Comparing the Effect of Coronal Mass Ejections on Earth's and Mercury's Magnetosphere

Although Mercury and Earth both have intrinsic magnetic field, their magnetosphere have very significant differences, due to the relative strength of the planetary dipole, the difference in the solar wind ram pressure and the interplanetary magnetic field at Mercury's vs. Earth's orbit. MESSENGER made it possible to study the effects of coronal mass ejections (CMEs) on Mercury's magnetosphere, revealing dayside compressions, substorms and, in rare instances, bow shock disappearance. While the magnetospheric timescales are very different from those at Earth, any comparative magnetospheric studies typically focus on different interplanetary drivers at Earth and Mercury or on statistical studies. Here, after reviewing insight gained from MESSENGER measurements of the effect of CMEs on Mercury's magnetosphere, we present a case, where Earth and Mercury were radially aligned during the passage of a long-duration CME. At Earth, the maximum speed at Earth was ~ 500 km/s, the magnetic field reached 16 nT with a North-South rotation of the magnetic field vector. The CME triggered a moderate geomagnetic storm and a moderate compression of the dayside magnetosphere. At Mercury, about 4 orbits of MESSENGER were affected, allowing us to study the effects of different parts of the CME on Mercury's bow shock, magnetopause and magnetosphere, and to compare the effects of this CME on the two planetary magnetospheres.