Occurrence of blanketing E_s layer (E_(sb)) over the equatorial region during the peculiar minimum of solar cycle 24

摘要

A thin and highly dense sporadic E layer, which can occasionally block the upper ionospheric layers, is called blanketing sporadic E (E_(sb)). We present the statistical seasonal local time occurrence pattern of E_(sb) at equatorial station Tirunelveli (8.7° N, 77.8° E, dip latitude 0.7° N) during the extended minimum of solar cycle 24 (2007-2009). In spite of nearly the same average solar activity during both 2007 and 2009, considerable differences are noticed in the seasonal occurrence of E_(sb) during this period. The percentage of E_(sb) occurrence is found to be the highest during the summer solstice (≥ 50 %) for both 2007 and 2009, which is in general accordance with the earlier studies. The occurrences of E_(sb) during the vernal equinox (～ 33 %) and January-February (～ 28 %) are substantial in 2009 as compared to those during the same seasons in 2007. We find that, during winter (January-February), ～ 75 % of E_(sb) occurred during or just after the period of sudden stratospheric warming (SSW). We suggest that enhanced E_(sb) occurrence during winter (January-February) and the vernal equinox of 2009 could be associated with SSW-driven changes in the E region ambient conditions. Furthermore, the close association of E_(sb) with counter equatorial electrojet (CEEJ) suggested by earlier studies is re-examined carefully using the scenario of E_(sb) occurrence on non-CEEJ days. Such an exercise is crucial as we are unaware whether the physical mechanisms driving E_(sb) and CEEJ are linked or not. We find that, of all the seasons, the association of E_(sb) and CEEJ is strongest during winter (November-December).