No abstract

No abstract

Following a brief discussion of Parker's spiral‐field model, the behavior of charged particles in this idealized interplanetary magnetic field is investigated. Defining equations are given for pertinent variables, and their values are plotted or tabulated as functions of heliocentric range for various values of the model parameters, proton energy, and pitch angle. The variables include the radius of curvature, arc length, gradients of the lines of force, particle drift and velocity, and flight time of protons. A modified magnetic moment that depends upon the drift velocity, and the energy limits for adiabatic motion that require that the magnetic moment remain invariant are determined for particles in this ideal field.

Field-aligned currents, which are known to play an important role in the dynamics of the earth's magnetosphere, would also be expected to flow in the Jovian magnetosphere during magnetically active periods. Such currents are difficult to observe largely because of their limited spatial extent and in. terreittent nature, but on the Pioneer I l inbound pass to Jupiter on day 336, 1800 GRT, near the L shell of Ganymede, the helium vector magnetometer recorded a structured perturbation with the characteristic signature of a field-aligned current. No analogous structure was found on a similar Pioneer l0 inbound pass (day 337, 1200 GRT). The perturbation could have been caused by turbulence within the wake of Ganymede through which the spacecraft was moving or the perturbation could represent the first observation of a Jovian substorm. The observations are discussed in terms of both •models, and difficulties are examined.