Asymmetry of the frequency response: poles vs. equator

The frequency response of the SR to solar disturbances is not uniform geographically or directionally. Energetic protons from SPEs enter the atmosphere preferentially in the magnetically unshielded polar regions, where the geomagnetic field is open. X-ray radiation, by contrast, acts rather globally over the entire day side of the cavity.

This difference leads to an anisotropic (direction-dependent) response: changes manifest themselves differently in the individual magnetic components of the measured field, depending on where and how the disturbance affected the cavity Satori 2016. Polar-localized proton ionization thus affects the resonance differently than the more uniform X-ray effect.

In practice this means that a complete picture of the SR's solar response requires the measurement of several field components and several stations; a single component at a single station may underestimate or overlook the disturbance, depending on the mutual geometry of the source, the cavity and the receiver.