Magnetic topology of quiet-Sun Ellerman bombs and associated ultraviolet brightenings

Nugget
Number:	485
1st Author:	Aditi BHATNAGAR
2nd Author:
Published:	6 January 2025
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Introduction

Ellerman "bombs" are small-scale magnetic reconnection events in the lower solar atmosphere. These were first observed in active regions, but now we also detect them in the quiet Sun as well (QSEBs). Recent work (Ref. [1]) has shown that a small percentage of them can occur co-spatially and co-temporally with ultraviolet (UV) brightenings in the transition region. In the quiet Sun, the UV brightenings are less intense than those in active regions.

Results

In our recent work we highlight four different magnetic topologies that can be associated with QSEBs and their related UV brightenings. The magnetic field topology was determined through potential field extrapolations performed on the line-of-sight component of the magnetic field (BLOS) derived from Fe I 6173 Å. We also used the Hα data from the Swedish Solar Telescope SST) to identify QSEBs, while UV brightenings were detected using co-aligned Si IV 1400 Å slit-jaw images from the Interface Region Imaging Spectrograph (IRIS). The four different topologies are illustrated in the schematic in Figure 1. The first topology involves a simple dipole configuration, where the QSEBs occur at polarity inversion lines (PILs) between opposite magnetic polarities. In this case, magnetic flux cancellation at the PIL would drive the energy release. The other three topologies feature a fan-spine configuration with a 3D magnetic null point, where UV brightenings are located near the null point. In these configurations, QSEBs could be observed at three locations, namely near the footpoints of the outer spine, inner spine, or the fan surface. In such cases, the QSEBs could be occurring due to the energy flow from the reconnection site. All these cases suggest that QSEBs and UV brightenings occur due to common reconnection scenarios.

Figure 1: Schematic of the magnetic field topologies derived from potential field extrapolations for co-occurring QSEBs and UV brightenings. Panel (a) shows a dipole configuration with a black vertical line marking the polarity inversion line (PIL). Panels (b), (c), and (d) show UV brightenings occurring at the null point of a fan-spine topology. These three panels highlight various possible locations for QSEBs: near the footpoint of the outer spine (b), near the footpoint of the inner spine (c), and near the dome footpoints (d). The orange-shaded parts highlight the possible regions influenced by the energy flow from the reconnection site.

For each of these cases, we show the observations along with their extrapolated field lines in Figure 2, which also highlights the location of the QSEBs and the UV brightenings. The heights of the 3D null points varied significantly, ranging from 0.2 Mm to 2.6 Mm above the solar photosphere which is governed by the magnetic field strength at the footpoints.

Figure 2: Observations with potential field extrapolated magnetic field lines corresponding to the four cases in Figure 1. The grey-scale image at the bottom in each panel is the line-of-sight component of the magnetic field, and the QSEB is shown in light blue and white shades in the Hα line wing. The UV brightening (1400 Å) is shown are shown as a yellow patch. Panel (a) shows a dipole configuration, with QSEB close to the photosphere, and the UV brightening at the top of the loops. In panels b), c) and d) the UV brightening is located near the 3D null point of a fan-spine topology, while QSEB is near the footpoint of the outer spine, inner spine and the fan surface, respectively.

Conclusion

The details of this work are available in Ref. [1]. Through this work we show that QSEBs can occur as part of complex magnetic topologies involving 3D nulls with fan-spine structures. These configurations also explain the observed co-spatial and co-temporal relationship between QSEBs and UV brightenings, providing valuable insights into the underlying reconnection-driven processes in the quiet Sun.

References

[1] "Magnetic topology of quiet-Sun Ellerman bombs and associated ultraviolet brightenings"