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.