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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.

After publication of this article (Nishitani et al. 2019), it was brought to our attention that the figure 5 is incorrect, where the positions of DCE and SPS were misplaced. The correct figure 5 is as below, the original publication has been corrected.

Backscatter data collected with a 398 MHz phased-array radar operated at Homer, Alaska (59.72 deg N, 151.53 deg W) have been analyzed for information on the height of radar auroral echoing. Altitude was resolved through the variation of backscattered power with antenna beam elevation angle. The mean height of backscattering could be determined with an accuracy of 1-2 km over small ((TURN)20 x 20 km('2)) areas and short ((TURN)1 min) periods. In this thesis, the results are presented in the form of maps of the spatial distribution of echo height. The data base encompassed approximately 40 hrs of observation carried out in 1973, 1976, and 1978. Echo activity most often spanned a 10-20 km range of height centered on 100-110 km. The echoing region was always sharply bounded from below at 96-98 km. The upper altitude limit of echo activity was 115-120 km. The height range in events of the post-midnight sector was 97-110 km vs. 97-118 km in events of the pre-midnight sector. Echoing was restricted to directions nearly perpendicular to the geomagnetic field, giving rise to systematic spatial and temporal variations of height. Magnetic aspect control of echo height was weaker in events of discrete radar aurora. The height range of the Homer echo activity is shown to be consistent with the onset of primary two-stream plasma instability within auroral electrojet current. The modulation of height within the altitude limits of echo activity by the magnetic aspect geometry is attributed to strong directional confinement of plasma wave growth. It is suggested that the spatial and temporal variabilities of radar auroral altitude derive from structure within auroral ionization and variability of auroral electrojet current.

Auroral observations in combination with radar observations of ionospheric flows have revealed much about the mesoscale flow bursts along nightside, plasmasheet magnetic field lines. The major disturbances within the magnetosphere‐ionosphere system include the disturbances associated with poleward boundary intensifications and aurora streamers themselves. Studies have shown that flow bursts can also lead to auroral arc formation during the sub‐storm growth phase and to the substorm onset instability in the near‐Earth plasmasheet. Additionally, during the substorm expansion phase, the flow bursts appear to have larger signatures than before substorms. They drive much of the expansion phase dynamics, including the substorm current wedge via a series of flow bursts, as well as much of the geomagnetic activity associated with expansion phase itself. Flow bursts that reach the subauroral polarization stream (SAPS) region lead to large SAPS region flow enhancements that propagate westward in association with westward‐moving auroral enhancement.