On the relationship between ELM filaments and solar flares

Abstract

Both solar flares and edge localized modes (ELMs) involve magnetized plasma eruptions which sporadically eject field-aligned filamentary structures into the surrounding, low density envelope: the far scrape-off layer (SOL) in the case of the tokamak and interplanetary space in the case of the sun. The erupting filamentary structures display many similarities and have been occasionally compared in the popular and specialist literature. In this contribution, the dynamical evolution of solar flares and ELM filaments is separately reviewed, after which the relationship between the two phenomena is examined. In particular, four families of dynamical theories of ELM filament evolution, classified according to the electric field ordering and the absence/presence of magnetic reconnection at the X-point, are compared with experimental measurements on tokamaks. This comparison reveals that theories, which encompass the drift ordering, offer better overall agreement with ELM filament observations than their magneto hydrodynamic (MHD) ordered counterparts. Although MHD ordered dynamics can describe the linear and early non-linear phases of ELM evolution, they must be supplemented by drift ordered dynamics to capture the saturation phase of the instability and the evolution of filamentary structures in the SOL. In other words, an integrated model of the ELM must include finite gyro-radius terms, in particular gradient-B and curvature guiding centre drifts arising from non-uniformities in the magnetic field and diamagnetic drifts arising from non-uniformities in the thermodynamic variables. This is consistent with the observed resemblance between ELM filaments and turbulent eddies, or blobs, observed in the SOL during Ohmic and low confinement mode (L-mode) operation. In contrast, the dynamical evolution of solar flares is shown to be predominantly MHD ordered, although drift ordered effects play a role in some aspects of solar flare physics, e.g. magnetic reconnection. It is concluded that ELM filaments and solar flares are most likely governed by different regimes of magnetized plasma physics.