The discovery that p-mode frequencies of low degree do not follow changes of solar surface
activity during the recent solar minimum offers the possibility of a new diagnostic signature of
the responsible pressure perturbation in the wave guiding medium, potentially rich of information
regarding the structure of the Sun and the cause of the unusually long solar minimum. Magnetic
fields, as well as temperature changes, introduce equilibrium pressure deviations that modify
the resonant frequencies of p-mode oscillations. Assuming the perturbation to be caused by
a horizontal layer of magnetic field located in a plane-stratified model of the Sun, we compile
analytical frequency shifts and process them to allow direct comparison with observations. The
effect of magnetism itself on the central p-mode frequencies can be neglected in comparison with
the thermal effect of a perturbative layer buried in the solar interior. A parametric study shows
that a layer as thin as 2100 km at subsurface depths is able to reproduce reported mean anomalous
frequency shifts (not correlated to the surface activity), while a layer of size around 4200 km
increasing by a small amount at depths near 0.08R⊙ can explain individual low-degree shifts. It
is also possible to obtain the mean shifts via the upward motion through depths near 0.03R⊙ of
a rising perturbative layer of thickness around 7000 km. Hence the anomalous frequency shifts
are best explained by thermal effects in the upper regions of the convection zone. The effects of
latitudinal distribution are not treated here.
Authors: C. Foullon and V.M. Nakariakov
Projects: None
Publication Status: ApJ Letter (in press)
Last Modified: 2010-03-16 08:28