Magnetopause expansions for quasi-radial interplanetary magnetic field: THEMIS and Geotail observations

TL;DR

This study uses THEMIS and Geotail data to analyze prolonged magnetopause expansions during steady quasi-radial IMF conditions, revealing a low-pressure magnetosheath mode that causes significant outward shifts of the magnetopause.

Contribution

It identifies a new low-pressure magnetosheath mode linked to specific upstream conditions, explaining large magnetopause expansions during steady quasi-radial IMF periods.

Findings

Magnetopause can expand over 7 RE beyond its nominal position during steady conditions.

A low-pressure magnetosheath mode reduces total pressure to as low as 0.16 nPa.

Magnetopause position is modulated by continuous wavy motions across various scales.

Abstract

We report THEMIS and Geotail observations of prolonged magnetopause (MP) expansions during long-lasting intervals of quasi-radial interplanetary magnetic field (IMF) and nearly constant solar wind dynamic pressure. The expansions were global: the magnetopause was located more than 3 RE and ~7 RE outside its nominal dayside and magnetotail locations, respectively. The expanded states persisted several hours, just as long as the quasi-radial IMF conditions, indicating steady-state situations. For an observed solar wind pressure of ~1.1-1.3 nPa, the new equilibrium subsolar MP position lay at ~14.5 RE, far beyond its expected location. The equilibrium position was affected by geomagnetic activity. The magnetopause expansions result from significant decreases in the total pressure of the high-beta magnetosheath, which we term the low-pressure magnetosheath (LPM) mode. A prominent LPM mode was observed for upstream conditions characterized by IMF cone angles less than 20 ~ 25 grad, high Mach numbers and proton plasma beta<1.3. The minimum value for the total pressure observed by THEMIS in the magnetosheath adjacent to the magnetopause was 0.16 nPa and the fraction of the solar wind pressure applied to the magnetopause was therefore 0.2, extremely small. The equilibrium location of the magnetopause was modulated by a nearly continuous wavy motion over a wide range of time and space scales.