Microwave emission from solar flares and comparison with observations

We have studied the evolution of electron energy and angular distributions using Monte Carlo technique for electron beams directed vertically downwards towards chromosphere for incident energies 30 keV, and 300 keV at different incidence angles. Using these distributions we have calculated microwave flux for different frequencies at a fixed column density as well as for a fixed frequency at different column densities. We have also calculated the total microwave flux coming out of solar atmosphere and have compared it with observations. Our results agree well with observational results and produce the observed nature of flux.     

Study of electron energy and angular distributions and calculations of X-ray,EUV line flux and rise times

Evolution of energy and angular distributions of electrons has been studied by combining small-angle analytical treatment with large-angle Monte Carlo calculations as a function of column density for initially monoenergetic and monodirectional electrons. The incident electron energies considered are 20, 30 and 60 keV at 0°, 30° and 60° angles of incidence. Using these distributions, time evolution of extreme ultraviolet (EUV) spectrum has been studied. The slopes of the curves calculated compare well with the experimentally observed curve.     

Energy loss of electrons in solar atmosphere taking many-body interactions into account

Role of many-body interactions on the energy loss of electrons accelerated at neutral point during solar flares has been studied. Energy loss with and without many-body interactions has been computed for different electron-density models as function of height. The energy loss increases by a factor of two by inclusion of many-body interactions for incident electron energies greater than 10 keV. Role of this on the generation of hard X-rays is discussed.     

Study of hard X-ray characteristics of solar flares - support for non-thermal processes

The spatial and angular distributions and also the energy spectrum of hard X-rays from solar flares have been studied in terms of the energy and angular distributions of the accelerated electron beam. The incident electron distributions as functions of column density have been computed by combining the analytical treatment of small-angle scattering with the Monte-Carlo calculations for large angle scattering. To start with monoenergetic electrons at 0°, 30°, and 60° incidence angles have been taken. Using the Bethe-Heitler total cross section and the Sauter differential cross section along with the calculated electron distributions, the bremsstrahlung flux and its angular distribution for different photon energies > 10 keV have been studied as function of column density. The shape of the calculated curves agrees with the observations of PVO/ISEE-3 lending support to the beamed thick-target model for X-ray generation with continuous injection.Physics Department, Vishwa Bharti Institution, Rainawari, Srinagar, India.     

Scattering of fast flare electrons in solar atmosphere and their X-ray spectrum

The evolution of the energy distributions of fast flare electrons injected towards the chromosphere are computed by the Monte Carlo method for different depths. Using these distributions, power law bremsstrahlung spectra having spectral indices increasing with photon energies are obtained.     

The Two-Component Virial Theorem and the Physical Properties of Stellar Systems

Using eigenmode expansion of the Mark III and SFI surveys of cosmological radial velocities, a goodness-of-fit analysis is applied on a mode-by-mode basis. This differential analysis complements the Bayesian maximum likelihood analysis that finds the most probable model given the data. Analyzing the surveys with their corresponding most likely models from the CMB-like family of models, as well as with the currently popular LambdaCDM model, reveals a systematic inconsistency of the data with these "best" models. There is a systematic trend of the cumulative chi(2) to increase with the mode number (where the modes are sorted by decreasing order of the eigenvalues). This corresponds to a decrease of the chi(2) with the variance associated with a mode and hence with its effective scale. It follows that the differential analysis finds that on small (large) scales the global analysis of all the modes "puts" less (more) power than actually required by the data. This observed trend might indicate one of the following: (1) the theoretical model (i.e., power spectrum) or the error model (or both) have an excess of power on large scales, (2) velocity bias, or (3) the velocity data suffers from systematic errors that have not yet been corrected.     

Vertical impulse measurements of mines buried in saturated sand

The ultimate aim of our overall task, of which the effort described in this paper is a part, is to be able to model the impulsive output of buried charges and the response of targets of interest. It is not practical or cost-effective to determine the response of all targets of interest to buried charges of all sizes by testing them. In order to have confidence in our models, however, they must be validated by a modest number of tests. A critical element in modelling the response of a target is the ability to model the loading function. The load a buried charge applies to a target above it when the charge detonates can be characterized in terms of the vertical impulse. The vertical impulse is a function of the size of the charge, its depth of burial, and the properties of the soil in which it is buried. The primary objective of the effort described in this paper is to determine the load a known charge places on a non-responding target so the data can be used to validate our models.

For model validation, a large number of detonator-scale experiments have been conducted by the University of Maryland (Fourney et al. [1]). It was also necessary to conduct a modest number of experiments at a larger scale, nine in total, to ensure that the results of the detonator-scale tests can be satisfactorily scaled up. Of the nine large-scale experiments conducted, seven were conducted with 5 or 10 lb cast TNT charges. All experiments were conducted in sand that was as nearly fully water-saturated as possible. The objective of the experiments was to determine the vertical impulse applied to a non-deforming target plate above the charge.

The large-scale experiments were conducted using the Vertical Impulse Measurement Fixture (VIMF) at the Army Research Laboratory, Aberdeen, MD. The VIMF is a unique facility that has been designed specifically to measure accurately the vertical impulse from buried charges weighing up to 8 kg.

This paper describes the VIMF and its instrumentation, test methods and test results. The results obtained demonstrate that in some cases, when the soil is saturated sand, explosive 'bubble' effects similar to those encountered in shallow water are encountered.     

On the triggering of a spotless double-ribbon flare

We have studied the evolution of the double-ribbon, spotless flare of 21 February, 1992, using Kodaikanal H and Kf1 observations. The analysis of the data shows that the H filament underwent a large change in shear prior to the day of the onset of the flare. We find considerable rotation of the plage region before the emergence of a small magnetic pore. It is concluded that shear plays an important role in the triggering of a spotless flare.     

Magnetic shear in flaring regions

We have evaluated the shear angle of the neutral line of the non-potential magnetic field for one or two days prior to and after the flare event for 10 cases. We have used the H filament positions to evaluate the shear in the neutral line. We find from the samples we have studied that it is the change in the shear that occurs a day prior to the flare that can lead to the event. This change can be in either direction, i.e., it can be a large increase from a small value or a decrease from a large initial value. Thus it is the change in the shear angle that seems to be a deciding criterion for a flare to occur and not a large value for the shear angle itself. We have one instance where there was no significant change in the shear angle over a period of a few days and this region, although similar to other active regions studied, did not produce any flare activity.