A rapid northward IMF turning produced a variety of magnetospheric processes on January 10, 1997. This rotation was followed by the formation of surface waves in the magnetopause/LLBL region via the K-H instability, resulting in the motion of GEOTAIL between two plasma regions, mainly between the plasma sheet and the LLBL. In the LLBL, large-scale magnetic field vortices were observed, whereas in the plasma sheet both vortices and fast mode waves were observed. The strongest compressional waves occurred at 1-15 mHz whereas the field-line resonances observed by POLAR occurred at 4-7 mHz. One open question is how a broad-band source produces only resonances with a narrow-band frequency range (see e.g., Allan et al. ).
Figure 6 presents wave characteristics of the field-line resonances as a function of POLAR's L shell. The panels from top to bottom are: phase difference between and (positive values mean that E-field leads B-field), oscillation periods , oxygen content, peak-to-peak amplitudes, and velocities. The phase difference between and is near 90, indicating that observed waves are standing modes.
, derived from E- and B-field measurements (diamonds - , circles - ), are in good agreement with each other and tend to increase with L shell, indicating that different L shells oscillate independently. At L = 7.4 and 8.5, has peaks that deviate from the trend over the interval. The solid line represents the model oscillation period, , of the fundamental mode of the field-line resonance of dipole field lines; is based on the local Alfven speed at POLAR, and on the assumptions that 5% of the ions are oxygen ions and is constant along each dipole field line. The electron density is derived from the spacecraft potential measurements. The differences between and suggest that there are more heavy ions on the field-lines where has peaks. The third panel in Figure 6 shows the O number density, assuming that and match. According to this result, the background O density is usually a few percent, but some field lines are filled with higher O densities.
The TIDE detector is turned off below about L = 8, but before that starting at 21:53 UT, it detected 5-10 eV ions streaming outward from the ionosphere along the magnetic field (see Figure 7). Note that ions flowing parallel to the field lines are observed later because they need to propagate from the southern hemisphere to the northern one. This suggests that the ion outflow has been initiated by field-line resonances. Unfortunately the ion masses cannot be determined. Therefore the predicted peaks in O density cannot be confirmed with TIDE. On the other hand, the TIMAS instrument on POLAR, which measures ions with energy greater than 15 eV, shows no enhancements in the O density (Bill Peterson, private communication, 1997). The explanation may be that the energies of the outflowing heavy ions are below 15 eV or high O densities exist at lower altitudes because of their lower speeds.
The fourth panel from the top in Figure 6 shows the wave amplitudes for and (units are and nT, respectively), which both are peaked at 7.4 ( inv. lat.) and 8.1 ( inv lat). It seems that oscillation periods and wave amplitudes are somewhat anticorrelated so that high O densities tend to reduce wave amplitudes.
In the bottom panel of Figure 6 the dashed line represents the ratio. The solid line represents the average along field lines, derived from of the field line resonances. The dotted line represents the local on POLAR, using the electron densities derived from the spacecraft potential measurements, and assuming a 5% O number density. The measured is related to , as expected; notice that these parameters have been derived from different data. The actual difference between these two parameters is not significant, because the ratio varies along the field line so that at magnetic field nodes it approaches infinity, and at electric field nodes it goes to zero.