On the electric field control of the MF radar scatterers in the lower E region over the magnetic equator

Abstract

Medium frequency (MF) spaced antenna radar observations from Tirunelveli (8.7°N, 77.8°E, geographic; 1.4°N magnetic dip) are used to examine the characteristics of lower E‐region echoes and their possible association with the electrodynamical processes that drive the equatorial electrojet (EEJ). For the period (January 2006) under study, intense echoes were observed at 90 km and above at times of strong EEJ, indicating a possible electric field control of the radar scatterers. Under strong EEJ conditions, smaller pattern lifetimes, larger pattern scale sizes and axial ratios and strong pattern alignment along the geomagnetic field are routinely observed during this period. During the afternoon counter electrojet conditions, the electric field control of radar scatterers appears to weaken and the echo intensities are diminished. The work presented here calls for a detailed investigation that should address fundamental issues like the nature of the scatterers responsible for the MF radar echoes and their underlying generation mechanisms.

Lower E-region echoes over the magnetic equator as observed by the MF radar at Tirunelveli (8.7° N, 77.8° E) and their relationship to <I>E<sub>sq</sub></I> and <I>E<sub>sb</sub></I>

“…On the other hand, when E sq was absent, the fading rates were too small implying slow fading, the drift direction was reversed to the east and the ground diffraction pattern was almost isotropic. The recent reports that made use of the MF radar observations from Tirunelveli were in conformity to the earlier HF spaced receiver findings (Gurubaran and Rajaram, 2000;Ramkumar et al, 2002;Gurubaran et al, 2007).…”

“…On certain days, the dynamical parameters revealed close connection to EEJ, whereas on certain other days the drifts at 98 km were different from those expected to be of EEJ origin. These peculiarities in observed drift motions were also noticed in the later work by Gurubaran et al (2007). Systematic investigations with colocated experiments like digital ionosonde and coherent VHF backscatter radar would provide various clues to understanding the complexities in the MF radar observations made at the magnetic equator.…”

“…Because the variation in the drift velocities, especially in the zonal component, often correlates well with H , the measured zonal drifts at these heights are expected to be dominated by the electric field that drives the EEJ. It is puzzling that there are several occasions when the zonal velocities are eastward that cannot be attributed to the electric field drive alone (Ramkumar et al, 2002;Gurubaran et al, 2007). During normal EEJ conditions and at EEJ heights, the vertical electric field is upward and drives the EEJ producing the westward electron drift motion.…”

“…Previous studies reported on the behavior of SNR and geometrical parameters during the afternoon hours on CEJ days, wherein the geometrical parameters were shown to deviate from the expected behaviour (shorter pattern lifetimes, pattern elongation along north-south direction and low SNR values), soon after H reversed and the electric field control was lost (Ramkumar et al, 2002;Gurubaran et al, 2007).…”

“…The MF radar operating at Tirunelveli has been providing useful data for more than a decade on mean winds, tides and planetary waves. Previous work using data from this site clearly revealed that the MF radar echoes at 90 km and above are influenced by electrodynamical processes (Gurubaran and Rajaram, 2000;Ramkumar et al, 2002;Gurubaran et al, 2007). For radar systems operating in the vicinity of the magnetic equator, complexities in interpreting the drift measurements in terms of neutral winds arise due to the presence of the equatorial electrojet (EEJ) that plays a dominant role in the generation and movement of electron density irregularities in the lower E region (Kelley, 1989).…”