Velocity Rotation Events in the Outer Magnetosphere Near the Magnetopause

摘要

We report velocity rotation events with amplitudes more than a few tens of kilometers per second observed by the Magnetospheric Multiscale mission. The events are detected in the outer magnetosphere near the magnetopause at approximately noon magnetic local time. When we examine one event, velocity and magnetic field variations are correlated. Accelerated electrons >100 eV are present. The azimuthal propagation direction is opposite to that of the background convection, implying that the structure and the background have different sources. The magnetic field variation can be approximately fitted to that of the line current. It is inferred that bundles of magnetic field lines with the same helicity propagate with the Alfvén velocity along the background magnetic field. One possible source is a flux transfer event. In addition, the variation in the solar wind dynamic pressure may also affect the velocity rotation.We have found 18 events during the first 2 years of Magnetospheric Multiscale operations. The derived properties are generally similar to those from the event study. For some events, we have identified a flux transfer event as a possible source. Again, the dynamic pressure variation may be the cause as well. The events reported resemble in some respects crewcuts and midday auroral brightening sequence, as measured by ground-based optical instruments.