N. P. Dmitrieva

On the conditions preceding sudden magnetotail magnetic flux unloading

According to the substorm paradigm, the tail magnetic flux F is accumulated during the growth phase and suddenly released afterward. However, only 33 out of 142 isolated midtail sudden unloading events identified by Geotail in 1995–1998 demonstrated monotonous F increase (type 1 substorms). In 32 cases, balanced tail flux (BTF) period (F ~constant) with enhanced dissipation and pseudobreakup activity during 0.5–2 h before unloading was registered (type 2, a new substorm class). The rest 77 substorms lie between types 1 and 2. Superposed epoch analysis revealed that during ~40 min before unloading, both groups demonstrate almost identical plasma sheet thinning and Bz decrease at Geotail. By the sudden unloading time, both groups have nearly identical Bz values in the midtail as well as at 6.6 RE. We conclude that pre-onset midtail configuration changes are common for all substorm types irrespective of flux growth.

Magnetosheath Propagation Time of Solar Wind Directional Discontinuities

Observed delays in the ground response to solar wind directional discontinuities have been explained as the result of larger than expected magnetosheath propagation times. Recently, Samsonov et al. (2017, https://doi.org/10.1002/2017GL075020) showed that the typical time for a southward interplanetary magnetic field (IMF) turning to propagate across the magnetosheath is 14 min. Here by using a combination of magnetohydrodynamic simulations, spacecraft observations, and analytic calculations, we study the dependence of the propagation time on solar wind parameters and near-magnetopause cutoff speed. Increases in the solar wind speed result in greater magnetosheath plasma flow velocities, decreases in the magnetosheath thickness and, as a result, decreases in the propagation time. Increases in the IMF strength result in increases in the magnetosheath thickness and increases in the propagation time. Both magnetohydrodynamic simulations and observations suggest that propagation times are slightly smaller for northward IMF turnings. Magnetosheath flow deceleration must be taken into account when predicting the arrival times of solar wind structures at the dayside magnetopause.