Flattened current sheet and its evolution in substorms

Abstract

[1] In this research, the properties of a tail current sheet, which has a flattened geometry, and its evolution during substorm activity have been investigated. The geometrical configuration of the magnetic field and the spatial distribution of the current density in a flattened current sheet have been revealed with certainty for the first time. It is found that such a flattened current sheet has sufficiently strong B y (GSM) within its neutral sheet that the magnetic field lines (MFLs) in the neutral sheet are lie almost in the GSM equatorial plane and that the normal directions are generally northward. Detailed analyses show that, the magnetic field lines are spiral-like, not plane curves, which are left-handed or right-handed spirals for B y > 0 or B y < 0. This magnetic rotation occurs predominantly in the neutral sheet. The flattened current sheet may be very thin, and the thickness of the neutral sheet is much less than the minimum radius of the curvature of the MFLs in the current sheet. The analysis also suggests that the neutral sheet current is field-aligned and lies mainly duskward. The curvature current makes little contribution to the total current in the flattened current sheet. The main current carriers in the neutral sheet of the flattened current sheet are electrons. A statistical survey shows that there is one positive correlation between B y in the flattened current sheet and IMF B y and penetration efficiency is 0.67. Flattened current sheets may occur in both quiet and disturbed periods and may appear at all phases of the substorms. During the growth phase of a substorm event, the neutral sheet of the flattened current sheet is shown to become progressively thinner, while the associated current density is increasing gradually. It is found that the northern turning of the IMF has triggered the explosive growth phase at the end of the growth phase, which lasts several minutes. At the explosive growth phase, the flattened current sheet becomes much thinner and the current density in the neutral sheet then increases considerably and reaches a value larger than 0.017 mAm À2 . Just after the onset of the substorm, the current density in the neutral sheet drops abruptly and varies turbulently.

Magnetic configurations of the tilted current sheets in magnetotail

“…As in previous investigations (Shen et al, 2007a), the neutral sheet (NS) is defined as the region within which B≤ √ 2B min in the current sheet. The magnetic field vector rotates about 90 • in the NS (Shen et al, 2008b), so the NS is actually the magnetic vector rotation layer in the current sheet. We have given the explicit and quantitative definition of the tilt angle of the tilted current sheet for the first time: the tilt angle δ of the tilted current sheet as the complementary angle of the angle between the normaln of the CS and the binormalN of the MFLs in the NS.…”

“…According to earlier investigations (Shen et al, 2007a), the neutral sheet (NS) can be defined as the region within which B≤ √ 2B min , where B min is the minimum value of the magnetic strength at the center of the current sheet. Within the NS defined above, the magnetic field vector rotates about 90 • totally from one boundary to another of the NS (Shen et al, 2008b). Because the main rotation of the magnetic vector occurs within the NS, we may regard the NS as the magnetic vector rotation layer in the current sheet.…”

“…Another approach for determining the half thickness of the onedimensional NS is based on the Ampere's Law (Shen et al, 2008b), that is…”

The scale of the magnetotail reconnecting current sheet in the presence of O+

“…A third method is to use the magnetic field gradient estimation, a method first proposed by Shen et al [2008]. With the Cluster curlometer technique, we are able to estimate the ∇B during the current sheet crossing.…”

Magnetic helicity of a flux rope in the magnetotail: THEMIS results

“…Under the interaction with solar wind, the magnetic field of the Earth is distorted from a normal dipolar field to form the magnetosphere (Dungey, 1961). Magnetic field lines in magnetotail become flattened, and especially those in the plasma sheet lean to the equator and are sheared (Shen et al, , 2008. Additionally, due to magnetic reconnection, the magnetic field lines in the transition layers such as the magnetopause and the plasma sheet change their configuration sharply.…”