10:30 - 11:00
The Flaring Lower Solar Atmosphere
Hugh Hudson (UC Berkeley and U. of Glasgow)
The interface region between the corona and photosphere dissipates most of the energy of a solar flare, and probably contains much of the important physics of the process. This flare dissipation occurs in a region with rapidly-changing physical parameters (beta, opacity, temperature, collisionality) and probably rich plasma physics. Thanks to new observations - both from space, and from ground-based observatories - we are now getting a much more complete picture of this region (the classical upper photosphere, chromosphere, and transition region). I describe recent observations (and inferences from them) about physical conditions and theories, and try to answer the question, Would Carrington be surprised at what we see?
11:00 - 11:30
White-light flares, sun-quakes and magnetic transients - kith or kin?
Sarah Matthews (UCL Mullard Space Science Lab.)
More than 150 years have now passed since Carrington and Hodgson made the first observations of a white-light flare. Over the course of those 150 years we have learnt much, especially with the advent of space-based observations in this wavelength range. However, these intriguing events still represent some of the most challenging observations to explain in the context of existing solar flare models, indicating there is still much to learn about how energy release in the corona then impacts the lower atmosphere, and indeed the solar interior. In this presentation I will review our current understanding of white-light flares, their relationship to other lower atmospheric phenomena such as photospheric magnetic transients and flare-related acoustic emission, and what the related implications for flare energy transport might be.
11:30 - 11:45
Physical Properties of Impulsive White-Light Sources in the 2011 Feb 15 Solar Flare
Graham Kerr (University of Glasgow)
White light flares (WLFs) are observational rarities, making them understudied events. However, WLFs are significant contributors to flare energy budgets and the emission mechanisms responsible could have important implications for flare models. Using Hinode SOT optical continuum data taken in broadband red, green and blue filters, we investigate impulsive white light emission from the 15 February 2011 X2.2 solar flare. We develop a technique to robustly identify enhanced flare pixels and, using a knowledge of the RGB filter transmissions, investigated two idealized models of WL emission - an optically thick photospheric source, and an optically thin slab. Under the optically thick assumption, values of the color temperature and brightness temperature of the flaring pixels were determined as a function of time and position, finding values in the range of 5000-6000K, corresponding to a temperature increase of a few hundred kelvins. The impulsive power emitted as white-light in each SOT continuum channel is in excess of 10^23 erg/s. Using measured effective temperatures (under the blackbody assumption), the total energy emitted over the observational window was ~10^(28-29) ergs. In some of the white-light sources the color temperatures and the blackbody temperature obtained are the same within the error bars, consistent with a blackbody emitter. In other regions this is not the case, indicating that some other continuum emission process is contributing. An optically thin slab model producing hydrogen recombination radiation is also discussed as a potential source of WL emission, with the result that due to density and energy constraints, the optically thick photospheric model was more favourable.
11:45 - 12:00
Chromospheric Hard X-ray Response to Nonthermal Electrons
Aidan O'Flannagain (Trinity College, Dublin)
Solar flare hard X-rays (HXR) are thought to be produced by nonthermally accelerated electrons braking in the dense chromosphere, with strong evidence that the release of energy required occurring high in the corona. However, the mechanism by which this energy is transferred to the chromosphere, and where acceleration occurs, remains unclear. Many competing models have been put forward to explain this missing link in flare initiation, such as beamed electrons and the propagation of torsional Alfven waves. In this presentation, an ongoing study is outlined, in which one such model known as the Collisional Thick Target Model (CTTM) is tested by searching for rarely-seen features in solar flares, as observed by RHESSI. Geometries of HXR emission are compared with CTTM predictions in order to verify whether this model can adequately explain how the distribution of HXR emission should appear during the observed phase of the flare.
12:00 - 12:15
An emission measure distribution analysis of impulsive phase flare footpoints
David Graham (University of Glasgow)
Diagnostics of the location and nature of flare heating in the solar atmosphere are imperative to understanding the heating mechanism. We present emission measure distributions (EMD) of the footpoints of six solar flares during the impulsive phase. The EMDs are obtained from Hinode/EIS observations in conjunction with a regularized inversion method. We find evidence for 8 MK plasma located in the footpoints, and an EMD gradient of EM(T) ~ T that is consistent with a flare atmosphere heated via conduction from a hot upper layer. Furthermore, using the same analysis on SDO/EVE data, hints of very high temperature plasma present early on in the impulsive phase are found. We also show preliminary results from a forward modelling method combining density sensitive line intensities from EIS, and the EMD, to determine the temperature and density profile with height, T(h), n(h), of the flare atmosphere.
12:15 - 12:30
Solar Flare Differential Emission Measures using SDO/EVE
Michael Kennedy (Queen's University Belfast)
The differential emission measure describes the amount of emitting material as a function of temperature and by comparing with theoretical flare models, it offers the potential to diagnose flare heating mechanisms. Using the Markov-Chain Monte Carlo method and observations from SDO/EVE we can construct the DEM over a temperature range of 1 - 20 MK every 10 seconds. We apply the technique to a number of X-class flares and investigate chromospheric heating during the impulsive phase.
14:15 - 14:30
Properties of a solar flare kernel observed by Hinode and SDO
Peter Young (George Mason University)
Flare kernels are compact features located in the solar chromosphere that are the sites of rapid heating and plasma upflow during the rise phase of flares. An example is presented from a M1.1 class flare observed on 2011 February 16 07:44 UT for which the location of the upflow region seen by EIS can be precisely aligned to high spatial resolution images obtained by the Atmospheric Imaging Assembly (AIA) and Heliospheric Magnetic Imager (HMI) on board the Solar Dynamics Observatory (SDO). A string of bright flare kernels is found to be aligned with a ridge of strong magnetic field, and one kernel site is highlighted for which an upflow speed of 400 km/s is measured in lines formed at 10-30 MK. The line-of-sight magnetic field strength at this location is 1000 G. Emission over a continuous range of temperatures down to the chromosphere is found, and the kernels have a similar morphology at all temperatures and are spatially coincident with sizes at the resolution limit of the AIA instrument (< 400 km). For temperatures of 0.3-3.0 MK the EIS emission lines show multiple velocity components, with the dominant component becoming more blue-shifted with temperature from a redshift of 35 km/s at 0.3 MK to a blueshift of 60 km/s at 3.0 MK. Emission lines from 1.5-3.0 MK show a weak redshifted component at around 60-70 km/s implying multi-directional flows at the kernel site. Significant non-thermal broadening corresponding to velocities of 120 km/s is found at 10-30MK, and the electron density in the kernel, measured at 2 MK, is 3.4 x 10^10 cm-3. Finally, the Fe XXIV 192.03/255.11 ratio suggests that the EIS calibration has changed since launch, with the long wavelength channel less sensitive than the short wavelength channel by around a factor two.
14:30 - 14:45
Spectral diagnostics of solar flares
Giulio Del Zanna (University of Cambridge)
The Hinode/EIS and SDO/EVE EUV spectrometers have provided a wealth of new and important information on the physics of flares, at unprecedented spatial and temporal resolution. Some results are highlighted, focusing on the ability of the EVE instrument to measure electron temperatures and elemental abundances from line ratios. Good agreement between the atomic data we provide and the EVE observations is found, if the EVE version 3 calibration is used and all the line blending is taken into account. The temperatures measured with the EVE lines are often near-isothermal and much lower than those obtained with the broad-band X-rays (GOES), especially for large flares. Argon and calcium abundances relative to iron are found photospheric. The chromospheric evaporation/impulsive phase is briefly discussed showing new observational characteristics.
14:45 - 15:00
Flare Energy Transport and Heating by Alfvénic Waves
Alexander Russell (University of Glasgow)
How do magnetohydrodynamic waves travel from the fully ionized corona, into and through the underlying partially ionized chromosphere, and what are the consequences for solar flares? I will show two fluid simulations of Alfvén waves in a one-dimensional, two-fluid (plasma and neutrals) model of the solar atmosphere with realistic density and temperature structure. Energy transmission from the corona to the chromosphere can exceed 20% of incident energy for wave periods of a few seconds or shorter. Waves subsequently heat the chromosphere, mostly through ion-neutral friction, with the majority of thermal energy depoited at the temperature minimum region. Wave damping is strongest for short period waves: for periods of 1 s or less, a substantial fraction (37%–100%) of wave energy entering the chromosphere is damped, however, waves with periods 10 s or longer pass into the interior with relatively little damping. We therefore conclude that Alfvénic waves with periods of a few seconds or less are capable of heating the chromosphere during solar flares, and speculate that they could also contribute to electron acceleration or exciting sunquakes.
15:00 - 15:15
Particle acceleration in reconnecting twisted coronal loops: the effect of magnetic field convergence
Mykola Gordovskyy (University of Manchester)
We study magnetic reconnection and particle acceleration in kink-unstable twisted coronal loops embedded into a gravitationally stratified atmosphere. the appearance of strong electric field with very fragmented structure results in particle acceleration. Based on the 3D time-dependent MHD model, proton and electron motion is considered, taking into account Coulomb collisions. In particular, we focus on the lower corona and chromosphere, where particles can be accelerated by currents appearing due to strong magnetic field convergence.
Based on the developed theoretical model we calculate bremsstrahlung emission intensities and determine the spatial structure of hard X-ray sources.
15:15 - 15:30
Hydrodynamic shocks in the lower atmosphere formed by electron, proton and mixed beams
Valentina Zharkova (University of Bradford, Department of Mathematics)
In this paper we will present the simulation results of hydrodynamic responses caused by injection of different agents (proton beams, electron beams, mixed beams, neutral jets) and explore their effects on the lower atmosphere versus those in the corona. We consider full radiative losses by the hydrogen plasma, present their depth variations for different times and times and compare with the hydrogen line and continuum emission at various depths. Based on the hydrodynamic simulations, we explore depths in the solar interior to which the shocks can propagate for each type of agents and discuss possible effects these shocks can impose on the associated seismic responses.