Dayside magnetospheric and ionospheric responses to a foreshock transient on June 25, 2008: 2. 2-D evolution based on dayside auroral imaging

TL;DR

This study uses high-resolution auroral imaging to analyze how a foreshock transient on June 25, 2008, affected the magnetosphere and ionosphere, revealing detailed propagation and response mechanisms.

Contribution

It provides detailed 2-D observations of auroral responses to a foreshock transient, enhancing understanding of magnetosphere-ionosphere coupling processes.

Findings

Diffuse aurora brightened and propagated duskward at ~100 km/s.

Discrete aurora brightened following diffuse aurora, extending duskward.

Field-aligned currents moved duskward consistent with auroral motion.

Abstract

The foreshock region involves localized and transient structures such as foreshock cavities and hot flow anomalies due to solar wind-bow shock interactions, and foreshock transients have been shown to lead to magnetospheric and ionospheric responses. In this paper, the interaction between a foreshock transient and the magnetosphere-ionosphere system is investigated using dayside aurora imagers revealing structures and propagation in greater detail than previously possible. A foreshock transient was detected by THEMIS-B and C during 1535-1545 UT on June 25, 2008. THEMIS-A, D and E observed magnetopause compression, cold plasma enhancement and ULF waves in the dayside magnetosphere. The all-sky imager (ASI) at South Pole observed that both diffuse and discrete aurora brightened locally soon after the appearance of this foreshock transient. The diffuse aurora brightening, which corresponded to a region a few Re size in GSM-Y in the equatorial plane, propagated duskward with an average speed of ~100 km/s. Soon after the diffuse aurora brightened, discrete aurora also brightened and extended duskward, which was consistent with the motion of the foreshock transient as it swept through the magnetosheath while impacting the magnetopause. Equivalent horizontal currents measured by magnetometers revealed a pair of field-aligned currents (FACs) moving duskward consistent with motion of the discrete aurora patterns. We conclude that the high-resolution and two-dimensional observation of auroral responses by ground-based ASI can help to estimate the evolution and propagation of upstream foreshock transients and their substantial impacts on the magnetosphere-ionosphere coupling system, including magnetospheric compression and currents in the ionosphere.