Solar flares: The impulsive phase

Only during the previous solar cycle have systematic observations begun to be made with the sensitivity and time resolution, and the continuous coverage required to catch the impulsive phase and measure the rapid variations present in many wavelength ranges. Observations in X-rays, gamma-rays, UV, H, and radio wavelengths all reveal rapid variations during the impulsive phase and have contributed to our understanding of the different phenomena involved. Results have been obtained from several spacecraft, from rocket and balloon flights, and from ground-based observations. These are reviewed in the context of a simple single loop flare model with a view to showing what results are consistent with this model and what the major problems are in our understanding of the impulsive phase. New instrumentation planned for observations during the present Cycle 22 will provide a concerted attack on the impulsive phase as part of the Max '91 program.     

Is the ‘superhot’ hard x-ray component in solar flares consistent with a thermal source?

It has been shown by Brown and Emslie (1988) that any optically thin thermal bremsstrahlung source must emit an energy spectrumL () (keV s^–1 keV^–1) which has the property that higher derivatives alternate in sign, i.e., (–) ^j L(^j)() > 0 for allj. In this short note, we apply this test to the superhot component discussed by Linet al. (1981) in order to determine whether a strictly thermal interpretation of this component is valid. We find that all statistically significant higher derivatives do indeed have the correct sign; this strengthens the identification of this component as due to a thermal source.Presidential Young Investigator.     

Finite-β effects on the radiative thermal instability in magnetized plasmas

The radiative thermal instability is investigated taking into account finite-, or electromagnetic, effects. The two-fluid model for magnetized plasmas together with the Maxwell equations are used to derive a general dispersion relation valid for compressional perturbations with frequency below the electron-cyclotron frequency. The growth rates of the radiative thermal instabilities involving fast magnetosonic flute-like and low-frequency hydromagnetic perturbations are presented.     

Photometry of the bright stars inside and outside associations in the spiral arm S5 of the andromeda galaxy

Photometry of more than 1000 bright stars of 20 associations and some fields in its vicinity is made, usingUBV plates taken with a 2-m Ritchey-Chrétien telescope of the National Observatory at the Bulgarian Academy of Sciences with limited magnitudes of about 20.6 inU andV, 21.6 inB. The colour-magnitudes of about 20.6 inU andV, 21.6 inB. The colour-magnitude and colour-colour diagrams of 12 associations and 8 fields are constructed and the brightest blue and red stars colour excesses, ages, and integrated luminosities of the associations are determined. The composite diagrams for all association and nonassociation measured stars are constructed too. Inside associations bright stars as well as non-stellar objects occur more often than outside. The brightest stars in S5 are withM _v7 and do not differ considerably from the brightest ones in the spiral arm S4 of M31. Approximately the same are the absolute magnitudes of the brightest stars in our Galaxy and M33. A small difference between the brightest stars of the latter galaxy with those of M31 is possible because the stars in M33 are younger. This is in agreement with the concept that Sc galaxies (for example M33) are younger than Sb ones (for example M31). The same could be the reason for the smaller ratio of blue to red supergiants in M33 than in M31. The age estimations of the associations in S5 together with those in S4 in M31 show that they are older than the associations in M33 which is in agreement with the above mentioned concept.     

Hall effects on hydromagnetic convection flow in a rotating fluid

An analysis of Hall effects on the hydromagnetic free convection resulting from the combined effects of thermal and mass diffusion of an electrically-conducting liquid passed an infinite vertical porous plate in a rotating frame of reference is carried out when a strong magnetic field is imposed in a plane which makes an angle with the normal to the plate. The expressions for the mean velocity, mean temperature in the boundary layer, and the mean skin-friction, the mean rate of heat transfer on the plate are derived. The influence of Hall currents on the flow is studied for various values of .     

The center-to-limb variations of four Ca i lines in the photospheric spectrum at λ6500

We study the center-limb (CL) variation of the average profiles of four Ca i lines near 6500 and compare these observations with synthetic data obtained from several line formation models having different thermal structures, line parameters, LTE and non-LTE conditions, and micro and macroturbulence values, to assess the formation characteristics of our Ca i lines in the solar photosphere.Comparison of numerical results with observations indicates that non-LTE is indispensable to fit the CL variation of the central residual intensity for the line 6493, and anisotropic microturbulence is indispensable to improve the CL behavior of the equivalent widths for all lines. The Ca i line analysis favors a cool photospheric model, but this cannot be disentangled clearly from the effects of non-LTE and small-scale velocity fields on the grounds of the present line formation models.     

The latitude of filament bands at the sunspot minimum and the activity level in the two following 11-year solar cycles

The zonal structure of the distribution of filaments is considered. The mean latitudes of two filament bands are calculated in each solar hemisphere at the minima of the sunspot cycle in the period 1924–1986: middle latitude _{2, m} and low latitude _{1, m} . It is shown that the mean latitude of the filament band _{2, m} at the minimum -m of the cycle correlates, with = 0.94, with the maximum - M sunspot area S(M) and maximum Wolf number W(M) in the succeeding solar cycle M. It is shown that the mean latitude of the low-latitude filament band _{1, m} is linearly dependent on the mean latitude filament band _{2, m + 1} at the succeeding minimum. We found a correlation of the latitude of the low-latitude filament band _{1, m} with the maximum sunspot area in the M + 1 cycle. This enables us to predict the power of two succeeding 11-year solar cycles on the basis of the latitude of filament bands at the minimum of activity, 1985–1986: W(22) - 205 ± 10, W(23) - 210 ± 10. The importance of the relationships found for theory and applied aspects is emphasized. An attempt is made to interpret the relationships physically.     

Hydrogen photoionization rates for chromospheric and prominence plasmas

New values of hydrogen photoionization rates for subordinate continua arising from bound levels with the principal quantum number i = 2–5 have been evaluated numerically, using an extensive compilation of the observed photospheric radiation fields. These rates can be directly incorporated into the equations of statistical equilibrium as so-called fixed rates. We tabulate the photoionization rates and equivalent radiation temperatures for various heights above the photosphere, which is particularly useful for chromospheric and prominence non-LTE modeling. Finally, we compare our results with those previously obtained by other authors.     

Hall effects on heat and mass transfer flow with variable suction and heat generation

A study of the combined buoyancy effects of thermal and mass diffusion on MHD convection flow in the presence of Hall currents with variable suction and heat generation has been carried out. Analytical expressions for the velocity and the temperature of the fluid are given. The effects of Hall currents, parameterm, and heat source parameter on the velocity are discussed.