A measure from line‐of‐sight magnetograms for prediction of coronal mass ejections

Abstract

From a sample of 17 vector magnetograms of 12 bipolar active regions we have recently found (1) that a measure of the overall nonpotentiality (the overall twist and shear in the magnetic field) of an active region is given by the strong shear length LSS, the length of the portion of the main neutral line on which the observed transverse fields is strong (>150 Guass (G)) and strongly sheared (shear angle >45°), and (2) that LSS is well correlated with the coronal mass ejection (CME) productivity of the active regions during the ±2‐day time window centered on the day of the magnetogram. In the present paper, from the same sample of 17 vector magnetograms, we show that there is a viable proxy for LSS that can be measured from a line‐of‐sight magnetogram. This proxy is the strong gradient length LSG, the length of the portion of the main neutral line on which the potential transverse field is strong (>150 G), and the gradient of the line‐of‐sight field is sufficiently steep (greater than ∼50 G/Mm). In our sample of active regions, LSG is statistically significantly correlated with LSS (correlation confidence level >95%), and LSG is as strongly correlated with active region CME productivity as is LSS (correlation confidence level ∼99.7%). Because LSG can be measured from line‐of‐sight magnetograms obtained from conventional magnetographs, such as the magnetograph mode of the Michelson Doppler Imager (MDI) on board the Solar and Heliospheric Observatory, it is a dependable substitute for LSS for use in operational CME forecasting. In addition, via measurement of LSG, the years‐long, nearly continuous sequence of 1.5‐hour cadence full disk line‐of‐sight magnetograms from MDI can be used to track the growth and decay of the large‐scale nonpotentiality in active regions and to examine the role of this evolution in active region CME productivity.