A statistical study of hot flow anomalies using Cluster data

TL;DR

This study analyzes hot flow anomalies using Cluster spacecraft data, confirming key features and identifying a new necessary condition involving increased solar wind speed for HFA formation, supported by observational and simulation data.

Contribution

It introduces a new necessary condition for HFA formation based on solar wind speed, validated by multi-spacecraft observations and hybrid simulations.

Findings

HFAs occur when the TD normal angle exceeds 45°.

Magnetic field changes are significantly large during HFAs.

A solar wind speed about 200 km/s above average is necessary for HFAs.

Abstract

Hot flow anomalies (HFAs) are studied using observations of the RAPID suprathermal charged particle detector, the FGM magnetometer, and the CIS plasma detector aboard the four Cluster spacecraft. Previously, we studied several specific features of tangential discontinuities on the basis of Cluster measurements in February-April 2003. In this paper, we confirm the following results: the angle between the Sun direction and the tangentional discontinuity (TD) normal is larger than 45{\deg} during HFAs, the magnetic field directional change is large. We then present evidence for a new necessary condition for the formation of HFAs, that is, the solar wind speed is significantly ( about 200 km/s or dMf=2.3) higher than the long-term average. The existence of this condition is also confirmed by simultaneous ACE MAG and SWEPAM solar wind observations at the L1 point 1.4 million km upstream of the Earth. The results are compared with recent hybrid simulations.