A review of lower hybrid solitary structures

The subject of lower-hybrid solitary structures (LHSS), sometimes called tower-hybrid cavities, is reviewed. LHSS are spatially localized structures embedded in space plasmas. They are characterized by localized electric field fluctuations at frequencies both above and below the ambient lower-hybrid resonance in a density depletion of a few percent to several tens of percent. The localized fluctuations are more intense than the ambient turbulence by a factor of three to five. The LHSS are observed to have dimensions across the magnetic field of a few to many thermal ion gyroradii, usually 10-100 m in the topside auroral ionosphere. Along the magnetic field the dimensions are inferred to be several kilometers to several hundred kilometers although there exist no direct measurements. Analysis of the electric field fluctuations within LHSS using plasma wave interferometers reveals that the phase fronts rotate relative to the geomagnetic field at the center of the LHSS: right-handed at frequencies above and left-handed at frequencies below the ambient lower-hybrid resonance. Within a gyrodiameter of LHSS, ions are found to be preferentially heated perpendicular to the geomagnetic field and local transit time acceleration is the likely mechanism for energization. Two classes of theories have been applied to LHSS: linear theories that assume that the density depletions are preexisting and use the observed spatial properties of the density depletions to compute the propagation characteristics of lower-hybrid waves, and nonlinear, strongly turbulent, collapse theories which assume that the density depletion and the electric field fluctuations are directly coupled through a ponderomotive force. The linear scattering theories successfully explain the rotational and frequency spectrum of LHSS electric field fluctuations. The nonlinear theories predict relations between field strength and cavity size, cavity spatial distributions, and electric field amplitude statistical distributions. None of these nonlinear predictions are validated by observations. Most observations of LHSS have been made by sounding rockets or satellites in the auroral ionosphere. However, recent observations demonstrate that LHSS exist at much higher altitudes in the magnetosphere. Overall, this evidence suggests that LHSS are a universal feature of whistler mode turbulence.