题目：On the current sheets surrounding dipolarizing flux bundles in the magnetotail: the case for wedgelets

主讲：刘江博士

时间：2013年01月15日（周二）上午10:00

地点：科学实验楼学术报告厅

摘要：Using THEMIS observations from four tail-seasons, we study the three-dimensional structure of the dipolarization front current sheet (DFCS), which demarcates the magnetic boundary of a dipolarizing flux bundle in Earth’s magnetotail. An equatorial cross section of the DFCS is convex; a meridional cross section is consistent with a dipolarized field line. The equatorial flow pattern in the ambient plasma ahead of the DFCS exhibits vorticities of opposite sense on its evening and morning sides. The magnetic field perturbations are consistent with local field-aligned current generation of region-2 sense ahead of the front and region-1 sense at the front. The median thickness of the DFCS increases from 800 to 2000 km with increasing distance from the neutral sheet, indicating bundle compression near the neutral sheet. On a meridional cross sect ion, the linear current density (1.2 - 1.8 nA/m) peaks ~±0.55l from the neutral sheet (where l is the ambient cross-tail current sheet half-thickness, l ~1.5 RE in our database). This peak, reminiscent of active-time cross-tail current sheet bifurcation noted in past studies, suggests that the intense but thin DFCS (10 to 20 nA/m2) may be produced by redistribution (diversion) of the extended but weaker cross-tail current (~1 nA/m2). Near the neutral sheet the average DFCS current over the DF thickness is perpendicular to both the magnetic field interior to the flux tube bundle and the average field direction over the DF thickness. Away from the neutral sheet the average current becomes progressively parallel to the internal field and the average field direction. The average current directions are indicative of region-1-sense current at the DF. As few as ~3 flux tube bundles can carry sufficient total current that, if redirected into the auroral ionosphere, can account for the substorm current wedge’s peak current for a sizable substorm (~1MA). A collapsing flux tube could thus be an elemental substorm current wedge, or “wedgelet”, that can divert a sizable portion of the ambient cross-tail current ahead of the dipolarization front into the auroral ionosphere.

简介：Education：

- University of California, Los Angeles, CA

M.S. in Space Physics, June 2010 (Advisor: Vassilis Angelopoulos)

- Peking University, Beijing, China July 2007

B.S. in Space Physics (Dissertation advisor: Prof. Pu Zuyin)

Research Experiences：

- 2006-2007: Studied the statistical behaviors of dayside magnetopause flux transfer events.

- 2007-2008: Investigated the flux transfer events’ modifications to the dayside magnetopause.

- 2008-2011: Analyzed multi-point substorm observations to solve the substorm sequence problem.

- 2011-present: Investigating the role that dipolarization fronts play in magnetotail dynamics.

Selected Publications：

- Liu, J., V. Angelopoulos, A. Runov, and X.-Z. Zhou (2013), On the current sheets surrounding dipolarizing flux bundles in the magnetotail: the case for wedgelets, J. Geophys. Res., accepted.

- Zhou, X.-Z., V. Angelopoulos, A. Runov, J. Liu, and Y. S. Ge (2012), Emergence of the active magnetotail plasma sheet boundary from transient, localized ion acceleration, J. Geophys. Res., 117, A10216, doi:10.1029/2012JA018171.

- Liu, J., et al. (2011b), Superposed epoch analysis of magnetotail flux transport during substorms observed by THEMIS, J. Geophys. Res., 116, A00I29, doi:10.1029/2010JA015886.

- Liu, J., et al. (2011a), Revised timing and onset location of two isolated substorms observed by Time History of Events and Macroscale Interactions During Substorms (THEMIS), J. Geophys. Res., 116, A00I17, doi:10.1029/2010JA015877.

- Ivanova, V., J. Liu, S. Kiehas, V. Semenov, and H. Biernat (2009), Inverse reconstruction technique based on time-dependent Petschek-type reconnection model: first application to THEMIS magnetotail observations, Ann. Geophys., 27, 4369-4377, doi:10.5194/angeo-27-4369-2009.

- Liu, J., et al. (2009), THEMIS observation of a substorm event on 04:35, 22 February 2008, Ann. Geophys., 27, 1831-1841.

- Liu, J., et al. (2008), THEMIS observations of the dayside traveling compression region and flows surrounding flux transfer events, Geophys. Res. Lett., 35, L17S07, doi:10.1029/2008GL033673.