Carl E. McIlwain

Substorm Injection Boundaries

An improved magnetospheric electric field model is used to compute the initial locations of particles injected by several substorms. Trajectories are traced from the time of their encounter with the ATS-5 satellite backwards to the onset time given by ground-based magnetometers. A spiral shaped inner boundary of injection is found which is quite similar to that found by a statistical analysis. This injection boundary is shown to move in an energy dependent fashion which can explain the soft energy spectra observed at the inner edge of the electrons plasma sheet.

A Kp dependent equatorial electric field model

Abstract A new model of the earths electric field in the equatorial plane has been constructed. While many models have been proposed, most of these have had serious shortcomings. They either have only a few parameters and cannot hope to match even large scale variations in the real field, or they are too complex to be suitable starting points for constructing time dependent three dimensional models which include inductive fields. Also, few previous models have a continuous functional dependence upon magnetic disturbance levels. The new model uses only a dozen parameters and yet fits many of the known dependences upon radial distance, local time, and magnetic disturbance level. One of the roles of electric field models is to predict and understand the motion of charged particles. Models like this one which are based upon a time dependent scalar potential can greatly help our understanding of the motion of equatorial particles during the time periods between substorms.

ATS-6 UCSD Auroral Particles Experiment

The University of California at San Diego (UCSD) Auroral Particles Experiment on the Applications Technology Satellite-6 (ATS-6) consists of five electrostatic charged particle detectors. The features which contribute to the uniqueness of the UCSD data include a rotation capability which often allows sampling very near the direction of the magnetic field, an energy range of five orders of magnitude with a lower extreme of less than 1 eV, and a very large geometric factor which results both from a postenergy analysis electrostatic lens and from the unique ovoidal shape of the analyzing plates. A preliminary look at a subset of UCSD magnetospheric data emphasizes those phenomena which are observed as a result of the new features described. These phenomena include intense magnetic field aligned auroral particles, a persistent and very low energy dusk region enhancement, and low energy 1-10-s fluctuations tentatively identified as Alfven waves.

Trapped radiation belts of Saturn: first look

Pioneer 11 has made the first exploration of the magnetosphere and trapped radiation belts of Saturn. Saturns magnetosphere is intermediate in size between Earths and Jupiters, with trapped particle intensities comparable to Earths. The outer region of Saturns magnetosphere contains lower energy radiation and is variable with time; the inner region contains higher energy particles. The pitch angle distributions show a remarkable variety of field-aligned and locally mirroring configurations. The moons and especially the rings of Saturn are effective absorbers of trapped particles; underneath the rings, the trapped radiation is completely absorbed. We confirm the discovery of a new ring, called the F ring, a new division, the Pioneer division, and a moon, called 1979 S 2. The latter has probably been seen from Earth. There may be evidence for more bodies like 1979 S 2, but at this stage the interpretation of the data is ambiguous. Using particle diffusion rates, we estimate that the cross-sectional area of the F ring is > 7 x 1013 square centimeters and that the opacity is > 10–5. Cosmic-ray albedo neutron decay should be looked into as a source of energetic particles in the inner magnetosphere of Saturn.

Radiation Belts of Jupiter: A Second Look

The outbound leg of the Pioneer 11 Jupiter flyby explored a region farther from the equator than that traversed by Pioneer 10, and the new data require modification or augmentation of the magnetodisk model based on the Pioneer 10 flyby. The inner moons of Jupiter are sinks of energetic particles and sometimes sources. A large spike of particles was found near lo. Multiple peaks occurred in the particle fluxes near closest approach to the planet; this structure may be accounted for by a complex magnetic field configuration. The decrease in proton flux observed near minimum altitude on the Pioneer 10 flyby appears attributable to particle absorption by Amalthea.

A digital multichannel photometer

A semiconductor diode image tube system has been built which has the characteristics of a good photomultiplier tube but which can simultaneously measure light intensity at 38 different positions. Tube dark current is 0.02 count/sec/position. A tube with up to 1000 elements appears feasible.

Radiation Belts of Jupiter

Pioneer 10 counted relativistic electrons throughout the magnetosphere of Jupiter, with the greatest fluxes being inside 20 Jupiter radii. The peak flux of electrons with energy greater than 50 million electron volts was 1.3 x 107 per square centimeter per second at the innermost penetration of the radiation belts.

Magnetic field‐aligned electrodynamics of Alfvén/ion cyclotron waves

We report on the first experimental determination of the magnetic field-aligned electric field (E∥) of an Alfven/ion cyclotron (A/I) wave. The wave was observed propagating in the geostationary regions of the Earths magnetosphere with a frequency of 1/3 the proton cyclotron frequency. Electron oscillations parallel to (B) are used in the determination of E∥. E∥ is found to be a factor of ∼18 greater than anticipated from cold plasma theory (i.e., electron inertial effects). The inclusion of electron pressure and pressure anisotropy brings reasonable agreement between the observations and linear wave theory.