Motoharu Nowada ( 野和田 基晴 : nowada@pku) 1 Sui-yan Fu 1 , George K. Parks 2 , Zu-yin Pu 1 ,

1. EGU2012-1991 Magnetic Flux Transport and Pressure Variations at Magnetotail Plasma Flow Bursts during Geomagnetically Quiet Times Motoharu Nowada (野和田 基晴: nowada@pku.edu.cn)1 Sui-yan Fu1, George K. Parks2, Zu-yin Pu1, Vassilis Angelopoulos3, Charles W. Carlson2, Hans-Ulrich Auster4 1: ISPAT/SESS, Peking University 2: SSL, University of California, Berkeley 3: IGPP, University of California Los Angeles 4: Technische Universität, Braunschweig, Deutsuland

2. Abstract The fast plasma flows in the geomagnetotail are observed during both geomagnetically active and quiet times. However, it has been unclear about the fundamental difference in the plasma fast flows between at two different geomagnetic conditions, that is, the generation mechanism of, and pictures of the energy transport and balance at the fast plasma flows. Magnetic reconnection in the magnetotail has been believed as one of the most possible mechanisms to generate the fast plasma flows regardless of the geomagnetic conditions. Recently, Nowada et al. [2012], however, demonstrated that the magnetotail magnetic reconnection does not always contribute to the generation of the fast plasma flows at geomagnetically quiet times based on the THEMIS measurements. It is very important to reveal how the energy transport and balance in the magnetotail in association with these plasma fast flows are on obtaining a clue to elucidate an essential difference in the plasma fast flows between during active and quiet geomagnetic conditions. Based on three events of the magnetotail plasma flow bursts, which are transient fast plasma flows with the durations between 1 and 2 minutes, during geomagnetically quiet times, observed by THEMIS, we examined detailed variations of the electric field as a proxy of the flux transport aspect, and associated pressure. The main characteristics of these events are shown as follows; 1) the GSM-X component of the plasma velocity (Vx) was higher than 300 km/s 2) associated parallel (V//) and perpendicular (V⊥) velocities to the local magnetic field line were higher than 200 km/s 3) the flow bursts were observed during which |AL| and AU indices were lower than 20 nT, and simultaneous Kp index range was between -1 and 1. For almost events, the parallel (E//) and perpendicular (E⊥) components of the electric field to the local magnetic field line were much stronger than the dawn-dusk electric field component (Ey). This result implies that a larger amount of the magnetic flux was transported into the parallel and perpendicular directions to the local magnetic field line than the dawn-dusk direction at the flow bursts. However, in the Ey component, the contribution from the dawn-to-dusk electric field (VxBz) was much greater than that from the dusk-to-dawn component (VzBx). Furthermore, for two events, significant reduction of the plasma pressure, and enhancement of the north-south magnetic field component (Bz) were observed at/near the flow bursts. Simultaneous total pressure was well-balanced, indicating that the magnetotail during the plasma flow bursts was in the state of equilibrium. Based on these results, “bubble” might play a crucial role for generating the plasma flow bursts at geomagnetically quiet times.

3. Based on our previous results …[Nowada et al., 2012] Under geomagnetically quiet conditions… 1. “Magnetotail Magnetic Reconnection” might not be necessary condition to generate the fast plasma flows (plasma flow bursts). 2. The fast plasma flows (plasma flow bursts) occur in association with not only “pure (typical) substorm” but also “substorm-like phenomena”, such as pseudo-substorm.

4. Locations and Orbits of the THEMIS probes

5. - Plasma flow bursts are periodically occurring. - The magnetic field perturbations are observed in association with the plasma flow bursts. Significant enhancements of the Bz components (magnetotail dipolarization signature)are observed at/near the plasma flow bursts. - Plasma density and temperature are variable during the plasma flow bursts. - The Vx component is almost consistent with the V// component. Event I: January 31, 2008

6. Event I: January 31, 2008 Each electric field component as a proxy parameter of the magnetic flux transport aspect is derived by following formulae: Parallel magnetic flux transport is the most dominant. Ey = VxBz – VzBx E⊥ = V⊥Bz E//= V//Bz E//> Eperp Variations of all electric field components are almost following those of the velocity components, suggesting that the magnetic flux (magnetic energy) is efficiently transported in association with the plasma flow bursts. Parallel magnetic flux transport is the most dominant. E//>Eperp

7. Event I: January 31, 2008 Magnetic Pressure (Pm): Plasma Pressure (Pp): NpkTi Total Pressure (Ptot): Pm + Pp Both Pp and Pm show significant variations associated with the plasma velocity. During the plasma flow bursts, Pp shows REDUCTION and simultaneous PmindicatesENHANCEMENT. In THEMIS-D, Ptot has been almost CONSTANT. The Ptotvaluesin THEMIS-Eare also almost background level(stable), except for a plasma flow burst on ～09:48 UT.

8. - The first plasma flow burst is observed by THEMIS-B, but is not observed by THEMIS-C. • - During all plasma flow bursts, the Bz enhancements(dipolarization signatures) are observed. • The second plasma flow burst observed by THEMIS-B is also seen by THEMIS-C, but associated magnetic field and plasma variations are different between the THEMIS-B and THEMIS-C observations. • The third plasma flow burst is observed by only THEMIS-B • -The Beta (β) values at the second plasma flow burst are different between the two probes’ observations. Event II: March 27, 2009

9. Event II: March 27, 2009 The electric field varies in association with some plasma flow burst events. However, during the other plasma flow bursts, the electric field does not show significant variations. The Eycomponent is the largest, although E//> Eperp and E// ≈ Eperpare observed during the plasma flow bursts.

10. Event II: March 27, 2009 In association with the plasma flow bursts… Pm: Enhancement Pp: Reduction Ptot: Background Level (stable) Pm: Reduction Pp: Enhancement Ptot: Variable

11. -Plasma flow bursts are observed during the periodical plasma velocity fluctuations. - Gradual (THEMIS-D) and sharp (THEMIS-E) enhancements of the Bz components are observed before the plasma flow bursts. -The magnetic field perturbations are observed in association with the plasma flow bursts -The plasma density and temperature of THEMIS-E are variable during the plasma flow burst. However, their variationsin THEMIS-D are almost stable. -The Vx component is almost consistent with the Vperp component. Event III: March 24, 2010

12. Event III: March 24, 2010 The electric field components in both THEMIS-D and THEMIS-E indicate significant enhancements in association with the plasma flow bursts. During the plasma flow bursts, the case of Eperp> E// is dominant.

13. Event III: March 24, 2010 During the plasma flow burst intervals, Pm showssignificantenhancement, and simultaneous Pp indicates reduction. Associated Ptot is stable (its value is background level).

14. Contribution of Dawn-Dusk Oriented Electric Field (VxBz) in the Ey component Ey = VxBz – VzBx Dawn-Dusk and Dusk-Dawn oriented electric field components  Earth-Sunward and North-Southward magnetic flux transport aspects Ey = VxBz Dawn-Dusk oriented electric field component  Earth-Sunward magnetic flux transport aspect (Contribution by the plasma flow bursts) Large contribution by the plasma flow bursts Large contribution Little contribution Large contribution by the plasma flow bursts

15. Conclusion (I) We investigated the magnetic flux transport aspects and pressure variations/balance during the plasma flow bursts at geomagnetically quiet times… - In Events I and III, E//, Eperp > Ey was seen, suggesting that the magnetic flux (magnetic energy) is dominantly transported to the field-aligned and perpendicular directions to the local magnetic field line. - In almost cases, the plasma flow bursts play an important role for the transport of the magnetic flux in the magnetotail.

16. Conclusion (II) - Significant enhancements of the Bz component were observed at/near the plasma flow bursts. - In Events I and III, significant reduction (enhancement) of the plasma pressure (the magnetic pressure) was seen in association with the plasma flow bursts. - Simultaneous total pressure was almost constant (was consistent with the background level), suggesting that the plasma flow bursts were occurring under the state of equilibrium. From these results, “bubble” might play a crucial role for the generation of the plasma flow bursts during the geomagnetically quiet times.

17. Back Up Slides

18. Plasma Flow Bursts in magnetotail… 1.The sun-earthward plasma velocity component (Vx) and the perpendicular/parallel plasma velocity components to the local magnetic field line (V⊥or V//), whose ranges are higher than 300 km/s,are frequently observed around-10 Re ~-30 Rein the magnetotail plasma sheet. 2. The durations for these fast plasma flows are typically ~ 10 min.,which are identified as Bursty Bulk Flow (BBF), but the fast plasma flows with much shorter duration (within 5 min.) are also observed. These transient fast plasma flows are called as “Plasma Flow Burst”.3. Interestingly, these fast plasma flows are observed duringgeomagnetically both active and quiet times.

19. Fast Plasma flows during geomagnetically quiet times Result of the GEOTAIL observations from January 1997 to April 1998 (1.4 years). Only 4 fast plasma flows from 41 geomagnetic quiet conditions, which were identified by absence of significant auroral brightening with POLAR UVI observations, were selected [Ieda et al., 2003]. Also statistical result with the GEOTAIL observations from September 1993 to December 1994 (1.4 years). The durations for the fast plasma flows at geomagentically quiet times, identified as |AL| < 100 nT, were 446 hours (corresponding to about 19 days) [Ohtani et al., 2002].

20. Main characteristics of these plasma flow bursts at geomagnetically quiet times - THEMIS 1. The Vx component is higher than 300 km/s, and associated parallel (V//)and perpendicular (V⊥) velocities to the local magnetic field line are higher than 200 km/s. The fast flow duration is between1 and 2 minutes. 2. GSM-X <0 [Re] (Nightside) and -10 < GSM-Y < 10 [Re] (to avoid the MPCL crossing data). 3. The β value is higher than or nearly 0.5. - Ground 4.Kp indexrange is between-1 and +1andAL/AU indicesarelower than ±20 nT.

21. 3 Events of Magnetotail plasma flow bursts at geomagnetically quiet times

22. Contribution of Dawn-Dusk Oriented Electric Field (VxBz) in the Ey component and comparison with the direct measurement Ey = VxBz – VzBx Dawn-Dusk and Dusk-Dawn oriented electric field components  Earth-Sunward and North-Southward magnetic flux transport aspects Ey = VxBz Dawn-Dusk oriented electric field component  Earth-Sunward magnetic flux transport aspect (Contribution by the plasma flow bursts) Ey: A direct measurement with EFI onboard THEMIS