Investigation of height gradient in vertical plasma drift at equatorial ionosphere using multifrequency HF Doppler radar

Abstract

[1] A multifrequency HF Doppler radar installed at the magnetic equatorial station Trivandrum provides an opportunity to study the height gradient in vertical plasma drift at the bottomside of equatorial F region during evening time. The multifrequency radar gives near-simultaneous observation of vertical plasma drift at three close by F region heights above the sounding station. The height gradient of the vertical drift shows a negative value during the prereversal enhancement (PRE) period and turns to positive value after the prereversal enhancement. The average height gradient in vertical plasma drift remains negative around PRE and its magnitude decreases with altitude, below F peak. This could be a clear-cut manifestation of the curl-free nature of the low-latitude electric field, and it could also indicate a partial signature of the postsunset velocity vortex at the equatorial F region. The magnitude of the mean height gradient around PRE exhibits a seasonal variation.

Electrodynamics of the equatorial F-region ionosphere during pre-sunrise period

“…These measurements appear to confirm the original Jicamarca study which found an occasional morningside enhancement in the vertical updrafting velocity at sunrise (Aggson et al, 1995). On seasonal averaging of both the Jicamarca data (Fejer, 1991) and the San Marco data, these morning-side enhancements appear to get averaged out and do not appear as a notable feature on the mean. Nicolls et al (2006) have shown that eastward electric field is not necessary to produce an upward enhancement but sometimes the uplift occurs as the ionospheric response to a decreasing westward electric field in conjunction with sufficient recombination.…”

“…As the observed drift is the sum of electrodynamic drift and apparent drift due to production, to extract the electrodynamic drift, the contribution due to ρH at every reflection height is calculated and is removed from the observed drift from 06:00 LT onwards (about half an hour prior to ground sunrise, at which the the value of foF2 starts increasing). The variation in the reflection heights of the three frequencies and the corresponding plasma drifts suggests the possible existence of the height gradient and velocity vortex during the pre-sunrise period similar to the one existing in the postsunset period (Nayar and Sreehari, 2004). The vertical drifts show an upward tendency from 05:30 LT onwards, reach a small maxima around 06:20 LT and then start decreasing.…”

“…The Multi-frequency HF Doppler radar installed at the University of Kerala is a coherent mono-static pulsed system capable of quadrature detection of ionospheric echoes (Nayar and Sreehari, 2004). It uses a single broadband transmitter and works with three fixed frequencies -2.5 MHz, 3.5 MHz and 4.5 MHz.…”

Relationships between electron density, height and sub-peak ionospheric thickness in the night equatorial ionosphere

“…7 by the spread in virtual velocity within each ionogram, seen most clearly in the bottom panel. The observed Doppler shift at each sounding frequency is an integrated function of ionisation change up to the reflection height (see discussion by Prabhakaran Nayar and Sreehari, 2004). In the present case, the variations in Doppler frequency and thus velocity appear to be dominated by changes in layer height.…”

Characteristics of the equatorial plasma drifts as obtained by using Canadian Doppler ionosonde over southern tip of India

“…Some notable instruments such as Jicamarca incoherent scatter radar and Jicamarca Unattended Long‐term Ionosphere and Atmosphere radar are often used to obtain equatorial zonal electric fields [e.g., Fejer et al ., ; Kudeki and Fawcett , ; Hysell and Burcham , ]. HF Doppler radars have also been used widely over Indian region to study the density irregularities and the drift motion of the ionosphere [e.g., Viswanathan et al ., ; Balan et al , ; Subbarao and Krishnamurthy , ; Sastri et al ., ; Prabhakaran Nayar and Sreehar , ; Reddi et al ., ]. These observations have proved that ionospheric drifts or electric fields are crucial to understand the electrodynamics of the ionospheric plasma irregularities and related variabilities in the ionosphere.…”