The planetary magnetic field and magnetosphere of Jupiter: Pioneer 10

Abstract

Data obtained by the Pioneer 10 vector helium magnetometer are presented along with models of the intrinsic magnetic field of JupiteL and its magnetosphere. Data acquired between 2.84 and 6.0 Rj, where the intensity of the planetary field ranged between 1900 and 18,400 'y, were used to develop a six-parameter eccentric dipole model of the field. The dipole so derived has a moment of 4.0 G Rfi and a tilt angle With respect to Jupiter's rotation axis of 11 ø. The system I!I (epOch 1957) longitude of the magnetic pole in the northern hemisPhere,•which is a north-seeking pole, is 222 ø. The dipole is displaced from the center of Jupiter by 0.11 Ra in the direction of iatitude 16 ø and system ili longitude 176 ø. The dipole tilt and the longitude of the pole ar e in goød agreement with values inferred from radio astronomy measurements. The magnetic moment and the offset derived from the Pioneer measurements represent a significant improvement in our knowledge of the planetary field. A model of the Jovian magnetosphere is presented in which the essential feature is an eastward current sheet that forms an annulus with Jupiter at the center. At Iarge distances from the planet the current sheet is nearly parallel to Jupiter's equator but, in general, does not lie in it. The Current sheet is warped, so that it is above the equator on one side and below it on the other. The current Sheet rotates with the planet, more or less lik e a rigid body; this behavior causes an apparent up and down motion and periodic crossings of the current sheet by Pioneer. The origin of the current sheet appears to be the very large centrifugal force, associated with Jupiter's great size and rapid rotation, acting on trapped low-energy magnetospheric plasma. The density of this plasma is estimated to be approximately 1 particle cm -3. A retrograde spiraling of field lines out of meridian planes is also observed, presumably as a result of azimuthal drag forces exerted on the outer magnetosphere. INSTRUMENT DESCRIPTION The magnetometer on board Pioneer !0 is an advanced version of the vector helium magnetometer previously used on the Mai'iner 4 and 5 missions to Mars and Venus [Slocum and Reilly, 1963; Connor, 1968]. The essential elements of the magnetometer include a helium lamp, a circular polarizer, a helium absorption cell, ß Helmholtz coils, a lens, and an !R detector. The basic operation of the instrument depends upon the effect of the magnetic field direction on the efficiency with which metastable helium can be optically pumped. The presence of an ambient magnetic field causes a sine wave modulation of the IR radiation passing through the gas cell at the fundamental frequency of the applied circular sweep field. The sensor output consists of the nulling currents that are applied in order to cancel this field 'signal.'The magnetometer measures the three field components over a frequency range from 0 to 10 Hz. At encounter the data rate was 1024 bits/s and the magnetometer sampling rate, corresponding to one vector measurement every 3/16 s, ...

New models of Jupiter's magnetic field constrained by the Io flux tube footprint

“…In addition to fitting the IFT footprints, we also require the magnetic field model to fit, in a least squares sense, the in situ magnetometer observations obtained by spacecraft. The data available include measurements obtained by the vector helium magnetometers on Pioneer 10 and 11 [Smith et al, 1974[Smith et al, , 1975 as determined previously [Connerney et al, 1982]. In the linearized system, y is a column vector of the model residuals (observed minus modeled field or equatorial crossing distance), x is a column vector of parameter corrections required to bring the model into closer agreement with the data, and the matrix A is a matrix of partial derivatives relating the observations to the model parameters.…”

“…In situ measurements of Jupiter's magnetic field were obtained by the Pioneer 10 and 11 spacecraft in 1973 and 1974 [Smith et al, 1974[Smith et al, , 1976; Acura and Ness, 1976] Baron et al, 1996]. While the distribution of these emissions in the polar regions could be mapped with relative ease, however, the limited accuracy of magnetic field models prevented one from associating these emissions with a particular source region in the Jovian magnetosphere [Connerney, 1992].…”

Jupiter's radiation belts

“…to account f o r the observationall y weak (Smith et al, 1974b) at low latitudes is the southward component , and it is difficult to cons t r u c t an "open " m a g n e t o s ph e r e unde r this observational c o n s t r a i n t .…”

Acceleration of protons at 32 Jovian radii in the outer magnetosphere of Jupiter

“…The exact position relative to the equator depends on the sweep back of the field and the tilt of the plasma sheet towards the geometric equator (Smith et al, 1974;Hill et al, 1974;Northrop and Goertz, 1974;Goertz, 1976). As the enhanced flux of particles decayed the field aligned part must have decayed more rapidly because at Time P we observe a pancake distribution which resembles the angular distributions normally observed in the plasma sheet near the equator.…”