Ionospheric longitudinal variations at midlatitudes: Incoherent scatter radar observation at Millstone Hill

Abstract

Incoherent scatter radar (ISR) extra-wide coverage experiments during the period of 1978-2011 at Millstone Hill are used to investigate longitudinal differences in electron density. This work is motivated by a recent finding of the US east-west coast difference in TEC suggesting a combined effect of changing geomagnetic declination and zonal winds. The current study provides strong supporting evidence of the longitudinal change and the plausible mechanism by examining the climatology of electron density Ne on both east and west sides of the radar with a longitude separation of up to 40 o for different heights within 300-450 km. Main findings include: 1) The east-west difference can be up to 60% and varies over the course of the day, being positive (East side Ne > West side Ne) in the late evening, and negative (West side Ne > East side Ne) in the pre-noon. 2) The east-west difference exists throughout the year. The positive (relative) difference is most pronounced in winter; the negative (relative) difference is most pronounced in early spring and later summer.3) The east-west difference tends to enhance toward decreasing solar activity, however, with some seasonal dependence; the enhancements in the positive and negative differences do not take place simultaneously. 4) Both times of largest positive and largest negative east-west differences in Ne are earlier in summer and later in winter. The two times differ by 12-13 h, which remains constant throughout the year. 5) Variations at different heights from 300-450 km are similar. Zonal wind climatology above Millstone Hill is found to be perfectly consistent with what is expected based on the electron density difference between the east and west sides of the site. The magnetic declination-zonal wind mechanism is true for other longitude sectors as well, and may be used to understand longitudinal variations elsewhere. It may also be used to derive thermospheric zonal winds.

Variations of the ionospheric TEC using simultaneous measurements from the China Crustal Movement Observation Network

“…The daytime density is normally lower in the east-most site than in the west site, and this turns opposite during the nighttime. These results agree well with the recent studies (Zhang et al, 2011(Zhang et al, , 2012. These longitudinal differences are caused by the difference in magnetic declination which gives rise to upward and downward ion drifts across the zero declination for a given thermospheric zonal wind direction.…”

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

“…It was suggested that this marked longitudinal differences in ionospheric TEC over the continent US and Ne in the F‐region heights over the eastern US at midlatitudes was caused by a combined effect of the varying declination and thermospheric zonal winds [ Zhang et al , 2011, 2012]. The longitudinal variation of magnetic declination (including its reversal in the central US) and the climatology in the zonal winds (which vary with local time, season and solar activity) could explain the observations.…”

Seasonal dependence of the longitudinal variations of nighttime ionospheric electron density and equivalent winds at southern midlatitudes

“…In both the northeast of East Asia and the continental US, the electron density diurnal variations show some similarity to the WSA in summer (Lin et al, 2009;Lin et al, 2010). Interestingly, Zhang et al (2011Zhang et al ( , 2012a showed that in the North American sector, the most significant longitudinal difference occurs during nighttime in winter and the solar minimum, which is possibly associated with the strongest nighttime zonal winds. Their results present a vital contribution of the zonal wind to electron density longitudinal variations under special magnetic configuration, i.e., a magnetic declination change between westward (negative) and eastward (positive) at midlatitudes.…”

“…Similar but much weaker summer evening enhancements also occur in the North American sector and East Asian sector (Lin et al, 2009(Lin et al, , 2010, known as midlatitude summer nighttime anomaly (MSNA). Several mechanisms are applied to explain these interesting phenomena, including the equatorward neutral wind, electric field, photoionization and the downward plasma diffusion from the plasmasphere (Burns et al, 2008(Burns et al, , 2011Jee et al, 2009;He et al, 2009;Chen et al, 2011Chen et al, , 2012Liu et al, 2010;Lin et al, 2009;Zhang et al, 2011Zhang et al, , 2012aZhao et al, 2013). Among those explanations, the magnetic configuration and the neutral winds are considered as important and even major contributors.…”

“…It can occur at wide latitudinal locations (∼ 30-70 • S) in summer in the Southern Hemisphere, and the strongest summer evening enhancement of F2-layer peak density occurs centered around 50-60 • S, 90 • W at midlatitudes (He et al, 2009;Lin et al, 2010;Liu et al, 2011;Burns et al, 2011). In recent years, longitudinal differences of electron density, or the density longitudinal asymmetries, have been studied over east-west regions of the continental US and the Far East area at northern midlatitudes (Zhang et al, 2011(Zhang et al, , 2012aZhao et al, 2013). In both the northeast of East Asia and the continental US, the electron density diurnal variations show some similarity to the WSA in summer (Lin et al, 2009;Lin et al, 2010).…”