On the Origin of the Extreme-Ultraviolet Late Phase of Solar Flares

Abstract

Solar flares typically have an impulsive phase that is followed by a gradual phase as best seen in soft X-ray emissions. A recent discovery based on the EUV Variability Experiment observations on board the Solar Dynamics Observatory (SDO) reveals that some flares exhibit a second large peak separated from the first main phase peak by tens of minutes to hours, which is coined as the flare's EUV late phase. In this paper, we address the origin of the EUV late phase by analyzing in detail two late phase flares, an M2.9 flare on 2010 October 16 and an M1.4 flare on 2011 February 18, using multi-passband imaging observations from the Atmospheric Imaging Assembly on board SDO. We find that (1) the late phase emission originates from a different magnetic loop system, which is much larger and higher than the main phase loop system. (2) The two loop systems have different thermal evolution. While the late phase loop arcade reaches its peak brightness progressively at a later time spanning for more than one hour from high to low temperatures, the main phase loop arcade reaches its peak brightness at almost the same time (within several minutes) in all temperatures. (3) Nevertheless, the two loop systems seem to be connected magnetically, forming an asymmetric magnetic quadruple configuration. (4) Further, the footpoint brightenings in UV wavelengths show a systematic delay of about one minute from the main flare region to the remote footpoint of the late phase arcade system. We argue that the EUV late phase is the result of a long-lasting cooling process in the larger magnetic arcade system.

On the Nature of the Extreme-Ultraviolet Late Phase of Solar Flares

“…In some events, such as the events analyzed by Dai et al (2013) and Sun et al (2013), there does exist additional heating that plays some role in producing the late phase. However, in other events like that studied by Liu et al (2013a), it appears that no additional heating is required.…”

“…In the events of Dai et al (2013) and Sun et al (2013), with additional heating, the AIA EUV light curves of late-phase loops show multiple peaks, especially in the lowtemperature channels, e.g., 21.1 nm, 19.3 nm, and 17.1 nm, in the late phase (see Figure 4(i) in Dai et al 2013 and Figure 7(d) in Sun et al 2013, respectively), while in the events of Liu et al (2013a), without additional heating, the AIA EUV light curves of late-phase loops show only one main peak during the late phase (see Figures 4 and 9 in Liu et al 2013a). Note that the multiple peaks are not obvious in AIA 33.5 nm emission because of the broad and flat temperature response function in this channel.…”

“…A number of case studies proposed two physical explanations for EUV late-phase emission: one is additional heating in the late-phase loops (Woods et al 2011;Hock et al 2012;Dai et al 2013;Sun et al 2013), and the other is longlasting cooling of late-phase loops (Liu et al 2013a;Sun et al 2013). In the first explanation, the EUV late phase is explained as the natural result of a separate energy release and heating event with a lower heating rate than the main phase of the flare, while in the second explanation, the EUV late phase is due to different cooling times in flare loops with different lengths.…”

SECONDARY FLARE RIBBONS OBSERVED BY THE SOLAR DYNAMICS OBSERVATORY

“…However, in events with complicated magnetic topology, SFRs are initiated. Similar to the flares with the late phase, 10 of the 11 SFRs may result from largescale secondary magnetic reconnection triggered by the initial magnetic reconnection in the main flare regions (Woods et al 2011;Liu et al 2013;Sun et al 2013). Another possibility is that the returning filament material interacts with the solar atmosphere and generates the observed SFRs.…”

“…The EUV emission of flares exhibits a second peak separated from the first peak by tens of minutes to hours (Woods et al 2011;Liu et al 2013;Sun et al 2013). The second peak originates from the cooling of the higher and larger-scale coronal loops.…”