Magnetic declination and zonal wind effects on longitudinal differences of ionospheric electron density at midlatitudes

A prominent ionospheric longitudinal variation at midlatitudes, in particular, over the continental US, was found recently. This variation is characterized as a higher east‐side electron density in the evening and a higher west‐side electron density in the morning, and with clear seasonal and solar activity dependencies. A combined effect of geomagnetic declination and changing zonal winds was proposed to explain it. This paper represents a comprehensive investigation of this effect by examining climatology for both electron density longitudinal differences and the nighttime zonal winds in the eastern US. Electron density is from incoherent scatter radar extra‐wide coverage experiments during 1978–2011 over Millstone Hill for which the spatial separation of the data can be up to 50° in longitude. The thermospheric zonal wind is from the on‐site Fabry‐Perot interferometer measurements during 1989–2001. The observed zonal wind climatology is found to be perfectly consistent with the expectation based on the east‐west electron density differences in terms of local time, seasonal, and solar cycle dependencies. The correlation between the zonal wind and the east‐west differential ratio is extremely high with an overall correlation coefficient of 0.93. The observed time delay of ∼3 hours in the response of electron density differences to zonal winds is a marked feature. Thus these results confirm positively the declination‐zonal wind mechanism and provide new insight into longitudinal variations at midlatitudes for other geographic sectors.