M. D. Desch

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.

On the possibility of coherent cyclotron emission from extrasolar planets

A model of the coherent cyclotron emission from extrasolar planets is presented. Scaling laws known to operate in our solar system (including scaling laws of planetary magnetic fields and the radiometric Bodes law of radio power generation) are applied to the extrasolar systems. We consider the possibility that each of the extrasolar planets possesses a substantial planetary magnetic field which is in quasi-continuous interaction with the local stellar wind. Cyclotron emission from extrasolar planets is then driven by the stellar wind/magnetospheric interaction, much like the coherent cyclotron radio emission processes associated with planets in our solar system. Based on the model results, the best candidate for solar-wind-driven cyclotron emission is Tau Bootes, with an expected median amplitude of about 2 janskys (1 Jy = 10−26 W m−2 Hz−1) at 28 MHz, an intensity level of about a factor of 100 below the current limit of detectability. However, variations in the local stellar medium could conceivably increase power levels by a factor of 100 for short periods of time. Like the solar planets, the extrasolar planets should radiate episodically, with emission reoccurring at the planetary rotation period. Thus spectral integration techniques could also be applied to improve the likelihood of detectability.

An Earth-like correspondence between Saturn's auroral features and radio emission

Saturn is a source of intense kilometre-wavelength radio emissions that are believed to be associated with its polar aurorae, and which provide an important remote diagnostic of its magnetospheric activity. Previous observations implied that the radio emission originated in the polar regions, and indicated a strong correlation with solar wind dynamic pressure. The radio source also appeared to be fixed near local noon and at the latitude of the ultraviolet aurora. There have, however, been no observations relating the radio emissions to detailed auroral structures. Here we report measurements of the radio emissions, which, along with high-resolution images of Saturns ultraviolet auroral emissions, suggest that although there are differences in the global morphology of the aurorae, Saturns radio emissions exhibit an Earth-like correspondence between bright auroral features and the radio emissions. This demonstrates the universality of the mechanism that results in emissions near the electron cyclotron frequency narrowly beamed at large angles to the magnetic field.

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.

Electric and magnetic signatures of dust devils from the 2000–2001 MATADOR desert tests

[1] Dust devils are significant meteorological phenomena on Mars: They are ubiquitous, continually gardening the Martian surface, and may be the primary atmospheric dustloading mechanism in nonstorm seasons. Further, dust grains in the swirling dust devils may become electrically charged via triboelectric effects. Electrical effects associated with terrestrial dust devils have been reported previously, but these were isolated measurements (electric fields only) with no corroborating measurements. To study the fluid and electrical forces associated with dust devils, NASA’s Human Exploration and Development of Space (HEDS) enterprise sponsored a set of desert field tests with a suite of mutually compatible and complementary instruments in order to determine the relationship between electric, magnetic, and fluid forces. The project (originally a selected flight project) was entitled ‘‘Martian ATmosphere And Dust in the Optical and Radio’’ (MATADOR). In this work, we present a number of interesting examples of the electromagnetic nature of the dust devil. We also describe potential hazards of the dust devil and how similar devil- and storm-related forces on Mars might affect any human occupation. INDEX TERMS: 6225 Planetology: Solar System Objects: Mars; 3304 Meteorology and Atmospheric Dynamics: Atmospheric electricity; 3379 Meteorology and Atmospheric Dynamics: Turbulence; 3394 Meteorology and Atmospheric Dynamics: Instruments and techniques; 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); KEYWORDS: triboelectricity, electric fields, dust devils, magnetic fields, atmospheric electricity

Detecting electrical activity from Martian dust storms

We present a model addressing the possible electrification of Martian dust storms based on the effective electrical charging of an individual dust grain. An upper charge bound on a grain can be determined based on the grain capacitance in the low-pressure Martian atmosphere. It is assumed that treiboelectric and inductive processes, like that presumed operating in terrestrial dust storms, can electrify the grain to significant levels. A collection of such grains charged in a dust cloud of many tens of kilometers in size can yield a substantial electric field moment. Given various grain charge and dust storm sizes, the electric moment will be determined along with estimates of electrical discharge and emitted radio power based upon known models. We also suggest the possibility that remote detection of discharge-related VLF emission propagating in the surface/ionosphere waveguide can be used to determine subsurface conductivity.

Quasiperiodic Jovian Radio bursts: observations from the Ulysses Radio and Plasma Wave Experiment

Abstract The Ulysses flyby of Jupiter has permitted the detection of a variety of quasiperiodic magnetospheric phenomena. In this paper, Unified Radio and Plasma Wave Experiment (URAP) observations of quasiperiodic radio bursts are presented. There appear to be two preferred periods of short-term variability in the Jovian magnetosphere, as indicated by two classes of bursts, one with ∼ 40 min periodicity, the other with ∼ 15 min periodicity. The URAP radio direction determination capability provides clear evidence that the 40 min bursts originate near the southern Jovian magnetic pole, whereas the source location of the 15 min bursts remains uncertain. These bursts may be the signatures of quasiperiodic electron acceleration in the Jovian magnetosphere; however, only the 40 min bursts occur in association with observed electron bursts of similar periodicity. Both classes of bursts show some evidence of solar wind control. In particular, the onset of enhanced 40 min burst activity is well correlated with the arrival of high-velocity solar wind streams at Jupiter, thereby providing a remote monitor of solar wind conditions at Jupiter.

Solar-cycle variation of low density solar wind during more than three solar cycles

The May, 1999 low density (< 1 cm -3 ) solar wind interval is one of a series of intervals of low density solar wind which have been detected since in-situ, near-Earth observations began. Examining the NSSDC OMNI database since 1965, covering solar Cycles 20-23, we show that such intervals, which are also periods of unusually low mass flux and low dynamic pressure, occur most frequently around sunspot maximum and are rarer at solar minimum. The occurrence rate of low-density plasma may be higher in weaker sunspot cycles (Cycle 20 and the current Cycle 23). Around two-thirds of periods with densities < 1 cm -3 are associated with transient solar wind structures, in particular with ejecta and post-shock flows. The majority of other events are associated with corotating streams. The May 1999 event is unusual because it is not associated with an ejecta or stream. A similar period was observed in July-August 1979.