Mid‐Latitude Spread‐F Structures Over the Geomagnetic Low‐Mid Latitude Transition Region: An Observational Evidence

Abstract

An observational evidence of a unique plasma depletion event was captured by an O( 1 D) 630.0 nm airglow imager on 13 June 2018 over a transition region of geomagnetic low-mid latitude, Hanle, Leh Ladakh, India (32.77°N, 78.97°E; Mlat.~24. 1°N). The observed plasma depletion structures are tilted at an angle of 13°± 2°west of the geomagnetic north and drifted toward west. Collocated Global Navigation Satellite System-Total Electron Content measurements confirm that the structures are indeed associated with TEC depletions. Simultaneous ionosonde measurements from Delhi, India (28.70°N, 77.10°E; Mlat.~20.2°N) shows spread-F signatures confirming that these structures are associated with the ionospheric irregularities. Interestingly, radar observations over the geomagnetic low-latitude station Gadanki, India (13.5°N; 79.2°E; Mlat.~6.5°N) reveal the absence of equatorial plasma bubbles on this night. Therefore, these observations strongly suggest that the observed structures in the airglow images over Hanle are associated with mid-latitude spread-F (MSF). These MSF structures are possibly affected by the shear in the zonal plasma drift that forces the field aligned plasma irregularity structures to tilt toward west. These observations, for the first time, bring out the presence of MSF structures over geomagnetic low-mid latitude transition region. It is suggested that the plasma distribution over low latitudes plays an important role in the occurrence of MSF structures over this transition region. Understanding the source and characteristics of the plasma irregularity structures over this transition region can help in understanding the spatio-temporal evolution of global L-band scintillation in a better way.Plain Language Summary Understanding the spatio-temporal distribution of the ionospheric plasma irregularities is important in the operational forecasting of L-band scintillation and therefore has important ramifications in the satellite-based communication and navigation systems. Traditionally, plasma irregularities in the low and mid-latitudes had received focused attentions in the past with very less attention has been paid over the low to mid-latitude transition region. The present investigation provides an attempt toward that direction and proposes a mechanism on the relationship between the plasma distribution over low latitudes and the occurrence of the mid-latitude plasma irregularities over the geomagnetic low-mid latitude transition region. Comprehensive investigations are further needed in the future to understand and characterize the ionospheric plasma irregularity structures over this region.

Evidence for the In‐Situ Generation of Plasma Depletion Structures Over the Transition Region of Geomagnetic Low‐Mid Latitude

“…Earlier investigation showed that the mid-latitude spread-F can be caused by the presence of wave undulation in the ionosphere alone or combination of E and F region coupling processes (Cosgrove, 2013 and reference therein;Earle et al, 2010). Recently, Sivakandan et al (2020);Yadav, Rathi, Gaur, et al (2021), and observed a mid-latitude spread-F/field aligned plasma structures over the low-mid-latitude transition region, Hanle and argued the possible supporting mechanism. However, to the best of our knowledge, there is no observational evidence that shows the onset and evolution of plasma depletion in the FOV of an airglow imager over the geomagnetic low-mid latitude transition region.…”

Westward Drift of Ionospheric Plasma Irregularities over a Low to Mid-latitude Transition Region in Indian Sector

“…where "s" represents the horizontal extension of the plasma irregularity under consideration [Sivakandan et al, 2020].…”

“…The question is whether the plasma depletions observed at latitudes > 20° N magnetic latitudes were the extension of equatorial plasma depletions or whether they were produced locally. Sivakandan et al [2020] has mentioned that the plasma depletion structures that appear over the low-mid latitude transition zone are difficult to classify in terms of their origin given that this region can be affected by both low and mid latitude processes in general. So, the detection of plasma depletion over such regions could be due to the equatorial plasma bubble mapping toward the low-latitudes, or it could be due to the mid-latitude irregularites extending toward the low-latitudes.…”

“…Airglow intensity modulations can occur in the mid-latitudes through gravity waves or under the action of Perkins instability mechanism. Gravity waves from the mesosphere-lower-thermosphere region (MLT-region) (~90-120 km) might penetrate into the F region of the ionosphere under suitable conditions of the background atmosphere [Sivakandan et al, 2020]. So, we analysed the 557.7 nm and 840 nm images corresponding to the peak altitudes of emission i.e.…”

“…Both the EPB and the MSF are capable of producing FAIs [Saito et al, 2008]. The EPBs tend to drift eastward, while the MSF structures tend to drift westward [Sivakandan et al 2020] and even though EPBs can map up to mid-latitudes, the propagation path can be used to distinguish between EPBs and MSF. The gradient drift instability is thought to be the most likely cause of FAIs on scales less than 50 km [Kelley et al, 2004;Otsuka et al, 2009].…”