Arrival time calculation for interplanetary coronal mass ejections with circular fronts and application to STEREO observations of the 2009 February 13 eruption

TL;DR

This study evaluates geometric models for predicting interplanetary CME arrival times using STEREO data, finding the harmonic Mean approximation yields more accurate results than the Fixed- method, especially for slow CMEs with certain shapes.

Contribution

It introduces a new formula for CME arrival time prediction using the harmonic Mean model and compares geometric assumptions, improving forecasting accuracy for specific CME configurations.

Findings

Harmonic Mean model predicts CME arrival times more accurately than Fixed-.

For slow CMEs, the HM approximation reduces errors by up to 12 hours.

Tracking CMEs beyond 30b0 elongation improves arrival time accuracy to within 5 hours.

Abstract

A goal of the NASA STEREO mission is to study the feasibility of forecasting the direction, arrival time and internal structure of solar coronal mass ejections (CMEs) from a vantage point outside the Sun-Earth line. Through a case study, we discuss the arrival time calculation of interplanetary CMEs (ICMEs) in the ecliptic plane using data from STEREO/SECCHI at large elongations from the Sun in combination with different geometric assumptions about the ICME front shape (Fixed-\Phi (FP): a point and harmonic Mean (HM): a circle). These forecasting techniques use single-spacecraft imaging data and are based on the assumptions of constant velocity and direction. We show that for the slow (350 km/s) ICME on 2009 February 13-18, observed at quadrature by the two STEREO spacecraft, the results for the arrival time given by the HM approximation are more accurate by 12 hours than those for FP in comparison to in situ observations of solar wind plasma and magnetic field parameters by STEREO/IMPACT/PLASTIC, and by 6 hours for the arrival time at Venus Express (MAG). We propose that the improvement is directly related to the ICME front shape being more accurately described by HM for an ICME with a low inclination of its symmetry axis to the ecliptic. In this case the ICME has to be tracked to > 30{\deg} elongation to get arrival time errors < 5 hours. A newly derived formula for calculating arrival times with the HM method is also useful for a triangulation technique assuming the same geometry.