Alfven waves in the inner polar coronal hole

Abstract

We investigate the Alfvén wave propagation characteristics in the inner zone (r < 1.35 R⊙) of the North Polar Coronal Hole. The linear, incompressible magnetohydrodynamic (MHD) approximation reveals two wave modes with different phase velocities. Alfvén wave with the slower phase velocity is quickly damped; in other words it cannot propagate in the coronal plasma. On the other hand, the higher phase velocity Alfvén waves can propagate along the magnetic field lines. Mechanical energy flux density of these waves is found to be high enough to replace the coronal energy which is lost via optically thin emission and through heat conduction to the chromosphere below. We show that Alfvén waves propagating along the magnetic flux tubes go through refraction and get damped via viscous dissipation and resistivity. The energy flux density of the waves is of the order of 106 erg cm−2 s−1. The radial profile of the energy flux density in the region 1.05–1.35 R shows a rise until about 1.15 R where it peaks and further on declines less steeply than rising arm. This result is used to interpret Si viii data taken by the Solar Ultraviolet Measurements of Emitted Radiation instrument on board Solar and Heliospheric Observatory. Si viii lines like many other ionic lines show non‐thermal broadening. The observed radial profile of the non‐thermal line‐of‐sight velocity of Si viii shows a plateau which coincides with the peak of the energy flux density we derived. We claim that this coincidence is not accidental but indicates the fact that the decrease in the energy flux density of damped Alfvén waves causes the plateau formation in Si viii non‐thermal line‐of‐sight velocity profile.

Solar off-limb line widths: Alfvén waves, ion-cyclotron waves, and preferential heating

“…The line-width variation with the altitude has been analysed with respect to this effect in Hassler et al (1990) for Mg x and Banerjee et al (1998) and Pekünlü et al (2002) for Si viii.…”

Comments on “Comment on Propagation and Dissipation of Alfven Waves in Coronal Holes” by Suresh Chandra

“…It should be noted that Pekünlü et al [1], and DS have reported the same form of their main dispersion relations [Eq. (13) of Pekünlü et al [1]; Eq.…”

“…are effectively presented [1,6]. This is the case for the ideal plasma in which no dissipation mechanism works, i.e., angular frequency ( ) is linearly proportional to the wave number (k) [7].…”

“…The group velocity and energy flux density of damped Alfvén waves in the resistive coronal hole plasma must be frequency and/or wavelength dependent [1], however, in less amount as these waves are weakly dispersive in the coronal context. Hence, SC's absolute claim for frequency-independent group velocity and energy flux in the dispersive coronal hole plasma is erroneous.…”

“…Recently, Pekünlü et al [1], and Dwivedi and Srivastava [2, henceforth DS], have interpreted respectively the plateau formation in the nonthermal velocity [3] and line-width decrement [4] in polar coronal holes as a likely signature of Alfvén wave dissipation which may play an important role in understanding the solar wind acceleration and coronal hole heating mechanisms.…”