Quantitative modeling of modulated ion injections observed by Polar-Thermal Ion Dynamics Experiment in the cusp region

On May 13, 1996, as the Polar spacecraft was traveling at high invariant la titudes (similar to 78 degrees -79 degrees) in the prenoon sector (similar to 1050 magnetic local time), the Thermal Ion Dynamics Experiment on board recorded successive injections of protons with clear energy-time dispersion . These dispersion structures spread over several minutes and extend from s everal hundreds of eV down to a few tens of eV. During this pass, simultane ous measurements from the Wind spacecraft revealed little variation of the solar wind dynamical pressure but a gradual turning of the interplanetary m agnetic field (IMF) from an essentially dawn-to-dusk orientation (i.e,, pre dominant positive By component and slighly negative B-Z) to a north-to-sout h one (predominant negative B-Z). We show that the observed injections resu lt from magnetosheath particle entry at higher and higher latitudes in the dawn sector. Using test particle calculations in a simple model of reconnec ted interplanetary and magnetospheric field, we show that the injection mod ulation likely follows fi-om changes in the dynamical regime experienced by the ions upon traversal of the magnetopause current sheet. That is, as the IMF gradually rotates, the time-varying B-Y and B-Z lead to changes in the adiabaticity parameter kappa in the region of entry and affect particle ac cess to the Polar location. In the morning sector where magnetosheath plasm a accelerates downtail, such an access to the inner magnetosphere requires magnetic moment damping and is thus favored during nonadiabatic episodes. T he flux variations obtained numerically are in qualitative agreement with t hose observed, both in terms of characteristic energy and overall time evol ution. This supports our interpretation of the modulated ion injections in terms of intermittent nonadiabatic entry from the magnetosheath followed by time of flight dispersion between the magnetopause and the spacecraft.