Sudden impulses in the magnetosphere observed by Explorer 12

Explorer 12 magnetic data show that a positive sudden impulse on the ground is accompanied by an increase in the field strength everywhere within the magnetosphere. Similarly, a negative sudden impulse on the ground is associated with a decrease in the field strength in the magnetosphere. Evidence is also found that the magnetospheric boundary moves outward at the time of a negative sudden impulse. The rise, and fall, time of a sudden impulse is the same everywhere in the magnetosphere (including that portion close to the boundary) and on the ground. Hence this time can be interpreted as the time in which the compression or the expansion of the magnetosphere takes place. The speed of propagation of the field variations through the magnetosphere seems to be close to the hydromagnetic wave speed. The magnitude of a sudden impulse at the satellite level relative to that on the ground is better explained by a magnetospheric model with a curved boundary than by an image dipole model.