False Positive Signatures of Agyrotropy Caused by Gradients

Any plasma detector takes time to collect its phase space distribution; during this time the spacecraft moves across (potentially) reorienting magnetic fields, and/or gradients of density, temperature, or flow velocity. This study reports on the magnitude of these false indications of agyrotropy for a spectrum of tangential and rotational discontinuity layer models that have differing degrees of (i) magnetic sheer, (ii) density variation, (iii) temperature variation, and (iv) various possible mixtures compatible with pressure balance for the TD and Walen condition for the RD. A separate study illustrates the sensitivity of the diagnostic to an incorrect determination of the electron bulk velocity, since agyrotropy is determined from the pressure tensor in the electron rest frame. These studies point to the importance of (a) adequate bulk velocity determinations and (b) the need for sub-moment cadence indicators of variation of the moments and the magnetic field direction when trying to use agyrotropy as a tool for discovery. Examples from the Polar Hydra instrument data products will illustrate this strategy for enhancing the integrity of physical agyrotropy detections and reducing the number of false positive ones. Examples of questionable and better detections of agyrotropy will be illustrated so that the method of rejection and acceptance are clarified.