Abstract
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 [EM] of “super-hot” (Ts≈ 30 – 50 MK) plasma but are almost invisible in the profiles of “hot” (Th≈ 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.
Original language | English (US) |
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Article number | 188 |
Journal | Solar Physics |
Volume | 296 |
Issue number | 12 |
DOIs | |
State | Published - Dec 2021 |
Externally published | Yes |
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science
Keywords
- Flares, dynamics
- Flares, relation to magnetic field
- Flares, waves
- Heating, in flares
- X-ray bursts, association with flares