Chromospheric condensation and quasi-periodic pulsations in a circular-ribbon flare
In this paper, we report our multiwavelength observations of the C3.1 circular-ribbon flare SOL2015-10-16T10:20 in active region (AR) 12434. The flare consisted of a circular flare ribbon (CFR), an inner flare ribbon (IFR) inside it, and a pair of short parallel flare ribbons (PFRs). The PFRs located to the north of the IFR were most striking in the Interface Region Imaging Spectrograph (IRIS) 1400 and 2796 Å images. For the first time, we observed the circular-ribbon flare in the Ca ii H line of the Solar Optical Telescope on board Hinode, which has a similar shape as observed in the Atmospheric Imaging Assembly 1600 Å on board the Solar Dynamic Observatory (SDO). Photospheric line-of-sight magnetograms from the Helioseismic and Magnetic Imager on board SDO show that the flare is associated with positive polarities with a negative polarity inside. The IFR and CFR were cospatial with the negative polarity and positive polarities, implying the existence of a magnetic null point (B=0) and a dome-like spine–fan topology. During the impulsive phase of the flare, “two-step” raster observations of IRIS with a cadence of 6 s and an exposure time of 2 s showed plasma downflow at the CFR in the Si iv λ1402.77 line (logT≈4.8), suggesting chromospheric condensation. The downflow speeds first increased rapidly from a few km s{sup −1} to the peak values of 45–52 km s{sup −1}, before decreasing gradually to the initial levels. The decay timescales of condensation were 3–4 minutes, indicating ongoing magnetic reconnection. Interestingly, the downflow speeds are positively correlated with the logarithm of the Si iv line intensity and time derivative of the GOES soft X-ray (SXR) flux in 1–8 Å. The radio dynamic spectra are characterized by a type III radio burst associated with the flare, which implies that the chromospheric condensation was most probably driven by nonthermal electrons. Using an analytical expression and the peak Doppler velocity, we derive the lower limit of energy flux of the precipitating electrons, i.e., 0.65 × 10{sup 10} erg cm{sup −2} s{sup −1}. The Si iv line intensity and SXR derivative show quasi-periodic pulsations with periods of 32–42 s, which are likely caused by intermittent null-point magnetic reconnections modulated by the fast wave propagating along the fan surface loops at a phase speed of 950–1250 km s{sup −1}. Periodic accelerations and precipitations of the electrons result in periodic heating observed in the Si iv line and SXR.
- OSTI ID:
- 22868448
- Journal Information:
- Astrophysical Journal, Vol. 832, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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IMAGING AND SPECTROSCOPIC OBSERVATIONS OF MAGNETIC RECONNECTION AND CHROMOSPHERIC EVAPORATION IN A SOLAR FLARE