Ion conics and electron beams associated with auroral processes on Saturn

Abstract

Ion conics accompanied by electron beams are observed regularly in Saturn's magnetosphere. The beams and conics are seen throughout the outer magnetosphere, on field lines that nominally map from well into the polar cap (L dipole > 50) to well into the closed field region (L dipole < 10). The electron beams and ion conics are often observed together but also sometimes separately. Typically, the ion conics are prominent at energies between about 30 keV and 200 keV. The electron beams extend from below the ∼20 keV lower energy threshold for the instrument to sometimes as high as 1 MeV. The electrons may be either unidirectional (upward) or bidirectional; the ions are exclusively unidirectional upward. The ion conics are usually seen in conjunction with enhanced broadband electromagnetic noise in the 10 Hz to few kHz frequency range. Most of the wave energy appears below the local electron cyclotron frequency, hence, is propagating in the whistler mode, although some extension to higher frequencies is sometimes observed, suggesting an electrostatic mode. Sometimes the particle phenomena and the broadband noise occur in pulses of roughly 5 min duration, separated by tens of minutes. At other times they are relatively steady over an hour or more. Magnetic signatures associated with some of the pulsed events are consistent with field aligned current structures. The ions are almost exclusively light ions (H, H 2 , H 3 , and/or He) with only occasional hints of oxygen or other heavier species, suggesting an ionospheric source. Taken together, the observations are strikingly similar to those made at Earth in association with auroral zone downward sheet currents, except that in the case of Saturn the particle energies are 20 to 100 times higher.