Propagation of Alfvénic Waves From Corona to Chromosphere and Consequences for Solar Flares

Abstract

How do magnetohydrodynamic waves travel from the fully ionized corona, into and through the underlying partially ionized chromosphere, and what are the consequences for solar flares? To address these questions, we have developed a 2-fluid model (of plasma and neutrals) and used it to perform 1D simulations of Alfvén waves in a solar atmosphere with realistic density and temperature structure. Studies of a range of solar features (faculae, plage, penumbra and umbra) show that energy transmission from corona to chromosphere can exceed 20% of incident energy for wave periods of one second or less. Damping of waves in the chromosphere depends strongly on wave frequency: waves with periods 10 seconds or longer pass through the chromosphere with relatively little damping, however, for periods of 1 second or less, a substantial fraction (37%-100%) of wave energy entering the chromosphere is damped by ion-neutral friction in the mid and upper chromosphere, with electron resistivity playing some role in the lower chromosphere and in umbras. We therefore conclude that Alfvénic waves with periods of a few seconds or less are capable of heating the chromosphere during solar flares, and speculate that they could also contribute to electron acceleration or exciting sunquakes.

Hαspectroscopy and multiwavelength imaging of a solar flare caused by filament eruption

“…Thermal radiation from soft X-rays, EUV, and UV can also contribute to this process, but this contribution was found to be very small (Allred et al 2005). Other more recent works have raised questions about the viability of this mechanism in the light of recent observations (Fletcher & Hudson 2008) and suggested Alfvén wave propagation as an alternate energy transport mechanism from the corona to chromosphere during flares (Russell & Fletcher 2013).…”

Solar flares and focused energy transport by MHD waves

“…For example, if this energy were later used to accelerate chromospheric electrons (Fletcher & Hudson 2008) or to re-accelerate energetic electrons in the chromosphere that had previously precipitated from the corona (Brown et al 2009), then the electron number problem could be disposed of. Alternatively, since MHD waves are capable of transporting energy to the chromosphere and heating it by ion-neutral friction (Emslie & Sturrock 1982;Russell & Fletcher 2013), they could also contribute directly to flare radiative emissions, thus reducing the energy flux that must be provided by electron beams.…”

“…Under these conditions, all of which apply to the Sun, Poynting flux or waves can be the dominant form of outgoing energy flux. As discussed by Fletcher & Hudson (2008) and Russell & Fletcher (2013), the Poynting flux carried by even small amplitude MHD waves in solar active regions can be of the order required to power the radiative emissions of a flare, so it is interesting to ask the general question of what happens to the waves produced by magnetic reconnection and the energy they carry.…”

Stellar flare oscillations: evidence for oscillatory reconnection and evolution of MHD modes

“…Some authors, e.g. Russell & Fletcher (2013), have even suggested that MHD waves may be responsible for energy transport from the reconnection region to the flare foot-points contradicting the conventional thick-target model. Three possible mechanisms exist: the oscillations are excited by the external flare trigger, within the flare itself, or in post-flare loops (Mathioudakis et al 2003(Mathioudakis et al , 2006 E-mail: jgd@arm.ac.uk Pandey & Srivastava 2009;McLaughlin et al 2012;Srivastava, Lalitha & Pandey 2013;Pucci, Onofri & Malara 2014).…”