML had a threshld value f 0.36 MeV and culd be triggered by prtns in the energy range MeV, by alpha particles in the energy range, MeV/nuclen, and by Z > 2 particles f energy > 0.7 MeV/nuclen ( 2C). The threshld setting fr MM was 2.8 MeV, crrespnding t alpha particles f MeV/nu- clen and t Z > 2 particles MeV/nuclen ( 2C). Rate MH had a threshld f 10.5 MeV and culd be triggered nly by Z > 2 particles in the interval MeV/nuclen ( 2C). Within the equatrial zne the ratis f the bserved cunting rates were MM/ML,- 10-3, MH/ML,- 10-4, and MH/MM , which reflect published values fr the cmpsitin [Spjeldt,'ik and Fritz, 1983] and indicate that the ML cunting rate was almst entirely due t prtns. An investigatin f the ML backgrund was carried ut by analysis f data frm regins between the equatr and the mid-latitude znes. This analysis shwed a negligible backgrund, less than 0.04 cunt per secnd (c/s) cmpared t an average peak rate f abut 1 c/s.

2 146 GuziK ET AL.' Lw-ALTITUDE TRAPPED PROTONS MONITOR /%'"'- Windw LiJ + 2OF...'!...'ML FLUX!,, i MAXIIV;,, A' ' i,,,i/' ' i,, i, ' I ' ' I l _. + ' -,,,,,, i'111,,, it' " ( --201"l I ] I i I i i I i i I,, I,, I I I I I I I I : : : I ZO MAGNETIC LONGITUDE I Fully depleted LI.I! [ /' si detectr Scle I i i 2cm Fig. 1. Diagram f the mnitr telescpe frm the Phenix 1 experiment. The hatched regin represents the passive shielding surrunding the telescpe. Glbal Distributin DATA ANALYSIS AND RESULTS Fr this analysis the equatrial zne was defined as -30 ø t + 30 ø in gemagnetic latitude, and the data taken ver this prtin f the rbit were first freed frm bviusly spurius readuts and frm the influence f the inner radiatin belt in the regin f the Suth Atlantic Anmaly by a (B, L) space cut. The clean data were then accumulated in 1 ø (latitude) by 5 ø (lngitude) bins, and the average ML cunts per readut (4.096 s) were determined fr each bin. Plts f the average cunting rate as a functin f latitude, illustrated in Figure 2 fr the gemagnetic lngitude range 180 ø < b < 185 ø, shwed apprximately Gaussian distributins. These were fit t determine the peak lcatin and the full width at half maximum (FWHM). The uncertainties shwn in Figure 2 were cmputed frm the cunting rates and the number f readuts included in each average. A glbal plt f the lcatin f the prtn flux maxima in gemagnetic crdinates is shwn in the tp panel f Figure 3, in which the hrizntal bars indicate the width f each lngitude bin and the vertical bars indicate the FWHM f the distributins. The bttm panel f Figure 3 shws the lcatin f the line f minimum magnetic field strength, Bmi n [Stassinpuls, 1970]. A cmparisn f the tw panels f -I0 MINIMUM ' -2, i I,, I,, I I,,I,,I,,I,,I,,I,,I,,I, MAGNETIC LONGITUDE Fig. 3. The lcatin f (tp) the peak ML cunting rate cmpared t (bttm) the line f minimum magnetic field strength [Stassinpuls, 1970] in gemagnetic crdinates. The vertical bars in the tp panel are the FWHM f the peaks. Figure 3 shws that the measured maxima fllw the Brain line. This prfile is expected frm the gemetry f the surce if these prtns reflect the ring current prtn distributin. T find the spatial extent f this radiatin, the latitude distributins frm individual lngitude bins were superpsed peak t peak. The FWHM f the resulting glbal distributin is -,, 10 ø in latitude, cnsistent wih the prfile measured by Hvestadt et al. [1972]. T investigate any pssible lngitude dependence f the flux, satellite passes were gruped in three altitude ranges, 180 km_<h_<215 km, 225 km_<h_<255 km, and 255 km < H < 285 km, based n the altitude at which the measured flux was greatest fr each pass. Fr each range the passes were binned in 30 ø lngitude intervals, and the average rate was cmputed fr each interval. Plts f the average rates fr the three altitude ranges are shwn in Figure 4 and indicate n statistically significant lngitude dependence, as expected frm the shrt prtn lifetime at these altitudes. 255KM <_ H <_ ' , m8-14 -IO IO GEOMAGNETIC LATITUDE(degree) Fig. 2. Awrage ML cunting rates as a functin f latitude bin (1 ø wide) fr the g magnetic lngitude range ø. 0 ' 0( GEOMAGNETIC LONGITUDE, (degree) Fig. 4. The lngitude dependence f the average peak cunting rate fr three altitude intervals.

3 GUZIK ET AL.' LW-ALTITUDE TRAPPED PROTONS '3 10 '4 _ AITITUDF' (Kin) Fig. 5. The altitude dependence f the prtn flux measured by Phenix 1. The line is a pwer law in altitude (h 5) nrmalized t the measured flux at 185 km. is taken t be abut 1 x 10-]6 cm 2, then the hydrgen flux is attenuated t 60% f its initial value by 280 km and t 6% by 185 km. A simple pwer law fit t six pints n this attenuatin curve gives an expnent f the rder f 5, cnsistent with ur bserved altitude dependence. This calculatin is strngly dependent n the details f the actual atmspheric density and cmpsitin encuntered by the neutral hydrgen in reaching a particular altitide. In additin, the neutrals created by the lss f trapped prtns by electrn capture at a particular altitude act as a secndary surce at ther altitudes. A cmplete mdel f the prtn ppulatin at lw altitude must cnsider the detailed surce gemetry, bth primary and secndary, as well as additinal lss prcesses such as inizatin energy lss and pssibly pitch angle scattering. The estimate given abve des indicate, hwever, that the charge exchange mdel can explain the grss features f the bserved altitude dependence when attenuatin f the neutral hydrgen flux is taken int accunt. A detailed mdel f the quasi-trapped prtn ppulatin incrprating all f these prcesses is currently being develped. Altitude Dependence The altitude range was binned in 5- t 15-km-wide intervals in rder t keep cmparable numbers f passes in each altitude bin. Passes with peak values ccurring in a given altitude bin were superpsed, and the average peak cunting rate was determined. Figure 5 shws a plt f the peak flux as a functin f altitude. The peak flux was fit t a pwer law in altitude, yielding an expnent f 5.0 _+ 0.2, shwn as the slid line in Figure 5, nrmalized at 185 km. Mritz [1972] reprted an bserved prtn distributin which was independent f altitude ver a range frm 400 km t 1000 km. This was explained by the simple mdel discussed abve, in which the prtn flux is given by jt,(e, h) -- ø'øi(e- ) jh(e, h) (1) ax(e) where j.(e, h) is the neutral hydrgen flux, ax is the electrn lss crss sectin averaged ver all atmspheric cnstituents, a] is the average electrn capture crss sectin, and h indicates altitude. Neglecting any dependence f a and a n altitude due t changes in cmpsitin, an altitude dependence fj, must be the result f an altitude dependent attenuatin f j.. Fr a 350-keV hydrgen atm, Mritz [1972] qutes an electrn lss crss sectin f 10-7 cm 2. At an altitude f 400 km the atmspheric clumn density is abut 1.3 x 10 5 cm -2, and s abut 99% f the initial hydrgen flux remains at this altitude. The estimate may be smewhat ptimistic. Tburen et al ] give electrn lss crss sectins f abut 4.5 x 10- x7 cm 2, 3 x 10-6 cm 2, and 2.8 x 10-6 cm 2 fr 350-keV atmic hydrgen n mlecular hydrgen, nitrgen, and xygen, respectively. Assuming the atmsphere at 400 km t be principally atmic xygen, then nly abut 83% f the initial hydrgen flux shuld remain at 400 km. This reductin, hwever, wuld prbably nt have been detectable by the instrument used by Mritz [1972]. Over the altitude range cvered by the current experiment, hwever, the atmspheric clumn density varies frm abut 2.8 x 10 ]6 cm -2 at 185 km t 5.3 x 10 x5 cm -2 at 280 km. If the average electrn lss crss sectin fr ur energy interval D!#krential Energy Spectrum Figure 6 shws a cmpilatin f the peak differential flux measured n previus missins [Mritz, 1972; Hvestadt et al., 1972; Mizera and Blake, 1973] and by Phenix 1 in These flux values were btained by dividing the bserved peak cunting rate by the quted gemetrical factr and energy range fr each instrument. Results frm quiet times and during gemagnetic strms are shwn fr cmparisn, and it is evident that the strm time enhancement f the flux is prnunced at lw energies. Abve,- 100 kev the data indicate a pssible pwer law behavir, and fitting the prestrm and average data yields the slid line with an index f _ Applied t the energy interval f the ML rate, 102 e _ ' ' ',,,,,i,, ''",,i, i,,ill, I, M.er nd Blke (1973) - Pststrm (3/25/69) - Prestrm (3/19/69) '* e Mritz (1972) - Pststrm (3/11/70) -- e 'ø:- -a _ Prestrm (3/5/70) -/ ercje(11/10-12/6/69) \ Hvestdt et l. (1972) \ *?- Aver.(xje(11/10-12/10/69): Phenix- I(This wrk) ' --'",¾ø' a - Averge(5/22-12/5/8?_j. ' ",, H 270km ",,._ =-Extrplted t --,, 450km., i i i1,1,1 i i i illill J I i,1,,,1 I i i Kinetic Energy (kev) Fig. 6. Differential energy spectrum f prtns cmpared t previus measurements, fr bth quiet (prestrm, average) cnditins and gemagnetically disturbed (pststrm) cnditins.

4 148 GUZIK ET AL.' LW-ALTITUDE TRAPPED PROTONS this pwer law gives a mean energy fr the prtns bserved by the mnitr telescpe f 1.3 MeV. Fr the S81-1 missin the highest altitude bin was centered at 277 km, whereas the lwest bservatin altitude f previus missins was 450 km. Using the bserved pwer law altitude dependence discussed abve, the Phenix 1 data were extraplated t 450 km, shwn as the slid square in Figure 6, crrespnding t a cunting rate f 108 q- 13 cunts per readut. Integrating the pwer law fit in energy ver the ML energy interval yields a predicted cunting rate f 38 q-5 cunts per readut, almst a factr f 3 belw the extraplated value. Alternatively, an extraplatin f the measured flux using the surce attenuatin mdel discussed abve yields an integrated cunt rate f abut cunts per readut, almst a factr f 3 belw the predicted cunting rate. This may indicate that the energy spectrum breaks r rlls ff abve 1 MeV, which wuld be cnsistent with the data shwn in Figure 6. Instrumental EJficiency The flux values discussed in the preceding sectin were btained under the simple assumptin that the flux culd be btained by dividing the cunting rate by the gemetric factr and energy interval, equivalent t the assumptin that the flux was mnidirectinal. As reprted previusly [Mritz, 1972; Mizera and Blake, 1973] and as shwn by the latitude distributin in Figure 2, this is nt the case. Rather, the distributin is strngly peaked tward 90 ø equatrial pitch angle. Thus in rder t make the crrect cmparisn between the Phenix 1 results and previus missins, it is necessary t evaluate the telescpe efficiency as a functin f particle pitch angle. In general, the cunting rate R f a particle telescpe in units per secnd can be expressed as R(x, t ) = dt ds? dr de j(e, t, x, t) (p) where j is the differential directinal flux in s-1 cm-2 st-1 MeV-, T is the sampling time, S is the area f the detectr, E 1 and E 2 are the limits f the energy respnse f the instrument, 2(p) is the dmain f as a functin f lcatin p n the detectr, x dentes the lcatin in space, and? is the unit vectr in the directin w [-cf. Sullivan, 1971]. At lw altitudes near the equatr the prtn ppulatin can be represented by a differential directinal flux which is axisymmetric abut the magnetic field directin and has the frm (2) j(e, w, x, t)= A(x, t)e - sin q % (3) where A(x, t) is a nrmalizatin cnstant in units f s- cm-2 sr- MeV - and is the equatrial pitch angle. The respnse f a telescpe t this highly directinal flux depends strngly n the gemetry f the telescpe and the detectr(s). We will cnsider the case f a telescpe such as the Phenix 1 mnitr (r the EI-92 telescpe f Mritz [1972]) in which the detectr(s) is (are) planar and in which shielding and/r cincidence cnditins define the aperture. Cnsider a crdinate system in which is the tangential angle frm the lcal magnetic field directin (pitch angle). Define fi t be the azimuthal angle measured frm the plane cntaining B and the nrmal t the detectr. Then, the element f slid angle is d v = sin s ds dfi (4) and if the angle between B and the detectr nrmal is 7, then ds? = ds(sin 7 sin s cs fi + cs 7 cs s) (5) Assuming fr simplicity that the pitch angle distributin and energy spectrum are independent and that the intensity des nt vary ver the sampling time f the instrument, then R(x, t) = A(x, t) E -ø de sin q+ s ds ds 1 ' " dfi (sin 7 sin s cs fi + cs 7 cs s) 1 (P.7, ) where the limits f the fl integratin reflect the dmain fl(p) at the pint p n the detectr. Thus fr a particular s the range f fl is just the range f azimuthal angle ver which particles with pitch angle s can reach pint p. If n trajectries with pitch angle s reach p, then the fl integral is identically zer. Nw if we identify 1 f$.f(s)= - n ds dfi (p.,, ) (6) dfi(sin 7sin scs fi+cs7css) (7) as the pitch angle dependent instument directinal efficiency r respnse functin, then we btain R(x, t)= 2hA(x, t) E - de sin q+ s f(s) ds (8) 1 Thus in rder t derive the flux measured by different instruments frm their measured cunting rates, the instrument respnse functin must be determined, and this requires knwing the directin 7 f the detectr nrmal relative t the lcal magnetic field as well as the variatin f the limits f fl integratin as a functin f s, 7, and p. The apprach which we have taken t find f(s) is t divide the detectr area int a large number f elemental subareas. Fr each f these subareas we determine the limits f fl as a functin f s and 7 (in practice we replace the s integratin in (8) by a sum, s we deal with discrete values f s). Using these limits, we evaluate the fl integral and sum ver elemental areas. The respnse functin depends strngly n 7 and thus, fr a satellite such as S81-1 which is nt magnetically aligned, n gemagnetic latitude. Near the equatr, hwever, 7 90ø fr Phenix 1, and the respnse functin becmes sinsfs f(s) -- ds cs dfl (9) The functin f(s) frm (7) was calculated alng the B = Bmi. line in a real gemagnetic field mdel (Internatinal Gemagnetic Reference Field 1975 extended t 1982) and was fund t peak at an average 92 ø pitch angle as cmpared t 90 ø fr (9). Thus t within a small uncertainty the respnse functin in (9) applies at B -- Bmi n and was used in calculating the peak flux. A cmparisn f differential flux intensities measured by different instruments can be made by finding the values f the altitude dependent nrmalizatin cnstant A(h, t) equivalent t the measured equatrial cunting rates. This requires an estimate f q and g as well as a determinatin ff(s) fr each instrument.

5 GUZIK ET AL.' Lw-ALTITUDE TRAPPED PROTONS 149 z - z '- EXPONENT q OF PITCH ANGLE DISTRIBUTION Fig. 7. Nrmalizatin cnstant A(h, t) in s- cm -2 st- kev - fr : pen circles, Phenix 1 at 277 km; squares, EI-92 [Mritz, 1972] at 450 km; triangles, Phenix 1 extraplated t 450 km using h s pwer law; and slid circles, Phenix I extraplated t 450 km using surce attenuatin mdel. T cmpare ur measured flux with that reprted by Mritz [1972], we fund the equatrial f( ) frm (9) fr bth instruments and tk the spectral index frm ur fit t the data in Figure 6. This value is applicable t the channel 3 data ( MeV) f Mritz [1972] as well as t the Phenix 1 data. In determining the values f A(h, t) we perfrmed the energy integratin ver the range apprpriate t the individual instrument. Fr the EI-92 calculatin we used the cunting rate frm Figure 2 f Mritz [1972]. Several pitch angle distributin functins fr lw altitudes have been reprted. Blake et al. [1973] measured a value f q 6.7 in the range 1.8 < L < 1.9 fr fur different prtn energy channels cvering kev/nuclen in the equatrial zne. Mizera and Blake [1973] measured the equatrial prtn pitch angle distributin at lw energies and btained a value f q q- 2. Mritz [1972] calculated the pitch angle distributin fr a simple particle surce mdel and btained q- 13 q- 3. Thus fr lw-altitude, lw-energy prtns near the equatr, q prbably falls in the range 5 < q < 15, with the mst likely value near the center f this interval. In rder t investigate the effect f q n the value f A(h, t) calculated frm the cunting rate, we fund A(h, t) fr bth the Phenix 1 data and the EI-92 results at a number f q values. The range 5 < q < 35 was chsen t investigate the pssible effect f a variatin in pitch angle anistrpy at lwer altitudes frm the values previusly reprted. Figure 7 shws the results f these calculatins fr (pen circles) the highest S81-1 altitude bin (277 km) and (squares) the EI-92 data (lwest altitude km). Als shwn are the Phenix 1 values extraplated t 450 km under the assumptin (triangles) that the pwer law altitude dependence shwn in Figure 5 hlds t this altitude and (slid circles) that the simple surce attenuatin mdel discussed abve can be applied. It is clear frm Figure 7 that within the uncertainties the flux measured by Mritz [1972] at 450 km and that measured by Phenix 1 at 277 km are indistinguishable even if there is a small change in the pitch angle anistrpy at lwer altitudes. The cmparisn f the Phenix 1 values extraplated t 450 km t the EI-92 values suggests that the flux measured n the S81-1 missin may reflect an enhancement f the prtn surce cmpared t that measured n Azur. This is certainly the case if the pwer law extraplatin is valid in this altitude range. The surce attenuatin extraplatin als supprts this cnclusin, althugh the required enhancement is much less and variatins in atmspheric density and cmpsitin must be taken int accunt. Assuming a reductin f the neutral hydrgen flux t 80% at 277 km, due nly t atmspheric attenuatin and nt t a change in the ring current surce, the extraplated Phenix 1 flux and the EI-92 measurement wuld be cnsistent fr an atmspheric clumn density (at 277 km) f 2.2 x 10 s cm -2, which is 42% f the estimate given earlier. Such a density reductin wuld nt, hwever, give the bserved altitude dependence belw 277 km. Therefre the best explanatin f the difference between the EI-92 and extraplated Phenix 1 measurements is an enhanced neutral hydrgen flux during the S81-1 missin in Explaining a flux enhancement such as this must invlve an investigatin f the exspheric hydrgen density, since this is the dminant factr in the lss f ring current prtns thrugh the charge exchange mechanism. Exspheric density can be changed by slar activity and increases with increasing slar EUV input t the atmsphere [Harris and Priester, 1962; Jacchia, 1977]. Mre hydrgen escapes t the gecrna during slar maximum cnditins than during slar minimum cnditins. The average exspheric temperatures ver the tw bservatin times, calculated frm the average slar EUV input, were, K in 1969 and, K in 1982 [Jacchia, 1977]. These temperatures crrespnd t Jeans' escape fluxes fr hydrgen f,-,300 in 1969 and,-,1000 in 1982!-Fahr and Shiz dal, 1983], suggesting that in 1982 mre hydrgen escaped t the exsphere, raising the rate f charge exchange in the ring current. We are investigating, in mre detail, this effect as well as the effect f variatins in atmspheric density and cmpsitin bth between 1969 and 1982 and ver the duratin f the S81-1 missin. CONCLUSIONS We have demnstrated the glbal character f the lwaltitude equatrial prtn ppulatin f energy,-- 1 Me¾. The maximum prtn flux prfile falls alng the minimum magnetic field line with a FWHM,--10 ø and is independent f lngitude. The peak prtn flux varies as the fifth pwer f altitude frm 170 t 290 km. As cmpared t previus data, by extraplatin f the 277-km flux t 450 km the prtn flux in 1982 is at least a factr f 1.5 larger than in 1969, prbably because f a difference in slar cnditins between the tw epchs. Acknwledgments. The wrk was supprted at Luisiana State University by the Office f Naval Research (ONR) under cntract N K The Phenix 1 instrument was built by the Labratry fr Astrphysics and Space Research at the University f Chicag under NASA cntract NAS , and spnsred by ONR, with launch supprt and telemetry cverage prvided by the Air Frce Space Test Prgram. The Editr thanks J. B. Blake and anther referee fr their assist- ance in evaluating this paper. REFERENCES Blake, J. B., J. F. Fennell, M. Schulz, and G. A. Paulikas, Gemagnetically trapped alpha particles, 2, The inner zne, J. Gephys. Res., 78, 5498, Fahr, H. J., and B. Shizgal, Mdern exspheric theries and their bservatinal relevance, ReF. Gephys., 21, 75, Harris, I., and W. Priester, Time-dependent structure f the upper atmsphere, NASA Tech. Nte, NASA TN D-1443, Hvestadt, D., B. Hausler, and M. Schler, Observatin f energetic prtns at very lw altitudes near the gemagnetic equatr, Phys. Rer. Lett., 28, 1340, 1972.

6 150 GUZIK ET AL.: Lw-ALTITUDE TRAPPED PROTONS Jacchia, L. G., Thermspheric temperature, density, and cmpsitin: New mdels, Spec. Rep. 375, Smithsn. Astrphys. Obs., Cambridge, Mass., Mizera, P. F., and J. B. Blake, Observatin f ring current prtns at lw altitudes, d. Gephys. Res., 78, 1058, Mritz, J., Energetic prtns at lw equatrial altitudes: A newly discvered radiatin belt phenmenn and its explanatin, Z. Gephys, 38, 701, Spjeldvik, W. N., and T. A. Fritz, Experimental determinatin f gemagnetically trapped energetic heavy in fluxes, in Energetic In Cmpsitin in the Earth's Magnetsphere, edited by R. G. Jhnsre p. 396, D. Reidel, Hingham, Mass., Stassinpuls, E.G., Wrld maps f cnstant B, L, and flux cnturs, NASA Spec. Publ., NASA SP-3054, Sullivan, J. D., Gemetrical factr and directinal respnse f single and multi-element particle telescpes, Nucl. lnstrum. Methds, 95, 5, Tburen, L. H., M. Y. Nakai, and R. A. Langley, Measurement f high-energy charge-transfer crss sectins fr incident prtns and atmic hydrgen in varius gases, Phys. Rev., 171, 114, T. G. Guzik, M. A. Miah, J. W. Mitchell, and J.P. Wefel, Department f Physics and Astrnmy, Luisiana State University, Batn Ruge, LA (Received January 4, 1988; revised August 2, 1988; accepted August 2, 1988.)