Although collisionless shocks primarily exist to mediate the flow of supermagnetosonic plasma, they also act as sites for particle acceleration. It is now well known that for certain magnetic field geometries, a portion of the inflowing plasma returns to the upstream region rather than being processed by the shock and passing irreversibly downstream. The combination of the inflowing plasma and this counterstreaming component upstream of the shock is subject to a number of plasma instabilities, leading to the generation of waves. These waves interact in a highly complex manner with the ions and electrons making up the plasma and can cause part of the plasma distribution to reach high energies.The region of space upstream of the bow shock, magnetically connected to the shock and filled with particles backstreaming from the shock is known as the foreshock. As discussed in Balogh et al. (2005), the bow shock can be classified into quasi-perpendicular and quasi-parallel shock regions according to the angle θ Bn between the shock normal n and the direction of the solar wind magnetic field B. For the quasi-perpendicular bow shock (θ Bn > 45 • ), the foreshock is restricted to the shock foot, while in the quasi-parallel part of the bow shock (θ Bn < 45 • ), it

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[1] We present observations of low-frequency waves (0.25 Hz < f < 10 Hz) at five quasi-perpendicular interplanetary (IP) shocks observed by the Wind spacecraft. Four of the five IP shocks had oblique precursor whistler waves propagating at angles with respect to the magnetic field of 20°-50°and large propagation angles with respect to the shock normal; thus they do not appear to be phase standing. One event, the strongest in our study and likely supercritical, had low-frequency waves consistent with steepened magnetosonic waves called shocklets. The shocklets are seen in association with diffuse ion distributions. Both the shocklets and precursor whistlers are often seen simultaneously with anisotropic electron distributions unstable to the whistler heat flux instability. The IP shock with upstream shocklets showed much stronger electron heating across the shock ramp than the four events without upstream shocklets. These results may offer new insights into collisionless shock dissipation and wave-particle interactions in the solar wind.

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Measurements provided by the Magnetometer and the Extreme Ultraviolet Monitor (EUVM) on board the Mars Atmosphere and Volatile EvolutioN (MAVEN) spacecraft together with atomic H exospheric densities derived from numerical simulations are studied for the time interval from October 2014 up to March 2016. We determine the proton cyclotron waves (PCWs) occurrence rate observed upstream from Mars at different times. We also study the relationship with temporal variabilities of the high‐altitude Martian hydrogen exosphere and the solar EUV flux reaching the Martian environment. We find that the abundance of PCWs is higher when Mars is close to perihelion and decreases to lower and approximately constant values after the Martian Northern Spring Equinox. We also conclude that these variabilities cannot be associated with biases in MAVEN's spatial coverage or changes in the background magnetic field orientation. Higher H exospheric densities on the Martian dayside are also found when Mars is closer to perihelion, as a result of changes in the thermospheric response to variability in the ultraviolet flux reaching Mars at different orbital distances. A consistent behavior is also observed in the analyzed daily irradiances measured by the MAVEN EUVM. The latter trends point toward an increase in the planetary proton densities upstream from the Martian bow shock near perihelion. These results then suggest a method to indirectly monitor the variability of the H exosphere up to very high altitudes during large time intervals (compared to direct measurements of neutral particles), based on the observed abundance of PCWs.