Oblique ``1-Hz'' whistler mode waves in an electron foreshock: The Cassini near-Earth encounter

Abstract

The Cassini spacecraft flew toward the Earth in a trajectory almost along the Sun-Earth line, giving a unique perspective of low-frequency plasma waves in the Earth's foreshock. At the time of the fly-through, the angle between the magnetic field and Sun-Earth direction was ~70°. For this orientation, only the most energetic (>=12 keV) ions could reach the upstream region, thus Cassini passed through an almost ``pure'' electron foreshock. We demonstrate the presence of nonlinear foreshock ``1-Hz'' whistler-mode waves and their properties. Power spectra demonstrate that the waves span a frequency range from ~0.3 to ~10 Hz with a center frequency of ~2 Hz. Close to the bow shock the waves had peak-to-peak transverse amplitudes as large as ~14 nT in a 35-nT ambient field. The waves were compressive with a maximum value of Δ|<bold>B</bold>|/B₀~0.25. The waves were propagating primarily along <bold>B</bold>₀. Of the waves, 82% were right-hand-polarized, 9% were linearly polarized, and 9% were left-hand-polarized (in the spacecraft frame). The wave amplitudes were largest near the bow shock and decreased with increasing distance in the upstream direction. Further from the shock the waves are determined to be propagating obliquely to <bold>B</bold>₀ (θ_kB>=20°). There is an equal mix of right-hand, left-hand, and linearly polarized (in the space-craft frame) waves. Left-hand-polarized waves propagating parallel to <bold>B</bold>₀ were not detected. Wave power changed dramatically as the interplanetary magnetic field rotated slightly. Arguments are presented for why low-energy (tens of eV) electrons are responsible for wave generation, rather than cyclotron resonant energetic (>13 keV) electrons.