Abstract. We investigate the stability to the development of plasma waves in the preflare situation of a loop structure at the chromospheric part of a current circuit of a loop. We investigate the conditions under which low-frequency plasma instabilities can develop, assuming the absence of beam instabilities.The large-scale quasi-static electric field in the loop circuit is assumed to be "subdreicer" and weak. Thus the percentage of "runaway" electrons is very small and their influence on the process of instability development is negligible. The pair Coulomb collisions are described by a BGK-model integral. We consider the situation when the plasma at the surface layer of a loop has a spatial gradient of density. In accordance with Heyvaerts-Priest-Rust theory, such a preflare situation would typically exist when the amplitude of the weak electric field |E 0 (t)| in the circuit of an "old" loop in an active region begins to increase when "new" magnetic flux emerges from under the photosphere. We have found that two types of waves are generated in such a plasma due to the growth of instabilities: the "kinetic Alfven-like" waves and new type of waves, in the range of magnetoacoustic ones. The instability of these latter waves has a clear threshold and it can be considered as an "indicator" of the development of a preflare situation in an active region.
Plasma-wave instability caused entirely by an external large-scale
electric field
E0 is considered for the pure electron Bernstein
mode. It is assumed for the
angle α between the wave vector k and the normal n to
the direction of the
external magnetic field B0 that the relation
α≈tanα[Lt ]1 is fulfilled. The
quasistatic time variation of the field E0, which
is parallel to B0, provides the
possibility of realizing the direct initiation
(by the large-scale field E0)
mechanism for the development of instability. An expression for the growth
rate is obtained for the case when the external electric
field E0 together with
Landau damping and pair Coulomb collisions are taken into account in a
modified dispersion relation (MDR). This expression is correct in the framework
of linear theory for the frequency ranges typical of electron cyclotron
waves and
Bernstein modes. The influence of the electric field in the MDR is taken
into
account through the method of Brinca and Dysthe. A numerical analysis of
obtained instability is performed for specific values of the plasma parameters.
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