T. D. Carr

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 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.

Microstructure of Jovian decametric S bursts

We have developed a method for displaying the spectral structure of Jupiters decametric radio S bursts on timescales down to a few microseconds, 2 orders of magnitude finer than has been achieved elsewhere. By employing an extremely sensitive antenna (640 dipoles, at 26 MHz) and selecting only relatively weak S bursts that possess the simplest possible spectral shape, we identify frequently occurring structural features that we associate with localized emission centers. On timescales having better than about 30 μs resolution we find that the S burst baseband oscillation (and therefore the RF oscillation) is modulated to form distinct pulses, which we refer to as subpulses. Still finer time resolution reveals that within individual subpulses the baseband (and RF) oscillation often displays segments in which the usually drifting phase term abruptly becomes essentially constant and, after remaining so for 10 to 100 μs, abruptly resumes its random drift. It is these abruptly starting and ending segments of phase coherence that we attribute to isolated powerful centers of cyclotron maser wave amplification, which happen for brief intervals to be the only ones that are active. We believe that the more usual phase-incoherent condition (i.e., one in which the instantaneous frequency drifts randomly within the emission band) is due to the fact that the resultant radiation is the sum of two or more components from neighboring wave amplification centers emitting at slightly different frequencies, with independently varying intensities. Possible models for the production of subpulses and phase coherent intervals are discussed.

Correlated variations of UV and radio emissions during an outstanding Jovian auroral event

An exceptional Jovian aurora was detected in the FUV on December 21, 1990, by means of Vilspa and Goddard Space Flight Center (GFSC) International Ultraviolet Explorer (IUE) observations. This event included intensification by a factor of three between December 20 and 21, leading to the brightest aurora identified in the IUE data analyzed, and, in the north, to a shift of the emission peak towards larger longitudes (these variations are even more dramatic once the actual source brightness distribution is retrieved from the raw data). The Jovian radio emission simultaneously recorded at decameter wavelengths in Nancay also exhibits significant changes, from a weak and short-duration emission on December 20 to a very intense one, lasting several hours, on December 21. Confirmation of this intense radio event is also found in the observations at the University of Florida on December 21. The emissions are identified as right-handed Io-independent “A” (or “non Io-A”) components from the northern hemisphere. The radio source region deduced from the Nancay observations lies, for both days, close to the UV peak emission, exhibiting in particular a similar shift of the source region toward larger longitudes from one day to the next. A significant broadening of the radio source was also observed and it is shown that on both days, the extent of the radio source closely followed the longitude range for which the UV brightness exceeds a given threshold (∼3 kW m−1). The correlated variations, both in intensity and longitude, strongly suggest that a common cause triggered the variation of the UV and radio emissions during this exceptional event. On one hand, the variation of the UV aurora could possibly be interpreted according to the Prange and Elkhamsi (1991) model of diffuse multicomponent auroral precipitation (electron and ion): it would arise from an increase in the precipitation rate of ions together with an inward shift of their precipitation locus from L ≈ 10 to L ≈ 6. On the other hand, the analysis of Ulysses observations in the upstream solar wind suggests that a significant disturbance in the solar wind, involving the generation of an interplanetary shock and the presence of a CME have interacted with the Jovian magnetosphere at about the time of the auroral event. Both arguments suggest that we may have observed for the first time a magnetic storm-type interaction in an outer planet magnetosphere, affecting simultaneously several auroral processes. Conversely, the observed relationship between the level of UV auroral activity and the detection of decameter emission (DAM), if it were a typical feature, might argue in favour of a more direct and permanent association between the auroral processes leading to UV and radio aurorae, possibly related to “discrete-arc”-like activity and electron precipitation.

Radio rotation period of jupiter.

The results of observations of Jupiter at 18 megacycles per second indicate that the apparent rotation period drifts cyclically about a constant mean value. The most probable drift period appears to be 11.9 years, Jupiters orbital period. The mean rotation period during one orbital period is about 0.3 second longer than that of the system III (1957.0) period. This is in close agreement with the rotation period deduced from decimetric observations and probably represents the true rotation period of the magnetic field. The cyclic drift in the rotation period of source A at 18 megacycles per second is explained on the basis of beaming of the escaping radiation at an angle 6 degrees north of the magnetic equator. The apparent rotation period of source A depends on the rate of change of the Jovicentric declination of Earth.

A redefinition of Jupiter's rotation period

We measured the rotation period of Jupiters inner magnetosphere with precision previously unattainable, using 35 years of observations of the Jovian decametric radiation at the University of Florida Radio Observatory at frequencies between 18 and 22.2 MHz. The new rotation period is the weighted mean of 13 independent 24-year average determinations. Each of these was found by measuring the drift of the histogram of occurrence probability versus System III (1965) central meridian longitude over an interval of approximately 24 years. The measured drift was used to correct the System III (1965) period to obtain the new value. Our weighted mean is 9 hours 55 min 29.6854 s, with a standard deviation of the weighted mean (σ) of 0.0035 s. This new rotation period is 7.4σ shorter than that of the System III (1965), indicating that the latter is in need of revision. Our measurements indicate an upper limit of about 4 ms/year on any possible Jovian rotation period drift.

SPECTRAL DISTRIBUTION OF THE DECAMETRIC RADIATION FROM JUPITER IN 1961

Abstract : Observations of the sporadic radiation from Jupiter were made in 1961 at observatories in Florida, in Australia, and in Chile, at various frequencies from 5 Mc/s to 85.5 Mc/s. A method was developed for computing the mean flux density for the apparition from measurements of peak flux density in consecutive 10-min intervals. Occurrence probability, peak flux density, and mean flux density for the apparition were determined as functions of frequency, and all three quantities were found to decrease monotonically with increase in frequency above 10 Mc/s. The decrease in mean flux density with increasing frequency indicated a spectral index in excess of 5.2 over much of the observed spectrum. Peak flux densities at 5 Mc/s and 10 Mc/s were about 10 to the minus 19th power W/sq m (c/s). No activity was detected at 85.5 Mc/s. The ratio of mean flux density to peak flux density for 1-min intervals during noise storms was found to be essentially independent of frequency, suggesting that there is no appreciable change of noise-burst shape with frequency. The mean power emitted at decameter wavelengths by Jupiter in 1961 was estimated to be about 5 X 10 to the 10th power watts, which is 1 or 2 orders of magnitude greater than that emitted at decimeter wavelengths. The average efficiency with which the solar-wind power incident on the Jovian magnetosphere is converted into decametric radiation power was estimated to be about 0.00001. (Author)

A new determination of Jupiter's radio rotation period

The result of a measurement of the period of rotation of Jupiters inner magnetosphere with unprecendented precision is presented. The measurement was made from the University of Florida database of 35 apparitions of Jovian decametric observations at frequencies of 18, 20, and 22 MHz between 1957 and 1994. The mean of our 24 independent measurements was 9h55m29s.685, and the standard deviation of the mean was 0s.0034. The System III (1965) Jovian rotation period value that is currently accepted by the International Astronomical Union is greater than our value by 7.4 times our standard deviation; it appears to be in need of revision. We set an upper limit of 27 milliseconds per year on a possible drift of the rotation period as measured by our method.

A model for the production of Jupiter's decametric modulation lanes

We present an interference-screen model for the production of the modulation lanes often observed in Jupiters decametric radiation. As radiation traverses a grid-like screen composed of field-aligned columns of enhanced plasma density downstream from Io, rays scattered on passing through the columns interfere constructively and destructively with those passing relatively unscattered between columns. Due to the geometrical relationship between the radio-emitting flux tube and the interference screen, the resulting rotating lobe pattern received at Earth synthesizes modulation lanes that very closely match those actually observed.

Observations of Jupiter at 26.3 MHz using a large array

Abstract A 640 element phase-steerable dipole array has been used to make highly sensitive observations of the planet Jupiter during the 1973 apparition. The satellite Io is found to have very little influence at the low flux levels, whereas the definition of sources A and B appears to be relatively flux independent. A two-dimensional analysis of the data in the Jupiter-Io plane has revealed considerable source B activity at low intensities which is not influenced by Io.