Space Environment

The Earth's external magnetic environment mainly comprises its ionosphere (altitude approximately from 48km to 965km) dynamically coupled with its magnetosphere (altitude >500km) of the ionized plasma region with finite electric conductivity. The magnetopause -- where the magnetic pressure resulting from the core geodynamo field balances the dynamic pressure due to the solar wind located at about ten Earth's radius on the dayside along the Sun-Earth line -- marks the outer boundary of the Earth's magnetosphere. Across the magnetopause, the solar wind transfers its energy into the magnetosphere, drives the highly active dynamics and results in the complex electric currents that can be indirectly measured by high-precision geomagnetic satellites. Sustained primarily by continuous interaction of the core geodynamo magnetic field with the solar wind, there exist complex electric current systems in the ionosphere and the magnetosphere which are rapidly changing.

Magnetopause currents, for example, are generated by the charge separation due to the opposite gyration direction of protons and electrons. The magnetotail current, which closes the circuit loop with the tail magnetopause currents, divides the magnetotail into two lobes with almost uniform magnetic fields in opposite directions. The ring current is maintained by the azimuthal drift of charged particles (~keV to hundreds of keV) that are trapped on the geomagnetic field lines whose strength may be indicated by an index calculated using the magnetic measurements of a network of ground-based stations. Intensive injections of high-speed solar-wind particles, along with ionospheric ions, into the Earth's magnetosphere can cause the rapid ring-current intensification/variation and lead to global-scale geomagnetic storms. The field-aligned currents, mainly carried by electrons along the geomagnetic field lines, connect the Earth's high-latitude ionosphere (the auroral zones) with the magnetosphere. The Solar Quiet (Sq) currents in the ionospheric region is mainly confined between 90km and 200 km height, referred to as the ionospheric dynamo, which is mainly driven by the solar tidal winds and manifests itself as regular variations on the Earth’s ground magnetic observations. The electric current systems in the Earth’s ionosphere and magnetosphere are highly nonlinear, complicated, and not fully understood. The schematic picture of different currents is shown in Figure 1.

Figure 1 Schematic diagram of different current systems in the near-Earth space. Revised from Ganushkina et al. (2018), and https://hamwaves.com/propagation.tutorials/doc/plates/ionospheric.currents.png

Below is a brief description of our progress on physically calculating the magnetic disturbances caused by the external space currents. Limited by the complexity of the inner and high-latitude magnetospheric structure and dynamics, empirical modeling has been an indispensable tool to describe the near-Earth magnetic fields so far. Empirical models, however, rely heavily on the quality of background field removal and lack real physical meaning. In order to physically establish the external magnetic field model, a coupled numerical model of the solar wind-magnetosphere-ionospheric is used to self-consistently solve the distribution and evolution of the electromagnetic fields and particles in regions of interest. By using the observed solar wind and Interplanetary Magnetic Field (IMF) at 1AU as input, MHD governing equations are used to describe the outer magnetosphere. The magnetic flux tube and the adiabatic drift theories are used to present the inner magnetosphere more realistically. By considering the Solar Extreme Ultraviolet (EUV) conductance and particle precipitation, the height-integrated ionospheric conductance is calculated. Also, the 2-D spherical electric potential is calculated by solving the current continuity equation. The logical diagram of the coupling process is shown in Figure 2.

Figure 2 Logical diagram of the coupling of the solar wind, magnetosphere, and ionosphere.

Using the numerical simulation described above, the global mapping of three components dBr (positive outward from the center of the Earth), dB theta (positive from the southern to the northern hemisphere), and dB phi (positive along the increasing longitude) at 3.0 Earth radii is shown in Figure 3. The colorbar at the bottom of each panel shows the magnitude of the corresponding component. Typical characteristics corresponding to space currents systems are captured very well by numerical simulation.

Figure 3 Radial, theta and phi components (from left to right) of perturbation B field due to all the relevant currents systems in the magnetosphere and ionosphere.