Radiophysics of Jupiter

For DIM, the problem of fixing the emission centroid remains, despite the very strong efforts by Roberts and Ekers, and by Berge. There are asymmetries in DIM and DAM, whose only explanation has been in terms of the displaced dipole. A satisfactory answer may depend on studies carried out in real time by a second-of-arc pencil beam. One might discern the thermal emission from Jupiter's disk embedded in the halo of radiation belt emission. The observations then would be self-calibrating. Such a measurement would require apparatus with multiple-pencil beams of the order of 5 seconds of arc. There might also be discernible local effects of Io and Amalthea on DIM (Rather, unpublished). More immediate problems for DIM certainly include-refinement of the rotational period, in view of its apparent disagreement with DAM's rotation period. Barber (1966) and Dickel (1967) believe that the period lies within 0.2 seconds of the system III (1957.0) period. Periodic checks of the rotational period seem important, and can be carried out with relatively simple equipment. Many stations around the world now observe DAM, although the concentration is heaviest in the U.S.A. There is value in 24-hour synoptic coverage at limited frequencies (Alexander, 1966). Such a study might possibly have suggested Io's modulation earlier, had it been available. The current tendency for observers of DAM to publish their data in summary form is also much to be recommended (see, for example, the catalogue of Morrow, Barrow, and Resch, 1965). However, the principal information needed is more refined data, especially on the fast-time resolution polarimetry and spectroscopy of millisecond bursts. The polarization diversity on these bursts as recorded at Arecibo needs confirmation. At Boulder equipment is being set up for continuing the study, but it may suffer from lack of antenna collecting area. In addition we plan to extend the swept-frequency receiver towards higher frequencies, from 40 to 80 Mc/s. Continuous coverage of that range, with high sensitivity, is required to establish the existence of possible localized spectral islands of emission. These would have escaped detection in any DAM surveys made to date. The ionospheric Faraday effect on Jupiter bursts should be observed with higher precision than so far accomplished. One possible result of such a study might be the detection of the effects of Jupiter's rotation in the orientation of DAM's polarization ellipse. Radar observations of Jupiter promise much for the future. The detection of echoes from this soft target is apparently variable (Pettengill, 1965). Pettengill (1966) also notes that improving radar system power may permit detection of echoes from Jupiter's Galilean satellites in the next decade, and suggests that the polarization should be measured as the satellite is occulted by Jupiter's ionosphere. Such measurements could provide an independent determination of Jupiter's magnetic field. If, as is likely the case, DAM is generated near the electron gyro frequency of Jupiter's ionosphere and magnetosphere, a lower limit of the emitted frequency is given by the weakest field containing emitting particles or waves. These fields lie at the outermost parts of the magnetosphere of Jupiter, whose extent is uncertain (say, 10–50 Jupiter radii; in the magnetospheric tail, the distance is still greater; this structure undoubtedly subtends degrees in our sky!). At the magnetopause, the gyro frequency is about 100 cps, and at Io, 150 kc/s. The interplanetary plasma frequency corresponding to one electron cm-3 is 9 kc/s. Observations of the lower limit of radio emission from Jupiter may succeed if sensitive radio telescopes are placed outside of the earth's magnetosphere. All known facts concerning non-thermal phenomena at Jupiter derive from radio astronomical data. In situ verification of the inferred particles and fields seems to most radioastronomers to be a priority item for space research. To design and fly apparatus to Jupiter is not easy. The equipment must survive not only a long voyage, several years in length, but also an obviously hostile environment upon its arrival. Benefits that might accrue to such a flight are: Objectives such as these justified space probes within the inner planetary system. A Jupiter probe, and especially a Jupiter orbiter, enhances the prospects of a pay-off, because this planet, uniquely aside from the earth, is known to involve the phenomena of interest. There remains an outstanding inconsistency in the data on the symmetry of Jupiter's magnetic field. Everyone agrees that DAM requires departure of the planetary field from a centered dipole. I find it necessary to review my reasons for suspecting that the nature of these departures is not resolved at present, despite the wonderful measurements by Roberts and Ekers. If the centroid of DIM is at the mass center, the evident asymmetry of the direction of DIM's polarization as function of longitude requires explanation, as does the variation of intensity, as a function of zenomagnetic latitude of the earth. At present, no explanation other than planetary shadowing of southward-shifted radiation belts has been advanced for the polarization effect. It now appears (Section 3.2) that the intensity effect shows that Jupiter's magnetic field is a very pure dipole. The suggestion that distortion of the dipole field produces the polarization effect therefore cannot be supported. In Section 3.2 we showed that Roberts and Komesaroff's (1965) determination of intensity as a function of latitude is symmetric around zenomagnetic latitude +1.2°. This latitude represents the effective magnetic equator of Jupiter, so far as the mirroring of the relativistic electrons is concerned. Assume that the observed asymmetry derives from a magnetic field made up of an axi-symmetric quadrupole field added to the pre-existing dipole field. The mirror-point equator (where the minimum magnetic field exists) lies north of the dipole equator. To achieve this, the southern pole of Jupiter's magnetic axis must have a stronger field (in agreement with the displaced dipole model for DAM). If the magnetic field is made up of an axi-symmetric quadrupole field added to a dipole field, the quadrupole has negative poles (with inwardly directed field lines) and a positive center (with outwardly directed fieldlines). Detailed calculations show that for synchrotron emission at 2.5 radii from the center of Jupiter, the required asymmetry is achieved if the ratio of quadrupole to dipole moment is .018 in units of Jupiter's radius. If both the dipole and the quadrupole lie at Jupiter's center, the ratio of equatorial field strengths is .0552. The southern polar field is slightly stronger than the northern, but the difference is too small to account for the asymmetries of DAM or the polarization asymmetry of DIM. The quadrupole field of the earth is .08 that of the dipole, and the earth has rather strong higher pole components as well. In other words, it seems that Jupiter's field is a more purely dipole field than is the earth's field. In the quadrupole model, there is a difference between the zenomagnetic latitudes in the northern and southern hemispheres where the line of force through Io intersects the surface of Jupiter. However, this difference amounts to only about four degrees, again emphasizing the purity of Jupiter's dipole field.