On the day‐to‐day variation of the equatorial electrojet during quiet periods

Abstract

It has been known for a long time that the equatorial electrojet varies from day to day even when solar and geomagnetic activities are very low. The quiet time day‐to‐day variation is considered to be due to irregular variability of the neutral wind, but little is known about how variable winds drive the electrojet variability. We employ a numerical model introduced by Liu et al. (2013), which takes into account weather changes in the lower atmosphere and thus can reproduce ionospheric variability due to forcing from below. The simulation is run for May and June 2009. Constant solar and magnetospheric energy inputs are used so that day‐to‐day changes will arise only from lower atmospheric forcing. The simulated electrojet current shows day‐to‐day variability of ±25%, which produces day‐to‐day variations in ground level geomagnetic perturbations near the magnetic equator. The current system associated with the day‐to‐day variation of the equatorial electrojet is traced based on a covariance analysis. The current pattern reveals return flow at both sides of the electrojet, in agreement with those inferred from ground‐based magnetometer data in previous studies. The day‐to‐day variation in the electrojet current is compared with those in the neutral wind at various altitudes, latitudes, and longitudes. It is found that the electrojet variability is dominated by the zonal wind at 100–120 km altitudes near the magnetic equator. These results suggest that the response of the zonal polarization electric field to variable zonal winds is the main source of the day‐to‐day variation of the equatorial electrojet during quiet periods.

Sq and EEJ—A Review on the Daily Variation of the Geomagnetic Field Caused by Ionospheric Dynamo Currents

“…Thus, perturbations in the wind-driven currents at low latitudes negatively correlate with perturbations in the equatorial electrojet. As noted by Yamazaki et al (2014a), the current system in Fig. 14b explains only less than 25 % of the total current shown in Fig.…”

“…The results were often taken as evidence that the equatorial electrojet is not a simple enhancement of Sq currents and hence other driving mechanisms exist for the EEJ. Yamazaki et al (2014a) noted, however, that the poor correlation in H between equatorial and off-equatorial stations can be reproduced by a numerical model in which the equatorial electrojet is treated as a part of the global wind dynamo current system. Therefore, different behaviors of H at equatorial and off-equatorial stations do not necessarily mean that Sq and EEJ have separate driving mechanisms.…”

“…The answer is "Yes". Figure 14a, from a simulation study by Yamazaki et al (2014a), shows the midday cross section of the eastward current density over the Indian sector, averaged over the period from 3 May through 30 Jun 2009. The equatorial electrojet is evident as an enhanced eastward current flow over the magnetic equator with the peak current density approximately 5 × 10 −6 A/m 2 at 105 km.…”

“…Later studies, using improved dynamo models with more realistic neutral winds, have also underscored the importance of lower-atmospheric drivers in the short-term variability of the ionospheric dynamo (Kawano-Sasaki and Miyahara 2008;Jin et al 2011;Fang et al 2013; Yamazaki et al 2014a). Among those studies, Yamazaki et al (2014a) attempted to clarify where the day-to-day variability of the equatorial electrojet is produced. They compared the current density of EEJ with neutral wind velocities at various latitude, longitude, and height.…”

“…The left panel of Fig. 43 (after Yamazaki et al 2014a) shows the correlation between the noon-time EEJ and eastward wind velocity in the Indian sector. The most significant correlation (R = −0.72) is found at ∼ 110 km near the magnetic equator.…”