Quasi-periodic energy release in a three-ribbon solar flare

Jul 8, 2021, 7:30 PM


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Working Group 1: Flare thermal response Working Group 1: Flare thermal response


Ivan Zimovets (Space Research Institute of the Russian Academy of Sciences (IKI RAS))


Quasi-periodic pulsations (QPPs) are found in solar flares of various magnetic morphologies, e.g. in two-ribbon or circular-ribbon flares, and mechanisms of their generation are not yet clear. Here we present the analysis of QPPs with a period P = 54±13 s found in RHESSI observations of a three-ribbon M1.1 class flare SOL2012-07-05T06:49. QPPs are manifested in the time profiles of temperature (T) and emission measure (EM) of superhot (Ts ~ 30-50 MK) plasma, but are almost invisible in the profiles of hot (Th ~ 15-20 MK) plasma parameters when approximating the X-ray spectrum of the flare with the bremsstrahlung spectrum of a two-temperature thermal plasma (the one-temperature approximation gives a poor approximation with a much higher residual). In addition, QPPs with a similar period are found in the time profiles of the flux and spectral index of nonthermal electrons if the observed X-ray spectrum is approximated by a combination of the bremsstrahlung spectra of a single-temperature Maxwellian plasma and nonthermal electrons. In this case, the power-law spectrum of nonthermal electrons is very soft, with an index from -7 to -10, and shows the known soft-hard-soft dynamics for each pulsation. QPPs are not expressed in the X-ray flux according to RHESSI and GOES data, as well as in radio data. Remarkable, QPPs are accompanied by apparent systematic movement of a “single” X-ray source at a speed below 120 km / s (average speed ~ 35 km / s) along the central flare ribbon over a narrow (<5 Mm) “tongue” of negative magnetic polarity, elongated (~ 20 Mm) between two areas of positive polarity. The results of magnetic extrapolation in the nonlinear force-free field (NLFFF) approximation show that the X-ray source could “move” along a separator and/or magnetic flux-rope in the corona. It is worth noting that in the homologous three-ribbon M6.1 flare SOL2012-07-05T11:39 that occurred in the same region about five hours later, X-ray sources “moved” much less systematically and did not produce similar QPPs. We interpret the observed QPPs as a result of successive episodes of energy release in different magnetic tubes (loops) of the flare region. In each pulsation approximately (4-7) x 10 ^ 29 erg is released in the form of thermal energy of hot and superhot plasmas (or accelerated electrons). The total energy release during all pulsations is ~ (3-6) x 10 ^ 30 erg, which is less than the value of the magnetic energy (~ 8 x 10 ^ 30 erg) released in the flare region. We discuss possible triggers of the “propagating” front of energy release (slow and fast magnetoacoustic waves, flapping oscillations and thermal instability of a reconnecting current sheet with strong guide-field formed around the separator, asymmetric eruption of a magnetic flux-rope, etc.), and argue that the detected QPPs are not an instrumental effect.

Email ivanzimovets@gmail.com

Primary authors

Ivan Zimovets (Space Research Institute of the Russian Academy of Sciences (IKI RAS)) Dr Ivan Sharykin (Space Research Institute of the Russian Academy of Sciences (IKI RAS)) Dr Ivan Myshyakov (Institute of Solar-Terrestrial Physics, Siberian Branch of the Russian Academy of Sciences (ISTP SB RAS))

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