Lower-hybrid cavity density depletions as a result of transverse ion acceleration localized on the gyroradius scale

[1] We explore a mechanism by which density depletions associated with lower-hybrid cavities and similar structures are a direct consequence of ion trajectories resulting from transverse energization localized on scales comparable to the ion gyroradius. Specifically, a heating region localized to a flux tube having a diameter of the order of a few ambient thermal ion gyroradii will be depleted of ions because of heated-gyroradius-scale excursions of the ions away from the heating region. Cooler ions outside will have smaller average gyroradii and will be unable to compensate for the depletion inside the heating region. The outflux of energized ions from the heated region results in a density enhancement at its periphery. Motivated by space observations, we characterize density perturbations resulting from an idealized, cylindrically symmetric Gaussian temperature enhancement, using Monte Carlo simulations and an orbit-averaged density calculation. Our model generates density depletions of the order of 10% for a two-fold increase in Ti over the background value and 50% for a fifty-fold increase. In addition to density depletions the model predicts density-enhanced shoulders around cavity perimeters and shows how heated ion tails superimposed on cool ionospheric plasmas can be explained by hot, Maxwellian ions originating on nonlocal flux tubes. We present examples of shoulders observed by sounding rockets and by the Freja satellite. Finally, we use sounding rocket measurements to demonstrate that the distribution of cavity widths (or chord lengths) is relatively narrow (16 ± 10 m), with an average width that remains nearly unchanged even as plasma density varies over five orders of magnitude. This result is consistent with gyroradius being the key scaling parameter, although it does not rule out other scaling predictions that do not depend on plasma density.