On the nature of the in-ecliptic interplanetary magnetic field's two-humped distribution at 1AU

Abstract

It was found out that the distribution's shape of the in-ecliptic (as well as radial) component of the interplanetary magnetic field (IMF) significantly changes with the heliocentric distance, which poorly corresponds to classical models of the solar wind and the interplanetary magnetic field (IMF) expansion. For example, distributions of the radial photospheric and the source surface's magnetic field in the ecliptic plane are Gaussian-like, the distribution of the radial IMF component at the Earth orbit demonstrates two-humped shape, and it becomes again Gaussian-like at 3-4 AU. These differences lead to lack of correspondence between simulations of the IMF behaviour at 1 AU and observations. Our results indicate that picture of the IMF expansion into space is more complicated than usually considered, and the sector structure is not the only source of the two-humped shape of the in-ecliptic or radial IMF component. We have analysed data from different spacecraft at the distances from 0.29 AU to 4 AU and found that the shape of the radial IMF component distribution strongly depends on a heliocentric distance and a heliolatitude. The "two-humped IMF" effect is most brightly expressed at low heliolatitudes at 0.7-2 AU, but it fully disappears at 3-4 AU. There is also dependence of the IMF distributions' view on a solar cycle due to active processes, such as solar flares and CMEs. We suppose that the in-ecliptic solar wind field at 1 AU is influenced by solar active regions in a high degree, and actually the distribution is the three-humped: two humps correspond to the IMF from the middle and high heliolatitudes and the third one is the theoretically expected distribution from the solar field nearby the heliomagnetic equator. Vanishing of the IMF zero-component with the distance from the Sun partially could be a result of a magnetic reconnection at the current sheets in the solar wind.