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Two important phenomena of the solar wind–magnetosphere–ionosphere coupling are auroral particle precipitation and the formation of ions flowing upward from the ionosphere. They have opposite transport directions of energy and substance. Based on the observations of particle precipitation and ion drift from the DMSP F13 satellite in January and July 2005, the ionospheric ion upflows in dayside auroral oval (0600–1800 MLT) can be divided into five types according to the velocity of ion upflows and the spectrum characteristics of auroral particle precipitation, and the distribution for different types of ion upflows is studied. The results show that the ion upflows mainly occur in the geomagnetic latitude (MLAT) range of 70–80°.The main magnetospheric source region of ion upflows (type A and D) caused by the accelerated electron (mainly the soft electron) corresponds to Low Latitude Boundary Layer (LLBL) and Cusp, and ion upflows of type B and C (related to the process of ambipolar diffusion caused by electron acceleration) mainly occur in LLBL and Boundary Plasma Sheet (BPS), while ion upflows of type E without electron acceleration mainly occur in the central plasma sheet (CPS).The dawn–dusk asymmetry is obvious in the winter season, with the ion upflows mainly occurring on the dawn/dusk side ionosphere. However, the ion upflows in summer mainly occur at the magnetic noon, with a symmetric distribution centered at the magnetic noon. The occurrence of ion upflow in winter is significantly higher than that in summer, and it is significantly enhanced during the period of moderate geomagnetic activity. The upward region expands to the lower latitude when the geomagnetic activity is enhanced. The effect of interplanetary magnetic field (IMF) components has also been studied in this paper. When IMF Bx is negative, the upflow occurrence increases in the region of 1500–1800 MLT and 0600–0900 MLT, with the MLAT range below 70°. The direction of IMF By may lead to the high-incidence area reverse at the prenoon or postnoon region. The occurrence of ion upflows with the MLAT range below 75° increases significantly when IMF is southward. Type A ion upflow has the highest velocity of ion upflows, followed by type E, and type D has the lowest. The average velocity of ion upflows in winter is significantly higher than that in summer. [ABSTRACT FROM AUTHOR]

Ion upflow in the F region and topside ionosphere can greatly influence the ion density and fluxes at higher altitudes and thus has significant impact on ion outflow. We investigated the statistical characteristics of ion upflow and downflow using a 3‐year (2011–2013) data set from the Poker Flat Incoherent Scatter Radar (PFISR). Ion upflow is twice more likely to occur on the nightside than on the dayside in PFISR observations, while downflow events occur more often in the afternoon sector. Upflow and downflow on the dayside tend to occur at altitudes ~500 km, higher than those on the nightside. Both upflow and downflow occur more frequently as ion convection speed increases. Upflow observed from 16 to 6 magnetic local time through midnight is associated with temperature and density enhancements. Occurrence rates of upflow on the nightside and downflow on the dayside increase with geomagnetic activity level. On the nightside, occurrence rate of ion upflow increases with enhanced solar wind and interplanetary magnetic field (IMF) drivers as well as southwestward local magnetic perturbations. The lack of correlation of upflow on the dayside with the solar wind and IMF parameters is because PFISR is usually equatorward of the dayside auroral zone. Occurrence rate of downflow does not show strong dependence on the solar wind and IMF conditions. However, it occurs much more frequently on the dayside when the IMF By > 10 nT and the IMF Bz < −10 nT, which we suggest is associated with the decaying of the dayside storm‐enhanced density (SED) and the SED plume. Key Points: The occurrence frequency of ion upflow increases with enhanced geomagnetic activity level and stronger solar wind and IMF drivingIon upflow at PFISR latitude is twice more likely to occur on the nightside than on the daysidePeak ion downflow occurrence rate reaches 30% on the dayside during strongly positive IMF By and negative Bz, associated with SED and plume [ABSTRACT FROM AUTHOR]

Auroral emission at 427.8‐nm from N2+ ions is caused by precipitation of energetic electrons, or by resonant scattering of sunlight by auroral N2+ ions. The latter often causes impressive purple aurora at high altitudes. However, statistical characteristics of auroral 427.8‐nm emission have not been well studied. In this paper we report occurrence characteristics of high 427.8‐nm emission intensities (more than 100 R) at subauroral latitudes, based on measurements by a filter‐tilting photometer over 14 years (2005–2018) at Athabasca, Canada (magnetic latitude: ~62°). We divided the data set into solar elevation angles (θs) more than and less than −24° (shadow height of sunlight: 600 km) to separate the 427.8‐nm emissions caused by resonant scattering of sunlight and those excited by auroral electrons, respectively. The occurrence rate of 427.8‐nm emissions of more than 100 R is 10.6% and 7.65% for θs more than and less than −24°, respectively, confirming that resonant scattering of sunlight by N2+ ions is a cause of the strong 427.8‐nm emissions of more than 100 R in the sunlit ionosphere. The occurrence rate is high in the postmidnight sector and increases with increasing geomagnetic activity, solar wind speed, and density. The occurrence rate is lowest in winter. A high occurrence rate was observed in 2015–2018, during the declining phase of the 11‐year solar activity. Superposed epoch analysis indicates that the 427.8‐nm emission exceeds 100 R when solar wind speed increases and solar wind density concurrently decreases, though the standard deviation of the data is rather large. Plain Language Summary: Purple auroras can be caused either by energetic electrons at low altitudes (about 100 km), or by scattering of sunlight at much higher altitudes where the auroral curtains are in the Sun although the ground is in darkness. We have studied both types by sorting measurements of the purple emission from ionized nitrogen molecules depending on how far the Sun is below the horizon. We used 14 years' worth of data in order to study the effects over more than a solar cycle (11 years typically). The emission is stronger when general auroral activity increases, which is in turn related to solar wind speed and density. Same result was obtained by binning solar wind and geomagnetic activity data, timing them relative to strong purple aurora. It is also highest in the declining phase of the solar cycle. The emission intensity becomes low in winter. Recently, a type of purple aurora outside of the normal auroral oval has been reported, called "STEVE." We note that we have studied the "traditional" type of purple aurora. Key Points: We made a statistical study of auroral/resonant‐scattering 427.8‐nm emission observed at subauroral latitudes over 14 yearsThe resonant‐scattering 427.8‐nm emission indicates upflow of ionospheric nitrogen ions to higher altitudesThe emission increases during magnetic active periods and is associated with increases of solar wind speed and density [ABSTRACT FROM AUTHOR]

Abstract: The large scale dynamics of the cusp ionosphere is directly controlled by the solar wind and the orientation of the interplanetary magnetic field. The IMF BY controlled east–west movement of the poleward moving forms (PMAFs), separating from the cusp, is consistent with the magnetic tension pull of newly opened flux, which is a key feature tying the auroral phenomenon of PMAFs to flux transfer events (FTEs) at the magnetopause. The central region of an FTE is a mesoscale flow channel, bounded by a pair of Birkeland current sheets on each flank. Systematic observations by EISCAT Svalbard Radar, meridian scanning photometers and all-sky imagers have provided us further insight into key processes of the cusp ionosphere such as how FTEs/PMAFs control ion upflow events, and how transient magnetopause reconnection processes segment a stream of high density solar EUV plasma into polar cap patches at the polar cap boundary. We have found a close causal relationship between formation of plasma patches and onset of scintillation by a previously unrecognized process. We have discovered a new category of flow channels, reversed flow channel events (RFEs) that may appear related to a signature of Birkeland currents arcs, and may be signature of the MI-coupling/inverted V rather than mapping directly to the magnetopause. This review paper concentrates on a niche of cusp studies made by high-resolution radar and optical measurements. [Copyright &y& Elsevier]