FIG. 8.— Top panels: CME morphology observed from COR2-A, C3, and COR2-B respectively at 10:54UT on April 11, 2013. Bottom panels: Overplot of the best fitted wireframe of MFR in GCS model. Note that in C3 the CME is halo directed toward Earth, whereas it directs away from COR-A on farside. Note the GCS parameters \( \gamma \) = −45o, \( _ \alpha \) = 50o, κ = 0.45 and \( h = 1 7 . 5 R _ { \odot } \) reproduce this CME morphology.

extremely difficult. However, to examine the effect of uncer-tainties in the estimated kinematics, we considered an error of 5 pixels in the measurements of elongation angle. Such an error corresponds to uncertainties of 0.02 degree, 0.1 degree and 0.35 degree in the derived elongation angles in COR2, HI1 and HI2 FOV, respectively. Corresponding to these elon-gation uncertainties, the calculated error for SSSE (with \( \lambda \) = 0) method maximally reaches up to \( 0 . 5 R _ { \odot } \) in distance, less than few degree in direction and few tens of km/s for speed. These errors certainly seem to be small but of course, they do not reflect the total error in derived kinematics. Further study is required to quantify the actual errors because of several in-valid idealistic assumptions in the methods.

Also, at higher elongation (greater than approx. \( 4 0 R _ { \odot } \)),larger variations in the estimated kinematics from SSSE method for different \( \lambda \) are noted. From Figure 11, it is clear that the estimated distance from GT method increased around 18:00 UT on April 11 at \( 9 0 R _ { \odot } \) from the Sun. Such an un-physically fast increase in distance and speed is meaningless due to the absence of forces capable of accounting for this acceleration at distances farther from the Sun (Cargill 2004;Vršnak et al. 2010). This is more likely because of improper use of SSSE with \( \lambda \) equal to 0 \( r \)(i.e. GT) method, especially for this CME which is propagating away from the observer. This late acceleration is significantly reduced (still unphysi-cal) if a higher value of \( \lambda \) is considered. SSSE with \( \lambda \) equal to \( 9 0 ^ { o } \)(i.e. TAS) method gives the lowest limit of the esti-mated distance and speed values. The kinematics from the TAS method is calculated up to \( 3 6 5 R _ { \odot } \) from the Sun.

The aforementioned facts highlight that the assumptions made in the GT method are not valid for a CME propagat-ing away from the observer. Also, the spherical front approx-imation in SSSE method with \( \lambda \) equal to nonzero value, be-comes worse due to flattening of the CME front on its inter-action with solar wind. These limitations start to play a cru-cial role much nearer to the Sun for a far-sided CME than for front sided CME (Liu et al. 2013). However, the distance be-yond which these effects are crucial depends on the direction of propagation of the CME and its size. We note that for a CME propagating at larger angle from the Sun-observer line, its kinematics depend more on the chosen value of \( \lambda \). This is because the flanks (not nose) for such CMEs are always ob-served from the observer and even a little change in its radius of curvature leads to huge difference in the estimated kine-matics from the SSSE method. It is noted that if direction of propagation of a CME from sun-observer line becomes more than 90 \( \gamma o \), an unphysical acceleration will be estimated from the SSSE methods irrespective of chosen \( \lambda \) value. However, such an acceleration is reduced with higher value of \( \lambda \) and gives more accurate kinematics.

To assess the relative performance, we also applied single spacecraft reconstruction methods on STEREO-A and B ob-servations. We employed the Fixed-Phi (FP: Kahler & Webb 2007), Harmonic Mean (HM: Lugaz et al. 2009), Self-Similar Expansion (SSE: Davies et al. 2012) methods on the derived time-elongation variations of the CME (Figure 10). These

FIG. 9.— top row: Left, middle, and right panels show the evolution of 2013 April 11 CME in the running difference image of HI2-A, HI1-A, and COR2-A respectively. bottom row: Running difference images from COR2-B, HI1-B, HI2-B in Left, middle and right panels, respectively. The contours of the elongation angle (green) and the position angle (blue) are also deployed on each image. In all panels, horizontal red line is along the ecliptic and indicates the position angle of the Earth and vertical red line (left panel) refers \( \mathcal { P } \) position angle.

methods essentially convert the elongation into distance from the Sun assuming a fixed direction (longitude, here \( 1 . 3 ^ { o } \) East) of CME propagation as an input.

On applying the above methods on the STEREO-A and B observations, we noticed that estimated kinematics also largely overestimates the speed (approx. 900-1200 km/s even beyond \( 1 0 0 R _ { \odot } , \)) of the CME and the method becomes com-pletely unreliable once the CME reaches higher elongations. The direction of propagation of CMEs is \( 1 4 6 ^ { c } \) and \( 1 2 8 ^ { o } \) away from the line connecting the Sun with STEREO-A and B spacecraft, respectively. As the CME propagating in East-ward is little closer to 90ofrom the STEREO-B spacecraft, the derived kinematics and arrival time from this spacecraft are more accurate than using STEREO-A observations. Among all the three single spacecraft methods, the most inaccurate re-sults are obtained from the FP method and less inaccurate are from the HM method. This probably confirms the assump-tion that the larger structure of CME is somewhat suitable for estimating the time varying profile of the CME kinemat-ics. However, the failure of these single spacecraft methods at higher elongation could be due to real deflection or artificial deflection because of expansion or/and due to changes in the approximated idealized structure (Wood et al. 2010; Howard 2011; Mishra & Srivastava 2014).

We further applied the fitting version of the three single spacecraft reconstruction methods, namely Fixed-Phi Fitting (FPF: Sheeley et al. 2008), Harmonic Mean Fitting (HMF: Möstl et al. 2011), and Self-Similar Expansion Fitting (SSEF: Davies et al. 2012). Noting the expressions for the elongation as a function of speed and direction from the earlier FP, HM and SSE methods, these methods fit the observed elongation-time profile of the CME to an analytical function. From the FPF method on STEREO-A, we found that CME speed as 654 km/s, propagation direction as 99ofrom STEREO-A space-craft and its launch time at 11 April 05:05. Similarly, we also applied the HMF and SSEF methods (with \( \lambda \)\( = 5 0 ^ { o } \)) and ob-tained the speed, propagation direction and launch time of the CME. The results obtained for STEREO-A and B are shown in Figure 13 and 14, respectively and summarized in Table 3. In these figures, the results for SSEF, which fall between the FPF and HMF methods, are not plotted to avoid cluttering.

Moreover, we have also analyzed the in-situ observations of this CME obtained from WIND spacecraft (Figure 12). The