COORDINATES. “The geocentric solar magnetospheric coordinate system (GSM) is used in the following discussion. In that system, the x-axis points Sunward along the Sun-Earth line, the z-axis is in the plane containing the Sun-Earth line and the dipole axis and points northward, and the y-axis completes the right-handed triad pointing duskward.”
https://link.springer.com/article/10.12942/lrsp-2007-1