An Explanation for the Observed Frequency Drift of Coherent ~1 Hz waves in Mercury's Inner Magnetosphere

Narrow-band coherent waves for which the frequency of the fundamental mode ranges from values less than to comparable to the proton cyclotron frequency are common in Mercury's nightside inner magnetosphere. The waves usually exhibit a distinct frequency drift with magnetic latitude (MLAT). On a typical MESSENGER orbit they are first detected at a frequency below the He+ cyclotron frequency at high magnetic latitudes, and the frequency drifts up to the proton cyclotron frequency (f_{cH+}) as the spacecraft approaches the magnetic equator. This drift is clearly visible in a plot of observed wave frequency normalized by the local f_{cH+} for measurements spanning two Mercury years. We have used magnetic field models to trace the wave observations from the point of observation along a magnetic field line to the magnetic equator of Mercury. When normalized by the equatorial f_{cH+} instead of the local f_{cH+}, the frequency shows no overall drift versus MLAT. This flat profile suggests that these waves are strongly confined to the field line on which they are generated. These waves are believed to be either long-wavelength electromagnetic ion cyclotron waves that can form field-line resonances or short-wavelength electromagnetic ion Bernstein waves. The group velocity for both modes can be highly directed along a magnetic field line. We also explore the path-integrated gain of the electromagnetic ion Bernstein wave in the plasma environment of Mercury, with its high ratio of thermal to magnetic pressure, to see if such waves become heavily damped as they propagate away from their source field line. This explanation for the frequency drift of waves near Mercury could explain the frequency drift observed for similar waves near the plasmapause in the Earth's magnetosphere. The statistical distribution of waves that are strongly confined to a field line could potentially be used to test the validity of planetary magnetospheric magnetic field models.