NASA Goddard Space Flight Center and Department of Astronomy, University of Maryland
What are the Open Problems and Opportunities in the Study of Solar Radio Bursts
Electron beams accelerated by solar flares and nanoflares are believed to be responsible for several types of solar radio bursts observed in the corona and interplanetary medium, including flare-associated coronal Type U and J and interplanetary Type III radio bursts, and nanoflare-associated weak coronal type III bursts. The coherent radio emission is a powerful probe of the plasma environment of solar corona and the mechanism of acceleration and energetic particle propagation. The characteristic of the radio bursts is their drifting frequency being consistent with the local electron plasma frequency. However, how electron two-stream instability driven by electron beams produces coherent emission with a duration of several orders of magnitude longer than the linear saturation time is a long-standing puzzle. I will review the essential problems associated with the observations of radio bursts and present a new mechanism proposed by Che et al. [2017, doi: 10.1073/pnas.1614055114]. This new mechanism shows that the continuous plasma coherent emission can be maintained by cyclic Langmuir collapse and the associated waves and spatial coherent structures can be tested by space probes such as Wind, ACE, the near-future Parker Solar Probe and Solar Orbiter. Also, electron beams are commonly discovered in the magnetosphere associated with magnetic reconnection and shock waves. The high spatial and time resolution field and particle data from the newly launched MMS provide possible in-situ observations to test the emission process near the acceleration source region. I will also discuss the open questions in the current theoretical study and observations.