On the Accuracy of Adiabaticity Parameter Estimations Using Magnetospheric Models

Abstract

AbstractRecent studies have found that even during quiet times, observed proton isotropic boundaries (IBs) are often projected to the region of high adiabaticity parameter (K≈30), where is the ratio of magnetic field line radius of curvature to the particle gyroradius. This contradicts the accepted hypothesis that current sheet scattering (CSS) is the dominant mechanism of IB formation because K≈8 would be expected for this mechanism. We used magnetohydrodynamic simulations and empirical models to compute K for 30‐keV proton IB observations within 3 hr of local midnight. We found that neither class of model reliably estimates K unless supported by magnetic field observations in the current sheet. magnetohydrodynamic simulations produced higher K values than expected for CSS (K = 15–30), and empirical models gave lower values (K < 4). We obtained reliable estimates of K by controlling for the accuracy of the normal component and the gradient of the radial component in the neutral sheet, using observations from three Time History of Events and Macroscale Interactions during Substorms satellites. For the first time, we demonstrated that both these variables should be taken into account for the accurate estimation of the curvature radius. This greatly reduced the spread of K values, indicating that much of the previous spread was due to errors in the magnetic field but also that these errors can be controlled. Most of the corrected values fall within the expected range for CSS, supporting the hypothesis that the IB's were formed by CSS. Accounting for all model results, we obtain an average corrected value of K = 6.0.