Definitions of the angles of the FR axis. Panel a: schema showing the observation, here GSE, and FR frames. The FR frame is defined by the vectors where is along the FR axis and in the direction of the axial magnetic field, is orthogonal to and in the plane defined by and , and completes the orthonormal direct basis. Two rotations are needed to pass from the GSE frame to the FR frame. They are defined by the angles i and λ which are respectively the inclination and the location angle (spherical coordinates with the polar axis ). Panel b: schema showing the meaning of the angle λ in the plane of the FR axis shown in panel a in light blue. , in green, is the projection of in the plane orthogonal to , see panel a.