Longitudinal dependence of the daily geomagnetic variation during quiet time

Abstract

The daily variation of the geomagnetic field during quiet time at equinox features a longitudinal dependence, which is observed in the dip‐latitude/longitude frame. Two explanations are considered: (1) the longitudinal changes of the geomagnetic latitude at constant dip‐latitude and (2) the longitudinal variations of the main field, which modulates the ionospheric dynamo e.m.f. and determines and constrains the field‐aligned currents (FAC) that flow between ionospheric hemispheres to maintain the current continuity. The Prairie View ionospheric dynamo model simulates this inter‐hemispheric coupling within the International Geomagnetic Reference Field (IGRF) by using magnetic coordinates, and is used for this investigation. Two distinct types of magnetic variation observed at midlatitudes in African and American sectors are explained by the differences in geomagnetic latitude. IGRF longitudinal effects are also in evidence. They account for one singular feature (a large positive excursion of the declination around 4 LT at Plateau). At African midlatitudes, they improve the horizontal component amplitude and the morning decrease of the declination, which are better reproduced with IGRF than with a dipolar main field. FAC, which generate a variation of the geomagnetic field of up to 40% of the horizontal current contribution around midday, are essential to simulate these three cases.