RESUMO
The interaction between Earth's magnetic field and the solar wind results in the formation of a collisionless bow shock 60,000-100,000 km upstream of our planet, as long as the solar wind fast magnetosonic Mach (hereafter Mach) number exceeds unity. Here, we present one of those extremely rare instances, when the solar wind Mach number reached steady values <1 for several hours on 17 January 2013. Simultaneous measurements by more than ten spacecraft in the near-Earth environment reveal the evanescence of the bow shock, the sunward motion of the magnetopause and the extremely rapid and intense loss of electrons in the outer radiation belt. This study allows us to directly observe the state of the inner magnetosphere, including the radiation belts during a type of solar wind-magnetosphere coupling which is unusual for planets in our solar system but may be common for close-in extrasolar planets.
RESUMO
Magnetometer data acquired by the MESSENGER spacecraft in orbit about Mercury permit the separation of internal and external magnetic field contributions. The global planetary field is represented as a southward-directed, spin-aligned, offset dipole centered on the spin axis. Positions where the cylindrical radial magnetic field component vanishes were used to map the magnetic equator and reveal an offset of 484 ± 11 kilometers northward of the geographic equator. The magnetic axis is tilted by less than 3° from the rotation axis. A magnetopause and tail-current model was defined by using 332 magnetopause crossing locations. Residuals of the net external and offset-dipole fields from observations north of 30°N yield a best-fit planetary moment of 195 ± 10 nanotesla-R(M)(3), where R(M) is Mercury's mean radius.