Confined or Eruptive?

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Nugget
Number: 414
1st Author: Ting LI et al
2nd Author:
Published: 16 August 2021
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Contents

Introduction

Solar flares and coronal mass ejections (CMEs) are the most catastrophic phenomena in the present solar system, driven by sudden releases of magnetic energy stored in the solar corona. Large solar flares are often, but not always, associated with CMEs. We dub flares with a CME ``eruptive" and flares without a CME ``confined". The aim of our study (Ref. [1]) was to investigate the flare-CME association rate as a function of the active region (AR) characteristics that produce the flare, specifically here its total unsigned magnetic flux. This result has important implications for the prediction of CMEs occurring in association with large flares as well as for the solar-stellar connection, where the solar flare-CME association rates are used to estimate stellar CME occurrence frequencies.

Note added: this material has subsequently appeared in an HMI Science Nugget, where it may be more legible.

Observations and data processing

We started with the GOES/XRS soft X-ray (SXR) flare catalog and selected flare events at class C5.0 and greater, choosing events occurring within 45ˆ of central meridian, from June 2010 to June 2019. This gave a total of 719 events, including 322 M-class (Ref. [2]) and 397 C-class flares. To determine whether a flare is associated with a CME, we use the CME catalog of the SOHO/LASCO. The observations from SDO/AIA and the twin STEREO spacecraft help to determine the CME association (see detailed description in Ref. [2]). Of these 719 flares, 251 events are eruptive and 468 are confined. For each event, we calculated the total unsigned magnetic flux AR of ARs before the flare onset by using the available vector magnetograms from the HMI Activve Region Patches.

Association Rate of Flare Intensity and Region Magnetic Flux

We investigated the flare-CME association rate R as function of both the flare class and the total flux of the source AR for 719 flares (see the scatter plot of AR magnetic flux versus flare peak SXR flux FSXR in left panel of Figure 1). The regions were then divided into five subintervals and the relations of the association rate R with SXR within the five subintervals are shown in middle panel of Figure 1. We find that, for each AR subinterval, R clearly increases with SXR class, i.e., larger flares are more likely associated with a CME. Our results show that the slope of the flare-CME association rate depends on the total flux of the AR that produces the flare, and reveals a steep monotonic decrease with AR magnetic flux (see the right panel of Figure 1). This means that flares of the same GOES class but originating from an AR of larger magnetic flux, are much more likely to be confined.

Figure 1: Left panel: Scatter plot of magnetic flux vs. SXR flare magnitude. Blue (red) circles are the eruptive (confined) flares. Middle panel: Association rate R as a function of SXR separately for five different subintervals of AR. Slopes and Spearman rank order correlation coefficients rs are shown at the bottom right. Right panel: Plot of slopes vs. AR.

Based on these solar observations, we can speculate on the association rate R for solar-type stars by taking a representative stellar AR magnetic fluxof 1024 Mx. For X100-class ``superflares" on solar-type stars, no more than 50% flares can generate stellar CMEs. This may help to provide an explanation why the detection of stellar CMEs is rare.

Conclusions

We have carried out the first statistical study that investigates the flare-CME association rate as a function of the flare intensity and the AR characteristics that produces the flare, in terms of its total unsigned magnetic flux (AR). Our results imply that the magnetic flux of an AR is a key factor determining the eruptive character of its solar flares, consistent with our previous studies (Ref. [2]). The region magnetic flux can be considered to be both a measure of the total flux that is in principle available for flaring, as well as being a measure of the background field overlying and containing the flaring region. Our findings imply that the latter is the more important factor here. Large flux means strong confinement and thus the flare-CME association rate is relatively low compared to that of regions with small fluxes.

Acknowledgements

The co-authors of this Nugget, and of Ref. [1], are Anqin CHEN, Yijun HOU, Astrid M. VERONIG, Shuhong YANG and Jun ZHANG.

References

[1] "Magnetic Flux and Magnetic Non-potentiality of Active Regions in Eruptive and Confined Solar Flares"

[2] "Magnetic Flux of Active Regions Determining the Eruptive Character of Large Solar Flares"

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