Flares and prominences

The principal goal of this group is to understand the energetics and dynamics of very complex plasma processes in flares and prominences, occurring on various spatial and temporal scales. However, small-scale processes observed with a high spatial resolution and on sub-second time scales are critical in evaluating the global physical behavior of these phenomena, this being the current trend in solar physics. Two complementary tools are used: (i) optical and UV spectral diagnostics to derive the basic structural and dynamical plasma parameters, and (ii) numerical simulations of plasma processes and radiation transfer. This work is further supplemented by X-ray and radio observations which provide information on hot plasmas.

Observations of the Multichannel Flare Spectrograph (MFS) operating since 1958 have been stopped and the device has been reduced into a one-channel spectrograph. A comprehensive archive of flares and prominences simultaneously registered in three spectral lines: H_{α, Hβ or He D3 and CaII 854.2 nm and slit-jaw Hα pictures is still available. List of observations since 29.05.1998 – 14.05.2004 with examples of the data can be found at the MFS homepage  http://www.asu.cas.cz/~pkotrc/index5.html.  A large horizontal telescope with a spectrograph (HSFA2) has been put in a operation after an extensive modernization. It is fully computer-controlled and replaces the MFS. A new diffraction grating enables spectral resolving power 247 000 in the 1st order. Data acquisition system has been changed and the spectrograph was converted from Czerny-Turner into a multichannel one. It works simultaneously in Hα, D3, Hβ, CaII K, (resp. CaII H) and CaII 854.2 nm lines. Five of 6 CCD cameras are placed at these lines and the sixth one in the reconstructed slit-jaw system with H-alpha filter. Auxiliary telescopes with cameras provide information about position of the solar image in white-light and the full disk in H-alpha line. Latest images and other technical details about HSFA2 can be found at http://www.asu.cas.cz/~pkotrc/2006.html. Most of the observations performed with the HSFA2 are concentrated on high temporal resolution of fast processes in solar flares, prominences and other active phenomena. As compared to the MFS, the HSFA2 has a larger spatial and spectral resolution. It takes part in collaborative campaigns with observatories in France (Meudon, Pic-du-Midi), Poland (Wroclaw), the Canary Islands and elsewhere as well with space-born devices.}

Solar radio emission is monitored by three radio telescopes. The 0.8–2.0 GHz radio spectrograph with 512 frequency channels was reconstructed in 2006 and now it is used for measuring dynamic spectra with 10 ms time resolution. The 2.0–4.5 GHz radio spectrograph with 512 frequency channels is used with 100 ms time resolution. The 3.0 GHz single frequency radiometer with 10 ms time resolution is used for monitoring solar radio activity and studying short-duration phenomena. All instruments are fully automatic, monitoring daily the solar activity, from sunrise to sunset. The goal is the study of fast dynamic phenomena, especially fine structures of solar radio bursts. The list of observed events, as well as pictures of observed radio bursts, are available to interested parties at http://www.asu.cas.cz/~radio/. Information about observed events is also regularly sent to Boulder, Colorado, USA, where it appears monthly in "Solar Geophysical Data".

EUV and X-ray bands represent a fundamental source of information on the state of solar plasma and physical processes taking place in the upper layers of the solar atmosphere – the transition region and corona. In these bands, we use top-quality satellite data obtained in broad international cooperation (missions YOHKOH, SOHO, TRACE, RHESSI, HINODE, etc.), as well as data from our own instrument (HXRS) launched in March 2000. The optical and UV spectral data are used for quantitative plasma diagnostics, which are performed by means of sophisticated non-LTE techniques. Non-LTE codes have been developed in close cooperation with the Institut d'Astrophysique Spatiale in Orsay (France) and with the Max-Planck-Institut für Astrophysik in Garching (Germany). Recently, they have been extended to time-dependent and 2D versions. As a result, we obtain information on the thermodynamic structure of the flaring atmosphere or prominence structures, as well as on dynamical processes (velocity fields). Numerical simulations of plasma processes also predict the X-ray and radio emissivity of flares. A so-called 'hybrid code', which consists of two parts, has been further developed: a simulation of accelerated-particle beams and the hydrodynamic part which solves the equations of 1D radiation hydrodynamics. The radiation part is now being calculated using fast non-LTE techniques based on accelerated lambda iterations. Numerical simulations of flare processes extended into interplanetary space, e.g. flare-shock propagation, are also carried out.

Soft X-ray images are used to understand the physics of solar flares (events preceding flares, evolution of hot post-flare loops, etc.) and other active processes in the solar corona (triggering of CMEs, formation of long trans-equatorial loops, etc.). From the diagnostic point of view, the electron temperature and emission measure of hot coronal plasma can be estimated using these measurements. Hard, mostly non-thermal, X-ray emission gives information on high-energy particle beams in the solar corona. Observations of this kind can identify regions of acceleration and thermalization of these beams and could also provide some clue to the still poorly understood physical mechanisms which produce these high-energy particle beams responsible for solar flares.

In 2005 the computer cluster OCAS (Ondrejov Cluster for Astrophysical Simulations, see http://wave.asu.cas.cz/ocas) was built at the Institute consisting of 16 double-processor working nodes (in total 32 AMD Opteron 252@2.6GHz, 64GB RAM) interconnected by the fast InfiniBand network. The cluster is used mainly for the numerical modelling of basic processes in solar flares (magnetic reconnection, plasmoid ejecta) and prominences (relaxation to the MHS equilibria, radiative transfer) using MPI-parallelised MHD and PIC codes (both 2D and 3D). The codes are extended of calculations of the modelled emission in X-rays, radio and Hα providing thus the connection with our observing facilities. In 2007 the OCAS cluster was extended to 80 CPU cores.