PhD thesis - Institute for Space Research - University of Calgary

100 5: Results from the Suprathermal Ion Imager
Figure 5.19: Frequency-domain investigation of polarization drift physics
between 334.4 s and 335.4 s after launch. (top) (v ix − v dx )/(E x/Ω O +B 0).
Black line is theoretical relative flow difference. (bottom) Relative phase between
flow difference and electric field. Polarization drift should lead electric
field by 90 ◦ .
the relative flow difference and the electric field is between 100 ◦ and
120 ◦ from 2 to 6 Hz. This is consistent with the predicted 90 ◦ phase
lead of the polarization drift over the electric field.
In Figure 5.20 I test Equation 5.6 in the time domain between 320 s
and 322 s. There is a significant correlation between the difference in
velocities and the time derivative of the electric field. The dashed line is
a χ 2 -minimized fit to the data by a straight line with intercept 0.4 ± 3.6
m s −1 and slope 2.3±1.0. The Pearson R (usual correlation coefficient) is
0.50. A more robust measure of association is the Spearman rank-order
correlation coefficient, which has a value of 0.55. The probability that a
value as high as this would occur by chance in the case of no correlation
is negligible [Press et al., 1992]. The advantage of using the Spearman
rank-order over the Pearson R as a measure of association is that it
makes no assumption about the form of the noise spectrum [Press et al.,
1992]. The correlation indicates that the difference between the ion flow
and E × B drift is in phase with the time derivative of the electric
field. These facts are consistent with polarization drift contributing
to the velocity differences. When I extend the analysis to cover data
between 300 s and 350 s, the differences are on average a factor of
3.4 ± 0.1 larger than the theoretically predicted polarization drift based