On the high-latitude limit of closed geomagnetic field lines

One ofthe investigations carried out with the energetic particle experiment on the Alouette satellites has been a study of the high-latitude boundary of 35-keV electrons in the outer radiation zone. A comparison of Λ
b
the average high-latitude boundary, with solar proton intensity versus latitude profiles, and with the location of the auroral oval, leads to the conclusion that Λ
b
indicates approximately the high-latitude limit of closed geomagnetic field lines, or alternatively the low-latitude limit of open field lines. The dependence of Λ
b
on local magnetic time and on the angle φ between the earth-sun line and the geomagnetic axis has been investigated. A noon-midnight asymmetry of about 8° is observed in A
b
as well as a marked dawn-dusk asymmetry. A dependence of A
b
on φ is also observed, in which Λ
b
has a minimum for φ ∼ 90° and is approximately symmetrical about this angle. The results are interpreted as indicating a stronger coupling between the solar wind and the magnetosphere when φ ∼ 90°, which results in more geomagnetic flux being swept back into the "tail" at these times. Some of the information included is presented in more detail in a recent paper by McDiarmid and Wilson [6].