Recent measurements by the incoherent scatter radar and by several other instruments in the polar cusp region at Sondre Stromfjord, Greenland, have shown features similar to previously published occurrences at lower, auroral-zone latitudes of high electric field related electron heating events ascribed to an E region plasma instability effect. Our results further substantiate previous suggestions on a plasma instability and high electric field intensities in the E region being closely related to the ionogram Slant E Condition (SEC) and its two main signatures: the slant sporadic E (diffuse E region backscatter) and the lacuna (abnormal E region radiowave damping). Our observations suggest E region absorption as a souce of weak riometer deflections, and as a possible basis for the creation of the lacuna-damping. The slant Es diffuse backscatter, most likely created by instability irregularities, has been found identical to diffuse radar aurora at HF and probably at higher frequencies too. It is suggested that the four phenomena, Slant E Condition, diffuse radar aurora, riometer E region absorption and E region electron heating events, are different, but often simultaneous effects related to a plasma instability and to high electric field and conductivity in the polar E region. It is emphasized that the routinely observed ionogram SEC phenomenon may be useful as a diagnostic tool for the study of the various associated phenomena mentioned above.
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