Physics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008epsc.conf..279b&link_type=abstract
European Planetary Science Congress 2008, Proceedings of the conference held 21-25 September, 2008 in Münster, Germany. Online a
Physics
Scientific paper
Abstract During the MESSENGER flyby of Mercury on 14 January 2008 [1] the Magnetometer [2] detected numerous narrow-band ultra-low-frequency (ULF) oscillations (Fig. 1). These narrow-band ULF waves occurred at frequencies between He+ cyclotron frequency (fcHe+) and the proton cyclotron frequency (fcH+) and were detected primarily outbound from closet approach (CA). A dayside boundary layer (BL) of solar wind plasma was detected a few minutes before the outbound magnetopause crossing [1,3]. In this boundary layer there was a step-like decrease in ULF frequency and an order of magnitude increase in wave power. There was a systematic increase in ULF frequency and amplitude from CA to the BL, starting at frequencies just above fcHe+ near CA and rising to frequencies just below fcH+ near the BL. Harmonics of these ULF waves were also detected. The frequency trend and the observation of harmonics are similar to terrestrial observations of field-line resonances (FLR) detected by geosynchronous satellites [4]. If these ULF waves are field-line resonances we infer a large mass density drop with altitude, followed by an increase in mass density in the BL. However, as pointed out by Othmer et al., [5] and Glassmeier et al. [6] the ULF waves observed at Mercury occur in a frequency range where the ion cyclotron resonances significantly modify the wave properties. Whereas, the polarization properties of the terrestrial ULF waves have a strong azimuthal component, at Mercury such a component was observed by MESSENGER only in the BL. In general MESSENGER detected a large variation in the wave polarization properties with ellipticity ranging from linear to right-hand circularly polarized. Near CA the parallel power tended to be larger than the perpendicular power, while away from CA the perpendicular power dominated. There was a large variation in the wave normal angle (WNA), with a slight tendency for the WNA to be perpendicular to the ambient magnetic field near CA and parallel away from CA. The MESSENGER flyby was at low latitudes. A brief burst of narrow-band ULF waves was detected by Mariner 10 during its first Mercury flyby while approaching CA at low latitude [7]. No such emissions were detected during the third flyby of Mercury by Mariner 10 at high latitudes. Observations of these waves occurring primarily at low latitudes suggests that these waves might be confined to the magnetic equator. References [1] Anderson B. J. et al. (2008) Science, in press. [2] Anderson B. J. et al. (2007) Space Sci. Rev., 131, 417-450. [3] Slavin J. A. et al. (2008) Science, in press. [4] Takahashi K. et al. (1984) J. Geophys. Res., 89, 6758-6774. [5] Othmer C. et al. (1999) J. Geophys. Res., 104, 10369-10378. [6] Glassmeier K.-H. et al. (2004) Adv. Space Res., 33, 1875-1883. [7] Russell C. T. (1989) Geophys. Res. Lett., 16, 1253-1256.
Acuña Mario Humberto
Anderson Benjamin J.
Boardsen Scott A.
Green James L.
Korth Haje
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