Alfvén wave dissipation in the solar chromosphere

Abstract

Magneto-hydrodynamic (MHD) Alfvén waves 1 have been a focus of laboratory plasma physics 2 and astrophysics 3 for over half a century. Their unique nature makes them ideal energy transporters, and while the solar atmosphere provides preferential conditions for their existence 4 , direct detection has proved difficult as a result of their evolving and dynamic observational signatures. The viability of Alfvén waves as a heating mechanism relies upon the efficient dissipation and thermalization of the wave energy, with direct evidence remaining elusive until now. Here we provide the first observational evidence of Alfvén waves heating chromospheric plasma in a sunspot umbra through the formation of shock fronts. The magnetic field configuration of the shock environment, alongside the tangential velocity signatures, distinguish them from conventional umbral flashes 5 . Observed local temperature enhancements of 5% are consistent with the dissipation of mode-converted Alfvén waves driven by upwardly propagating magneto-acoustic oscillations, providing an unprecedented insight into the behaviour of Alfvén waves in the solar atmosphere and beyond.The solar surface hosts a web of diverse magnetic fields, from sunspots exhibiting sizes that dwarf the Earth, to dynamic bright grains only a few hundred km across. The magnetic nature of the Sun's atmosphere supports the plethora of MHD wave activity observed in recent years 6 . Such wave motion is predominantly generated near the surface of the Sun, with the creation of upwardly propagating MHD waves providing a conduit for the transportation of heat, from the vast energy reservoir of the solar photosphere, to the outermost extremities of the multi-million degree corona.In comparison to other MHD modes, Alfvén waves are the preferred candidates for energy transport since they do not reflect or dissipate energy freely 3 . Observational studies have been limited by the challenging requirements on instrumentation needed to identify the Doppler line-of-sight (LOS) velocity perturbations and non-thermal broadening associated with Alfvén waves, thus there is only tentative evidence of their existence within the Sun's magnetized plasma 7-9 . Given the difficulties associated with resolving the intrinsic wave signatures, to date there has been no observational evidence brought forward to verify the dissipative processes associated with Alfvén waves. Theoretical studies have proposed multiple dissipation methods that would allow the embedded mechanical energy of Alfvén waves to be converted into localized heat 10,11 . Unfortunately, most act on unobservable scales, providing no clear signatures that can be identified with even the largest current solar telescopes. However, one distinct mechanism revolves around the formation of macroscopic shock fronts, which naturally manifest as a result of the propagation of waves through the solar atmosphere 12 . Shock behavior induced by slow magneto-acoustic waves is ubiquitously observed in sunspots, manifesting as umbral flashes 5 (UFs), giv...

The Magnetic Response of the Solar Atmosphere to Umbral Flashes

“…For locations defined as "edge" UF detections, this produces temperature fluctuations, with the magnetic pressure allowed to vary through the increased magnetic field strength in that location (Figure 6(c)), producing detectable perturbations in the derived strength of the magnetic field. This, of course, assumes that the umbral atmosphere is dominated by magnetic pressure (i.e., the plasma-β = 1), which is consistent with previous observational studies (e.g., Jess et al 2013;Aschwanden et al 2016;Grant et al 2018). According to thermodynamic theory for adiabatic expansions, pressure fluctuations produce a subsequent change in temperature through the relationship,…”

“…A polarimetric uniformity across quiescent and UF Stokes profiles suggests that the less energetic phase of UF morphology is being sampled; a consequence resulting from the upper-chromospheric formation height of the He I 10830 Å spectral line (Vernazza et al 1981;Avrett et al 1994). This is in contrast to upper-photospheric and lowerchromospheric observations of UF phenomena, which are obtained close to the formation heights of the UFs themselves (Grant et al 2018), hence producing a strong polarity change (de la Cruz Rodríguez et al 2013;Henriques et al 2017). When considered on a statistical basis (i.e., not isolating individual profiles that may inadvertently bias subsequent analyses), the excellent quality of the FIRS data and synthetic HAZEL spectra means that a complete study of vector magnetic field fluctuations can be undertaken.…”

“…Furthermore, the He I 10830 Å plasma is often treated in an optically thin manner, and, as a consequence, is less responsive to temperature fluctuations when compared to the optically thick lower solar atmosphere (Andretta & Jones 1997). Grant et al (2018) showed that the strongest UFs can form as low as ∼250km above the solar surface. Therefore, at the formation height of He I 10830 Å (expected to be up to ∼2100 km; Vernazza et al 1981;Avrett et al 1994), the induced shock signatures will have abated, thus providing less energy dissipation, in the form of heat, to the upper chromosphere.…”

“…A whitelight camera, in sync with the narrowband feed, was also employed to enable processing of the narrowband image sequence, alongside the implementation of high-order adaptive optics to improve quality (Rimmele 2004). Data reduction of the IBIS observations followed standard techniques (i.e., dark subtraction and flat fielding), yet also included a radial blueshift correction that is required due to the use of a classical etalon mounting (Cauzzi et al 2008), in addition to the alignment and destretching of the resulting image sequences (Jess et al 2010b(Jess et al , 2012Grant et al 2018).…”

Relevant heating of the quiet solar corona by Alfvén waves: a result of adiabaticity breakdown

“…shocks, shock-shock collisions, cumulative effects, reconnections etc., that are observed and theoretically proved to be responsible for heating different parts of corona 1 . Among these, ion heating by Alfvén Waves (AWs) looks as an efficient mechanism 4,5 , since these waves propagate effectively in the corona 6–8 , and with a high energy flux. This paper deals with the quiet corona and coronal holes considered by McIntosh et al .…”