1. Magnetic Field and Plasma Responses in the Near-Earth Magnetotail and Magnetospheric Boundary Layer During an Encounter of Heliospheric Current Sheet Motoharu Nowada 1 [nowada@pku.edu.cn] Motoharu Nowada 1 [nowada@pku.edu.cn], Ching-huei Lin 2, Zu-yin Pu 1, Sui-yan Fu 1, Howard J. Singer 3, Vassilis Angelopoulos 4, Charles W. Carlson 5, Hans-Ulrich Auster 6 1: ISPAT/SESS, Peking University, People Republic of China 2: Department of Electric Engineering, Chin Yun University, Taiwan 3. NOAA Space Environment Center, Boulder, Colorado, USA 4. IGPP, UCLA, California, USA 5. SSL, UCLA, California, USA 6. IGEP, Technishche Universität, Braunschweig, Deutschland Z131 EGU2011-2340

2. Abstract We examined responses for the magnetic field and plasma in the near-earth magnetotail and boundary layer due to an encounter of Heliospheric Current Sheet (HCS) indicated as a sharp discontinuity in the B x component of the Interplanetary Magnetic Field (IMF) without striking variation of the solar wind dynamic pressure and the polarity change of the IMF-B z component. In this study, the magnetic field and plasma data obtained from THEMIS, Cluster, GOES-11 and -12 to investigate the magnetospheric responses, and those taken from ACE and GEOTAIL to monitor the solar wind conditions are utilized. Simultaneous geomagnetic field variations at high-latitude ground observatories were also examined. When HCS encountered with the magnetosphere, THEMIS-D, -E and -A observed the abrupt and transient magnetic field and plasma variations in the dawnside near-earth magnetotail. Although Cluster was oppositely located in the duskside magnetotail, much smaller magnetic field perturbations were seen. Simultaneous substorm- related dipolarization and negative bay variations with the Pi 2 waves were observed by GOES-11, -12 and the ground observatories. THEMIS-C, which was located in the magnetosheath near the dawnside magnetopause, experienced the single/multiple magnetopause crossings due to the magnetopause surface waves. These results suggest to attribute the abrupt and transient magnetic field and plasma variations observed by THEMIS-D, -E and -A, and the magnetopause undulations observed by THEMIS-C to substorm and associated inner-magnetospheric configuration changes. Also, they indicate that clear magnetospheric disturbances can be brought even without significant variations of the solar wind dynamic pressure and the polarity change of the IMF-B z component.

3. The 9 satellites’ Locations at 07:06 UT on November 25 2007

4. Solar Wind Observations by WIND and GEOTAIL from 5:30 UT - 8:10 UT A : Duration for the THEMIS-A ’s Magnetospheric Magnetic Field and Plasma Variations D/E : Duration for THEMIS-D/-E ’s Magnetospheric Magnetic Field and Plasma Variations

5. MF Rotation and Plasma Variations: Heliospheric Current Sheet (HCS) B nz had been the northward direction before and after an encounter of HCS with ACE. See [Crooker et al. 1993; Crooker and Intriligator 1996].

6. THEMIS-A Observation from 6:30 UT - 8:10 UT 1 st Magnetic Field (MF)/Plasma (PL) variations: 7:01:35 UT – 7:06:09 UT 2 nd MF/PL variations: 8:01:03 UT – 8:04:53 UT

7. THEMIS-D and -E and Cluster Observations 1 st MF/PL variations: 7:05:48 UT – 7:13:19 UT No 2 nd MF/PL variations ✓ The duration of the MF/PL variations and the aspects for them are totally different from those of THEMIS-A’s observation. ★ The physical mechanisms to cause the variations observed by between THEMIS-A and –D/E are different ? No particular magnetic field variations observed by Cluster

8. THEMIS’s Footpoint Magnetic Field Variations CARISMA Magnetometer Chain Magnetic Field Fluctuations and SYM-H Variation TH-A TH-D/-E

9. A GOES-11/-12 Magnetic Field Observation Bz Enhancement due to dipolarization and Pi 2 waves related with substorm Θ inc = tan -1 (Bz/Bx) Tailward magnetic field stretching due to substorm AD/E

10. Before

11. Within

12. Comparison of the observation of THEMIS-C in the magnetosheath with that of THEMIS-A in the magnetosphere THEMIS-A (Ion) THEMIS-C (Ion) THEMIS-A (electron) THEMIS-C (electron)

13. Time Sequence of Observed Events

14. Discussion-I By an encounter of HCS with the magnetosphere... - Small-scale (weakly intensified) substorm was occurred. - Rapid change for the inner-magnetospheric configuration was observed. Furthermore, magnetopause surface waves or motion was induced by the substorm- associated geomagnetic condition change. These phenomena were observed during non southward IMF-B z period without particular solar wind dynamic pressure enhancement !

15. Discussion-II Aspects for magnetic field and plasma variations observed between THEMIS- A and -D/E in the dawnside magnetosphere are totally different. ★ Abrupt and transient magnetic field and plasma variations observed by… ✓ THEMIS-A  Intermittent magnetopause crossings/deformations induced by magnetopause wave (motions) due to inner-magnetospheric condition change - No striking plasma density and velocity variations, and dynamic pressure enhancement in the solar wind… Simultaneous IMF-B z was not southward. The inner-magnetospheric configuration (condition) changes due to substorm induced the magnetopause’s surface waves (motions) [e.g., Aubry et al. 1970; Meng 1970; Nikolaeva et al. 2006]. ✓ THEMIS-D/E  Direct Influence by the substorm - Consistency between Pi 2 waves, dipolarization (B z enhancement) observed by GEOES-11/12 and at ground-based observatories, and the magnetic field/plasma variations in the magnetosphere. - The slight enhancement in the background of the B z magnetic field was also found (dipolarization signature ?). ✓ Cluster in the duskside magnetosphere observed no outstanding magnetic field fluctuations as THEMIS, but clearly substorm-associated MHD waves (e.g., Pi 2 waves) were detected, although their spectrum’s intensity was much smaller than those observed by THEMIS.

16. Discussion-III Why did the magnetic field and plasma variations caused by a HCS-associated substorm localized (the dawnside magnetotail region in this case) ?  Substorm locally occurred, and its intensity was so moderate (small-scaled). Therefore, observed magnetic field and plasma variations were transient and localized. ★ The source of this substorm would be related with the magnetotail lobe magnetic reconnection (MLMX) because “MLMX” occurrence rate is sensitive to the IMF-B x orientation.

17. Conclusion 1. Because the abrupt and transient magnetic field and plasma variations were clearly localized in the dawnside near-earth magnetotail and their durations were also short-lived, substorm triggered by an encounter of HCS occurs locally, and its intensity is so moderate. 2. Inner-magnetospheric configuration can be easily changed by only one solar wind parameter's striking variation, if the solar wind conditions do not have significant variations that would make the magnetospheric condition dramatically change, such as a sudden or long-term variation of the solar wind dynamic pressure, and an encounter of the impulsive or large southward (northward) IMF- Bz structure to cause large-scale geomagnetic disturbance.

18. Extra Figures

19. Incidental FTEs (Flux Transfer Events) occurred during 7:05:15 UT – 7:05:50 UT as bracketed by two magenta broken lines ✓ Bn bipolar perturbation ✓ Bt prominent Enhancement ✓ No clear correlation between the Bn and Vn ✓ Prominent or transient variations in both Np and Ptot (or Pm) Magnetic field and plasma signatures bracketed by two gray broken lines are not associated with FTEs

20. Discussion-III Why did the magnetic field and plasma variations caused by a HCS- associated substorm localized (the dawnside magnetotail region in this case) ? ★ The source of this substorm would be related with magnetotail lobe magnetic reconnection (MLMX). - The positive (sunward) IMF-Bx interval (before an HCS observation): MLMX occurs in the lobe region in the southern hemisphere. - The negative (tailward) IMF-Bx interval (after an HCS observation): The location of MLMX would switch from the southern to northern hemispheres. This sudden change for MLMX location might trigger the substorm.  Resultant substorm occurred in only southern hemisphere (namely, locally occurred) and its intensity was so moderate. Therefore, the magnetic field and plasma variations associated with substorm might be observed locally (in the dawnside magnetotail region).