Statistical visualization of the Earth's magnetotail based on Geotail data and the implied substorm model

Abstract

Abstract. We investigated the temporal and spatial development of the near-Earth magnetotail during substorms based on multi-dimensional superposed-epoch analysis of Geotail data. The start time of the auroral break-up (t=0) of each substorm was determined from auroral data obtained by the Polar and IMAGE spacecraft. The key parameters derived from the plasma, magnetic-field, and electric-field data from Geotail were sorted by their meridional X(GSM)-Z(proxy) coordinates.The results show that the Poynting flux toward the plasmasheet center starts at least 10 min before the substorm onset, and is further enhanced at X∼−12 R E (Earth radii) around 4 min before the onset. Simultaneously, large-amplitude fluctuations occurred, and earthward flows in the central plasma sheet between X∼−11 R E and X∼−19 R E and a duskward flow around X=−10 R E were enhanced. The total pressure starts to decrease around X=−16 R E about 4 min before the onset of the substorm. After the substorm onset, a notable dipolarization is observed and tailward flows commence, characterised by southward magnetic fields in the form of a plasmoid.We confirm various observable-parameter variations based on or predicted by the relevant substorm models; however, none of these can explain our results perfectly. Therefore, we propose a catapult (slingshot) current-sheet relaxation model, in which an earthward convective flow produced by catapult current-sheet relaxation and a converted duskward flow near Correspondence to: S. Machida (machida@kugi.kyoto-u.ac.jp) the Earth are enhanced through flow braking around 4 min before the substorm onset. These flows induce a ballooning instability or other instabilities, causing the observed current disruption. The formation of the magnetic neutral line is a natural consequence of the present model, because the relaxation of a highly stretched catapult current-sheet produces a very thin current at its tailward edge being surrounded by intense earthward and tailward magnetic fields which were formerly the off-equatorial lobe magnetic fields. This location is the boundary between a highly stressed catapult current sheet and a Harris-type current sheet characterized by little stress. In addition, the flows induced around the boundary toward the current-sheet center may enhance the formation of the magnetic neutral line and the efficiency of magnetic reconnection. After magnetic reconnection is induced, it plays a significant role in driving the substorm.

The location and rate of occurrence of near-Earth magnetotail reconnection as observed by Cluster and Geotail

“…7d) with the simple model for DR formation and evolution discussed in Section 1. In this simple model the DR forms near Earth (within ≥ − * X R 25 GSM E ), where for high solar wind energy input the DR forms within ≥ − * X R 20 GSM E and for low solar wind energy input the DR forms tailward of this region ( ≤ − * X R 20 GSM E ) (Nagai et al, 2005;Machida et al, 2009). After formation, the DR is expelled tailward towards larger negative * X GSM (Oka et al, 2011).…”

“…Observations of the reconnection site have been made over a wide range of locations along the 2D magnetotail neutral sheet, where observations are typically discussed in terms of their dependence on * X GSM and * Y GSM . Machida et al (2009) analyzed several years worth of Geotail-observed substorm-related changes to the structure of the magnetotail magnetic field and plasma. They found that the average region where the reconnection site formed was ∼ R 20 E (Earth radii) down the magnetotail ( = − X R 20 = − * X R 15 GSM E (Miyashita et al, 2009;Eastwood et al, 2010).…”

Tearing stability of a multiscale magnetotail current sheet

“…This is consistent with the conclusion based on the analysis of Geotail observations [Asano et al, 2004;Miyashita et al, 2009], that the magnetic reconnection in the tail starts near the tailward edge of an intense cross-tail current, which extends from X ∼ −5 to −20 R E in the GSM coordinate system. Moreover, a distribution of the northsouth magnetic field B z similar to the specific profile shown in Figure 2a has recently been inferred from the statistical vizualization of the magnetotail Geotail data prior to the substorm onset (Machida et al [2009], see in particular, their Figure 2c). The formation of a thin current sheet embedded into a thicker plasma sheet and extended from near geosynchronous orbit to the mid -magnetotail (∼30 R E ) during the growth phase of a substorm has been shown using multisatellite measurements by Pulkkinen et al [1999].…”

Full-particle simulations of instabilities in a thin current sheet of the magnetospheric system prior to substorm onset

“…Recent simulation studies that impose the convective electric field as a boundary condition at the magnetospheric lobe have shown that tearing instability and magnetic reconnection can be driven (Liu et al 2014;Pritchett 2005). Sitnov and Swisdak (2011) showed that the "X-point" (the point where oppositely directed magnetic-field lines reconnect) of magnetic reconnection forms on the tailward side of the B z -hump in the CS, as was found by Machida et al (2009;, because the electric energy of the Poynting flux is locally concentrated. The observational results reported in Machida et al (2009;, however, require simulations of both the DL field and the CS region.…”

“…A magnetic reconnection arises at X ∼ −20 Re as the consequence of the process. Machida et al (2009Machida et al ( , 2014 performed superposedepoch analyses with Geotail and THEMIS data to investigate the physical conditions in the magnetotail around the substorm onset. They have suggested a new substormtriggering model, namely, the "Catapult Current Sheet Relaxation" (CCSR) model, based on the results.…”