Direct EUV Imaging of Mode Conversion in the Solar Corona.

Magnetohydrodynamic (MHD) waves play a crucial role in the dynamics of the solar atmosphere, particularly in the processes of wave propagation, energy transfer, and heating of the solar chromosphere and corona. The phenomenon of mode conversion—where one type of MHD wave transforms into another—can significantly affect these processes. This typically occurs in regions where the Alfvén speed matches the sound speed, such as at a 3D magnetic null point.

Observation Overview.

Report on the direct extreme ultraviolet (EUV) imaging of mode conversion from a fast-mode to a slow-mode MHD wave near a 3D null point. These observations were made using data from the Solar Dynamics Observatory's Atmospheric Imaging Assembly (SDO/AIA).

An incident fast EUV wavefront, triggered by an adjacent eruptive flare, was observed to propagate laterally through a neighboring pseudostreamer. As this fast wave traversed the null point, a slow-mode wave emerged, propagating both upward along the open magnetic structures and downward along the separatrix that encompasses the fan loops at the base of the pseudostreamer.

Implications of Observations.

These observations are significant as they provide empirical evidence for mode conversion in the solar corona, a phenomenon that has been theorized and simulated in MHD models. The transformation of fast-mode waves into slow-mode waves at null points can have substantial implications for the transfer and dissipation of energy in the solar atmosphere.

Additionally, the detection of decaying transverse oscillations in both the open and closed structures of the pseudostreamer, coupled with quasiperiodic type III radio bursts, suggests repetitive episodes of electron acceleration. These findings offer new insights into the behavior of MHD waves and their interactions with the solar magnetic field, contributing to our understanding of solar energetic events and the heating of the solar corona.

Conclusion.

The direct imaging of mode conversion from fast-mode to slow-mode MHD waves near a 3D magnetic null point, as captured by SDO/AIA, supports theoretical predictions and simulations. These observations underscore the complex interplay between MHD waves and the magnetic structures in the solar corona, highlighting the importance of mode conversion in solar atmospheric dynamics. Further studies will enhance our comprehension of these processes, potentially leading to improved models of solar activity and its impact on space weather.

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