The Earth's magnetic field is not static but constantly changing. The variations 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 from the surface) and magnetosphere (several Earth radii from the surface). Studies of those geomagnetic variations 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. Our current focus topics include:
There are occasionally times when a record of the geomagnetic field at a ground observatory shows a smooth regular daily variation that repeats its pattern for many days. Such days are identified as geomagnetically quiet days and have been found to correspond to the periods when solar wind disturbances are absent. During geomagnetically quiet days, the frequency spectrum of the geomagnetic field is dominated by the 24-, 12-, 8-, and 6-hour period components, and those solar variations in the geomagnetic field are called "Sq" for solar-quiet. The Sq source electric current flows in the ionosphere, where the atmosphere is electrically conducting due to the existence of free electrons and ions. Fig. 1 shows the global electric current system derived from geomagnetic data. The arrows represent the horizontal current flow at 110km altitude, while the color indicates the current flowing along the magnetic field lines from one hemisphere to the other. The Sq current system is mostly confined to the sunlit side of the ionosphere and consists of a counterclockwise vortex in the Northern Hemisphere and a clockwise vortex in the Southern Hemisphere. Little current flows on the nightside (the shaded area in Fig. 1), where the electrical conductivity of the ionosphere is very small. There is a particularly strong current flow along the magnetic equator (the red line in Fig. 1), which is referred to as the equatorial electrojet or "EEJ". The goal of our Sq/EEJ studies is to estimate the global wind system at ionospheric heights, which drives those currents. The neutral wind in the upper atmosphere is important for various space operation purposes (for example, for the calculation of satellite drag) but difficult to measure directly.
The geomagnetic field is known to vary with the phase of the Moon, which is denoted as "L" for lunar variations. Fig. 2 presents an example of L in the horizontal geomagnetic component observed at ground station in the equatorial region. Like Sq, the source electric current for L flows in the ionosphere. The driving force of the L current is the atmospheric lunar tide. The atmospheric lunar tide is a global-scale wave in the Earth's atmosphere generated by the gravitational force of the Moon. Studies of L can provide information not only about the atmospheric lunar tide but also about the state of the background atmosphere through which atmospheric lunar tide propagates.
We currently work on following funded projects