Constructing mean electron distributions from hard X-ray spectra of solar flares

The Ramaty High Energy Solar Spectroscopic Imager (RHESSI) mission provides information on high resolution X-ray spectra emitted by collisional bremsstrahlung of thermal and non-thermal electrons with ions in solar flares. One of the aims of the mission is to infer information about the acceleration and transport mechanisms of hard X-ray emitting electrons and 7-ray emitting ions in solar flares. I investigate events observed during distinct attenuator states of the satellite, which have unique characteristics in their photon and mean electron distributions. Such characteristics include evidence of low energy cutoff features and other such physically real bump and dip features. In the framework of a colli-sionally thin bremsstrahlung model and an adjustable thermal function I forward fit these to the background-removed count flux data, using least squares minimization via OSPEX (Object Spectroscopy Executive). Along with Chi-Square Tests, I present random and non- systematic residuals to show goodness of fit. Conversion from count rate to count flux and then to photon flux spectra is a traditional approach to modelling hard X-ray spectra, but is significantly unreliable due to its dependence on parametric electron distribution approximations. Model independent hard X-ray spectra can be non-trivially calculated using the Detector Response Matrix (DRM) and are computed using a sequence of algorithms solving a linear system of equations, which present an inversion problem characterized by numerical instability due to the non-diagonal nature of the DRM. The extent of such instability is unique for different DRM configurations, hence different under each attenuator state. We then address the solution of this inversion problem- by using a regularization algorithm with the aim of inferring accurate and useful electron distribution spectra. Adjustment of this procedure to convert directly between counts and electrons, which is a one step rather than two step process, presents more robust and detailed information about features of electron dynamics during flares. Significant features such as low and high energy cutoffs tell us much about electron acceleration properties and energy losses in the flare evolution. Comparing regularized solutions in each event through different approaches will allow for confirmation of the existence or non-existence of such features. This innovative regularization technique is capable of portraying a truer interpretation of both hard X-ray and mean electron spectra

Unresolved fine-scale structure in solar coronal loop-tops

New and advanced space-based observing facilities continue to lower the resolution limit and detect solar coronal loops in greater detail. We continue to discover even finer sub-structures within coronal loop cross sections, in order to understand the nature of the solar corona. Here, we push this lower limit further to search for the finest coronal loop sub-structures, through taking advantage of the resolving power of the Swedish 1- m Solar Telescope (SST) / CRisp Imaging Spectro-Polarimeter (CRISP), together with co-observations from the Solar Dynamics Observatory (SDO) / Atmospheric Image Assembly (AIA). High resolution imaging of the chromospheric H-alpha 656.28 nm spectral line core and wings can, under certain circumstances, allow one to deduce the topology of the local magnetic environment of the solar atmosphere where its observed. Here, we study post-flare coronal loops, which become filled with evaporated chromosphere that rapidly condenses into chromospheric clumps of plasma (detectable in H-alpha) known as a coronal rain, to investigate their fine-scale structure. We identify, through analysis of three datasets, large-scale catastrophic cooling in coronal loop-tops and the existence of multi-thermal, multi-stranded sub-structures. Many cool strands even extend fully-intact from loop-top to foot-point. We discover that coronal loop fine-scale strands can appear bunched with as many as 8 parallel strands, within an AIA coronal loop cross-section. The strand number density vs cross-sectional width distribution, as detected by CRISP within AIA-defined coronal loops, most-likely peaks at well below 100 km and currently 69% of the sub-structure strands are statistically unresolved in AIA coronal loops.Comment: 10 pages, 10 figures, submitted to Ap

Are giant tornadoes the legs of solar prominences?

Observations in the 171 AA channel of the Atmospheric Imaging Assembly of the space-borne Solar Dynamics Observatory show tornadoes-like features in the atmosphere of the Sun. These giant tornadoes appear as dark, elongated and apparently rotating structures in front of a brighter background. This phenomenon is thought to be produced by rotating magnetic field structures that extend throughout the atmosphere. We characterize giant tornadoes through a statistical analysis of properties like spatial distribution, lifetimes, and sizes. A total number of 201 giant tornadoes are detected in a period of 25 days, suggesting that on average about 30 events are present across the whole Sun at a time close to solar maximum. Most tornadoes appear in groups and seem to form the legs of prominences, thus serving as plasma sources/sinks. Additional Halpha observations with the Swedish 1-m Solar Telescope imply that giant tornadoes rotate as a structure although clearly exhibiting a thread-like structure. We observe tornado groups that grow prior to the eruption of the connected prominence. The rotation of the tornadoes may progressively twist the magnetic structure of the prominence until it becomes unstable and erupts. Finally, we investigate the potential relation of giant tornadoes to other phenomena, which may also be produced by rotating magnetic field structures. A comparison to cyclones, magnetic tornadoes and spicules implies that such events are more abundant and short-lived the smaller they are. This comparison might help to construct a power law for the effective atmospheric heating contribution as function of spatial scale.Comment: 15 pages, 12 figures; ApJ, accepted versio

Beam electrons as a source of Hα flare ribbons

The observations of solar flare onsets show rapid increase of hard and soft X-rays, ultra-violet emission with large Doppler blue shifts associated with plasma upflows, and Hα hydrogen emission with red shifts up to 1–4 Å. Modern radiative hydrodynamic models account well for blue-shifted emission, but struggle to reproduce closely the red-shifted Hα lines. Here we present a joint hydrodynamic and radiative model showing that during the first seconds of beam injection the effects caused by beam electrons can reproduce Hα line profiles with large red-shifts closely matching those observed in a C1.5 flare by the Swedish Solar Telescope. The model also accounts closely for timing and magnitude of upward motion to the corona observed 29 s after the event onset in 171 Å by the Atmospheric Imaging Assembly/Solar Dynamics Observatory

2D and 3D Analysis of a Torus-unstable Quiet-Sun Prominence Eruption

The role of ideal-MHD instabilities in a prominence eruption is explored through 2D and 3D kinematic analysis of an event observed with the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory between 22:06 UT on 2013 February 26 and 04:06 UT on 2013 February 27. A series of 3D radial slits are used to extract height–time profiles ranging from the midpoint of the prominence leading edge to the southeastern footpoint. These height–time profiles are fit with a kinematic model combining linear and nonlinear rise phases, returning the nonlinear onset time (t nl) as a free parameter. A range (1.5–4.0) of temporal power indices (i.e., β in the nonlinear term ${(t-{t}_{\mathrm{nl}})}^{\beta }$) are considered to prevent prescribing any particular form of nonlinear kinematics. The decay index experienced by the leading edge is explored using a radial profile of the transverse magnetic field from a PFSS extrapolation above the prominence region. Critical decay indices are extracted for each slit at their own specific values of height at the nonlinear phase onset (h(t nl)) and filtered to focus on instances resulting from kinematic fits with ${\chi }_{\mathrm{red}}^{2}\lt 2$ (restricting β to 1.9–3.9). Based on this measure of the critical decay index along the prominence structure, we find strong evidence that the torus instability is the mechanism driving this prominence eruption. Defining any single decay index as being "critical" is not that critical because there is no single canonical or critical value of decay index through which all eruptions must succeed

Signatures of the quiet Sun reconnection events in Ca II, Hα and Fe I

We use observations of quiet Sun (QS) regions in the Hα 6563 Å, Ca ii 8542 Å and Fe i 6302 Å lines. We observe brightenings in the wings of the Hα and Ca ii combined with observations of the interacting magnetic concentrations observed in the Stokes signals of Fe i. These brightenings are similar to Ellerman bombs (EBs), i.e. impulsive bursts in the wings of the Balmer lines which leave the line cores unaffected. Such enhancements suggest that these events have similar formation mechanisms to the classical EBs found in active regions, with the reduced intensity enhancements found in the QS regions due to a weaker feeding magnetic flux. The observations also show that the quiet Sun Ellerman bombs (QSEBs) are formed at a higher height in the photosphere than the continuum level. Using simulations, we investigate the formation mechanism associated with the events and suggest that these events are driven by the interaction of magnetic field-lines in the upper photospheric regions. The results of the simulation are in agreement with observations when comparing the light-curves, and in most cases we found that the peak in the Ca ii 8542 Å wing occurred before the peak in Hα wing. Moreover, in some cases, the line profiles observed in Ca ii are asymmetrical with a raised core profile. The source of heating in these events is shown by the MURaM simulations and is suggested to occur 430 km above the photosphere

Explosive events in active region observed by IRIS and SST/CRISP

Transition-region explosive events (EEs) are characterized by non-Gaussian line profiles with enhanced wings at Doppler velocities of 50–150 km s−1. They are believed to be the signature of solar phenomena that are one of the main contributors to coronal heating. The aim of this study is to investigate the link of EEs to dynamic phenomena in the transition region and chromosphere in an active region. We analyse observations simultaneously taken by the Interface Region Imaging Spectrograph (IRIS) in the Si iv 1394 Å line and the slit-jaw (SJ) 1400 Å images, and the Swedish 1-m Solar Telescope in the Hα line. In total 24 events were found. They are associated with small-scale loop brightenings in SJ 1400 Å images. Only four events show a counterpart in the Hα−35 km s−1 and Hα+35 km s−1 images. Two of them represent brightenings in the conjunction region of several loops that are also related to a bright region (granular lane) in the Hα−35 km s−1 and Hα+35 km s−1 images. 16 are general loop brightenings that do not show any discernible response in the Hα images. Six EEs appear as propagating loop brightenings, from which two are associated with dark jet-like features clearly seen in the Hα−35 km s−1 images. We found that chromospheric events with jet-like appearance seen in the wings of the Hα line can trigger EEs in the transition region and in this case the IRIS Si iv 1394 Å line profiles are seeded with absorption components resulting from Fe ii and Ni ii. Our study indicates that EEs occurring in active regions have mostly upper-chromosphere/transition-region origin. We suggest that magnetic reconnection resulting from the braidings of small-scale transition region loops is one of the possible mechanisms of energy release that are responsible for the EEs reported in this paper

First simultaneous SST/CRISP and IRIS observations of a small-scale quiet Sun vortex

Context. Ubiquitous small-scale vortices have recently been found in the lower atmosphere of the quiet Sun in state-of-the-art solar observations and in numerical simulations. Aims. We investigate the characteristics and temporal evolution of a granular-scale vortex and its associated upflows through the photosphere and chromosphere of a quiet Sun internetwork region. Methods. We analyzed high spatial and temporal resolution ground- and spaced-based observations of a quiet Sun region. The observations consist of high-cadence time series of wideband and narrowband images of both Hα 6563 Å and Ca II 8542 Å lines obtained with the CRisp Imaging SpectroPolarimeter (CRISP) instrument at the Swedish 1-m Solar Telescope (SST), as well as ultraviolet imaging and spectral data simultaneously obtained by the Interface Region Imaging Spectrograph (IRIS). Results. A small-scale vortex is observed for the first time simultaneously in Hα, Ca II 8542 Å, and Mg II k lines. During the evolution of the vortex, Hα narrowband images at −0.77 Å and Ca II 8542 Å narrowband images at −0.5 Å, and their corresponding Doppler signal maps, clearly show consecutive high-speed upflow events in the vortex region. These high-speed upflows with a size of 0.5–1 Mm appear in the shape of spiral arms and exhibit two distinctive apparent motions in the plane of sky for a few minutes: (1) a swirling motion with an average speed of 13 km s-1 and (2) an expanding motion at a rate of 4–6 km s-1. Furthermore, the spectral analysis of Mg II k and Mg II subordinate lines in the vortex region indicates an upward velocity of up to ~8 km s-1 along with a higher temperature compared to the nearby quiet Sun chromosphere. Conclusions. The consecutive small-scale vortex events can heat the upper chromosphere by driving continuous high-speed upflows through the lower atmosphere

Observations and 3D Magnetohydrodynamic Modeling of a Confined Helical Jet Launched by a Filament Eruption

We present a detailed analysis of a confined filament eruption and jet associated with a C1.5 class solar flare. Multiwavelength observations from the Global Oscillations Network Group and Solar Dynamics Observatory reveal the filament forming over several days following the emergence and then partial cancellation of a minority polarity spot within a decaying bipolar active region. The emergence is also associated with the formation of a 3D null point separatrix that surrounds the minority polarity. The filament eruption occurs concurrently with brightenings adjacent to and below the filament, suggestive of breakout and flare reconnection, respectively. The erupting filament material becomes partially transferred into a strong outflow jet (∼60 km s−1 ) along coronal loops, becoming guided back toward the surface. Utilizing high-resolution Hα observations from the Swedish Solar Telescope/CRisp Imaging SpectroPolarimeter, we construct velocity maps of the outflows, demonstrating their highly structured but broadly helical nature. We contrast the observations with a 3D magnetohydrodynamic simulation of a breakout jet in a closed-field background and find close qualitative agreement. We conclude that the suggested model provides an intuitive mechanism for transferring twist/helicity in confined filament eruptions, thus validating the applicability of the breakout model not only to jets and coronal mass ejections but also to confined eruptions and flares