The Morphology of Decimetric Emission from Solar Flares: GMRT Observations

Abstract

Abstract.Observations of a solar flare at 617 MHz with the Giant Meter-wave Radio Telescope are used to study the morphology of flare radio emssion at decimetric wavelengths. There has been very little imaging in the 500 -1000 MHz frequency range, but it is of great interest since it corresponds to densities at which energy is believed to be released in solar flares. This event has a very distinctive morphology at 617 MHz: the radio emission is clearly resolved by the 30 beam into arc-shaped sources seeming to lie at the tops of long loops, anchored at one end in the active region in which the flare occurs, with the other end lying some 200000 km away in a region of quiet solar atmosphere. Microwave images show fairly conventional behaviour for the flare in the active region: it consists of two compact sources overlying regions of opposite magnetic polarity in the photosphere. The decimetric emission is confined to the period leading up to the impulsive phase of the flare, and does not extend over a wide frequency range. This fact suggests a flare mechanism in which the magnetic field at considerable height in the corona is destabilized a few minutes prior to the main energy release lower in the corona. The radio morphology also suggests that the radiating electrons are trapped near the tops of magnetic loops, and therefore may have pitch angles near 90 • .

Decimetric Emission 500″ Away from a Flaring Site: Possible Scenarios from GMRT Solar Radio Observations

“…In the present paper, using the first high time cadence images at 610 MHz from the GMRT, we have studied the decimetric radio activity associated with C-and M-class flares and a coronal mass ejection (CME) onset that occurred on 2015 June 20 and reported a decimetric radio source during the C-class flare to be located about 500″ away from the flaring site. The time cadence of the present GMRT observations is 0.5 s, while the earlier reported GMRT observations had a maximum time cadence of 16.9 s (Subramanian et al 2003), 2.11 s (Kundu et al 2006), or 1 s (Mészárosová et al 2013). The high time cadence images of GMRT 610 MHz during the C-class flare have also been used, in combination with the coronal images of extreme ultraviolet (EUV) and X-ray wavelengths, to investigate the genesis of radio-emitting electrons of decimetric sources and their temporal association with the Type III burst activity at metric wavelengths.…”

“…The GMRT actually observes at six distinct frequencies: 150, 236, 327, 610, 1280, and 1400 MHz. Though the GMRT is not a dedicated solar telescope, it has been used successfully for observing and studying the Sun (Subramanian et al 2003;Kundu et al 2006;Mercier et al 2006;Mészárosová et al 2013;Mercier et al 2015). Snapshot solar images with unprecedented resolution and high dynamic range, far better than any images previously obtained, have been produced by combining visibilities from the GMRT and NRH (Mercier et al 2006).…”

“…Only a few studies have been reported at GMRT 610 MHz. The first study (Kundu et al 2006) of GMRT 610 MHz radio emission reported an arc-shaped radio source morphology of decimetric flare emission, describing its origin as due to trapped radiating electrons at the magnetic loop tops. In another study of GMRT 610 MHz (Mészárosová et al 2013), during a weak B8.9 flare, the authors reported very weak radio sources located in the fan structure of coronal magnetic field lines originating from a coronal magnetic null point.…”

“…In an earlier study of radio emission at a decimetric wavelength range using the GMRT at 610 MHz, Kundu et al (2006) also reported radio sources located about 200″ from the flaring active region. In the present paper, using the first high time cadence images at 610 MHz from the GMRT, we have studied the decimetric radio activity associated with C-and M-class flares and a coronal mass ejection (CME) onset that occurred on 2015 June 20 and reported a decimetric radio source during the C-class flare to be located about 500″ away from the flaring site.…”

Flare Observations

“…This range is a possible indication for the density in the acceleration region. Although initially interpreted as loss-cone emissions of trapped particles near footpoints, they have been found at high altitudes Khan et al 2002;Kundu et al 2006). As the emission mechanism of spikes and pulsations is not well understood, the origin of the radiating high-frequency waves is not known.…”