The computed trajectories of the O+ ions, as projected in the (X, Z)GSM plane, are shown in Figure 3. Polar was at the triangle labeled B. At
= 2345:18 UT, the parallel velocity component of the ion was 25 km/s. The black curve depicts the tracing of this ion back to the ionosphere, where the co-latitude and MLT were 15.29and 0836:13. At the altitude of 500 km, the circular energy of the ion was 35.2 eV and its parallel velocity component was 0 ( criterion for the selection of the magnetic moment). By applying Liouville's theorem, its flux was found to be
(
s sr keV)-1 at 
= 90.
The red curves show the trajectory of the ion measured at
= 2347:18 UT. At that time the parallel velocity component of the ion was 100 km/s. The trajectory of this ion was traced backward through the compression a distance of 2.64
from the spacecraft (2.37
perpendicular to the magnetic field), to the point marked A, which is at (X, Y, Z)
= (0.2018,-2.193, 8.417)
. At this point the parallel velocity component of the ion was 30.5 km/s. All the ion species that were at this point at the onset of the compression were rapidly flung along the magnetic field. From point A the ion was traced to the altitude of 500 km in the ionosphere, where the co-latitude and MLT were 15.12and 0924:55. At that altitude, the parallel velocity of the ion was 19.2 km/s, its circular energy was 36.4 eV, and its flux was
(
s sr keV)-1 at = 47. The blue lines drawn through points A and B depict the magnetic field line just before- and just after- the compression.