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(c) American Geophysical Union, reprinted with permission. Article also available at source: https://doi.org/10.1002/2016JA023080

[1] We have analyzed Cluster magnetic field and plasma data during high-altitude cusp crossing on 14 February 2003. Cluster encountered a diamagnetic cavity (DMC) during northward interplanetary magnetic field (IMF) conditions, and as IMF rotated southward, the spacecraft reencountered the cavity more at the sunward side. The DMC is characterized by a high level of magnetic field fluctuations and high-energy electrons and protons. Ultralow-frequency turbulence has been suggested as a mechanism to accelerate particles in DMC. We demonstrate in this paper for the first time that many of the low-frequency fluctuations in the cavity are back and forth motion of the DMC boundaries over the spacecraft and transient reconnection signatures. We also find examples of some isolated high-amplitude waves that could possibly be nonlinear kinetic magnetosonic modes. The lack of strong wave power at the vicinity of local ion cyclotron frequency in the DMC suggests that perhaps a mechanism other than wave-particle heating is a dominant source for ion heating in DMCs.

[1] We analyze a hot flow anomaly (HFA) event that the Cluster spacecraft encountered on 22 April 2008. HFAs are regions of hot, highly deflected ion flow near planetary bow shocks that are created from interactions between tangential discontinuities in the solar wind and the bow shock. This HFA event is studied in detail because it is flanked by suprathermal ion beams that contain signatures of particle dispersion and where the particles have a nongyrotropic distribution. The composition of the beam ions is similar to that of the solar wind, and it implies that the beams consist of reflected solar wind plasma. The particle pitch angles in the beam are found to evolve over time so that the pitch angle decreases closer to the discontinuity. A simple model of particle motion at a tangential discontinuity is developed, which successfully predicts many of the observed features of the particle beams. The model also shows that the best conditions for formation of similar beams occur when the field rotation in the tangential discontinuity corresponding to the HFA is between 25° and 40° in the case where the magnetic field magnitude is the same on both sides of the discontinuity. The particle beams are coincident with observations of “1-Hz waves” that are frequently found in the foreshock region. On the leading side of the HFA event, these waves are shown to be generated by the beam, while on the trailing side, the properties of the waves indicate that they are whistler waves generated at the bow shock through electron dynamics and propagating upstream. The second mechanism is the standard one that has been used to explain most observations of the 1-Hz waves in the literature.

[1] Low-frequency wave properties inside two hot flow anomalies (HFAs) at different stages of evolution are, for the first time, studied applying the k-filtering technique on multipoint measurements from the Cluster satellites. The observed wave activity in an HFA cavity in an early stage of its evolution was interpreted as the combination of inherent fluctuation in the solar wind and those of a plasma component specularly reflected at the Earth's bow shock, where the amplitude of the fluctuations had been enhanced by a plasma beam instability. The wave field of a more evolved HFA was found to be less complex but contained a periodicity in the wave number distribution with a period that is suggested to come from the geometry of the HFA cavity.