G. C. Zhou

Joint observations by Cluster satellites of bursty bulk flows in the magnetotail

[1] Using the observations of three satellites of Cluster (C1, C3, and C4) during the periods July to October 2001 and July to October 2002, we study 209 active time bursty bulk flows (BBFs), the difference between single satellite observations and multisatellite observations, and the difference among three selection criteria (two about BBFs and one about rapid convection event). Single satellite observations show that the average duration of BBFs selected by the criterion of Angelopoulos et al. is 604 s, while multisatellite observations show that the average duration of BBFs is 1105 s. Single satellite sometimes misses the BBFs. The missing ratio of single satellite is 22.4% for the criterion of Angelopoulos et al. and 44.9 % for the criterion of Raj et al. Therefore the single satellite observations cannot tell the true number of BBFs. The multisatellite observations are more important for the criterion of Raj et al. The single satellite observations also show that 22% of substorms are not accompanied by BBFs, while multisatellite observations show that only 4.5% of substorms are not accompanied by BBFs. Thus it seems possible that all substorms are accompanied by BBFs. The occurrence frequency of RCEs in the central plasma sheet obtained by multisatellites is 12.2%. The occurrence frequency of BBFs in the central plasma sheet is 9.5% for single satellite observations and 19.4% for multisatellite observations. So BBFs may contribute more to the transport of magnetic flux, mass, and energy than what was estimated by previous studies based on single satellite observations.

Multiple responses of magnetotail to the enhancement and fluctuation of solar wind dynamic pressure and the southward turning of interplanetary magnetic field

During the interval from 06:15 to 07:30 UT on 24 August 2005, the Chinese Tan-Ce 1 (TC1) satellite observed the multiple responses of the near-Earth magnetotail to the combined changes in solar wind dynamic pressure and interplanetary magnetic field (IMF). The magnetotail was highly compressed by a strong interplanetary shock because of the dynamic pressure enhancement (similar to 15 nPa), and the large shrinkage of magnetotail made a northern lobe and plasma mantle move inward to the position of the inbound TC1 that was initially in the plasma sheet. Meanwhile, the dynamic pressure fluctuations (similar to 0.5-3 nPa) behind the shock drove the quasi-periodic oscillations of the magnetopause, lobe-mantle boundary, and geomagnetic field at the same frequencies: one dominant frequency was around 3 mHz and the other was around 5 mHz. The quasi-periodic oscillations of the lobe-mantle boundary caused the alternate entries of TC1 into the northern lobe and the plasma mantle. In contrast to a single squeezed or deformed magnetotail by a solar wind discontinuity moving tailward, the compressed and oscillating magnetotail can better indicate the dynamic evolution of magnetotail when solar wind dynamic pressure increases and fluctuates remarkably, and the near-Earth magnetotail is quite sensitive even to some small changes in the solar wind dynamic pressure when it is highly compressed. Furthermore, it is found that a considerable amount of oxygen ions (O(+)) appeared in the lobe after the southward turning of IMF.

Density depletion and Hall effect in magnetic reconnection

Density depletions were detected by Wind, Cluster, and Polar spacecrafts in the observations of diffusion region encounters at the Earths magnetotail and magnetopause. In this report we investigate the layers of density depletion in magnetic reconnection using a 2.5-dimensional Hall MHD code developed from a multistep implicit scheme. The numerical results at the quasi-steady state of the Hall MHD reconnection with d(i)/L-c >= 1.0, where d(i) is the ion inertial length and L-c is the half thickness of the initial current sheet, show not only the density depletions along the magnetic separatrices but also a density dip in the x direction near the X neutral line. The comparative tests with various Hall terms demonstrate that the density depletion in the magnetic reconnection is a peculiar feature of the case with a strong Hall effect (d(i)/L-c >= 1.0). The layers of low density following the shape of separatrices are in coincidence with the regions of high magnetic pressure. In the spatial profile of density rho along z, which is in quantitative agreement with Cluster observation, the obvious dips located at the separatrices coincide with the peak and valley in the profile of K-H/rho (J x B)(z) for the case with d(i)/L-c = 2.0. It indicates the major role of the Hall term in the formation of the density depletion layers near the separatrices. On the basis of the comparison between Wind observation and simulation results, we argue that the density dip observed by Wind would be distributed around the reconnection X-line, rather than along the magnetic separatrix. In the case with a strong Hall effect, the in-plane ion flows go around the diffusion region, and enterable ions in this region are significantly reduced due to the action of the in-plane Lorentz force. A density dip in the vicinity of the X-line is attributed to the hard entry of in-plane ion flow and might be related to an increase of the ion drift velocity in the y direction.