Spatial features of geomagnetic cutoff rigidity secular variation using analytical approaches

Abstract

The secular variation spatial pattern of geomagnetic cutoff rigidity, Rc, is analyzed considering analytical expressions. Störmer analytic Rc equation in terms of geomagnetic latitude is used considering a pure dipolar field for the centered and eccentric dipole coordinates, together with a dipole-quadrupole superposition simplified equation. Linear Rc time trend for the period 1900-2020 in a 5° × 15° latitude-longitude grid is assessed in each case and a comparative analysis is made with trends based on trajectory based Rc estimations. Common features with some analytic approaches are the two patches of Rc positive and negative trends in the western hemisphere, even though maximum and minimum trend values are underestimated in all analytical cases. Global mean values are almost coincident, except for the dipole-quadrupole superposition case. The salient patches pattern may be due mainly to the dipole axis rotation, more than to its center displacement, its decreasing momentum or the increasing quadrupolar component role. Even though with the advent of better computational tools the trajectory based estimation would be a natural choice to analyze Rc variations, we still consider that an appropriately modified Störmer equation, with its natural advantages of its analytical form and extremely fast way of computation, is sufficient for many applications.