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Discussion

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. [1986]).

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 tex2html_wrap_inline438 and tex2html_wrap_inline448 (positive values mean that E-field leads B-field), oscillation periods tex2html_wrap_inline484, oxygen content, peak-to-peak amplitudes, and velocities. The phase difference between tex2html_wrap_inline448 and tex2html_wrap_inline438 is near 90tex2html_wrap_inline446, indicating that observed waves are standing modes.

tex2html_wrap_inline484, derived from E- and B-field measurements (diamonds - tex2html_wrap_inline438, circles - tex2html_wrap_inline448), 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, tex2html_wrap_inline484 has peaks that deviate from the trend over the interval. The solid line represents the model oscillation period, tex2html_wrap_inline500, of the fundamental mode of the field-line resonance of dipole field lines; tex2html_wrap_inline500 is based on the local Alfven speed tex2html_wrap_inline504 at POLAR, and on the assumptions that 5% of the ions are oxygen ions and tex2html_wrap_inline504 is constant along each dipole field line. The electron density is derived from the spacecraft potential measurements. The differences between tex2html_wrap_inline484 and tex2html_wrap_inline500 suggest that there are more heavy ions on the field-lines where tex2html_wrap_inline484 has peaks. The third panel in Figure 6 shows the Otex2html_wrap_inline514 number density, assuming that tex2html_wrap_inline484 and tex2html_wrap_inline500 match. According to this result, the background Otex2html_wrap_inline514 density is usually a few percent, but some field lines are filled with higher Otex2html_wrap_inline514 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 Otex2html_wrap_inline514 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 Otex2html_wrap_inline514 density (Bill Peterson, private communication, 1997). The explanation may be that the energies of the outflowing heavy ions are below 15 eV or high Otex2html_wrap_inline514 densities exist at lower altitudes because of their lower speeds.

The fourth panel from the top in Figure 6 shows the wave amplitudes for tex2html_wrap_inline448 and tex2html_wrap_inline438 (units are tex2html_wrap_inline534 and nT, respectively), which both are peaked at 7.4 (tex2html_wrap_inline538 inv. lat.) and 8.1 (tex2html_wrap_inline540 inv lat). It seems that oscillation periods and wave amplitudes are somewhat anticorrelated so that high Otex2html_wrap_inline514 densities tend to reduce wave amplitudes.

In the bottom panel of Figure 6 the dashed line represents the tex2html_wrap_inline544 ratio. The solid line represents the average tex2html_wrap_inline504 along field lines, derived from tex2html_wrap_inline484 of the field line resonances. The dotted line represents the local tex2html_wrap_inline504 on POLAR, using the electron densities derived from the spacecraft potential measurements, and assuming a 5% Otex2html_wrap_inline514 number density. The measured tex2html_wrap_inline544 is related to tex2html_wrap_inline504, as expected; notice that these parameters have been derived from different data. The actual difference between these two parameters is not significant, because the tex2html_wrap_inline544 ratio varies along the field line so that at magnetic field nodes it approaches infinity, and at electric field nodes it goes to zero.


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Next: Summary Up: Field-Line Resonances Triggered by Previous: GEOTAIL observations

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