Large electric fields near the nightside plasmapause observed by the Polar spacecraft

1. Large electric fields near the nightside plasmapause observed by the Polar spacecraft K.-H. Kim1, F. Mozer2, and D.-H. Lee1 1Department 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 1 2 3 4 5 6 7 Event 1 L = 2.3 MLAT = 32.2 ALT = 3900 km 80 51 32 54 125 30 48 mV/m (Okada et al., JGR, 1993)

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

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

5. Previous study: Akebono E-field observations

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. • 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 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.

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 April 18, 1997 (event B) 04/18/97 14:35-15:10 UT L = 4.3, MLT = ~23.5 hrs, MLAT = ~11.5, ILT = ~61.3 • 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.

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

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

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

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.

16. Plasma sheet Plasmasphere • The magnetic field perturbations in Bx and Bz 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 = 0J, X = ~23 km, Z = ~268 km, Bx = 6.5 nT, and Bz = 4.2 nT. • Then, J at the plasmapause is about 0.1 A/m2. • 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].

17. Polar observations and SAPS/SAID E-field DE-2 observations J|| = 2.1 A/m2 J|| = 0.4 A/m2 Plasma sheet Plasmasphere (Anderson et al., JGR, 1993)

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.