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In the last decade, two advanced modular incoherent scatter radars (AMISRs) have been installed deep in the polar cap at Resolute Bay, Canada, named Resolute Bay ISR‐North face (RISR‐N) and Canadian face (RISR‐C). These two ISRs provide new opportunities for in‐depth investigation of the patch plasma characteristics, such as altitude profiles of key plasma parameters (e.g., Dahlgren et al., 2012a and 2012b; Gillies et al., 2016; Lamarche & Makarevich, 2017; Perry & St. Maurice, 2018; Ren et al., 2018).

Using a special 25‐beam imaging mode of RISR‐N, Dahlgren et al. (2012a,b) revealed, for the first time, the 3‐D density structure of a patch and its temporal evolution. Figure 4.4 shows a volumetric image of the patch studied in Dahlgren et al. (2012b) with measured 630 nm redline emission shown at the bottom. They identified up to 10% density variability even though the patch acted as a closed system with no additional plasma transported horizontally into the patch. A comprehensive discussion is provided in Dahlgren et al. (2012a) trying understand the source of this density variation, including field‐aligned motion and local precipitation, but none of them seem to be supported by observations and, thus, they concluded that internal plasma structuring is responsible for the density variability and plasma irregularities develop rapidly as the patch drifts across the polar cap. Perry et al. (2015) later postulated that the density variations may be a signature of several patches with scale sizes below the spatial resolution of the radar system.

Figure 4.4 Volumetric image of a patch using RISR‐N data on 11 December 2009 at 22:12:36–22:13:46 UT. The horizontal slices show the electron density at 220, 250, 280, 310, and 340 km altitude. The contemporary 630.0 nm all‐sky image is projected onto the 200 km plane. The locations of the radar beams at each altitude slice are indicated as black circles

(from Dahlgren et al., 2012b; Reproduced with permission of John Wiley and Sons).

Using the most field‐aligned beams from RISR‐C, Ren et al. (2018) automatically identified over 400 patches in 2016 and statistically constructed the patch electron density, electron and ion temperatures, vertical flow and flux profiles in the noon, dusk, midnight, and dawn sectors. Figure 4.5 selectively presents the median profiles of those parameters at the center of patches (blue curves) in the noon and midnight sectors, compared with the sector (red curves) and overall (black curves) median profiles. As expected, the patch median density is higher than the sector median, and the F‐region peak density (h_mF₂) decreases by ~42% from the dayside (~3.6 x 10¹¹ m^‐3) to the nightside (~2.1 x 10¹¹ m^‐3). In the noon sector, the bottom F‐region density inside the patch is actually lower than that of the surrounding region, consistent with the SED density profile observation shown in Zou et al. (2013). As suggested in Zou et al. (2013), these SED plasma can be lifted to higher altitudes due to a combination of poleward convection flow and equatorward thermospheric wind. When the production in the lower F region is not fast enough to replenish the transported plasma, this lifting motion will result in a lower F‐region density than the surrounding region. This observation has also been reproduced in the numerical simulation performed by (Zou & Ridley, 2016).

Figure 4.5 Polar cap patch median profiles compared with sector median and all‐sector median profiles. From left to right, plasma density, electron temperature, ion temperature, and ion flux are shown. The ion flux profiles are based on measurements from the RISR‐C vertical beam

(modified based on Ren et al., 2018; Reproduced with permission of John Wiley and Sons).

In addition, Ren et al. (2018) found that the median electron temperature at the center of the patch is suppressed by as much as ~380 K compared with the sector median temperature in the noon sector. The patch median ion temperature is very close to the background ion temperature in all sectors except ~100 K higher in the noon sector, and the ion vertical flows and fluxes within patches are typically downward. Plasma temperature is often used to infer whether local particle precipitation is present in the patch. It can also shed light on the patch generation mechanisms, since higher electron temperature would be expected if local precipitation is responsible for the patch formation. Together with the evidence that the bottom F‐region patch density is smaller than that of the surrounding region in the noon sector, these observations suggest that the major plasma source for patches is the dayside solar EUV produced plasma transported into the polar cap region, which is consistent with the statistical UT dependence found in both modeling (Sojka et al., 1994) and observation (David et al., 2016). However, we also need to realize that the RISR‐C is deep in the center of the polar cap, where it takes time for the patch to drift from the dayside cusp region to reach, and the electron temperature can change along the trajectory. Therefore, a careful numerical modeling study is required to quantify the cooling rate inside the patch and evaluate whether the electron temperature is a good and sufficient indicator of the origin of the patch plasma.

Although statistically the polar cap patches have lower electron temperature than that of the surrounding regions, “hot” patches, which have higher electron temperature than the surrounding regions, have also been identified using DMSP observations at ~850 km (Zhang et al., 2017). It is found that this type of patch is associated with local particle precipitation and convection flow shear, and might be produced when the traditional patches convect into the particle precipitation region.