Auroral ion acceleration from lower hybrid solitary structures: A summary of sounding rocket observations

In this paper we present a review of sounding rocket observations of the io n acceleration seen in nightside auroral zone lower hybrid solitary structu res. Observations from Topaz3, Amicist, and Phaze2 are presented on various spatial scales, including the two-point measurements of the Amicist missio n. From this collection of observations we will demonstrate the following c haracteristics of transverse acceleration of ions (TAI) in lower hybrid sol itary structures (LHSS). The ion acceleration process is narrowly confined to 90 degrees pitch angle, in spatially confined regions of up to a few hun dred meters across B, The acceleration process does not affect the thermal core of the ambient distribution and does not directly create a measurable effect on the ambient ion population outside the LHSS themselves. This prec ludes observation with these data of any nonlinear feedback between the ion acceleration and the existence or evolution of the density irregularities on which these LHSS events grow. Within the LHSS region the acceleration pr ocess creates a high-energy tail beginning at a few times the thermal ion s peed. The ion acceleration events are closely associated with localized wav e events. Accelerated ions bursts are also seen without a concurrent observ ation of a localized wave event, for two possible reasons. In some cases,th e pitch angles of the accelerated tail ions are elevated above perpendicula r; that is, the acceleration occurred below the observer and the mirror for ce has begun to act upon the distribution, moving it upward from the source . In other cases, the accelerated ion structure is spatially larger than th e wave event structure, and the observation catches only the ion event. The occurrence rate of these ion acceleration events is related to the ambient environment in two ways: its altitude dependence can be modeled with the p arameter B-2/n(e), and it is highest in regions of intense VLF activity. Th e cumulative ion outflow from these LHSS TAI is consistent with Freja stati stics for VLF-type premidnight auroral upflow.