- Email: [email protected]
Temperature variation of the plasma sheet during substorms
TEMPERATURE
VARIATION OF THE PLASMA DURING SU~ST~RMS
SHEET
A. T. Y. LUI and C.-L MENG
The Johns Hopkins University,
Applied Physics Laboratory, Laurel,
MD 20810, U.S.A.
and
Department
of
t. A. FRANK and K. L. ACKERSON Physics and Astronomy, University of IA 52242, U.S.A.
Iowa, Iowa City,
and
S.-I. AKASO~ ~eophys~caj fnstitute, Uni~ersj~~ of Alaska, ~a~rb~nks~ AK 99701, U.S.A.
Abstract-The temperature and density of the plasma in the Earth’s distant plasma sheet al the downstream distances of about 20_25R, are examined during a high geomagnetic disturbance period. It is shown that the plasma sheet cools when magnetospheric substorm expansion is indicated by the AE index. During cooling, the plasma sheet temperature, T, and the number density, N, are related by T a N*” (adiabatic process) in some instances, while by 7’ a N-’ (isobaric process) in other cases. The total plasma and magnetic pressure decreases when T a Nz” and increases when T = N-‘. Observation also indicates that the dawn-dusk component of plasma flow is frequently large and comparable to the sunward-tailward ffow component near the central plasma sheet during substorms.
~ons~erabIe efforts have been made in the past to investigate the morp~o~ogi~a1 nature of the Earth’s plasma sheet in the downstream distances of about 15-40 & Several dynamic features of this portion of the plasma sheet during m~~netospheric substorms, such as its configuratiaual changes, ptasma flow patterns, magnetic field var~tions, and energetic particle anisotropies have been extensively examined and reported in the literature (Hones et al., 1975; Frank et of., 1976; Akasofu, 1978; Coroniti d al., 1978; Frank and Ackerson, 1975; M~P~erron~ 1979; Lui, 1979: Krimigis and Sarris, 1979). On the other hand, the variation of the pfasma sheet temperature during substorms has not been studied to the same extent, partly due to the limited reports in which the plasma sheet at these downstream distances are monitored throughout a substorm interval. The scarcity of reports is due largely to the occurrence of plasma sheet thinning during substorm expansion which often leads to a spacecraft exitting from the plasma sheet into the tail lobe near the peak of the substorm activity. Brief glimpses of the thinned plasma sheet has been reported for a few cases from Veia observations, but it is difficult to determine accurately the temperature variation from these previous observations because the average proton energy,
presented in these and other subsequent reports, includes both the thermal energy and the bulk flow energy and is not read@ identifiable with the plasma temperature. From the proton velocity distribution function, Frank et al. (1976f have noted that the temperature associated with the magnetotail fireball events is lower than the typical plasma sheet temperature. Their result leads to an interesting question. How does the plasma sheet temperature at the downstream distances beyond about 15 R, vary as a function of time during magnetospheric substorms? This study is an attempt to supprement our plasma sheet observations in this aspect by examining an interval in which the IMP-6 spacecraft remains embedded within the plasma sheet throughout a period of more than one substorm. 2. INSTRUMENT DESCRIPTIONS Plasma abservations at IMP-6 are made with the low energy proton and electron differential electrostatic analyzer (Cepedea) from the University of Iowa {Frank, 1967). This instrument measures the directional differential fluxes of positive ions and electrons over the energy range between 50 eV and 40 keV in I5 energy steps. The fuii-wjdth-halfmaximum (FWHM~ field of view is -8” x 30”. with the larger angular dimension paratfel to the spin axis. The macroscopic plasma parameters are 837
838
A. T. Y. Lul et
derived from 10 instrument cycles of Lepedea in order to achieve a good angular determination of the particle velocity distribution function. This results in a temporal resolution of -7 min.
al.
- 24 Re. Figure I shows plasma observation during this 8-h interval, together with the aurora1 electrojet (AE) index in the top panel of the figure. There are two large substorms with peak AE values larger than 1000 nT between 1500 and 1800 U.T., followed by a smaller substorm with AE index less than 500nT between 1800 and 2000 U.T. Examination of individual ground magnetograms indicates that the magnetic disturbances for these substorms are quite widespread. Disturbances are recorded at least from -2000 M.L.T.
3. OBSERVATIONS
The period in this study is taken from the IMP-6 outbound pass of Orbit 224 on 24 September 1973 between 1400 and 2200 U.T. The spacecraft is located very near the midnight meridian (I Y,, 1s 2.5 R,) at the downstream distances of X = - 19 to
1973 SEPT 24 1500(nT)
AEOI)
INDEX I
IOOO-
1 IMP-6
> > n
UT
5001
14
xgy -19.1
LEPEDEA
I
I
I
t
ti
I
I5
16
I7
I8
I9
20
-20.7 l-2 5.1
I
21
I 22
-22%2
-23.5
-24.7
0.0
-0.9
-1-7
5.0
4.6
4.2
FIG. 1. THE AE(l1) INDEXAND PLASMAMEASUREMENTS FROMLEPEDEAON IMP-6 FORTHE INTERVAL ON 24 SEPTEMBER1973. Positive V, denotes flow component towards the sun, and positive V, denotes the flow component towards dusk. The spacecraft lies within the plasma sheet throughout the three substorms in this
interval and plasma flows with sunward component is predominantly detected during these substorm expansions. Plasma flows with large dawnward components are also detected.
839
Temperature variation of the plasma sheet during substorms
(Dixon Island) to -0800 M.L.T. (Fort Churchill) for the first substorm, from -2100 M.L.T. (Dixon Island) to -0600 M.L.T. (College) for the second substorm, and from - 1800 M.L.T. (Narssarssuaq) to -0200 M.L.T. (Cape Chelyuskin) for the third one. Therefore, under any reasonable magnetic field model, the spacecraft is within the M.L.T. sector of substorm disturbance in the magnetotail during these substorms. Below the AE index in Fig. 1 are the plasma measurements from Lepedea. The number density measured during this interval is generally above 0.1 cmm3. This value may be compared with the number density of -0.005 cmm3 or less in the tail lobe, e.g. as observed at - 2200 U.T. Two noticeable decreases in density occur in association with the peaks of the two substorms between 1500 and 1800U.T. The lowest densities observed in the first and second decreases are 0.04 and 0.02cmm3, respectively. These density decreases are indicative of plasma sheet thinning. In spite of thinning, the observed plasma density is still well above the plasma density in the tail lobe, indicating that the IMP-6 spacecraft has not left the plasma sheet. The average proton energies show variations in association with the AE index. Noticeable decreases in the proton average energy occur near the peaks of the AE index, in the intervals - 1700-1730 U.T., - 1845-1920 U.T., and -2010-2200 U.T. During the entire interval of the first large substorm (- 1520-1620 U.T.), plasma flow is observed to have sunward components of 200-6OOkms-‘. In the second substorm intervai, the observed flow has a sunward component for an interval of more than 1 h during the period when the substorm was intensified. Small tailward flow at speeds of less than 150 kms-’ is also observed for a brief interval of less than 1.5min. Earthward flow is again detected during the expansion phase of the third substorm. There is an extended period of large tailward flow between 1924 and 2026U.T. This interval of tailward flow occurs during decreasing AE index and thus corresponds to the substorm recovery phase. A noteworthy feature in the plasma flow is that the dawn-dusk flow component is comparable with or even larger than the sunward-tailward flow component during the interval of 1400-1700 U.T. 4. SUBSTORM VARIATION
fivj m-62
24 September.1973
I
10-1'
I 10-12
L.AL~~ -3x 106 -2 x 106 -1 x 106
0
1x106
2x106
3x106
V Ims-'I
FIG. 2. COMFARISON OF THE PROTON VELOCITY DISTRIBUTIONS NEAR EARLY SUBSTORM EXPANSION AND SUBSTORM MAXIMUMEPOCH FORTHREESUBSTORMSON 24 SEPTEMBER 1973. Cooling of the plasma sheet during substorm expansion is indicated.
shows the proton velocity distributions during three substorms on 24 September 1973. The distributions in solid lines are taken at times near the onset of AE enhancement for the three substorms on that day, while the distributions in dashed lines are taken near the times of peak AE index for these substorms. ft is clear that the velocity distributions in solid lines have larger spread than the corresponding distributions in dashed lines, indicating the rather unexpected result that the plasma temperature is lower near the substorm peak than near the onset of AE enhancement. In a more quantitative manner, one may define a measure of the plasma temperature by kT=$$
Kux - uxf2 -I-(u, -
u,Yl
OF PLASMA
SHEET TEMPERATURE
The temperature variation of the plasma sheet during substorms is examined in Fig. 2, which
where N is the proton number density, m,, is the proton mass, (uX, u,) is the proton bulk flow velo-
A. T. Y. LUI et al.
840
city, and f(uX, 0,) is the distribution function. The values of kT computed numerically from Lepedea measurements for these distributions are shown under the time labels. It can be seen that the kT values are indeed in agreement with the result arrived by simply judging the spread of the distribution function. In order to understand better the physical processes which cause the thinning of the plasma sheet, it is important to examine whether the observed cooling is related to an adiabatic process or to an isobaric process. The relationship between the number density N and temperature T for an adiabatic process is given by
1
15:25
lO.O15:39
1.0
s :
10.0
\
11
T-N-’ 16:20
/
4 -r
k
1.0
N’-‘T
= constant
T=N-’ ‘\
where y is the ratio of specific heats. For an isobaric process, the relationship is given by
10.0
18:14
NT = constant.
Figure 3 plots the variations of N and T during the substorm expansions in Fig. 2. In addition, we have added observations during three other substorm expansions on 25 September 1973 when the spacecraft encounters the neutral sheet several times, as indicated by the onboard magnetometer (Lui, 1979). Solid lines are drawn between successive points to indicate the sequence of temporal variations. Times at the start and end of the sequence are also shown. It is readily seen that in association with cooling, N increases in some cases and decreases in others. For the first two substorms on 24 September 1973 and the third substorm on 25 September 1973, T varies approximately as N2’3 (shown by the dashed lines) which is consistent with an adiabatic process. For the other cases, T varies approximately as N-’ (shown also by the dashed lines), indicating an isobaric process. This analysis suggests that cooling of the plasma sheet can be achieved by both the isobaric and the adiabatic processes. If one considers the total pressure, i.e. plasma and magnetic pressure (BX2/2pO),for the events in Fig. 3, it is consistently found that when T 3~N-‘, the total pressure increases, whereas when T m N2’3, the total pressure decreases. It may be mentioned that the measured plasma temperature decreases regardless of an increase or decrease of the measured density. Since the number density can be regarded as a measure of the proximity to the neutral sheet, the observation indicates that plasma sheet cooling occurs
N (cm-31
FIG. 3. A PLOT TOSHOWTHERELATIONSHIP BETWEENTHE PLASMASHEETTEMPERATURE T ANDTHEPLASMANUMBER DENSITY N DURING THE INTERVALS OF PLASMA SHEET COOLINGON 24 SEPTEMBER 1973 AND 25 SEPTEMBER 1973.
The events on the former and latter dates are shown at the left and right columns, respectively. throughout the plasma sheet and that the observed variation in temperature is indeed a temporal effect rather than a spatial effect. Figure 4 shows the variation of N and T during the recovery phase of those substorms studied in Fig. 3. Again, solid lines are drawn between successive points to indicate the sequence of temporal variations. It is seen that the temperature returns closely to the pre-expansion phase values, sometimes to a higher temperature, e.g. the third substorm on 24 September 1973, and sometimes to a lower one, e.g. the third substorm on 25 September 1973. Unlike the situation in Fig. 3, the variation of N and T in Fig. 4 deviates considerably from a simple adiabatic process or a simple isobaric process, possibly suggesting that plasma detected during plasma sheet expansion may have originated from a source which is associated with vastly different number density and temperature from that of the thinned plasma sheet. 5. DISCUSSIONS
In analyzing the physical process associated with the plasma sheet cooling, an important aspect
Temperature
16:33
16:33 7 16:M
15:39
!
1.0I10.0
1754
-i y21:01
841
later substorm phase when the thinned plasma sheet expands. Substantial delays (> 1 h) in the heating of the plasma sheet at the lunar distance has been noted previously (Rich el at., 1974; Hardy ef al., 1979). Also consistent with this finding is the report by Meng and Akasofu (1971) that there is 15-113 min delay in the enhancement of energetic electron flux near the neutral sheet after the onset of magnetospheric substorms.
73 iieptember 25
73 September'24 10.0
variation of the plasma sheet during substorms
r
j
FIG. 4. A
PLOT TO SHOW THE RECOVERY OF THE PLASMA SHEETTEMPERAT~REA~ERTHEPER~ODSOFFLASMASHEET COOLING IN FIG. 3.
of the substorm dynamics is revealed. The behavior of the distant (X,, - - 20 R,) thinned plasma sheet is governed by more than one physical process. This result is likely to be essentiat in comprehending the complex variability of plasma sheet dynamics during quiet and substorm intervals. In addition, plasma sheet cooling observed during substorm expansion in this study is not consistent with substorm models which invoke the tearing mode instability or of any model proposing single or multiple magnetic field merging regions in the plasma sheet; according to recent numerical simulations (e.g. Sato, 1979; Cheng, 1979; Birn, 1980), such a process will invariably heat the plasma. It therefore appears that future modelling of the substorm process needs to account for the apparent cooling of the distant plasma sheet in the early substorm expansion phase. In addition, the observation that the dawn-dusk component of plasma flow near the central plasma sheet is frequently large and comparable to the sunwardtailward flow component during substorms needs future understanding. If substorm energy is derived from processes in the magnetotail, then the present observation indicates that the substorm process does not heat the distant plasma sheet initially. The heating of the distant plasma sheet occurs only during the
Acknowledgements-This work is supported in part by the Atmospheric Sciences Section of the National Science Foundation Grants ATM 79-25987 to the Applied Physics Laboratory, and ATM 77-26522, to the Geophysical Institute, University of Alaska. One of us (C.I.M.) is supported by the Atmospheric Sciences Section of the National Science Foundation Grant ATM 79-23240 and the Air Force Office of Scientific Research, Air Force Systems Command, under Grant 79-0010 to The John Hopkins University. The research at the University of Iowa was supported in part by the National Aeronautics and Space Administration under Contract NASS-11039 and Grant NGL-16-001-002.
REFERENCES Akasofu, S.-I. (1978). The interaction between a magnetized plasma flow and a magnetized cellestial body: A review of magnetospheric studies. Space Sci. Rev. 21, 489. Birn, J. (1980). Computer studies of the dynamic evolution of the geomagnetic tail. J; geophys. Res. 85, 1214. Cheng. A. F. (1979). Unsteady magnetic merging in one dimension. I; geophys. Res. 84, 2129. Coroniti, F. F., Frank, L. A., Lepping, R. P., Scarf, F. L. and Ackerson, K. L. (1978). Plasma flow pulsations in Earth’s magnetic tail. J. geophys. Res. 83, 2162. Frank, L. A. (1967). Initial observations of low-energy electrons in the earth’s magnetosphere with Ogo 3. f. geap~ys. Res. 72, 185. Frank, L. A. and Ackerson, K. L. (1976). Examples of plasma flows within the earth’s magnetosphere. In Mannetosuheric Particles and Fields (Ed. B. M. McCormac), p. 29. D. Reidel, MA. Frank, L. A., Ackerson, K. L. and Lepping, R. P. (1976). On the hot tenuous plasmas, fireballs, and boundary layers in the Earth’s magnetotail. J. geophys. Res. 81, 5859. Hardy, D. A., Reiff, P. H. and Burke, W. J. (1979). Response of magnetotail plasma at lunar distance to changes in the interplanetary magnetic field, the solar wind plasma, and substorm activity. J. geophys. Res. 84, 1382. Hones, E. W., Bame, S. J. and Asbridge, J. R. (1976). Proton flow measurements in the magnetotail plasma sheet made with IMP-6. J. geophys. Res. 81, 227. Krimigis, S. M. and Sarris, E. T. (1979). Energetic particle bursts in the earth’s magnetotail in Dynamics of the Magnetosphere (Ed. S.-I. Akasofu), p. 599. Lui, A. T. Y. (1979). Observations on plasma sheet dynamics during magnetospheric substorms. In Dynamics of the Magnetosphere (Ed. S.-I. Akasofu), p. 563.
842
McPherron,
A. T. Y. LUI et al. R. L. (1977). Magnetospheric
substorms.
Rev. geophys. Space Phys. 17,657.
Meng, C.-I. and Akasofu, S.-I. (1971). Magnetospheric substorm observations near the neutral sheet. J. geophys. Res. 76.4679. Rich, F. J., Reasoner, D. L.. Burke, W. J. and Hones, E.
W. Jr. (1974). Plasma sheet at lunar distance during magnetospheric substorms. J. geophys. Rex 79, 1981. Sato, T. (1979). Strong plasma acceleration by slow shocks resulting from magnetic reconnection. J. geophys. Rex 84, 7177.
VARIATION OF THE PLASMA DURING SU~ST~RMS
SHEET
A. T. Y. LUI and C.-L MENG
The Johns Hopkins University,
Applied Physics Laboratory, Laurel,
MD 20810, U.S.A.
and
Department
of
t. A. FRANK and K. L. ACKERSON Physics and Astronomy, University of IA 52242, U.S.A.
Iowa, Iowa City,
and
S.-I. AKASO~ ~eophys~caj fnstitute, Uni~ersj~~ of Alaska, ~a~rb~nks~ AK 99701, U.S.A.
Abstract-The temperature and density of the plasma in the Earth’s distant plasma sheet al the downstream distances of about 20_25R, are examined during a high geomagnetic disturbance period. It is shown that the plasma sheet cools when magnetospheric substorm expansion is indicated by the AE index. During cooling, the plasma sheet temperature, T, and the number density, N, are related by T a N*” (adiabatic process) in some instances, while by 7’ a N-’ (isobaric process) in other cases. The total plasma and magnetic pressure decreases when T a Nz” and increases when T = N-‘. Observation also indicates that the dawn-dusk component of plasma flow is frequently large and comparable to the sunward-tailward ffow component near the central plasma sheet during substorms.
~ons~erabIe efforts have been made in the past to investigate the morp~o~ogi~a1 nature of the Earth’s plasma sheet in the downstream distances of about 15-40 & Several dynamic features of this portion of the plasma sheet during m~~netospheric substorms, such as its configuratiaual changes, ptasma flow patterns, magnetic field var~tions, and energetic particle anisotropies have been extensively examined and reported in the literature (Hones et al., 1975; Frank et of., 1976; Akasofu, 1978; Coroniti d al., 1978; Frank and Ackerson, 1975; M~P~erron~ 1979; Lui, 1979: Krimigis and Sarris, 1979). On the other hand, the variation of the pfasma sheet temperature during substorms has not been studied to the same extent, partly due to the limited reports in which the plasma sheet at these downstream distances are monitored throughout a substorm interval. The scarcity of reports is due largely to the occurrence of plasma sheet thinning during substorm expansion which often leads to a spacecraft exitting from the plasma sheet into the tail lobe near the peak of the substorm activity. Brief glimpses of the thinned plasma sheet has been reported for a few cases from Veia observations, but it is difficult to determine accurately the temperature variation from these previous observations because the average proton energy,
presented in these and other subsequent reports, includes both the thermal energy and the bulk flow energy and is not read@ identifiable with the plasma temperature. From the proton velocity distribution function, Frank et al. (1976f have noted that the temperature associated with the magnetotail fireball events is lower than the typical plasma sheet temperature. Their result leads to an interesting question. How does the plasma sheet temperature at the downstream distances beyond about 15 R, vary as a function of time during magnetospheric substorms? This study is an attempt to supprement our plasma sheet observations in this aspect by examining an interval in which the IMP-6 spacecraft remains embedded within the plasma sheet throughout a period of more than one substorm. 2. INSTRUMENT DESCRIPTIONS Plasma abservations at IMP-6 are made with the low energy proton and electron differential electrostatic analyzer (Cepedea) from the University of Iowa {Frank, 1967). This instrument measures the directional differential fluxes of positive ions and electrons over the energy range between 50 eV and 40 keV in I5 energy steps. The fuii-wjdth-halfmaximum (FWHM~ field of view is -8” x 30”. with the larger angular dimension paratfel to the spin axis. The macroscopic plasma parameters are 837
838
A. T. Y. Lul et
derived from 10 instrument cycles of Lepedea in order to achieve a good angular determination of the particle velocity distribution function. This results in a temporal resolution of -7 min.
al.
- 24 Re. Figure I shows plasma observation during this 8-h interval, together with the aurora1 electrojet (AE) index in the top panel of the figure. There are two large substorms with peak AE values larger than 1000 nT between 1500 and 1800 U.T., followed by a smaller substorm with AE index less than 500nT between 1800 and 2000 U.T. Examination of individual ground magnetograms indicates that the magnetic disturbances for these substorms are quite widespread. Disturbances are recorded at least from -2000 M.L.T.
3. OBSERVATIONS
The period in this study is taken from the IMP-6 outbound pass of Orbit 224 on 24 September 1973 between 1400 and 2200 U.T. The spacecraft is located very near the midnight meridian (I Y,, 1s 2.5 R,) at the downstream distances of X = - 19 to
1973 SEPT 24 1500(nT)
AEOI)
INDEX I
IOOO-
1 IMP-6
> > n
UT
5001
14
xgy -19.1
LEPEDEA
I
I
I
t
ti
I
I5
16
I7
I8
I9
20
-20.7 l-2 5.1
I
21
I 22
-22%2
-23.5
-24.7
0.0
-0.9
-1-7
5.0
4.6
4.2
FIG. 1. THE AE(l1) INDEXAND PLASMAMEASUREMENTS FROMLEPEDEAON IMP-6 FORTHE INTERVAL ON 24 SEPTEMBER1973. Positive V, denotes flow component towards the sun, and positive V, denotes the flow component towards dusk. The spacecraft lies within the plasma sheet throughout the three substorms in this
interval and plasma flows with sunward component is predominantly detected during these substorm expansions. Plasma flows with large dawnward components are also detected.
839
Temperature variation of the plasma sheet during substorms
(Dixon Island) to -0800 M.L.T. (Fort Churchill) for the first substorm, from -2100 M.L.T. (Dixon Island) to -0600 M.L.T. (College) for the second substorm, and from - 1800 M.L.T. (Narssarssuaq) to -0200 M.L.T. (Cape Chelyuskin) for the third one. Therefore, under any reasonable magnetic field model, the spacecraft is within the M.L.T. sector of substorm disturbance in the magnetotail during these substorms. Below the AE index in Fig. 1 are the plasma measurements from Lepedea. The number density measured during this interval is generally above 0.1 cmm3. This value may be compared with the number density of -0.005 cmm3 or less in the tail lobe, e.g. as observed at - 2200 U.T. Two noticeable decreases in density occur in association with the peaks of the two substorms between 1500 and 1800U.T. The lowest densities observed in the first and second decreases are 0.04 and 0.02cmm3, respectively. These density decreases are indicative of plasma sheet thinning. In spite of thinning, the observed plasma density is still well above the plasma density in the tail lobe, indicating that the IMP-6 spacecraft has not left the plasma sheet. The average proton energies show variations in association with the AE index. Noticeable decreases in the proton average energy occur near the peaks of the AE index, in the intervals - 1700-1730 U.T., - 1845-1920 U.T., and -2010-2200 U.T. During the entire interval of the first large substorm (- 1520-1620 U.T.), plasma flow is observed to have sunward components of 200-6OOkms-‘. In the second substorm intervai, the observed flow has a sunward component for an interval of more than 1 h during the period when the substorm was intensified. Small tailward flow at speeds of less than 150 kms-’ is also observed for a brief interval of less than 1.5min. Earthward flow is again detected during the expansion phase of the third substorm. There is an extended period of large tailward flow between 1924 and 2026U.T. This interval of tailward flow occurs during decreasing AE index and thus corresponds to the substorm recovery phase. A noteworthy feature in the plasma flow is that the dawn-dusk flow component is comparable with or even larger than the sunward-tailward flow component during the interval of 1400-1700 U.T. 4. SUBSTORM VARIATION
fivj m-62
24 September.1973
I
10-1'
I 10-12
L.AL~~ -3x 106 -2 x 106 -1 x 106
0
1x106
2x106
3x106
V Ims-'I
FIG. 2. COMFARISON OF THE PROTON VELOCITY DISTRIBUTIONS NEAR EARLY SUBSTORM EXPANSION AND SUBSTORM MAXIMUMEPOCH FORTHREESUBSTORMSON 24 SEPTEMBER 1973. Cooling of the plasma sheet during substorm expansion is indicated.
shows the proton velocity distributions during three substorms on 24 September 1973. The distributions in solid lines are taken at times near the onset of AE enhancement for the three substorms on that day, while the distributions in dashed lines are taken near the times of peak AE index for these substorms. ft is clear that the velocity distributions in solid lines have larger spread than the corresponding distributions in dashed lines, indicating the rather unexpected result that the plasma temperature is lower near the substorm peak than near the onset of AE enhancement. In a more quantitative manner, one may define a measure of the plasma temperature by kT=$$
Kux - uxf2 -I-(u, -
u,Yl
OF PLASMA
SHEET TEMPERATURE
The temperature variation of the plasma sheet during substorms is examined in Fig. 2, which
where N is the proton number density, m,, is the proton mass, (uX, u,) is the proton bulk flow velo-
A. T. Y. LUI et al.
840
city, and f(uX, 0,) is the distribution function. The values of kT computed numerically from Lepedea measurements for these distributions are shown under the time labels. It can be seen that the kT values are indeed in agreement with the result arrived by simply judging the spread of the distribution function. In order to understand better the physical processes which cause the thinning of the plasma sheet, it is important to examine whether the observed cooling is related to an adiabatic process or to an isobaric process. The relationship between the number density N and temperature T for an adiabatic process is given by
1
15:25
lO.O15:39
1.0
s :
10.0
\
11
T-N-’ 16:20
/
4 -r
k
1.0
N’-‘T
= constant
T=N-’ ‘\
where y is the ratio of specific heats. For an isobaric process, the relationship is given by
10.0
18:14
NT = constant.
Figure 3 plots the variations of N and T during the substorm expansions in Fig. 2. In addition, we have added observations during three other substorm expansions on 25 September 1973 when the spacecraft encounters the neutral sheet several times, as indicated by the onboard magnetometer (Lui, 1979). Solid lines are drawn between successive points to indicate the sequence of temporal variations. Times at the start and end of the sequence are also shown. It is readily seen that in association with cooling, N increases in some cases and decreases in others. For the first two substorms on 24 September 1973 and the third substorm on 25 September 1973, T varies approximately as N2’3 (shown by the dashed lines) which is consistent with an adiabatic process. For the other cases, T varies approximately as N-’ (shown also by the dashed lines), indicating an isobaric process. This analysis suggests that cooling of the plasma sheet can be achieved by both the isobaric and the adiabatic processes. If one considers the total pressure, i.e. plasma and magnetic pressure (BX2/2pO),for the events in Fig. 3, it is consistently found that when T 3~N-‘, the total pressure increases, whereas when T m N2’3, the total pressure decreases. It may be mentioned that the measured plasma temperature decreases regardless of an increase or decrease of the measured density. Since the number density can be regarded as a measure of the proximity to the neutral sheet, the observation indicates that plasma sheet cooling occurs
N (cm-31
FIG. 3. A PLOT TOSHOWTHERELATIONSHIP BETWEENTHE PLASMASHEETTEMPERATURE T ANDTHEPLASMANUMBER DENSITY N DURING THE INTERVALS OF PLASMA SHEET COOLINGON 24 SEPTEMBER 1973 AND 25 SEPTEMBER 1973.
The events on the former and latter dates are shown at the left and right columns, respectively. throughout the plasma sheet and that the observed variation in temperature is indeed a temporal effect rather than a spatial effect. Figure 4 shows the variation of N and T during the recovery phase of those substorms studied in Fig. 3. Again, solid lines are drawn between successive points to indicate the sequence of temporal variations. It is seen that the temperature returns closely to the pre-expansion phase values, sometimes to a higher temperature, e.g. the third substorm on 24 September 1973, and sometimes to a lower one, e.g. the third substorm on 25 September 1973. Unlike the situation in Fig. 3, the variation of N and T in Fig. 4 deviates considerably from a simple adiabatic process or a simple isobaric process, possibly suggesting that plasma detected during plasma sheet expansion may have originated from a source which is associated with vastly different number density and temperature from that of the thinned plasma sheet. 5. DISCUSSIONS
In analyzing the physical process associated with the plasma sheet cooling, an important aspect
Temperature
16:33
16:33 7 16:M
15:39
!
1.0I10.0
1754
-i y21:01
841
later substorm phase when the thinned plasma sheet expands. Substantial delays (> 1 h) in the heating of the plasma sheet at the lunar distance has been noted previously (Rich el at., 1974; Hardy ef al., 1979). Also consistent with this finding is the report by Meng and Akasofu (1971) that there is 15-113 min delay in the enhancement of energetic electron flux near the neutral sheet after the onset of magnetospheric substorms.
73 iieptember 25
73 September'24 10.0
variation of the plasma sheet during substorms
r
j
FIG. 4. A
PLOT TO SHOW THE RECOVERY OF THE PLASMA SHEETTEMPERAT~REA~ERTHEPER~ODSOFFLASMASHEET COOLING IN FIG. 3.
of the substorm dynamics is revealed. The behavior of the distant (X,, - - 20 R,) thinned plasma sheet is governed by more than one physical process. This result is likely to be essentiat in comprehending the complex variability of plasma sheet dynamics during quiet and substorm intervals. In addition, plasma sheet cooling observed during substorm expansion in this study is not consistent with substorm models which invoke the tearing mode instability or of any model proposing single or multiple magnetic field merging regions in the plasma sheet; according to recent numerical simulations (e.g. Sato, 1979; Cheng, 1979; Birn, 1980), such a process will invariably heat the plasma. It therefore appears that future modelling of the substorm process needs to account for the apparent cooling of the distant plasma sheet in the early substorm expansion phase. In addition, the observation that the dawn-dusk component of plasma flow near the central plasma sheet is frequently large and comparable to the sunwardtailward flow component during substorms needs future understanding. If substorm energy is derived from processes in the magnetotail, then the present observation indicates that the substorm process does not heat the distant plasma sheet initially. The heating of the distant plasma sheet occurs only during the
Acknowledgements-This work is supported in part by the Atmospheric Sciences Section of the National Science Foundation Grants ATM 79-25987 to the Applied Physics Laboratory, and ATM 77-26522, to the Geophysical Institute, University of Alaska. One of us (C.I.M.) is supported by the Atmospheric Sciences Section of the National Science Foundation Grant ATM 79-23240 and the Air Force Office of Scientific Research, Air Force Systems Command, under Grant 79-0010 to The John Hopkins University. The research at the University of Iowa was supported in part by the National Aeronautics and Space Administration under Contract NASS-11039 and Grant NGL-16-001-002.
REFERENCES Akasofu, S.-I. (1978). The interaction between a magnetized plasma flow and a magnetized cellestial body: A review of magnetospheric studies. Space Sci. Rev. 21, 489. Birn, J. (1980). Computer studies of the dynamic evolution of the geomagnetic tail. J; geophys. Res. 85, 1214. Cheng. A. F. (1979). Unsteady magnetic merging in one dimension. I; geophys. Res. 84, 2129. Coroniti, F. F., Frank, L. A., Lepping, R. P., Scarf, F. L. and Ackerson, K. L. (1978). Plasma flow pulsations in Earth’s magnetic tail. J. geophys. Res. 83, 2162. Frank, L. A. (1967). Initial observations of low-energy electrons in the earth’s magnetosphere with Ogo 3. f. geap~ys. Res. 72, 185. Frank, L. A. and Ackerson, K. L. (1976). Examples of plasma flows within the earth’s magnetosphere. In Mannetosuheric Particles and Fields (Ed. B. M. McCormac), p. 29. D. Reidel, MA. Frank, L. A., Ackerson, K. L. and Lepping, R. P. (1976). On the hot tenuous plasmas, fireballs, and boundary layers in the Earth’s magnetotail. J. geophys. Res. 81, 5859. Hardy, D. A., Reiff, P. H. and Burke, W. J. (1979). Response of magnetotail plasma at lunar distance to changes in the interplanetary magnetic field, the solar wind plasma, and substorm activity. J. geophys. Res. 84, 1382. Hones, E. W., Bame, S. J. and Asbridge, J. R. (1976). Proton flow measurements in the magnetotail plasma sheet made with IMP-6. J. geophys. Res. 81, 227. Krimigis, S. M. and Sarris, E. T. (1979). Energetic particle bursts in the earth’s magnetotail in Dynamics of the Magnetosphere (Ed. S.-I. Akasofu), p. 599. Lui, A. T. Y. (1979). Observations on plasma sheet dynamics during magnetospheric substorms. In Dynamics of the Magnetosphere (Ed. S.-I. Akasofu), p. 563.
842
McPherron,
A. T. Y. LUI et al. R. L. (1977). Magnetospheric
substorms.
Rev. geophys. Space Phys. 17,657.
Meng, C.-I. and Akasofu, S.-I. (1971). Magnetospheric substorm observations near the neutral sheet. J. geophys. Res. 76.4679. Rich, F. J., Reasoner, D. L.. Burke, W. J. and Hones, E.
W. Jr. (1974). Plasma sheet at lunar distance during magnetospheric substorms. J. geophys. Rex 79, 1981. Sato, T. (1979). Strong plasma acceleration by slow shocks resulting from magnetic reconnection. J. geophys. Rex 84, 7177.