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Smith, D. F.

Publications and source records attributed to Smith, D. F..

30 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1981

Solar radio emission

Active areas of both observational and theoretical research in which rapid progress is being made are discussed. These include: (1) the dynamic spectrum or frequency versus time plot; (2) physical mechanisms in the development of various types of bursts; (3) microwave type 1, 2, 3, and moving type 4 bursts; (4) bursts caused by trapped electrons; (5) physics of type 3bursts; (6) the physics of type 2 bursts and their related shocks; (7) the physics of both stationary and moving traps and associated type 1 and moving type 4 bursts; and (8) the status of the field of solar radio emission.

Goldman, M. V.↗
NASA NTRS1980

Limits on the streaming and escape of electrons in thermal models for solar hard X-ray emission

Upper limits on the number of fast electrons streaming through and escaping from a plasma whose electrons have been heated to approximately 100 billion K and confined by a collisionless ion-acoustic thermal conduction front are determined. It is shown that such a front is fairly transparent to fast electrons with velocities much larger than the thermal velocity because the anisotropic ion-acoustic waves cannot scatter them, making them collisionless on a scale much larger than the thickness of the front. The collisionless analog of the collisional thermoelectric field is derived self-consistently and shown to offer a significant impediment to fast electrons because they must climb over a larger potential barrier than in the collisional case. The only factors limiting the escape of electrons able to surmount this barrier are their rate of production and the requirement that they carry less heat flux than the maximum heat flux allowable. The rate of production is determined for the case of a Maxwellian whose tail is being filled collisionally.

Smith, D. F.↗
NASA NTRS1980

Thermal models for solar hard X-ray bursts

Thermal models for hard X-ray bursts consisting of a one-dimensional flux tube whose central electrons are heated to about 400 million K are examined. It is found that the evolution of a thermal X-ray source is a sensitive function of the electron-ion thermal coupling and the state of the plasma into which the source expands. When this coupling is weak, the heated electrons separate into a region of high temperature of about 400 million K and a region of lower temperature of about 100 million K, a process which leads to a power-law X-ray spectrum. In the case of strong coupling there is only one dominant temperature, about 200 million K, and the X-ray spectrum resembles a true thermal spectrum.

Smith, D. F.↗
NASA NTRS1980

First phase acceleration mechanisms and implications for hard X-ray burst models in solar flares

The paper considers the number of nonthermal electrons which must be accelerated in the impulsive phase of a flare, as well as possible acceleration mechanisms. Direct electric field acceleration in a plasma is considered, taking explicit account of magnetohydrodynamic processes; it is shown that significant heating is involved. Attention is also given to acceleration by electron plasma waves, which requires the waves to be driven to large amplitude by already streaming electrons or anisotropic ion-acoustic waves. It is concluded that at most 0.1% of the flare energy can be deposited into nonthermal streaming electrons with the proposed acceleration and energy conversion mechanisms. Therefore, hard X-ray production above 10 keV primarily by hot thermal electrons is the only choice compatible with existing primary energy release models.

Smith, D. F.↗
NASA NTRS1979

Confinement of hot, hard X-ray producing electrons in solar flares

Possible thermal models for solar, hard X-ray emission, consisting of small volumes in which the electrons are rapidly heated to 400 million K, are examined to determine under what conditions such models can be more efficient than nonthermal models. The analysis is restricted to one dimension by supposing that the hot electrons are confined in a flux tube and that conduction and convection act only along the magnetic field. A conduction front, thin in relation to other length scales involved, separates the hot electrons from the relatively cold plasma. The range of conduction-front velocity is studied by systematizing the present knowledge of heat conduction and applying it first to a simple model in which convection is neglected, and then to a detailed numerical model in which convection is included. Convection in which mass motion goes to zero at the conduction front does not lead to a change in the conduction-front velocity compared to conduction alone, but does constitute another nonradiative energy loss which reduces the source efficiency.

Smith, D. F.↗
NASA NTRS1979

Plasma radiation diagnostics of the primary energy release region in solar flares

The possibility is investigated that the plasma turbulence used in many recent models of the primary energy release and acceleration in solar flares should be detectable by radiation near the fundamental and second harmonic of the plasma frequency. Formulae are derived for fundamental emission due to the combination of ion-acoustic and Langmuir plasma turbulence and for second harmonic emission due to the combination of two Langmuir waves. These results are applied to recent primary energy release and acceleration models which shows that either such radiation should be detectable and possibly distinguishable with suitable microwave interferometers or that its absence places fairly stringent constraints on the possible level of Langmuir or Langmuir and ion-acoustic waves in these models.

Smith, D. F.↗
NASA NTRS1976

Report on the solar physics-plasma physics workshop

The paper summarizes discussions held between solar physicists and plasma physicists on the interface between solar and plasma physics, with emphasis placed on the question of what laboratory experiments, or computer experiments, could be pursued to test proposed mechanisms involved in solar phenomena. Major areas discussed include nonthermal plasma on the sun, spectroscopic data needed in solar plasma diagnostics, types of magnetic field structures in the sun's atmosphere, the possibility of MHD phenomena involved in solar eruptive phenomena, the role of non-MHD instabilities in energy release in solar flares, particle acceleration in solar flares, shock waves in the sun's atmosphere, and mechanisms of radio emission from the sun.

Sturrock, P. A.↗
NASA NTRS1974

Solar Physics - Plasma Physics Workshop

A summary of the proceedings of a conference whose purpose was to explore plasma physics problems which arise in the study of solar physics is provided. Sessions were concerned with specific questions including the following: (1) whether the solar plasma is thermal or non-themal; (2) what spectroscopic data is required; (3) what types of magnetic field structures exist; (4) whether magnetohydrodynamic instabilities occur; (5) whether resistive or non-magnetohydrodynamic instabilities occur; (6) what mechanisms of particle acceleration have been proposed; and (7) what information is available concerning shock waves. Very few questions were answered categorically but, for each question, there was discussion concerning the observational evidence, theoretical analyses, and existing or potential laboratory and numerical experiments.

Baum, P. J.↗
NASA NTRS1973

Type 2 and type 3 burst theory

The present state of the theory of type 3 bursts is reviewed by dividing the problem into the exciting agency, radiation source, and propagation of radiation between the source and the observer. In-situ measurements indicate that the excitors are electron streams of energy about 40 keV which are continuously relaxing. An investigation of neutralization of an electron stream indicates that n sub s is much less than 100,000 n sub e, where n sub s is the stream density and n sub e the coronal electron density. In situ observations are consistent with this result. An analysis of propagation of electrons in the current sheets of coronal streamers shows that such propagation at heights greater than 1 solar radius is impossible. The mechanisms for radiation are reviewed; it is shown that fundamental radiation at high frequencies (approximately 100 MHz) is highly beamed in the radial direction and that near the earth second harmonic radiation must be dominant. Because of beaming of the fundamental at high frequencies, it can often be quite weak near the limb so that the second harmonic is dominant. In considering propagation to the observer, the results of scattering of radiation are discussed. The present state of the theory of type 2 bursts is reviewed in the same manner as type 3 bursts.

Smith, D. F.↗
NASA NTRS1971

Maximum temperatures for radiation from plasma waves.

Upper limits to radiation temperatures from plasma waves for emissions near the fundamental and second harmonic of the electron plasma frequency are derived in terms of effective temperature for plasma waves. Results are obtained that differ from those of Melrose (1970) by a factor that can exceed 40,000 for some plasmas.

Smith, D. F.↗