Which Photospheric Characteristics Are Most Relevant to Active-Region Coronal Mass Ejections?

Abstract

We investigate the relation between characteristics of coronal mass ejections and parameterizations of the eruptive capability of solar active regions widely used in solar flare-prediction schemes. These parameters, some of which are explored for the first time, are properties related to topological features, namely, magnetic polarity-inversion lines (MPILs) that indicate large amounts of stored non-potential (i.e. free) magnetic energy. We utilize the Space Weather Database of Notifications, Knowledge, Information (DONKI) and the Large Angle and Spectrometric Coronograph (LASCO) databases to find flare-associated coronal mass ejections and their kinematic characteristics, while properties of MPILs are extracted from Helioseismic and Magnetic Imager (HMI) vector magnetic-field observations of active regions to extract the properties of source-region MPILs. The correlation between all properties and the characteristics of CMEs ranges from moderate to very strong. More significant correlations hold particularly for fast CMEs, which are most important in terms of adverse space-weather manifestations. Non-neutralized currents and the length of the main MPIL exhibit significantly stronger correlations than the rest of the properties. This finding supports a causal relationship between coronal mass ejections and non-neutralized electric currents in highly sheared, conspicuous MPILs. In addition, non-neutralized currents and MPIL length carry distinct, independent information as to the eruptive potential of active regions. The combined total amount of non-neutralized electric currents and the length of the main polarity-inversion line, therefore, reflect more efficiently than other parameters the eruptive capacity of solar active regions and the CME kinematic characteristics stemming from these regions.

Appendix: CME Linear Speed Threshold Selection

In this analysis we examined the different correlations exhibited between eruptive-flare characteristics and MPIL properties for fast and slow events. The speed threshold chosen to distinguish between these two classes was 750 km s⁻¹, based on the conclusions of the statistical study of Sheeley et al. (1999). As such, 750 km s⁻¹ is a threshold in a statistical sense, meaning that there is a degree of confluence between the two groups around this value. In order to cross-check the validity of this threshold, we examine here the correlation between the 24-hour averaged MPIL property values with the CME linear speed, for various threshold values ranging from 500 to 900 km s⁻¹. The results for all parameters are shown in Figure 14. All parameters (with the exception of $E_{\mathrm{Ising}}$) exhibit a systematic increase in the correlation coefficient between 700 and 800 km s⁻¹ after which the correlation coefficient drops and the size of the sample decreases. For most parameters, the maximum correlation is found within this range. Therefore, we deem that 750 km s⁻¹ is a reasonable threshold choice that also ensures that the two populations have comparable sizes (14 vs. 18). Additionally, Figure 14 demonstrates that any threshold selection above 700 km s⁻¹ would not alter significantly the conclusions of the study.

Figure 14

Correlation coefficients between the CME linear speed and the 24-hour MPIL properties for different CME linear-speed thresholds. The horizontal red lines mark the corresponding correlation coefficients for a threshold equal to zero, i.e. considering all values of the sample.