Origin of the Submillimeter Radio Emission During the Time-Extended Phase of a Solar Flare

Abstract

Solar flares observed in the 200 -400 GHz radio domain may exhibit a slowly varying and time-extended component which follows a short (few minutes) impulsive phase and can last for a few tens of minutes to more than one hour. The few examples discussed in the literature indicate that such long-lasting submillimeter emission is most likely thermal bremsstrahlung. We present a detailed analysis of the time-extended phase of the 27 October 2003 (M6.7) flare, combining 1 -345 GHz total-flux radio measurements with X-ray, EUV, and Hα observations. We find that the time-extended radio emission is, as expected, 340 G. Trottet et al. radiated by thermal bremsstrahlung. Up to 230 GHz, it is entirely produced in the corona by hot and cool materials at 7 -16 MK and 1 -3 MK, respectively. At 345 GHz, there is an additional contribution from chromospheric material at a few 10 4 K. These results, which may also apply to other millimeter-submillimeter radio events, are not consistent with the expectations from standard semiempirical models of the chromosphere and transition region during flares, which predict observable radio emission from the chromosphere at all frequencies where the corona is transparent.

The 6 September 2017 X9 Super Flare Observed From Submillimeter to Mid‐IR

“…However, it is not possible to determine whether the submillimeter emission comes from a nonthermal source or not; if thermal, however, each frequency would form at different plasma temperatures, that is, a multithermal source is required. In this latter case, a similar result for the extended phase of SOL2003‐10‐27 was found by Trottet et al () who modeled the emission in the range 8–230 GHz with a multithermal greater than megakelvin optically thin source. They also observed that 345 GHz is optically thick and should come from a lower height.…”

“…Krucker et al () give a comprehensive and critical analysis of all possible processes that may be responsible for the submillimeter emission. On the other hand, the extended or gradual phase of flares can be attributed to thermal bremsstrahlung (Lüthi, Lüdi, & Magun, ; Lüthi, Magun, & Miller, ; Trottet et al, , ).…”

Lower Ionosphere Sensitivity to Solar X‐ray Flares Over a Complete Solar Cycle Evaluated From VLF Signal Measurements

“…CHIANTI version 7.1 (Landi et al, 2013), used in this study, includes a new set of coronal abundances taken from Schmelz et al (2012) and two new sets of photospheric abundances from Lodders et al (2009) and Caffau et al (2011). In principle, for solar X-ray flares the coronal set of abundances is more suitable than the photospheric one because the emitted plasma recorded by GOES resides in coronal loops rather than in chromospheric foot points (Trottet et al, 2011). Nevertheless, we used both sets of abundances for comparison.…”

On the origin of 140 GHz emission from the 4 July 2012 solar flare

“…Also it should give negligibly small contribution to centimeter emission from the 4 July 2012 flare at S < 10 18 cm 2 . However, the SDO plasma can give significant but not decisive contribution to millimeter emission if the thermal source area S > 10 18 cm 2 (see also White & Kundu, 1992;Trottet et al, 2002Trottet et al, , 2008Trottet et al, , 2011.…”

“…Therefore observations in poorly explored wavelength ranges can be very fruitful and important. Specifically, sub-THz observations corresponding to the frequency range of 10 2 − 10 3 GHz (3 − 0.3 mm) can give us valuable information about the acceleration of electrons with energy E 1 MeV as well as the flare coronal and chromospheric thermal plasma (Raulin et al, 1999;Lüthi et al, 2004;Giménez de Castro et al, 2009;Fleishman & Kontar, 2010;Trottet et al, 2011;Krucker et al, 2013).…”