Inside Jupiter's Magnetosphere

January 12, 2001

These readings obtained on Jan. 10, 2001, by the radio and plasma wave science instrument on NASA's Cassini spacecraft show that the craft was, as had been hoped, inside Jupiter's magnetosphere during a portion of the 18-hour span covered by the data.

A similar instrument on NASA's Galileo spacecraft showed that on Jan. 10 Galileo was also inside the Jupiter's magnetosphere, a vast surrounding environment of charged particles moving under the influence of the planet's magnetic field. This marks the first time humankind has placed two spacecraft within the magnetosphere of an outer planet at the same time. Cassini was 14 million kilometers (8.7 million miles) from Jupiter and Galileo was half as far from the planet. Apparently, a rapid shrinking of the magnetosphere soon left both spacecraft outside if it. Cassini and Galileo scientists are collaborating in studies of how the magnetosphere is affected by fluctuations in the solar wind of particles speeding away from the Sun. Rapid changes in the size of the magnetosphere may be one of the most dramatic showings of that interaction.

The sign that Cassini was inside the magnetosphere comes from the detection of radio waves caught within the magnetosphere, the "trapped radio waves" indicated on this chart. While central regions of Jupiter's magnetosphere contain very dense plasmas (with some 2000 electrons per cubic centimeter, or 30,000 per cubic inch), the outer magnetosphere can have very low densities. In fact, the outer magnetosphere can have densities 100 times lower than that of the surrounding solar wind, which consists of particles speeding away from the Sun. Because of that sharp difference in density between the outer magnetosphere and what lies just beyond it, the outer wall of the magnetosphere, called the magnetopause, serves as a boundary to very low-frequency radio emissions.
Since such low-frequency radio emissions are generated inside the magnetosphere, the radio waves are effectively trapped inside. The radio emissions in the frequency range of a few hundred Hertz to a few thousand Hertz in this chart are just such trapped waves, and they provide clear evidence that Cassini moved inside of the magnetosphere during this time interval.

The lower frequency boundary of the trapped waves can be used to determine the electron density in the Jovian magnetosphere. The fact that it is as low as 300 Hertz for much of the plotted time interval indicates that the density is only one electron per 1,000 cubic centimeters (60 cubic inches). During the time periods when the low-frequency edge has moved to higher frequencies, the electron density has increased, indicating that Cassini has moved very close to the magnetopause.

The lack of trapped waves at the right edge of the chart shows that, by that later point in the time interval, Cassini had left the magnetosphere. Interestingly, Galileo's plasma wave instrument showed that Galileo stopped detecting trapped waves at nearly the same time, indicating it, too, was no longer inside the magnetosphere. The close timing despite the different distances the two spacecraft were from Jupiter suggests there was a rapid and very dramatic decrease in the size of the magnetosphere, leaving both spacecraft outside the magnetopause.

Near the bottom of this image appear bursts of waves with frequencies below about 20 Hertz. These electrostatic waves are actually structures similar to bubbles in the plasma. These bubbles are often found on boundaries, such as the magnetopause, where there are strong electrical currents or beams of charged particles flowing.

Cassini is a cooperative mission of NASA, the European Space Agency and the Italian Space Agency. The Jet Propulsion Laboratory, a division of the California Institute of Technology in Pasadena, manages Cassini and Galileo for NASA's Office of Space Science, Washington, D.C.

For higher resolution image, click here.

Additional information about Cassini-Huygens is online at

Cassini passed its closest to Jupiter on Dec. 30, 2000, en route to reach its primary destination, Saturn, in 2004. Cassini is a cooperative project of NASA, the European Space Agency and the Italian Space Agency. JPL manages the Cassini and Galileo missions for NASA¿s Office of Space Science, Washington, D.C. JPL is a division of the California Institute of Technology in Pasadena.

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