Feasibility study to estimate the ion velocity field in the F region from EISCAT_3D radar observations

We conducted a feasibility study to estimate the horizontal ion velocity field in the ionospheric F region from ion velocities observed by the EISCAT_3D radar. We assumed a 27-beam configuration with a minimum elevation angle of 30 degrees. The ion velocity observation data from 200 km to 500 km altitude were projected to 200 km altitude, assuming ions above 200 km altitude follow the E x B drift. Then, we reconstructed ion velocity vectors for ±250 km in the east-west direction and ±500 km in the north-south direction at 200 km altitude. The resolution in north-south and east-west directions was 25 km. The reconstruction was conducted by maximizing a posterior probability based on Bayes’ theorem. The constraints were set to minimize the L2 norms of the following four vectors in the horizontal plane: (1) the first and (2) the second derivative of three components of the ion velocity vector, (3) the divergence of the ion velocity perpendicular to the magnetic field lines, and (4) the ion velocity parallel to the magnetic field lines. We investigated which combination of the four constraints would reconstruct the shear flow field most correctly and found that the best combination was (2), (3), and (4).