X-Rays from a Type I Radio Burst

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Number:	416
1st Author:	R. RAMESH
2nd Author:
Published:	20 September 2021
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Introduction

We can study a variety of small-scale energy releases on the Sun (flaring bright points, active-region transient brightenings, coronal jets, etc.) by combining X-ray and radio-wavelength observations. This array of phenomena embraces most of the well-known forms of coronal magnetic activity, with one major exception to date...

While solar soft X-rays (E < 10 keV) are dominated by thermal emission from hot coronal plasmas, the radio observations are sensitive to non-thermal emission also. The observations of low-frequency type III radio bursts in association with X-ray bright-point flares (Ref. [1]) clearly indicated that the latter are capable of accelerating particles to non-thermal energies, as well as producing the heated material detected in soft X-rays. The detection of type III bursts together with coronal X-ray jets strengthened the above argument. Therefore, radio observations are a useful complementary tool for observing signatures of weak, transient energy releases in the solar atmosphere since the related non-thermal emission can be easily detected.

Type I radio bursts represent the smallest discrete releases of energy observable. They are extremely common and are considered to be evidence of successive electron accelerations, implying heating. So, establishing its association with activities in the other regions of the solar atmosphere would be useful to understand the acceleration processes of the non-thermal electrons at the sites of elementary/weakest energy releases. In this context, we report observations of weak type I radio burst emission during the same time as soft X-ray observations of a GOES sub-A-classflare and an extreme-ultraviolet (EUV) brightening from the 'quiet' solar corona in the complete absence flares and/or coronal mass ejection (CME) activity.

Observations and data processing

The radio observations were carried out using the facilities operated by the Indian Institute of Astrophysics (IIA) in the Gauribidanur Observatory The imaging and spectral observations were carried out with the Gauribidanur RAdioheliograPH (GRAPH) and the Gauribidanur LOwfrequency Solar Spectrograph (GLOSS), respectively. The X-ray and EUV observations (at 94 Â) were from the X-ray Solar Monitor (XSM) onboard the Chandrayaan-2 space observatory and the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO), respectively.

Analysis and results

Figure 1 shows our observations of a type I burst event on 2020 April 21. There was also an EUV brightening observed with the SDO/AIA at 94{\AA} around the same time as the X-ray and radio observations (see Figure 2).

Figure 1: The 'green' colour plot is the frequency averaged (40-85 MHz) time profile of the GLOSS dynamic spectrum. Its amplitude values are indicated in the left hand side ordinate axis. The 'red' colour line is the fit to the data points. The labels 1, 2 indicate the epochs of maximum radio emission from the regions 1, 2 in Figure 2, respectively. The 'blue' colour profile is the light curve of the soft X-ray emission from the Sun close to the same epoch as the radio observations. Its amplitude values are indicated in the right hand side ordinate axis. The data were obtained with the Chandrayaan-2/XSM in the energy range ~1 - 5 keV with a time binning of ~120 sec.

Figure 2: A composite of the GRAPH difference image of the type I bursts at 80 MHz and the EUV observations at 94Å with the SDO/AIA around the same time as the radio and X-ray observations in Figure 1 on 2020 April 21. The contours 1, 2 correspond to the GRAPH observations. The background is the EUV image. The bigger and smaller boxes in the left panel image indicate the region around the EUV \brightening and the location of maximum emission, respectively. A zoomed version of the same brightening is shown in the right side panel. The peak flux density in the GRAPH observations is ~241 Jy. It's nearly the same for the contours 1, 2, which correspond to the two maxima 1, 2 in the radio time profile in Figure 1, respectively. The contours shown are at 80% level. The 'dots' inside the contours 1, 2 indicate the centroid locations of the individual type I bursts.

The peak flux of the XSM flare is ~6 x 10^-9 Wm^-2. It was thus a very weak event. The Gauribidanur radio observations indicate the presence of type I or noise storm bursts during the same time as the X-ray flare. The bursts are widely believed to be due to plasma radiation at the fundamental plasma frequency. The location of the radio bursts correlate well with that of the EUV brightening. No Hα and/or GOES soft X-ray flares were reported during the radio burst period. The Sun was totally free of any active regions and/or CMEs. The total duration of the radio event is smaller (~2 min) compared to the X-ray flare (~5 min). Assuming that both the X-ray and radio events are related to a common primary phenomenon, the comparatively shorter duration of the radio event indicates that the electrons responsible for its occurrence are probably thermalized quickly. As a result they cannot travel to larger heights in the corona from where the low frequency radio emission primarily originate. The shorter duration of the radio bursts could be also due to the emission being non-thermal in nature as compared to the soft X-ray emission. We estimated the total energy associated with the type I radio burst in the present case to be in the range 10²² - 10²⁵ erg. The upper limit to the associated X-ray microflare is in the range 10²⁶ - 10²⁷ erg. Interestingly, Ref. [2] had earlier reported EUV flares in the quiet solar corona with energy budget ~10²⁴ - 10²⁶ erg. These confirm that the type I radio bursts are an independent ground-based observational tool to probe weak activity in quiet regions of the corona, in addition to their known association with sunspot activity.

Summary

We have presented co-temporal/co-spatial observations of weak type I radio bursts, X-ray microflare, and EUV brightening from a quiet Sun completely devoid of any active regions. As far as we know, this is the first time such simultaneous observations of such transient activity in the quiet Sun have been reported. Considering that type I radio bursts like those described in this work hint activity in the outer layers of the solar corona which is currently inaccessible to observations in X-rays and extreme ultra-violet (EUV), combined investigations of weak energy releases observed at the same time in all domains would be helpful to understand the energies deposited at different levels in the solar corona in addition to the associated mechanisms themselves.

References

[1] "Association of flaring X-ray bright points with type III bursts"

[2] "Energy Distribution of Microevents in the Quiet Solar Corona"