What controls the local time extent of flux transfer events?

TL;DR

This study compares flux transfer event signatures under different solar wind conditions, revealing that solar wind speed and IMF components influence the local time extent of FTEs, impacting magnetosphere coupling.

Contribution

It demonstrates how variations in solar wind speed and IMF orientation affect the local time extent of FTEs during similar geomagnetic conditions.

Findings

FTEs extended over 7 hours of local time with higher solar wind speed.

Lower solar wind speed resulted in FTEs spanning about 2 hours.

Reconnection rates correlated with the extent of FTE signatures.

Abstract

Flux transfer events (FTEs) are the manifestation of bursty and/or patchy magnetic reconnection at the magnetopause. We compare two sequences of the ionospheric signatures of flux transfer events observed in global auroral imagery and coherent ionospheric radar measurements. Both sequences were observed during very similar seasonal and interplanetary magnetic field (IMF) conditions, though with differing solar wind speed. A key observation is that the signatures differed considerably in their local time extent. The two periods are 26 August 1998, when the IMF had components $\mathit{B}_{\mathit{z}}$ $\approx$ -10 nT and $\mathit{B}_{\mathit{y}}$ $\approx$ 9 nT and the solar wind speed was $\mathit{V}_{\mathit{x}}$ $\approx$ 650 km $s^{-1}$, and 31 August 2005, IMF $\mathit{B}_{\mathit{z}}$ $\approx$ -7 nT, $\mathit{B}_{\mathit{y}}$ $\approx$ 17 nT, and $\mathit{V}_{\mathit{x}}$ $\approx$ 380 km $s^{-1}$. In the first case, the reconnection rate was estimated to be near 160 kV, and the FTE signatures extended across at least 7 h of magnetic local time (MLT) of the dayside polar cap boundary. In the second, a reconnection rate close to 80 kV was estimated, and the FTEs had a MLT extent of roughly 2 h. We discuss the ramifications of these differences for solar wind-magnetosphere coupling.