Azimuthal magnetic field disturbances and spiral structure of the interplanetary magnetic field

A 2.5 dimensional, ideal MHD model in spherical coordinates is used to numerically simulate the propagation of azimuthal magnetic field disturbances with emphasis upon its influence on the spiral angle of the interplanetary magnetic field (IMF). The observed fact that the IMF spiral angle is larger than that predicted by the Parker model is attributed to a ceaseless launching of azimuthal magnetic field disturbances of the same sign from the solar surface. The differential rotation of the Sun induces an azimuthal magnetic field inside the Sun, providing a source for these disturbances. The simulation results show that azimuthal magnetic field disturbances of the same sign which have a duration of one tenth of the period and an amplitude of order of 10(3) nT is capable of increasing the IMF spiral angle at lAU by 2degrees so as to match with relevant observations. It is also shown that the azimuthal magnetic field disturbances mentioned above have little effect on the solar wind properties and the magnetic configuration in the heliospheric meridional plane.