The penultimate steps in an aurora's journey take place at terrestrial altitudes of 80,000 to 100,000 km, where the solar wind and the interplanetary magnetic field
first encounter the magnetosphere.
"Any space physicist, including me, would have said a year ago there could not be substorms when the interplanetary magnetic field was staying northward, but that's wrong," Lyons said.
"Generally, it's correct, but when you have a fluctuating interplanetary magnetic field, you can have substorms going off once per hour," he added.
One tantalizing result, reported by Margaret Kivelson (University of California, Los Angeles), is that there was a disturbance in the interplanetary magnetic field
just as Galileo passed closest to Ida.
If enough ions from the comet are trapped on the lines of the interplanetary magnetic field
, Fainberg says, the resulting "mass loading" may cause the field lines separated by the comet to recombine behind it, producing radio emissions.
Kivelson, who heads the magnetometer team, an intrinsic field is the best explanation for an abrupt change in the orientation of the interplanetary magnetic field
recorded when Galileo and Gaspra were closest.
Yet the interplanetary magnetic field
, which originates in the sun, does not reflect this activity.
Analysis of the magnetometer record showed that Gaspra created a large distortion in the interplanetary magnetic field
flowing around it.
In addition, she reports in the April GEOPHYSICAL RESEARCH LETTERS, there was a disturbance in the interplanetary magnetic field
of just the sort that would be produced by a trailing "filament' of the solar wind blowing past the spacecraft.
One problem for the AMPTE researchers trying to decide whether to go ahead with the barium release was that they could not know in advance about the condition of the interplanetary magnetic field
that transports the solar-wind particles that ultimately populate earth's magnetic tail.
In general, the solar wind's ability to penetrate into near-Earth space is thought to rely on the magnetic alignment of the interplanetary magnetic fields
, often shortened to IMF.