Erosion of the inner magnetosphere during geomagnetic storms

Using the empirical magnetic field model dependent on the Dst index and solar wind dynamic pressure, we calculated the behaviour of the contour B = B_s in the equatorial plane of the magnetosphere where B_s is the magnetic field in the subsolar point at the magnetopause. The inner domain of the magnetosphere outlined by this contour contains the bulk of geomag-netically trapped particles. During quiet time the boundary of the inner magnetosphere passes at the distance ∼10R_E at noon and at ∼7R_E at midnight. During very intense storms this distance can be reduced to 4–5 R_E for all MLT. The calculation results agree well with the satellite measurements of the magneto-pause location during storms. The ionospheric projection of the B = B_s contour calculated with the Euler potential technique is close to the equatorward edge of the auroral oval.