The Earth's magnetic field is not static but constantly changing. Perturbations in the geomagnetic field on time scales of minutes to days are primarily of external origin; that is, the source electric currents flow in the ionosphere (a few hundreds of kilometers above the Earth's surface) and magnetosphere (several Earth radii above the Earth's surface). Studies of geomagnetic perturbations are, therefore, important for understanding the physical processes in the Earth's upper atmosphere and near-Earth space, which is necessary for the predictions of space weather and climate. Working Group "Geomagnetic Variations" mainly focuses on the ionospheric currents and their forcing mechanisms. The following paragraphs describe some features of geomagnetic variations and associated ionospheric currents.
Figure 1 gives examples of geomagnetic field records at St. Helena (16.0˚S, 5.7˚W), Guam (13.6˚N, 144.9˚E) and Apia (13.8˚S, 171.8˚W) in January 2019. The figure shows only "perturbations" of the geomagnetic field, defined here as the deviation of the geomagnetic field from its monthly mean. The red and blue lines indicate the northward and eastward components of the geomagnetic field, respectively. Regular daily variations on the order of several tens of nano teslas are seen in the data. These daily variations are commonly known as "Sq" for solar-quiet. Sq source electric currents flow in the ionosphere, where the atmosphere is electrically conductive due to the existence of free electrons and ions. Figure 2 depicts the electric current system in the ionosphere derived from a numerical simulation.
In Figure 2, ionospheric currents are viewed from 30˚W longitude. Each panel shows the ionospheric current system at a certain universal time (UT). The local solar time (LST) at 30˚W longitude is also indicated. There is a global-scale current system over middle and low latitudes, which is confined to the dayside. This current system is responsible for Sq variations (e.g., Figure 1) and is often referred to as Sq current system. The Sq current system has a counterclockwise vortex in the Northern Hemisphere and a clockwise vortex in the Southern Hemisphere. In the polar region, there is another current system, called DP2 current system. The DP2 current system also has two current vortices; one is in the dusk sector and the other in the dawn sector. Unlike the Sq current system, the DP2 current system is visible during both daytime and nighttime. Although Figure 2 shows only the DP2 current system in the Northern Hemisphere, there is a Southern Hemisphere counterpart over the Antarctic region.
The electric current systems in the ionosphere vary from day to day under the influence of external forces. Figure 3 describes how the Sq and DP2 current systems respond to a "geomagnetic storm". A geomagnetic storm is a major disturbance of the Earth's magnetosphere, which is caused by enhanced energy input from the solar wind. In Figure 3, the ionospheric current system presents a typical quiet-day pattern on 3 and 9 April 2010, which are before and after the storm, respectively. During the main phase and recovery phase of the storm (5 and 7 April 2010), the intensity of DP2 currents is significantly increased. As the DP2 currents leak into middle and low latitudes, the distinction between DP2 and Sq is no longer obvious on the dayside.
The bottom panels of Figure 3 show perturbations in the northward-component geomagnetic field, associated with ionospheric currents. The black lines indicate the location of the "magnetic equator", where the vertical component of the geomagnetic field vanishes. It may be noticed that geomagnetic perturbation amplitudes are enhanced near the magnetic equator during both quiet and storm times. This is due to a strong zonal current along the dayside magnetic equator, which is known as "equatorial electrojet" or "EEJ". The EEJ is driven by a vertical electric field, which owes its existence to the unique geometry of the geomagnetic field over the magnetic equator; that is, the geomagnetic field is completely horizontal. Thus, the EEJ is confined to the region closed to the magnetic equator.
The Sq-EEJ currents undergo day-to-day changes even without a geomagnetic storm. For instance in Figure 3 (bottom panels), the magnitude of the magnetic perturbations over the dayside magnetic equator due to the EEJ is different between 3 and 9 April 2010 despite the fact that the magnetosphere was comparably quiescent at these times. The difference in the EEJ intensity is largely due to neutral winds. The weather of the Earth's upper atmosphere is highly variable, as it is subject to forcing by various types of atmospheric waves from the lower atmosphere. One example of such waves is tidal waves of the atmosphere that are generated by the gravitational force of the moon. Analysis of geomagnetic field data can reveal the effect of the lunar tide on Sq-EEJ currents, as demonstrated in Figure 4.
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