Jovian magnetopause breathing

Five closely spaced magnetic field directional discontinuities observed by Voyager 1 late on 2 March 1979 are analyzed using a variance technique. These discontinuities showed magnetic field signatures characteristic of magnetopause crossings occurring periodically at ≈ 36 min intervals but without dramatic changes in the low energy (<6 keV) plasma density. The locally obtained variance estimates of the discontinuity normal directions show no systematic variation consistent with magnetopause surface waves, are always within 19° of the average normal direction, and their average gives excellent agreement, to within 2°, with a well-known large-scale model of Jupiterʹs magnetopause. In addition, energetic ions (>5 MeV) of magnetospheric origin were present at elevated levels relative to typical magnetosheath rates throughout all crossings. These results suggest the discontinuities are magnetopause crossings resulting from a possibly semi-global breathing mode of the magnetopause boundary with a period of 66 min, an amplitude of 1.2 RJ, and a mean position of 49.9 RJ. The maximum boundary speed is 140 km s−1 with a magnetopause thickness of 21 ± 11 × 103 km.