Hydrogen Balmer Continuum in Solar Flares Detected by the Interface Region Imaging Spectrograph (IRIS)

We present a novel observation of the white-light flare (WLF) continuum, which was significantly enhanced during the X1 flare on March 29, 2014 (SOL2014-03-29T17:48). Data from the Interface Region Imaging Spectrograph (IRIS) in its NUV channel show that at the peak of the continuum enhancement, the contrast at the quasi-continuum window above 2813 \AA\ reached 100 - 200 % and can be even larger closer to the Mg II lines. This is fully consistent with the hydrogen recombination Balmer continuum emission, which follows an impulsive thermal and non-thermal ionization caused by the precipitation of electron beams through the chromosphere. However, a less probable photospheric continuum enhancement cannot be excluded. The light curves of the Balmer continuum have an impulsive character with a gradual fading, similar to those detected recently in the optical region on Hinode/SOT. This observation represents a first Balmer-continuum detection from space far beyond the Balmer limit (3646 \AA), eliminating seeing effects known to complicate the WLF detection. Moreover, we use a spectral window so far unexplored for flare studies, which provides the potential to study the Balmer continuum, as well as many metallic lines appearing in emission during flares. Combined with future ground-based observations of the continuum near the Balmer limit, we will be able to disentangle between various scenarios of the WLF origin. IRIS observations also provide a critical quantitative measure of the energy radiated in the Balmer continuum, which constrains various models of the energy transport and deposition during flares.Comment: accepted by ApJ

Prospects of solar magnetometry - from ground and in space

In this review we present an overview of observing facilities for solar research, which are planned or will come to operation in near future. We concentrate on facilities, which harbor specific potential for solar magnetometry. We describe the challenges and science goals of future magnetic measurements, the status of magnetic field measurements at different major solar observatories, and provide an outlook on possible upgrades of future instrumentation.Comment: Accepted for publication in Space Science Review

Unusual Filaments Inside the Umbra

We analyze several unusual filamentary structures, which appeared in the umbra of one of the sunspots in AR 11302. They do not resemble typical light bridges, neither in morphology, nor in evolution. We analyze data from SDO/HMI to investigate their temporal evolution, Hinode/SP for photospheric inversions, IBIS for chromospheric imaging, and SDO/AIA for the overlying corona. Photospheric inversions reveal a horizontal, inverse Evershed flow along these structures, which we call umbral filaments. Chromospheric images show brightenings and energy dissipation, while coronal images indicate that bright coronal loops seem to end in these umbral filaments. These rapidly evolving features do not seem to be common, and are possibly related to the high flare-productivity of the active region. Their analysis could help to understand the complex evolution of active regions

High-Density Off-Limb Flare Loops Observed by SDO

The density distribution of flare loops and the mechanisms of their emission in the continuum are still open questions. On September 10, 2017 a prominent loop system appeared during the gradual phase of an X8.2 flare (SOL2017-09-10), visible in all passbands of SDO/AIA and in the white-light continuum of SDO/HMI. We investigate its electron density by taking into account all radiation processes in the flare loops, i.e. the Thomson continuum, hydrogen Paschen and Brackett recombination continua, as well as free-free continuum emission. We derive a quadratic function of the electron density for a given temperature and effective loop thickness. By absolutely calibrating SDO/HMI intensities, we convert the measured intensities into electron density at each pixel in the loops. For a grid of plausible temperatures between cool (6000 K) and hot (10^6 K) structures, the electron density is computed for representative effective thicknesses between 200 and 20 000 km. We obtain a relatively high maximum electron density, about 10^13 cm^-3. At such high electron densities, the Thomson continuum is negligible and therefore one would not expect a significant polarization degree in dense loops. We conclude that the Paschen and Brackett recombination continua are dominant in cool flare loops, while the free-free continuum emission is dominant for warmer and hot loops.Comment: 11 pages, 8 figure

A parameter study for modeling MgII h and k emission during solar flares

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 MgII h and k lines. We aim at understanding the formation of the chromospheric and optically thick MgII 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 velocities, we study their effects on the emergent intensity spectra. We can reproduce the typical single-peaked MgII h and k flare spectral shape and their approximate intensity ratios to the subordinate MgII lines by either increasing densities, temperatures or velocities at the line core formation height range. Additionally, by combining unresolved up- and downflows up to ~250 km/s within one resolution element, we also 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.Comment: Accepted in ApJ. 12 pages, 14 figure

How important are electron beams in driving chromospheric evaporation in the 2014 March 29 flare?

We present high spatial resolution observations of chromospheric evaporation in the flare SOL2014-03-29T17:48. Interface Region Imaging Spectrograph (IRIS) observations of the FeXXI 1354.1 A 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 km s$^{-1}$ and 200 km s$^{-1}$. Hard X-ray (HXR) footpoints were observed by RHESSI 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 FeXXI by 30-75 seconds. However, in other parts of the southern ribbon and in the northern ribbon the observed upflows were not coincident with a HXR source in time nor 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.Comment: 6 figures, ApJ, accepte

Continuum Enhancements in the Ultraviolet, the Visible and the Infrared during the X1 flare on 2014 March 29

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 IRIS, SDO/HMI, and the ground-based FIRS instrument, covering wavelengths in the far-UV, near-UV, visible, and infrared during the X1 flare SOL20140329T17:48. Fits of blackbody spectra to infrared and visible wavelengths are reasonable, yielding radiation temperatures $\sim$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 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 $\sim$10-20% of electrons $>$20 keV is sufficient to explain the extra continuum emission of $\sim4-8 \times 10^{10}$ 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.Comment: Accepted for publication in Ap

On helium line polarization during the impulsive phase of an X1 flare

We analyze spectropolarimetric data of the He I 1083~nm multiplet ($1s2s~^3\!S_1 - 1s2p~^3\!P^o_{2,1,0}$) during the X1 flare SOL2014-03-29T17:48, obtained with the Facility Infrared Spectrometer (FIRS) at the Dunn Solar Telescope. While scanning active region NOAA 12017, the FIRS slit crossed a flare ribbon during the impulsive phase, when the helium line intensities turned into emission at $<$ twice the continuum intensity. Their linear polarization profiles are of the same sign across the multiplet including 1082.9 nm, intensity-like, at $<5$\% of the continuum intensity. Weaker Zeeman-induced linear polarization is also observed. Only the strongest linear polarization coincides with hard X-ray (HXR) emission at 30-70 keV observed by the Reuven Ramaty High Energy Solar Spectroscope Imager. The polarization is generally more extended and lasts longer than the HXR emission. The upper $J=0$ level of the 1082.9~nm component is unpolarizable, thus lower level polarization is the culprit. We make non-LTE radiative transfer calculations in thermal slabs optimized to fit only intensities. The linear polarizations are naturally reproduced, through a systematic change of sign with wavelength of the radiation anisotropy when slab optical depths of the 1082.9 component are $< 1$. Collisions with beams of particles are neither needed nor can they produce the same sign of polarization of the 1082.9 and 1083.0 nm components. The He I line polarization merely requires heating sufficient to produce slabs of the required thickness. Widely different polarizations of H$\alpha$, reported previously, are explained by different radiative anisotropies arising from slabs of different optical depths.Comment: Accepted for publication by the Astrophysics Journa

Mg II Lines Observed during the X-class Flare on 29 March 2014 by the Interface Region Imaging Spectrograph

Mg II lines represent one of the strongest emissions from the chromospheric plasma during solar flares. In this article, we studied the Mg II lines observed during the X1 flare on March 29 2014 (SOL2014-03-29T17:48) by IRIS. IRIS detected large intensity enhancements of the Mg II h and k lines, subordinate triplet lines, and several other metallic lines at the flare footpoints during this flare. We have used the advantage of the slit-scanning mode (rastering) of IRIS and performed, for the first time, a detailed analysis of spatial and temporal variations of the spectra. Moreover, we were also able to identify positions of strongest HXR emissions using RHESSI observations and to correlate them with the spatial and temporal evolution of Mg II spectra. The light curves of the Mg II lines increase and peak contemporarily with the HXR emissions but decay more gradually. There are large red asymmetries in the Mg II h and k lines after the flare peak. We see two spatially well separated groups of Mg II line profiles, non-reversed and reversed. In some cases, the Mg II footpoints with reversed profiles are correlated with HXR sources. We show the spatial and temporal behavior of several other line parameters (line metrics) and briefly discuss them. Finally, we have synthesized the Mg II k line using our non-LTE code with the MALI technique. Two kinds of models are considered, the flare model F2 of Machado et al. (1980) and the models of Ricchiazzi and Canfield (1983). Model F2 reproduces the peak intensity of the unreversed Mg II k profile at flare maximum but does not account for high wing intensities. On the other hand, the RC models show the sensitivity of Mg II line intensities to various electron-beam parameters. Our simulations also show that the microturbulence produces a broader line core, while the intense line wings are caused by an enhanced line source function.Comment: 25 pages, 18 figures, accepted for publication in Solar Physic

On the nature of off-limb flare continuum sources detected by SDO/HMI

The Helioseismic and Magnetic Imager onboard the Solar Dynamics Observatory has provided unique observations of off-limb flare emission. White-light (WL) continuum enhancements were detected in the "continuum" channel of the Fe 6173 A line during the impulsive phase of the observed flares. In this paper we aim to determine which radiation mechanism is responsible for such an enhancement being seen above the limb, at chromospheric heights around or below 1000 km. Using a simple analytical approach, we compare two candidate mechanisms, the hydrogen recombination continuum (Paschen) and the Thomson continuum due to scattering of disk radiation on flare electrons. Both mechanisms depend on the electron density, which is typically enhanced during the impulsive phase of a flare as the result of collisional ionization (both thermal and also non-thermal due to electron beams). We conclude that for electron densities higher than $10^{12}$ cm$^{-3}$, the Paschen recombination continuum significantly dominates the Thomson scattering continuum and there is some contribution from the hydrogen free-free emission. This is further supported by detailed radiation-hydrodynamical (RHD) simulations of the flare chromosphere heated by the electron beams. We use the RHD code FLARIX to compute the temporal evolution of the flare heating in a semi-circular loop. The synthesized continuum structure above the limb resembles the off-limb flare structures detected by HMI, namely their height above the limb, as well as the radiation intensity. These results are consistent with recent findings related to hydrogen Balmer continuum enhancements, which were clearly detected in disk flares by the IRIS near-ultraviolet spectrometer.Comment: 8 pages, 8 figures, to be published in Ap