2.1 Differential method
Ionospheric perturbations can be computed by numerically differentiating the TEC between two successive epochs known as differential TEC (dTEC) (Liu et al. 2004, 2006 and Catherine et al. 2015). This method is widely used to study the traveling ionospheric perturbations caused by forcing from below the ionosphere like tsunami, earthquakes, volcanic eruptions, cyclones, rocket launching, nuclear detonation, etc.
– (1)
where is leveled vertical TEC along the ray path of a Satellite(S)-Receiver(R) pair at epoch t, and is time difference between the two consecutive epochs.
– (2)
where and
Where is elevation of satellite; sTEC is leveled slant TEC;TECϕ and TECP are TEC computed from phase and code measurements, respectively;Re is radius of the Earth andhmax is ionospheric shell height.
The sTEC along the ray path of satellite-receiver link from ground based GPS observations were computed using the software “IONODETECT” developed at CSIR-4PI (Vijayan et al., 2013; Catherine et al., 2015). IONODETECT computes sTEC by carrying out phase leveling along the phase connected arc for each satellite-receiver link using both code and phase measurements to remove carrier phase ambiguity. Further, the effect of slant ray path is corrected by mapping the sTEC to vertical TEC (vTEC), by assuming a single-layer model (Astafyeva et al., 2015; Jakowski and Hoque, 2019) where the ionosphere is approximated by a thin shell with a maximum ionization at a height of 350 km (Rao et al., 2006). The dTEC is measured in TECU s-1 (1 TECU = 1016electrons/m2).
In this method, the time-varying distance between the successive IPPs or sampling points are not accounted while computing dTEC (Eq. 1). The unaccounted inter-IPP distances alter the amplitude of the detected perturbation and alias as signal (Artru et al., 2005). Removing such signal aliases, manually, would be a laborious and time consuming process. In order to alleviate this problem, Galvan et al. (2011) introduced a residual method in which the ionospheric perturbations are obtained as residual TEC (rTEC) by detrending the regular characteristic variations of TEC along the IPP track formed by satellite-receiver link.