Alfvén wave
In plasma physics, an Alfvén wave, named after Hannes Alfvén, is a type of plasma wave in which ions oscillate in response to a restoring force provided by an effective tension on the magnetic field lines.
Alfvén waves propagate in the direction of the magnetic field, and the motion of the ions and the perturbation of the magnetic field are transverse to the direction of propagation.
However, Alfvén waves existing at oblique incidences will smoothly change into magnetosonic waves when the propagation is perpendicular to the magnetic field.
over all species of charged plasma particles (electrons as well as all types of ions).
The phase velocity of an electromagnetic wave in such a medium is
(The formula for the phase velocity assumes that the plasma particles are moving at non-relativistic speeds, the mass-weighted particle velocity is zero in the frame of reference, and the wave is propagating parallel to the magnetic field vector.)
The Alfvén wave velocity in relativistic magnetohydrodynamics is[4]
The study of Alfvén waves began from the coronal heating problem, a longstanding question in heliophysics.
Intuitively, it would make sense to see a decrease in temperature when moving away from a heat source, but this does not seem to be the case even though the photosphere is denser and would generate more heat than the corona.
In 1942, Hannes Alfvén proposed in Nature the existence of an electromagnetic-hydrodynamic wave which would carry energy from the photosphere to heat up the corona and the solar wind.
Owing to the magnetic field, these currents give mechanical forces which change the state of motion of the liquid.
The convection zone of the Sun, the region beneath the photosphere in which energy is transported primarily by convection, is sensitive to the motion of the core due to the rotation of the Sun.
Together with varying pressure gradients beneath the surface, electromagnetic fluctuations produced in the convection zone induce random motion on the photospheric surface and produce Alfvén waves.
The waves then leave the surface, travel through the chromosphere and transition zone, and interact with the ionized plasma.
The wave itself carries energy and some of the electrically charged plasma.
In the early 1990s, de Pontieu[6] and Haerendel[7] suggested that Alfvén waves may also be associated with the plasma jets known as spicules.
It was theorized these brief spurts of superheated gas were carried by the combined energy and momentum of their own upward velocity, as well as the oscillating transverse motion of the Alfvén waves.
In 2007, Alfvén waves were reportedly observed for the first time traveling towards the corona by Tomczyk et al., but their predictions could not conclude that the energy carried by the Alfvén waves was sufficient to heat the corona to its enormous temperatures, for the observed amplitudes of the waves were not high enough.
[8] However, in 2011, McIntosh et al. reported the observation of highly energetic Alfvén waves combined with energetic spicules which could sustain heating the corona to its million-kelvin temperature.
[9] The short period of the waves also allowed more energy transfer into the coronal atmosphere.
The 50,000 km-long spicules may also play a part in accelerating the solar wind past the corona.
The role of Alfvénic oscillations in the interaction between fast solar wind and the Earth's magnetosphere is currently under debate.
[11][12] However, the above-mentioned discoveries of Alfvén waves in the complex Sun's atmosphere, starting from the Hinode era in 2007 for the next 10 years, mostly fall in the realm of Alfvénic waves essentially generated as a mixed mode due to transverse structuring of the magnetic and plasma properties in the localized flux tubes.
In 2009, Jess et al.[13] reported the periodic variation of H-alpha line-width as observed by Swedish Solar Telescope (SST) above chromospheric bright-points.
They claimed first direct detection of the long-period (126–700 s), incompressible, torsional Alfvén waves in the lower solar atmosphere.
After the seminal work of Jess et al. (2009), in 2017 Srivastava et al.[14] detected the existence of high-frequency torsional Alfvén waves in the Sun's chromospheric fine-structured flux tubes.
They discovered that these high-frequency waves carry substantial energy capable of heating the Sun's corona and also originating the supersonic solar wind.
In 2018, using spectral imaging observations, non-LTE (local thermodynamic equilibrium) inversions and magnetic field extrapolations of sunspot atmospheres, Grant et al.[15] found evidence for elliptically polarized Alfvén waves forming fast-mode shocks in the outer regions of the chromospheric umbral atmosphere.
They provided quantification of the degree of physical heat provided by the dissipation of such Alfvén wave modes above active region spots.
In 2024, a paper was published in the journal Science detailing a set of observations of what turned out to be the same jet of solar wind made by Parker Solar Probe and Solar Orbiter in February 2022, and implying Alfvén waves were what kept the jet's energy high enough to match the observations.
