Separating Contributions to Plasma Vorticity in the High‐Latitude Ionosphere From Large‐Scale Convection and Meso‐Scale Turbulence

Abstract

Measurements of ionospheric flow vorticity can be used for studying ionospheric plasma transport processes, such as convection and turbulence, over a wide range of spatial scales. Here, we analyze probability density functions (PDFs) of ionospheric vorticity for selected regions of the northern hemisphere high‐latitude ionosphere as measured by the Super Dual Auroral Radar Network over a 6‐year interval (2000–2005 inclusive). Subdividing these PDFs for opposite polarities of the By component of the prevailing interplanetary magnetic field allows the separation into two distinct components: (a) A single‐sided Weibull distribution which relates to the large‐scale convection driven by magnetic reconnection; (b) A double‐sided and symmetric q‐exponential distribution which arises from meso‐scale plasma flow related to processes such as turbulence.

The Spatial Variation of Large‐ and Meso‐Scale Plasma Flow Vorticity Statistics in the High‐Latitude Ionosphere and Implications for Ionospheric Plasma Flow Models

“…There are also other factors that may be controlling the meso-scale flow that we have not considered. As discussed in Chisham and Freeman (2023), large vorticity on the nightside may be associated with short substorm intervals of strong flows, which are interspersed with longer, quieter intervals. These flows would not be directly ordered by the IMF direction.…”

“…Recently, Chisham and Freeman (2023) developed a new method to statistically separate the contributions of large and meso-scale variations in the measured flow vorticity PDFs. Their results suggested that there are regions of the high-latitude ionosphere where the effects of meso-scale vorticity outweigh those of the large-scale vorticity.…”

“…Owing to the smaller area of the spatial bins used in this study, the typical number of vorticity values measured in any particular spatial bin is reduced compared to Chisham and Freeman (2023). In previous studies (Chisham & Freeman, 2010, 2023, we assumed that at least N = 100 vorticity values are required for an accurate MLE determination of the q-exponential and Weibull estimators but presented no assessment of how the level of uncertainty in the estimators varied with N. To address this, we simulate a large number of q-exponential and Weibull samples with different occurrence numbers, apply the MLE process, and assess the accuracy of the estimators.…”

“…Hence, in these regions, present ionospheric flow models are severely lacking. However, Chisham and Freeman (2023) only determined the separation of these components in two small high-latitude regions, to illustrate the potential of the methodology. In this paper, we extend this method to almost the whole of the northern hemisphere high-latitude region in order to study the spatial variation of these two vorticity components.…”

“…Hence, these studies have analyzed the positive and negative vorticity portions of the PDFs independently. From a comparison of the PDFs observed during opposing IMF B y conditions, Chisham and Freeman (2023) concluded that this asymmetry was a result of different combinations of the 'large-scale' (LS) vorticity inherent in the ionospheric convection driven by magnetic reconnection, and the 'meso-scale' (MS) vorticity associated with fluid processes, such as turbulence, and measurement errors. Chisham and Freeman (2023) separated the different contributions to the vorticity PDFs by making the following assumptions.…”