Solar wind propagation delay: Comment on “Minimum variance analysis-based propagation of the solar wind observations: Application to real-time global magnetohydrodynamic simulations” by A. Pulkkinen and L. Raststätter

Abstract

[1] Many space weather phenomena, like geomagnetic storms, aurora etc., are typically associated with disturbances in the solar wind, in particular directional changes in the interplanetary magnetic field (IMF). A southward directed IMF interacts with the geomagnetic field at the dayside magnetopause and causes enhanced energy and momentum transfer from the solar wind to the magnetosphere. The subsequent entry of this energy into the terrestrial ionosphere can lead to disturbances in radio communication, navigation systems and power grids. The ability to predict such consequences is therefore a central topic for space weather applications. [2] Since actual measurements of the solar wind and IMF are typically taken at large distances from the Earth, e.g., from the Advanced Composition Explorer (ACE) spacecraft orbiting the L1 libration point some 1.5 * 10 km upstream of the Earth, any measurements need to be time shifted to be representative for the conditions near the upstream magnetopause where the interaction takes place. The propagation time depends on both solar wind velocity, orientation of the IMF and the location of the solar wind monitor. [3] A lot of effort has therefore been made to be able to predict the propagation time of solar wind disturbances between a monitor and a target position. One of the most successful methods in terms of prediction accuracy is the phase front model introduced by Weimer et al. [2003, hereafter W03] and later benchmarked by, e.g., Weimer and King [2008] and Mailyan et al. [2008]. [4] W03 noted that variations in the IMF primarily occur within surfaces that can be arbitrarily tilted with respect to the IMF orientation. They use the term phase front normal (PFN) to describe the orientation of these surfaces, and use minimum variance analysis on sliding time segments of IMF measurements to determine the PFNs. In many aspects, the analyzed time segments are thus treated as discontinuities, although IMF variations within a time segment usually do not fulfill more formal classifications of a discontinuity suggested by, e.g., Tsurutani and Smith [1979] and Lepping and Behannon [1986]. [5] As pointed out by Pulkkinen and Raststatter [2009, hereafter PR09], one problem with the W03 method arises from the use of quality criteria imposed on the minimum variance analysis. Failure to satisfy these quality criteria can lead to “locking” to certain orientations of the phase fronts, and therefore an erroneous propagation time estimation. To circumvent this problem, PR09 suggested a modification which does not rely on these quality criteria, and which prevents abrupt changes in the phase front orientation. In this comment we point out some undesirable effects of the PR09 approach, and present an alternative solution. [6] The outline of this paper is as follows. In section 2 we give a brief review of the W03 method and the alterations and optimizations proposed by PR09. In section 3 we point Department of Physics and Technology, University of Bergen, Bergen, Norway. Also at Max‐Planck Institute for Solar Systems Research, Lindau, Germany. Department of Physics, Alexandru Ioan Cuza University of Iasi, Iasi, Romania. Physics Faculty, Yerevan State University, Yerevan, Armenia. SPACE WEATHER, VOL. 8, S06005, doi:10.1029/2009SW000542, 2010