One important task connected with the corona heating problem is exploring the physical mechanisms that produce, accelerate, and transport energetic particles in the upper corona. The magnetic connectivity between the sources of meter emission in the upper corona and electron acceleration sites in low corona are studied using the temporal correlation between hard X-ray (HXR) bursts and type III emission (Kane 1972; Aschwanden et al. 1995; Reid & Vilmer 2017). In many cases, Type III bursts are observed without signs of activity in the hard X-ray emission in the lower corona. It should be noted that microwave observations on large radioheliographs can be more sensitive to the appearance of nonthermal electrons to the lower corona (Altyntsev et al. 2020).
Recently, during the first Parker Solar Probe perihelion, an unusual narrowband radio emission in the 5 – 7 GHz band originating from the weak X-ray bright point on 13 April 2019 was discussed by Altyntsev et al. (2022). An analysis showed that characteristics of the spectrum of microwave emission indicated a coherent emission mechanism produced by electrons trapped in low coronal loops. The emission was generated by electrons with energies of several tens of keV at the harmonic of the plasma frequency. In addition, numerous bursts were observed by LOFAR over this time but there were no signatures of the nonthermal emissions in hard X-rays. The goal of this study is to find the low corona source of type III and/or J bursts at meter wavelengths using the spatially resolved microwave observations.
Observations
The numerous short bursts cover the receiving band of the LOFAR Low Frequency Array (Fig.1a) during the interval 05:43:30–07:39:59 of common observations with the BBMS spectropolarimeter in the microwave range 4–8 GHz (Fig.1b). We have found a box in the LOFAR dynamic spectrum (Fig.1a) for which the cross-correlation coefficient with an average microwave profile over the range of 5- 7 GHz profile is high and exceeds 0.7.
Figure 1. Dynamic spectra on 13 April 2019, recorded by LOFAR (a) and BBMS in microwaves (b). The box in panel a) with the time bounds from 05:45:15 till 06:56:40 and frequency range 53–80 MHz gives the spectrum region for which there is a high cross-correlation coefficient with the microwave emission at 6 GHz.
Figure 2. Images on 13 April 2019. a) Solar disk at 171 A (SDO/AIA) and contours of brightness temperature at 50, 60, 70 MHz at half height at 06:48 (LOFAR); b) AR 12635 in soft X-rays (XRT/Hinode) at 06:04 and 06:24 (box in the upper right corner) together with the line of sight magnetic field component ±100, ±300, ±1000, ± 1500 G). Red and blue contours show positive and negative components, respectively. White contours at (0.2, 0.5, 0.9)×0.6 MK corresponds to brightness temperature at 6.25 GHz (SRH) in intensity at 05:44:49. The SRH beam width is 52×35 arcsec. Black solid and dashed contours indicate the brightness temperature in the right and left polarization, respectively. The levels are (0.5, 0.7)×2.8・104 K and -(0.7, 0.5)×1.6・105 K.
Altyntsev et al. (2022) showed that the microwave emission in the frequency band 5–7 GHz was generated by a relatively small number of accelerated electrons, trapped in the loop, linked with bright X-ray point A. To study the magnetic field structure around the positive polarity inclusion and the bright XRT loop we assumed local non-potentiality of the magnetic field and performed a nonlinear force-free field reconstruction in the small subvolume near point A. Small-scale magnetic field lines near this point A correspond to the bright feature in the XRT image. Potential-field extrapolation reveals that the small-scale magnetic flux, anchored to the magnetic inclusion, is overlapped by the relatively large-scale magnetic flux, anchored to the surrounding magnetic polarity. The vertical components of the magnetic field vectors of the large and small loops are anti-parallel at closely spaced regions, which creates favorable conditions for magnetic reconnection. This is referred to as “interchange reconnection” if a field line that is open to interplanetary space switches the location of its photospheric footpoint (Cairns et al. 2018). Post-reconnection field lines take the shape of a small hot loop and field lines that are open to interplanetary space. Thus, it is shown that the source of a series of type III and/or J bursts in the frequency range 50–80 MHz is the interaction of two loop systems with significantly different lengths.
We can conclude that the use of large multiwave radioheliographs makes it possible to reveal non-thermal processes in the solar corona not only in weak flares, but also in transient events. The combination of cross correlation analysis of the temporal profiles of radio emission from the lower and upper corona with the localization of sources makes it possible to verify the methods of reconstruction of coronal magnetic fields.
Based on the recent paper by Altyntsev, A. T., Reid, H., Meshalkina, N. S., Myshyakov, I. I., Zhdanov, D. A. Temporal and Spatial Association Between Microwaves and Type III Bursts in the Upper Corona: 2023, Astronomy& Astrophysics, 671, A30, DOI: 10.1051/0004-6361/202244599.
References
Altyntsev, A.T., Meshalkina, N.S., Fedotova A.Ya. & Myshykov I.I.: 2020, ApJ, 905, 149, 13pp.
Altyntsev, A.T., Meshalkina, N.S., &Myshykov I.I.: 2022, Solar-Terrestrial Physics, 8,3.
Aschwanden, M. J., Benz, A. O., Dennis, B. R., & Schwartz, R. A.: 1995, ApJ, 455, 347.
Cairns, I. H., Lobzin, V. V., Donea, A., et al.: 2018, Scientific Reports, 8, 1676.
Kane, S. R.: 1972, Sol. Phys., 27, 174.
Reid, H. A. S. & Vilmer, N.: 2017, A&A, 597, A77.
*Complete list of authors: Altyntsev1, A. T., Reid2, H., Meshalkina1, N. S., Myshyakov, I. I.1, ., Zhdanov1, D. A.
1Institute of Solar-Terrestrial Physics SB RAS, Lermontov St. 126A, Irkutsk 664033, Russia
2 Mullard Space Science Laboratory, Department of Space and Climate Physics, University College London, Dorking, United Kingdom