Simultaneous observation of sporadic E with a rapid-run ionosonde and VHF coherent backscatter radar

Abstract. During the SEEK 2 rocket campaign, ionograms were recorded every minute at the Yamagawa Radio Observatory at about 90km west of the region monitored by a VHF (very high frequency) coherent backscatter radar. Sporadic E-layer parameters, which include the critical (foEs) and blanketing (fbEs) frequencies, the layer height (h'Es), and the width of the range spread of sporadic E-traces, were compared with RTI (range-time-intensity) plots of VHF quasi-periodic (QP) and continuous coherent backscatter echoes. A close relationship was found between the appearance of QP echoes in the RTI plots and the level of spatial inhomogeneity in sporadic E plasma, signified here by the difference between foEs and fbEs. During QP echo events, foEs increased while fbEs decreased, so that the difference foEs-fbEs was enhanced, indicating the development of strong spatial structuring in electron density within a sporadic E-layer. On the other hand, increases in sporadic E range spreading also correlated with the occurrence of QP echoes but the degree of correlation varied from event to event. Continuous radar echoes were observed in association with low altitude sporadic E-layers, located well below 100 km and at times as low as 90 km. During the continuous echo events, both foEs and fbEs were less variable, and the difference foEs-fbEs was small and not as dynamic as in the QP echoes. On the other hand, the Es-layer spread intensified during continuous echoes, which means that some patchiness or corrugation in those low altitude layers is also necessary for the continuous backscatter echoes to take place.