Bursts of Narrow-Band Magnetospheric Waves near the He+ to H+ Cyclotron Frequencies
Observed During MESSENGER’s Mercury Flybys
Scott A. Boardsen 1,2, James A. Slavin1, Brian J. Anderson3, Haje Korth3, Jim M. Raines4, Sean C.
Solomon5, George Gloeckle 6,4, and Thomas H. Zurbuchen4
1
Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA
Goddard Earth Science and Technology Center, University of Maryland, Baltimore County,
Baltimore, MD, USA
3
The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA
4
Dept. of Atmospheric, Oceanic and Space Sciences, The Univ. of Michigan, Ann Arbor, USA
5
Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC, USA
6
Dept. of Astronomy, University of Maryland, College Park, MD, USA
2
The three Mercury flybys by the MErcury Surface, Space ENvironment, GEochemistry, and
Ranging (MESSENGER) spacecraft occurred on 14 January 2008, 6 October 2008, and 29
September 2009. For all flybys the spacecraft followed similar equatorial trajectories, entering
the dusk side of the magnetosphere downstream of the planet and exiting just sunward of the
dawn terminator. No science data were taken after closest approach (CA) during the third flyby.
The first flyby (M1) occurred during steady northward interplanetary magnetic field (IMF) and
showed no evidence of reconnection or substorm activity. In contrast, the second flyby (M2)
occurred during steady southward IMF, and intense reconnection was observed in the
magnetotail and on the dawnside magnetopause. Despite the different IMF geometry during M1
and M2, bursts of narrow-band waves at frequencies in the range between the He+ and H+
cyclotron frequencies were observed during both flybys. These bursts were seen almost
continuously from a few minutes before closest approach (CA) and to the dawnside
magnetopause crossing. Outbound from CA, trends in wave power, wave-normal angle, and
ellipticity were similar for both flybys. Also, during both flybys a dawnside "boundary layer"
(BL) was marked by a step decrease in magnetic field strength but no change in field orientation.
During both flybys a factor of ~10 increase in wave power was observed in this BL relative to
that before BL entrance. However, during M1 a frequency drift was observed in these waves
from just above the He+ cyclotron frequency to just below the H+ cyclotron frequency between
CA and the inner edge of the BL. In contrast, no such drift was apparent during M2. Overall
wave power was ~4 times larger during M2 than during M1. For both flybys the wave-normal
angles were generally greater than 45°, and power parallel to the local magnetic field was often
greater than power in the perpendicular direction, suggesting a strong magnetosonic component
for these waves.