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This study explores the behavior of machine-learning-based flare forecasting models deployed in a simulated operational environment. Using Georgia State University’s Space Weather Analytics for Solar Flares benchmark data set, we examine the impacts of training methodology and the solar cycle on decision tree, support vector machine, and multilayer perceptron performance. We implement our classifiers using three temporal training windows: stationary, rolling, and expanding. The stationary window trains models using a single set of data available before the first forecasting instance, which remains constant throughout the solar cycle. The rolling window trains models using data from a constant time interval before the forecasting instance, which moves with the solar cycle. Finally, the expanding window trains models using all available data before the forecasting instance. For each window, a number of input features (1, 5, 10, 25, 50, and 120) and temporal sizes (5, 8, 11, 14, 17, and 20 months) were tested. To our surprise, we found that, for a window of 20 months, skill scores were comparable regardless of the window type, feature count, and classifier selected. Furthermore, reducing the size of this window only marginally decreased stationary and rolling window performance. This implies that, given enough data, a stationary window can be chosen over other window types, eliminating the need for model retraining. Finally, a moderately strong positive correlation was found to exist between a model’s false-positive rate and the solar X-ray background flux. This suggests that the solar cycle phase has a considerable influence on forecasting.

 

The first significant sunquake event of Solar Cycle 25 was observed during the X1.5 flare of 2022 May 10, by the Helioseismic and Magnetic Imager (HMI) on board the Solar Dynamics Observatory. We perform a detailed spectro-polarimetric analysis of the sunquake photospheric sources, using the Stokes profiles of the Fei6173 Å line, reconstructed from the HMI linear and circular polarized filtergrams. The results show fast variations of the continuum emission with rapid growth and slower decay lasting 3–4 minutes, coinciding in time with the hard X-ray impulses observed by the Konus instrument on board the Wind spacecraft. The variations in the line core appeared slightly ahead of the variations in the line wings, showing that the heating started in the higher atmospheric layers and propagated downward. The most significant feature of the line profile variations is the transient emission in the line core in three of the four sources, indicating intense, impulsive heating in the lower chromosphere and photosphere. In addition, the observed variations of the Stokes profiles reflect transient and permanent changes in the magnetic field strength and geometry in the sunquake sources. Comparison with the radiative hydrodynamics models shows that the physical processes in the impulsive flare phase are substantially more complex than those predicted by proton and electron beam flare models currently presented in the literature.

 

Solar Dynamics Observatory (SDO)/Helioseismic and Magnetic Imager (HMI) observations reveal a class of solar flares with substantial energy and momentum impacts in the photosphere, concurrent with white-light emission and helioseismic responses, known as sunquakes. Previous radiative hydrodynamic modeling has demonstrated the challenges of explaining sunquakes in the framework of the standard flare model of “electron beam” heating. One of the possibilities to explain the sunquakes and other signatures of the photospheric impact is to consider additional heating mechanisms involved in solar flares, for example via flare-accelerated protons. In this work, we analyze a set of single-loop Fokker–Planck and radiative hydrodynamics RADYN+FP simulations where the atmosphere is heated by nonthermal power-law-distributed proton beams which can penetrate deeper than the electron beams into the low atmospheric layers. Using the output of the RADYN models, we calculate synthetic Fei6173 Å line Stokes profiles and from those the line-of-sight observables of the SDO/HMI instrument, as well as the 3D helioseismic response, and compare them with the corresponding observational characteristics. These initial results show that the models with proton beam heating can produce the enhancement of the HMI continuum observable and explain qualitatively the generation of sunquakes. The continuum observable enhancement is evident in all models but is more prominent in ones withE_c≥ 500 keV. In contrast, the models withE_c≤ 100 keV provide a stronger sunquake-like helioseismic impact according to the 3D acoustic modeling, suggesting that low-energy (deka- and hecto-keV) protons have an important role in the generation of sunquakes.

 

Spectral lines formed at lower atmospheric layers show peculiar profiles at the “leading edge” of ribbons during solar flares. In particular, increased absorption of the BBSO/GST Heiλ10830 line, as well as broad and centrally reversed profiles in the spectra of the Mgiiand Ciilines observed by the IRIS satellite, has been reported. In this work, we aim to understand the physical origin of such peculiar IRIS profiles, which seem to be common of many, if not all, flares. To achieve this, we quantify the spectral properties of the IRIS Mgiiprofiles at the ribbon leading edge during four large flares and perform a detailed comparison with a grid of radiative hydrodynamic models using theRADYN+FPcode. We also studied their transition region (TR) counterparts, finding that these ribbon front locations are regions where TR emission and chromospheric evaporation are considerably weaker compared to other parts of the ribbons. Based on our comparison between the IRIS observations and modeling, our interpretation is that there are different heating regimes at play in the leading edge and the main bright part of the ribbons. More specifically, we suggest that bombardment of the chromosphere by more gradual and modest nonthermal electron energy fluxes can qualitatively explain the IRIS observations at the ribbon leading front, while stronger and more impulsive energy fluxes are required to drive chromospheric evaporation and more intense TR emission in the bright ribbon. Our results provide a possible physical origin for the peculiar behavior of the IRIS chromospheric lines in the ribbon leading edge and new constraints for the flare models.

 

Abstract This study presents a C3.0 flare observed by the Big Bear Solar Observatory/Goode Solar Telescope (GST) and Interface Region Imaging Spectrograph (IRIS) on 2018 May 28 around 17:10 UT. The Near-Infrared Imaging Spectropolarimeter of GST was set to spectral imaging mode to scan five spectral positions at ±0.8, ±0.4 Å and line center of He i 10830 Å. At the flare ribbon’s leading edge, the line is observed to undergo enhanced absorption, while the rest of the ribbon is observed to be in emission. When in emission, the contrast compared to the preflare ranges from about 30% to nearly 100% at different spectral positions. Two types of spectra, “convex” shape with higher intensity at line core and “concave” shape with higher emission in the line wings, are found at the trailing and peak flaring areas, respectively. On the ribbon front, negative contrasts, or enhanced absorption, of about ∼10%–20% appear in all five wavelengths. This observation strongly suggests that the negative flares observed in He i 10830 Å with mono-filtergram previously were not caused by pure Doppler shifts of this spectral line. Instead, the enhanced absorption appears to be a consequence of flare-energy injection, namely nonthermal collisional ionization of helium caused by the precipitation of high-energy electrons, as found in our recent numerical modeling results. In addition, though not strictly simultaneous, observations of Mg ii from the IRIS spacecraft, show an obvious central reversal pattern at the locations where enhanced absorption of He i 10830 Å is seen, which is consistent with previous observations. 

Abstract While solar flares are predominantly characterized by an intense broadband enhancement to the solar radiative output, certain spectral lines and continua will, in theory, exhibit flare-induced dimmings. Observations of transitions of orthohelium He i λ λ 10830 Å and the He i D3 lines have shown evidence of such dimming, usually followed by enhanced emission. It has been suggested that nonthermal collisional ionization of helium by an electron beam, followed by recombinations to orthohelium, is responsible for overpopulating those levels, leading to stronger absorption. However, it has not been possible observationally to preclude the possibility of overpopulating orthohelium via enhanced photoionization of He i by EUV irradiance from the flaring corona followed by recombinations. Here we present radiation hydrodynamics simulations of nonthermal electron-beam-driven flares where (1) both nonthermal collisional ionization of helium and coronal irradiance are included, and (2) only coronal irradiance is included. A grid of simulations covering a range of total energies deposited by the electron beam and a range of nonthermal electron-beam low-energy cutoff values were simulated. In order to obtain flare-induced dimming of the He i 10830 Å line, it was necessary for nonthermal collisional ionization to be present. The effect was more prominent in flares with larger low-energy cutoff values and longer lived in weaker flares and flares with a more gradual energy deposition timescale. These results demonstrate the usefulness of orthohelium line emission as a diagnostic of flare energy transport.