Solar flares at submillimeter wavelengths

Abstract

International audienceWe discuss the implications of the first systematic observations of solar flares at submillimeter wavelengths, defined here as observing wavelengths shorter than 3 mm (frequencies higher than 0.1 THz). The events observed thus far show that this wave band requires a new understanding of high-energy processes in solar flares. Several events, including observations from two different observatories, show during the impulsive phase of the flare a spectral component with a positive (increasing) slope at the highest observable frequencies (up to 405 GHz). To emphasize the increasing spectra and the possibility that these events could be even more prominent in the THz range, we term this spectral feature a "THz component". Here we review the data and methods, and critically assess the observational evidence for such distinct component(s). This evidence is convincing. We also review the several proposed explanations for these feature(s), which have been reported in three distinct flare phases. These data contain important clues to flare development and particle acceleration as a whole, but many of the theoretical issues remain open. We generally have lacked systematic observations in the millimeter-wave to far-infrared range that are needed to complete our picture of these events, and encourage observations with new facilities

Hydrogen Balmer Line Broadening in Solar and Stellar Flares

“…Our modeling of the hotspot implies that the heating may result from ultrarelativistic electrons. Synchrotron emission from ultrarelativistic (E 500 keV > ) electrons are a possible source for the sub-mm/THz radiation in solar flares (see the review in Krucker et al 2013). Sub-THz radiation attributed to synchrotron emission has also been detected from stellar flares in active binary systems (Massi et al 2006).…”

SSALMON – The Solar Simulations for the Atacama Large Millimeter Observatory Network

“…Still there are many open questions concerning flares. For instance, the physical mechanism behind an observed emission component at sub-THz frequencies remains unclear (Kaufmann et al 2001;Trottet et al 2002;Lüthi et al 2004;Kaufmann et al 2004;Cristiani et al 2008;Fleishman and Kontar 2010;Krucker et al 2013;Zaitsev et al 2014;Klopf et al 2014). ALMA's spectral and spatial resolution for observations in the sub-THz range promise ground-breaking discoveries in this respect.…”

New Eyes Looking at Solar Activity: Challenges for Theory and Simulations – Placing It into Context

“…The solar atmosphere is a strong radio emission source, with the characteristic that shorter wavelengths probe deeper into the atmosphere and closer to the solar surface (Shibasaki, Alissandrakis, and Pohjolainen, 2011;Krucker et al, 2013). Solar transients like flares and coronal mass ejections can also generate strong radio emission (Pick and Vilmer, 2008).…”