Front Boundary of the Magnetic Cloud

The front boundary of the magnetic cloud at day 291.79 was well-defined. The magnetic field strength increased, the magnetic field direction rotated southward, the density decreased by a large amount, and the temperature decreased (Figure 1). The large abrupt changes in the field and plasma parameters suggest that the boundary was a tangential discontinuity in the MHD limit.

A higher resolution profile shows that the boundary was a magnetic hole (a small-scale structure in which the magnetic field strength is low and across which the total pressure was approximately constant), which is not unusual at magnetic clouds [see, e.g., Burlaga, 1995]. If one takes the beginning and end times of the magnetic hole at 18:51:25.5 UT and 19:01:04.5 UT, respectively, the magnetic field direction rotated through 222⁰ in a plane. A minimum variance analysis using the unnormalized magnetic field vectors shows that the normal to this plane had the direction (_n = 230.3⁰, _n = 1.7⁰). Recall that the axis of the magnetic cloud was approximately (c = 291⁰, _n = -10⁰), so that the angle between the normal to the front boundary of the magnetic cloud and its axis was approximately 60S⁰. The component of the magnetic field normal to the plane of rotation of the field at the front boundary is 0.09 nT 0.40 nT (two sigma uncertainty), consistent with zero, which is additional evidence that the boundary was a tangential discontinuity.