Magnetic properties of the high-latitude tail boundary: Draping of magnetosheath field lines and tail-aligned current

Magnetic properties of the high-latitude tail boundary are examined with IMP 8 magnetometer data. The high-latitude tail boundary separates the tail lobe from the magnetosheath. Magnetic fields are stable in the tail lobe, but very irregular in the magnetosheath. Boundary crossings are marked by the rotation of magnetic components parallel to the boundary plane. The magnetic component normal to the boundary, if any, is very small in comparison to this rotational chnage. Despite large magnetic fluctuations in the magnetosheath, the magnetosheath-side field orientation is consistent with the draping of the interplanetary magnetic field (IMF) against the magnetotail. The boundary current has a component parallel to the lobe field (tail-aligned current), as well as a circumferential component. The IMF orientation controls the sheath-side By, while the lobe-side field has a more rigid configuration flaring antisunward. Accordingly, the polarity of the tail-aligned current is primarily determined by the sign of IMF By; in other words, the tail-aligned current shields the magnetotail against the IMF Y component. The current polarity is antisunward for positive IMF By and sunward for negative IMF By in the northern hemisphere. The tail-aligned current is also discussed in terms of the cusp (mantle) current system