报告人：Prof. dr. Rony Keppens

Centre for mathematical Plasma-Astrophysics, KU Leuven, Belgium
Visitor at: School of astronomy and space science, Nanjing University - Purple Mountain Observatory

时间：2019年6月20日（周四）上午9:30

地点：教学行政楼702会议室

报告摘要：Particle acceleration processes in reconnecting magnetic fields are at the heart of all high energy phenomena, whether they occur in accretion disks and jets surrounding black holes, in pulsar winds and nebulae, or in our own Solar corona. Solar flares provide spatially and temporally resolved testbeds for all astrophysical counterparts where magnetic reconnection mediates energy conversion and release. In the standard solar flare model, the basic ingredients for flare processes are well-identified and they have been verified against multi-wavelength observational views. Still, some aspects remain debated, and this is true especially for the source regions of hard X-rays (above order 25 keV).

We identified a previously overlooked ingredient to the standard solar flare model, which is a consequence of the sudden energy deposition near the chromosphere by downwards accelerated particles. While it is known that these lead to evaporation flows that invade the flaring loop, multi-dimensional magnetohydrodynamic modelling revealed that one frequently encounters situations where the upflows from both loop legs meet up in a loop-top localized, turbulent fashion. At the loop apex, Kelvin-Helmholtz instability (KHI) of the interacting flows sets in and thermal soft X-ray (SXR) photons are abound in this interaction zone. We modelled this process in isolation of the overarching reconnection site, and found that the intrinsically fragmented magnetic field topology due to the KHI vortical disruption can explain hard X-ray (HXR) sources in loop apexes. A 2.5D numerical MHD simulation is performed in which we incorporate the penetration of high energy electrons as a spatio-temporal dependent trigger for chromospheric evaporation flows. Extreme Ultra-violet (EUV), SXR and HXR emission are synthesized based on the evolving plasma parameters and given energetic electron spectra. KHI turbulence leads to clear brightness fluctuations in the EUV, SXR and HXR emission, with the SXR lightcurve demonstrating a clear quasi-periodic pulsation (QPP) with period of 26 s. This QPP derives from a locally trapped, fast standing wave that resonates in between KHI vortices.