Robert L. Carovillano

Axisymmetric Plasmasphere Resonances: Toroidal Mode

The usual magnetohydromagnetic approach is used to investigate axisymmetric perturbations in a plasma permeated by a static dipole field. The model plasmasphere used has stationary, perfectly reflecting boundaries concentric with the dipole, and the plasma density is assumed to be spherically symmetric. The poloidal wave equation is reduced to a hierarchy of coupled differential equations in the radial variable. The toroidal wave equation is solved exactly using a density law which gives a good order of magnitude fit to recent whistler data. Because the extent of the plasma is finite, the eigenperiods are everywhere finite, including the poles. The stretched string model of vibrating lines of force is derived using the Wentzel‐Kramers‐Brillouin approximation, which fortuitously gives the exact periods for the density law utilized. Two unusual results are the prediction of shorter periods in the polar zones than at some high middle latitudes and discontinuous eigenperiods across the line of force which jus...

Average Electron Precipitation in the Polar Cusps, Cleft and Cap

Results are given of the statistical properties of electron precipitation measured at low altitudes by polar orbiting satellites. The morphology of high latitude electron precipitation is determined in the polar cap, the polar cusp, and the polar cleft as a function of magnetic latitude, local time, and activity, Kp. Results show that a polar cusp region highly confined in latitude and local time can be Identified by a minimum in the precipitating electron average energy. This region lies very close to local noon, but is not spatially coincident with the maximum in precipitating electron flux which occurs several hours earlier. Surrounding the cusp is a well-defined region of low energy precipitation whose contours of constant integral flux have crescent shapes centered about the flux maxima. This is best seen in cases of low magnetic activity when boundary plasma sheet electron precipitation is not strong. We refer to this region as the cleft. The average energy of the cleft electrons increases steadily as one moves away from the cusp in local time and in latitude. The polar cap appears to have two states: an active state, when the IMF is northward; and a quiet state, characterized by polar rain precipitation and occurring when the oval is active. Two-dimensional maps of integral flux and average energy for polar rain occurrence show that the basic vatiation in this precipitation is from dayside to nightside. The axis ot symmetry for the variation is pre-noon to pre-midnight with the integral flux (average energy) increasing (decreasing) from day to night. These variations in number flux and average energy exhibit a symmetry that is an extension of that seen in the cleft.

Magnetic Energy Relationships in the Magnetosphere

Using a more physical method of calculating magnetic energy changes, various theorems relating to geomagnetic processes have been reformulated. In particular, we have determined : (a) the energy of confinement of a magnetic dipole field by a perfectly conducting surface (e.g., the steady state containment of the Earth’s field by the solar wind); (b) the energy of transient compression of a shielded dipole field by a diamagnetic medium (such as the sudden commencement of a magnetic storm); and (c) the zero order energy of trapped particles in an undistorted dipole field (e.g., the energy of the radiation belts during quiet times or the main phase in a weak magnetic storm). In all three instances the magnetic and kinetic energies are directly proportional to M·b(0), where b(0) is the total perturbation field at the position of the dipole moment M due to sources exterior to the shielding volume (if any). A new result shows that (c) follows directly from a more general theorem which phenomenologically includes the non-linear effects of particle interactions within the belt. A simple model is developed which demonstrates that the zero order theorem (c) leads to an overestimate of the kinetic energy of the particle distribution when the non-linear magnetic distortions of the particles are important.

Poloidal Hydromagnetic Plasmaspheric Resonances

Under conditions of axisymmetry, the vector hydromagnetic wave equation of an infinitely conducting, stationary plasma permeated by a magnetic dipole field decouples into independent toroidal and poloidal modes. A general treatment is provided of the poloidal mode for a wide variety of radial and latitude‐dependent plasma densities. A harmonic decomposition of the poloidal wave equation is made, resulting in radial equations which are coupled except for the special case where the Alfven speed is latitude independent. Exact solutions are obtained in the latter case, and these are used to provide a general solution to the coupled equations for the general case. The method is applied to a model plasma confined between spherical surfaces concentric with the dipole. Resonance periods are calculated for the separable case with various positions of the outer plasma boundary. Periods are also obtained for a spherically symmetric density, and an evaluation of earlier approximate studies of this case is made.

Latitude‐Dependent Plasmasphere Oscillations

For a perfectly conducting plasma magnetized by a dipole field, the magnetohydrodynamic toroidal mode describes waves which propagate energy along the field lines. This characteristic causes the resonance oscillations of this mode to be strongly latitude‐dependent. The periods of these oscillations are determined for a generalized plasma density of the form ρ = ρ0(a/r) s D(φ) and compared with previously obtained results for the poloidal or isotropic mode. A discussion is provided of the validity of the model plasmasphere and the conditions for experimental observation.

Optical giant resonances in nuclear reactions

Abstract In this paper we discuss the physical reasons for optical giant resonances being a very general feature of nuclear reactions. Optical giant resonances, so familiar in the scattering of nucleons on nuclei, have also been observed in elastic cross sections of composite particles scattered by nuclei and in certain reaction channels of a compound nucleus decay. That one should expect this optical giant resonance structure to occur in such a variety of nuclear reactions is mainly attributed, in our arguments, to the effect of the indistinguishability of the nucleons on the interactions between two proximate nuclear clusters. Because of the indistinguishability of the nucleons, the lifetime of the nuclear clusters in a compound nuclear state can often be prolonged to times, of order 10 −20 to 10 −22 sec, of sufficient duration compared to the transit times to account for the observed resonances. The relationship of the giant resonance structure and the Coulomb barrier is also discussed.

Hydromagnetic Eigenmodes in Multipole Fields

A formalism is developed providing an exact analytical procedure for determining linearized hydromagnetic wave phenomena in a perfectly conducting plasma permeated by a static magnetic multipole field. All perturbations are harmonically represented by scalar and vector spherical harmonics. The vector wave equation for the perturbed electric field is then reduced to a set of coupled equations in the radial variable alone. Different harmonic modes are coupled because the vector wave equation is not simply separable in spherical coordinates (or any other known system of coordinates). The physical significance of operative couplings derives from parity and angular momentum conservation requirements. It is shown that the radial equations are generally soluble in principle.

Robert Eather receives first Athelstan Spilhaus award

Robert H. Eather received the Athelstan Spilhaus Award at the Joint Assembly Honors Ceremony, which was held on 25 May 2006 in Baltimore, Md. The award honors individuals who have devoted portions of their lives to expressing the excitement, significance, and beauty of the Earth and space sciences to the general public.

Correlations of Auroral Kilometric Radiation with Visual Auroras and with Birkeland Currents

The first satellite measurements of intense radio emissions from the earth’s magnetosphere were made seventeen years ago by Benediktov et al. (1965, 1968). They observed 725 kHz and 2.3 MHz radio emissions with the Elektron satellite receivers during geomagnetic storms. Five years later Dunkel et al. (1970) reported OGO 1 observations of strong radio wave intensities at frequencies below 100 kHz also related to magnetic disturbances. Brown (1973), using the Goddard Space Flight Center (GSFC) radio astronomy experiment on the IMP 6 satellite, observed earth related radio emissions in the frequency range 150–300 kHz. It was not, however, until the University of Iowa plasma wave experiments, on the IMP 6 and IMP 8 satellites, had measured intense electromagnetic radiation (in the frequency range of 50–500 kHz) that the first complete observational study of terrestrial kilometric radiation was produced. Gurnett (1974) was able to present a comprehensive documentation of the location of the radio source near the earth because his instruments afforded him directional resolution, dynamic range, high sensitivity, and broad frequency range. Other authors contributing to the completion of the picture of radio emissions from the earth’s magnetosphere were Stone (1973) and Frankel (1973).