Changes in magnetopause positions and cosmic-ray intensity

Abstract

Magnetic field and solar plasma experiments on satellites have shown that the position of the magnetopause (magnetospheric boundary) is variable. During a magnetic storm it may move as close as 8 geocentric earth radii. Even during quiet geomagnetic activity it may change as much as 2 earth radii. The Explorer 12 magnetic field experiment allows continuous observation of the magnetopause position twice a day for the period of August–December 1961. We have studied the changes in cosmic-ray intensity associated with such motion of the magnetopause using neutron monitor data from several ground observatories. The method of superposition of epochs shows a definite relationship, viz. inward motion of the magnetopause is associated with a decrease and outward motion with an increase in the cosmic-ray intensity. However, this relationship becomes weak after 20 days from the decrease in the magnetopause distances, i.e. increase in the energy density of the solar plasma responsible for such changes. The effect of the plasma density increase corresponds to the passage of the sector boundary of the interplanetary field.