Magnetic Field Models From Energetic Particle Data at Neptune

The locations of features in the Voyager 2 energetic particle data from Neptune are combined with uncertainties in the multipole expansion of the planetary magnetic field to derive new magnetic field models that are consistent both with various interpretations of the particle features and with the magnetic field data. While assumptions as to the origin of the features must be made, they do not provide sufficient constraints to obtain significant new information on any of the unknown multipole coefficients. However, the magnetic L shell positions of the particle features, which are interpreted primarily as absorption signatures of Neptune's satellites, can, in general, be brought into agreement with expected values

Modelling He and H Isotopes in the Radiation Belts

Nuclear interactions between inner zone protons and atoms in the upper atmosphere produce energetic H and He nuclei that are an additional radiation belt source. We calculate production rates of these isotopes from models of the inner zone proton intensity, the upper atmosphere drift averaged composition and densities, and cross-sections for the various interaction processes. For comparison with observations of radiation belt H and He isotopes, the production rates are combined with a model of the energy loss rate in the residual atmosphere to calculate particle intensities. Although the calculations are in principle straightforward, they depend on a detailed knowledge of the various model inputs, including models for radiation belt protons, and may also depend on the phase of the solar cycle. On the other hand, the results of the calculations, when compared with the observational data, can provide useful tests of the model inputs. Preliminary results show that the atmosphere is a significant source for inner zone ^4He, ^3He, and d

Energetic Electrons at Uranus: Bimodal Diffusion in a Satellite Limited Radiation Belt

The Voyager 2 cosmic ray experiment observed intense electron fluxes in the middle magnetosphere of Uranus. High counting rates in several of the solid-state detectors precluded the normal multiple coincidence analysis used for cosmic ray observations, and we have therefore performed laboratory measurements of the single-detector response to electrons. These calibrations allow a deconvolution from the counting rate data of the electron energy spectrum between energies of about 0.7 and 2.5 MeV. We present model fits to the differential intensity spectra from observations between L values of 6 and 15. The spectra are well represented by power laws in kinetic energy with spectral indices between 5 and 7. The phase space density at fixed values of the first two adiabatic invariants generally increases with L, indicative of an external source. However, there are also local minima associated with the satellites Ariel and Umbriel, indicating either a local source or an effective source due to nonconservation of the first two adiabatic invariants. For electrons which mirror at the highest magnetic latitudes, the local minimum associated with Ariel is radially displaced from the minimum L of that satellite by ∼0.5. The latitude variation of the satellite absorption efficiency predicts that if satellite losses are replenished primarily by radial diffusion there should be an increasing pitch angle anisotropy with decreasing L. The uniformity in the observed anisotropy outside the absorption regions then suggests that it is maintained by pitch angle diffusion. The effective source due to pitch angle diffusion is insufficient to cause the phase space density minimum associated with Ariel. Model solutions of the simultaneous radial and pitch angle diffusion equation show that the displacement of the high-latitude Ariel signature is also consistent with a larger effective source. This source may be created by inelastic scattering, leading to diffusion in energy as well as pitch angle

Solar/Interplanetary Energetic Particles in the Outer Heliosphere

The intense solar activity that occurred in the 1988/89 solar maximum period of cycle 22 produced long-lived solar/interplanetary energetic particle events that were observed out to 50 AU. This paper presents a phenomenological view of these events and their relation to energetic particle increases in the previous cycle

Atmosphere losses of radiation belt electrons

A numerical model of the low-altitude energetic electron radiation belt, including the effects of pitch angle diffusion into the atmosphere and azimuthal drift, predicts lifetimes and longitude-dependent loss rates as a function of electron energy and diffusion coefficient. It is constrained by high-altitude (�20,000 km) satellite measurements of the energy spectra and pitch angle distributions and then fit to low-altitude (�600 km) data that are sensitive to the longitude dependence of the electron losses. The fits provide estimates of the parameterized diffusion coefficient. The results show that the simple driftdiffusion model can account for the main features of the low-altitude radiation belt inside the plasmasphere during periods of steady decay. The rate of pitch angle diffusion is usually stronger on the dayside than on the nightside, frequently by a factor �10. The average derived lifetimes for loss into the atmosphere of �10 days are comparable to the observed trapped electron decay rates. Considerable variability in the loss rates is positively correlated with geomagnetic activity. The results are generally consistent with electron scattering by plasmaspheric hiss as the primary mechanism for pitch angle diffusion

Neptune's cosmic ray cutoff

The Voyager 2 cosmic ray experiment observed the signature of a magnetic cutoff of cosmic ray protons immediately after the closest approach to Neptune. The cutoff signature shows that the simultaneous observation of a trapped particle dropout could not have occurred at high magnetic latitudes, but is likely to be a drift shadow of the planet caused by the large offset of the particle drift shells. The OTD2 dipole model of Neptune's magnetic field predicts a drift shadow at approximately the correct times. In addition, the cutoff signature is similar to that predicted by the OTD2 model, although the model is not accurate throughout the observation period. The similarity is consistent with small estimated corrections to the predicted signature based on the locally observed magnetic field values, indicating that the L shell values derived from the model are approximately valid after the closest approach time

Observations of Geomagnetically Trapped Anomalous Cosmic Rays by SAMPEX

he first detailed measurements of a belt of geomagnetically trapped heavy ions that originated as interplanetary anomalous cosmic rays (ACRs) are being made by the polar orbiting satellite SAMPEX. The singly ionized interplanetary ACRs are trapped after losing electrons in the upper atmosphere. Their subsequent lifetime against energy loss by ionization of the atmosphere allows them to reach a substantially higher intensity than in interplanetary space. The ACR composition, which includes only elements with high first ionization potentials, is reflected in the trapped ACRs with some bias due to the trapping mechanism. The elements O, N, and Ne are present, while the lower atomic number elements, He and C, are either absent or substantially depleted relative to their interplanetary abundances. The trapping mechanism also determines the location of the ACR belt, which is confined to a narrow region near L=2, and the pitch‐angle distribution of the trapped ACRs, which is nearly isotropic except for the well‐defined loss cones. The intensities of the trapped and interplanetary ACRs have been measured by SAMPEX since its July, 1992 launch. Both have been steadily increasing with the approach of the minimum of the solar sunspot cycle

A model of the secondary radiation belt

Products of nuclear reactions between primary radiation belt protons and constituents of the tenuous upper atmosphere form a collocated secondary radiation belt. A calculation of the time-dependent secondary intensity provides a model specification of this environmental component for low- and medium-altitude satellite orbits. It is based on an earlier model of the radiation belt protons, the novel feature being a determination of the secondary source function from nuclear reaction cross sections. All long-lived secondary products are included, isotopes of H and He being dominant while the heavier Li to O isotopes are present at relatively low levels. Secondary protons are shown to be a minor correction to the primary radiation belt