Use of Twenty Years CLUSTER/FGM Data to Observe the Mean Behavior of the Magnetic Field and Current Density of Earth's Magnetosphere

The data from the CLUSTER FGM magnetometer, recorded for 20 years at ESA's Cluster Science Archive, as well as the position of the spacecraft, have been used to form a database aligned in time. It allows the calculation of curl(B) over all the lifetime of the mission (representing the current density via mu(0)(J) over bar = c (u) over bar rl((B) over bar). The (B) over bar and (J) over bar data are then bin averaged, as a function of the dipole tilt angle, to form a 3D grid of spatial extent of about 20 RE, and for any spatial resolution. From these data grids, maps of the direction of the magnetic field and of the current density can be produced, allowing the observation of the average behavior of the magnetic field and the current density on a large scale. The validity of the calculation of (J) over bar is discussed. By means of spatial interpolation, the grids are used to provide a measurement of the magnetic field at any point in space where the grid is filled. This allows the possibility of ray tracing to obtain empirical plots of the magnetic field lines, that is, modeled from experimental data. Field lines near the cusp can be visualized, although smoothed by the averaging of the IMF and solar wind parameters. In future work it would be possible to add other classification criteria than just the dipole tilt angle, such as various activity indices and solar wind parameters. The prospect of adding data from other missions (such as MMS?) would extend the regions that have been covered by Cluster, and increase the spatial extent of the 3D grid and its resolution.