The interplanetary medium is filled with the solar wind and the interplanetary magnetic field (IMF).

When the supersonic solar wind plasma flow finds an obstacle on its way out, such as a magnetosphere, a shock wave arises upstream of the obstacle. In case of a magnetospheric interaction it is called the “bow shock”. The bow shock slows the solar wind to subsonic speeds, so that the solar wind can flow around themagnetopause,which is the surface enclosing the magnetosphere. The magnetosphere is the region, where the inner planetary magnetic field plays the main role.

The magnetopause boundary is determined by the equation of balance of the magnetic field and plasma pressure sum inside and outside of the magnetopause:

IMF pressure + solar wind plasma pressure =

planetary and magnetospheric magnetic field pressure + magnetospheric plasma pressure

In the case of Mercury, in front of the magnetopause the solar wind ram pressure, and inside it the magnetospheric magnetic field pressure play the main roles. Mercury's weak internal magnetic field, combined with its proximity to the Sun, means that the magnetosphere of the innermost planet is small and highly dynamic. Specifically, the magnetopause subsolar standoff distance is estimated to be 1.4 – 1.6 R_M, where R_M is the radius of Mercury.Sometimes, during a high-density plasma flow, the magnetopause can appear closer to the Hermean surface, or even disappear. The bow shock changes its shape and stands closer or farther from the planet in response to variations in the solar wind speed and, to a lesser extent, IMF direction. The magnetopause location and shape are determined principally by the pressure exerted on the magnetopause by the shocked solar wind plasma, which scales with the solar wind ram pressure, balanced by the magnetospheric magnetic field pressure.

The MESSENGER spacecraft has completed over 4000 orbits around Mercury. The initial orbit had a 200 km periapsis altitude, 82.5 inclination, 15,300 km apoapsis altitude, and 12 h period. The MESSENGER’s orbit around Mercury was approximately fixed in inertial space so that the orbit completes a local time rotation once every Mercury year (88 Earth days). The periapsis altitude and latitude of the orbit also drifted during every Mercury year, and the MESSENGER orbit varied between two extremes, the dawn-dusk terminator and noon-midnight orbits.

During each orbit, MESSENGER typically spent 1–2 h inside the magnetosphere; the rest of the time was spent in the magnetosheath and in the interplanetary medium. The interplanetary magnetic field (IMF) magnitude assigned to each crossing can be evaluated as a 1 h average of magnetometer (MAG) data upstream of the outermost bow shock encounter.

Magnetic field data are analyzed in Mercury solar orbital (MSO) coordinates. In MSO coordinates, X_MSO is positive sunward, Z_MSO is positive northward, Y_MSO is positive duskward and completes the right-handed system, and the origin is at the center of the planet.

The main task in this science case is to determine the bow shock and magnetopause crossings in MESSENGER data. Also, it is necessary to filter out the intersections of the current sheet inside the magnetosphere. Magnetopause and bow shock crossings should be identified on every orbit, both before and after the magnetospheric transit. They will be denoted as the inbound and outbound crossings, respectively.