A stability analysis of the spiral sector transition region in the interplanetary magnetic field

We use a four-layer model in a stability analysis of the ME type spiral sector transition in the interplanetary magnetic field. Our results show that (1) three kinds of large-scale waves may be excited in the region and for all three, there exists a low-frequency cut-off. (2) In all three, the rate of growth of instability increases with k; in Model A only, the rate of growth has a maximum and a minimum. (3) As the angle between k and the solar wind velocity vector $\text{V}{}_{\mathrm{q}}$ increases, the cut-off frequency increases, and the excitation of waves gets more and more difficult, until it becomes impossible when k is perpendicular to $\text{V}{}_{\mathrm{q}}$. (4) when the angle between k and $\text{V}{}_{\mathrm{q}}$ is 75°, waves with a wavelength of 5 × 10⁴ km and a phase velocity of 340 km/s may be excited; this agrees with the observations by Voyager 1 at the Earth's magnetopause. Hence we deduce that waves in the spiral sector transition region may be a source that triggers off the Kelvin-Helmholtz instability of the magnetopause.