Sporadic E: Irregular scattered patches of relatively dense ionization that develop seasonally within the E region and that reflect and scatter frequencies up to 150 MHz.
Note 1: The sporadic E is a regular daytime occurrence over the equatorial regions and is common in the temperate latitudes in late spring, early summer and, to a lesser degree, in early winter.
Note 2: At high, i.e., polar, latitudes, sporadic E can accompany auroras and associated disturbed magnetic conditions.
Note 3: The sporadic E can sometimes support reflections for distances up to 2,400 km at frequencies up to 150 MHz. Synonym sporadic E propagation.
Source: from Federal Standard 1037C