Sporadic E: Current views and recent progress

Sporadic E (E_s) and related processes are reviewed as functions of viewing system, latitude and altitude. We find that the "windshear theory" - perhaps with a small added external electric field - appears sufficient along with the tidal wind system to explain the mid-latitude layers which we refer to as tidal ion layers (TILs) - layers that have often been identified as sporadic E or as sequential sporadic E. Additionally, it has become clear that the high-latitude, altitude-narrow layers, also often identified as sporadic E, are consistently explained as being formed in appropriate large-scale convective electric field structures with the wind system playing a lesser role. Finally, we find that "true" sporadic E - an altitude-thin E region layer at an unpredictable altitude and/or an unexpected intensity - is found in a rich context of related phenomena that includes the tidal ion layers, electric-field-induced layers, HF/VHF radar quasi-periodic echoing (QPE) regions, and an apparently newly observed phenomenon descriptively termed "ion rain." We conclude that the QPEs and "ion rain" indicate small horizontal scales and find considerable other evidence of order 10-100 km scale horizontal-structuring of layers which at least hints at an E/F region coupled-electrodynamic process, or processes. These processes apparently include instability-generated 10-100 km horizontal-scale E-fields that - we hypothesize - generate true sporadic E and the related complex layer structures (CLS) via horizontal redistribution of ions.