Cold ion outflow response to the solar wind energy input and the geomagnetic dipole tilt angle

Abstract:

The cold ions (total energy lower than 70 eV) from the ionosphere dominate the ion population in the magnetosphere/magnetotail lobes. They play important roles in both the magnetospheric dynamics and evolution of the Earth's atmosphere. Measuring cold ions is difficult. For decades, measurements of cold ion outflow in the magnetosphere have to be done usually in situations when the spacecraft gains a positive electric charge due to solar illumination. This keeps cold ions away from the detectors onboard spacecraft. Quantitative studies on cold ion outflow in terms of its response to the solar wind energy inputs and solar extreme ultraviolet (EUV) emissions have been hindered until recent advances in technique of measurements. In this paper, we have utilized a comprehensive data set with measurements of velocities and densities of cold ions to study the fluxes and the energies of cold ion outflow in their source region of the ionosphere. We found that: 1) around 0.1% of the solar wind energy input in to the magnetosphere (parameter epsilon) is transferred to the total kinetic energy of cold ion outflow and solar EUV emissions supply cold ion outflow with kinetic energies of about 10⁷ W; 2) We also found that when epsilon is lower than ~10¹⁰ W, the magnetic tilt angle altering illumination of ionospheric source of cold ion outflow is the main controller of the cold ion outflow.