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Astronomical Institute

Academy of Sciences of the Czech Republic

Plasma Processes in Solar 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
 high  spatial resolution  and  on  subsecond time  scales are
 critical in evaluating the global physical behaviour of these
 phenomena - this  is the current trend in  solar physics. Two
 complementary tools  are used by this  group: (i) optical and
 UV spectral  diagnostics to derive  the basic structural  and
 dynamical plasma  parameters, and (ii)  numerical simulations
 of  plasma processes  and radiation  transport. This  work is
 further  supplemented by  X-ray and  radio observations which
 provide information  on hot plasmas. The  key instrument used
 by this  working group is a  newly reconstructed Multichannel
 Flare Spectrograph  (MFS). MFS allows us  to record the flare
 and  prominence  optical  spectra  simultaneously  in several
 cameras, covering the most important optical lines of various
 chemical  species. These  spectra are  used for  quantitative
 plasma   diagnostics,  which   are performed   by  means   of
 sophisticated  non-LTE  techniques.  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).  This  analysis   is  further
 complemented by using UV-data  from satellites, OSO-8 and SMM
 in the past, and SOHO  launched in 1995. Optical observations
 are    performed   within    collaborative   campaigns   with
 observatories   in  France   (Meudon,  Pic-du-Midi),   Poland
 (Wroclaw),  the U.S.A  and  others.  Non-LTE codes  have been
 developed  in  close  and  long-lasting  cooperation with the
 Institut   d'Astrophysique  Spatiale   in  Orsay.   Numerical
 simulations of  plasma processes are parallel with the  above
 spectral analysis, but they also  predict the X-ray and radio
 emissivity  of  flares.  A  so-called  'hybrid  code',  which
 consists of  two parts, has  been developed: a  simulation of
 accelerated-particle beams and  the hydrodynamical part which
 solves the equations of  1D radiation hydrodynamics. This was
 developed in  collaboration with Observatoire  de Meudon. The
 radiation  part  is  now  being  improved  using fast non-LTE
 techniques based  on accelerate lambda  iterations. Numerical
 simulations of  flare processes extended  into interplanetary
 space, e.g. flare-shock propagation, are also made.

        Recent results:

 In  the framework  of a  coordinated international  campaign,
 a system of post-flare loops was  observed on June 25 and 26,
 1992  by   several  instruments:  MFS,   MSDP  (Pic-du-Midi),
 SXT-Yohkoh, coronagraphs. In  collaboration with Observatoire
 de Meudon, a  complex study of the structure  and dynamics of
 this  loop  system  was  performed.  For  the first time, the
 relationship  between hot  X-ray loops  and cool  H ones was
 investigated in  a great detail. From  a coalignment analysis
 of images we conclude that cool loops are situated just below
 the hot ones and the whole  system grows up with the velocity
 of  about   1  km/sec.  This   is  explained  by   a  gradual
 reconnection process.  For the whole gradual  phase of seveal
 hours, we  have determined the  temperature and the  emission
 measure from  SXT data. About  the same emission  measure was
 also found  in the cool  loops. For the  electron density, we
 found   n(hot) < n(cool) = 2.1010  cm-3.  However, n(hot)  was
 decreasing  during  the  whole  gradual  phase  by almost two
 orders   of  magnitude.   Using  these   densities  and   SXT
 temperatures, we  have computed lower  limits of the  cooling
 time which increases from several minutes at the beginning to
 more  than 2  hours at  the end  of the  gradual phase of the
 flare.  Furthermore, an  evolution of  superthermal electrons
 and protons were  studied in solar flare loops.  It was found
 that an asymmetry of the  proton distribution function at the
 Halphaline  formation  layer  causes  a  polarization  of  this
 chromospheric optical line.  Moreover, superthermal electrons
 propagating  in large  coronal loops  generate U  (N) type of
 radio bursts. The complicated trajectories of these electrons
 were explained by their scattering in zones of whistler waves
 near the loop top. The method for the force-free current path
 calculation were suggested. This method  can also be used for
 a computation of the non-linear force-free magnetic field.