Effects of Coronal Structures on the Dynamics of the Global Coronal Wave of SOL2017-09-10

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Number:	438
1st Author:	Huidong HU
2nd Author:
Published:	October 17, 2022
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Contents 1 Introduction 2 Propagation on the opposite side of the Sun 3 Interactions with Dim Regions and a Small Bipolar Structure 4 Conclusions 5 Acknowledgements 6 References

Introduction

A "solar extreme ultraviolet (EUV) wave" reveals a large-scale disturbance propagating in the corona and thus observable by SDO/AIA, for example. Observations and simulations suggest that such global waves have close associations with coronal mass ejections (CMEs). We interpret them as fast-mode magnetohydrodynamic shocks. This "bow shock" will probably decouple from the driving CME and take the form of a blast wave after the expansion ceases in the lower corona. Previous observations show EUV wave motion in directions seemingly opposite to the initial motion of the CME, spreading globally even onto the solar hemisphere opposite to that of the eruption itself (e.g Ref. [1]).

Multiple coronal magnetic structures can affect the propagation of a global coronal wave. A coronal-hole boundary can in principle reflect and also transmit wave energy, and this has been observed previously (Ref. [2]). Diffraction, refraction and reflection may similarly occur occur when the wave interacts with the magnetic field of a remote active region. The propagation speed of the wave can be elevated inside a coronal cavity. So far, it is still unclear how large and persistent such a wave can be, and what contributes to its global nature and remarkable persistence.

In this Nugget we describe the global wave structures associated with the flare SOL2017-09-10. This remarkable event showed interactions between its global wave and multiple coronal structures. The global wave resulted from a huge CME in one hemisphere, and the wave propagated into the opposite hemisphere. It impinged upon both north and south polar coronal holes and extended to the unprecedented full range of latitudes (Ref. [3] and Animation 1). The transmitted secondary wave probably pushes streamers to collide on the opposite side of the Sun. The EUV wave shows different behavior when it encounters a small bright point with a bipolar structure and on-disk dim regions. The EUV wave also interacts with multiple active regions on both sides of the Sun

Propagation on the opposite side of the Sun

After the CME-driven shock has propagated beyond the limb, as seen from the direction of the eruption, it curves towards the photosphere while still connected with the wave transmitted by the coronal hole. A collision between two streamers is observed (see Animation 2 and Figure 1c), which suggests that the lateral parts of the shock probably also collide on the opposite side. Including the transmission within the polar coronal holes, the wave eventually has extended to all latitudes, both north and south.

Interactions with Dim Regions and a Small Bipolar Structure

The EUV waves (from both polar coronal holes) propagates roughly along meridians. Figure 2 shows that the speed of the wavefront drops noticeably after it has left the south-polar coronal hole, and is elevated when it enters an on-disk coronal cavity. Part of the primary EUV wavefront bends around a small bright bipolar magnetic structure when the wavefront approaches a filament channel near the bipolar structure. As shown in Figure 3, the bright bipolar structure halts part of the primary wave, and the low-density filament channel, with a high characteristic wave speed, accelerates the wave.

Conclusions

Global coronal waves excited by flare/CME events can have very large scales. We have discussed the EUV wave associated with SOL2017-09-10. In this remarkable event the disturbance traversed all longitudes and entered both polar coronal holes; these interactions and others altered the structure of the disturbance.

Acknowledgements

Ying D. LIU, and Bei ZHU share the authorship of this Nugget and of Ref. [3].

References

[1] "New Insights into the Physical Nature of Coronal Mass Ejections and Associated Shock Waves within the Framework of the Three-dimensional Structure"

[2] "Secondary Waves and/or the ``Reflection from and ``Transmission through a Coronal Hole of an Extreme Ultraviolet Wave Associated with the 2011 February 15 X2.2 Flare Observed with SDO/AIA and STEREO/EUVI"

[3] "Effects of Coronal Density and Magnetic Field Distributions on a Global Solar EUV Wave"