.
Quasi-periodic pulsations (QPPs; also understood as quasi-periodic oscillations, i.e., QPOs) are phenomena that electromagnetic emissions vary quasi-periodically with time. In the standard flare model, magnetic reconnection plays an essential function in releasing energy from the coronal magnetic field, ejecting plasmoid, and accelerating electrons. And once the magnetic reconnection happens quasi-periodically, the electron acceleration will also be quasi-periodic and QPPs will be created.
It is likewise prevalently thought that magnetohydrodynamic (MHD) waves could likewise regulate some plasma parameters such as density, temperature, and magnetic field strength, and etc., which may improve electro-magnetic radiations in a regular form. The above two systems are still in argument for a long time, since direct imaging observations on microwave or HXR sources, which are highly related to energetic electrons, at the energy release site (reconnection existing sheet) are still absent. In the current paper by Kou et al, 2022, we report the imaging observation of microwave QPPs at the flare present sheet region for the very first time, by utilizing the microwave radioheliograph Expanded Owens Valley Solar Array (EOVSA) with the state-of-art spectral-imaging capability (Chen et al, 2020).
Figure 1. Microwave emissions from the flare. The upper group is from flaring loops, and the lower group extends along reconnection existing sheet. The background is an EUV image at 211 A. The gold curve represents the temporal variation curve of existing sheet source at 8.4 GHz.
Quasi-periodic pulsations (QPPs; likewise known as quasi-periodic oscillations, i.e., QPOs) are phenomena that electro-magnetic emissions differ quasi-periodically with time. In the recent paper by Kou et al, 2022, we report the imaging observation of microwave QPPs at the flare existing sheet area for the very first time, by using the microwave radioheliograph Expanded Owens Valley Solar Array (EOVSA) with the state-of-art spectral-imaging capability (Chen et al, 2020).
Figure 2. Temporal variation curves of microwave sources at 8.4 GHz (left) and the Wavelet Analysis outcome (right).
It has actually been discovered that the microwave sources at the existing sheet are much weaker than those at flaring loops, and therefore tend to be overwhelmed by the background noise. Thanks to EOVSAs high dynamic variety and spectral-imaging capability, the authors discovered that temporal variations of brightness temperature levels and spectral indices (smaller sized spectral indices, i.e., more difficult spectra, represent greater non-thermal quality) of all those sources, no matter weak or strong, present comparable quasi-periodicities. A 2.5-dimensional MHD simulation even more confirms that magnetic islands generated in the current sheet regulate magnetic reconnection into a quasi-periodic type. All these outcomes strongly support the scenario that quasi-periodic magnetic reconnection in the current sheet speeds up electrons quasi-periodically and after that produces QPPs.
Based upon the recent paper by Kou, Y., Cheng, X., Wang, Y. et al. “Microwave imaging of quasi-periodic pulsations at flare current sheet”, Nature Communications, 13, 7680 (2022 ). DOI: 10.1038/ s41467-022-35377-0.
References.
Kou, Y., Cheng, X., Wang, Y. et al., Nature Commun. 13, 7680 (2022 ).
Zimovets, I. V. et al., Space Sci. Rev. 217, 66 (2021 ).
Chen, B. et al., Nat. Astronomy 4, 1140– 1147 (2020 )
It has actually been discovered that the microwave sources at the existing sheet are much weaker than those at flaring loops, and hence tend to be overwhelmed by the background noise. A 2.5-dimensional MHD simulation further validates that magnetic islands created in the existing sheet regulate magnetic reconnection into a quasi-periodic form. All these outcomes highly support the scenario that quasi-periodic magnetic reconnection in the current sheet accelerates electrons quasi-periodically and then creates QPPs.