Impact Angle Control of Interplanetary Shock Geoeffectiveness: A Statistical Study

TL;DR

This study analyzes how the impact angle of interplanetary shocks influences their ability to trigger geomagnetic activity, revealing that frontal, high-speed shocks are most effective in energizing Earth's magnetosphere.

Contribution

It provides a comprehensive statistical analysis linking shock impact angles and speeds to geomagnetic responses, confirming simulation predictions with observational data.

Findings

Frontal, high-speed shocks are more geoeffective.

Strong correlation (R=0.70) between impact angle and geomagnetic response.

Symmetrical compression by frontal shocks facilitates magnetic energy release.

Abstract

We present a survey of interplanetary (IP) shocks using Wind and ACE satellite data from January 1995 to December 2013 to study how IP shock geoeffectiveness is controlled by IP shock impact angles. A shock list covering one and a half solar cycle is compiled. The yearly number of IP shocks is found to correlate well with the monthly sunspot number. We use data from SuperMAG, a large chain with more than 300 geomagnetic stations, to study geoeffectiveness triggered by IP shocks. The SuperMAG SML index, an enhanced version of the familiar AL index, is used in our statistical analysis. The jumps of the SML index triggered by IP shock impacts on the Earth's magnetosphere is investigated in terms of IP shock orientation and speed. We find that, in general, strong (high speed) and almost frontal (small impact angle) shocks are more geoeffective than inclined shocks with low speed. The strongest correlation (correlation coefficient R = 0.70) occurs for fixed IP shock speed and varying the IP shock impact angle. We attribute this result, predicted previously with simulations, to the fact that frontal shocks compress the magnetosphere symmetrically from all sides, which is a favorable condition for the release of magnetic energy stored in the magnetotail, which in turn can produce moderate to strong auroral substorms, which are then observed by ground based magnetometers.