James W. Warwick

Planetary radio astronomy observations from Voyager 2 near Saturn

Planetary radio astronomy measurements obtained by Voyager 2 near Saturn have added further evidence that Saturnian kilometric radiation is emitted by a strong dayside source at auroral latitudes in the northern hemisphere and by a weaker source at complementary latitudes in the southern hemisphere. These emissions are variable because of Saturns rotation and, on longer time scales, probably because of influences of the solar wind and Dione. The electrostatic discharge bursts first discovered by Voyager 1 and attributed to emissions from the B ring were again observed with the same broadband spectral properties and an episodic recurrence period of about 10 hours, but their occurrence frequency was only about 30 percent of that detected by Voyager 1. While crossing the ring plane at a distance of 2.88 Saturn radii, the spacecraft detected an intense noise event extending to above 1 megahertz and lasting about 150 seconds. The event is interpreted to be a consequence of the impact, vaporization, and ionization of charged, micrometer-size G ring particles distributed over a vertical thickness of about 1500 kilometers.

Voyager 1 Planetary Radio Astronomy Observations Near Jupiter

We report results from the first low-frequency radio receiver to be transported into the Jupiter magnetosphere. We obtained dramatic new information, both because Voyager was near or in Jupiters radio emission sources and also because it was outside the relatively dense solar wind plasma of the inner solar system. Extensive radio spectral arcs, from above 30 to about 1 megahertz, occurred in patterns correlated with planetary longitude. A newly discovered kilometric wavelength radio source may relate to the plasma torus near Ios orbit. In situ wave resonances near closest approach define an electron density profile along the Voyager trajectory and form the basis for a map of the torus. Detailed studies are in progress and are out-lined briefly.

Voyager detection of nonthermal radio emission from Saturn

The planetary radio astronomy experiment on board the Voyager spacecraft has detected bursts of nonthermal radio noise from Saturn occurring near 200 kilohertz, with a peak flux density comparable to higher frequency Jovian emissions. The radiation is right-hand polarized and is most likely emitted in the extraordinary magnetoionic mode from Saturns northern hemisphere. Modulation that is consistent with a planetary rotation period of 10 hours 39.9 minutes is apparent in the data.

Planetary radio astronomy experiment for Voyager missions

The planetary radio astronomy experiment will measure radio spectra of planetary emissions in the range 1.2 kHz to 40.5 MHz. These emissions result from wave-particle-plasma interactions in the magnetospheres and ionospheres of the planets. At Jupiter, they are strongly modulated by the Galilean satellite Io.As the spacecraft leave the Earths vicinity, we will observe terrestrial kilometric radiation, and for the first time, determine its polarization (RH and LH power separately). At the giant planets, the source of radio emission at low frequencies is not understood, but will be defined through comparison of the radio emission data with other particles and fields experiments aboard Voyager, as well as with optical data. Since, for Jupiter, as for the Earth, the radio data quite probably relate to particle precipitation, and to magnetic field strength and orientation in the polar ionosphere, we hope to be able to elucidate some characteristics of Jupiter auroras.Together with the plasma wave experiment, and possibly several optical experiments, our data can demonstrate the existence of lightning on the giant planets and on the satellite Titan, should it exist. Finally, the Voyager missions occur near maximum of the sunspot cycle. Solar outburst types can be identified through the radio measurements; when the spacecraft are on the opposite side of the Sun from the Earth we can identify solar flare-related events otherwise invisible on the Earth.

Voyager Planetary Radio Astronomy at Neptune

Detection of very intense short radio bursts from Neptune was possible as early as 30 days before closest approach and at least 22 days after closest approach. The bursts lay at frequencies in the range 100 to 1300 kilohertz, were narrowband and strongly polarized, and presumably originated in southern polar regions ofthe planet. Episodes of smooth emissions in the frequency range from 20 to 865 kilohertz were detected during an interval of at least 10 days around closest approach. The bursts and the smooth emissions can be described in terms of rotation in a period of 16.11 � 0.05 hours. The bursts came at regular intervals throughout the encounter, including episodes both before and after closest approach. The smooth emissions showed a half-cycle phase shift between the five episodes before and after closest approach. This experiment detected the foreshock of Neptunes magnetosphere and the impacts of dust at the times of ring-plane crossings and also near the time of closest approach. Finally, there is no evidence for Neptunian electrostatic discharges.

Radiophysics of Jupiter

ConclusionsTypes of data that are neededFor 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 Jupiters 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 DAMs 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 Ios 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 Jupiters rotation in the orientation of DAMs 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 Jupiters Galilean satellites in the next decade, and suggests that the polarization should be measured as the satellite is occulted by Jupiters ionosphere. Such measurements could provide an independent determination of Jupiters magnetic field.Space observationsIf, as is likely the case, DAM is generated near the electron gyro frequency of Jupiters 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 earths magnetosphere.Observations of jupiters radiation beltsAll 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:(1)deeper understanding of plasma physical processes of generation and acceleration of energetic particles;(2)understanding of non-linear mechanisms for creation of radio emission from plasmas;(3)understanding of the origin of magnetic fields in rotating bodies;(4)observations of the solar wind and energetic particles at radically different places within the solar system. 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.Asymmetries in Jupiters magnetic fieldThere remains an outstanding inconsistency in the data on the symmetry of Jupiters 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 DIMs 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 Jupiters 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 Komesaroffs (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 Jupiters 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 Jupiters radius. If both the dipole and the quadrupole lie at Jupiters 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 Jupiters field is a more purely dipole field than is the earths 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 Jupiters dipole field.

Redefinition of system III longitude

Abstract There is a current need for a redefinition of the Jovian System III longitude measure. We report on a proposed new definition which has been widely circulated among users and has met with general acceptance. Some errors in current calculations of System III [1957.0] are not noted so that these errors can be avoided in future calculations.

FARADAY ROTATION ON DECAMETRIC RADIO EMISSIONS FROM JUPITER

Some decametric radio emissions from Jupiter observed recently exhibit effects which are attributable to Faraday rotation within the earths ionosphere. We present the evidence for the presence of Faraday rotation and discuss its origin. Identification of the magnetoionic mode in which the emission is generated at Jupiter appears possible, although near the limit of accuracy of our present observations. The emission seems to be generated in the extraordinary mode.

Shock waves and type II radiobursts in the interplanetary medium

The planetary radio astronomy experiment on the Voyager spacecraft observed several type II solar radiobursts at frequencies below 1.3 MHz; these correspond to shock waves at distances between 20R⊙ and 1 AU from the Sun. We study the characteristics of these bursts and discuss the information that they give on shock waves in the interplanetary medium and on the origin of the high energy electrons which give rise to the radioemission. The relatively frequent occurence of type II bursts at large distances from the Sun favors the hypothesis of the emission by a longitudinal shockwave. The observed spectral characteristics reveal that the source of emission is restricted to only a small portion of the shock. From the relation between type II bursts, type III bursts and optical flares, we suggest that some of the type II bursts could be excited by type III burst fast electrons which catch up the shock and are then trapped.

The effect of magnetic topography on high‐latitude radio emission at Neptune

Occultation by a local elevation on the surface of constant magnetic field is proposed as a new interpretation for the unusual properties of Neptune high-latitude emission. Abrupt changes in intensity and polarization of this broadband smooth radio emission were observed as the Voyager 2 spacecraft passed near the north magnetic pole before closest approach. The observed sequence of cutoffs with polarization reversal would not occur during descent of the spacecraft through regular surfaces of increasing magnetic field. The sequence can be understood in terms of constant-frequency (constant-field) surfaces that are not only offset from the planet center but are locally highly distorted by an elevation that occults the outgoing extraordinary-mode beam. The required occulter is similar to the field enhancement observed directly by the magnetometer team when Voyager reached lower altitude farther to the west. Evidence is presented that the sources of the high-altitude emission are located near the longitude of the minimum-B anomaly associated with the dipole offset and that the local elevation of constant-B surfaces extends eastward from the longitude where it is directly measured by the magnetometer to the longitude where occultation of the remote radio source is observed. Together, the radio and magnetometer experiments indicate that the constant-frequency surfaces are distorted by an elevation that extends 0.3 rad in the longitudinal direction.