Numerical simulations of coronal particle trapping

In this paper the trapping of high energy particles in solar coronal loops is addressed. Using simulations, the time evolution of electrons and protons trapped in a magnetic bottle is calculated under various scattering conditions and the results compared with loss-cone analysis. Thereafter the case of time-dependent injection into a magnetic loop is addressed, and the results compared with previous analytic work on X and γ-ray delay times

Generation of solar Hα impact polarization by fragmented evaporative upflows

In this paper a novel mechanism is proposed for the generation of Hα impact polarization observed during some solar flares. Rather than being generated by the primary particle beams transporting energy from the chromosphere to the corona, we suggest that following heating, the solar chromosphere evaporates in a fragmented manner, and that impact excitations in the regions of interaction of hot evaporating and cool non-evaporating material locally generates impact-polarized Hα emission. This thermal upflow model is more consistent with the large areas and times over which polarization is observed than are beam models. A simple model for the process is given, and the resulting polarization is calculated and compared with observations, under two assumptions about the number density of neutral particles in the interaction regions

Physical Properties of White-Light Sources in the 2011 Feb 15 Solar Flare

White light flares (WLFs) are observational rarities, making them understudied events. However, optical emission is a significant contribution 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 white-light emission from the X2.2 flare SOL2011-02-15T01:56:00. We develop a technique to robustly identify enhanced flare pixels and, using a knowledge of the RGB filter transmissions, determined the source color temperature and effective temperature. We investigated two idealized models of WL emission - an optically thick photospheric source, and an optically thin chromospheric slab. Under the optically thick assumption, the color temperature and effective temperature of flare sources in sunspot umbra and penumbra were determined as a function of time and position. Values in the range of 5000-6000K were found, corresponding to a blackbody temperature increase of a few hundred kelvin. The power emitted in the optical was estimated at $\sim 10^{26}$ergs s$^{-1}$. In some of the white-light sources the color and blackbody temperatures are the same within uncertainties, consistent with a blackbody emitter. In other regions this is not the case, suggesting 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; it requires temperatures in the range 5,500 - 25,000K, and total energies of $\sim 10^{27}$ergs s$^{-1}$.Comment: Accepted for publication in the Astrophysical Journal, 15 pages, 15 figure

The Impulsive Phase in Solar Flares: Recent Multi-wavelength Results and their Implications for Microwave Modeling and Observations

This short paper reviews several recent key observations of the processes occurring in the lower atmosphere (chromosphere and photosphere) during flares. These are: evidence for compact and fragmentary structure in the flare chromosphere, the conditions in optical flare footpoints, step-like variations in the magnetic field during the flare impulsive phase, and hot, dense 'chromospheric' footpoints. The implications of these observations for microwaves are also discussed.Comment: 6 pages, 5 figures, presented at 'Solar Physics with Radio Observations' Symposium, November 2012, Nagoya, Japa

Cycle 23 Variation in Solar Flare Productivity

The NOAA listings of solar flares in cycles 21-24, including the GOES soft X-ray magnitudes, enable a simple determination of the number of flares each flaring active region produces over its lifetime. We have studied this measure of flare productivity over the interval 1975-2012. The annual averages of flare productivity remained approximately constant during cycles 21 and 22, at about two reported M or X flares per region, but then increased significantly in the declining phase of cycle 23 (the years 2004-2005). We have confirmed this by using the independent RHESSI flare catalog to check the NOAA events listings where possible. We note that this measure of solar activity does not correlate with the solar cycle. The anomalous peak in flare productivity immediately preceded the long solar minimum between cycles 23 and 24

Optical Spectral Observations of a Flickering White-Light Kernel in a C1 Solar Flare

We analyze optical spectra of a two-ribbon, long duration C1.1 flare that occurred on 18 Aug 2011 within AR 11271 (SOL2011-08-18T15:15). The impulsive phase of the flare was observed with a comprehensive set of space-borne and ground-based instruments, which provide a range of unique diagnostics of the lower flaring atmosphere. Here we report the detection of enhanced continuum emission, observed in low-resolution spectra from 3600 \AA\ to 4550 \AA\ acquired with the Horizontal Spectrograph at the Dunn Solar Telescope. A small, $\le$0''.5 ($10^{15}$ cm$^2$) penumbral/umbral kernel brightens repeatedly in the optical continuum and chromospheric emission lines, similar to the temporal characteristics of the hard X-ray variation as detected by the Gamma-ray Burst Monitor (GBM) on the Fermi spacecraft. Radiative-hydrodynamic flare models that employ a nonthermal electron beam energy flux high enough to produce the optical contrast in our flare spectra would predict a large Balmer jump in emission, indicative of hydrogen recombination radiation from the upper flare chromosphere. However, we find no evidence of such a Balmer jump in the bluemost spectral region of the continuum excess. Just redward of the expected Balmer jump, we find evidence of a "blue continuum bump" in the excess emission which may be indicative of the merging of the higher order Balmer lines. The large number of observational constraints provides a springboard for modeling the blue/optical emission for this particular flare with radiative-hydrodynamic codes, which are necessary to understand the opacity effects for the continuum and emission line radiation at these wavelengths.Comment: 54 pages, 13 figures, accepted for publication in the Astrophysical Journa