Global properties of magnetotail current sheet flapping: THEMIS perspectives

Abstract

Abstract.A sequence of magnetic field oscillations with an amplitude of up to 30 nT and a time scale of 30 min was detected by four of the five THEMIS spacecraft in the magnetotail plasma sheet. The probes P1 and P2 were at X=−15.2 and −12.7 R E and P3 and P4 were at X=−7.9 R E . All four probes were at −6.5>Y >−7.5 R E (major conjunction). Multi-point timing analysis of the magnetic field variations shows that fronts of the oscillations propagated flankward (dawnward and Earthward) nearly perpendicular to the direction of the magnetic maximum variation (B 1 ) at velocities of 20-30 km/s. These are typical characteristics of current sheet flapping motion. The observed anti-correlation between ∂B 1 /∂t and the Z-component of the bulk velocity make it possible to estimate a flapping amplitude of 1 to 3 R E . The cross-tail scale wave-length was found to be about 5 R E . Thus the flapping waves are steep tail-aligned structures with a lengthwise scale of >10 R E . The intermittent plasma motion with the cross-tail velocity component changing its sign, observed during flapping, indicates that the flapping waves were propagating through the ambient plasma. Simultaneous observations of the magnetic field variations by THEMIS ground-based magnetometers show that the flapping oscillations were observed during the growth phase of a substorm.

Current sheet flapping in the near-Earth magnetotail: peculiarities of propagation and parallel currents

“…Thus, we can conclude that flapping CSs in the near-Earth magnetotail are more or less uniform in the (X, Y ) plane within the spatial scales about 1-2 R E . This result confirms the previously reported THEMIS observations of large-scale flapping motion (Runov et al, 2009;Kubyshkina et al, 2014).…”

“…Further, THEMIS (Runov et al, 2009) and Cluster/Double Star (Zhang et al, 2005) observations have confirmed the large-scale nature of the flapping motion. Cluster spacecraft separation in [2001][2002][2003][2004] was varied between 300 and 4000 km.…”

Numerical linearized MHD model of flapping oscillations

“…This mechanism has a substantial advantage, because it may explain the non-zero xcomponent of the wave vector of some flapping wave fronts observed in the magnetotail. 10 On the other hand, the DG model represents a long-wavelength limit of the general ballooning mode. 34 Generally speaking, the wave vector of the latter has k x 6 ¼ 0, while k ¼ ð0; k y ; 0Þ is a limiting case only.…”

“…Flapping oscillations are long-wavelength perturbations propagating in the magnetotail current sheet in transversal direction, mainly from the central part of the sheet towards the flanks, registered in the Earth, Venus, Jupiter, and Saturn magnetospheres. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] Typically (for Earth magnetotail conditions), flapping waves are observed in the central part of the midtail at distances of 10 À 30 Re, where Re is the Earth radius, propagating with a speed of 20 À 60 km=s ($10 times less than the Alfv en velocity), an amplitude of 1 À 2 Re, and a quasiperiod of several minutes. Flapping motions in the distant magnetotail are reported in Refs.…”

Profiles of electron temperature and <I>B</I><sub>z</sub> along Earth's magnetotail

“…THB spacecraft crossed CS; then THC and THE (as well as THD) crossed B x = 0 in agreement with the distribution of z-coordinates of spacecraft. Synchronous crossings correspond to the vertical flapping motions of the CS (see the detailed study of CS flapping observed by several THEMIS spacecraft in Runov et al, 2009).…”