Flickering aurora is an optical phenomenon in which luminosity rapidly oscillates at frequencies of approximately 3–15 Hz within localized, patch-like regions with horizontal scales of a few kilometers . These patches are known to appear below bright discrete auroras and may exhibit drifting or rotational motions . The amplitude of the luminosity modulation is typically about 10 %–20 % of the brightness of the background discrete aurora .

Based on previous observational results, the origin of flickering aurora is widely regarded as being closely related to electron acceleration by electromagnetic ion cyclotron (EMIC) waves. Using simultaneous observations with sounding rockets and ground-based cameras, reported field-aligned oscillations of the electron flux, known as field-aligned bursts (FABs), occurring simultaneously with flickering aurora. From the velocity dispersion of these electrons, they inferred that the source region was located at altitudes of approximately 4000–8000 km. It has been pointed out that the cyclotron frequencies of oxygen EMIC waves generated at these altitudes are in good agreement with the typical frequencies observed in flickering aurora .

Several theoretical interpretations have been proposed to explain how EMIC waves accelerate electrons. suggested that EMIC waves excited in the lower part of the acceleration region generate an ambipolar potential through the ponderomotive force. This potential transports cold electrons from the ionosphere upward to the lower part of the acceleration region, where the electrons subsequently resonate with the EMIC waves and are precipitated back into the ionosphere, leading to FABs. However, this model requires unrealistically large wave amplitudes. As an alternative scenario, proposed that EMIC waves periodically perturb and release cold electrons trapped within the acceleration region, allowing them to be accelerated by the potential drop and precipitated into the ionosphere, thereby producing flickering aurora.

demonstrated that the spatiotemporal patterns of flickering aurora observed with an imaging photometer can be explained by interference between two dispersive Alfvén waves (EMIC waves or inertial Alfvén waves), and showed that their results are consistent with the model proposed by , in which flickering aurora is generated through resonance between EMIC waves and electrons in the lower part of the acceleration region. In addition, used numerical simulations to show that the complex spatiotemporal structures of flickering aurora can be reproduced by considering interference among multiple EMIC waves.

Within this theoretical framework, observed chirps in which the flickering frequency varied on time scales of about 1 s, using narrow-field cameras and photometers. They suggested that higher flickering frequencies correspond to lower electron energies inferred from auroral emission intensity ratios, whereas lower frequencies are associated with higher electron energies. Their observations can be explained by the model of , in which electron energy is primarily controlled by the parallel phase velocity of dispersive Alfvén waves (such as EMIC waves), and are also consistent with the picture proposed by and , in which electron flux is modulated through resonance between EMIC waves and electrons.

If EMIC waves excited at altitudes of several thousand kilometers are responsible for flickering aurora, then not only oxygen ion modes at frequencies of about 3–15 Hz but also higher-frequency modes associated with hydrogen and helium ions may be observable. In earlier periods, however, the temporal resolution of TV cameras was limited to 30 frames s⁻¹, making it difficult to record the rapid luminosity variations of flickering aurora in video. overcame this limitation by performing photometric observations of flickering aurora at 40 kHz, and found amplitude modulations in the 35–60 Hz range, as well as fluctuation components reaching up to 180 Hz. With the advent of electron-multiplying CCD (EMCCD) detectors, further demonstrated, using high-speed imaging at 100 Hz, that variations above 30 Hz were frequently observed, accounting for roughly one third of all events. Using multispectral EMCCD imaging, demonstrated fine-scale spatiotemporal variations of flickering aurora near the magnetic zenith and discussed their consistency with interference-based EMIC/DAW scenarios. More recently, using a scientific CMOS (sCMOS) camera, which provides higher-fidelity images without electron multiplication, conducted imaging observations at 160 frames s⁻¹ and suggested that, in addition to patches flickering at typical frequencies of around 10 Hz, patch-like regions can also flicker over only a few frames (i.e., near 80 Hz). They interpreted this as the first evidence that flickering patches can be generated by multi-ion-mode EMIC waves.

As summarized above, a wide range of previous studies has suggested that interactions with EMIC waves play an essential role in the generation of flickering aurora and the accompanying FABs. However, the observational evidence used to construct these models has almost exclusively been obtained with narrow-field cameras. This is partly unavoidable for clearly resolving flickering patches, which are small-scale phenomena with horizontal sizes of a few kilometers and rapid intensity variations at frequencies of order 10 Hz, and for minimizing projection and line-of-sight superposition effects when characterizing small-scale structuring.

At the same time, there have been efforts to extend high-speed imaging to moderately larger fields of view (FoV). For example, used a 15° × 15° high-speed camera oriented toward the magnetic zenith and reported compound auroral microstructures, including cases where localized flickering appeared together with pulsating aurora; they noted that the larger FoV was an important factor in identifying such simultaneous features. Recent observations have also linked pre-onset wave-like auroral forms to auroral kilometric radiation precursors and dispersive-Alfvén-wave-driven double layers , highlighting the importance of mesoscale structuring in discrete auroral arcs.

Nevertheless, when the FoV is limited, it remains difficult to continuously follow flickering aurora over longer periods, particularly when it appears together with discrete auroral arcs that are narrow in the latitudinal direction and evolve dynamically. In this study, we successfully observed flickering aurora over a much wider FoV than in previous studies by using a ground-based camera equipped with a diagonal fisheye lens. While wide-FoV imaging provides improved contextual coverage, interpreting off-zenith (i.e., oblique) observations may require additional care because line-of-sight mixing can become more pronounced. We evaluate the consistency of our observations with existing generation models for flickering aurora and introduce newly revealed spatiotemporal characteristics: although individual examples may suggest a dispersion-like relationship between flickering frequency and patch size, this apparent trend does not persist statistically, implying that dispersion-like behavior inferred from generation models does not always translate directly into the optical signatures of flickering aurora.

The detector used in the camera system was an ORCA-Flash 4.0 V2 manufactured by Hamamatsu Photonics. A NIKKOR 8 mm f/2.8 lens was employed, providing a diagonal FoV of 180°. No dedicated optical filters were used to block specific wavelengths. In this paper, we refer to this system simply as a scientific complementary metal–oxide–semiconductor (sCMOS) camera. The camera was installed at the Poker Flat Research Range, Alaska (65.12° N, 147.43° W), and oriented toward the geographic zenith. During the event analyzed in this study, images were recorded at 80 Hz with a frame size of 512×512 pixels. For the analyses presented after Fig. 1, 2×2 software binning was applied to improve the signal-to-noise ratio, and the resulting 256×256 pixel images were used.

Unless otherwise stated, spatial distances reported below are apparent horizontal distances obtained by projecting the images to an assumed emission altitude of 110 km. Because the observations were made with a single camera, these mapped distances should not be interpreted as uniquely reconstructed true horizontal scales.

Flickering aurora was observed with the sCMOS camera on 8 February 2016. Selected intervals of the observation are provided as Video 1 accompanying the electronic version of this paper to illustrate representative examples of flickering aurora . Figure  summarizes the event. Panel (a) shows a keogram constructed by extracting a vertical cross section at x=256 pixels from each frame of the 512×512-pixel image sequence and stacking these cross sections in time. Panels (b)–(e) present all-sky images corresponding to the times indicated by the black arrows above panel (a).

The aurora initially appeared as a faint east–west arc around 06:20 UT (panel b). It subsequently intensified, developing into a typical discrete auroral arc by approximately 06:30 UT (panel c). From this period onward, localized, patch-like blinking areas became detectable within the arc on scales of several kilometers. The discrete arc and its associated flickering patches remained within the FoV for about 1 h, exhibiting temporal variations in their average brightness, and eventually faded after 07:00 UT as a bulge-like bright auroral structure propagated westward. During the event, the magnetic coordinates of the observation site were approximately 65.3° MLAT and 19.2–20.0 MLT. The broader geophysical and optical context of this event is summarized in Appendices  and . The solar-wind and geomagnetic data indicate enhanced auroral activity during the 06:25–07:10 UT interval, while the keograms show that the flickering aurora was embedded in a dynamically evolving auroral activation rather than occurring under quiet conditions.

Panel (f) shows the sum of spectra obtained by applying the fast Fourier transform (FFT) for every 4 s window to the pixels within the region outlined by the black dashed line in panel (b). From around 06:30 UT, when the aurora began to brighten, intervals of enhanced power appeared intermittently in the frequency range from a few hertz up to about 20 Hz. The frequencies exhibiting high power were broadly consistent with previous studies . However, the occurrence of flickering was intermittent, and the dominant frequencies varied with time. The spike-like vertical structures in Panel (f) at approximately 06:48:00, 06:55:44, and 07:03:44 UT are artifacts caused by short data gaps of about 1 s in the image acquisition. They should not be interpreted as broadband rapid temporal variations of the auroral luminosity.

At 06:34–06:39, 06:43–06:45, and 06:52–06:55 UT (intervals indicated by white arrows), the signals in this frequency band dropped to the noise level. This occurred because the background discrete aurora weakened, reducing the amplitude of the flickering to a level at which it was no longer detectable as a signal. Weak flickering can nevertheless be identified in Video 1, especially in the video interval from 00:00 to about 11:50. Around 06:58–07:02 UT, the arc propagated equatorward and narrowed in its north–south extent, forming a typical curtain-like aurora just before the substorm onset. During this interval, no flickering was detected on the arc. However, flickering was observed in a latitudinally broader discrete aurora that appeared in a region farther north, as well as in the bulge observed afterward.

To investigate spatial variations in the periodicity, we examined the frequency distribution within a 1 s interval. Figure  presents the results for the 1 s interval starting at 06:50:34.5 UT (Fig. d), during which flickering aurora was clearly observed. The grayscale background image represents the average of the 80 frames acquired during this one-second period. The fast Fourier transform (FFT) was applied to the 1 s time series at each pixel whose mean count exceeded a predefined threshold, indicating the presence of auroral emissions. Among these spectra, pixels for which the spectral power in the 3–20 Hz range exceeded a threshold determined by visual inspection were identified as exhibiting flickering. For these pixels, the frequency corresponding to the maximum amplitude is shown in color.

Flickering was detected over broad portions of the arc, although not uniformly across its entire extent. This spatially intermittent pattern is consistent with visual inspection. The dominant frequencies varied spatially, generally ranging from 5 to 15 Hz. The distribution was not completely random: in some regions extending over more than 10 km, spatially coherent clusters exhibited similar peak frequencies. These clusters were larger than individual flickering patches; that is, multiple patches sharing similar frequencies formed a single cluster. In Fig. , for example, frequencies around ∼8 Hz were prevalent in regions with x coordinates below ∼80, whereas frequencies around ∼13 Hz were more common in regions with x coordinates above ∼120. The apparent mapped separation between these two regions was approximately 150 km when projected to the assumed emission altitude of 110 km.

As shown in Fig. , clusters at a given peak frequency were sometimes observed to move together with the background auroral arc. Panel (a) shows the average of 80 images acquired during a 1 s interval starting at 06:50:20.0 UT. An arc extending in the east–west direction is visible; although the flickering cannot be discerned from the still image, the luminosity modulation is clearly seen in Video 1. The FFT was applied to the time series obtained during this 1 s interval at each pixel, and, as in Fig. , the frequency with the maximum amplitude in the 3–20 Hz range is shown in panel (b). Panels (c) and (d) show the same analyses applied to data acquired 4 s later, corresponding to panels (a) and (b), respectively.

Comparing panels (b) and (d) of Fig. , an eastward propagation is evident in the region enclosed by the red dotted line, in the direction indicated by the white arrow in panel (c). Specifically, a slightly folded, twisted structure becomes more linear in panel (c). At the same time, comparison of panels (b) and (d) shows that the 7–8 Hz cluster corresponding to this folded structure (outlined by the red dotted line in panel b) shifted eastward in concert with the motion of the auroral arc.

In the analyses presented so far, frequency analysis was performed on a pixel-by-pixel basis, without explicitly considering individual patch structures. However, to systematically investigate the relationship between patch size and periodicity, it is necessary to automatically detect the flickering patches themselves. Although two-dimensional region extraction directly from the video data is in principle possible, no optical filter was used in the present observations. As a result, the strong 557.7 nm oxygen emission can blur the boundaries of flickering patches, potentially introducing uncertainty in the estimated patch sizes. Therefore, in this study, patch detection was performed using keograms constructed from the video data. This approach has several limitations. First, a keogram cross section does not necessarily pass through the center of every patch; a slice intersecting only the edge of a patch can underestimate its apparent size. Therefore, the derived patch-size distribution should be interpreted as a distribution of apparent cross-slice extents rather than exact two-dimensional patch diameters. Second, off-zenith viewing may increase line-of-sight superposition, so multiple flickering components can contribute to a single pixel and complicate the apparent patch morphology. Nevertheless, keograms offer the practical advantage that even when the brightening of a single patch persists over multiple frames, it can be continuously tracked as a single patch. In this study, we prioritized this advantage.

Figure  illustrates the workflow for the automatic detection of flickering patches. As an example, we show a relatively stable interval with little motion, observed over a 23 s period starting at 06:50:15 UT. Panel (a) shows a keogram extracted at x=150. Fine vertical stripe structures on time scales shorter than 1 s are visible over much of the panel (minor ticks on the horizontal axis are given at 1 s intervals); these stripes represent the luminosity modulation of flickering aurora, and our first goal is to measure their vertical extent.

Panel (b) is obtained by removing dark regions in panel (a) using a threshold and subtracting a 0.5 s moving average from the remaining regions, thereby enhancing the high-frequency variations. Panel (c) shows a binary image derived from panel (b), where regions with ΔCount exceeding 30 are set to 1 and the others to 0. Structures extending over more than four pixels in the vertical direction are detected and overlaid with red lines. Each red line corresponds to the apparent cross-slice extent of an individual patch. The threshold ΔCount>30 was chosen because a relatively high threshold reduces false detections from background fluctuations and weak non-flickering brightness variations, although not all flickering structures are detected.

Panel (d) shows the frequency with the maximum amplitude obtained by applying the FFT to the time series extracted from panel (b) at 2 s intervals. The time windows were shifted in steps of 0.2 s, resulting in overlapping segments. Regions with small amplitudes (i.e., low brightness) were regarded as non-flickering and are left blank.

Comparison of panels (a) and (c) shows that, although not all flickering structures are detected, the identified structures (highlighted in red in Fig. c) correspond to the bright phases of the flickering and their lengths are generally consistent with the structures seen in panel (a). Furthermore, comparison of panels (c) and (d) indicates that, as illustrated in the figure, relatively long patches in the y direction (corresponding to about 7–10 km at an altitude of 110 km) were observed during the earlier part of the interval (approximately 18–24 s), when the peak frequency was around 8 Hz. In contrast, during the later intervals (approximately 29–32 and 36–38 s), shorter patches (about 4–6 km at an altitude of 110 km) were detected, with corresponding frequencies of about 12 Hz. In other words, larger patches tended to exhibit longer flickering periods (i.e., lower frequencies).

The same analysis as that shown in Fig.  was also applied to a brighter and more dynamically evolving discrete aurora observed during a 23 s interval starting at 07:06:17 UT (corresponding to Fig. e). The results are presented in Fig. . The figure format and analysis procedure are the same as in Fig. ; however, note that the range of the y axis and the color-map scales in panels (a) and (b) are slightly different.

As shown in Fig. a, compared with Fig. a, the aurora occupied a much broader region, and flickering was correspondingly observed over a wider area. During the earlier part of the interval, two structures with comparable background brightness were observed above and below (north and south of) y≈125 pixels. In the region enclosed by the red dotted line, the flickering patches appeared at relatively long intervals, corresponding to a frequency of about 4 Hz as seen in panel (d). The apparent patch sizes in this region were typically about 7–11 km at an altitude of 110 km.

At the same time, in the region enclosed by the blue dotted line, located approximately 20 km away, the patches flickered at much shorter intervals, with a corresponding frequency of about 14 Hz in panel (d), roughly three times higher than in the red region. The apparent patch extents in this region were mostly about 3–5 km, representing roughly a factor-of-two difference compared with the red region. The tendency for larger patches to exhibit longer flickering periods (lower frequencies) is consistent with the results shown in Fig. .

In Fig. a, during a few seconds starting at 07:06:22 UT, structures extending toward the upper right were observed three times, as indicated by the black arrows. From the inclination of these structures, it can be inferred that the bright portion of the aurora propagated from south to north at a speed of approximately 10 km s⁻¹. Figure b and c show that, even as these bright regions moved, flickering continued to accompany them. However, the patches themselves did not remain continuously bright while drifting northward.

In Fig. d, a region in which the peak frequency remains nearly constant along an upward-rightward trend can be identified, as outlined by the white dotted line. Although such a region does not correspond one-to-one with the structures seen in Fig. a, they may indicate that the flickering period is not strongly modulated during the propagation of the discrete aurora.

After the interval during which north–south-propagating auroral structures were observed (after 07:06:25 UT), patches with a wide range of apparent mapped extents, from approximately 2 to 15 km, were detected. Correspondingly, the peak-frequency panel in Fig. d shows a broad distribution of values ranging from about 3 to 20 Hz. In contrast to the period up to 07:06:22 UT and the cases shown in Fig. , it was difficult to identify a systematic relationship between patch size and frequency during this later interval. Even when focusing on individual moments, multiple isolated patches sometimes appeared without continuity in the north–south direction.

In addition, as indicated by the black arrows in Fig. b, patch structures resembling a laterally reversed J shape were observed. This implies that the patches propagated in the north–south direction at speeds of up to approximately 30 km s⁻¹, rather than expanding from a fixed point. Although not shown in the figures, numerous patches propagating in the east–west direction were also identified in Video 1, particularly in the interval after about 15:53.

We statistically examined the relationship between the apparent north–south extent of patches and their flickering frequency observed in Figs.  and . We used the north–south extent because the discrete auroral arc was elongated mainly in the east–west direction, so this direction approximately sampled the arc across its narrow dimension. In addition, Video 1 shows that some flickering patches propagated along the east–west direction within the arc; therefore, an east–west extent would be more sensitive to along-arc motion. The north–south extent is thus used here as a practical one-dimensional measure of apparent patch size, rather than as a full two-dimensional patch diameter. Specifically, for the software-binned 256×256 pixel image sequences, the same analysis as in Figs.  and  was applied at x coordinates from 25 to 250 in steps of 25 pixels, at 1 min intervals over the period from 06:20 to 07:10 UT. For each analysis window, the length of each patch (in pixels) obtained from panel (c) was associated with the corresponding peak frequency from the frequency map in panel (d), yielding pairs of apparent patch length and peak frequency. The patch lengths were then converted to apparent mapped distances at an assumed altitude of 110 km.

Figure  shows the occurrence frequency map of the inverse of the apparent north–south patch length at an altitude of 110 km and the corresponding peak flickering frequency, based on the 23 704 patches obtained through the procedure described above. In the figure, the mean value and the ±1σ range for each bin on the horizontal axis are indicated by white solid and dashed lines, respectively. According to Fig. , the flickering frequency remains nearly constant at about 8±4 Hz, regardless of patch size. Although the examples shown in Figs.  and  suggested a tendency for larger patches to exhibit lower frequencies (i.e., longer flickering periods), this trend is not reproduced statistically. This result is unchanged even when the analysis is restricted to shorter time intervals; that is, patch size does not depend on the flickering period over time. The typical north–south patch size is 4.4±2.4 km. We performed additional robustness checks for viewing geometry, detection threshold, finite emission thickness, and the assumed projection altitude; the details are given in Appendix .

The inverse patch size shows a pattern of repeated local minima and maxima, particularly on the right-hand side of the distribution (corresponding to smaller patch sizes); however, this behavior is an artifact of the analysis arising from the fact that the spatial resolution of the observations is lower than the bin resolution along the horizontal axis.

The absence of a statistical relationship between flickering frequency and patch size suggests that the observed optical scale does not directly reflect a simple single-mode dispersion relation. Instead, it is naturally interpreted in terms of interference among multiple EMIC waves, as proposed in previous studies .

We analyzed a flickering aurora event observed above the Poker Flat Research Range, Alaska, during the interval from approximately 06:20 to 07:10 UT on 8 February 2016. The main results can be summarized as follows:

Flickering occurred intermittently in both space and time over a frequency range of 3–20 Hz.

Using high-time resolution, wide-field imaging, we have characterized the mesoscale organization of flickering aurora over spatial scales of several hundred kilometers, placing previous narrow-field results into a broader mesoscale context. Flickering was observed intermittently in the 3–20 Hz range, with most power between 4 and 12 Hz, consistent with O⁺ EMIC waves generated at altitudes of several thousand kilometers. The occurrence and visibility of flickering depended strongly on the evolution of the discrete arc: it disappeared as the background arc narrowed and intensified before substorm onset, while persisting on a wider polar-side discrete aurora and in the subsequent bulge, suggesting that suitable plasma and acceleration conditions are not simply controlled by brightness alone but also by the latitudinal width and structure of the arc. Spatial mapping revealed multiple frequency clusters along the same arc, separated by ∼150 km. These features indicate that multiple EMIC sources can coexist along neighboring field lines, each shaped by local plasma parameters and inverted-V potential structures, in line with interference-based models and with beam-driven instability . Finally, although individual examples sometimes show an inverse relation between patch size and frequency reminiscent of the EMIC dispersion relation, our statistical analysis demonstrates that flickering frequencies are distributed within a similar band regardless of patch size, consistent with the idea that the optical scale reflects interference patterns rather than a simple one-to-one mapping from a single-mode dispersion curve.