[en] Stepwise changes of the photospheric magnetic field, which often becomes more horizontal, have been observed during many flares. Previous interpretations include coronal loops that contract, and it has been speculated that such jerks could be responsible for sunquakes. Here we report the detection of stepwise chromospheric line-of-sight magnetic field (B${}_{\mathrm{L}\mathrm{O}\mathrm{S}}$) changes obtained through spectropolarimetry of Ca ii 8542 Å with DST/IBIS during the X1-flare SOL20140329T17:48. These changes are stronger (<640 Mx cm⁻²) and appear in larger areas than their photospheric counterparts (<320 Mx cm⁻²). The absolute value of $B_{\mathrm{L}\mathrm{O}\mathrm{S}}$ more often decreases than increases. Photospheric changes are predominantly located near a polarity inversion line, and chromospheric changes near the footpoints of loops. The locations of changes are near, but not exactly co-spatial to hard X-ray emission and neither to enhanced continuum emission nor to a small sunquake. Enhanced chromospheric and coronal emission is observed in nearly all locations that exhibit changes of $B_{\mathrm{L}\mathrm{O}\mathrm{S}}$, but the emission also occurs in many locations without any $B_{\mathrm{L}\mathrm{O}\mathrm{S}}$ changes. Photospheric and chromospheric changes of $B_{\mathrm{L}\mathrm{O}\mathrm{S}}$ show differences in timing, sign, and size and seem independent of each other. A simple model of contracting loops yields changes of the opposite sign to those observed. An explanation for this discrepancy could be increasing loop sizes or loops that untwist in a certain direction during the flare. It is yet unclear which processes are responsible for the observed changes and their timing, size, and location, especially considering the incoherence between the photosphere and the chromosphere.

[en] With machine learning entering into the awareness of the heliophysics community, solar flare prediction has become a topic of increased interest. Although machine-learning models have advanced with each successive publication, the input data has remained largely fixed on magnetic features. Despite this increased model complexity, results seem to indicate that photospheric magnetic field data alone may not be a wholly sufficient source of data for flare prediction. For the first time, we have extended the study of flare prediction to spectral data. In this work, we use Deep Neural Networks to monitor the changes of several features derived from the strong resonant Mg II h and k lines observed by the Interface Region Imaging Spectrograph. The features in descending order of predictive capability are: the triplet emission at 2798.77 Å, line core intensity, total continuum emission between the h and k line cores, the k/h ratio, line width, followed by several other line features such as asymmetry and line center. Regions that are about to flare generate spectra that are distinguishable from non-flaring active region spectra. Our algorithm can correctly identify pre-flare spectra approximately 35 minutes before the start of the flare, with an AUC of 86% and an accuracy, precision, and recall of 80%. The accuracy and AUC monotonically increase to 90% and 97%, respectively, as we move closer in time to the start of the flare. Our study indicates that spectral data alone can lead to good predictive models and should be considered an additional source of information alongside photospheric magnetograms.

[en] A three-dimensional picture of the solar atmosphere’s thermodynamics can be obtained by jointly analyzing multiple spectral lines that span many formation heights. In Paper I, we found strong correlations between spectral shapes from a variety of different ions during solar flares in comparison to the quiet Sun. We extend these techniques to address the following questions: which regions of the solar atmosphere are most connected during a solar flare, and what are the most likely responses across several spectral windows based on the observation of a single Mg ii spectrum? Our models are derived from several million IRIS spectra collected from 21 M- and X-class flares. We applied this framework to archetypal Mg ii flare spectra and analyzed the results from a multiline perspective. We find that (1) the line correlations from the photosphere to the transition region are highest in flare ribbons. (2) Blueshifted reversals appear simultaneously in Mg ii, C ii, and Si iv during the impulsive phase, with Si iv displaying possible optical depth effects. Fe ii shows signs of strong emission, indicating deep early heating. (3) The Mg ii line appears to typically evolve a blueshifted reversal that later returns to line center and becomes single peaked within 1–3 minutes. The widths of these single-peaked profiles slowly erode with time. During the later flare stages, strong red-wing enhancements indicating coronal rain are evident in Mg ii, C ii, and Si iv. Our framework is easily adaptable to any multiline data set and enables comprehensive statistical analyses of the atmospheric behavior in different spectral windows.

[en] Spectral lines allow us to probe the thermodynamics of the solar atmosphere, but the shape of a single spectral line may be similar for different thermodynamic solutions. Multiline analyses are therefore crucial, but computationally cumbersome. We investigate correlations between several chromospheric and transition region lines to restrain the thermodynamic solutions of the solar atmosphere during flares. We used machine-learning methods to capture the statistical dependencies between six spectral lines sourced from 21 large solar flares observed by NASA’s Interface Region Imaging Spectrograph. The techniques are based on an information-theoretic quantity called mutual information (MI), which captures both linear and nonlinear correlations between spectral lines. The MI is estimated using both a categorical and numeric method, and performed separately for a collection of quiet Sun and flaring observations. Both approaches return consistent results, indicating weak correlations between spectral lines under quiet Sun conditions, and substantially enhanced correlations under flaring conditions, with some line-pairs such as Mg ii and C ii having a normalized MI score as high as 0.5. We find that certain spectral lines couple more readily than others, indicating a coherence in the solar atmosphere over many scale heights during flares, and that all line-pairs are correlated to the GOES derivative, indicating a positive relationship between correlation strength and energy input. Our methods provide a highly stable and flexible framework for quantifying dependencies between the physical quantities of the solar atmosphere, allowing us to obtain a three-dimensional picture of its state.

[en] Solar flares show highly unusual spectra in which the thermodynamic conditions of the solar atmosphere are encoded. Current models are unable to fully reproduce the spectroscopic flare observations, especially the single-peaked spectral profiles of the Mg ii h and k lines. We aim to understand the formation of the chromospheric and optically thick Mg ii h and k lines in flares through radiative transfer calculations. We take a flare atmosphere obtained from a simulation with the radiative hydrodynamic code RADYN as input for a radiative transfer modeling with the RH code. By iteratively changing this model atmosphere and varying thermodynamic parameters such as temperature, electron density, and velocity, we study their effects on the emergent intensity spectra. We reproduce the typical single-peaked Mg ii h and k flare spectral shape and approximate the intensity ratios to the subordinate Mg ii lines by increasing either densities, temperatures, or velocities at the line core formation height range. Additionally, by combining unresolved upflows and downflows up to ∼250 km s⁻¹ within one resolution element, we reproduce the widely broadened line wings. While we cannot unambiguously determine which mechanism dominates in flares, future modeling efforts should investigate unresolved components, additional heat dissipation, larger velocities, and higher densities and combine the analysis of multiple spectral lines.

[en] Continuum (“white-light,” WL) emission dominates the energetics of flares. Filter-based observations, such as the IRIS SJI 2832 filter, show WL-like brightenings during flares, but it is unclear whether the emission arises from real continuum emission or enhanced spectral lines, possibly turning into emission. The difficulty in filter-based observations, contrary to spectral observations, is to determine which processes contribute to the observed brightening during flares. Here we determine the contribution of the Balmer continuum and the spectral line emission to IRIS ’ SJI 2832 emission by analyzing the appropriate passband in simultaneous IRIS NUV spectra. We find that spectral line emission can contribute up to 100% to the observed slitjaw images (SJI) emission, that the relative contributions usually temporally vary, and that the highest SJI enhancements that are observed are most likely because of the Balmer continuum. We conclude that care should be taken when calling SJI 2832 a continuum filter during flares, because the influence of the lines on the emission can be significant.

[en] Continuum emission, also called white-light emission (WLE), and permanent changes of the magnetic field (ΔB _LOS) are often observed during solar flares. However, their relation and precise mechanisms are still unknown. We study statistically the relationship between ΔB _LOS and WLE during 75 solar flares of different strengths and locations on the solar disk. We analyze SDO/HMI data and determine for each pixel in each flare if it exhibited WLE and/or ΔB _LOS. We then investigate the occurrence, strength, and spatial size of the WLE, its dependence on flare energy, and its correlation to the occurrence of ΔB _LOS. We detected WLE in 44/75 flares and ΔB _LOS in 59/75 flares. We find that WLE and ΔB _LOS are related, and their locations often overlap between 0% and 60%. Not all locations coincide, thus potentially indicating differences in their origin. We find that the WL area is related to the flare class by a power law, and extend the findings of previous studies, that the WLE is related to the flare class by a power law, to also be valid for C-class flares. To compare unresolved (Sun-as-a-star) WL measurements with our data, we derive a method to calculate temperatures and areas of such data under the blackbody assumption. The calculated unresolved WLE areas improve, but still differ to the resolved flaring area by about a factor of 5–10 (previously 10–20), which could be explained by various physical or instrumental causes. This method could also be applied to stellar flares to determine their temperatures and areas independently.

[en] We present high spatial resolution observations of chromospheric evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging Spectrograph observations of the Fe xxi $\mathrm{\lambda <!--\; ??\; -->}1354.1$ line indicate evaporating plasma at a temperature of 10 MK along the flare ribbon during the flare peak and several minutes into the decay phase with upflow velocities between 30 and 200 km s⁻¹. Hard X-ray (HXR) footpoints were observed by the Ramaty High Energy Solar Spectroscopic Imager for two minutes during the peak of the flare. Their locations coincided with the locations of the upflows in parts of the southern flare ribbon but the HXR footpoint source preceded the observation of upflows in Fe xxi by 30–75 s. However, in other parts of the southern ribbon and in the northern ribbon, the observed upflows were not coincident with an HXR source in time or space, most prominently during the decay phase. In this case evaporation is likely caused by energy input via a conductive flux that is established between the hot (25 MK) coronal source, which is present during the whole observed time-interval, and the chromosphere. The presented observations suggest that conduction may drive evaporation not only during the decay phase but also during the flare peak. Electron beam heating may only play a role in driving evaporation during the initial phases of the flare.

[en] A statistical study of the correlation between hard X-ray and white light emission in solar flares is performed in order to search for a link between flare-accelerated electrons and white light formation. We analyze 43 flares spanning GOES classes M and X using observations from the Reuven Ramaty High Energy Solar Spectroscopic Imager and Helioseismic and Magnetic Imager. We calculate X-ray fluxes at 30 keV and white light fluxes at 6173 Å summed over the hard X-ray flare ribbons with an integration time of 45 s around the peak hard-X ray time. We find a good correlation between hard X-ray fluxes and excess white light fluxes, with a highest correlation coefficient of 0.68 for photons with energy of 30 keV. Assuming the thick target model, a similar correlation is found between the deposited power by flare-accelerated electrons and the white light fluxes. The correlation coefficient is found to be largest for energy deposition by electrons above ∼50 keV. At higher electron energies the correlation decreases gradually while a rapid decrease is seen if the energy provided by low-energy electrons is added. This suggests that flare-accelerated electrons of energy ∼50 keV are the main source for white light production

[en] Enhanced continuum brightness is observed in many flares (“white light flares”), yet it is still unclear which processes contribute to the emission. To understand the transport of energy needed to account for this emission, we must first identify both the emission processes and the emission source regions. Possibilities include heating in the chromosphere causing optically thin or thick emission from free-bound transitions of Hydrogen, and heating of the photosphere causing enhanced H"− continuum brightness. To investigate these possibilities, we combine observations from Interface Region Imaging Spectrograph (IRIS), SDO/Helioseismic and Magnetic Imager, and the ground-based Facility Infrared Spectrometer instrument, covering wavelengths in the far-UV, near-UV (NUV), visible, and infrared during the X1 flare SOL20140329T17:48. Fits of blackbody spectra to infrared and visible wavelengths are reasonable, yielding radiation temperatures ∼6000–6300 K. The NUV emission, formed in the upper photosphere under undisturbed conditions, exceeds these simple fits during the flare, requiring extra emission from the Balmer continuum in the chromosphere. Thus, the continuum originates from enhanced radiation from photosphere (visible-IR) and chromosphere (NUV). From the standard thick-target flare model, we calculate the energy of the nonthermal electrons observed by Reuven Ramaty High Energy Solar Spectroscope Imager (RHESSI) and compare it to the energy radiated by the continuum emission. We find that the energy contained in most electrons >40 keV, or alternatively, of ∼10%–20% of electrons >20 keV is sufficient to explain the extra continuum emission of ∼(4–8) × 10"1"0 erg s"−"1 cm"−"2. Also, from the timing of the RHESSI HXR and the IRIS observations, we conclude that the NUV continuum is emitted nearly instantaneously when HXR emission is observed with a time difference of no more than 15 s