Fast‐moving diffuse auroral patches: A new aspect of daytime Pc3 auroral pulsations

Abstract

Auroral pulsations are a convenient diagnostic of wave‐particle interactions in the magnetosphere. A case study of a daytime Pc3 (22–100 mHz) auroral pulsation event, measured with a ~2 Hz sampling all‐sky camera at South Pole Station (74.4°S magnetic latitude) on 17 May 2012, is presented. The daytime Pc3 auroral pulsations were most active in a closed field line region where the aurora was dominated by diffuse green‐line emissions and within ±2 h of magnetic local noon. Usually, but not always, the corresponding periodic variations were recorded with a colocated search coil magnetometer. Of particular interest is the two‐dimensional auroral signature, indicating that the temporal luminosity variations at a given point were due to repeated formation and horizontal motion of faint, nonpulsating auroral patches with scale sizes of ~100 km. The individual patches propagated equatorward with speeds of 15 km s−1 up to 20–25 km s−1 one after another along the magnetic meridian through local magnetic zenith. These properties differ considerably from typical pulsating aurorae, being periodic on‐off luminosity variations in a particular auroral patch and drifting in accordance with the convection electric field in the magnetosphere. We speculate that such repetitive patterns of the fast‐moving auroral patches, being another aspect of the daytime Pc3 auroral pulsations, may be a visible manifestation of compressional Pc3 waves which propagate earthward and cause modulation of precipitating keV electron fluxes in the dayside outer magnetosphere.

On the Driver of Daytime Pc3 Auroral Pulsations

“…Unfortunately, the calibrated monochromatic all‐sky imager at SPA did not operate in the 2016 season, which made it impossible to quantitatively estimate the absolute values of the auroral luminosity for Event 1. However, our earlier study has revealed that the green line emission of daytime diffuse auroral pulsations has several hundred Rayleighs (see Figure b of Motoba et al, ), which is in good agreement with the small‐amplitude chorus wave modulation observed in space, suggesting it is responsible for the daytime auroral pulsations. On the other hand, no chorus data were available at TH‐E for Event 2, which leaves room for considering another possibility, for example, compressional Pc3 magnetic pulsations may act as a direct driver for scattering keV electrons (Brito et al, , ; Rae et al, ).…”

“…This means that the daytime auroral pulsations measured at a given point (i.e., the zenith of SPA) were produced by the fast spatial motion of nonpulsating auroral patches not by the temporal luminosity variation in a pulsating auroral patch. Such 2‐D morphological signatures of the daytime auroral pulsations are very similar to those reported by Motoba et al (), as was mentioned in the introduction.…”

“…The earthward propagation of the Pc3 waves in the magnetosphere matches well with the 20‐ to 25‐km/s equatorward propagation speed of the auroral patches estimated from the auroral intensities along a north‐south keogram (please also see supporting information). The simultaneous space‐ground observations provide evidence that the 2‐D behavior of daytime auroral pulsations is consistent with the earthward propagation of compressional Pc3 waves in the dayside outer magnetosphere, as suggested by Motoba et al ().…”

“…Geophysical Research Letters emission of daytime diffuse auroral pulsations has several hundred Rayleighs (see Figure 3b of Motoba et al, 2017), which is in good agreement with the small-amplitude chorus wave modulation observed in space, suggesting it is responsible for the daytime auroral pulsations. On the other hand, no chorus data were available at TH-E for Event 2, which leaves room for considering another possibility, for example, compressional Pc3 magnetic pulsations may act as a direct driver for scattering keV electrons (Brito et al, 2012(Brito et al, , 2015Rae et al, 2018).…”

Development of low-cost multi-wavelength imager system for studies of aurora and airglow

“…The 630-nm emission data are used not only for auroral study, but also airglow observation such as polar patches in the polar cap region. Panchromatic and color all-sky camera observations have been also continuously conducted for dayside auroral pulsation study [e.g., Motoba et al, 2017] even during new moon periods, as well as narrow/wide FOV camera observations mentioned in the previous section. At the South Pole and McMurdo stations, all the cameras can be operated for 24 hours a day between April and August.…”