Journal Article

Quasi-Periodic Energy Release in a Three-Ribbon Solar Flare

Zimovets, Ivan, Sharykin, Ivan, Myshyakov, Ivan

2021

Overview

Quasi-periodic pulsations (QPPs) are found in solar flares of various magnetic morphologies, e.g. in two-ribbon or circular-ribbon flares, and the mechanisms of their generation are not yet clear. Here we present the first detailed analysis of QPPs (with a period P = 54 ± 13 seconds) found in the Ramaty High Energy Solar Spectroscopic Imager (RHESSI) observations of a relatively rare three-ribbon M1.1 class flare that occurred on 5 July 2012 (SOL2012-07-05T06:49). QPPs are manifested in the temporal profiles of temperature [ T ] and emission measure [ E M ] of “super-hot” ( T s ≈ 30  – 50 MK) plasma but are almost invisible in the profiles of “hot” ( T h ≈ 15  – 20 MK) plasma parameters when approximating X-ray spectra of the flare with the bremsstrahlung spectrum of a two-temperature thermal (Maxwellian) plasma. In addition, QPPs with a similar period are found in the temporal profiles of the flux and spectral index of nonthermal electrons if the observed X-ray spectra are approximated by a combination of the bremsstrahlung spectra of a single-temperature plasma and nonthermal electrons with a power-law energy distribution. QPPs are not well expressed in the X-ray flux according to RHESSI and GOES data, or in radio data. The QPPs are accompanied by apparent systematic movement of a single X-ray source at a low speed of 34 ± 21  km s −1 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 curved and twisted field lines between two sheared flare arcades. It is worth noting that in the homologous three-ribbon M6.1 flare (SOL2012-07-05T11:39), which occurred in the same region about five hours later, the 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 spatial locations. In each pulsation, ≈(1 – 4) × 10 29  erg is released in the form of thermal energy of hot and super-hot plasmas (or accelerated electrons), which is comparable with the energy of a microflare. The total kinetic energy released during all QPPs is ≈(0.7 – 3.5) × 10 30  erg, which is about an order of magnitude less than the free magnetic energy ≈ 1.56 × 10 31  erg released in the flare region. We discuss possible propagating triggers of the quasi-periodic energy release (slow magnetoacoustic waves, asymmetric rise of curved/twisted field lines, flapping oscillations, and thermal instability in a reconnecting current sheet) and argue that the current state of available mechanisms and observations does not allow us to reach an unambiguous conclusion.

• Publisher: Springer Netherlands,Springer Nature B.V
• Subject: Approximation / Astrophysics and Astroparticles / Atmospheric Sciences / Bremsstrahlung / Current sheets / Electrons / Emission measurements / Energy / Energy distribution / Flapping / Homology / Kinetic energy / Low speed / Magnetoacoustic waves / Magnetohydrodynamic (MHD) Waves and Oscillations in the Sun’s Corona and MHD Coronal Seismology / Physics / Physics and Astronomy / Plasma / Plasmas (physics) / Polarity / Solar energy / Solar flares / Solar physics / Space Exploration and Astronautics / Space Sciences (including Extraterrestrial Physics / Thermal energy / Thermal instability / Wave propagation / X ray sources / X ray spectra / X-ray fluxes / X-rays