N. S. Meshalkina

On the origin of microwave zebra pattern

The results of the first observations of a zebra pattern at frequencies around 5.6 GHz are presented. The fine structures in the emission spectrum were recorded simultaneously by the Siberian Solar Radiotelescope and the spectropolarimeters of the National Astronomical Observatories, which allowed us to study the presented event with high spatial, temporal and spectral resolution. The apparent source size does not exceed 10 arcsec, and the sources of the different stripes of the zebra structure coincide spatially. The circular polarization degree reaches 100%, and the polarization sense corresponds to the extraordinary wave. We argue that the most probable generation mechanism of the zebra pattern is nonlinear coupling of Bernstein waves. In this case the value of the magnetic field in the burst source, determined by the frequency separation between the adjacent stripes, is 60-80 G.

On the wave mode of subsecond pulses in the cm-range

In this paper we determined the wave mode of subsecond pulses (SSP). We used data on pulses with a degree of polarization over 30%, with the sources located at −60 to +60 deg from the central meridian, for the period 2000–2002. The superposition of SSRT radio maps and MDI magnetograms has shown that radio SSP sources are typically located near the polarity inversion line of the active region magnetic field. Such an arrangement indicates that SSP sources are located at the tops of magnetic loops. The ordinary mode of electromagnetic radiation is recorded in SSP sources located from the inversion line by no less than about 10 arc sec.

The microwave subsecond pulse of September 17, 2001: The spectrum, location and size of the source

We examine simultaneous observations of microwave subsecond pulses with high temporal, spatial and spectral resolution from the Siberian Solar Radio Telescope (5.7 GHz, 14 ms resolution) and from the spectropolarimeters (5.2-7.6 GHz, 6 ms) of the National Astronomical Observatories. The September 17, 2001 flare is discussed in detail. The subsecond pulse (SSP) was observed in the initial phase of the flare, and its HWFH duration was 40 ms. The pulse was accompanied by a rapid change of the dynamic spectrum whose width was about 1%, and the degree of polarization made up 35%. The time profile at the SSRT frequency depends substantially on the SSPs spectral features. The SSP was observed by both the NS (North-South) and EW (East-West) arms (in two interference orders in the latter case). SSP sources locations in burst structures were determined. We also find that the source was not a point-like one, but its apparent size was about the beam width (15 arcsec) for the NS scanning direction. In the EW direction the SSP was less than 10 arcsec in size.

On the origin of microwave type U-bursts

An analysis is made of the observations of U-type cm-bursts recorded simultaneously with high spatial (Siberian solar radio telescope) and spectral resolution (National Astronomical Observatories/Beijing spectropolarimeters). It is shown that the source positions of increasing and decreasing branches of the U-burst coincide within a few arcsec. The suggestion is made that the occurrence of the observed type U-bursts at microwave frequencies is associated with a pressure and density response to rapid heating pulses in flare loops.

Relation between the active region magnetic field and solar flares

A weak active region (NOAA 11158) appeared on the solar disk near the eastern limb. This region increased rapidly and, having reached the magnetic flux higher than 1022 Mx, produced an X-class flare. Only weak field variations at individual points were observed during the flare. An analysis of data with a resolution of 45 s did not indicate any characteristic features in the photospheric field dynamics during the flare. When the flux became higher than 3 × 1022 Mx, active region NOAA 10720 produced six X-class flares. The field remained quiet during these flares. An increase in the magnetic flux above ∼1022 Mx is a necessary, but not sufficient, condition for the appearance of powerful flares. Simple active regions do not produce flares. A flare originates only when the field distribution in an active region is complex and lines of polarity inversion have a complex shape. Singular lines of the magnetic field can exist only above such active regions. The current sheets, in the magnetic field of which the solar flare energy is accumulated, originate in the vicinity of these lines.

Effect of a self-induced electric field on the electron beam kinetics and resulting hard X-ray and microwave emissions in flares

The kinetics of beam electron precipitation from the top of a loop into the solar atmosphere with density gradients and an increasing magnetic field have been generally described. The Fokker-Planck equations are solved with regard to Coulomb collisions and the effect of the electric field induced by this beam. The photon spectra and polarization degree in hard X-ray (10–300 keV) and microwave (1–80 GHz) emissions are simulated under different assumptions regarding the beam electron distribution function. The simulation results are compared with the flare observations on March 10, 2001, and July 23, 2002, visible at different position angles. It has been indicated that the coincidence of the theoretical photon spectra with simultaneous observations of the hard X-ray and microwave emissions of these flares is the best for models that not only take into account collisions, but also the electric field induced by electron fluxes propagating in flare loops with very weakly or moderately converging magnetic fields.

Diagnostics of electron beam properties from the simultaneous hard X-ray and microwave emission in the 10 March 2001 flare

Context. Microwave (MW) and hard X-ray (HXR) data are thought to be powerful means for investigating the mechanisms of particle acceleration and precipitation in solar flares reflecting di!erent aspects of electrons interac tion the ambient particles in a presence of magnetic field. Simultaneous simulation of HXR and MW emission with the same populations of electrons is still a great challenge for interpretation of observations in real events. Recent progress in simulations of particle kinetics with time-dependent Fokker‐Planck (FP) approach o!ers an opportunity to produce such the interpretation. Aims. In this paper we apply the FP kinetic model of precipitation o fe lectron beam with energy range from 12 keV to 1.2 MeV to the interpretation of X-ray and microwave emissions observed in the flare of 10 March 2001. Methods. The theoretical HXR and MW emissions were calculated by usin gt he distribution functions of electron beams found by solving time-dependent Fokker‐Planck approach in a converging magnetic field (Zharkova at al., 2010; Kuznetsov and Zharkova, 2010) for anisotropic scattering o fb eam electrons on the ambient particles in Coloumb collisions and Ohmic losses. Results. The simultaneous observed HXR photon spectra and frequency distribution of MW emission and polarization were fit by those simulated from FP models which include the e! ects of electric field induced by beam electrons and precipitation into a converging magnetic loop. Magnetic field strengths in the footpoints on the photosphere were updated with newly calibrated SOHO/MDI data. The observed HXR energy spectrum above 10 keV is shown to be ad ouble power law which was fi t precisely by the photon HXR spectrum simulated for the model including the selfinduced electric field but without magnetic convergence. Th eM W emission simulated for di!rent models of electron precipitation revealed a better fit(above 90% confidence level) to the observed distribution at higher frequencies for the models combining collisions and electric field e!ects wi th a moderate magnetic field convergence of 2. The MW simulations were able to reproduce closely the main feature so f the MW emission observed at higher frequencies: the spectral index, the frequency of peak intensity and the frequency of the MW polarisation reversal while at lower frequencies the simulated MW intensities are lower than the observed ones.

Dynamics of magnetic fields of active regions in pre-flare states and during solar flares

Solar flares observed in the active regions NOAA 10656, NOAA 11429, and NOAA 10930 are analyzed. The magnetic fluxes were constant to within 2% during these flares, as well as the distribution of the magnetic fields in the active regions. The analysis supports earlier conclusions that large (class X) solar flares arise when the magnetic fluxes of the active regions exceed 1022 Mx. The observation of a high magnetic flux in an active region is not sufficient for the appearance of a large flare: complex ßγδ field structures must also be observed before flares. Such active regions can generate singular lines of the magnetic field in the corona, in whose vicinities current sheets form. Magnetic-field lines above simple dipolar active regions have arched forms; singular lines are absent and no current sheets are created. Dipolar-type active regions do not generate flares. Imbalances in the magnetic flux of an active region and the growth rate of the magnetic flux are not any indications of the imminent appearance of a flare.

Numerical MHD simulations of the appearance of a series of current sheets above the active region AR 0365

The first attempt at numerical MHD simulations of the appearance of several current sheets above an active region before a series of elementary flares is described. Energy accumulates in the field of each sheet that can be released during one of the flares. The computations started three days before the appearance of a series of flares, i.e., before the emergence of a new magnetic flux in the active region. The initial (potential) magnetic field was calculated by solving the Laplace equation with an oblique derivative. The boundary conditions on the photosphere were specified from maps of the measured magnetic field in the active region for various instants of time. The Peresvet program solving the full system of MHD equations with dissipative terms was used in the computations. An absolutely implicit scheme conservative relative to the magnetic flux was used. The problem of properly choosing the size of the computational domain and finding the positions of singular magnetic field lines is discussed.