Solar radio spikes are short duration, narrowband radio bursts that are signature of the velocity of non-thermal electrons in solar flares. Their brief periods represent an upper limitation for the energy release time, and paired with their narrow frequency bandwidths, spikes are indicative of processes that occur on millisecond timescales, supplying an avenue to study the fastest procedures in the solar corona. Utilizing the exceptional time and frequency resolution of LOFAR, we were able to resolve private radio spikes between 30-70 MHz (e.g. Figure 1), and analyse their numerous characteristics such as period, frequency width, frequency drift, area, and obvious movement over 10s of millisecond scales. Spikes were observed both prior to and after the CME; with the bulk of the observed spikes taking place within the CME wake. (b) Type IIIb burst (post-CME) (c) Spike cluster (d) A private spike.
Figure 2: Temporal homes of the spike shown in Figure 1d at 34.5 MHz. (a) Spike centroid motion (coloured triangles) overlaid on an SDO/AIA 171 Å image. The blue plus signs show the peak centroid position of other spikes pre-CME, whilst white plus symbols show those post-CME. The grey lines with diamond (pre-CME) and triangle (post-CME) markers represent the centroid movement of 2 individual striae from Figure 1( b, e). (b) Observed FWHM location in time. (c) Spike vertical centroid movement with time. The red curves represent the normalised spike lightcurve. Figure adjusted from Clarkson et al. (2021 ).
Conclusions
It is revealed that low-frequency radio spikes are strongly affected by scattering due to the radiation escaping through anisotropic density turbulence, with spreading preferentially along the directing magnetic field. For this event, the spike and striae motions show that the magnetic field lines are parallel to the solar limb. Combined with the coronal height of the emission where the condition for ECM emission in not likely to be pleased, the spikes are likely to be produced via the plasma emission system near the plasma frequency.
Based on the recent paper by: Clarkson, D. L., Kontar, E. P., Gordovskyy, M., Chrysaphi, N., Vilmer, N. First Frequency-time-resolved Imaging Spectroscopy Observations of Solar Radio Spikes, ApJL, 917, L32. DOI: 10.3847/ 2041-8213/ ac1a7d.
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Benz, A. O., Su, H., Magun, A., Stehling, W. 1992, A&AS, 93, 3.
Chrysaphi, N., Reid, H. A. S., Kontar, E. P. 2020, ApJ, 898, 115.
Kontar, E. P., Yu, S., Kuznetsov, A. A., Emslie, G. A., et al. 2017, NatComm, 8, 1515.
Kontar, E. P., Chen, X., Chrysaphi, N., et al. 2019, ApJ, 884, 122.
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Complete list of authors: Clarkson, D. L., Kontar, E. P., Gordovskyy, M., Chrysaphi, N., Vilmer, N.
Solar radio spikes are brief duration, narrowband radio bursts that are signature of the velocity of non-thermal electrons in solar flares. They are observed over a large range of frequencies from the tens of MHz (Melnik et al. 2014) to the GHz variety (Benz et al. 1992), and have a few of the shortest durations and narrow bandwidths of any solar radio bursts. The origin of spikes is not completely comprehended. Their brief durations represent an upper limit for the energy release time, and coupled with their narrow frequency bandwidths, spikes are indicative of processes that take place on millisecond timescales, offering an opportunity to study the fastest processes in the solar corona. The high brightness temperature levels connected with spikes indicate coherent systems; particularly, plasma emission or electron cyclotron maser (ECM) emission.
In the recent paper, Clarkson et al. (2021) have reported for the very first time, spatially, frequency, and time dealt with observations of specific radio spikes connected with a coronal mass ejection (CME).
Observations and Analysis
Using the exceptional time and frequency resolution of LOFAR, we were able to solve individual radio spikes between 30-70 MHz (e.g. Figure 1), and evaluate their numerous qualities such as duration, frequency width, frequency drift, area, and obvious motion over 10s of millisecond scales. Spikes were observed both before and after the CME; with the bulk of the observed spikes occurring within the CME wake. Both the spikes and striae program comparable characteristics; that being a reducing period, increasing bandwidth, and reducing location, with frequency.
Figure 1: (a) Dynamic spectra of spike and Type IIIb bursts. (b) Type IIIb burst (post-CME) (c) Spike cluster (d) A private spike. (e) Type IIIb burst (pre-CME).
One of the intriguing observations is that the spike (and striae) centroid movements are not radial, but parallel to the solar limb (Figure 2a). Analysing the temporal variation of the spike location and vertical movement in the image airplane (Figure 2b, c), we discover that both the change in areal degree and movement are most pronounced throughout the decay stage. The spikes show superluminal velocities between 0.76-1.8 c, and superluminal growth of the FWHM source sizes.