Transition Region and Chromospheric Signatures of Impulsive Heating Events. I. Observations

Abstract

We exploit the high spatial resolution and high cadence of the Interface Region Imaging Spectrograph (IRIS) to investigate the response of the transition region and chromosphere to energy deposition during a small flare. Simultaneous observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager provide constraints on the energetic electrons precipitating into the flare footpoints, while observations of the X-Ray Telescope, Atmospheric Imaging Assembly, and Extreme Ultraviolet Imaging Spectrometer (EIS) allow us to measure the temperatures and emission measures from the resulting flare loops. We find clear evidence for heating over an extended period on the spatial scale of a single IRIS pixel. During the impulsive phase of this event, the intensities in each pixel for the Si IV 1402.770 Å, C II 1334.535 Å, Mg II 2796.354 Å, and O I 1355.598 Å emission lines are characterized by numerous small-scale bursts typically lasting 60 s or less. Redshifts are observed in Si IV, C II, and Mg II during the impulsive phase. Mg II shows redshifts during the bursts and stationary emission at other times. The Si IV and C II profiles, in contrast, are observed to be redshifted at all times during the impulsive phase. These persistent redshifts are a challenge for one-dimensional hydrodynamic models, which predict only short-duration downflows in response to impulsive heating. We conjecture that energy is being released on many small-scale filaments with a power-law distribution of heating rates.

Spectroscopic Observations of Magnetic Reconnection and Chromospheric Evaporation in an X-shaped Solar Flare

“…Tian et al (2015) also reported that the Si IV line is often not entirely redshifted but just shows an evident red-wing enhancement at the ribbons. It is worth mentioning that some fully redshifted Si IV profiles were reported by Warren et al (2016) and also reproduced in numerical simulations by Reep et al (2016). However, those Si IV profiles generally exhibit multiple components and might be better fitted by a double-or multiple-Gaussian function.…”

The development of lower-atmosphere turbulence early in a solar flare

“…The presence of turbulent signatures over ~60 s means that the driver of turbulence persists for longer. Si IV emission in flares is usually well correlated with flare hard x-rays (HXRs) ( 19 ), which provide the most direct signature of intense energy deposition by flare-accelerated electrons in the lower atmosphere. In this flare, however, there is no detectable high-energy HXR emission, although the 6- to 12-keV x-rays rise rapidly as the Si IV intensity peaks, possibly indicating low-energy nonthermal electrons.…”

A New View of the Solar Interface Region from the Interface Region Imaging Spectrograph (IRIS)

“…Tian et al, 2015;Brosius, Daw, and Inglis, 2016;Graham and Cauzzi, 2015;Graham et al, 2020). The Si IV and C II spectra sometimes show persistent red-shifts or red-asymmetries of a few tens of km s −1 during the whole impulsive phase (Warren et al, 2016;Yu et al, 2020), which provide challenging constraints to the 1D loop models (e.g. Reep et al, 2016bReep et al, , 2018.…”