New Solar Extreme-Ultraviolet Irradiance Observations During Flares

Abstract

New solar extreme-ultraviolet (EUV) irradiance observations from the NASA Solar Dynamics Observatory (SDO) EUV Variability Experiment provide full coverage in the EUV range from 0.1 to 106 nm and continuously at a cadence of 10 s for spectra at 0.1 nm resolution and even faster, 0.25 s, for six EUV bands. These observations can be decomposed into four distinct characteristics during flares. First, the emissions that dominate during the flare's impulsive phase are the transition region emissions, such as the He ii 30.4 nm. Second, the hot coronal emissions above 5 MK dominate during the gradual phase and are highly correlated with the GOES X-ray. A third flare characteristic in the EUV is coronal dimming, seen best in the cool corona, such as the Fe ix 17.1 nm. As the post-flare loops reconnect and cool, many of the EUV coronal emissions peak a few minutes after the GOES X-ray peak. One interesting variation of the post-eruptive loop reconnection is that warm coronal emissions (e.g., Fe xvi 33.5 nm) sometimes exhibit a second large peak separated from the primary flare event by many minutes to hours, with EUV emission originating not from the original flare site and its immediate vicinity, but rather from a volume of higher loops. We refer to this second peak as the EUV late phase. The characterization of many flares during the SDO mission is provided, including quantification of the spectral irradiance from the EUV late phase that cannot be inferred from GOES X-ray diagnostics.

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

“…After the reconnections, the small arcade cools down quickly, while the large arcade cools down more slowly, which produces the observed late phase. Woods et al (2011) also pointed out that the EUV late phase may be caused by another reconnection, which has a different location and rate from the flare reconnection. In this study, we further propose that there is a cause-effect relationship between the two reconnections.…”

“…This gradual phase is also called the decay phase of a flare (Moore et al 1980). Recently, Woods et al (2011) discovered a new phase of solar flare evolution using EUV irradiance observations on the EUV Variability Experiment (EVE; Woods et al 2012) on board the Solar Dynamics Observatory (SDO; Pesnell et al 2012). They found that, in addition to a peak during the gradual phase in EUV "warm" coronal emissions (e.g., ∼2.5 MK from Fe xvi 33.5 nm), some flares also exhibit a second peak separated by tens of minutes to hours from the first peak; the second peak in the EUV is completely absent in the usual smooth profiles of the soft X-ray gradual phase (thus evading detection in the past).…”

“…They found that, in addition to a peak during the gradual phase in EUV "warm" coronal emissions (e.g., ∼2.5 MK from Fe xvi 33.5 nm), some flares also exhibit a second peak separated by tens of minutes to hours from the first peak; the second peak in the EUV is completely absent in the usual smooth profiles of the soft X-ray gradual phase (thus evading detection in the past). This second peak was coined as the "EUV late phase" by Woods et al (2011). They also pointed out that the late phase originates from a second set of higher loops within the flaring active region (AR), i.e., where the fast magnetic reconnection occurs, which can be called the flare main region, comparing to the flare late phase region.…”

Failed filament eruption inside a coronal mass ejection in active region 11121

“…In this paper, we observationally study a failed filament eruption associated with an M5.4-class flare (Schrijver 2011;Woods et al 2011). We describe the morphology and dynamic of the loop arcades and filament before and after the eruption, and interpret our observations in terms of the magnetic breakout and kink instability models.…”

Post-Flare Ultraviolet Light Curves Explained With Thermal Instability of Loop Plasma

“…EVE flare decays showed qualitatively the same features described by Feldman et al (2003), with a series of intensity enhancements of lines from progressively colder ions, down to around 1 MK (Woods et al 2011). Since EVE observes the entire solar disk continuously, flare observations are being made routinely, although at a reduced spectral resolution.…”