Charles A. Higgins

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.

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.

Structure within Jovian hectometric radiation

Observations of Jovian hectometric radio emission (HOM) by the Voyager planetary radio astronomy (PRA) experiment at frequencies from 300 kHz to 1.3 MHz indicate persistent dynamic spectral features that had not been previously studied. The features of interest appear as “lanes” of decreased emission intensity within the otherwise persistent HOM. The lanes are apparent in intensity and occurrence probability spectrograms of frequency versus Jovian System III (1965) longitude. In the investigation of the morphology of these features, we use inbound and outbound Voyager 2 data at Jupiter to show that the lane occurrence and characteristics do not depend on local time over the range sampled. Occurrence probability spectrograms of frequency versus magnetic latitude are created from the portion of the data when the spacecraft was between 0° and +10° magnetic latitude. These spectrograms represent both the inbound and outbound passes and are quite similar despite the different longitude ranges. A simple extension of decametric (DAM) arc features into the HOM wavelength does not account for all the lane features, giving further evidence that HOM is an independent emission component. Polarization signatures for the data show that the polarization is predominantly right-hand circular and that it does not reverse across the lanes, suggesting the emission is from the same hemisphere. In addition, we investigate possible effects due to solar wind variations and find that the occurrence of the lanes appears to be independent of times of low and high solar wind densities. The intensity of the HOM emission on either side of the lanes is comparable, implying that the lane is probably not a result of a gap between fundamental and second harmonic emission regions. We present these data and analyses as a morphological study to establish that the lane features are an important part of the HOM emission and should be considered in HOM emission models. At this time, no theory of the source of the lanes explains all the observed features.

Latitudinal structure within Jovian hectometric radiation

Jovian hectometric radio emission (HOM: 300–3000 kHz) has a number of persistent structural features associated with it as observed by the Voyager 1, Voyager 2, Ulysses, and Galileo spacecraft for specific jovigraphic latitudes (−4° to +7.1°) and local times (0.3 to 10.5 hours). Most notable are the presence of HOM emission between 270° and 120° central meridian longitude (CML) and the region of reduced emission intensity (a “gap”) between 120° and 270°. We displayed the Ulysses and Galileo data using time-frequency occurrence probability spectrograms and show that the observed HOM emission features are nearly identical to those observed by the Voyager spacecraft. This implies that the HOM structure is long-lived and fixed in its longitudinal position within the Jovian magnetosphere. HOM structure depends on small changes in the observers jovigraphic latitude, so the different jovigraphic latitudes of the spacecraft were used to probe the HOM beam structure. Prom this analysis we found that the CML width of the main HOM gap is directly correlated to the latitude of the spacecraft. We conclude that the latitudinal thickness of the HOM beam is about 12°, extending from −5° to +7° magnetic latitude.

Results of decametric monitoring of the comet collision with Jupiter

Decametric observations of Jupiter were made at frequencies from 16.7 to 32 MHz from the Maipu Radio Astronomy Observatory in Chile, the University of Florida Radio Observatory, and the Owens Valley Radio Observatory in California before, during, and after the collision of comet Shoemaker-Levy 9 with the planet. No significant change in the general level of Jovian decametric activity that might be attributed to the comet was observed. However, single bursts of possibly Jovian origin appeared at two of the 12 fragment impact times during which we were observing. We are attempting to establish more definitely whether these two bursts really were Jovian, and assuming that they were, we are tentatively modeling the circumstances of their emission.

Simple ray tracing of Galileo-observed hectometric attenuation features

Observations of persistent structural features within Jovian hectometric (HOM) radio emission have been made with the Galileo spacecraft. Two well-defined sinusoidal-shaped “band” features of reduced emission intensity and occurrence probability exist at all Jovian longitudes and nearly cover the entire spectrum of HOM radio emission from ∼500 kHz to 3000 kHz. These two sinusoidal lanes have a bandwidth of 200–400 kHz and are 180° out of phase with one another, suggesting that they are a result of HOM radio emission propagation processes from opposite hemispheres. These features become more apparent when presented as intensity or occurrence probability spectrograms added together over multiple Jovian rotations. Enhancements in the HOM intensity and occurrence are seen along the edges of one of the observed sinusoidal lane features which may indicate caustic surfaces due to refraction along the propagation path. We present some simple ray tracing analyses to show that refraction from density enhancements in the Io torus flux tube may explain some of the observations. Using this simple method, we approximate the density enhancements in the Io flux tube to be 100 cm−3.

Search for effects of comet S-L 9 fragment impacts on low radio frequency emission from Jupiter

Decametric radio observations of Jupiter were made before, during, and after the impacts of the fragments of the comet S-L 9 with the planet, from the University of Florida Radio Observatory, the Maipu Radio Astronomy Observatory of the University of Chile, and the Owens Valley Radio Observatory of the California Institute of Technology. The decametric radiation was monitored at frequencies from 16.7 to 32 MHz. The minimum detectable flux densities were on the order of 30 kJy, except for that of the large 26.3 MHz array in Florida, which was about 1 kJy. There was no significant enhancement or suppression of the decametric L-burst or S-burst emission with respect to normal activity patterns that might be attributed to the fragment entries. However, a burst of left-hand elliptically polarized radiation having a considerably longer duration than an L-burst was observed almost simultaneously with the impact of the large fragment Q2, and another with right-hand elliptical polarization was observed simultaneously with Q1. We consider the possibility that these two bursts were emitted just above the local electron cyclotron frequencies from the southern and northern ends, respectively, of magnetic flux tubes that had been excited in some way by the proximity of fragments Q2 and Q1.In addition to the monitoring of the decametric radiation, a search was conducted for possible comet-enhanced Jovian synchrotron radiation at 45 MHz using a large dipole antenna array at the observatory in Chile. This frequency is above the cutoff of the decametric radiation, but is considerably below the lowest frequency at which the synchrotron emission has previously been detected. The minimum detectable flux density with the 45 MHz antenna was about 5 Jy. No synchrotron emission at all was found before, during, or after the entry of the comet fragments.