Astronomical Institute
Academy of Sciences of the Czech Republic
Plasma Processes in Solar Flares and Prominences
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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 HÐ 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 . 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
line 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.