On the use of a sunward libration‐point‐orbiting spacecraft as an interplanetary magnetic field monitor for magnetospheric studies

Abstract

Magnetospheric studies often require knowledge of the orientation of the interplanetary magnetic field (IMF). In order to test the accuracy of using magnetometer data from a spacecraft orbiting the sunward libration point for this purpose, the angle between the IMF at ISEE 3, when it was positioned around the libration point, and at ISEE 1, orbiting the earth, has been calculated for a data set of 2‐hour periods covering 4 months. For each period, a 10‐minute average of ISEE 1 data is compared with 10‐minute averages of ISEE 3 data at successively lagged intervals. At the lag time equal to the time required for the solar wind to convect from ISEE 3 to ISEE 1, the median angle between the IMF orientation at the two spacecraft is 20°, and 80% of the angles are less than 38°. The results for the angles projected on the y‐z plane are essentially the same. The median minimum angle between the IMF orientation at the two spacecraft attained during each 2‐hour segment is 11°, with 80% of the angles less than 19°. These low values indicate relatively little temporal or small scale variation between the spacecraft. The minimum angle generally occurs at a lag time different from the convection time. The sign of the difference depends on IMF orientation in the sense that magnetic features tend to arrive sooner when the IMF is directed along the line between the spacecraft. However, the difference between a lag time appropriate to this corotation geometry and the convection lag time is not large enough to produce a significant decrease in the angles between the IMF vectors at the two spacecraft. We conclude that the IMF orientation at a libration‐point‐orbiting spacecraft, lagged by the time required for the solar wind to convect to the earth, is a convenient predictor of IMF orientation near the earth, to within ∼20° accuracy.