The Super Dual Auroral Radar Network (SuperDARN) is a network of high-frequency (HF) radars located in the high-and mid-latitude regions of both hemispheres that is operated under international cooperation. The network was originally designed for monitoring the dynamics of the ionosphere and upper atmosphere in the high-latitude regions. However, over the last approximately 15 years, SuperDARN has expanded into the mid-latitude regions. With radar coverage that now extends continuously from auroral to sub-auroral and mid-latitudes, a wide variety of new scientific findings have been obtained. In this paper, the background of mid-latitude SuperDARN is presented at first. Then, the accomplishments made with mid-latitude SuperDARN radars are reviewed in five specified scientific and technical areas: convection, ionospheric irregularities, HF propagation analysis, ion-neutral interactions, and magnetohydrodynamic (MHD) waves. Finally, the present status of mid-latitude SuperDARN is updated and directions for future research are discussed.

A general dispersion relation is derived that integrates the Farley‐Buneman, gradient‐drift, and current‐convective plasma instabilities (FBI, GDI, and CCI) within the same formalism for an arbitrary altitude, wave propagation vector, and background density gradient. The limiting cases of the FBI/GDI in the E region for nearly field‐aligned irregularities, GDI/CCI in the main F region at long wavelengths, and GDI at high altitudes are successfully recovered using analytic analysis. Numerical solutions are found for more general representative cases spanning the entire ionosphere. It is demonstrated that the results are consistent with those obtained using a general FBI/GDI/CCI theory developed previously at and near E region altitudes under most conditions. The most significant differences are obtained for strong gradients (scale lengths of 100 m) at high altitudes such as those that may occur during highly structured soft particle precipitation events. It is shown that the strong gradient case is dominated by inertial effects and, for some scales, surprisingly strong additional damping due to higher‐order gradient terms. The growth rate behavior is examined with a particular focus on the range of wave propagations with positive growth (instability cone) and its transitions between altitudinal regions. It is shown that these transitions are largely controlled by the plasma density gradients even when FBI is operational.

Abstract. In this study, a focused investigation of the potential for the King Salmon (KS) SuperDARN HF radar to monitor high-velocity flows near the equatorial edge of the auroral oval is undertaken. Events are presented with lineof-sight velocities as high as 2 km/s, observed roughly along the L-shell. Statistically, the enhanced flows are shown to be typical for the dusk sector (16:00-23:00 MLT), and the average velocity in this sector is larger (smaller) for winter (summer) conditions. It is also demonstrated that the highvelocity flows can be very dynamical with more localized enhancements existing for just several minutes. These shortlived enhancements occur when the luminosity at the equatorial edge of the auroral oval suddenly decreases during the substorm recovery phase. The short-lived velocity enhancements can be established because of proton and ion injections into the inner magnetosphere and low conductance of the ionosphere and not because of enhanced tail reconnection. This implies that some KS velocity enhancements have the same origin as subauroral polarization streams (SAPS).

The cubic dispersion relation describing inertial modes of fundamental ionospheric instabilities at arbitrary altitude is demonstrated to yield three distinct solutions, and their stability is analyzed using a combination of numerical and analytic techniques. A robust numerical method is developed for obtaining all three solutions for arbitrary altitude, and analytic expressions are developed for two solutions. In the E region, one unstable and two stable modes are found for strong electric fields, with the unstable mode being the Farley‐Buneman instability. In the F region, zero, one, or two unstable modes are found depending on the plasma density gradient. The first unstable mode represents a finite‐temperature generalization of the inertial mode of the gradient drift instability (GDI) in the F region that has been previously considered for cold‐plasma case. The instability cone width and the gradient strength cutoff values are analyzed analytically and inertial effects are shown to drastically alter their behavior for decameter‐scale waves. In particular, progressively stronger gradients are required to excite the instability with an increasing electric field. Another strongly unstable mode is found at high altitudes and for sufficiently sharp gradients, although the applicability of this solution is limited due to its high‐frequency nature. The results strengthen the case for analyzing different ionospheric instabilities within the same formalism and provide an additional framework for interpreting the experimentally observed irregularity formation times that are inconsistent with those predicted by the standard GDI theory.

A general dispersion relation for the gradient drift instability (GDI) in the lower ionosphere is derived and solved analytically for the oscillation frequency and growth rate of unstable GDI waves. The approach presented is applicable in the broad range of altitudes, both within the E region and the lower F region, and for an arbitrary background density gradient. The ion and electron fluids are treated in the same way, and linearized system of fluid equations is solved exactly, with no geometry‐ or altitude‐specific approximations required. It is demonstrated that, in the short‐wavelength limit, the GDI growth rate is maximized along the bisector between the electric current and the cross product of the gradient vector and magnetic field. This result holds at all considered altitudes, including a transitional region between the E and F regions. Symmetries of the resulting expression for the growth rate are discussed, and numerical calculations for representative gradient and current configurations are presented to illustrate and validate analytical results.