On the spin-up of a stably stratified, electrically conducting fluid. Part 2: Spin-up controlled by the mac layer

Abstract

Abstract The linear spin-up of a stably stratified, electrically conducting fluid within an electrically insulating cylindrical container in the presence of an applied axial magnetic field is analyzed for those cases in which electric currents generated within the steady MAC layer control the fluid interior, The MAC layer is a new boundary layer first studied by Loper (1976a) which controls the fluid in the parameter range E/α2 ≪ σS ≪ α2/E, α2 ≪ 1 Where E = vωL2, 2α2 = σB2/pω and σS = vN2/κω;2. The problem is solved using the Laplace transform and four new spin-up times are obtained. Combined into one expression they are t = ω;−1E-½[1+(σSE/α6)½ + δα-2] [1+(σSE/α6 1/4]−1 where δ = σμv. The internal spin-up mechanisms for this problem are shown to be very similar to those discussed in part 1 (Loper, 1976b). The ten known spin-up times are summarized and their inter-relationships are investigated. It is shown how to obtain the seven hydromagnetic spin-up times from a simple torsional Alfvén wave model involving a single parameter which measures the strength of the boundary layer dissipation. Finally, the present theory is applied to the solar spin-down problem and it is found that if the magnetic field in the solar interior is at least as strong as the interplanetary field of 10-5 gauss, then the hydromagnetic spin-down time is much shorter than the Eddington-Sweet time and is comparable to the age of the sun.