Observations of lower hybrid cavities in the inner magnetosphere by the Cluster and Viking satellites

Abstract

Abstract.Observations by the Viking and Cluster satellites at altitudes up to 35 000 km show that Lower Hybrid Cavities (LHCs) are common in the inner magnetosphere. LHCs are density depletions filled with waves in the lower hybrid frequency range. The LHCs have, until recently, only been found at altitudes up to 2000 km. Statistics of the locations and general shape of the LHCs is performed to obtain an overview of some of their properties. In total, we have observed 166 LHCs on Viking during 27 h of data, and 535 LHCs on Cluster during 87 h of data. These LHCs are found at invariant latitudes from the auroral region to the plasmapause. A comparison with lower altitude observations shows that the LHC occurrence frequency does not scale with the flux tube radius, so that the LHCs are moderately rarer at high altitudes. This indicates that the individual LHCs do not reach from the ionosphere to 35 000 km altitude, which gives an upper bound for their length. The width of the LHCs perpendicular to the geomagnetic field at high altitudes is a few times the ion gyroradius, consistent with observations at low altitudes. The estimated depth of the density depletions vary with altitude, being larger at altitudes of 20 000-35 000 km (Cluster, 10-20%), smaller around 1500-13 000 km (Viking and previous Freja results, a few percent) and again larger around 1000 km (previous sounding rocket observations, 10-20%). The LHCs in the inner magnetosphere are situated in regions with background electrostatic hiss in the lower hybrid frequency range, consistent with investigations at low altitudes. Individual LHCs observed at high altitudes are stable at least on time scales of 0.2 s (about the ion gyro period), which is consistent with previous results at lower altitudes, and observations by the four Cluster satellites show that the occurrence of LHCs in a region in space is a stable phenomenon, at least on time scales of an hour.

Nonlinear ion-acoustic (IA) waves driven in a cylindrically symmetric flow

“…The magnetosphere was highly compressed due to an intense injection of electrons ∼14 min earlier (Baker et al 2002), which could enhance the local magnetic field from a few mG of the background strength to tens even hundreds of mG (Alfaro et al 2004;Tjulin et al 2004;Hall et al 2009). Although the Cluster has no single probe measurements to give nonlinear wave speeds, all four panels reveal that they excellently recorded all the bursts by double probes of high resolutions Cattell et al 2003).…”

Formation of lower-hybrid solitary structures by modulational interaction between lower-hybrid and dispersive Alfvén waves

“…ion transverse heating (Knudsen et al, 2004), set in before the collapse is complete. An interesting question not explicitly addressed in the present work is if the inclusion of electron thermal effects introduces a preferred formation scale size transverse to the magnetic field on the order of the ion sound gyroradius, ρ s , as LHSS typically show a width of this order (Knudsen et al, 2004;Tjulin et al, 2004a). However, as the collapse in our simulations proceeds through all scales and no stationary stage is reached, we believe that the perpendicular size is determined by the processes leading to arrest of the collapse rather than by the formation mechanisms studied here.…”

“…These elongated density structures with localized waves in the LH frequency range are termed LH cavities (LHC) or LH solitary structures (LHSS). The relative decrease in the plasma number density inside the cavities is typically a few percent at altitudes of 1500-13 000 km (Dovner et al, 1997;Tjulin et al, 2004a) to a few tens of percent below 1500 km (Kintner et al, 1992;Knudsen et al, 2004) and above 20 000 km (Tjulin et al, 2004a). The perpendicular width is observed to be of the order of the ion gyroradius and to scale with this parameter better than with the electron inertial length (Knudsen et al, 2004).…”

“…Spacecraft measurements have shown that localized bursts of lower hybrid (LH) waves are common events in the ionosphere (LaBelle et al, 1986;Kintner et al, 1992;Eriksson et al, 1994;Pécseli et al, 1996;Bonnell et al, 1998) and in the magnetosphere (Tjulin et al, 2003;Tjulin et al, 2004a). It has been confirmed that the wave activity coincides with density depletions that are elongated along the geomagnetic Correspondence to: J. O.…”

“…The plasma β is extremely low in all plasmas where LHSS have been observed, in the ionosphere (Schuck et al, 2003) and (with less margin) in the magnetosphere (Tjulin et al, 2004a). This means that parallel electron kinetics effects, which give rise to dispersion on the ρ s scale, are potentially more important than effects of electron inertia, depending on which length scales are excited.…”