On a summer maximum in the occurrence frequency of 150 km (F1 radar echoes over Pohnpei

Abstract

Using three and a half years of 50 MHz radar data collected from Pohnpei, Federated States of Micronesia (6.96°N, 158.19°E geographic, 0.3° magnetic dip latitude), we show for the first time that the occurence frequency of 150 km (or F1) echoes has a broad but conspicuous maximum during northern summer in this longitude sector with minimal activity in other months. Given a seasonal similarity to sporadic E (Es), we suggest that an Es-layer instability [Cosgrove and Tsunoda, 2002] generates a polarization electric field (→E), which maps along geomagnetic field lines to the F1 region. There, →E forms thin plasma sheets that provide gradients to excite 3-m-scale plasma waves via an interchange process. Copyright 2004 by the American Geophysical Union.