Infrared spectropolarimetry of a C-class solar flare footpoint plasma -- I. Spectral features and forward modelling

Abstract

We performed high-spatial resolution spectropolarimetric observations of active region NOAA 13363 during a C-class flare with the Gregor Infrared Spectrograph (GRIS) on 16 July 2023. We examine the coupling between the photosphere and the chromosphere, studying the polarimetric signals during a period that encompasses the decaying phase of a C-class flare and the appearance of a new C-class flare at the same location. We focus on the analysis of various spectral lines. In particular, we study the Si I 10827 Å, Ca I 10833.4 Å, Na I 10834.9 Å, and Ca I 10838.9 Å photospheric lines, as well as the He I 10830 Å triplet. GRIS data revealed the presence of flare-related red- and blueshifted spectral line components, reaching Doppler velocities up to 90 km/s, and complex Si I profiles where the He i spectral line contribution is blueshifted. In contrast, the photospheric Ca i and Na i transitions remained unchanged, indicating that the flare did not modify the physical conditions of the lower photosphere. We combined that information with simultaneous imaging in the Ca ii H line and TiO band with the improved High-resolution Fast Imager (HiFI+), finding that the flare emission did not affect the inverse granulation or nearby plage, in agreement with the results from GRIS. We also complement the previous studies with a forward modelling computation, concluding that the He I spectral line emission reflects a complex response of the flaring chromosphere. Radiative excitation from coronal EUV irradiation, energy deposition by flare-accelerated electrons, and dynamic field-aligned plasma flows likely act together to produce the observed supersonic downflows and upflows. We plan to expand these findings through inversions of the He I 10830 Å triplet signals in the future.

Figures (8)

• Figure 1: Temporal evolution of the Fried-parameter $r_0$ (black bullets) and air mass (solid line) during the 2023 July16 observations with the 1.5-metre Gregor solar telescope. The light grey rectangle corresponds to the time interval of the HiFI+ observations. The black bullets were taken from the Gregor status monitor and the colour-coded bullets are recorded in the headers of the GRIS scans. The colour-coded bullets refer to the Fried-parameter $r_0$ for each step during the five GRIS scans. The air mass was obtained by using the web interface to the JPL Horizons On$-$Line Ephemeris System.
• Figure 2: Collection of SDO observations taken at around 08:48 UT. Co-aligned Gregor/GRIS slit-scan and HiFI+ FOVs are shown as red and green (TiO) as well as white (Caii H) rectangles, respectively. The locations of the first flare (FL1) and second flare (FL2) are labelled in each panel. Bottom left image displays an HMI continuum image of the solar disk at the time of the observation, highlighting with a black square the location of the active region of interest. The lower right panel shows the temporal evolution of the X-ray flux recorded by GOES in the 1.0 -- 8.0 Å (red) and 0.5 -- 4.0 Å (blue) channels for the period from 08:30 UT -- 09:10 UT on July 16, 2023. Gregor's observations are indicated by the blue shaded region. The X-ray maxima are indicated by a solid green line for FL1 and a dotted blue line for FL2, corresponding to the flare peaks.
• Figure 3: Speckle-restored TiO image taken at 08:45:18 UT at the start of the second flare FL2. The white rectangle indicates the size and location of the speckle-restored Caii H image shown in Figure \ref{['CAIIH']}. The red solid rectangle shows the area covered by the Gregor/GRIS slit-scan.
• Figure 4: Time series of speckle-restored Caii H images showing the temporal evolution of the second flare FL2 at a cadence of about 12 s (time interval $-$ 24 s). The small white rectangle in the lower left corner has a size of $5\arcsec \times 1\arcsec$. Gregor/GRIS slit scans are indicated by the red solid rectangle. The black arrow and the label "FL2" indicate the identified location of the second flare. Note that the field of view of these HiFI+ Caii H observations does not cover the location of FL1, which lies outside these panels.
• Figure 5: Logarithmic BaSAM of the Caii H intensity. The horizontal colour bar indicates, from left to right, increased activity. The black dashed rectangle highlights the region of the Gregor/GRIS slit-scan while the encircled area displays a region with small-scale Caii H brightenings (see more details in the text).