Following a coronal mass ejection from the sun, the magnetosphere was suddenly compressed on September 24, 1998 at about 2344 UT when the ram pressure of the solar wind increased from 2 to 15 nPa [Moore et al., 1999; Russell et al., 1999]. At that time the Polar satellite was near apogee over the northern polar cap, at (X, Y, Z) . The ion measurements discussed here were made with the Toroidal Imaging Mass-Angle Spectrograph (TIMAS). As discussed by Shelley et al., , this instrument measures nearly a full three-dimensional velocity distribution of several species twice during a spin period (6s) of the spacecraft, and it covers an energy-per-charge range of 0.016 to 33.3 keV/e in two spin periods. Prior to the compression of the magnetosphere and for several hours afterwards, this instrument was measuring highly field-aligned ions (beams) in the mantle region.
In Figure 1 the energies per unit charge (keV/e) of H+, O+, He+, and He++ are plotted as a function of time (UT). The color code at the right indicates the ion flux in units of cm . These energies were averaged over all ion directions. Clearly evident in the figure is the jump in the energies of the H+, O+, and He+ ions, from 2345:18 UT to 2347:18 UT, due principally to their velocity increase of about 75 km/s along the magnetic field after the compression. The jump in the energy of He++ can also be discerned in the figure even though the flux was very low. While the magnetosphere remained compressed (several hours), the ion velocities along the field continued to be high.
After the compression, at about 2347:30 UT, note that the "temperature" of H+ (and possibly He++) increased, as evidenced by its broader energy spectrum relative to those of O+ or He+. Moreover, the H+ flux increased at that time reflecting its density increase. Moore et al.  and Russell et al.  have noted that these changes occurred because of the displacement of the field line (about 2.3 ) during the event, bringing cusp field lines closer to Polar. The increased fluxes resulted from the enhanced injection of magnetosheath H+ and He++ ions attending the increased solar-wind pressure.
The Polar measurements directly pertaining to the centrifugal acceleration of the ions are plotted together in Figure 2. In panel (a) the magnetic-field components in GSM coordinates and the total field intensity, as measured by the UCLA magnetometer, are plotted versus time (UT). During the compression the total field intensity increased smoothly from 126 nT to 182 nT in the time interval 2345:18 UT to 2347:18 UT. Most of this increase was due to the X-component of the field, indicating the field became directed more sunward. The full rotation of the field direction was through an accumulated angle of 19.87. Since the magnetic moment of the ions was very small, as discussed below, the betatron acceleration resulting from the intensified magnetic field (by factor of 1.44) increased the circular energy of the ions by less than a few eV. In panel (b) the ion drifts perpendicular to are shown. Here, the perpendicular O+ drift components are plotted before the compression (23.76 hr) and the average perpendicular drift components of H+ and O+ are plotted after the compression These components, as well as the average parallel velocity components shown in panel (c), were determined from the TIMAS velocity-space measurements averaged over 2 spin periods. The H+ components prior to the compression are not included because the 15 eV lower limit of the TIMAS measurements precludes the determination of the H+ velocity moments. However, the two-dimensional velocity moments from the TIDE instrument show that the H+ perpendicular drift components before 23.76 hr are consistent with those of O+. Also, after 23.76 hr the TIMAS measurements of these H+ and O+ components were very similar, as expected for drifts that are principally due to the motion. Note that the ion drifts in panel (b) were principally tailward ( ) and earthward ( ) during the compression, with both and decreasing rapidly to a minimum of about -90 km/s and recovering slowly after the compression. The average H+ and O+ velocities parallel to , as shown in panel (c), increased sharply from 25 to 100 km/s in the direction during the compression and remained high for several hours afterwards.
The centrifugal acceleration term, , based on the local measurements of and , is also shown in panel (c). At any point of the magnetosphere, this term is non-zero only if the field is changing in time. In a steady field it is appreciable only when evaluated along the particle motion, especially in high field-curvature regions. Prior to the compression, the centrifugal acceleration of the ions at Polar was only about 0.01 km/s2. However, as shown in panel (c), the magnitude of this term locally, which increased to 50 times this value, clearly illustrates its importance during the compression.