Abstract
Abstract Solar radio spikes are short duration and narrow bandwidth fine structures in dynamic spectra observed from the GHz to tens of MHz range. Their very short duration and narrow frequency bandwidth are indicative of subsecond small-scale energy release in the solar corona, yet their origin is not understood. Using the LOw Frequency ARray, we present spatially, frequency, and time resolved observations of individual radio spikes associated with a coronal mass ejection. Individual radio spike imaging demonstrates that the observed area is increasing in time and the centroid positions of the individual spikes move superluminally parallel to the solar limb. Comparison of spike characteristics with that of individual Type IIIb striae observed in the same event show similarities in duration, bandwidth, drift rate, polarization, and observed area, as well the spike and striae motion in the image plane suggesting fundamental plasma emission with the spike emission region on the order of ∼108 cm, with brightness temperature as high as 1013 K. The observed spatial, spectral, and temporal properties of the individual spike bursts are also suggestive of the radiation responsible for spikes escaping through anisotropic density turbulence in closed loop structures with scattering preferentially along the guiding magnetic field oriented parallel to the limb in the scattering region. The dominance of scattering on the observed time profile suggests the energy release time is likely to be shorter than what is often assumed. The observations also imply that the density turbulence anisotropy along closed magnetic field lines is higher than along open field lines.
Highlights
Solar activity sporadically releases magnetic energy via solar flares and coronal mass ejections (CMEs) that brightly manifest via electromagnetic radiation from X-rays to radio waves (e.g., Holman et al 2011, as a review)
Spike durations are observed to decrease with increasing frequency to below 10 ms at gigahertz frequencies (Benz 1986; Staehli & Magun 1986). Their short duration and narrow frequency range are indicative of processes that occur on millisecond timescales and provide a unique avenue to study the fastest processes in the solar corona (e.g., Aschwanden 2002; Karlický et al 2021)
Tarnstrom & Philip (1972b) proposed that weak electron beams with lower densities and smaller spatial sizes than those that produce Type III bursts are responsible for spike bursts. Another proposed mechanism is electron cyclotron maser (ECM) emission (Holman et al 1980) due to spikes being observed in conjunction with Type IV radio bursts, and could serve to explain spike emission in regions of strong magnetic fields and/or low densities
Summary
Solar activity sporadically releases magnetic energy via solar flares and coronal mass ejections (CMEs) that brightly manifest via electromagnetic radiation from X-rays to radio waves (e.g., Holman et al 2011, as a review). Solar radio bursts are a signature of electrons accelerated in flares and CMEs. Solar radio spikes are short duration (10–1000 ms) bursts with narrow spectral widths from Δf/f ; 0.002 to 0.01, observed from 7 to 8 GHz (Staehli & Magun 1986; Benz et al 1992) down to decametric frequencies (Melnik et al 2014). Tarnstrom & Philip (1972b) proposed that weak electron beams with lower densities and smaller spatial sizes than those that produce Type III bursts are responsible for spike bursts. Another proposed mechanism is electron cyclotron maser (ECM) emission (Holman et al 1980) due to spikes being observed in conjunction with Type IV radio bursts, and could serve to explain spike emission in regions of strong magnetic fields and/or low densities. The results confirm the similarity of the observed spike properties with Type IIIb striae, but the cospatial character of the Type IIIb and spike sources
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