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The features of the auroral topside ionosphere-magnetosphere coupling induced by the time-dependent magnetic field
Pefgamon 0273-l 177(9S)OO98@-9
Adv. Space Res. Vol. 18, No. 8. pp. (8)233-(8)236, 1996 Copyright 0 1995 COSPAR Printed in Great Britain. All rights rcsetwd 0273-I 177/96 $9.50 + 0.00
THE FEATURES OF THE AURORAL TOPSIDE IONOSPHEREMAGNETOSPHERE COUPLING INDUCED BY THE TIME-DEPENDENT MAGNETIC FIELD E. B. Wodnicka and M. Banaszkiewicz Space Research Centre, Polish Academy of Sciences, Bartycka 18A. 00716 Warsaw, Poland
ABSTRACT It has been revealed in numerous studies that the topside aurora1 ionosphere is an important source of ions for the magnetosphere (e. g. /l/). The transport of ionospheric ions couples the aurora1 ionosphere and the near-equatorial regions. The source intensity and the coupling is strongest during substorms when at least three types of magnetospheric field reconfiguration occur in the equatorial region: stretching,collapse and oscillations. In this study the transport of the ionospheric oxygen ions is investigated by tracing their motion in the time-dependent magnetic field modeled for stretching and collapse type reconfigurations. Different types of orbits are obtained depending on the initial particle characteristics (energy,pitch angleposition) and on the temporal and spatial changes of the field. The ionospheric oxygen ion population is energized up to tens of keV energy range. Final pitch angle values are confined in narrow cones around Oo and 1800. The energetic and collimated ionospheric component couples the south and north ionospheres and can contribute to the cross-tail current changes during substorms. INTRODUCTION At substorm time short ( about 1 min.) magnetic field reconfiguration events develop when the Bx and B, components change beeing a decreasing function of each other. During a collapse Bx decrease coincides with B, increase,during stretching the change is reverse. In the aurora1 topside ionosphere during substonns the fluxes of the upflowing ions are enhanced /l/ and an abundent field-aligned keV component of oxygen ion population has been observed /2/. The issue to be considered in this report is the problem of oxygen ion motion in the quickly reconfigurating field. TRACING IONOSPHERIC OXYGEN ION MOTION The model of a time-dependent magnetospheric field proposed by /3/ is adopted in this study. The Mead-Fairfield magnetosphere is applied with time introduced into the kp-scaled coefficients of the external part of the field Bex!
C ( 4, kPf,t ) = C ( kPi) + f ( t ) * ( C ( kPf ) _ C ( kpi ) )
(1)
The function f ( t ) = 0 at the beginlng of the event and C ( kpi, kpf, t = 0 ) reduces to c ( kpi ). At the end of the event f ( t = b) =l and the magnetic field is in its final configuration. The collapse is simulated supposing kpl = 4, kpf = 0 , for stretching the values were reversed. The (8)233
(8)234
E. B. Wodnicka and M. Hanaszkiewicz
event duration time td = 2 min. The induced electric field Eind = -dNdt expressed by A - the vector potential of Bext is introduced. An example of fields encountered by ions along the trajectory during 2 min collapse is presented on Figure 1.
2-S 2.0 1s 1.0 0.1 0.0 0
80120
40
0
[sJ
the
120
40
80
kw
[sJ
0
40
a0
12c
time Is]
Fig. 1. Fields seen by the moving particle. B,, B,
components (a), induced electric field components parallel (b) and perpendicular (c) to B for an oxygen ion ejected at aurora1 latitudes (67O-69O MtAT) at 21.30 MLT with initial energy and pitch angle values: 3.2 keV and 900. As for the oxygen ions the temporal adiabacity criterion is not fulfilled trajectory the full equation of motion dv/dt=q/m*(E(r,
t)+v
x B(r,
along the whole
t))
(2)
is solved for O+ particles upflowing from the ionosphere. Ions are ejected along the aurora1 field lines at the altitude of 2 RE with energies ranging from 0.4 eV to 3.2 keV and with pitch angles from 90° to 1800 interval. RESULTS The details of the motion are displayed on Figure 2
--rl
.
Q
0 0
-2
-4
sarl{r*2+y-2)
-8
-0-10 [rEJ
-4
-2
0 2 IrEI
2
4
-4
-2
0
2
4
2 IrEI
Fig. 2. Trajectories (a), energy (b) and pitch angle (c) of ions ejected from both ionospheres. Initial energy and pitch angle values: 3.2 keV and 900.
Auroral Topside Ionosphere-Magnetosphhere
(8)235
Coupling
The ion final energy in the near-equatorial region reaches values from 40 keV for 67°MLAT ejection up to 60 keV for an ion ejected at 69OMlAT. ions from the south have their final pitch angle value around O” and the final pitch angle of northern ions is confined in a narrow cone around 1800 . As the ion approaches regions of low magnetic field intensity its cyclotron period increases and the motion becomes non-adiabatic. It is manifested by the oscillation amplitude and the oscillation period enhancement of the magnetic moment displayed on the left panel of Figure 3 . Oscillations of the magnetic moment start when the cyclotron period begins to increase and the surging electric field is approaching its maximum value. On the right panel of this figure the results of sampling the oxygen ion velocity distribution function reported by /2/, tracing the ions during the 2 min. collapse and extrapolating the trajectory to the equatorial plane adiabatically in the relaxed field are presented. Ions move in the phase space into the regions of tens of keV energies and their pitch angles evolve depending on the initial energy and pitch angle.
0
40
80
iimc
IsJ
120
0
4
60
t-ms
Is]
120
0
200 vper
400
600
[km/s]
Fig. 3. Non-adiabacity of the ion motion and the ion velocity distribution function. Magnetic moment (left panel) and cyclotron period (middle panel) change along the trajectory. The symbols show the final velocities of ions in the equatorial plane.
0
-2
-4
sarl(W+y-2)
-6-B-10 IrE]
-4
-2
0
2
A
1 IrEI
Fig. 4. Stretching in the post midnight sector of lo×. quiet state into an elongated configuration.
-4
-2
0 z
2
A
IrEI
The field line transforms from the
The magnetic field By component changes its sign in the postmidnight local times and it affects the motion of the ionospheric ion. Not the collapse but the stretching extracts the ions
(8)236
E. B. Wodnicka and M. Banaslkiewicz
from the aurora1 altitudes. On Figure 4 plotted in the same format as Figure 2 the motion during stretching is illustrated. Ions are ejected at 2.30 MLT from both ionospheres. Oscillations defined as sequences of collapses and stretchings of the magnetic field are not effective in supplying the low latitude regions with the ionospheric ions if the duration time of one pulse in the oscillation is short (30 set). Data analysis is needed to proceed with the oscillating regimes and this issue will be addressed in the future study. CONCLUSION It follows from the study that the field-aligned 10-100 keV component of 0+ ions with butterfly PADS measured by the geostationary satellites during substorms 141can come from the aurora1 ionosphere as a result of short collapse/stretching events. Aknowledamenf. The panels were completed during a short visit of one of the authors at MaxPlanck lnstitut fur Aeronomie in Lindau. The authors thank all their collaborators and the staff at the MPAE for their kind help and assistance. REFERENCES 1. Yau,A. W.,B. A. Whalen,W. K. Peterson and E. G. Shelley,Distribution of upflowing ionospheric ions in the high-altitude polar cap and aurora1 ionosphere,J. Geophys. Res., 89, 5507 (1984). 2. Reiff,P. H.,H. L. Collin,J. D. Craven,J. L. Burch,J. D. Winningham,E. G. Shelley,L. A. Frank and M. A. FriedmanDetermination of aurora1 electrostatic potentials using high- and lowaltitude particle distribution, J. Geophys. Res., 93, 7441 (1988). 3. Delcourt,D. C.,J. A. Sauvaud,A. Pedersen, Dynamics of single-particle substorm expansion phase, J. Geophys. Res., 95, 20853-20865 (1990).
orbits during
4. Daglis,l. A.,E. T. San-&B. Wilken,AMPTE/CCE CHEM observations of the energetic ion population at geosynchronous altitudes, Ann. Geophysicae, 11, 685898 (1993).
Adv. Space Res. Vol. 18, No. 8. pp. (8)233-(8)236, 1996 Copyright 0 1995 COSPAR Printed in Great Britain. All rights rcsetwd 0273-I 177/96 $9.50 + 0.00
THE FEATURES OF THE AURORAL TOPSIDE IONOSPHEREMAGNETOSPHERE COUPLING INDUCED BY THE TIME-DEPENDENT MAGNETIC FIELD E. B. Wodnicka and M. Banaszkiewicz Space Research Centre, Polish Academy of Sciences, Bartycka 18A. 00716 Warsaw, Poland
ABSTRACT It has been revealed in numerous studies that the topside aurora1 ionosphere is an important source of ions for the magnetosphere (e. g. /l/). The transport of ionospheric ions couples the aurora1 ionosphere and the near-equatorial regions. The source intensity and the coupling is strongest during substorms when at least three types of magnetospheric field reconfiguration occur in the equatorial region: stretching,collapse and oscillations. In this study the transport of the ionospheric oxygen ions is investigated by tracing their motion in the time-dependent magnetic field modeled for stretching and collapse type reconfigurations. Different types of orbits are obtained depending on the initial particle characteristics (energy,pitch angleposition) and on the temporal and spatial changes of the field. The ionospheric oxygen ion population is energized up to tens of keV energy range. Final pitch angle values are confined in narrow cones around Oo and 1800. The energetic and collimated ionospheric component couples the south and north ionospheres and can contribute to the cross-tail current changes during substorms. INTRODUCTION At substorm time short ( about 1 min.) magnetic field reconfiguration events develop when the Bx and B, components change beeing a decreasing function of each other. During a collapse Bx decrease coincides with B, increase,during stretching the change is reverse. In the aurora1 topside ionosphere during substonns the fluxes of the upflowing ions are enhanced /l/ and an abundent field-aligned keV component of oxygen ion population has been observed /2/. The issue to be considered in this report is the problem of oxygen ion motion in the quickly reconfigurating field. TRACING IONOSPHERIC OXYGEN ION MOTION The model of a time-dependent magnetospheric field proposed by /3/ is adopted in this study. The Mead-Fairfield magnetosphere is applied with time introduced into the kp-scaled coefficients of the external part of the field Bex!
C ( 4, kPf,t ) = C ( kPi) + f ( t ) * ( C ( kPf ) _ C ( kpi ) )
(1)
The function f ( t ) = 0 at the beginlng of the event and C ( kpi, kpf, t = 0 ) reduces to c ( kpi ). At the end of the event f ( t = b) =l and the magnetic field is in its final configuration. The collapse is simulated supposing kpl = 4, kpf = 0 , for stretching the values were reversed. The (8)233
(8)234
E. B. Wodnicka and M. Hanaszkiewicz
event duration time td = 2 min. The induced electric field Eind = -dNdt expressed by A - the vector potential of Bext is introduced. An example of fields encountered by ions along the trajectory during 2 min collapse is presented on Figure 1.
2-S 2.0 1s 1.0 0.1 0.0 0
80120
40
0
[sJ
the
120
40
80
kw
[sJ
0
40
a0
12c
time Is]
Fig. 1. Fields seen by the moving particle. B,, B,
components (a), induced electric field components parallel (b) and perpendicular (c) to B for an oxygen ion ejected at aurora1 latitudes (67O-69O MtAT) at 21.30 MLT with initial energy and pitch angle values: 3.2 keV and 900. As for the oxygen ions the temporal adiabacity criterion is not fulfilled trajectory the full equation of motion dv/dt=q/m*(E(r,
t)+v
x B(r,
along the whole
t))
(2)
is solved for O+ particles upflowing from the ionosphere. Ions are ejected along the aurora1 field lines at the altitude of 2 RE with energies ranging from 0.4 eV to 3.2 keV and with pitch angles from 90° to 1800 interval. RESULTS The details of the motion are displayed on Figure 2
--rl
.
Q
0 0
-2
-4
sarl{r*2+y-2)
-8
-0-10 [rEJ
-4
-2
0 2 IrEI
2
4
-4
-2
0
2
4
2 IrEI
Fig. 2. Trajectories (a), energy (b) and pitch angle (c) of ions ejected from both ionospheres. Initial energy and pitch angle values: 3.2 keV and 900.
Auroral Topside Ionosphere-Magnetosphhere
(8)235
Coupling
The ion final energy in the near-equatorial region reaches values from 40 keV for 67°MLAT ejection up to 60 keV for an ion ejected at 69OMlAT. ions from the south have their final pitch angle value around O” and the final pitch angle of northern ions is confined in a narrow cone around 1800 . As the ion approaches regions of low magnetic field intensity its cyclotron period increases and the motion becomes non-adiabatic. It is manifested by the oscillation amplitude and the oscillation period enhancement of the magnetic moment displayed on the left panel of Figure 3 . Oscillations of the magnetic moment start when the cyclotron period begins to increase and the surging electric field is approaching its maximum value. On the right panel of this figure the results of sampling the oxygen ion velocity distribution function reported by /2/, tracing the ions during the 2 min. collapse and extrapolating the trajectory to the equatorial plane adiabatically in the relaxed field are presented. Ions move in the phase space into the regions of tens of keV energies and their pitch angles evolve depending on the initial energy and pitch angle.
0
40
80
iimc
IsJ
120
0
4
60
t-ms
Is]
120
0
200 vper
400
600
[km/s]
Fig. 3. Non-adiabacity of the ion motion and the ion velocity distribution function. Magnetic moment (left panel) and cyclotron period (middle panel) change along the trajectory. The symbols show the final velocities of ions in the equatorial plane.
0
-2
-4
sarl(W+y-2)
-6-B-10 IrE]
-4
-2
0
2
A
1 IrEI
Fig. 4. Stretching in the post midnight sector of lo×. quiet state into an elongated configuration.
-4
-2
0 z
2
A
IrEI
The field line transforms from the
The magnetic field By component changes its sign in the postmidnight local times and it affects the motion of the ionospheric ion. Not the collapse but the stretching extracts the ions
(8)236
E. B. Wodnicka and M. Banaslkiewicz
from the aurora1 altitudes. On Figure 4 plotted in the same format as Figure 2 the motion during stretching is illustrated. Ions are ejected at 2.30 MLT from both ionospheres. Oscillations defined as sequences of collapses and stretchings of the magnetic field are not effective in supplying the low latitude regions with the ionospheric ions if the duration time of one pulse in the oscillation is short (30 set). Data analysis is needed to proceed with the oscillating regimes and this issue will be addressed in the future study. CONCLUSION It follows from the study that the field-aligned 10-100 keV component of 0+ ions with butterfly PADS measured by the geostationary satellites during substorms 141can come from the aurora1 ionosphere as a result of short collapse/stretching events. Aknowledamenf. The panels were completed during a short visit of one of the authors at MaxPlanck lnstitut fur Aeronomie in Lindau. The authors thank all their collaborators and the staff at the MPAE for their kind help and assistance. REFERENCES 1. Yau,A. W.,B. A. Whalen,W. K. Peterson and E. G. Shelley,Distribution of upflowing ionospheric ions in the high-altitude polar cap and aurora1 ionosphere,J. Geophys. Res., 89, 5507 (1984). 2. Reiff,P. H.,H. L. Collin,J. D. Craven,J. L. Burch,J. D. Winningham,E. G. Shelley,L. A. Frank and M. A. FriedmanDetermination of aurora1 electrostatic potentials using high- and lowaltitude particle distribution, J. Geophys. Res., 93, 7441 (1988). 3. Delcourt,D. C.,J. A. Sauvaud,A. Pedersen, Dynamics of single-particle substorm expansion phase, J. Geophys. Res., 95, 20853-20865 (1990).
orbits during
4. Daglis,l. A.,E. T. San-&B. Wilken,AMPTE/CCE CHEM observations of the energetic ion population at geosynchronous altitudes, Ann. Geophysicae, 11, 685898 (1993).