A global study of hot flow anomalies using Cluster multi-spacecraft measurements

G Facskó,I Dandouras,A Kis,Z Németh,G Erdős

doi:10.5194/angeo-27-2057-2009

Abstract

Abstract. Hot flow anomalies (HFAs) are studied using observations of the magnetometer and the plasma instrument aboard the four Cluster spacecraft. We study several specific features of tangential discontinuities on the basis of Cluster measurements from the time periods of February–April 2003, December 2005–April 2006 and January–April 2007, when the separation distance of spacecraft was large. The previously discovered condition (Facskó et al., 2008) for forming HFAs is confirmed, i.e. that the solar wind speed and fast magnetosonic Mach number values are higher than average. Furthermore, this constraint is independent of the Schwartz et al. (2000)’s condition for HFA formation. The existence of this new condition is confirmed by simultaneous ACE magnetic field and solar wind plasma observations at the L1 point, at 1.4 million km distance from the Earth. The temperature, particle density and pressure parameters observed at the time of HFA formation are also studied and compared to average values of the solar wind plasma. The size of the region affected by the HFA was estimated by using two different methods. We found that the size is mainly influenced by the magnetic shear and the angle between the discontinuity normal and the Sun-Earth direction. The size grows with the shear and (up to a certain point) with the angle as well. After that point it starts decreasing. The results are compared with the outcome of recent hybrid simulations.

Highlights

Hot flow anomalies (HFAs), explosive events near the Earth’s bow shock have been known more than 20 years (Schwartz et al, 1985; Thomsen et al, 1986), their theoretical explanation needs further studies (Burgess and Schwartz, 1988; Thomas et al, 1991; Lin, 2002)
The most reliable description of hot flow anomalies (HFAs) is so far based on hybrid plasma simulations where electrons are considered as a massless and neutralizing fluid
Another prediction is that the size of HFAs is a monotonically increasing function of the magnetic field vector direction change angle ( ) across the discontinuity (Lin, 2002)

Summary

Introduction

Hot flow anomalies (HFAs), explosive events near the Earth’s bow shock have been known more than 20 years (Schwartz et al, 1985; Thomsen et al, 1986), their theoretical explanation needs further studies (Burgess and Schwartz, 1988; Thomas et al, 1991; Lin, 2002). Lin’s hybrid simulation (Lin, 2002) uses a larger simulation box than in other studies mentioned above, and inserts a zero-resistivity surface (magnetopause) to the super-Alfvenic plasma flow when the simulation is initialized. This plasma flow moves parallel to the x-axis of the box and a shock is formed. A prediction of her theory is that the size of HFAs increases monotonically with γ until 80◦ and begins to decrease Another prediction is that the size of HFAs is a monotonically increasing function of the magnetic field vector direction change angle ( ) across the discontinuity (Lin, 2002). The goal of this study was to check the validity of these predictions based on simulation results

Objectives

Discussion

Conclusion