Soft electron precipitation in collaboration with ion upwelling is investigated as a possible driver of the cusp neutral density enhancement discovered by the CHAMP satellite. A time-dependent, three-uid numerical model is used to simulate the vertical ionospheric and thermospheric response for this type of event. Particle data from the FAST satellite from a single favorable conjunction alignment event with CHAMP are input to the model. Results are given which suggest that altitude-dependent neutral density enhancements accompany ion upwelling that is driven by soft electron precipitation. The enhancement mechanism is summarized as follows. Su ciently soft electron precipitation transfers energy to the ambient electron gas , which subsequently undergoes a thermal (upward) expansion. This establishes a vertical ambipolar electric eld which pulls the ions upward. The momentum carried by up-owing ions is signi cant and results show it to be capable of dragging neutral gas upward to create density structures above the F-region.

The Fast Auroral Imager (FAI) consists of two charge-coupled device (CCD) cameras: one to measure the 630 nm emission of atomic oxygen in aurora and enhanced night airglow; and the other to observe the prompt auroral emissions in the 650 to 1100 nm range. High sensitivity is realized through the combination of fast lens systems (f/0.8) and CCDs of high quantum efficiency (>90 % max). The cameras have a common 26 degree field-of-view to provide nighttime images of about 650 km diameter from apogee at 1500 km. The near infrared camera provides up to two images of 0.1 s exposure per second with a spatial resolution of a few km when the camera is pointing in the nadir direction, making it suitable for studies of dynamic auroral phenomena. The 630-nm camera has been designed to provide one image of 0.5 s exposure every 30 seconds. Launch of the satellite occurred on September 29, 2013. Following a description of the instrument, sample auroral images are presented.

The Rocket Experiment for Neutral Upwelling 2 (RENU2) rocket was launched on 13 December 2015 at 07:34 UT. The payload transited the cusp region during a neutral upwelling event, supported by a comprehensive set of onboard and ground-based instrumentation. RENU2 data highlight two important processes. One is that a proper understanding of neutral upwelling by Poleward Moving Auroral Forms (PMAFs) requires a treatment that mimics the quasiperiodic passage of a sequence of PMAFs. As a PMAF reaches a flux tube, its physical consequences must be determined including the residual history of effects from previous passages, implying that understanding such a process requires an accounting of the system hysteresis. Second, RENU2 observations suggest that neutral density enhancements driven by precipitation and/or Joule heating can be highly structured in altitude and latitude. In addition, timescales involving neutral dynamics suggest that the structuring must be slowly changing, for example, over the course of 10 to tens of minutes. 1.1. Altitudinal Structuring Additional modeling constraints have arisen from recent observations, those that show not only density enhancements but also depletions at higher altitudes. For example, the Streak satellite observed relative density depletions near the southern cusp in the altitude range 123-325 km when averaged over all orbits

At 0735 UT on 13 December 2015, the Rocket Experiment for Neutral Upwelling-2 experiment launched north toward the auroral cusp region from Andoya, Norway. The instrumented rocket included an electron spectrometer, photometers that measured the auroral redline and greenline, and an instrument that measured ionospheric thermal electron temperature. On the down leg, just south of Svalbard, the rocket entered a region of poleward moving auroral forms that were characterized by narrow structures due to a combination of spatial and temporal variations. A noticeable feature was that the redline to greenline brightness ratio was much smaller than expected. A model is developed that shows that these emissions can be used to estimate the lifetimes of bursty electron precipitation. This model is shown to be consistent with some poleward moving auroral form lifetimes being on the order of 100 ms. The correlation between the precipitation and temperature bursts suggest that some transport occurred.

The cusp is one of the most dynamic regions in the polar ionosphere. The cusp is located near noon just poleward of the open and closed magnetic field lines of the dayside magnetosphere-ionosphere system, and a variety of energy input processes from the magnetosheath and magnetopause characterize the cusp. Precipitation in the cusp is dominated by soft electrons and energetic protons, and impact ionization and precipitation heating primarily occur in the F-region ionosphere and thermosphere (Hardy et al., 1985;Skjaeveland et al., 2017). The cusp is located at the dayside convection throat, where fast flows transport plasma from closed to open field lines (Moen et al., 2001;Sandholt et al., 2003) and increases Joule heating (Shi et al., 2017). The heating in the cusp increases plasma upflows and thermospheric density (Carlson et al., 2012;Ogawa et al., 2009). In addition to the large-scale quasi-steady convection, pulsed flows propagate poleward in association with localized field-aligned currents. The footprint of the upward FACs is illuminated as discrete auroral arcs, and it is referred to as poleward moving auroral forms (PMAFs) (Oksavik et al., 2004). This transient system is characterized as a flow channel and a wedge-like current system.An important consequence of the cusp dynamics is formation of polar cap patches. During a large southward IMF, the open-closed boundary moves equatorward and photoionized plasma from the sunlit ionosphere becomes an important source of polar cap patches (