Analysis of close conjunctions between dayside polar cap airglow patches and flow channels by all-sky imager and DMSP

Abstract

Recent imager and radar observations in the nightside polar cap have shown evidence that polar cap patches are associated with localized flow channels. To understand how flow channels propagate from the dayside auroral oval into the polar cap, we use an all-sky imager in Antarctica and DMSP (F13, F15, F16, F17 and F18) to determine properties of density and flows associated with dayside polar cap patches. We identified 50 conjunction events during the southern winter seasons of 2007-2011. In a majority (45) of events, longitudinally narrow flow enhancements directed anti-sunward are found to be collocated with the patches, have velocities (up to a few km/s) substantially larger than the large-scale background flows (~500 m/s) and have widths comparable to patch widths (~400 km). While the patches start with poleward moving auroral forms (PMAFs) as expected, many PMAFs propagate azimuthally away from the noon over a few hours of MLT, resulting in formation of polar cap patches quite far away from the noon, as early as ~6 MLT. The MLT separation from the noon is found to be proportional to the IMF |By|. Fast polar cap flows of >~1500 m/s are predominantly seen during large IMF |By| and small |Bz|. The presence of fast, anti-sunward flow channels associated with the polar cap patches suggests that the flow channels form in the dayside auroral oval through transient reconnection and can be the source of flow channels propagating into the polar cap.

The Ion/Electron Temperature Characteristics of Polar Cap Classical and Hot Patches and Their Influence on Ion Upflow

“…In the noon sector or the central polar cap, the convection speed is lower than in the dawn/dusk region (before 9 and after 17 MLT) around 80° MLAT (Figure b). After comparing the averaged IMF and convection speed data for each patch (Figures S2 and S3), and consistent with Wang et al (), the patch occurrence rate is higher near the polar cap boundary (around 80° MLAT) where there is larger convection speed for strongly negative IMF B y . The field‐aligned current is higher near 80° MLAT than in the central polar cap (>85° MLAT), and it is downward in the postnoon sector and upward in the prenoon sector (Figure c), which may be associated with the region 0 cusp current system (Iijima & Potemra, ).…”

“…They found that some of airglow patches are associated with enhanced polar cap flow, strong localized filed‐aligned currents, and low‐energy precipitation. For large negative IMF B y , the airglow patch near the polar cap boundary can be associated with very fast antisunward flow (>~1,500 m/s; Wang et al, ). Recently, Zhang et al () used DMSP F16 in situ measurements to identify a similar enhanced density structure (electron density 6.6 × 10 5 cm −3 at 860‐km altitude) in the topside F region polar cap.…”

A Statistical Study of Polar Cap Flow Channels and Their IMF By Dependence

“…Figure 3 shows that the flow channels were mostly between 200 and 300 km in latitudinal width with peak velocities of 1.1–1.3 km s −1 . The width values are of a similar order to previous studies, such as Pinnock et al (1993) (100 km), Provan et al (1998) (250 km), Wang et al (2016) (300 km), and Zou et al (2015) (200–300 km).…”

“…As stated, the look direction of the SuperDARN Longyearbyen radar could be biasing these results, as it is orientated in a favorable direction to detect zonal flow channels. However, Wang et al (2016) also find that they detect flow channels associated with airglow patches most frequently during IMF By dominant conditions. In that study, satellite data were used to determine the flow characteristics, removing the look direction bias that applies to single SuperDARN radars and still noting the same IMF By dependence.…”

Identifying the evolution of Southern Hemisphere poleward moving auroral forms (PMAFs) in the context of plasma convection and magnetic reconnection

“…The imager uses a single detector in conjunction with a special dual‐wavelength interference filter with two narrow passbands corresponding to each wavelength. The ASC captures one image every minute and has a 10 km geographic resolution over most of the field of view and sensitivity of 20 Rayleighs per 8 s of exposure [ Wang et al, ]. The mapping height used in our analysis for 630.0 nm is 230 km, while it is 110 km for 427.8 nm, and the images use arbitrary intensity scaling from 0 to 255.…”