1. Large electric fields near the nightside plasmapause observed by the Polar spacecraft K.-H. Kim 1, F. Mozer 2, and D.-H. Lee 1 1 Department of Astronomy & Space Science, Kyung Hee Univ., Korea 2 Space Science Laboratory, UC Berkeley, USA K.-H. Kim 1, F. Mozer 2, and D.-H. Lee 1 1 Department of Astronomy & Space Science, Kyung Hee Univ., Korea 2 Space Science Laboratory, UC Berkeley, USA

2. Previous study: Akebono observations of enhanced electric fields in the magnetosphere Event 1234567 L = 2.3 MLAT = 32.2  ALT = 3900 km Event 1 48 mV/m 80 51 32 54 125 30 (Okada et al., JGR, 1993)

3. Event 2 L = 2.8 MLAT = 41.9  ALT = 3600 km Plasmapause: L = 2.4 The enhanced E field was observed outside the plasmapause. Previous study: Akebono observations

4. Event 3 Event 4 Event 5 Event 7 Event 6 Previous study: Akebono observations

5. Previous study: Akebono E-field observations EventDst (nT) KpPeak amp. (mV/m) ILAT (deg.) MLAT (deg.) MLT (hrs) LLpp (Plasmapause location) 1-2318+4850.532.319.72.3No data 2-1938-8053.641.919.42.82.4 3-10055156.047.819.53.32.9 4-627-3256.345.419.53Unclear 5-1175+5455.142.420.12.7Unclear 6-1054+12556.54619.53Unclear 7-934+3055.342.220.12.7Unclear

6. Motivation of this study Akebono obs.: Intense electric fields are dominant in the GSE-Z component. They were observed in the magnetic latitudes (MLAT) higher than ~32 , in the region of L = ~2.3-3, and outside the plasmapause when the location of the plasmapause was identified. Akebono obs.: Intense electric fields are dominant in the GSE-Z component. They were observed in the magnetic latitudes (MLAT) higher than ~32 , in the region of L = ~2.3-3, and outside the plasmapause when the location of the plasmapause was identified. In this study: To examine whether intense electric fields exist in the region lower than MLAT = ~32 . To examine where intense electric fields occur. (at plasmapause? or outside plasmapause?) What is magnetic field variation associated with the intense electric field? In this study: To examine whether intense electric fields exist in the region lower than MLAT = ~32 . To examine where intense electric fields occur. (at plasmapause? or outside plasmapause?) What is magnetic field variation associated with the intense electric field?

7. Polar observations: enhanced electric field with a spike signature L = 3 4 4 5 5

8. Polar observations: event A April 25, 1998 (event A) 04/25/98 03:55-04:25 UT L = 3.5, MLT = ~23.0 hrs, MLAT = ~23.0 , ILT = ~57.6  Enhanced electric field was observed at the plasmapause. The electric field is dominant in Ez with a peak value of ~60 mV/m. The Ez component is approximately perpendicular to the dipole magnetic field. There is magnetic field perturbation associated with the enhanced electric field. Enhanced electric field was observed at the plasmapause. The electric field is dominant in Ez with a peak value of ~60 mV/m. The Ez component is approximately perpendicular to the dipole magnetic field. There is magnetic field perturbation associated with the enhanced electric field.

9. Polar observations: event A April 25, 1998 (event A) Plasma sheet Plasmasphere

10. Geomagnetic conditions for event A April 25, 1998 (event A) Kp = 4, Dst = -30 nT Apr. 25, 1998

11. Polar observations: event B Enhanced electric field was observed outside the plasmapause. The electric field is 14 mV/m in Ez, -10 mV/m in Ex, and 3mV/m in Ey. The electric field is approximately perpendicular to the dipole magnetic field. There is magnetic field perturbation associated with the enhanced electric field. Enhanced electric field was observed outside the plasmapause. The electric field is 14 mV/m in Ez, -10 mV/m in Ex, and 3mV/m in Ey. The electric field is approximately perpendicular to the dipole magnetic field. There is magnetic field perturbation associated with the enhanced electric field. L = 4.3, MLT = ~23.5 hrs, MLAT = ~11.5 , ILT = ~61.3  04/18/97 14:35-15:10 UT April 18, 1997 (event B)

12. Polar observations: event B April 18, 1997 (event B)

13. Apr. 18, 1997 Kp = 3+, Dst = -37 nT Geomagnetic conditions for event B

14. Comparison Polar obs. and Akebono obs. Akebono Polar All events were observed in the dusk-to-midnight MLT sector.

15. Polar observations and SAPS/SAID E-field (Goldstein et al., JGR, 2005) Ionospheric SAPS (subauroral polarization stream) occurs when the equatorial boundaries of ion and electron plasma sheets separate, leading to a poleward flowing Pedersen current in the subauroral ionosphere. Because of the low conductivity in the subauroral ionosphere, the poleward Pedersen current generates an intense poleward E-field that is mapped via geomagnetic field line to a strong radial E-field in the equtorial plane between the ion and electron plasma sheet edges. SAPS forms a radially narrow (1 to 2 Re) flow channel just outside or overlapping the dusk-to-midnight plasmasphere. Ionospheric SAPS (subauroral polarization stream) occurs when the equatorial boundaries of ion and electron plasma sheets separate, leading to a poleward flowing Pedersen current in the subauroral ionosphere. Because of the low conductivity in the subauroral ionosphere, the poleward Pedersen current generates an intense poleward E-field that is mapped via geomagnetic field line to a strong radial E-field in the equtorial plane between the ion and electron plasma sheet edges. SAPS forms a radially narrow (1 to 2 Re) flow channel just outside or overlapping the dusk-to-midnight plasmasphere.

16. The magnetic field perturbations in  B x and  B z may be due to the dawnward plasmapause current, which is caused by the balance of forces (  P ~ J  B) between hot plasma sheet plasma and cold plasmaspheric plasma, perpendicular to the background magnetic field. Assuming that the plasmapause is not moving in the earth-fixed frame, current density can be calculated using  B =  0 J,  X = ~23 km,  Z = ~268 km,  B x = 6.5 nT, and  B z = 4.2 nT. Then, J  at the plasmapause is about 0.1  A/m 2. This current density is comparable to or one order of magnitude smaller than field- aligned currents associated with a SAID event [Anderson et al., 1993]. The magnetic field perturbations in  B x and  B z may be due to the dawnward plasmapause current, which is caused by the balance of forces (  P ~ J  B) between hot plasma sheet plasma and cold plasmaspheric plasma, perpendicular to the background magnetic field. Assuming that the plasmapause is not moving in the earth-fixed frame, current density can be calculated using  B =  0 J,  X = ~23 km,  Z = ~268 km,  B x = 6.5 nT, and  B z = 4.2 nT. Then, J  at the plasmapause is about 0.1  A/m 2. This current density is comparable to or one order of magnitude smaller than field- aligned currents associated with a SAID event [Anderson et al., 1993]. Plasma sheetPlasmasphere

17. Polar observations and SAPS/SAID E-field (Anderson et al., JGR, 1993) DE-2 observations Plasma sheetPlasmasphere J || = 2.1  A/m 2 J || = 0.4  A/m 2

18. Summary Polar observed the enhanced electric fields with a spike signature at the plasmapause (event A) and outside the plasmapause (event B) during substorm recovery. They are predominantly perpendicular to the ambient magnetic field and their peaks are coincident with the inner edge of the electron plasma sheet. The electric fields in our study may be associated with SAPS/SAID in the midlatitude ionosphere. The enhanced E fields were accompanied by a negative (outside the plasmapause)-then-positive (inside the plasmapause) magnetic field perturbation in the magnetic meridian. This is not the field-aligned current-associated magnetic field perturbations but may be due to dawnward plasmapause current, which is caused by the balance of forces (  P ~ J  B) between cold plasmaspheric plasma and hot plasma sheet plasma. The location of a negative-then-positive magnetic perturbation in the magnetic meridian is probably a good indicator of the inner edge of the electron plasma sheet. Polar observed the enhanced electric fields with a spike signature at the plasmapause (event A) and outside the plasmapause (event B) during substorm recovery. They are predominantly perpendicular to the ambient magnetic field and their peaks are coincident with the inner edge of the electron plasma sheet. The electric fields in our study may be associated with SAPS/SAID in the midlatitude ionosphere. The enhanced E fields were accompanied by a negative (outside the plasmapause)-then-positive (inside the plasmapause) magnetic field perturbation in the magnetic meridian. This is not the field-aligned current-associated magnetic field perturbations but may be due to dawnward plasmapause current, which is caused by the balance of forces (  P ~ J  B) between cold plasmaspheric plasma and hot plasma sheet plasma. The location of a negative-then-positive magnetic perturbation in the magnetic meridian is probably a good indicator of the inner edge of the electron plasma sheet.