Temporal and spatial variations in TEC using simultaneous measurements from the Indian GPS network of receivers during the low solar activity period of 2004–2005

Abstract

Abstract. With the recent increase in the satellite-based navigation applications, the ionospheric total electron content (TEC) and the L-band scintillation measurements have gained significant importance. In this paper we present the temporal and spatial variations in TEC derived from the simultaneous and continuous measurements made, for the first time, using the Indian GPS network of 18 receivers located from the equator to the northern crest of the equatorial ionization anomaly (EIA) region and beyond, covering a geomagnetic latitude range of 1° S to 24° N, using a 16-month period of data for the low sunspot activity (LSSA) years of March 2004 to June 2005. The diurnal variation in TEC at the EIA region shows its steep increase and reaches its maximum value between 13:00 and 16:00 LT, while at the equator the peak is broad and occurs around 16:00 LT. A short-lived day minimum occurs between 05:00 to 06:00 LT at all the stations from the equator to the EIA crest region. Beyond the crest region the day maximum values decrease with the increase in latitude, while the day minimum in TEC is flat during most of the nighttime hours, i.e. from 22:00 to 06:00 LT, a feature similar to that observed in the mid-latitudes. Further, the diurnal variation in TEC show a minimum to maximum variation of about 5 to 50 TEC units, respectively, at the equator and about 5 to 90 TEC units at the EIA crest region, which correspond to range delay variations of about 1 to 8 m at the equator to about 1 to 15 m at the crest region, at the GPS L1 frequency of 1.575 GHz. The day-to-day variability is also significant at all the stations, particularly during the daytime hours, with maximum variations at the EIA crest regions. Further, similar variations are also noticed in the corresponding equatorial electrojet (EEJ) strength, which is known to be one of the major contributors for the observed day-to-day variability in TEC. The seasonal variation in TEC maximizes during the equinox months followed by winter and is minimum during the summer months, a feature similar to that observed in the integrated equatorial electrojet (IEEJ) strength for the corresponding seasons. In the Indian sector, the EIA crest is found to occur in the latitude zone of 15° to 25° N geographic latitudes (5° to 15° N geomagnetic latitudes). The EIA also maximizes during equinoxes followed by winter and is not significant in the summer months in the LSSA period, 2004–2005. These studies also reveal that both the location of the EIA crest and its peak value in TEC are linearly related to the IEEJ strength and increase with the increase in IEEJ.

The comparative study of chaoticity and dynamical complexity of the low-latitude ionosphere, over Nigeria, during quiet and disturbed days

“…These effects can also be seen more clearly in the Tsallis entropy values for the five window period for quiet days of January 2011, because the night-time value is higher and also shows a much higher series of fluctuations during this period compared with other periods. As mentioned in Unnikrishnan and Ravindran (2010), the irregular changes in the dynamical characteristics of TEC from the results of the Lyapunov exponent and Tsallis entropy may also be due to the collisional Raleigh-Taylor instability that may give rise to a few large irregularities in L band measurements (Rama Rao et al, 2006;Sripathi et al, 2008). All these can be seen as internal factors responsible for variations in the dynamical response of TEC, as recorded from the values of the Lyapunov exponents and Tsallis entropy completed for days without storms, which might be quiet or disturbed according to classification and which could also account for higher values of these qualifiers during disturbed days compared with quiet days.…”

“…The fountain effect being controlled by the EXB drift shows the dynamics of the diurnal variation equatorial anomaly (Abdu, 1997;Unnikrishnan, 2010). There is a reduction in the F region ionization density at the magnetic Equator and much enhanced ionization density at the two anomaly crests within ±15 • of the magnetic latitude north and south of the Equator (Rama Rao et al, 2006).…”

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

“…Evidence for the electrodynamic coupling between the E-region and F-region ionosphere has also been found in plasma density data in the equatorial anomaly region. Studies have shown that the intensity and latitude of the equatorial anomaly crests vary with the intensity of the equatorial electrojet (Dunford 1967;MacDougall 1969;Rastogi and Rajaram 1971;Rush and Richmond 1973;Raghavarao et al 1978;Balan and Iyer 1983;Huang et al 1989;Rastogi and Klobuchar 1990;Rama Rao et al 2006;Stolle et al 2008). The most comprehensive study was made by Stolle et al (2008), involving 5 years of CHAMP electron density observations along with radar and geomagnetic field measurements.…”

On the performance of the IRI‐2012 and NeQuick2 models during the increasing phase of the unusual 24th solar cycle in the Brazilian equatorial and low‐latitude sectors

“…The morphological features of TEC such as the diurnal, seasonal, latitudinal, and solar activity variations have been investigated by several workers using various techniques over different parts of the globe [Lanyi and Roth, 1988;Goodwin et al, 1995;Abdu et al, 1996;Davies and Hartman, 1997;Mannucci et al, 1998;Afraimovich et al, 2001;Brunini et al, 2003;Jee et al, 2005;Rama Rao et al, 2006;Bagiya Mala et al, 2009;Prasad et al, 2012]. One of the most important electrodynamical features of the equatorial ionosphere is the double-humped latitudinal distribution of ionization centered at the dip equator, which is popularly known as the equatorial ionization anomaly (EIA) [Appleton, 1946].…”