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Compositional changes, time and density variations in the magnetosphere associated with Birkeland currents and particle acceleration
Adv. Space Res. Vol. 6, No. 3, pp. 113—116, 1986 Printed in Great Britain. All rights reserved.
0273—1177/86 $0.00 + .50 Copyright © COSPAR
COMPOSITIONAL CHANGES, TIME AND DENSITY VARIATIONS IN THE MAGNETOSPHERE ASSOCIATED WITH BIRKELAND CURRENTS AND PARTICLE ACCELERATION E. Ungstrup,* W. J. Heikkila,** J. C. Foster*** and W. 0. Lennartsson~ *Danish Space Research Institute, DK-2800 Lyngby, Denmark * *Danish Space Research Institute and Center for Space Sciences, University of Texas at Dallas, Richardson, TX 75080, U.S.A. ***MIT Haystack Observatory, Westford, MA 01866, U.S.A. tLockheed Palo Alto Research Laboratory, 3251 Hanover St., Palo Alto, CA 94304, U.S.A. ABSTRACT Birkeland currents, parallel electric fields and plasma instabilities often occur together in time and space and play an important role in large scale plasma motions between the ionosphere and magnetosphere and along the magnetic field in the magnetosphere. The results of the plasma movements are large density and composition variations in the ionosphere and magnetosphere. Observations from ISIS-2 at 1400 km altitude show large densities with heavy ions dominating in regions with upward Birkeland currents, and low densities and light ions in regions with downward currents. Observations from ISEE-l in field aligned current regions at 10000 to 15000 km altitude show transverse heating of protons and oxygen ions to 250 eV. Because of the different mobility of the protons and oxygen ions the proton flow is important in the beginning of the events but later the outflow becomes almost pure oxygen. Similarly ISEE-l observations of outgoing field aligned ion beams at 10000 to 15000 km altitude show time variations in the N+/O+ ratios and a dominance of 0+ later in the events. INTRODUCTION Block and F~lthamar /1/ studied the effect of Birkeland currents on the structure of the ionosphere in 1968; they limited themselves to the steady state solution and to altitudes below ‘-~90O km. They concluded that the Birkeland currents would lead to a reduction in total electron content and in density if the charge carriers have to be produced in the ionosphere, but that particle precipitation would lead to an increase. They estimated the time constant to be of the order of three hours. We will show in this paper some satellite observations of field aligned ion flows when important density and compositional changes occur on time scales of minutes. Our observations are made during weakly disturbed or disturbed conditions and we do not have a steady state situation; we feel that the observations are closely connected to Birkeland currents and their effects on the ionosphere and magnetosphere. We believe that the ions are the determining factor in the density changes in the magnetosphere, and that the main role of the (thermal) electrons is to neutralize the magnetosphere except for the small residual net charge associated with the parallel electric fields. OBSERVATIONS In Figure 1 we show an observation from ISIS-2 during an evening pass (‘~2O.2 MLT) in local winter. In the upper panel we see the magnetic 2field signature of the Birkeland currents. The but in spite of that we see pronounced currents are very weak only about 0.2 pA/m effects in the density profiles and composition of the topside ionosphere. In the third panel we show the electron density profiles from 300 to 1400 km altitude observed by the topside sounder; it is clearly seen that the density is increased in the topside ionosphere in the region with outgoing Birkeland current and decreased in the region with inward Birkeland current. The upwelling in the outgoing Birkeland current region can be caused by heating of the ionosphere by precipitating particles or by (weak) upward parallel electric fields. In the lowest panel we see the composition measured at 1400 km and it is clear that oxygen is the dominating ion in the outgoing Birkeland current region. The density profiles show that there is no change in scale height in the ingoing Birkeland current region from the 113
E. Ungstrsip eta!.
114
21. DEC. 1972 1435-1442 liT 20.2 MLT
r
OUT.--. IN-
1500
::°~ 3— ~2
A -f\~-~I \
I \
I
//H+
~/
\~‘~
/
0
0
3)
ELECTRON DENSITY (cisc
I 79.2
74.0
I 62.8
68.6
INVARIANT LATITUDE
Fig. 1. ISIS-2 observations of Birkeland currents, low energy particle precipitation, electron density profiles and ion composition. Note the effects of the Birkeland currents on the electron density profiles and on the composition.
ISEE-1
5.FEBRUARY 1981
ISEE-1 5. FEBRUARY 1981
LOG PHASE SPACE DENSITY (s’fkm’) 2112:45—2113:50
LOG PHASE SPACE DENSITY (s~/kn,~)
2112:40—2113:50
2117:02—3110:05
V 5
~
1000
0
3000 km/s
250
2113:50—2314:54
-tOO0L~ 1000
o
-23~ 1000 km/s 250
250 km/s
o v,
1000
2113:S0-2114:s4
_______ 0
250 •16/S
Fig. 2. Logarithm of ion phase space density observed from ISEE-l at 10000 km altitude,
2117.02—2113:00
k:OL
1000 km/s
0
250
2~, 2123:26—2124:30
1~ 1000
~
0 23
250 km/s 212326-212430
~ 0
1000 km/s
250
0
250 km/S
Fig. 3. Logarithm of ion phase space density observed at 12000 and 13500 km altitude.
Compositional Changes, Time and Density Variations
115
ionosphere peak up to 1000 km altitude and thus O~ is probably dominant at least up to that altitude, but unfortunately the density is too low to be measured at the satellite and also too low for the composition to be measured. The ISIS-2 observations show that Birkeland currents are associated with large density and compositional changes in the topside ionosphere, but ISEE—l observations reveal that the influence of Birkeland currents also is important at much larger distances (‘—‘10000 km) in the magnetosphere. Figures 2 and 3 show phase space density distributions during an event with field aligned beams of ions.. ISEE-l is on field lines connected to northern Scandinavia and magnetometers in that area show a sudden enhancement of eastward auroral currents at 2112 that lasts until 2144 when a moderate (150 nfl expansion phase occurred. The enhancement occurs simultaneously with the onset of ion beams at ISEE-l at 2112 UT; the first beam observations are shown in Figure 2. The oxygen phase space density is 2~orders of magnitude larger than for protons so that the density of protons and oxygen ions are about equal. Also the beam energy is about equal for the two species as can be seen from the two lower frames in Figure 2. About five minutes into the event the situation is changed completely as we see in the two upper frames in Figure 3. There -is now only a weak indication of a beam in the protons whereas we have a well developed beam for the oxygen ions. Further five minutes later we still have the same situation as shown in the lower two frames in Figure 3. Now the phase space density of oxygen ions is 3 orders of magnitude larger than for protons and the beam is dominated by the oxygen ions. Simultaneously with the beam observations in Figure 3 we have also intense downward beams of electrons. We find it difficult to explain these oppositely directed ion and electron beams by wave acceleration of the involved particles. Instead we find these observations consistent with acceleration by parallel electric fields. Figure 4 shows the phase space density of protons with energies >130 eV at the time an intense current sheet has just passed ISEE-l on another orbit just at the onset of a major substorm. The first frame (upper left) shows the phase space density immediately after the first current sheet or filament passed ISEE—l. (In the previous measurement 52 seconds earlier the ion mass spectrometer showed practically only background levels in all energy channels.) In the first frame the phase space density has jumped by more than three orders of magnitudes above the background level in the lowest energy channels and the figure shows a conical distribution with a cone angle of lOO_llO0 from the (downward) magnetic field direction. We have 52 seconds between frames and as time progresses (to lower left, upper right, lower right) we see a clear development of the conical distribution where the cone angle increases towards 1800 at the same time as the faster protons disappear first and we end up having a cone angle of 1500 and only protons in the lowest energy channel (137-281 eV). In the following frame the proton conic had completely disappeared. Figure 5 shows the development of the phase space distribution for the oxygen ions. Immediately after the current system has passed over the satellite we have a strong conic with a cone angle of 1000 which in 9 minutes develops to a cone angle of ‘-~l50°and as for the protons the higher energies disappear first and we end up with an oxygen conic only in the lowest channel.
20.APRIL 1981
SEE—i
ISEE—1 20 APRIL 1981
LOG PROTON PHASE SPACE DENSITY V.4 2348:45-2349:37
V::423S0:29~.2351:21
-
-
-
:1000
0
-1000 1000
-
LOG O~PHASESPACE DENSITY
—
I
—
— km/i
- I 0
—
2349:37—2350.25
-. —3000 1000 km,s 1000
-
-
a
234S:45~2349:37
-
-
~.
:.
0,,
V,
— 1000 km/s
250
I _z~~— 250 km/S zoo
_._.
V,:
‘4,, 2351:21—2352:13
.
-
kfll/5
— —
-
0
2351.23~235l.13
—
~
—250 0
‘4::
V~42353:57—2354:4I km/s —.
—
—
—
-
.
—. 250
kin/s
2300033—2357:25
—
lvi C
-10001000
—
°.
0
-a
-1000 1000 km s 1000
— 0 .
—
—
1000 km/S
Fig. 4. Logarithm of proton phase space density observed at ISEE-l at 10000 to 11000 km altitude.
-250
250
0
sr”
°
250 km/s 250
:
—.
— 0 ——
— 250 km/s
Fig. 5. Logarithm of oxygen ion phase space density observed at ISEE-l at 10000 to 13000 km altitude.
116
E. Ungstrup
et al.
The development of the conical ion distributions shown in Figures 4 and 5 is consistent with a model in which we have an almost instantaneous transverse heating of the ambient plasma along a distance of the order of one Earth radius along the magnetic field below the satellite followed by an adiabatic expansion caused by the mirror force acting on the transversely heated plasma. The idea of instantaneous heating is also in agreement with the magnetometer observations which show that the main current structure passes the satellite in ‘--‘30 seconds as the first and strongest in a complicated sequence of up and downgoing current structures that lasts ‘—‘180 seconds. From the slope of the distribution function we derive a temperature of --‘250 eV immediately after the passage of the main current for both the H~and 0+ ions. With the same energy the oxygen ions are four times slower than the protons and therefore the development of the oxygen conic should be four times slower than the hydrogen conic which is in agreement with the observations. (In Figure 5 we show the 0+ phase space densities at 3x52 s intervals but that is because the data reduction programs had difficulties handling data across midnight. The oxygen conic lasted 15 minutes). During this period of about 4 minutes for the H+ and about 15 minutes for the 0+ ions the pitch angle changed from ~~,9O0 to ~~l5O0 and this means that the mirror points moved down by 1 Earth radius below the satellite or to ‘--‘4000 km altitude. The phase space density of the oxygen ions was —‘3.5 orders of magnitude larger than for the protons even in the first frame where the ions are almost locally heated. Taking the larger volume in phase space for protons into consideration we find that oxygen was the dominating ion in the ambient plasma at the satellite at the time the heating took place. At -the time of this event ISEE-l was moving ve~ry slowly eastward in B-L space at a pearly constant L-value and we see a mixed although 0 dominated conic change to a pure 0 conic in a matter o~ minutes. Furthermore the region eastward of the current sheet continued to be dominated by 0+ for the following hour, where ISEE-l moved out beyond 20000 km. We observed periodically 0 beams in this period when ISEE-1 slowly started to move westward again and changed very slowly in magnetic latitude. We have demonstrated in this paper that even weak Birkeland currents can result in density differences of an order of magnitude in the topside F-region and even larger differences at 1400 km altitude. They also result in similar density changes at 10000 km altitude /2/. Another important result is the compositional changes that occur in a few minutes at altitudes of 10000 to 13000 km altitude. The field aligned event was observed with a quiet aurora at the foot of the field lines, and ended in a substorm of --‘ 150 nI about 20 minutes after the observations in Figure 3. The conic event (Figures 4 and 5) occurred during a rather large substorm. We thus see that even a weak auroral event can have a profound influence on the density and composition of the magnetosphere up to altitudes of a couple of Earth radii and that changes can occur on the time scale of minutes. We believe that such observations should be taken into account in future modelling of the magnetosphere where Birkeland currents and electric fields play an important role. Acknowledgment: This research was partially supported by NASA under contract NAS 5-28702. REFERENCES 1.
L.P. Block and C.-G. F~lthammarEffects of Field Aligned Currents on the Structure of the Ionosphere, ,J. Geophys. Res. 73, 4807 (1968)
2.
E. Ungstrup, R.D. Sharp, C.A. Cattell, R.R. Anderson, D.S. Evans and D.N. Baker, Observation of a Westward Travelling Surge from Satellites at Low, Medium and High Altitudes, Submitted to 13. Geophys. Res.
0273—1177/86 $0.00 + .50 Copyright © COSPAR
COMPOSITIONAL CHANGES, TIME AND DENSITY VARIATIONS IN THE MAGNETOSPHERE ASSOCIATED WITH BIRKELAND CURRENTS AND PARTICLE ACCELERATION E. Ungstrup,* W. J. Heikkila,** J. C. Foster*** and W. 0. Lennartsson~ *Danish Space Research Institute, DK-2800 Lyngby, Denmark * *Danish Space Research Institute and Center for Space Sciences, University of Texas at Dallas, Richardson, TX 75080, U.S.A. ***MIT Haystack Observatory, Westford, MA 01866, U.S.A. tLockheed Palo Alto Research Laboratory, 3251 Hanover St., Palo Alto, CA 94304, U.S.A. ABSTRACT Birkeland currents, parallel electric fields and plasma instabilities often occur together in time and space and play an important role in large scale plasma motions between the ionosphere and magnetosphere and along the magnetic field in the magnetosphere. The results of the plasma movements are large density and composition variations in the ionosphere and magnetosphere. Observations from ISIS-2 at 1400 km altitude show large densities with heavy ions dominating in regions with upward Birkeland currents, and low densities and light ions in regions with downward currents. Observations from ISEE-l in field aligned current regions at 10000 to 15000 km altitude show transverse heating of protons and oxygen ions to 250 eV. Because of the different mobility of the protons and oxygen ions the proton flow is important in the beginning of the events but later the outflow becomes almost pure oxygen. Similarly ISEE-l observations of outgoing field aligned ion beams at 10000 to 15000 km altitude show time variations in the N+/O+ ratios and a dominance of 0+ later in the events. INTRODUCTION Block and F~lthamar /1/ studied the effect of Birkeland currents on the structure of the ionosphere in 1968; they limited themselves to the steady state solution and to altitudes below ‘-~90O km. They concluded that the Birkeland currents would lead to a reduction in total electron content and in density if the charge carriers have to be produced in the ionosphere, but that particle precipitation would lead to an increase. They estimated the time constant to be of the order of three hours. We will show in this paper some satellite observations of field aligned ion flows when important density and compositional changes occur on time scales of minutes. Our observations are made during weakly disturbed or disturbed conditions and we do not have a steady state situation; we feel that the observations are closely connected to Birkeland currents and their effects on the ionosphere and magnetosphere. We believe that the ions are the determining factor in the density changes in the magnetosphere, and that the main role of the (thermal) electrons is to neutralize the magnetosphere except for the small residual net charge associated with the parallel electric fields. OBSERVATIONS In Figure 1 we show an observation from ISIS-2 during an evening pass (‘~2O.2 MLT) in local winter. In the upper panel we see the magnetic 2field signature of the Birkeland currents. The but in spite of that we see pronounced currents are very weak only about 0.2 pA/m effects in the density profiles and composition of the topside ionosphere. In the third panel we show the electron density profiles from 300 to 1400 km altitude observed by the topside sounder; it is clearly seen that the density is increased in the topside ionosphere in the region with outgoing Birkeland current and decreased in the region with inward Birkeland current. The upwelling in the outgoing Birkeland current region can be caused by heating of the ionosphere by precipitating particles or by (weak) upward parallel electric fields. In the lowest panel we see the composition measured at 1400 km and it is clear that oxygen is the dominating ion in the outgoing Birkeland current region. The density profiles show that there is no change in scale height in the ingoing Birkeland current region from the 113
E. Ungstrsip eta!.
114
21. DEC. 1972 1435-1442 liT 20.2 MLT
r
OUT.--. IN-
1500
::°~ 3— ~2
A -f\~-~I \
I \
I
//H+
~/
\~‘~
/
0
0
3)
ELECTRON DENSITY (cisc
I 79.2
74.0
I 62.8
68.6
INVARIANT LATITUDE
Fig. 1. ISIS-2 observations of Birkeland currents, low energy particle precipitation, electron density profiles and ion composition. Note the effects of the Birkeland currents on the electron density profiles and on the composition.
ISEE-1
5.FEBRUARY 1981
ISEE-1 5. FEBRUARY 1981
LOG PHASE SPACE DENSITY (s’fkm’) 2112:45—2113:50
LOG PHASE SPACE DENSITY (s~/kn,~)
2112:40—2113:50
2117:02—3110:05
V 5
~
1000
0
3000 km/s
250
2113:50—2314:54
-tOO0L~ 1000
o
-23~ 1000 km/s 250
250 km/s
o v,
1000
2113:S0-2114:s4
_______ 0
250 •16/S
Fig. 2. Logarithm of ion phase space density observed from ISEE-l at 10000 km altitude,
2117.02—2113:00
k:OL
1000 km/s
0
250
2~, 2123:26—2124:30
1~ 1000
~
0 23
250 km/s 212326-212430
~ 0
1000 km/s
250
0
250 km/S
Fig. 3. Logarithm of ion phase space density observed at 12000 and 13500 km altitude.
Compositional Changes, Time and Density Variations
115
ionosphere peak up to 1000 km altitude and thus O~ is probably dominant at least up to that altitude, but unfortunately the density is too low to be measured at the satellite and also too low for the composition to be measured. The ISIS-2 observations show that Birkeland currents are associated with large density and compositional changes in the topside ionosphere, but ISEE—l observations reveal that the influence of Birkeland currents also is important at much larger distances (‘—‘10000 km) in the magnetosphere. Figures 2 and 3 show phase space density distributions during an event with field aligned beams of ions.. ISEE-l is on field lines connected to northern Scandinavia and magnetometers in that area show a sudden enhancement of eastward auroral currents at 2112 that lasts until 2144 when a moderate (150 nfl expansion phase occurred. The enhancement occurs simultaneously with the onset of ion beams at ISEE-l at 2112 UT; the first beam observations are shown in Figure 2. The oxygen phase space density is 2~orders of magnitude larger than for protons so that the density of protons and oxygen ions are about equal. Also the beam energy is about equal for the two species as can be seen from the two lower frames in Figure 2. About five minutes into the event the situation is changed completely as we see in the two upper frames in Figure 3. There -is now only a weak indication of a beam in the protons whereas we have a well developed beam for the oxygen ions. Further five minutes later we still have the same situation as shown in the lower two frames in Figure 3. Now the phase space density of oxygen ions is 3 orders of magnitude larger than for protons and the beam is dominated by the oxygen ions. Simultaneously with the beam observations in Figure 3 we have also intense downward beams of electrons. We find it difficult to explain these oppositely directed ion and electron beams by wave acceleration of the involved particles. Instead we find these observations consistent with acceleration by parallel electric fields. Figure 4 shows the phase space density of protons with energies >130 eV at the time an intense current sheet has just passed ISEE-l on another orbit just at the onset of a major substorm. The first frame (upper left) shows the phase space density immediately after the first current sheet or filament passed ISEE—l. (In the previous measurement 52 seconds earlier the ion mass spectrometer showed practically only background levels in all energy channels.) In the first frame the phase space density has jumped by more than three orders of magnitudes above the background level in the lowest energy channels and the figure shows a conical distribution with a cone angle of lOO_llO0 from the (downward) magnetic field direction. We have 52 seconds between frames and as time progresses (to lower left, upper right, lower right) we see a clear development of the conical distribution where the cone angle increases towards 1800 at the same time as the faster protons disappear first and we end up having a cone angle of 1500 and only protons in the lowest energy channel (137-281 eV). In the following frame the proton conic had completely disappeared. Figure 5 shows the development of the phase space distribution for the oxygen ions. Immediately after the current system has passed over the satellite we have a strong conic with a cone angle of 1000 which in 9 minutes develops to a cone angle of ‘-~l50°and as for the protons the higher energies disappear first and we end up with an oxygen conic only in the lowest channel.
20.APRIL 1981
SEE—i
ISEE—1 20 APRIL 1981
LOG PROTON PHASE SPACE DENSITY V.4 2348:45-2349:37
V::423S0:29~.2351:21
-
-
-
:1000
0
-1000 1000
-
LOG O~PHASESPACE DENSITY
—
I
—
— km/i
- I 0
—
2349:37—2350.25
-. —3000 1000 km,s 1000
-
-
a
234S:45~2349:37
-
-
~.
:.
0,,
V,
— 1000 km/s
250
I _z~~— 250 km/S zoo
_._.
V,:
‘4,, 2351:21—2352:13
.
-
kfll/5
— —
-
0
2351.23~235l.13
—
~
—250 0
‘4::
V~42353:57—2354:4I km/s —.
—
—
—
-
.
—. 250
kin/s
2300033—2357:25
—
lvi C
-10001000
—
°.
0
-a
-1000 1000 km s 1000
— 0 .
—
—
1000 km/S
Fig. 4. Logarithm of proton phase space density observed at ISEE-l at 10000 to 11000 km altitude.
-250
250
0
sr”
°
250 km/s 250
:
—.
— 0 ——
— 250 km/s
Fig. 5. Logarithm of oxygen ion phase space density observed at ISEE-l at 10000 to 13000 km altitude.
116
E. Ungstrup
et al.
The development of the conical ion distributions shown in Figures 4 and 5 is consistent with a model in which we have an almost instantaneous transverse heating of the ambient plasma along a distance of the order of one Earth radius along the magnetic field below the satellite followed by an adiabatic expansion caused by the mirror force acting on the transversely heated plasma. The idea of instantaneous heating is also in agreement with the magnetometer observations which show that the main current structure passes the satellite in ‘--‘30 seconds as the first and strongest in a complicated sequence of up and downgoing current structures that lasts ‘—‘180 seconds. From the slope of the distribution function we derive a temperature of --‘250 eV immediately after the passage of the main current for both the H~and 0+ ions. With the same energy the oxygen ions are four times slower than the protons and therefore the development of the oxygen conic should be four times slower than the hydrogen conic which is in agreement with the observations. (In Figure 5 we show the 0+ phase space densities at 3x52 s intervals but that is because the data reduction programs had difficulties handling data across midnight. The oxygen conic lasted 15 minutes). During this period of about 4 minutes for the H+ and about 15 minutes for the 0+ ions the pitch angle changed from ~~,9O0 to ~~l5O0 and this means that the mirror points moved down by 1 Earth radius below the satellite or to ‘--‘4000 km altitude. The phase space density of the oxygen ions was —‘3.5 orders of magnitude larger than for the protons even in the first frame where the ions are almost locally heated. Taking the larger volume in phase space for protons into consideration we find that oxygen was the dominating ion in the ambient plasma at the satellite at the time the heating took place. At -the time of this event ISEE-l was moving ve~ry slowly eastward in B-L space at a pearly constant L-value and we see a mixed although 0 dominated conic change to a pure 0 conic in a matter o~ minutes. Furthermore the region eastward of the current sheet continued to be dominated by 0+ for the following hour, where ISEE-l moved out beyond 20000 km. We observed periodically 0 beams in this period when ISEE-1 slowly started to move westward again and changed very slowly in magnetic latitude. We have demonstrated in this paper that even weak Birkeland currents can result in density differences of an order of magnitude in the topside F-region and even larger differences at 1400 km altitude. They also result in similar density changes at 10000 km altitude /2/. Another important result is the compositional changes that occur in a few minutes at altitudes of 10000 to 13000 km altitude. The field aligned event was observed with a quiet aurora at the foot of the field lines, and ended in a substorm of --‘ 150 nI about 20 minutes after the observations in Figure 3. The conic event (Figures 4 and 5) occurred during a rather large substorm. We thus see that even a weak auroral event can have a profound influence on the density and composition of the magnetosphere up to altitudes of a couple of Earth radii and that changes can occur on the time scale of minutes. We believe that such observations should be taken into account in future modelling of the magnetosphere where Birkeland currents and electric fields play an important role. Acknowledgment: This research was partially supported by NASA under contract NAS 5-28702. REFERENCES 1.
L.P. Block and C.-G. F~lthammarEffects of Field Aligned Currents on the Structure of the Ionosphere, ,J. Geophys. Res. 73, 4807 (1968)
2.
E. Ungstrup, R.D. Sharp, C.A. Cattell, R.R. Anderson, D.S. Evans and D.N. Baker, Observation of a Westward Travelling Surge from Satellites at Low, Medium and High Altitudes, Submitted to 13. Geophys. Res.