## 3. Diverging magnetic field and no gravitational stratification: no dissipationTo understand each physical effect clearly, we first examine the effect of a diverging field on the phase mixing of Alfvén waves. Hence, we eliminate gravitational effects by setting the pressure scale height to infinity, i.e. . In the rest of this paper, we assume that the plasma is structured in the -direction. When referring to stratification, we mean radial stratification due to gravity while a diverging atmosphere refers to the area change due to spherical geometry. If we neglect dissipation, i.e. , the solution for the magnetic field and velocity perturbations are given by where and The ideal MHD solutions in spherical coordinates may be approximated by a simple WKB solution (see Appendix) of the form where . Although the exact analytical solutions for the perturbed magnetic field and velocity differ, the approximate WKB solutions are both the same since the leading asymptotic terms of the Bessel functions agree on using the large argument approximation and (Abramowitz & Stegun). The solutions (15) and (16) show that area divergence decreases the wavelength in agreement with the numerical solutions shown in Figs. 1 and 2. Figs. 2 (a) and 2(b) confirm that the solutions for the perturbed magnetic field and the perturbed velocity are the same as predicted by Eq. (17). When , Eq. (14) becomes the standard wave equation for phase mixing in a Cartesian, non-dissipative system, i.e.
Therefore, the solutions for the perturbed velocity and magnetic field in Cartesian coordinates are given by where . From Fig. 2 (c) we see that low down, i.e. near , and for small initial wavelengths (see Eq. (6)), the spherical and the Cartesian case indeed agree extremely well. The results in Fig. 1 show clearly that, unlike gravitational stratification which lengthens the wavelengths, area divergence shortens the wavelengths while the wavelengths remain constant in the Cartesian case. Indeed, in this case the Alfvén speed and the wavelength behave like . The amplitude of both the perturbed magnetic field and the perturbed velocity are constant in height as we expected. Wright & Garman (1998) and Torkelsson & Boynton (1998) showed that in the large wavenumber limit the amplitudes of the Alfvén waves behave as and and as is constant with height in this case, the result follows. This suggests that phase mixing will be more efficient in a diverging medium than in a non diverging medium as the short length scales, necessary for efficient dissipation, will be created much faster. Therefore, heat could now be deposited at lower heights. Similar results were obtained by Ruderman et al. (1998). These results also show that, unlike the results due to stratification, the effect of the divergence of the background field is the same whether resistive or viscous heating is considered. To obtain an estimate of where this heat would be deposited if dissipation were included, we now consider the current density, . In spherical coordinates, only including the dominant terms, is given by while in Cartesian coordinates, is given by where and . The numerical results obtained for both the magnetic field and the velocity indicate that the behaviour of the current density and the vorticity will be similar. Therefore we concentrate on the current density alone. From Fig. 3 (a) we see that even when there is no phase
mixing, i.e. , the current density
builds up very rapidly in a diverging magnetic field. In the
non-diverging atmosphere, i.e. the Cartesian case, this current
density remains constant as gradients in the vertical or horizontal
direction do not build up. As there is no phase mixing, the growth in
in the diverging atmosphere is
entirely due to the flaring out of the background field lines, i.e.
the radial derivatives are building up. When we include phase mixing,
i.e. , we see from Fig. 3 (b)
that the build up of the current density,
, increases when
increases. However, when the
magnetic field is diverging, the effect of increasing
is larger than in the Cartesian
case. Indeed, from comparing Eqs. (17) and (19), we see that a change
in causes a bigger change in the
spherical -derivatives than in the
Cartesian
Fig. 4 (a) shows the change in the current density
when we change the initial
wavelength , through changes to
either the frequency or the
background Alfvén velocity .
Since (Eq. (6)), doubling
has the same effect as doubling
or halving
and so on. Therefore we concentrate
on the effect of varying just the one parameter
. Changing
also causes
to change as
. We notice that
starts off with a higher initial
value when the initial wavelength is
smaller, which is clear from Eqs. (17) and (19). We still see that the
current density builds up faster in
the spherical case and even that the difference between the spherical
and the Cartesian case is more pronounced as the initial wavelength
gets smaller. From Eqs. (17) and
(19) we see that all derivatives, apart from the Cartesian
From studying the non-dissipative case, we expect phase mixing to be enhanced when we increase the phase mixing parameter or decrease either the intial wavelength or the parameter . We also expect more heat to be deposited into the plasma when we consider a diverging magnetic field compared to the Heyvaerts and Priest model in Cartesian coordinates. We now want to examine if these effects remain the same when we include dissipation in the system. © European Southern Observatory (ESO) 2000 Online publication: January 31, 2000 |