Engineering topics

Sturrock, P. A.

Publications and source records attributed to Sturrock, P. A..

At least 91 records · Page 5

NASA NTRS1973

Evaluation of astrophysical hypotheses.

The aim of this article is to set out a bookkeeping procedure for formalizing the process of assessing a hypothesis by comparison of conclusions drawn theoretically from this hypothesis with facts obtained by reduction of observational data. The formalism used is that of probability theory. A key role is played by Bayes's rule representing the inductive process of adjusting a degree of belief in response to new information. Formulae are derived which show (1) how the probability of each hypothesis should be adjusted in response to information concerning one item, and (2) how such estimates concerning more than one item may be combined. The procedure is illustrated by a work sheet showing how a few facts and conclusions concerning pulsars were combined to appraise the neutron-star and white-dwarf hypotheses.

Sturrock, P. A.↗

NASA NTRS1973

Pulsar magnetospheres, braking index, polar caps, and period-pulse-width distribution.

An investigation is conducted concerning the trapping of gas and its effect on the structure of the magnetic field. The aligned and orthogonal cases are discussed in detail. In each case, there are small collection zones which are both microscopically and macroscopically stable. By determining where macroscopic stability changes to instability, the separatrix between closed and open field lines is investigated together with the sizes and shapes of the polar caps. This information makes it possible to reexamine the period-pulse-width distribution and to compare it with observational data.

Roberts, D. H.↗

NASA NTRS1973

Mass motion in solar flares

Mass motions in solar flares are here considered in terms of a previously proposed model. Particle acceleration occurs during reconnection of a current sheet located at coronal heights. The downward component of the particle flux produces an impulsive hard X-ray burst and heats the upper layers of the chromosphere sufficiently to lead to explosive evaporation. Some of the evaporated gas remains trapped in newly closed magnetic field lines and is responsible for the soft thermal component of X-ray emission. Gas which flows along open magnetic field lines subsequently forms a plasmoid which is ejected by magnetic stresses into interplanetary space and may subsequently cause a geomagnetic storm. Analysis of a highly simplified model leads to formulas for the density, temperature, and other parameters of the flare-produced plasma in terms of a length scale and mean magnetic field strength for the flare.

Sturrock, P. A.↗

NASA NTRS1972

Activity in galaxies and quasars.

Activity in galaxies and quasars is interpreted in terms of plasma processes occurring in the magnetosphere of a certain magnetoid model. This magnetoid comprises a core and an annulus rotating about a common axis with different angular velocities. The magnetic field linking the core to the annulus may begin in an initial current-free state but will be distorted along a sequence of force-free configurations. After a finite differential rotation, the force-free configuration has higher energy than a corresponding open-field configuration. It is conjectured that the transition from the closed configuration to an open configuration will be effected by an MHD eruptive instability, and that such eruptions lead to high-velocity clouds of cool gas identified with clouds producing absorption lines in quasars. It is proposed that minor instabilities of the current sheets are responsible for fluctuations in the nonthermal luminosity and for small-scale radio bursts sometimes observed in galaxies and quasars.

Sturrock, P. A.↗

NASA NTRS1972

Force-free magnetic-field structures and their role in solar activity.

Magnetic-field structures in solar active regions are expected to be substantially force-free. A method is proposed for calculating such structures by numerical methods. The method is applied to the study of the magnetic-field pattern associated with a sunspot of one polarity surrounded by a magnetic region of opposite polarity when the sunspot rotates with respect to the surrounding region. Rotation introduces a toroidal component of magnetic field, and the associated pressure leads to inflation of the magnetic field pattern. If the differential rotation exceeds about 180 deg, the force-free magnetic field has energy greater than that of an open magnetic-field configuration with the same photospheric boundary conditions. It is concluded that, beyond this point, the force-free field structure is metastable and can be converted into an open field structure by an explosive MHD instability.

Barnes, C. W.↗

NASA NTRS1972

A classification of magnetic field configurations associated with solar flares.

On the assumption that solar flares are due to instabilities which occur in current sheets in the sun's atmosphere, one may classify magnetic-field configurations associated with flares into two types. One is characterized by 'closed' current sheets, magnetic-field lines adjacent to these sheets beginning and ending at the sun's surface. The other is characterized by 'open' current sheets, magnetic-field lines adjacent to these sheets beginning at the sun's surface but extending out into interplanetary space. Flares associated with open current sheets can produce Type III radio bursts and high-energy-particle events, but flares associated with closed current sheets cannot. The flare of July 6, 1966 apparently consisted of one flare of each type.

Sturrock, P. A.↗

NASA NTRS1972

Magnetic models of solar flares.

A solar flare is interpreted as the explosive release of magnetic energy associated with a current sheet in the solar atmosphere. The release is due to field-line reconnection caused by the tearing-mode instability. Current sheets may be classified into closed and open, depending on whether the adjacent field lines are closed (contained in the sun's atmosphere) or open, extending into interplanetary space. A high-energy event requires an open current sheet. It is shown that certain types of photospheric motion may lead to the formation of current sheets. One of these is associated with high-energy events, another with filaments, and a third with surge flares.

Sturrock, P. A.↗

NASA NTRS1971

Activity in galaxies and quasars

Activity in galaxies and quasars is interpreted in terms of plasma processes occurring in the magnetosphere of a magnetoid model. The magnetoid is comprised of a core and an annulus rotating about a common axis with different angular velocities. It is conjectured that the transition from a closed force field configuration to an open one will be effected by an MHD eruptive instability, and that such eruptions lead to high velocity clouds of cool gas, identified with clouds producing absorption lines in quasars. The ejection of radio clouds from galaxies and quasars is attributed to galactic flares. Current sheets contain mildly relativistic electrons moving in directions partially transverse to the magnetic field. Synchrotron radiation from these electrons is held responsible for the nonthermal radiation from quasars and certain galaxies. It is proposed that minor instabilities of the current sheets are responsible for fluctuations in the nonthermal luminosity and for small scale radio bursts sometimes observed.

Sturrock, P. A.↗

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.↗

NASA NTRS1971