Tests and Comparisons of Velocity‐Inversion Techniques

Abstract

Recently, several methods that measure the velocity of magnetized plasma from time series of photospheric vector magnetograms have been developed. Velocity fields derived using such techniques can be used both to determine the fluxes of magnetic energy and helicity into the corona, which have important consequences for understanding solar flares, coronal mass ejections, and the solar dynamo, and to drive time-dependent numerical models of coronal magnetic fields. To date, these methods have not been rigorously tested against realistic, simulated data sets, in which the magnetic field evolution and velocities are known. Here we present the results of such tests using several velocity-inversion techniques applied to synthetic magnetogram data sets, generated from anelastic MHD simulations of the upper convection zone with the ANMHD code, in which the velocity field is fully known. Broadly speaking, the MEF, DAVE, FLCT, IM, and ILCT algorithms performed comparably in many categories. While DAVE estimated the magnitude and direction of velocities slightly more accurately than the other methods, MEF's estimates of the fluxes of magnetic energy and helicity were far more accurate than any other method's. Overall, therefore, the MEF algorithm performed best in tests using the ANMHD data set. We note that ANMHD data simulate fully relaxed convection in a high-plasma, and therefore do not realistically model photospheric evolution.

Sequential Coronal Mass Ejections from AR8038 in May 1997

“…All three CME speeds given in Table 1 are measured in the plane-of-the-sky. The frontal speed of the halo CME on 12 May was estimated to be about 600 km s −1 after correcting for the projection effects Zhao, Plunkett, and Liu, 2002). All three CMEs had moderate speeds.…”

“…Eruptions may originate from emerging or decaying active regions (ARs) and well-decayed magnetic regions with quiescent filaments. In the case of emerging ARs, the free magnetic energy carried by electric current emerges from the solar interior where solar dynamo is operating (Leka et al, 1996;Fisher et al, 2000;Schrijver, 2007;Welsch and Li, 2008). In decaying ARs or decayed magnetic regions, it becomes arguable whether the free energy was generated below the photosphere or above it by flows, magnetic flux cancellation and other processes (Forbes et al, 2006;Welsch, 2006).…”

“…In the case of emerging ARs, the free magnetic energy carried by electric current emerges from the solar interior where solar dynamo is operating (Leka et al, 1996;Fisher et al, 2000;Schrijver, 2007;Welsch and Li, 2008). In decaying ARs or decayed magnetic regions, it becomes arguable whether the free energy was generated below the photosphere or above it by flows, magnetic flux cancellation and other processes (Forbes et al, 2006;Welsch, 2006). In addition to the energy source, it has been suggested that the coronal magnetic topology may play a role in CME initiation (Antiochos, DeVore, and Klimchuk, 1999;Lynch et al, 2008).…”

“…He also reported flux cancellation and emergence of magnetic features near the PIL. Li et al (2004) reported converging flow towards the PIL and the temporal evolution of the flow along a single perpendicular cut through the center of the sunspot using their preliminary flow field obtained by applying Fourier local correlation tracking (FLCT) (Welsch et al, , 2007Fisher and Welsch, 2008) to the line of sight (LOS) magnetic field without projection corrections. LCT techniques measure magnetic field-related flow field from a different perspective than magnetic feature tracking (further description in Section 3).…”

“…The results of LCT have the potential to either inspire or serve as the input to numerical modeling for data-driven CME predictions. The FLCT method and pre-processing of magnetograms have been improved in the last few years in our group (Welsch et al, 2007(Welsch et al, , 2009Fisher and Welsch, 2008). Further analysis and updated results are discussed later in this paper.…”