Upstream particle spatial gradients and plasma waves

The upstream electron and ion fluxes detected by our experiment on ISEE 1/2 spacecraft undergo frequent time variations, from a few seconds to minutes. Many flux variations correlate with directional changes of the interplanetary magnetic field (IMF). Particles propagating in the upstream region acted on by the solar wind electric field creates a quasi-stationary particle pattern in space. Evidently, the spacecraft frequently crosses the boundaries of these particle patterns. Our analysis strongly suggests that the particle time variations are usually spatial variations that have been convoluted into our data. Estimates of the thickness of the particle boundaries deduced is ≳1 Larmor radius (for both the upstream electron and the ion events). Plasma waves are observed in association with the upstream particle fluxes and a correlation between the amplitudes and the particle boundaries is suggested. We will theoretically show that the ion and electron density gradients across the boundary play an important role in exciting the ion acoustic-like and plasma waves.

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