Observations concerning the relationship between the quiet-time ring current and electron temperatures at trough latitudes

Planet. Space Sci., Vol. 27. pp. 1175-1185. 0

00324633/79/0901-1175$02.00/0

Pergamon Press Ltd., 1979. Printed in Northern Ireland

OBSERVATIONS CONCERNING THE RELATIONSHIP BETWEEN THE QUIET-TIME RING CURRENT AND ELECTRON TEMPERATURES AT TROUGH LATITUDES WILLIAM 1.BURKE*,

HANS J. BRAUNt,

(Receioed

J. W. MfJNCHt

22 March

and RITA C. SAGALyN*

1979)

Abstract-The relationship between the simultaneously observed positions of the maximum omnidirectional flux of the quiet-time ring current positive ions (Am) and the maximum electron temperature AT in the trough is studied in the midnight sector of the topside ionosphere. A, maps to the inner edge of the plasma sheet where ring current fluxes change from nearly isotropic to trapped. At altitudes near 2500 km, the electron temperature at trough latitudes were always sharply peaked. Although A, varied with the level of geomagnetic activity, (A, -AT) did not. These observations support the hypothesis that the quiet-time ring current is the source of elevated electron temperatures found near the plasmapause. Below 1300 km, peaked electron temperature distributions in the trough

were not consistent features of the data. It is shown that (A, -AT) increased with decreasing altitude. The nossible influences of a westward comnonent to the convective electric field and ionospheric refraction of ion cyclotron waves are discussed. INTRODUCI-ION

Previous investigations of the thermal structure of the night-time topside ionosphere (Mahajan and Brace, 1969; Brace and Theis, 1974; Titheridge 1976a, b) have revealed the existence of an electron temperature peak in the vicinity of the plasmapause. This observation is attributed either to the dissipation of energy stored in the night-time protonosphere (Mahajan and Brace, 1969) or to the heating of thermal electrons by resonant cyclotron waves generated by the quiet-time ring current (Brace and Theis, 1974; Titheridge, 1976a). The later explanation extends the theory of stable auroral red arc formation to magnetically quiet-times (Cornwall et al., 1972). An alternate theory for heating electrons near the plasmapause by the quiet-time ring current via Coulomb collisions has been proposed by Cole (1975). It is argued that the wave-particle interaction (Cornwall et al., 1971) requires densities ~100 crnw3 at the equator where the Coulomb process does not. Characteristics of ring current particles have been studied by meads of a number of satellites at ionospheric altitudes and near the magnetospheric equator. Frank (1971) has shown that in the midnight sector the ring current extends earthward of both the inner edge of the plasma sheet and the *Regis College Research Center, Weston, MA 02193, U.S.A. tMax Planck Institute for Aeronomy, D-3411 Kattenburg-Lindan 3, Germany. SAFGL, Hanscom AFB, MA 01731, U.S.A. 1175

plasmapause. During a magnetic storm Williams and Lyons (1974) found that just outside the plasmapause the ring current protons have flat (isotropic) distributions except for nearly empty loss cones. Inside the plasmapause the distributions are anisotropically peaked at a pitch angle of 90”. The transition occurs where the particle energy is approximately equal to the magnetic energy per particle (Williams and Lyons, 1974; Sorras and Berg, 1974). This is consistent with particles being lost due to ion cyclotron turbulence (Kennel1 and Petschek, 1966). The energy lost by the ring current in this instance was more than sufficient to drive a simultaneously observed SAR arc (Williams et al., 1976). It should be noted that at distances from the Earth greater than that studied by Williams and Lyons the proton distributions are isotropic (DeForest and McIlwain, 1971) or field aligned (McIlwain, 1975). The latitudinal distribution of the ring current protons with energies between 1 and 14 keV have been studied at ionospheric altitudes by Hultquist et al., (1976). According to the pitch angle characteristics of the protons, three spatial regions are distinguished. The high latitude region, which is marked by isotropic proton fluxes, extends slightly equatorward of measurable plasma sheet electrons in the ionosphere. Equatorward of this position proton fluxes decrease significantly and become strongly peaked at pitch angles of 90”. Between the ionospheric projections of the plasmapause and -50” invariant latitude the measured fluxes again increase and become anisotropic. The time period

1176

W. J. BURKE et ai.

studied by Hultquist et al., (1976) was magnetically quiet and partially overlaps one of the periods studied in this paper. Here we report on the lati~din~ dist~bution of electron temperatures observed in the midnight sector at altitudes between 800 and 2500 km by means of the Injun 5 satellite. Due to the versatile nature of the satellite we are in a unique position to evaluate the ring current source hypothesis. This is done by comparing electron temperature distributions near the ionospheric projection of the plasmapause with the simultaneously measured position of the ring current. In the following section we describe the Injun 5 experimental package. In the section on observations we first describe our technique for identifying the position of the ring current. The dist~bution of electron temperatures at trough latitudes in the 800-2500 km range are presented and compared with the ring current position. These Injun 5 observations are found to support the ring current rather than the protonospheric source hypothesis. The data are more easily explained by a wave-particle rather than a Coulomb interaction. ‘JXE

EXPERIMENT

Injun 5 is a magnetically aligned, polar orbiting satellite with an apogee of 2543 km and a perigee of 677 km. Data used in this report come from the Low Energy Proton and Electron Differential Energy Analyzers (LEPEDEA), two spherical electrostatic analyzers (SEA) and a double-probe electric field experiment. The LEPEDEA systems consist of two continuous-channel electron multipliers so mounted as to observe fluxes at pitch angles ~30” (LEPEDEA A) and 90” (LEPEDEA B). Each LEPEDEA is capable of measuring proton and electron spectra in 117 energy ranges between 50 eV and 15 keV every 2 s (Frank and Ackerson, 1971). The two boom-mounted SEAS consist of solid collectors surrounded by two concentric wire mesh grids. The electron sensor’s collector has a potential of +lOOV; the grids are electrically connected and operate in a repetitive two-mode sequence, each of 15.9 s duration. In Mode 1 the grids are held at either 1.6 or 6.0 V; in Mode 2, they are swept from -10 to +3 V. The outer grid of the ion sensor is held at satellite ground and the inner grid is set at +28 V, thus screening thermal ions from the collector. The ion sensor’s collector also acts in a repetitive two-mode sequence. In Mode 1, the collector is held at -2000 V for 47.7 s; it is then stepped 14 times between -11.2 and -2000 V in a

15.9 s interval. The only ion data presented or discussed come from Mode 1 periods. In these modes of operation we are able to measure the electron temperature (T) and density and, while the satellite is in darkness, the omnidirectional flux of ions with E >28 eV. At midlatitudes, the ‘I’ measurements have a latitudinal resolution of 51.2”. A more detailed description of the instruments and the data reduction procedures is given by Burke et al. (1978). The meas~ements presented in this paper come from regions where the satellite was in darkness and away from intense aurora1 fluxes. In sunlight, photoelectrons emitted by the ion sensor’s collector overwhelms the flux of energetic ions. Aurora1 electrons frequently drive the satellite to such negative potentials that thermal electrons are shielded from detection (Sagalyn and Burke, 1977). It should also be noted that only relative variations in the ion flux can be considered as having geophysical significance. The measured ion current is amplified due to the efficient production of secondary electrons by energetic ions impacting the collector. The double-probe experiment is capable of measuring one component of the convective electric field. The method used to reduce these data is discussed by CaufIman and Gurnett (1971). Simultaneous electric field, LEPEDEA and SEA measurements are available from a very limited number of orbits during February 1971. OBSERVATIONS

Thermal electron observations presented in this section come from three periods when the Injun 5 orbit was in the midnight sector (cf. Table 1). The first data set was taken during the NovemberDecember 1968 period while the satellite was near apogee over the northern hemisphere. The second set comes from the late February-early March, 1971 period in the 800-1OOOkm altitude range. The third set, which comes from the December 1969-January 1970 period, spans the 850-2000 km altitude range over the northern hemisphere trough. Simultaneous LEPEDEA proton and SEA ion observations are available from two of the first, eight of the second and none of the third sets of orbits. We first consider the relationship between the LEPEDEA proton and the SEA hyperthermal ion observations. It is shown that the SEA ion llux can be used to determine the position of the ring current. The relationship between the ring current position and other electron trough parameters are then studied with a larger data set than would be

Ring current and trough latitude electron temperature TABLE

Data Set I II III

Date

Altitude range (km)

1.

INVARIANT

A,

NovemberDecember 1968 2200-2500 66.3+2.6 FebruaryMarch 1971 1000-800 63.4k1.2 December 1969January 1970 2000-850 65.5~t2.6

the case if we were restricted to cases where only LEPEDEA and thermal electron data were available. Proton fluxes, which are due to ring current particles reaching the satellite altitude, are observed with the Injun 5-LEPEDEA system throughout and equatorward of the region of plasma sheet electron precipitation (Frank and Ackerson, 1971, 1972). During two Injun 5 orbits when simultaneous LEPEDEA and SEA observation were available, it was found that the regions of ring current and enhanced hyperthermal ion fluxes were latitudinally coextensive. On this basis, it was suggested that SEA ion signatures could be used to determine the positions of the ring current (Burke et al., 1978). Eight more examples of simultaneous LEPEDA proton and SEA hyperthermal ion measurements, with the satellite in darkness, were provided during the February-March 197 1 period. Unfortunately, at this time only the LEPEDEA-A (field-aligned) system was functioning. Four examples of simultaneous observations from these detectors are given in Fig. 1. Here we have plotted the proton directional flux from LEPEDEA energy channels between 1 and 15 keV, and the omnidirectional flux of ions with E >28 eV as functions of invariant latitude. The hourly averaged aurora1 electrojet indices (AE) for the periods of observations are given for reference. Periodic gaps in the SEA ion data result from deletions of measurements taken while the sensor’s collector was being stepped in voltage. During these orbits, the proton count rate slowly rose to a maximum value found between 65” and 62”. In the case of orbit No. 11380 a second, lesser proton flux is found a full degree equatorward of the ring current’s main body. A sharp gradient is found at the low latitude boundary of the proton flux. LEPEDEA-A count fell from maximum values to instrument background in less than 2”. The SEA energetic ion fluxes display quantitatively similar features, reaching maximum val-

1177

LATITUDE

&-

A,

A+--A2,

M

AEa

63.3h2.3

63.3+2.2

2.3~t~l.4”

135

135

57.1zt2.3

55.4zt2.0

5.7ztl.4”

171

207

61.7k2.2

61.5k2.3

3.851.4”

85

77

ues at the same latitudes as the LEPEDEA. However, the SEA ion fluxes decreased at a much slower rate. Energetic ion fluxes >107 cm’s_‘. were measured to invariant latitudes of 50”. This difference is explained by the fact that the SEA intergrates ion distributions over all energies >28 eV and all pitch angles. The steep gradient at the low latitude boundary of LEPEDEA measurements reflects a rapidly changing pitch angle distribution of ring current protons with E < 14 keV as they go from nearly isotropic to trapped (Hultquist et al., 1976). Since SEA and LEPEDEA flux measurements have maxima at the same latitude, this is approximately true for protons of all energies. The qualitative similarities of the pitch angle distributions for protons with E < 14 keV (Hultquist et al., 1976) and with E> 100 keV (Sorras and Berg, 1974) are consistent with this conclusion. The invariant latitudes of the maximum ring current fluxes (A,) in both the LEPEDEA and SEA measurements lie between 62” and 65”. This lies poleward of the ionospheric projections of the plasmapause. The proximity of A, to the cut off in measurable plasma sheet electron fluxes indicates that the transition from nearly isotropic to trapped populations occurs at the inner boundary of the plasma sheet (Frank and Ackerson, 1971, 1972; Hultquist et al., 1976). Finally it should be noted that in these examples from February to March 1971 no secondary maxima in SEA fluxes were found between 50” and 60”. This is contrary to the results reported by Hultquist et al., (1976). In the following analysis A, is the invariant latitude of maximum SEA ion flux and/or maximum LEPEDEA proton flux rates. We interpret A+ as the position at which the ring current changes from an isotropic to a trapped distribution. For the sake of simplicity we introduce the notation AT and A,.., to designate the invariant latitudes at which the electron temperature in the trough has a maximum value and d In Nldh = 0, the base of the equatorward wall of the electron trough, respectively.

1178

W. J. BURKE et al. I’S

”

II

”

8

II

c

’

REV

LEPEDEA

IOJ

”

11320

DAY 56.1971,~04 AE

SO ,103

REV

11320

?

t

106 z

1

t

J/\k---_

1

,1,

log

F

I05

REV II401 : DAY AE 62,1971, 3211 -103 : 104

LEPEDEA I

SEA

REV

II401

REV

11414 j’05

109 r +

-’

--1

I - LEPEDEA

DAY 63.1971: I o4 AE IO3 : : I03 #

I lo9

f

108 r

REV

SEA .-/I, I

10

II414

t .

I

66

1

-a I

.

0

’ 56

00

INVARIANT

a

-w ’

*

’

c

’

’

60

LATITUDE

FIG. 1. FOUREXAMPLESOFSIMULTANEOUSMEASUREMENTSOFPRECIPITATINGPROTONSWITHENERGIES 15 EsXI keV AND THE OMNIDIREcTIONAL nuxOFPOsmv~ IONS WITH E> 28eV AS FUNCTIONS OF INVAFUANTLATITLIDE. Thehourlyaveraged value of AEis givenforreference.

(a) Results near 2500 km Before considering the possible heating effects of the ring current it is necessary to recognize that conjugate photoelectrons (CF’Es) also contribute to the night-time ionospheric heat budget (Hanson, 1963). The influences of these two heat sources are readily distinguished by comparing latitudinal profiles of T with the conjugate ionosphere in sunlight and in darkness. Two such examples are provided in Fig. 2. Electron densities and temperatures and the omnidirec-

tional hyperthermal ion fluxes are plotted as functions of invariant latitude and the conjugate solar zenith angle. AE values of 52 (No. 1325) and 31 (No. 1331) indicate quiet geomagnetic conditions. In both cases the maximum T was found in the trough. Equatorward of the trough T fell to 2300 K in the case of No. 1325 (conjugate ionosphere in darkness), whereas it remained between 4000 and 4500 K during No. 1331 to A= 51” where the conjugate ionosphere passed into darkness. Of the

1179

Ring current and trough latitude electron temperature REV. 1325

~~

tNVA~lANT t I

L

a

110*

i2u

$300

CON& FIG.

2. TEE?ELECTRON

SERVED

DUmNG

0RBZT-S

DENSITY

No.

AND

LATITUDE

1400

SOLAR

I 90'

I

1

,OO'

IW

* 120.

ZEN. ANOLE

TEMF-ERAWKE?

1325 ANDNo. 1331 CONJUGATE

I LIW

SOLAR

AND

HYPERTHERMAL

AS FuNcTfONS ZENITH

ION

OF INVARIANT

FWJX

(E

>

LATITUDE

28 ev) AND

OBTHE

AblGLE.

The electron density and the ion flux are given in the top panels and are designatedby the symbol X and the dashed lines, respectively.The electron temperature, given in the bottom panels, is designated by the symbol 0. The altitude, longitude and local time of the satellite at h=60” are given for reference.. 25 Injun 5 orbits from the November-December 1968 period 22 (3) had the conjugate ionosphere in sunlight (darkness). Sharply peaked T d~s~~butions were found at trough latitudes, independent of whether conjugate ionosphere was in sunlight or darkness. In the 22 cases with the conjugate ionosphere in sunlight T remained high to conjugate sunset. These observations indicate that a source other than CFEs is primarily responsible for heating topside electrons at trough latitudes. For this reason we concern ourselves only with T values in the electron trough. The maximum ion flux during No. 1331 is found at A = W”, -2” poleward of maximum observed

value of T. That is, AT occurred along the steep gradient in the ring current flux associated with a rapidly changing pitch angle distribution. A maximum value of Te = 6200 K at AT = 63” during No. 1325 is also found in the region of a steep ring current gradient. Because the satellite was in sunlight for h~65.5” it is not possible to specify the separation of A, and &, in this instance. The two examples given in Fig. 2 come from the same period and approximately the same local time sector studied by Hultqnist et at. (1975). In agreement with the observat,ions of Hultquist and coworkers a secondary maximum is found in the SEA ffux measurements near an invariant latitude of 55”.

W. J. BURKE et al.

1180

A comparison with simultaneous electron density measurements shows that the secondary ion flux maximum lies equatorward of the ionospheric projection of the plasmapause. This is a stable feature of Injun 5 observations from the November to December 1968 period. A study by Rao et al. (1978) shows that this population was injected to L-shells 2-3 during the great magnetic storm of early November 1968. Thus, the quiet-time ring current is made up of two distinct populations. The more prominent is made up of plasma sheet particles. The second, low latitude component is made up of a slowly decaying, stably trapped population that was injected to relatively low L-shells during a period of high magnetic activity. The highest values of T in Fig. 2 are found near the inner edge of the plasma sheet portion of the ring current. An example taken from a period of geomagnetic activity (AE=498) is given in Fig. 3. Both the positions of the ring current and the trough have REV.

1452 I’

_ -

2466 100.

km w

_

22.12

hrs.

1. 12/6/66

:

moved equatorward relative to their quiet-time positions during No. 1331, at the same geographic longitude. However, the maximum value of T (7800 K) is found 2.5” equatorward of Am. Although the maximum value of T during No. 1452 is 3000” higher than in the case of No. 1331, no consistent relationship was found between T in the trough and AE. The highest electron temperatures were measured at A = 67” during two orbits when AE had a value of 14. In all 25 cases AT was found in regions of steep ring current gradients. In the cases where they could be measured at -2500kmA,=66.3+2.6”, AT =63.6+2.3” and AN =63.3=t2.2”. The separation between A, and A, was 2.3 f 1.4” and on the average, AT was within 1” of AN. (b) Results near 1000 km Data are available from 31 Injun 5 orbits between 21 February and 11 March 1971. Due to sunlight effects on the instruments, A, could be identified in only 13 cases and the values of T in the trough were measured during only 22 orbits. The values of N and T as functions of invariant latitude from the orbits for which we presented ring current flux data in Fig. 1, are given in Fig. 4. In these four examples maximum values of T in the trough are clearly identifiable. However, in 50% of the orbits near 1000 km AT could not be identified. Figure 5 provides an example where no maximum value of T was measured in the trough. The mean values of A,, A, and A, observed during this period were 63.4+ 1.2”, 57.1*2.3” and 55.4*2.0”, respectively. Of the eleven cases where A, could be identified, A, was measured in only 7 instances. The separation between A, and A, was 5.7 * 1.4”, a value larger than that found near satellite apogee. (c) Results at intermediate altitudes

70.

60°

50.

INVARIANT FIG. 3.

THE

ELECTRON

HYPOTHERMAL

ION

DENSrIY

FLUX

40’

LATITUDE AND

OBSERVED

same

format

invariant

as Fig. latitudes

2 is used.

<55”

AND

ORBIT

No.

LATITUDE.

~~~~AsFuN~TIONSOF~~ARIANT The

TEMPERATURE DURING

No

during

data

were

this orbit.

taken

at

Observations were taken during 42 Injun 5 orbits during the 13 December 1969-22 January 1970 period when the satellite was in the 850-2000 km altitude range. Maximum values of T were measured in all cases when the satellite altitude was >1300 km, and in 70% of the cases below this height. Values of T in the trough at altitudes ~1200 were comparable to those measured near 1000 km during the 1971 period. The values of A,, AT and AN were 65.5+2.6”, 61.7h2.2” and 61.5+ 2.3”, respectively. In the 18 cases where both A.+ and AT were measured, their separation was 3.8+ 1.4”, a value intermediate to that measured near 2500 and 1OOOkm.

Ring current and trough latitude electron temperahne

1181

I04

3000

2000

1000

IO3

? ” c) ;b g -I

70=’

IO5

60°

SO”

r: -0

40.

~i5000

i 4000

i 3000

I04

c 2

2000

4

70°

1

60’

I

I SO0

*

I 40”

INVARIANT FIG. 4. EIXTRON

IO00

I03

I03

’

70°

’

1 50°

*

1

i

40’

LATITUDE

DENSITY AND TE%WER!.TXJRES. MEASURED

(d) Synopsis of results The mean values and standard deviations of A&, A,, AN, (A+ --A,) and AE for the three data sets are listed in Table 1. The mean value of A,+ is fairly uniform over the three data sets. The lowest value of A,, 63.4”, is partially attributed to the relatively high Ievel of geomagnetic activity during the second data set period and partially to a seasonal effect. A scatter plot of A& as a function of AE (Fig. 6) shows that for AE< 100, A& is randomly distributed (rt 3”) about 67”. For AE> 100, A, tends toward lower values. A linear regression analysis of A, as a function of days from the solstice (D) for AE
1 60°

DURING

THE FOUR ORBITS OF FIG.

1, AS

latitude boundary for electron precipitation (Kamide and Win~ngham, 1977). There is a trend toward lower values of A, and A, at lower altitudes that is not entirely explained by increased levels of geomagnetic activity. A large part of this equatorward shift of AN (the equatorward wall of the electron trough) with altitude is explained in terms of the different latitudinal distributions of heavy and light ions at 2500 and 100 km (Taylor, 1972). At 2500 km the electron and light ion troughs are coincident but not at 1000 km. A seasonal effect is also present. Data sets II and III of Table 1 are from the late and early part of the winter respectively. There is a range of altitudes in which data sets II and III overlap. In the cases from data set III with hi 1000 km, AN = 60 ~tr2” which is significantly higher

W. J. BURKE et al. REV.

of AE occurred during the three observation periods. This point was illustrated in the cases of orbits Nos. 1331(AE= 35) and 1452(AE=498) where (A+ --A,) was 2” and 2.5” respectively. The increasing value of (Am-AT) with decreasing altitude does not appear to be a seasonal effect. The trend towards higher values of (A, -A,) is evident in the low altitude observation portion of data set III. A possible explanation is given in the following section.

11377

~+Joo*

;

DICUSSION The observation of elevated electron temperatures in the vicinity of the plasmapause have been explained in terms of heating by quiet-time ring current via processes analogous to those responsible for SAR areas. Near the inner boundary of the plasma sheet portion of the ring current, the proton distribution change from nearly isotropic to mostly trapped. The precipitation of protons results from resonant pitch angle scattering caused by unstable waves (Kennel and Petscheck, 1966). Sorras and Berg (1974) have shown that during magnetically quiet periods the thermal plasma density gradients at the plasmapause are quite diffuse. Under these circumstances the conditions required for the onset of ion-cyclotron turbulence is met near the inner edge of the plasma sheet. According to the model of Cornwall et al.(1971), the waves heat ambient, thermal electrons through Landau damping. The energy transferred to the electrons is conducted along magnetic field lines into the ionosphere. Cole

Q Te

x Ne

ww ‘00

60”

ii

50*

INVARIANT

LATITUDE

FIG. 5. ELECTRON DENSITIES AND TEMPEFCATLJRES MEASURED DURING ORBIT No. 11377 AS FUNCIlONS OF INVARL4NTLATITUDE. This

plot illustrates a case where no peak value of be found in the trough.

T

can

than the 55.4+2” found in the late winter (set II) orbits. The separation between A, and A, appears to depend on altitude but not the level of geomagnetic activity. The latter point is demonstrated by noting that the standard deviation of (A+ -A.,) was 1.4” for all three data sets, even though wide variations

.

l

66

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:*

.

.

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.

. :.

.

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.

. 60

. .

-

I

56 0

I

200

* I

. I

400

1

1

600

I

I

600

AE FIG. 6. SCATIZR

PLOT OF THE INVARIANT

LATITUDE OF MAXIMUM FUNCIIONOFL‘%.

RING CURRENT

FLUX

(A,) AS A

Ring current and trough latitude electron temperature (1975) pointed out that the ambient density required at the equator is -100 crnm3 and that because the waves can only propagate along the magnetic field lines the elevated electron temperatures should be confined to a very narrow latitudinal range. He proposed that the energy is transferred to the electrons by means of Coulomb collisions with ring protons. At altitudes near 2500 km the electron temperatures are sharply peaked (Figs. 2 and 3). In the case of No. 1325 the electron temperature peak had a latitudinal width of -1”. This measurement is consistent with the predictions of the Cornwall et al. (1971) model. The broader maximum of No. 1331 and No. 1452 seem to favor Cole’s model. The fact that Ar 2 AN in these cases does not suggest a protonospheric heat source (Mahajan and Brace 1969). Despite a wide range of magnetic activity (14 5 AE 5 498) the maximum electron temperature was 2.3O~t1.4” equatorward of the position where the ring current distribution changed from isotropic to trapped. We conclude that the Injun 5 observations near 2500 km strongly support the ring current heating hypothesis. The present data do not allow us to determine whether the transfer of energy occurs by way of wave-particle or Coulomb interactions. Although the observations down to an altitude of 1300 km continue to support this conclusion, this interpretation is not so clearly evident at lower altitudes.

1183

(1) Below 1300 km elevated electrons temperatures in the trough are not consistent features of the observations. (2) The separation between A, and AT increases with decreasing altitude. It may be asked why the ring current can produce clearly discernable heating effects below 1300 km at some but not all times. No simple connections between the presence or absence of a maximum T in the trough and the strength of the ring current, as evidenced by AE, could be established. Elevated electron temperature appeared in the trough during both quiet and disturbed times (e.g. orbits No. 11320 and No. 11401, Fig. 4. Most of the time when a T maximum was absent in the trough AE was (100. There were however, five instances of no maximum with AE between 105 and 190. Between 2500 km and 1000 km the average value of (A+ -A,) increased from 2.3” to 5.7”. We believe that there are two possible ways by which this observation can be reconciled with a ring current heating model. In Fig. 7 we have plotted the electric field observations made during orbit No. 11380 as a function of invariant latitude. A grey scale representation of LEPEDEA proton observations as well as N and T are provided for reference. The dashed line represents the Vx B electric field due to satellite motion. Departures from this line are due to the component of the electric field along the dipole antenna. There is a westward component

REV. 11380

600

55’ INVARIANT

FIG. 7.

kOT

FUNCTION

4

OF ONE

COMPONFSVT

OF INVARIANT

OF THE

LATITUDE;

THE

ELECTRIC

SW

450

LATITUDE

FIELD

MEASURED

LEPEDEA AM3SEA

DATA

DURING

ORBIT

ARE GIVEN

No. 11380 As A

FOR REFERENCE.

W. J. BURKE et al.

1184

EQUATORIAL

PLANE

E sooo -*

L

A+-4

A++4

A,

INVARIANT

LATITUDE

FIG. 8. SIMPLIFIED MODELOFELECTRON HEATCONDUCTION PATHFROMMAGNETOSPHERE TO 1000 km IN THE PRESENCE

The average value of (Am-A,)

OF A WESTWARD

ELECTRIC

FIELD.

at 2500,140O and 1000 km are marked by * symbol.

of the electric field that extends from A=62” to 56”. In the magnetotail the E x B drift has an earthward component and in the ionosphere an equatorward component. We next consider the consequences of this drift on the distribution of electron temperatures in the topside ionosphere at trough latitudes. The situation is shown schematically in Fig. 8. Magnetic field lines are represented by straight lines extending from the ground (at aurora1 and subauroral latitudes) to the equatorial plane. In the ionosphere the electric field is westward and the E x B drift equatorward. Poleward of the field line at A, the ring current particle distribution is assumed to be nearly isotropic. The hatched marked box represents the region of resonant pitch angle scattering and subsequent electron heating The * symbols at A, -2.3, Am-3.8 and A, - 5.7 represent the average positions of AT measured near 2500, 1400 and 1000 km. The observations suggest that between the time that energy was given to the thermal electrons near the equatorial plane and when it was conducted to 2500 km the field line had drifted equatorward by -2.3” and 5.7” when the energy reached 1000 km. A drift of 1” in

latitude requires -4min if the westward electric field component at 1000 km is 10 mv-‘m. It would thus require -8 min for the energy to conduct from the equatorial plane to 2500 km. About 12 min would be required to conduct energy from 2500 to 1000 km. It is beyond the scope of this study to

RG. 9. DIWLE FIELD The

LINES CORRESPONDING

TO INVARIANT

62” AND58.6” (TOSCALE). vector connecting the two lines at altitudes of 2500 km and 1000 km are indicated. LATlTUDES

Ring current

and trough

latitude

explain why these transport times are required. The time required for thermal energy to flow between 2500 and 1000 km can be estimated using average, measured thermal energy fluxes and plasma densities. The lapse time estimated by such a calculation is - 10 s, rather than 12 min. Another possible explanation is that the observed temperature maxima result from local heating of the ionosphere by damping waves. In this model, large amplitude waves are generated in the magnetosphere and propagate largely unattenuated to the topside ionosphere. Ih the ionosphere the waves encounter a steep gradient in the index of refraction. In accordance with Snell’s law the wave vectors should bend away from the direction of the field lines toward that of the index of refraction gradient. In the ionosphere the index of refraction increases with increasing plasma density, that is, with decreasing latitude. The waves should bend toward the radially downward direction, heating the ambient electrons via the Landau damping. This model is illustrated in Fig. 9 where we have sketched the dipole field lines corresponding to invariant latitudes 62” and 58”. The vector connecting these two field lines at altitudes of 2500 km and 1000 km to indicate the path of wave propagation required for the ATvs altitude profiles given in Table 1. We note that this line is close to the radial downward direction, as expected from Snell’s law. If this hypothesis is correct it might also be invoked to explain the frequent lack of a maximum T below 1300 km. During relatively quiet times the amplitude of waves generated near the inner edge of the plasma sheet could be relatively small so that they have been completely damped by the time that they reach an altitude of 1300 km. Acknowledgements-We wish to thank Drs. L. A. Frank and D. A. Gurnett of the U. of Iowa for allowing access to LEPEDEA and double-probe measurements. Special thanks is extended to W. P. Sullivan of AFGL for his role in the development of the spherical electrostatic analyzers. This work was supported in part by Air Force contracts F19628-77-C-0122. REFERENCES

Brace, L. H. and Theis, R. F. (1974). The behavior

of the plasmapause at midlatitudes: ISIS I Langmuir probe measurements. J. geophys. Res. 79, 1871. Burke, W. J., Donatelli, D. E. and Sagalyn, R. C. (1978). Injun 5 observations of low-energy plasma in the high latitude topside ionosphere. J. geophys. Res. 83,2047. Cauffman, D. P. and Gurnett, D. A. (1971). Doubleprobe measurements of convection electric fields with the Injun 5 satellite. J. geophys. Res. 76, 6014.

electron

temperature

1185

Cole, K. D. (1975). Coulomb collisions of ring current particles-indirect source of heat for the ionosphere. Goddard Space Flight Center, Rep. X621-75-108 Greenbelt, MD. Cornwall, J. M., Coroniti, F. V. and Thorne, R. M. (1971). Unified theory of SAR arc formation at the plasmapause. J. geophys. Res. 76, 4428. DeForest, S. E. and McIlwain, C. E. (1971). Plasma clouds in the magnetosphere. J. geophys. Res. 76, 3587. Frank, L. A. (1971). Relationship of the plasma sheet, ring current, trapping boundary and plasmapause near the magnetic equator and local midnight. J. geophys. Res. 76, 2265. Frank, L. A. and Ackerson, K. L. (1971). Observations of charged particle precipitation in the aurora1 zone. J. geophys. Res. 76, 3612. Frank, L. A. and Ackerson, K. L. (1972). Local time survey at low altitudes over the aurora1 zone. J. geophys. Res. 77,4116. Hanson, W. B. (1963). Electron temperatures in the upper atmosphere. Space Res. 3, 282. Hultquist, B., Riedler, W. and Borg, H. (1976). Ring current protons on the upper atmosphere within the plasmapause. Planet. Space Sci. 24, 783. Kamide, Y. and Winningham, J. D. (1977). A statistical study of the “instantaneous” nightside aurora1 oval: the equatorward boundary of electron precipitation as observed by the ISIS I and II satellites. J. geophys. Res. 82, 5573. Kennel, C. F. and Petschek, H. E. (1966). Limit on stably trapped particle fluxes. J. geophys Res. 71, 1. Mahajan, K. K. and Brace, L. H. (1969). Latitudinal observations of the thermal balance in the night-time protonosphere. J. geophys. Res. 74, 5099. McIlwain, C. E. (1975). Plasma convection in the vicinity of the geosynchronous orbit, in Earth’s Magnetospheric Processes (Ed. B. M. McCormac), pp. 268-279. Reidel, Dordrecht. Rao, L. D. V., Burke, W. J., Kanal, M. and Sagalyn, R. C. (1978). Injun 5 low-energy plasma observations during a major magnetic storm. J. geophys. Res. 83, 3217. Sagalyn, R. C. and Burke, W. J. (1977). Injun 5 observations of vehicle potential fluctuations at 2500 km. Proc. of the Spacecraft Charging Technology Conference (Eds. C. P. Pike and R. R. Lovell), AFGL-TX-77-0051. S&ass, F. and Berg, L. E. (1974). Correlated satellite measurements of proton precipitation and plasma density. J. geophys. Res. 79, 5171. Spitzer, L. (1962). Physics of Fully Ionized Gases, 2nd edition, p. 144. Interscience, New York. Taylor, H. A. (1972). The light ion trough. Planet. Space Sci. 20, 1593. Titheridge, J. E. (1976a). Plasmapause effects in the topside ionosphere. J. geophys. Rks. 81,3227. Titheridge, J. E. (1976b). Plasma temneratures from Alouejte I: electron density profiles. Planet. Space Sci. 24, 247. Williams, D. J. and Lyons, L. E. (1974). The proton ring current and its interaction with the plasmasphere. J. geophys. Res. 79, 4195. Williams, D. J., Hernandey, G. and Lyons, L. R. (1976). Simultaneous observations of the proton ring current and stable aurora1 red arcs. J. geophys. Res. 81,608.