...

Permanent address: Swedish Institute of Space Physics, Umeå Division, Sörfors 634, S-905 88 Umeå, Sweden.

.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.