Motion and orientation of magnetic field dips and peaks in the terrestrial magnetosheath

Abstract

Using data from the four Cluster spacecraft, estimates of the motion of isolated magnetic field dips and peaks in the magnetosheath are presented. Their motion and orientation are deduced using four‐spacecraft timing, with typical spacecraft separations of ∼100 km. A set of 39 clear isolated dips and 12 peaks, spread over 4 days, is considered. The calculated velocities in the plasma frame are scattered around zero, with a width of ∼30 km/s and an average absolute value of 21 km/s. This places an upper limit on the speed of these structures, and within timing errors these values are consistent with the structures being stationary in the plasma frame. Two events had estimated speeds of over half the local Alfvén speed; it is shown that these two estimates were unreliable as a result of uncertainties in timings. The inferred propagation speeds were typically slower than local wave speeds (95% less than half the local Alfvén speed), even allowing for their orientations (85% less than the slow mode speed and 56% less than half of it). These results are consistent with the structures being stationary in the plasma frame, as expected for mirror modes, rather than being propagating waves or solitons. In addition, they are not consistent with structures standing in the flow. However, it is not possible to eliminate the possibility that a small number of the structures may have been slowly propagating. The normals of the structures deduced from the timings suggest that they are cylinders rather than sheets and that the maximum variance direction is a reliable estimate of their axis orientation, with a precision of ∼20° for maximum/intermediate eigenvalue ratios of 10 or more.