Characteristics of low‐latitude TEC during solar cycles 23 and 24 using global ionospheric maps (GIMs) over Indian sector

Abstract

The characteristics of quiet time equatorial and low-latitude total electron content over the Indian sector using global ionospheric map (GIM) data (1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013)(2014) are obtained filtering out the solar flare and storm effects. The results are examined and interpreted in the context of large number of previous studies. The newly found features from this study are as follows. Marked difference in nature of equinoctial asymmetry is noted between solar cycles 23 and 24. Long absence of winter anomaly both during low and high solar activity (HSA) in LL (low-latitude) regions is found. Climatology of the diurnal cycle is provided in four categories using new criteria for demarcation of solar activity levels. Highest correlation (~77%) between GIM ionospheric electron content (IEC) and PI (solar EUV proxy index) is noted over equator in contrast to previous studies. The minimum positive contribution of PI in variation of IEC requires minimum of 2 years of data, and if more than 7-8 years of data are used, it saturates. Root-mean-square width of PI can be used to define the HSA. Strong QBO (quasi-biennial oscillations) in IEC is noted in tune with the one in PI over both LL locations but QBO remains surprisingly subdued over equator. The semiannual oscillations in GIM-IEC are found to be stronger at all locations during high solar activity and weaker between 2005 and 2011, whereas the annual oscillations are found to be substantially stronger only during HSA-23 and weakest over southern LL location throughout 17 years.

Interhemispheric Asymmetry in Response of Low‐Latitude Ionosphere to Perturbation Electric Fields in the Main Phase of Geomagnetic Storms

“…In case if there were no background meridional winds the equatorial plasma fountain formed due to vertical E × B drift shall almost equally distribute the plasma in low-latitude regions in both the hemispheres. This has been an effective practice which is used in several previous reports to decipher the effects of a geomagnetic storm in comparison to quiet background over given latitude and time (Balan et al, 1995;Balan, Rajesh, et al, 2013;Dashora & Suresh, 2015;Huang et al, 2013;Luan et al, 2015;Rishbeth, 1972;Su et al, 1997). They have explained possible scenarios under which the asymmetry in VTEC can be formed by transport of plasma to very high altitudes in a particular hemisphere, which corroborates the well-known underlying mechanisms using numerical simulations and observations in effective manner.…”

“…To cover the EIA zone in this sector, geomagnetic latitudes between 30°north and 30°south in flanks of the dip equator are selected with separation of 2°l atitude. Bilinear interpolation is performed using nearest four GIM VTEC grid points surrounding the desired coordinate for a given UT, and then hourly time series is obtained again by bilinear temporal interpolation from subsequent GIM maps (Dashora & Suresh, 2015). GIM data are downloaded from the public web server at cddis.gsfc.nasa.gov and GIM prepared by Centre for Orbit Determination Europe is used.…”

“…Since the aim of this work is to highlight the simultaneous enhancements/depressions in TEC over several stations, VTEC curves for all the visible satellites are shown instead of deriving mean VTEC for a given time unlike many other reports. The quiet days are selected following Dashora and Suresh (2015) and the day-to-day variability is represented by the standard deviation (1 sigma). A three-layer feed-forward artificial neural network (ANN) with back propagation algorithm including eight inputs and five neurons with one hidden layer is indigenously developed.…”

“…Further, the instantaneous PPEF that originates in respective northern and southern polar regions traversing toward dip equator suffers geometric attenuation (Hashimoto et al, 2017;Kikuchi & Araki, 1979). Simultaneous observations of asymmetry in hemispheric distribution of electron density or TEC in the EIA (equatorial ionization anomaly) zone during quiet periods have been explained by seasonal changes in background neutral meridional wind across hemispheres (Balan et al, 1995;Dashora & Suresh, 2015;Huang et al, 2013;Luan et al, 2015;Rishbeth, 1972;Su et al, 1997). Similarly, the presence of LSTID (large-scale TIDs; Tsugawa et al, 2004) during the MP and its modulating impact on low-latitude electrodynamic processes needs quantifications.…”

Analysis of local ionospheric variability based on SVD and MDS at low-latitude GNSS stations

“…But in January 2013, winter solstice is remained subdued. Recent studies using GIM TEC model values have the same immense detections (Dashora and Suresh 2015;Raghunath and Ratnam 2015). The correlation between GPS-TEC and IRI model data for the first three singular Table 1.…”