Foreshock cavitons and spontaneous hot flow anomalies: a statistical study with a global hybrid-Vlasov simulation

Vertti Tarvus,Maxime Dubart,Maxime Grandin,Xóchitl Blanco-Cano,Minna Palmroth,Urs Ganse,Jonas Suni,Markku Alho,Lucile Turc,Yann Pfau-Kempf,Markus Battarbee,Andreas Johlander,Konstantinos Papadakis

doi:10.5194/angeo-39-911-2021

Abstract

Abstract. The foreshock located upstream of Earth's bow shock hosts a wide variety of phenomena related to the reflection of solar wind particles from the bow shock and the subsequent formation of ultra-low-frequency (ULF) waves. In this work, we investigate foreshock cavitons, which are transient structures resulting from the non-linear evolution of ULF waves, and spontaneous hot flow anomalies (SHFAs), which are thought to evolve from cavitons as they accumulate suprathermal ions while being carried to the bow shock by the solar wind. Using the global hybrid-Vlasov simulation model Vlasiator, we have conducted a statistical study in which we track the motion of individual cavitons and SHFAs in order to examine their properties and evolution. In our simulation run where the interplanetary magnetic field (IMF) is directed at a sunward–southward angle of 45∘, continuous formation of cavitons is found up to ∼11 Earth radii (RE) from the bow shock (along the IMF direction), and caviton-to-SHFA evolution takes place within ∼2 RE from the shock. A third of the cavitons in our run evolve into SHFAs, and we find a comparable amount of SHFAs forming independently near the bow shock. We compare the properties of cavitons and SHFAs to prior spacecraft observations and simulations, finding good agreement. We also investigate the variation of the properties as a function of position in the foreshock, showing that transients close to the bow shock are associated with larger depletions in the plasma density and magnetic field magnitude, along with larger increases in the plasma temperature and the level of bulk flow deflection. Our measurements of the propagation velocities of cavitons and SHFAs agree with earlier studies, showing that the transients propagate sunward in the solar wind rest frame. We show that SHFAs have a greater solar wind rest frame propagation speed than cavitons, which is related to an increase in the magnetosonic speed near the bow shock.

Highlights

As the supermagnetosonic solar wind interacts with Earth’s magnetosphere, a curved bow shock forms upstream of Earth
We have found that under such conditions, cavitons and spontaneous hot flow anomalies (SHFAs) are a common occurrence in the foreshock, with ∼ 1–4 new transients forming every second and 29–72 transients populating the foreshock in the noon– midnight meridional plane at any given time
We find cavitons to be more numerous than SHFAs, with cavitons making up ∼ 75 % of the simultaneously observed transients on average

Summary

Introduction

As the supermagnetosonic solar wind interacts with Earth’s magnetosphere, a curved bow shock forms upstream of Earth. The bow shock slows the solar wind down to submagnetosonic speeds while compressing and heating it. This allows the solar wind to flow around the magnetopause that separates the solar wind from the magnetosphere. Depending on the angle θBn, a range of interesting features are found in Earth’s foreshock. These include different shock-reflected suprathermal ion populations (e.g., Fuselier, 1995; Kempf et al, 2015), different

Objectives

Methods

Results

Discussion

Conclusion