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We present the results of a study of the non-eruptive C2.8 class flare in the active region (AR) NOAA 13256, which occurred on March 19,2023 from 02:12 to 02:19 UTC. This event was chosen on the basis of the test launches of the Irkutsk Solar Radio Spectropolarimeter (SOLARSPEL), Badary (ISTP RAS). Despite the low X-ray class and short duration, according to SOLARSPEL data, this impulsive flare had a complex multi-peak fine time structure, recorded at different frequencies in the microwave range. The presence of a photospheric disturbance in the vicinity of the sunspot penumbra, recorded using HMI/SDO, was of great interest for physics and motivated this study. There are few detailed multi-wavelength studies in the literature of low-power flares accompanied by a response at the photosphere level. Notably, this event was observed simultaneously by four X-ray instruments: SoLO/STIX, ASO-S/HXI, FERMI/GBM, and Konus-Wind. As a result, the unique observation conditions of this flare, from the point of view of the available instruments and various recorded physical high-energy processes, motivated us to carry out detailed research. We found that the photospheric perturbations are mostly associated with the stronger magnetic field in the penumbra rather than with the distribution of the HXR sources. The observed flare ribbons were located in the penumbral PIL region, which revealed the complexity of the larger events in terms of the spatial and temporal structure of the energy release. We also briefly discuss the observed quasi-periodic pulsations.
Sun: solar flare, radio emission, X-ray emission, photospheric perturbations, magnetic fields
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\title{
Multi-wavelength observations of the impulsive C2.8 class flare producing photospheric impact at the penumbra region of the leading sunspot in NOAA 13256
}
%
\titlerunning{
Multi-wavelength observations of the impulsive C2.8 class flare
} % abbreviated title (for running head)
%also used for the TOC unless
%\toctitle is used
\author{
G.~Koynash\inst{1}\and
I.~Sharykin\inst{1}\and
I.~Zimovets\inst{1}\and
E.~Ivanov\inst{2}\and
V.~Kiselev\inst{2}\and
B.~Nizamov\inst{3}
}
%
\authorrunning{Koynash et al.} % abbreviated author list (for running head)
%
%%%% list of authors for the TOC (use if author list has to be modified)
%\tocauthor{}
%
\institute{
Space Research Institute of Russian Academy of Sciences, Moscow, 117997, Russia
\and Institute of Solar-Terrestrial Physics of Siberian Branch of Russian Academy of Sciences, Irkutsk, 664033, Russia
\and Sternberg Astronomical Institute, Moscow, 119234, Russia
}
\abstract{
We present the results of a study of the non-eruptive C2.8 class flare in the active region (AR) NOAA 13256, which occurred on 2023 March 19 from 02:12 to 02:19 UTC. This event was chosen on the basis of the test launches of the Irkutsk Solar Radio Spectropolarimeter (SOLARSPEL), Badary (ISTP RAS). Despite the low X-ray class and short duration, according to SOLARSPEL data, this impulsive flare had a complex multi-peak fine time structure, recorded at different frequencies in the microwave range. The presence of a photospheric disturbance in the vicinity of the sunspot penumbra, recorded using HMI/SDO, was of great interest for physics and motivated this study. There're few detailed multi-wavelength studies in the literature of low-power flares accompanied by a response at the photosphere level. Notably, this event was observed simultaneously by four X-ray instruments: SoLO/STIX, ASO-S/HXI, FERMI/GBM, and Konus-Wind. As a result, the unique observation conditions of this flare, from the point of view of the available instruments and various recorded physical high-energy processes, motivated us to carry out detailed research. We found that the photospheric perturbations are mostly associated with the stronger magnetic field in the penumbra rather than with the distribution of the HXR sources. The observed flare ribbons were located in the penumbral PIL region, which revealed the complexity of the larger events in terms of the spatial and temporal structure of the energy release. We also briefly discuss the observed quasi-periodic pulsations.
\keywords{ Sun: solar flare, radio emission, X-ray emission, photospheric perturbations, magnetic fields
}
\doi{
10.26119/VAK2024-ZZZZ
}
}
\maketitle
\section{Introduction}
The selected solar flare (briefly described in the abstract) is investigated due to the following peculiarities that motivated us for this research: relatively low GOES class, multi-peak impulsive energy release in the MW data, presence of photospheric impacts, and relatively simple AR morphology. Moreover, we have an abundance of observational data, making this flare unique for collecting detailed physical information. Today, there are not as many complex case studies of solar flares below the C3.0 class that are accompanied by photospheric perturbations \citep[see e.g.][]{1} in active regions with simple Hale and McIntosh classes. Furthermore, flares with white-light emission and photospheric perturbations are quite rare in the case of C-class flares \citep[e.g. see, for example, the statistical work of ][]{2}. The main aim is to study the morphological properties of this particular solar flare: the configuration of the emission sources in the multi-wavelength data relative to each other in the AR and to the positions of the photospheric impacts and magnetic field structure.
%We compare the microwave (MW) data received at the very begin of the SOLARSPEL observations with NoRP's MW data and ultraviolet (UV), extreme ultraviolet (EUV), and X-ray data from the world's major space observatories. We also analyzes the features of the morphology of the C2.8\thinspace class flare region from the point of view of magnetic field structure, configuration of UV/EUV, X-ray and MW sources relative to areas of photospheric disturbances.
\section{Observations and Instruments}
A selected C2.8 solar flare occurred near to the east limb (S23E58) in AR NOAA\thinspace 13256, on March\thinspace 19,\thinspace 2023. It began at 02:12\thinspace UT, reached its maximum at 02:14:54\thinspace UT, and ended at 02:19\thinspace UT. The parent AR, where the flare was located, had a Hale $\beta$ class and a ``simple'' McIntosh class of Eho, with the magnetic field flux concentrated in a large single leading sunspot where flare emission sources were also detected. We also registered photospheric disturbances seen in HMI 45 second data: line-of-sight (LOS) magnetograms and Dopplergrams.
%The sunspot area was 0250/0060 [millionths], and the number of spots was 02/01.
This flare was simultaneously observed by the Solar Radio Spectropolarimeter (SOLARSPEL), the Nobeyama Radio Polarimeters \citep[NoRP, ][]{3}; Siberian Radio Heliograph \citep[SRH, ][]{4}, the Atmospheric Imaging Assembly \citep[AIA, ][]{5}, the Helioseismic and Magnetic Imager \citep[HMI, magnetic field and mapping of photospheric impacts, ][]{6} onboard the Solar Dynamics Observatory (SDO); the X-Ray Sensor (XRS) onboard the Geostationary Operational Environmental Satellite (GOES), the Gamma Ray Burst Monitor \citep[GBM, ][]{7} onboard the FERMI, the Konus/Wind \citep[][]{8}, the Spectrometer Telescope for Imaging X-rays \citep[STIX, ][]{9} on the Solar Orbiter and the Hard X-Ray Imager \citep[HXI, ][]{10} on the Advanced Space-based Solar Observatory.
SOLARSPEL is a new radio broadband spectropolarimeter located at the Badary Radioastrophysical Observatory, and its data obtained from test runs allowed us to identify the studied flare due to its impulsive multi-peak MW time series (from event catalog of the SOLARSPEL test runs). SOLARSPEL and NoRP measures microwave (MW) emission from the entire Sun in multiple frequency channels. The MW flux is measured for a frequency grid of 48\thinspace channels in the range from 3\thinspace GHz to 24\thinspace GHz (for the studied flare, only the 3–12 GHz range is available) with circular polarization for SOLARSPEL with a time cadence of 1\thinspace s, and intensity and circular polarization at six frequencies (we detected a polarization signal only at 9.4 GHz) for NoRP with a time cadence of 0.1\thinspace s. Periodic background variations in SOLARSPEL data are due to not well-compensated instrumental effects.
% The Konus–Wind's omnidirectional detector array locates in interplanetary space far outside the Earth’s magnetosphere, and observes the all sky in a wide energy range from $\sim{20}$ keV to $\sim{14}$ MeV \citet{5}.
% The STIX Solar Orbiter is a hard X-ray imaging spectrometer, which covers the energy range from 4 to 150 keV. The STIX instrument provides imaging spectroscopy of solar flare X-ray emission \citet{7, 8}.
\begin{figure*}
\centerline{\includegraphics[width=0.8\textwidth]{fig_1.pdf}}
\caption{Overview of the time profiles (TP) of the C2.8 GOES-class solar flare on 19 March 2023 from 02:12 to 02:19 UT. Left side: SOLARSPEL, NoRP, GOES/XRS the TS and the time derivative of GOES/XRS. Right side: Fermi/GBM, Konus-Wind, SoLO/STIX and ASO-S/HXI.}
\label{fig:1}
\end{figure*}
Fig~\ref{fig:1} shows the light curves produced by the mentioned instruments. In top left corner, we present the uncalibrated SOLARSPEL MW data at five frequencies (Stokes\thinspace I). Below them are the NoRP's MW data for 9.4\thinspace GHz and the SOLARSPEL MW data at 9.6\thinspace GHz, calibrated with the NoRP's 9.4\thinspace GHz data.
% SDO/AIA provide nearly simultaneously observation of full-disk solar images in multiple wavelengths \citet{2}, and SDO/HMI is designed to investigate the solar magnetic field and oscillatory features on the Sun \citet{3}.
\section{Data Analysis and Results}
We analysed SOLARSPEL and NoRP MW data, finding pronounced polarization in the radio burst. SOLARSPEL showed good sensitivity and temporal resolution with consistent signals across a wide frequency range. Although we lacked sufficient data to fully calibrate the SOLARSPEL
% MW data, we will attempt a simple spectrum approximation based on Zirin's method.
The MW and X-ray data reveal that the flare's impulsive phase consisted of a few peaks. In general, X-ray observations are consistent with the Neupert effect. Based on those data, we detected quasi-periodic pulsations (QPP) with a repeat period $P_{R} \approx 12.3 \pm 2.5$\thinspace s. QPP is a common feature of flaring energy releases in the solar atmosphere, observed in all bands, from radio and MW to hard X-rays and gamma-rays \citep{11}. The physical mechanisms responsible for QPP can be explained by various models \citep[e.g., ][]{12}. For this episode, we plan to investigate it more carefully and in detail later.
\begin{figure*}
\centerline{\includegraphics[width=0.83\textwidth]{aia_hmi.pdf}}
\caption{The sequence of images snapshots with a field of view (FOV) is $130"\times130"$ of the C2.8 flare observed by SDO/AIA (EUV 131\thinspace Å - top row; UV 1600\thinspace Å - bottom row) includes SDO/HMI contours of magnetic field [1.0, 1.5, 2.0, 2.5, 3.0]\thinspace kG, and contours for X-ray energy bands: 4-10\thinspace keV (red), 10-15\thinspace keV (yellow), 15-25\thinspace keV (green), 25-50\thinspace keV (blue).}
\label{fig:2}
\end{figure*}
However, these peaks could be connected with the multiple magnetic loops forming a magnetic arcade. To analyse the spatial structure of the flare energy release with the best available spatial resolution, we used AIA images (Fig~\ref{fig:2}) at 131\thinspace Å (top-Fig~\ref{fig:2}) with a time cadence of 12 s, and low-temperature chromospheric UV band around 1600\thinspace\AA (bottom-Fig~\ref{fig:2}), with a time cadence of 24\thinspace s. In the 1600~\AA{}, we observe many localized quasi-static brightnings forming large-scale flare ribbons, with the northern one located in the sunspot penumbra. It is noteworthy that the small event shows ``complexity'' characteristic of large flares and cannot be considered as an ``elementary'' episode of flare energy release.
The STIX X-ray contour images are shown in Fig~\ref{fig:2}~c2.
We observe a typical situation with two HXR sources corresponding to the flare ribbons. A more detailed vizualization of high-energy flare processes is presented in Fig~\ref{fig:3}. MW contour images from SRH at a few frequencies, X-ray STIX and HXI contour maps cover AIA~1600~\AA{} map at the flare MW peak in the corresponding figure panels a-c. The spatial resolution of X-ray images is sufficient to resolve paired HXR sources. In the case of SRH, we observe a single flare source covering the flare UV ribbons or located between the ribbons at the highest frequencies. Possibly, we are observing MW coronal sources.
%We also used the HMI's contours for [0.5, 1.0, 1.5, 2.0, 2.5, 3.0]\thinspace kG levels of magnetic field.
\begin{figure*}
\centerline{\includegraphics[width=0.8\textwidth]{AIA_SRH_STIX_HXI.pdf}}
\centerline{\includegraphics[width=0.85\textwidth]{HMI.eps}}
\caption{Top panels show AIA 1600~\AA{} image with countours of SRH (a), STIX(b), HXI(c) sources. The bottom panels present photospheric perturbation observed in the HMI LOS magnetograms (d-f) and Dopplergram (g). Panel d corresponds to the flare onset, while panels e-g show the flare MW peak time. Red contours show the AIA 1600~\AA{} emission sources. STIX contours are also shown in panels e-f. Contours highlight magnetic field strengths of 1.0, 1.5, 2.0, 2.5, and 3.0 kG, with the PIL location marked by the thick white line. Different polarities are shown by white and cyan.
}
\label{fig:3}
\end{figure*}
Weak photospheric perturbations were best observed in HMI 45-second LOS magnetograms and Dopplergrams (Fig~\ref{fig:3}). The strongest impacts were detected in the northern ribbon (highlighted by the thick red contour in panel e), associated with the stronger penumbral magnetic fields compared to the larger ribbon on the other side of the PIL outside the sunspot. It's worth noting that some photospheric perturbations are outside of the centroids of the HXR sources (Fig~\ref{fig:3}~f).
It seems that the photospheric perturbation are related to the presence of the strong magnetic field. It is possible that we observe MHD motions of the magnetized plasma due to the magnetic reconnection process. However, it's also possible that some dense narrow beams of energetic particles are not seen in HXR due to the limited dynamic ranges of STIX and HXI. Additional research is needed to understand physics of the changing photospheric magnetic field and Doppler velocity in the flare ribbons.
\section{Conclusions}
% Using MW observations, we studied the simultaneous observations of the impulsive C2.8 class flare with MW quasi-periodic pulsation (QPP) and photospheric impact at the leading sunspot.
The following conclusions for the studied flare were drawn:
\begin{enumerate}
\item The studied C2.8 flare reveals a complexity typical of large events despite the relatively simple morphology of the parent AR. There are two ribbons on either side of the penumbral polarity inversion line (PIL), corresponding hard X-ray sources, and a fine temporal and spatial structure of the energy release.
\item The nature of the observed MW and HXR QPP is uncertain. The multi-peak time series may be explained by subsequent energy release in separate thin magnetic loops rooted at the flare ribbons, forming the observed fine spatial structure.
\item Photospheric perturbations are registered in the flare ribbon of the sunspot penumbra. There is no exact coincidence with the HXR emission sources. Are the flare photospheric impacts primarily connected with the magnitude and structure of the magnetic field?
\end{enumerate}
%As one of the significant results of the review of these data that is the SOLARSPEL is very sensitive and modern radio-astronomical instrument.
%\acknowledgements{The observations were carried out with the Radio Spectropolarimeters 0.05-24\thinspace GHz (Badary, ISTP RAS)\footnote{\url{https://badary.iszf.irk.ru/SP_3_24.php}}, the Nobeyama Radio Polarimeters (NoRP), the Solar Dynamics Observatory (SDO), GOES/XRS, SoLO/STIX, ASO-S/HXI, FERMI/GBM and Konus-Wind. This research has made use of the NASA/SDO Database, SunPy is a python scientific library \footnote{\url{https://sunpy.org/}} for collected and research observation's data, Solar Orbiter STIX Data Center\footnote{\url{https://datacenter.stix.i4ds.net/}}}
\acknowledgements{We thank the numerous team members of the SRH and SOLARSPEL (Badary, ISTP RAS)\footnote{\url{https://badary.iszf.irk.ru}}, NoRP, SDO, GOES, STIX, HXI, GBM, and Konus-Wind for providing the data and corresponding analysis software.}
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