Main
Solar-terrestrial coupling involves energy transfer from the magnetosphere into the ionosphere and atmosphere below. A critical component of this magnetosphere-ionosphere coupling (MIC) involves large scale field-aligned currents (FACs) which flow in patterns of upwards and downwards sheets in response to solar wind forcing [1][2], and which are related to convection plasma flows in the magnetosphere arising from coupling to the solar wind through magnetic reconnection [3]. Such FACs are established and change dynamically as a result of the field-aligned propagation of Alfvén waves [4]. Such waves are also linked with the formation of some types of auroral features [5][6].
Recent research has addressed the question of whether the aurora are symmetric between the northern and southern hemispheres. For example, the aurora in each hemisphere can be differentially distorted as due to non-zero dawn-dusk component of the interplanetary magnetic field (IMF) [7]. Evidence for a seasonal dependence in the aurora was also presented by [8][9]. Asymmetries in the aurora may also occur as a result of differential solar illumination [10], from potential interhemispheric differences in ionosphere-thermosphere coupling as due to the offset of the magnetic dipole from the Earth’s centre, as well as from higher-order multipole terms [11].
These studies demonstrate that the auroral forms and their intensities in the two hemispheres can be asymmetric. However, a systematic study of asymmetries in the incoming Poynting flux from electromagnetic plasma waves has not been completed. Interestingly, recent work found that FACs in the auroral zone tend to be stronger in the north [12]. The ionospheric conductance is known to have a strong influence on the strength of the FACs [12]. However, in order to assess in-situ electromagnetic energy transfer one requires both electric and magnetic field measurements in order to compute the Poynting vector and this has heretofore not been analysed in detail. Under common assumptions the magnetic-field-aligned component of the Poynting vector is equal to the height-integrated Joule dissipation below the satellite [13].
Here we use data from the European Space Agency (ESA) Swarm mission [14] to assess the seasonal dependence of the electromagnetic energy input associated with MIC at Swarm altitudes, and thereby assess the response of space weather in geospace to solar wind forcing. Preliminary statistics [15] demonstrated a northern preference for electromagnetic energy input during the northern summer. As that study only considered northern summer months, they were unable to assess whether such asymmetry would reverse six month later, nor whether there was any systematic seasonably averaged interhemispheric asymmetry.
In contrast to the standard paradigm of interhemispheric symmetry, we demonstrate using data from the Swarm satellite, in polar low-Earth orbit (LEO) at an altitude of around 450 km, that there is persistently higher electromagnetic energy input in the northern hemisphere even when averaged over season. This preference for stronger northern electromagnetic energy input is observed in both the dayside and nightside. Indeed, on the nightside there is a dominance of energy transfer into the north in both near-summer and near-winter solstice seasons.