Rayleigh‐Taylor type instability in auroral patches

Shiokawa, K.; Nakajima, Akira; Ieda, A.; Sakaguchi, Kaori; Nomura, Reiko; Aslaksen, T.; Greffen, M.; Donovan, E.

doi:10.1029/2009ja014273

Cited by 23 publications

(40 citation statements)

References 23 publications

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“…In addition to the ionospheric feedback, highly structured diffuse auroras in the non‐patch shape were suggested to be a result of interchange instability in the magnetosphere [ Ebihara et al , 2010]. A Rayleigh‐Taylor type instability was also assumed to be the cause of finger‐like patches [ Shiokawa et al , 2010]. The spatial structures of PA1–3 may be determined by a balance between magnetic tensions and plasma pressures.…”

Section: Discussionmentioning

confidence: 99%

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

Nishiyama¹,

Sakanoi²,

Miyoshi³

et al. 2012

J. Geophys. Res.

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We have carried out ground‐based observations, optimized to temporal and spatial characteristics of pulsating auroras (PAs) in the micro/meso scale, using an electron multiplying charge coupled device (EMCCD) camera with a wide field of view corresponding to 100 × 100 km at an altitude of 110 km and a high sampling rate up to 100 frames per second. We focus on transient PAs propagating southward around 1100 UT, in the early recovery phase of the substorm, on 4th March 2011. Three independent patches (PA1–3) each with different periods between 4 and 7 s were observed, which means that the periodicity was not explained by the electron bounce motion and strongly depended on local plasma conditions in the magnetosphere or in the ionosphere. One more insight is that only PA1 had also a sharp peak of modulations around 1.5 Hz, with a narrow frequency width of 0.30 Hz, and the strong modulations existed as a small spot in the center of PA1. We have also conducted cross spectrum analysis and have obtained coherence and phase distributions for auroral variations between 0.1 and 3.0 Hz. The results indicated that low frequency variations from 0.2 to 0.5 Hz inside PA1–3 propagated as a collective motion in well‐defined directions. The estimated horizontal propagation velocities ranged from 50 to 120 km/s at the auroral altitude. The velocities are almost consistent with the Alfven speed at the magnetic equator, which suggests that compressional waves have an effect on PA via modulations of the ambient plasma environment.

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Section: Discussionmentioning

confidence: 99%

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

Nishiyama¹,

Sakanoi²,

Miyoshi³

et al. 2012

J. Geophys. Res.

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“…However, Lui and colleagues also pointed out that there exist eastward drifting auroral patches embedded within the region of diffuse aurora [ Lui et al , 1973]. In addition, modern sensitive high resolution optical measurements from the ground showed that diffuse aurora sometimes consist of small‐scale patches of irregular shape [ Sergienko et al , 2008; Shiokawa et al , 2010]. Such auroral patches are observed in the equatorward part of the auroral oval during substorm recovery phase.…”

Section: Introductionmentioning

confidence: 99%

Plasma irregularities adjacent to auroral patches in the postmidnight sector

et al. 2010

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[1] We demonstrate a close association between decameter-scale plasma irregularities in the E region ionosphere and auroral patches in the postmidnight sector. In September 2009, campaign-based measurements of the aurora were conducted in Iceland with a white light all-sky camera (ASC) at Tjörnes (66.20°N, 17.12°W) and the SuperDARN radar at þykkvibaer (63.77°N, 20.54°W). On one night during the campaign period, the ASC observed the successive passage of auroral patches in the postmidnight sector after a small substorm-like activity. The patches were drifting predominantly eastward across the field-of-view of the ASC with a speed of approximately 360-450 m s, which is consistent with the sunward convection in the postmidnight westward electrojet. The simultaneous radar measurements recorded strong radar backscatter echoes (>15 dB) within the gaps between adjacent auroral patches, while such echoes were not observed or were very weak in the region of the aurora. The Doppler velocity estimation showed that the electric field was clearly reduced within the patches, which was probably the result of the enhanced conductance associated with auroral precipitation. Thus, this reduction in the electric field suppressed the generation of irregularities (i.e., radar echoes) in the regions of auroral patches. This suggests that the conductance enhancement associated with precipitating electrons not only modified the electric field within the aurora but also affected the generation of small-scale plasma structures in the vicinity of the patch-type optical auroral forms.

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“…Blixt et al (2006) developed a method using optical flow analysis that extracts object motion from sequences of images by assuming that variations in brightness are solely related to object motion. For pulsating aurora, this is a significant limitation due to their prominent pulsations and constantly evolving shape (Shiokawa et al, 2010). This study details a new method of routinely and quantitatively tracking auroral forms, focusing particularly on pulsating auroral patches.…”

Section: Introductionmentioning

confidence: 99%

“…Past studies have tracked aurora by hand (e.g. Yang et al, 2015;Shiokawa et al, 2010), but there have been few attempts to apply an automatic algorithm. Blixt et al (2006) developed a method using optical flow analysis that extracts object motion from sequences of images by assuming that variations in brightness are solely related to object motion.…”

Section: Introductionmentioning

confidence: 99%

Tracking patchy pulsating aurora through all-sky images

2017

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Abstract. Pulsating aurora is frequently observed in the evening and morning sector auroral oval. While the precipitating electrons span a wide range of energies, there is increasing evidence that the shape of pulsating auroral patches is controlled by structures in near-equatorial cold plasma; these patches appear to move with convection, for example. Given the tremendous and rapidly increasing amount of auroral image data from which the velocity of these patches can be inferred, it is timely to develop and implement techniques for the automatic identification of pulsating auroral patch events in these data and for the automatic determination of the velocity of individual patches from that data. As a first step towards this, we have implemented an automatic technique for determining patch velocities from sequences of images from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imager (ASI) and applied it to many pulsating aurora events. Here we demonstrate the use of this technique and present the initial results, including a comparison between ewograms (eastwest keograms) and time series of patch position as determined by the algorithm. We discuss the implications of this technique for remote sensing convection in the inner magnetosphere.

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Rayleigh‐Taylor type instability in auroral patches

Cited by 23 publications

References 23 publications

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

Plasma irregularities adjacent to auroral patches in the postmidnight sector

Tracking patchy pulsating aurora through all-sky images

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