Mercury’s Magnetospheric Response to the Dynamics of the Space Environment
Haje Korth1, Brian J. Anderson1, James A. Slavin2, Scott A. Boardsen2,3, Thomas. H.
Zurbuchen4, Daniel N. Baker5, Sean C. Solomon6, and Ralph L. McNutt, Jr.1
1
The Johns Hopkins University Applied Physics Laboratory, Laurel, MD, USA.
Heliophysics Science Division, NASA Goddard Space Flight Center, Greenbelt, MD, USA.
3
Goddard Earth Science and Technology Center, University of Maryland, Baltimore County,
Baltimore, MD, USA.
4
Dept. of Atmospheric, Oceanic and Space Sciences, Univ. of Michigan, Ann Arbor, MI, USA.
5
Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO, USA.
6
Dept. of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC, USA.
2
Mercury is exposed to the most dynamic heliospheric space environment of any planet in the
solar system. The magnetosphere is particularly sensitive to variations in the interplanetary
magnetic field (IMF), which control the intensity and geometry of the magnetospheric current
systems that are the dominant source of uncertainty in determinations of the internal planetary
magnetic field structure. The Magnetometer on the MErcury Surface, Space ENvironment,
GEochemistry, and Ranging (MESSENGER) spacecraft has made extensive magnetic field
observations in the inner heliosphere over the heliocentric distances of Mercury's orbit, between
0.31 and 0.47 AU. Magnetometer data from MESSENGER, obtained at rates of 2 and 20 vector
samples per second, are used together with previous observations in the inner heliosphere by
Helios, to estimate the response of Mercury’s magnetosphere to the dynamics of the space
environment. The IMF magnitude at Mercury’s orbit is not only larger than at Earth but also
exhibits considerably greater variability which is comparable to the total field magnitude. Using
models for the external current systems, we evaluate the impact of IMF variability on the
magnetospheric field. From the IMF observations we also infer the spectral characteristics of
magnetospheric field fluctuations. Plasma fluid-mode waves require a finite time to
communicate solar wind and IMF conditions through the magnetosphere, so the magnetospheric
response to IMF variability is not instantaneous but can be regarded as a low-pass filter. At
Mercury, the characteristic response time, i.e., the Dungey-cycle time, is a few minutes, within
the period range over which the IMF is observed to be turbulent. This implies that the IMF will
strongly affect the magnetosphere of Mercury so that distinguishing intrinsic magnetospheric
processes from external driving will require particular care. In addition, identifying periods of
quiescence will be of particular importance for analyses of the intrinsic planetary field.