Narrow-Band Ultra-Low-Frequency Wave Observations During the October 6, 2008, Flyby of Mercury.

During MESSENGER's first flyby of Mercury on 14 January 2008 (M1), numerous observations were made of narrow-band ultra-low frequency (ULF) waves having frequencies between the He+ and H+ cyclotron frequencies. These magnetospheric waves were observed primarily outbound between closest approach and the magnetopause and exhibited systematic variations in frequency and amplitude with distance from the planet, while the polarization properties of these waves were quite variable. The trajectories of the M1 encounter and the second flyby (M2, 6 October 2008) are similar, both located in the equatorial nightside magnetosphere crossing from dusk to dawn. However, the M2 trajectory will enter the magnetosphere ~1.5 RM (Mercury radii) tailward and exit the magnetosphere ~0.5 RM sunward of the penetration points for M1. The flybys M1 and Mariner 10 I (29 March 1974) sampled similar flux tubes during their magnetospheric passages, while the Mariner 10 III (16 April 1975) flyby sampled flux tubes at much higher latitudes. Only one brief burst of these waves was detected during the Mariner 10 I flyby. The difference in prevalence of ULF waves found for M1 relative to that of the Mariner 10 flybys is intriguing, and we use observations from M2 to investigate whether this prevalence is consistent along comparable trajectories. In this paper the ULF waves detected during M2 will be analyzed and compared with the previous observations of ULF waves in Mercury's magnetosphere to assess, to the extent possible, whether their occurrence or properties depend on prevailing IMF and activity conditions within Mercury's magnetosphere.