Discussing the Formation of Artificial Phase Space Density Peaks in the Outer Radiation Belt During Storms

During geomagnetic storms, the magnetic field in the outer belt is known to be significantly distorted by the solar wind, such that the outer belt dynamics can be greatly impacted by the magnetic field topology. In this work, we will explain analytically, with simple examples, and, then, numerically, why the use of a dipole field in the conversion of the adiabatic invariants can either produce artificial peaks in the electron distribution function (PSD), or artificially enhance existing PSD peaks. The artificial formation or enhancement of these peaks leads to incorrect interpretations of the local enhancement, erroneously attributed to local acceleration, usually originating from wave particle interactions with chorus mode whistler waves. This results in inaccurate simulation results at L-shells larger than ~4 in the outer belt, during storm times when the magnetic field is far from the dipolar configuration. We will show that the artificial enhancement of the distribution can originate from the conversion of the first invariant (μ), while this effect was also known but when in the conversion step of the second adiabatic invariant (K). Our findings show here how essential a real magnetic field model is to compute not only L* during storm times, as done broadly in the community, but even more to convert (E, α0) to (μ, K) to avoid creating or enhancing artificially PSD peaks. With a realistic magnetic field model, a significant part of the acceleration of the electrons induced by the storm can then be described by radial transport. This crucial importance of the magnetic field geometry will be further emphasized with the numerical calculation of various electron fluxes during a storm.