Upstream ULF waves and energetic electrons associated with the lunar wake: Detection of precursor activity

We present observations of precursor ULF wave activity and energetic electron flows detected by the WIND spacecraft just prior to entry of the lunar wake on 27 December 1994. This activity occurs upstream of the wake on field lines directly connected to the wake penumbra region. The activity ceases near the penumbra entrance. The observations of upstream ULF wave activity and solar wind counterstreaming electron flows is similar to observations made upstream of collisionless bow shocks. Analogously, the wake precursor region is characterized by thermalization and information propagation ahead of the wake structure

Observations of the lunar plasma wake from the WIND spacecraft on December 27, 1994

On December 27, 1994, the WIND spacecraft crossed the lunar wake at a distance of 6.5 lunar radii ( RL ) behind the moon. The observations made were the first employing modem instruments and a high data rate. The SWE plasma instrument on WIND observed new aspects of the interaction between the solar wind and unmagnetized dielectric bodies. The plasma density decreased exponentially from the periphery of the wake towards its center as predicted by simple theory. Behind the moon two distinct cold ion beams were observed refilling the lunar cavity. The ions were accelerated along the direction of the magnetic field by an electric field of the order 2 × 10−4 volts/m. The region of plasma depletion was observed to extend beyond the light shadow, consistent with a rarefaction wave moving out from the wake into the undisturbed solar wind

Near-simultaneous bow shock crossings by WIND and IMP 8 on December 1, 1994

Near‐simultaneous dawn‐side bow shock crossings by WIND and IMP 8 on December 1, 1994 are analyzed to determine shock location and shape and to examine the changes in shock structure and the foreshock MHD wave properties with increasing downstream distance. The WIND and IMP 8 crossings took place at sun‐Earth‐spacecraft angles of 64.7° and 115.3°, respectively. The solar wind speed and interplanetary magnetic field magnitude were near their long‐term average values. However, the orientation of the IMF was unusual in that it rotated from an angle of ∼50–60° to the sun‐Earth line at the beginning of the interval of shock crossings to less than 20° just after the final crossings. The ratio of the downstream to upstream components of the magnetic field tangential to the shock decreases from 4.1 at WIND to 3.1 at IMP 8 in general agreement with theory. In addition, the overshoot in the shock magnetic ramp observed at WIND is greatly diminished by the downstream distance of IMP 8. In the foreshock, MHD waves with periods of 10–20 s and amplitudes of 3–6 nT were observed at both spacecraft. However, at WIND they have a strong compressional component which is much weaker farther downstream at IMP 8. Unexpectedly, the radial distance of the shock at both spacecraft is only ∼80–85% of that predicted by recent models. Motivated by this event, we have statistically analyzed a larger data set of bow shock crossings which took place under quasi‐field‐aligned flow conditions. On this basis it is suggested that magnetosheath thickness may decrease by ∼10% as the IMF becomes increasingly flow aligned