Z. Y. Li

THEMIS observations of the near-Earth plasma sheet during a substorm

We present observations of a substorm on 13 March 2008 by Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft in the near-Earth tail during the plasma sheet expansion evidenced by increase in the plasma density and temperature. The main features of the event are as follows: (1) Cross-tail current reduction or current disruption (CD) was observed in the near-Earth tail at X similar to -8.0 R-E and Y similar to 2.0 R-E, marked by a sharp drop of vertical bar B-x vertical bar and accompanied by sharp increases in the plasma density and temperature, manifesting a rapid expansion (recovery) of the local plasma sheet. During the course of the plasma sheet expansion, the propagation speed of the dipolarization is similar to 48 km/s in tailward direction and similar to 35 km/s in azimuthal direction. ( 2) In the inner edge of the plasma sheet, slow flux pileup is observed. The magnetic flux pileup is characterized by continuous enhancement of B-z and B-t and a reduction of the plasma density, pressure P-th, and beta in the flow-braking region. The tailward moving plasma sheet expansion (CD) is also passing across the flow-braking region. In short, the dipolarization in the inner edge of the plasma sheet can be attributed to CD, while the tailward progression of dipolarization in the flow-braking region can be attributed to magnetic flux pileup. (3) A sharp decrease in the magnitude of B-x at P1 (-13.1 R-E, 2.5 R-E, -0.56 R-E) prior to the dipolarization at P5 is difficult to explain as part of the outward evolution of the CD. The rapid change in the magnetic field topology, and signatures of earthward flows and dipolarization observed at P1 prior to Pi2 onset may be caused by inward motion of flux from magnetic reconnection in the midtail (20-30 R-E). Tail reconnection prior to substorm expansion can result in a sudden change of the near-Earth configuration, which may result in instabilities related to the onset of CD.

A comparison of Neutral Atom Detector Unit neutral atom image inversion with a comprehensive ring current model

We present energetic neutral atom (ENA) images in the energy range 45 to 50 keV for H and 92 to 138 keV for O measured by the Neutral Atom Detector Unit (NUADU) onboard Double Star TC-2 during a geomagnetic storm on 8 May 2005. We compare the ion fluxes deduced from inversion of the NUADU image with those calculated using the Comprehensive Ring Current Model (CRCM). This comparison shows that the two approaches are consistent when used to derive the configuration of the corresponding global ion distribution and the peak ion fluxes. The deduced peak ion flux is located in the premidnight sector at 1540 UT, while the deduced ion peak flux is located in the midnight sector at 1610 UT. There are strong ion fluxes in the region between L = 2 and L = 4 which form a closed loop configuration. The ion peak flux is about 2.2 x 10(6)/cm(2)/sr/keV/s. The deduced ion distribution agrees well with the NUADU measurement. The agreement between the inverted ion distributions and the CRCM results give us confidence in applying our ENA imaging and modeling techniques to the study of the evolution of the inner magnetosphere plasma distribution and the global dynamics of the ring current during magnetic storms.