Deriving solar transient characteristics from single spacecraft STEREO/HI elongation variations: a theoretical assessment of the technique

Abstract

Abstract.Recently, a technique has been developed whereby the radial velocity, V r , and longitude direction, β, of propagation of an outward-moving solar transient, such as a Coronal Mass Ejection (CME), can be estimated from its track in a time-elongation map produced using Heliospheric Imager (HI) observations from a single STEREO spacecraft. The method employed, which takes advantage of an artefact of projective geometry, is based on the evaluation of the best fit of the time-elongation profile of the transient, extracted from a time-elongation map, to a set of theoretical functions corresponding to known combinations of radial velocity and direction; here we present an initial theoretical assessment of the efficacy of this technique. As the method relies on the manual selection of points along the time-elongation profile, an assessment of the accuracy with which this is feasible, is initially made. The work then presented assesses theoretically this method of recovering the velocity and propagation direction of solar transients from their time-elongation profiles using a Monte-Carlo simulation approach. In particular, we assess the range of elongations over which it is necessary to make observations in order to accurately recover these parameters. Results of the Monte-Carlo simulations suggest that it is sufficient to track a solar transient out to around 40 • elongation to provide accurate estimates of its associated radial velocity and direction; the accuracy to which these parameters can be estimated for a transient tracked over a particular elongation extent is, however, sensitive to its velocity and direction relative to the Sun-Spacecraft line. These initial results suggest that this technique based on single spacecraft STEREO/HI observations could prove extremely useful Correspondence to: A. O. Williams (aow2@ion.le.ac.uk) in terms of providing an early warning of a CME impact on the near-Earth environment.

Corotating Interaction Regions as Seen by the STEREO Heliospheric Imagers 2007 – 2010

“…In this example, the overplotted family of tracks convincingly fits the underlying CIR structure (with the proviso that the fit is poorer at lower elongation values, as noted by Williams et al (2009)) and yields a radial of speed 450 km s −1 . The in-situ measured speed of the transient is around 400 km s −1 .…”

ARRIVAL TIME CALCULATION FOR INTERPLANETARY CORONAL MASS EJECTIONS WITH CIRCULAR FRONTS AND APPLICATION TOSTEREOOBSERVATIONS OF THE 2009 FEBRUARY 13 ERUPTION

“…We implemented the fitting by using a downhill simplex method (Nelder & Mead (1965), IDL function amoeba.pro) which is, from our experience, robust for this three-parameter minimization. It can also be done by creating an "error map" for V FP and φ FP and looking for the minimum residual between the theoretical and observed ICME tracks (Williams et al 2009;Lugaz 2010). The same procedure can be applied using the so-called harmonic mean approach.…”

“…However, a major goal of future missions concerned with real-time space weather forecasting should be to have the longest possible prediction lead times. It is commonly quoted that the ICME must be observed at at least 30 • from the Sun (e.g., Williams et al 2009;Lugaz et al 2010) to obtain reliable results with HMF and FPF. To check this statement for our case study, we plot in Figure 7(a) the error in the arrival time at STEREO-B as a function of the angle of the maximum measurement taken as an input for the fitting methods.…”

Observational Tracking of the 2D Structure of Coronal Mass Ejections Between the Sun and 1 AU

“…The tracks corresponding to the front and rear edges are approximated by manually selecting a number of sample points along the J-map, N; where N is typically between 30 and 60 points Lugaz 2010). The typical error associated with the manual selection procedure has been investigated by Williams et al (2009) and . It has been found that the error in the direction of propagation is usually of the order of 5°, but that uncertainty decreases as the maximum observed elongation angle increases.…”

“…Savani et al (2009) displayed a different technique than the manual selection by creating an automatic tracking procedure for the density feature found in the J-map. Although this method lowers the uncertainty by reducing the standard error of the residuals, as defined by Rouillard et al (2010a) and Williams et al (2009), in practice, the uncertainty is limited by the physical assumptions used to create the formulae. This uncertainty in the true direction of propagation and velocity is more clearly visible when the results from the point-P, FP and HM methods are compared (Lugaz 2010;Wood et al 2010).…”