Quasiperiodic acceleration of electrons by a plasmoid-driven shock in the solar atmosphere

Abstract

Cosmic rays and solar energetic particles may be accelerated to relativistic energies by shock waves in astrophysical plasmas. On the Sun, shocks and particle acceleration are often associated with the eruption of magnetized plasmoids, called coronal mass ejections (CMEs). However, the physical relationship between CMEs and shock particle acceleration is not well understood. Here, we use extreme ultraviolet, radio and white-light imaging of a solar eruptive event on 22 September 2011 to show that a CME-induced shock (Alfvén Mach number 2.4 +0.7 −0.8 ) was coincident with a coronal wave and an intense metric radio burst generated by intermittent acceleration of electrons to kinetic energies of 2-46 keV (0.1-0.4 c). Our observations show that plasmoid-driven quasi-perpendicular shocks are capable of producing quasi-periodic acceleration of electrons, an effect consistent with a turbulent or rippled plasma shock surface.Coronal mass ejections (CMEs) are spectacular eruptions of magnetized plasma from the low solar atmosphere into interplanetary space [1,2]. With kinetic energies of ∼10 25 J [3], they are the most energetic explosive events in the solar system and are often associated with plasma shocks and the acceleration of particles to relativistic speeds [4,5]. However, the underlying mechanism relating CMEs, shocks, and particle acceleration is still a subject of intense debate [6]. By clarifying the inherent characteristics of these phenomena we learn not only about the nature of explosive plasma events but also about how they drive shocks and accelerate particles to high energies. Such processes are ubiquitous in the universe, playing a role in the acceleration of cosmic rays in supernovae and active galactic nuclei shocks [7]. CME-associated shocks are often observed over a variety of spectral bands. At radio frequencies, high intensity (∼10 8 Jy) emissions, known as type II and type III bursts, are associated with coronal shocks and accelerated particles in the solar corona [8,9]. Fine structure in these radio bursts can often reveal a 'bursty' nature to the shock particle acceleration [10], which can reveal details of the internal shock structure [11,12]. At extreme ultraviolet (EUV) wavelengths, the shock or pressure pulse response of the corona to an eruption may be imaged as a bright pulse propagating across the entire solar disk at typical velocities of 200-400 km s −1 [13]. These 'coronal bright fronts' (CBFs) are a regular feature of solar eruptive events and often display wave-like properties such as reflection [14], refraction [15] and pulse broadening [16]. Like CMEs, CBFs are often accompanied by type II and type III radio bursts, with EUV and radio images revealing a spatial link between the phenomena that is suggestive of a common origin [17,18,19].It has been proposed that the common origin for these myriad phenomena may be a CME-driven shock [5,20]. In this scenario, the CME eruption drives a pressure pulse, observable in the low corona as a propagating wavelike CBF. Higher in the corona this...

Large-scale Globally Propagating Coronal Waves

“…This provides strong evidence for a single physical disturbance creating both the type II burst and the observed coronal wave features. This scenario has been supported by the recent observation of a similar propagating radio source associated with a limb wave seen with AIA (Carley et al, 2013).…”

The Coronal Analysis of SHocks and Waves (CASHeW) framework

“…A strong connection between fast CBFs and shock waves was made by observations, which showed temporal and spatial overlap of CBFs and drifting metric type II radio emission, indicative of a coronal shock (Gopalswamy & Yashiro, 2011;Ma et al, 2011;Bain et al, 2012;Carley et al, 2013). Some CBFs may be sub-Alfvénic compressive waves (not necessarily shocks), or may steepen into shocks depending on the relative speed of the driver to the local Alfvén speed (Mann et al, 2003).…”

The density compression ratio of shock fronts associated with coronal mass ejections

“…There is a growing amount of evidence for the association of CME-driven shocks with SEPs in the corona (see recent papers, e.g., Rouillard et al, 2012Rouillard et al, , 2016Carley et al, 2013;Lario et al, 2014Lario et al, , 2016Salas-Matamoros et al, 2016, and references therein) but it remains unclear whether such shock waves are capable of accelerating particles at the observed energies Mancuso, 2010, 2011). An important shock parameter that could be accessible from coronagraphic observations is the ratio between the downstream and upstream electron densities or density compression ratio, X.…”