Investigation of the Gradient Drift Instability as a Cause of Density Irregularities in Subauroral Polarization Streams

Abstract

Subauroral polarization streams (SAPS) are regions of enhanced westward plasma flow associated with an E × B drift, which is driven by a strong poleward electric field caused by magnetosphere-ionosphere coupling through the closure of Region 2 field-aligned currents (Foster & Burke, 2002). These plasma flows are accompanied by density, or conductivity, troughs (Spiro et al., 1978) that are formed due to increased recombination caused by larger ion frictional heating from the increased plasma velocity (Schunk et al., 1976). This decreased conductivity then further increases the electric field which creates a feedback loop that results in a continued lowering of the conductivity (Anderson et al., 1993). SAPS occur in the dusk-midnight sector and have a latitudinal width of a few degrees. In the literature, subauroral ion drifts and polarization jets are other terms that have been used to describe narrow SAPS. The SAPS velocities can range from Abstract Density irregularities have been observed in subauroral polarization streams (SAPS). One hypothesis of the cause of this ionospheric turbulence, based on the background morphology, is the gradient drift instability (GDI). This work models the GDI using a two-dimensional electrostatic fluid model to determine if it is a viable cause of turbulence generation in SAPS. A statistical study of different velocity profiles, based on SuperDARN radar and Global Positioning System total electron content data, is used to prescribe parameters in the numerical model. The parameter space of different SAPS profiles is explored to study the effect on GDI development. As the velocity shear is initialized closer to the unstable density gradient, the GDI becomes increasingly damped. For these cases, the density and electric potential turbulence cascades obtained from the numerical model follow power laws of about −5/3 or −2, which is in agreement with observational data. If the region of sheared velocity overlaps the density gradient, the GDI becomes stabilized. The latitudinal location of maximum GDI growth depends on the density profile, the velocity profile, and the neutral wind direction. Using velocity profiles with regions of low velocity shear can cause instabilities that grow inside SAPS which have turbulence cascades with different behavior. In all parameter regimes considered, the GDI turbulence is precluded from extending through regions of velocity shear. Turbulence is generated for a variety of SAPS relevant conditions; therefore, the GDI has been shown to be a viable candidate for generating ionospheric irregularities in SAPS.Plain Language Summary Turbulence in the ionosphere is important to understand because it can negatively impact radio communication signals such as Global Positioning System (GPS) signals.For example, when GPS signals travel through a turbulent region in the ionosphere, a device's position estimates become less accurate. This work helps scientists understand how turbulence is generated in the ionosphere by simulating a phenomenon called subauroral pol...

Stratified Subauroral Ion Drift (SSAID)

“…For instance, a numerical model by Rathod et al. (2021) based on the gradient drift instability successfully reproduced the spectrum in the Mishin and Blaunstein (2008) observations.…”

“…Additionally, the formation of PJS can be caused both by plasma instabilities generated by strong (supersonic) ion drift and by various small-scale fluctuations in the measured parameters of the ionospheric plasma. For instance, a numerical model by Rathod et al (2021) based on the gradient drift instability successfully reproduced the spectrum in the Mishin and Blaunstein (2008) observations.…”

Long Lifetime Hiss Rays in the Disturbed Plasmasphere

“…Without these plasmaspheric density ducts, the quasi‐parallel plume hiss waves would be difficult to enter the plasmaspheric core. In the low‐altitude ionosphere, the gradient drift instability (Gondarenko & Guzdar, 2004; Keskinen et al., 2004; Rathod et al., 2021) and temperature gradient instability (Greenwald et al., 2006; Hudson & Kelley, 1976) can produce density fluctuations on a scale from several to tens of km, which may further cascade into fluctuations of 10 m scale (Eltrass et al., 2016; Heine et al., 2017; Nishimura et al., 2021). When mapped from the ionosphere to the magnetosphere, these density fluctuations appear as the field‐aligned density ducts with cross‐field sizes of ∼0.1–100 km (Gu et al., 2022; Nishimura et al., 2022).…”

Effect of charged dust grains on the electrojet instabilities

“…Type-II or gradient drift(GD) instability (which is driven by the electron density and the potential gradients) is linearly unstable at the long wavelength that is much longer than the FB instability [26][27][28][29][30][31][32][33]. In order to explain the physical mechanism of gradient drift instability first let us consider the electron density gradient is along the y ˆdirection and the relative drift of the electrons as compared to the ions is parallel to the density gradient.…”