Jupiter'S Synchrotron Radiation:
Observed Variations Before, During,
and After the Impacts of Comet SL-9

Responding to predictions that periodic comet Shoemaker-Levy-9 (SL-9) would impact Jupiter in July of 1994, radio astronomers worldwide carried out a campaign to observe and monitor the synchrotron emission from the Jovian radiation belts. Substantial increases in the planet's synchrotron emission were reported by several research teams (Ref. 1) during the week of July 16-23 at numerous wavelengths spanning the decimetric spectrum.

The NASA/JPL Jupiter Patrol, a long-term radio astronomy monitoring program begun in 1971 (Ref. 2), participated in the campaign to monitor the effects of the SL-9 impacts. The Jupiter Patrol observations are made at 2295 MHz (13 cm wavelength) using the large filled-aperture antennas of NASA's Deep Space Network (DSN). The patrol data clearly show that the intensity of Jupiter's microwave radio emission varies smoothly +/-15 percent over timescales of years. While short-term variations with timescales of a few days have been reported in the past, none have ever been confirmed; at least not until the impacts of Shoemaker-Levy-9.

The primary source of Jupiter's microwave radio emission at frequencies below ~6000 MHz is synchrotron radiation from electrons with relativistic energies, i.e., their velocity is near the speed of light. The negatively charged electrons radiate as they spiral up and down the magnetic field lines of the planet's inner magnetosphere. The intensity of the emission depends on the orientation and strength of the magnetic field, as well as the number and energy distribution of the relativistic electrons. At frequencies above ~6000 MHz, thermal radiation from the deep Jovian atmosphere emerges as the primary source of microwave radio emission.

A radio brightness map of the synchrotron emission measured with the Very Large Array (VLA) (Ref. 3) is shown in Figure 1. Note that the emission is concentrated near the magnetic equator with secondary brightening at low latitudes above and below the equatorial plane. The maximum brightness occurs near 1.5 Jovian radii (RJ) from the planet and very little microwave emission is observed beyond 3 RJ. Single-dish radio telescopes, like those used for the Jupiter Patrol, measure the spatially-averaged intensity of the synchrotron emission, because the solid angle of the antenna beam is typically much larger than the angular dimensions of the emitting region.

The results of the Jupiter Patrol observations from 1994 to 1997 are shown in Figure 2. The data points represent the peak intensity of Jupiter measured at Goldstone primarily with the 34-m antenna, located at the DSS 13 research and development station. The 70-m antenna was used for approximately 10 percent of the observations. The data show that the synchrotron flux at 13-cm increased 27 percent during the week of the impacts in July 1994 and was followed by a steady decline that began in August and continued throughout 1995.

The dotted curve in Figure 2 is an estimate of the baseline made by fitting a second order polynomial to the data taken before July 15, 1994 and after September 1995. The downward sloping baseline is consistent with the long-term decline in Jupiter's synchrotron emission that began in 1991-92, and may be related to the current minimum in the 11-year cycle of sunspots and solar magnetic storms. Figure 3 is a plot of the data after the baseline is subtracted. The exponential curve is given by e^(-70/t), where t is the number of days since July 16, 1994.

The rapid outburst that began the first day of the week-long series of impacts was accompanied by dramatic changes in the radio maps produced from observations at the VLA, as well as radio astronomy observatories in Australia and Europe. The results were surprising because nearly all theorists had predicted that cometary dust would scatter and deplete the energetic electrons and the net effect would be a decrease, not an increase, in the synchrotron radio emission.

The Jupiter Patrol observations revealed a second, unexpected result. The data in Figure 3 seem to indicate that a second outburst with a peak increase of approximately 10 percent was observed in late August of 1995. There is no reason to assume this outburst is related to the much stronger SL-9 event observed 15 months earlier. The Jupiter Patrol observations have been intensified to search for other short-term outbursts that would confirm their existence and reveal new information about the physics of the inner magnetosphere.

The surprising observational results that followed the SL-9 impacts have spawned new theories, as teams of scientists compete to explain the data. Two of the leading candidate processes are: (a) high-speed shock waves that temporarily energize the radiating electrons, and (b) the generation of low-frequency waves known as "whistlers" that alter the spiral motion of the relativistic electrons so they reach higher magnetic latitudes where the magnetic field is stronger.

Sophisticated computer programs are being developed to model the details of the radio astronomy observations. As they use these models, scientists hope to sort out the physical processes at work in Jupiter's magnetosphere and, perhaps, apply the "lessons learned" to advance knowledge of the magnetosphere here on earth. The models may also be used to evaluate and design spacecraft radiation protection for future missions that may spend long periods of time in Jupiter's inner magnetosphere. One such possible mission would send a spacecraft to orbit Europa and land a robotic probe on the ice-crusted surface.

References

1. dePater, I., et al. "Outburst of Jupiter Synchrotron-Radiation After the Impact of Comet Shoemaker-Levy-9," Science 286, (1995): 1679-1883.
2. Klein, M. J., Gulkis, S., and Stelzried, C. T. "Jupiter-New Evidence of Long-Term Variations of its Decimeter Flux Density," Astrophysical Journal, 176, (1972): 85-88.
3. dePater, I., and Jaffe, W. J. "Very Large Array Observations of Jupiter's Nonthermal Radiation," Astrophysical Journal Supplement Series, 54, (1984): 405-419.

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