The top figure shows the angle that the magnetic field made with the
northward direction for the outbound passes through the magnetopause and
bow shock for the mission’s first (blue) and second (orange) Mercury
flybys. Although the magnetic field strengths were comparable for the two
flybys (see PIA11404), the direction of the field outside the magnetosphere,
imposed by the solar wind, was opposite: northward for flyby 1 and
southward for flyby 2. The two encounters therefore present us with a
nearly ideal “controlled” experiment to contrast Mercury’s magnetosphere
under these two opposite extremes in its interaction with the solar wind.
The bottom figure illustrates the profound difference in magnetic
connection between Mercury and the solar wind when the magnetic field in
the solar wind is southward (left) as for flyby 2 versus northward (right)
as for flyby 1. These views from the Sun show a notional cross section of
the magnetic lines of force in the dawn-dusk meridian plane. For southward
solar wind magnetic fields, the solar wind and planetary magnetic fields
are connected over the poles and Mercury’s magnetosphere is tightly
coupled and strongly driven by the solar wind. By contrast, for northward
solar wind magnetic fields, the magnetosphere is “closed” and there is
minimal inter-connection between the solar wind and planetary magnetic
fields.
Date Acquired: January 14 and October 6, 2008
Instrument: Magnetometer (MAG)
These images are from MESSENGER, a NASA Discovery mission to conduct the
first orbital study of the innermost planet, Mercury. For information
regarding the use of images, see the MESSENGER image use policy.