Jessica S. Zhao¹ & Yang Liu²
1 Homestead High School, Cupertino, CA 95014
2 W. W. Hansen Experimental Physics Laboratory, Stanford University, Stanford, CA 94305-4085

We have thoroughly surveyed all the flares that occurred near the solar limb during May 2010 to August 2023 and were stronger than M2.0, and found a total of 78 flares that had white-light off-limb features detected in SDO/HMI continuum intensity data. We maintain an online catalogue at http://jsoc.stanford.edu/data/off_limb_WLF/index.html that records all these events and will update the catalogue as new events are available.

Off-limb eruptive events were reportedly observed by the SDO/HMI continuum intensity previously by various authors^[1,2], and the emission mechanisms associated with such events were also discussed by some authors^[3,4]. But it is unclear how popular such events are: are they only associated with some eruptions, or they are more popularly detectable? In our recent paper^[5], we surveyed all the flaring events that occurred between May 2010 and August 2023 with a magnitude stronger than M2.0 and a heliographic longitude larger than 65◦. We found that among the 189 flares that met our selection criteria, 78 (41.3%) had off-limb features associated with them.

Figure 1| Histogram showing our statistical results on how many solar flares of different magnitudes are with or without off-limb white-light features.

To get a better sense of how the off-limb features are related to the magnitude of flares, we further divided all the candidate events into three categories: M2.0 – M4.9, M5.0 –M9.9, and X1.0+. As shown in Figure 1, we found that among 119 M2.0 – M4.9 flares we have examined, 37 (31.1%) had off-limb white-light features associated with them; among 40 M5.0 – M9.9 flares, 23 (57.5%) had off-limb features; and among 30 X1.0+ flares, 18 (60%) had off-limb features. This demonstrates an unsurprising fact, i.e., the stronger the flare, the higher the chance it has an off-limb feature.

Figure 2| Sample images from SDO/HMI continuum intensity observations, showing the evolution of a Type II event after an X1.1 flare that started at 03:17 UT on 17 April 2022.

Based on the movies of the 78 events that have off-limb white-light features, we further categorized the events into four different types. Type I events are the most popular among the detected off-limb events, shown as evolving loop structures without the loop opened or materials ejected. Type II events are quite similar to Type I events, but usually develop more rapidly with the loop top opened and materials ejected into space, likely producing coronal mass ejection events. Type III events are fast ejections. Materials are seen rapidly ejected from the Sun, sweeping rapidly past the limited field of view without showing loop structures, likely causing a coronal mass ejection event. Type IV events are flare arcades. In this type of events, an arcade becomes visible about 10 – 20 minutes after the onset of the flare, slowly but steadily rising until gradually fading away from the field of view. Figure 2 shows one example of Type II events.

Figure 3| Sample images from SDO/HMI continuum intensity observations, showing the evolution of a Type II event after an X1.1 flare that started at 03:17 UT on 17 April 2022.

The white-light observations of off-limb events provide valuable information that can be coupled together with SDO/AIA UV/EUV observations for more comprehensive studies. To demonstrate their usefulness, we show in Figure 3 a confined eruption observed in selected wavelength channels. It can be found that the white-light emissions of the event coincide in location with the UV 1600Å observation, but are located in the darkest area in the EUV 171Å and 335Å observations. This will offer us some insight into the emission mechanism of white light in off-limb events.

References

[1] Martínez Oliveros, J.-C., et al., 2014, ApJ Lett, 780, L28
[2] Zhao, J., Liu,W., Vial, J.-C.: 2021, ApJ Lett, 921, L26
[3] Heinzel, P., Kleint, L., Kašparová, J., Krucker, S. 2017, ApJ, 847, 48
[4] Jejˇciˇc, S., Kleint, L., Heinzel, P. 2018, ApJ, 867, 134
[5] Zhao, J. S., Liu, Y. 2023, Solar Phys., 298, 148