A survey of low frequency waves at Jupiter: The Ulysses encounter

Abstract

We report the results of a survey of low‐frequency (LF) plasma waves detected during the Ulysses Jupiter flyby. In the Jovian foreshock, two predominant wave periods are detected: 102‐s and 5‐s, as measured in the spacecraft frame. The 102‐s waves are highly nonlinear propagate at large angles to (typically 50°), are steepened, and sometimes have attached whistler packets. For the interval analyzed the 102‐s waves had mixed right‐and left‐hand polarizations. We argue that these are all consistent with being right‐hand magnetosonic waves in the solar wind frame. The 102‐s waves with attached whistlers are similar to cometary waves. The trailing portions are linearly polarized and the whistler portions circularly polarized with amplitudes decreasing linearly with time. The emissions are generated by ∼2‐keV protons flowing from the Jovian bow shock/magnetosheath into the upstream region. The instability is the ion beam instability. Higher Z ions were considered as a source of the waves but have been ruled out because of the low sunward velocities needed for their resonance. The 5‐s waves have , are compressive and are left‐hand polarized in the spacecraft frame. Local generation by three different resonant interactions were considered and have been ruled out. One possibility is that these waves are whistler mode by‐products of the steepened lower‐frequency magnetosonic waves. Mirror mode structures were detected throughout the outbound magnetosheath passes. For these structures, the θkB values were consistently in the range of 80° to 90°, exceptionally high values. Assuming 1‐keV protons, the spacing between the magnetic decreases is ∼10‐20 rp. Close to the magnetopause, small amplitudes , transverse waves were detected. Within the wave packet, right, left‐ and linear polarizations were found. These may be similar to waves detected in the Earth's plasma depletion layer. Within the Jovian low‐latitude boundary layer, enhanced transverse spectral power was detected at frequencies just below the proton cyclotron frequency. A total magnetic power of 10−1 nT2 was determined. Cross‐field diffusion of ∼1‐keV protons yields a calculated boundary layer thickness of 0.15 RJ. This is in excellent agreement with previous measurements. Transverse left‐hand (spacecraft frame) waves were detected within the Jovian magnetosphere. These waves have periods of 5‐8 min, are elliptically to circularly polarized and propagate in a range of 10° to 43° relative to . It is most likely that these waves are generated by an ion beam instability. Assuming S+, we get a resonant parallel energy of 60 keV.