On 24 March 1991 instrumentation aboard CRRES observed the almost instantaneous injection of electrons and protons with energies above 15 MeV into the L‐region 2 < L < 3. The energy spectrum of the injected electrons, a power law (E−6) peaked at 15 MeV and continued to at least 50 MeV

A study of the major changes in the solar wind during the Pioneer 10 and 11 encounters and their influence on the size of the Jovian magnetosphere is reported. Simultaneous sets of encounter data acquired by the Jet Propulsion Laboratory vector helium magnetometer, the Ames Research Center plasma analyzer, and the University of California at San Diego trapped radiation detector have been compared with data acquired simultaneously in interplanetary space by the sister spacecraft. Of particular interest were four intervals during which it appeared that the spacecraft had reentered the magnetosheath near 50 Rj after having first entered the magnetosphere near 100 Rj. The principal outcome of the study is that in three of these cases the reentries into the magnetosheath occurred when high-speed solar wind streams and their associated interaction regions were expected to arrive at Jupiter. Thus the study supports the hypothesis advanced previously that the Jovian magnetosphere had undergone a large-scale compression. The results are contrary to an alternative hypothesis that the Pioneers had traversed a spatial region located inside the magnetosphere possibly associated with plasma outflow. The fourth case, which was observed by Pioneer 11 outbound, appears to have occurred during quiet interplanetary conditions. However, a detailed reinvestigation of magnetic field and plasma data during this interval shows that the spacecraft had reentered the magnetosheath and not a region interior to the magnetosphere. The reentry into the magnetosheath and the subsequent return to the magnetosphere were separated by an interval of 10 hours and would have been expected to occur when the spacecraft was at its highest magnetic latitude. It is concluded, tentatively, that this reentry was the result of a large-scale north-south motion intrinsic to the Jovian magnetosphere. The question of whether or not the magnetic field just inside the magnetopause is sufficiently strong to withstand the pressure of the incident solar wind has been reexamined within the context of this present study. The field appears able to hold off the solar wind both at 100 R• and near 50 R•. The compressibility of the Jovian magnetosphere is enhanced because the field inside the magnetopause is not the planetary field but is principally caused by currents inside the magnetosphere, presumably the equatorial current sheet. The possible acceleration of energetic trapped radiation when the magnetosphere was compressed has been investigated. Comparison of the increased particle fluxes and the magnetic field shows that gyrobetatron acceleration can be discounted. Based on the measured time difference between the particle enhancement and the arrival of the magnetopause at the spacecraft, an estimate is derived for the average plasma density inside the magnetosphere of 1-10 cm -8 Finally, the characteristic time constants appropriate to an electric circuit model of Jupiter's magnetosphere have been estimated as being in the range between 15 and 50 hours. 1973].The que...

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 10(7) per square centimeter per second at the innermost penetration of the radiation belts.

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.

The University of California at San Diego trapped radiation detector measured proton and electron fluxes, angular distributions, and energy spectra throughout the Pioneer 10 flyby of Jupiter last December.Here the instrumentation and calibrations are described, and good values for particle fluxes in the inner and outer regions are presented. The major features of the Jovian radiation belts are described, with preliminary discussions of their meanings.The existence of Van Allen radiation belts at Jupiter has been known for almost as long as the existence of Van Allen radiation belts at earth [Van Allen, 1958; Drake and Hvatum, 1959]. However, whereas rockets, satellites, and spacecraft have had access to the earth's radiation belts for the last 15 years, it was only last December that a spacecraft, Pioneer 10, first probed the immediate vicinity of Jupiter [Hall, 1974]. On board this spacecraft were several instruments designed to make direct measurements of the Van Allen belts. Preliminary notes on experiments were written 2 weeks after encounter and appeared together in the January 25, 1974, issue of Science [Van Allen et at., 1974; Simpson et at., 1974; Trainor et at., 1974; Fittius and Mcltwain, 1974]. The present paper gives a more thorough account of results from one of these experiments, the University of California at San Diego (UCSD) trapped radiation detector. INSTRUMENTATIONConstraints and objectives. It is obvious that one hyperbolic cut through Jupiter's magnetosphere cannot bring us the same level of knowledge that we enjoy of the earth's environment. Also, the instruments on Pioneer 10 cannot approach the sophistication of those flown today in the neighborhood of earth. The constraints on weight, power, and telemetry forced too much simplification on our design, and the range of environmental unknowns was too wide. We committed our instrument to obtain just the basic features of the radiation belts with minimum ambiguity as to interpretation of the data. Specifically, our instrument design objectives were to distinguish trapped protons and electrons, to measure absolute intensities within known energy ranges, to obtain several is a preamplifier, a delay line pulse shaper, and one or two voltage amplifiers. After this the signals go to a linear gate that commutates one of the three to a shared amplifier string. The amplifiers drive three integral pulse height discriminators that are set to the ratios 1:2.12:4.5, and the outputs of the discriminators go to a commutator that gates one of them to a 23bit accumulator. A particular channel is determined by the linear gate, which chooses the sensor, and by the commutator, which selects the pulse height discriminator: e.g., E3, M 1, etc.At high counting rates, two characteristic times associated with the electronics become important: the width of the analog pulse and the dead time of the discriminator. The pulse width determines the probability that two pulses, each of which is too small to trigger the discriminator separately, will add in height and produc...