The magnetic field of Jupiter: A generalized inverse approach

Abstract

The estimation of planetary magnetic fields from observations of the magnetic field gathered along a spacecraft flyby trajectory is examined with the aid of generalized inverse techniques, with application to the internal magnetic field of Jupiter. Model non-uniqueness resulting from the limited spatial extent of the observations and noise on the data is explored and quantitative estimates of the model parameter resolution are found. The presence of a substantial magnetic field of external origin due to the currents flowing in the Jovian magnetodisc is found to be an important source of error in estimates of the internal Jovian field, and new models explicitly incorporating these currents are proposed. New internal field models are derived using the vector helium magnetometer observations and the high field fluxgate observations of Pioneer 11, and knowledge of the external current system gained from the Pioneer 10 and Voyagers 1 and 2 encounters. (Smith et al., 1974;Van Allen et al., 1974), a thin annular disc of tenuous plasma and charged particles encircling the giant planet to distances approaching 100 Jovian radii. Large scale azimuthal currents flowing in this magnetodisc, subsequently observed by the Voyagers 1 and 2 spacecraft (;Jess et al., 1979a; Ness et al., 1979b; Bridge et al., 1979) lead to a substantial magnetic field of ext .., rnal origin throughout the entire inner magnetosphere. The high-inclination retrograde approach of the Pioneer 11 spacecraft yielded measurements of the inner magnetosphere at high latitudes spanning a wide range of Jovian. longitude. In addition, the close approach of s1.6 Jovian radii (R J ) made this trajectory the most favorable for the purposes of estimating the internal magnetic field of Jupiter. Thus, models of the internal field based on the Pioneer 10 measurements (Smith et al., 1974) were quickly supplanted by models based on the Pioneer 11 observations 2 r^ //+ .^^ t c^ 11 ,0114"A Iwo n i (Davis and Smith, 1976; Acuna and Ness, 1976). The near equatorial approaches of the Voyager 1 and 2 spacecraft, at, the close approaches of P5 and 10 Rio have permitted more detailed studies of the external azimuthal current system but have thus far a . 4ded little to the present understanding of the internal magnetic field of Jupiter (Ness et al., 1979b) .As the number of available Jovian internal field models grows it becomes increasingly important to evaluate the model non-uniqueness inherent in the estimation of planetary magnetic fields from flyby observations. While some of the proposed mot1#*l3 represented an intermediate stage of data processing and analysis, in general differences between the proposed models reflect differences in the number and kind of coefficients used in the hasic models, differences in the actual observations, data intervals eWsen for analysis, r and the choice of weights applied to the observations in the least squares minimization process. A recent summary of the magnetic field observations of Jupiter (Smith and Gulkis, 1979) lists...

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