Growth‐phase thinning of the near‐Earth current sheet during the CDAW 6 substorm

Abstract

The thinning of the near-Earth current sheet during the growth phase of the CDAW 6 magnetospheric substerm is studied. The exp,'msion onset of the substerm occurred at 1054 UT, March 22, 1979. During the growth phase, two spacecraft, ISEE 1 and ISEE 2, were within the current sheet approximately 13R e from the Earth and obtained simultaneous highresolution magnetic dam at two points in the current sheet. Plasma data were also provided by the ISEE spacecraft and solar wind dam by IMP 8. To facilitate the analysis, the GSM magnetic field data are transformed to a "neutral sheet coordinate system" in which the new x axis is parallel to the average magnetic field above m•d below the neutral sheet and the new y axis lies in the GSM equatorial pl,-me. A model based on the assumption that the current sheet is a •ne-invariant structure fails to predict neutral sheet crossing times. Consequently, the Harris sheet model, which allows one to remove the restriction of time invariancy, is used instead. It is found that during the growth phase, a model p,-tr,'uneter corresponding to the thickness of the current sheet decreased exponentially from about 5R e to IR e with a time constant of about 14 min. In addition, the ISEE 1 ,'red ISEE 2 neutral sheet crossings after expansion onset indicate that the neutral sheet was moving upward at 7 km/s relative to the spacecraft. Since both crossings occurred in approximately 80 s, the current sheet thickness is estimated to be about 500 km. These results demonstrate that the near-Earth current sheet undergoes dramatic thinning during the substerm growth phase m•d expansion onset.

Models of Three‐dimensional Flux Ropes

“…Most of these models are based on the assumption that the magnetic configuration prior to an eruption is force-free (i.e., with the current flowing along the magnetic field), but such an assumption is not valid in the vicinity of the helmet streamer current sheet (Li et al 1998), and it is also possible that localized non-force-free current sheets develop prior to an eruption. Recent results in the modeling of the Earth's magnetotail prior to the onset of a magnetospheric substorm indeed suggest that the formation of a thin current sheet may be a late phase leading to the loss of equilibrium (Birn & Schindler 2002), consistent with observations (e.g., McPherron, Nishida, & Russell 1987;Sergeev et al 1993;Sanny et al 1994). Also, only some of the models (e.g., Gibson & Low 1998) include the prominence, the surrounding coronal cavity, or the helmet streamer above, all of which are typically associated with CMEs.…”

Circulation of Heavy Ions and Their Dynamical Effects in the Magnetosphere: Recent Observations and Models

“…As a result, the thickness of the CS can decrease from several Earth radii down to ∼250 to 2000 km, comparable to a few proton gyroradii (Fairfield 1984;McPherron et al 1987;Lui et al 1992;Sergeev et al 1993;Sanny et al 1994;. Such a thin CS (TCS) has metastable properties, i.e., it can be stable during relatively long time periods from several minutes to 2 hours, but then may be spontaneously destroyed due to plasma instabilities (e.g.…”

Spectral characteristics of protons in the Earth's plasmasheet: statistical results from Cluster CIS and RAPID

“…Since the plasma sheet sometimes gets extremely thin during disturbed conditions (see e.g., Sanny et al, 1994;Pulkkinen et al, 1999;Sergeev et al, 2008), Cluster often completely crosses or enters and exits the central plasma sheet several times within the 3 h interval used to determine the Kp. The minor difference in β criteria for identifying the central plasma sheet between the present study and that ofÅsnes et al (2008) sheet does not play any role.…”