Abstract
Solar flares often involve the acceleration of particles to relativistic energies and the generation of high-intensity bursts of radio emission. In some cases, the radio bursts can show periodic or quasiperiodic intensity pulsations. However, precisely how these pulsations are generated is still subject to debate. Prominent theories employ mechanisms such as periodic magnetic reconnection, magnetohydrodynamic (MHD) oscillations, or some combination of both. Here we report on high-cadence (0.25 s) radio imaging of a 228 MHz radio source pulsating with a period of 2.3 s during a solar flare on 2014-April-18. The pulsating source is due to an MHD sausage mode oscillation periodically triggering electron acceleration in the corona. The periodic electron acceleration results in the modulation of a loss-cone instability, ultimately resulting in pulsating plasma emission. The results show that a complex combination of MHD oscillations and plasma instability modulation can lead to pulsating radio emission in astrophysical environments.
Highlights
Solar flares often involve the acceleration of particles to relativistic energies and the generation of high-intensity bursts of radio emission
On 18 April 2014, a flux rope eruption and M7.3 class flare commenced at 12:35 Universal Time (UT) from active region AR12036 (labelled flare active region in Fig. 1a), observed using the Atmospheric Imaging Assembly (AIA)[35]; on-board Solar Dynamics Observatory (SDO)
We examine the emission characteristics in detail to establish which emission mechanisms are at play and how the MHD oscillation and electron acceleration brings about the pulsations
Summary
Solar flares often involve the acceleration of particles to relativistic energies and the generation of high-intensity bursts of radio emission. Solar flares are the most energetic phenomena in the solar system, thought to be due to the release of up to 1025 J of magnetic energy over the course of tens of minutes[1] This energy release results in the acceleration of particles to relativistic energies and the generation of electromagnetic radiation from gamma rays to high-intensity bursts of radio emission[2]. The radio emission at metric wavelengths is thought to be generated by a plasma instability due to the presence of energetic electrons in the solar corona[3], and can sometimes show periodic or quasiperiodic pulsations in intensity[4,5]. 16,18,23 have attributed the pulsations to periodic injection of electrons into loop footpoints in the active region, which act as a magnetic trap This results in modulated maser or plasma emission from a loss-cone instability induced in the trap. Due to the generally fast dynamics caused by reconnection and MHD oscillations, it is an ongoing challenge in observational astrophysics to temporally and spatially resolve such mechanisms
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