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.