June Solstice Equatorial Spread F in the American Sector: A Numerical Assessment of Linear Stability Aided by Incoherent Scatter Radar Measurements

Previous studies have suggested that weakening downward plasma drifts can produce favorable conditions for the ionospheric Generalized Rayleigh‐Taylor (GRT) instability and explain the occurrence of postmidnight equatorial spread F (ESF). We evaluated this hypothesis using numerical simulations aided by measurements and attempted to explain ESF events observed in the American sector during June solstice, low solar flux conditions. We analyzed plasma drifts and ESF measurements made by the incoherent scatter radar of the Jicamarca Radio Observatory (11.95° S, 76.87° W, ∼1° dip). We found adequate measurements during a prototypical, quiet time event on 4–5 June 2008 when the downward drifts weakened and a fully developed ESF appeared. The measured drifts were used as input for the SAMI2 model. SAMI2 reproduced an “apparent” uplift of the ionosphere based on h ′F measurements that was consistent with expectations and observations. SAMI2 also provided parameters for estimation of the flux tube linear growth rates of GRT instability associated with the weakening drift event. We found that the weakening drifts did produce unstable conditions with positive growth rates. The growth rates, however, were slower than those obtained for typical, premidnight ESF events and those obtained for similar drift conditions in other longitude sectors. We show, however, that departures in the wind pattern, from climatological model predictions, can produce favorable conditions for instability development. Following the hypothesis of Huba and Krall (2013) and using SAMI2 simulations, we show that equatorward winds, when combined with weakening drifts, could have contributed to the unstable conditions responsible for the postmidnight ESF events. Key Points Analysis of a weakening downward drift event followed by June solstice, postmidnight ESF radar plume is presented Numerical analyses show that weakening drifts only weakly destabilize the F region in the American sector Weakening drifts and converging winds cause linear growth rates similar to those of postsunset ESF conditions