Radar observations of density gradients, electric fields, and plasma irregularities near polar cap patches in the context of the gradient‐drift instability

Abstract

We present observations of plasma density gradients, electric fields, and small‐scale plasma irregularities near a polar cap patch made by the Super Dual Auroral Radar Network radar at Rankin Inlet (RKN) and the northern face of Resolute Bay Incoherent Scatter Radar (RISR‐N). RKN echo power and occurrence are analyzed in the context of gradient‐drift instability (GDI) theory, with a particular focus on the previously uninvestigated 2‐D dependencies on wave propagation, electric field, and gradient vectors, with the latter two quantities evaluated directly from RISR‐N measurements. It is shown that higher gradient and electric field components along the wave vector generally lead to the higher observed echo occurrence, which is consistent with the expected higher GDI growth rate, but the relationship with echo power is far less straightforward. The RKN echo power increases monotonically as the predicted linear growth rate approaches zero from negative values but does not continue this trend into positive growth rate values, in contrast with GDI predictions. The observed greater consistency of echo occurrence with GDI predictions suggests that GDI operating in the linear regime can control basic plasma structuring, but measured echo strength may be affected by other processes and factors, such as multistep or nonlinear processes or a shear‐driven instability.

Statistical Analysis of the Electron Density Gradients in the Polar Cap F Region Using the Resolute Bay Incoherent Scatter Radar North

“…In addition, Figure shows a distribution of the horizontal gradient strength at 308 km shown in dark gray found in the present study (the same distribution as that shown by the green line in Figure b). From this distribution, the typical gradient strength at this height is 2 − 5×10 −6 m −1 , which is in agreement with a previous study by Lamarche and Makarevich (). From the data presented by Lamarche and Makarevich () only ∼5% of measurements exceed G hor =10 −5 m −1 (Lamarche & Makarevich, ).…”

“…From this distribution, the typical gradient strength at this height is 2 − 5×10 −6 m −1 , which is in agreement with a previous study by Lamarche and Makarevich (). From the data presented by Lamarche and Makarevich () only ∼5% of measurements exceed G hor =10 −5 m −1 (Lamarche & Makarevich, ). This is in contrast with the current study that showed occurrence of gradients with G hor >10 −5 m −1 is 15.5% (out of ∼1.6 × 10 8 measurements).…”

“…This limitation was due to limited instrumental capabilities, such as the direction of the radar beam (Haldoupis et al, ) or the rocket's flight path (Moen et al, ; Lynch et al, ). One exception is a recent study by Lamarche and Makarevich () where the 2‐D density gradient vectors were estimated using ISR density measurements in the plane with the fixed height. It was demonstrated that gradients at the height of 300 km have spatial scale on the order of 500 km (Lamarche & Makarevich, ) but sometimes can reach 50 km (Lamarche & Makarevich, ).…”

“…One exception is a recent study by Lamarche and Makarevich () where the 2‐D density gradient vectors were estimated using ISR density measurements in the plane with the fixed height. It was demonstrated that gradients at the height of 300 km have spatial scale on the order of 500 km (Lamarche & Makarevich, ) but sometimes can reach 50 km (Lamarche & Makarevich, ). All previous direct measurements of the gradients were event based and did not have a large enough statistics to address the question of how often gradients exceed the threshold for a direct generation of GDI waves.…”

Critical density gradients for small‐scale plasma irregularity generation in the E and F regions

“…More generally, critical gradients and growth rate also depend strongly on the flow angle and this directional dependence is not traditionally considered. The two exceptions were recent studies by Lamarche and Makarevich [] and Lamarche and Makarevich [] that have considered this dependence as predicted by the fluid theory in the long‐wavelength limit [ Makarevich , ]. The current study allows to improve accuracy of predictions by considering effects of the ion inertia and thermal diffusion.…”