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

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 occurrence 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 (), which maps along geomagnetic field lines to the F1 region. There, forms thin plasma sheets that provide gradients to excite 3‐m‐scale plasma waves via an interchange process.