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Lower hybrid cavities (LHC), i. e., small-scale density depletions coinciding with enhanced lower hybrid waves, are frequent in the upper ionosphere. It is found that they exist in all regions the Freja satellite has scanned, as long as lower hybrid waves are present. They are found most frequently at magnetic latitudes between 55° to 65°, preferably on the morning side. In the auroral oval, the LHCs do not occur as densely as at lower latitudes but they usually form larger structures.

Various electrostatic solitary structures are observed by the Freja satellite in the upper ionosphere in the auroral zone. The use of multiple probes makes the determination of their propagation characteristics possible. Some of them are possibly similar to the solitary waves and weak double layers previously observed in the magnetosphere, but the Freja examples move much slower. Their size is less than 100 meters. Another type is the lower hybrid cavities, density depletions with a lower hybrid wave packet inside. The depletions are about 5%, and they have little or no speed relative to the ambient plasma in the direction perpendicular to the geomagnetic field. A third type is related to the lower hybrid cavities, but inside the density depletion there is a solitary electrostatic structure, as well as lower hybrid waves.

Localized density depletions filled with electrostatic waves in the lower hybrid frequency range are commonly observed by the wave instrument on the Freja satellite. We refer to these phenomena by the phenomenological name Lower Hybrid Cavities (LHCs). Typically, the amplitude of the density depletion is a few per cent of the ambient plasma density, and its width is around 50 m. The structures are identified at all magnetic latitudes we have searched (60–75 degrees), and at all local times at the satellite altitude (around 1,700 km). Clear examples are found in regions with fairly low or moderate wave activity, but not where the highest wave amplitudes are encountered. We do not investigate the detailed small-scale correlation between LHCs and ion heating in this letter, but note that up to now, we have no LHC observations in intense ion heating regions.

Citation: Cairns, R. A., A. A. Mamun, R. Bingham, R. Bostrom, R. O. Dendy, C. M. C. Nairn, and P. K. Shukla (2011), Correction to “Electrostatic solitary structures in non‐thermal plasmas,” Geophys. Res. Lett., 38, L13309, doi:10.1029/2011GL048422. [1] In the paper “Electrostatic solitary structures in non‐ thermal plasmas” by R. A. Cairns et al. (Geophysical Research Letters, 22(20), 2709–2712, 1995) the second author’s name was misspelled. The correct spelling appears above.

A cesium-doped high explosion was detonated at 165 km altitude in the auroral ionosphere during quiet conditions. An Alfven wave pulse with a 200-mV/m electric field was observed, with the peak occurring 135 ms after the explosion at a distance of about 1 km. The count rate of fixed energy 2-keV electron detectors abruptly increased at 140 ms, peaked at 415 ms, and indicated a downward field-aligned beam of accelerated electrons. An anomalously high-field aligned beam of backscattered electrons was also detected. The acceleration is interpreted as due to production of an electrostatic shock or double layer between 300 and 800 km altitude. The structure was probably formed by an instability of the intense field-aligned currents in the Alfven wave launched by the charge-separation electric field due to the explosion.