Conjugate observation of auroral finger‐like structures by ground‐based all‐sky cameras and THEMIS satellites

Nishi, Katsuki; Shiokawa, K.; Frühauff, Dennis

doi:10.1002/2016ja023774

Cited by 4 publications

(16 citation statements)

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“…The absolute value of the magnetic pressure is several times higher than that of the ion thermal pressures. This feature differs from the previously reported conjugate observation made using the THEMIS‐E satellite (Nishi et al, ). Figure d shows the deviations of magnetic pressure and plasma thermal pressures, which we obtained by subtracting 10‐min running averages from the original data shown in Figure c.…”

Section: Observationcontrasting

confidence: 99%

“…In this paper, we report another unique conjugate observation of auroral finger‐like structures observed at the equatorward edge of the oval by using the Radiation Belt Storm Probes (RBSP)‐A satellite and the THEMIS ground‐based all‐sky cameras. The observed characteristics of pressure variations by RBSP‐A at a radial distance of ∼5.8 R E are different from those reported by Nishi et al () by THEMIS at 8–9 R E . Based on these observations, we propose another possibility for the generation of auroral finger‐like structures by ionospheric gradient drift instability.…”

Section: Introductioncontrasting

confidence: 99%

“…According to Figure , the RBSP‐A satellite arrived at the conjugate point of the finger‐like structures slightly outside the inner edge of the ion plasma sheet. In the previous study by Nishi et al (), the THEMIS‐E satellite also conjugated in such a region. Therefore, we consider that the instability occurred slightly outside the inner boundary of the ion plasma sheet and near the equatorward boundary of the aurora.…”

Section: Discussionmentioning

confidence: 63%

“…We have reported herein a conjugate observation of auroral finger‐like structures using the RBSP‐A satellite in the inner magnetosphere. Nishi et al () reported a simultaneous observation using the THEMIS‐E satellite. However, the observational facts that pertain in the present RBSP‐A case differ from those in the THEMIS‐E case as regards magnetic pressure, plasma pressure, and the location of the finger‐like structures in the auroral oval.…”

Section: Discussionmentioning

confidence: 99%

“…Before we discuss the differences between the THEMIS‐E and RBSP‐A results, we give an overview of the THEMIS‐E conjugate event reported by Nishi et al (). The THEMIS‐E event was observed at Narsarsuaq, Greenland (MLAT: 65.3°N) at 0720–0820 UT on 17 February 2012 in the diffuse auroral region.…”

Section: Discussionmentioning

confidence: 99%

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Magnetospheric Source Region of Auroral Finger‐like Structures Observed by the RBSP‐A Satellite

Nishi

Shiokawa

2018

JGR Space Physics

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Auroral finger‐like structures appear equatorward of the auroral oval in the diffuse auroral region and contribute to the auroral fragmentation into patches. A previous report of the first conjugate observation of auroral finger‐like structures using a Time History of Events and Macroscale Interactions during Substorms (THEMIS) Ground‐Based Observatories camera and the THEMIS‐E satellite at a radial distance of ∼8 RE showed antiphase oscillations of magnetic and plasma pressures in the dawnside plasma sheet. In the present study, we report another simultaneous observation of auroral finger‐like structures at Gillam, Canada, at ∼0900 UT (0230 magnetic local time) on 14 November 2014 with the Radiation Belt Storm Probes satellites at 5.8 RE in the inner magnetosphere. From this simultaneous observation event, we obtained the following observations. (1) Auroral finger‐like structures developed poleward in the equatorward moving auroral arc at the equatorward edge of the auroral oval. (2) Both the electron and ion OMNI fluxes measured by HOPE increased at ∼0900 UT as the satellite footprint entered the auroral region, indicating that the satellite was crossing the observed auroral finger‐like structures. (3) The absolute value of magnetic pressure was several times that of the plasma pressure, and no systematic phase relationship was identified between the magnetic and plasma pressures, unlike that in the THEMIS case. Based on these observations, we discuss two possible causes of the observed finger‐like structures, namely, pressure‐driven instability in the magnetosphere and gradient drift instability in the ionosphere. In this paper, the latter possibility is newly suggested to develop in the equatorward moving aurora associated with the westward electric field in the equatorward ionospheric density gradient.

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Section: Observationcontrasting

confidence: 99%

Section: Introductioncontrasting

confidence: 99%

Section: Discussionmentioning

confidence: 63%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Magnetospheric Source Region of Auroral Finger‐like Structures Observed by the RBSP‐A Satellite

Nishi

Shiokawa

2018

JGR Space Physics

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Statistical Study of Phase Relationship Between Magnetic and Plasma Pressures in the Near‐Earth Nightside Magnetosphere Using the THEMIS‐E Satellite

et al. 2018

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Distributions of plasma and magnetic pressures are the basic information to investigate macroscopic dynamics of the Earth's magnetosphere. Several studies have been made on magnetic and plasma pressures and macroscopic plasma instabilities in the magnetosphere. However, correlation between magnetic and plasma pressure variations has not been statistically investigated. In this paper, we analyze the statistical characteristics of the phase relationships between variations of magnetic and plasma pressures at frequencies of 4-15 mHz using 2 years of the THEMIS-E satellite data in the nightside magnetosphere. Spectral peaks with coherence greater than 0.85 between magnetic and plasma pressures for 1-hr time segments were selected. The average occurrence rates of the phase relationships are antiphase (within ±10 ∘ from 180 ∘ ), 39.75%; in-phase (within ±10 ∘ from 0 ∘ ), 0.73%; and other phases (10-170 ∘ ), 49.83%. For the other-phase events, the phase differences are much closer to antiphase rather than to in-phase. Thus, we conclude that the two pressure variations tend to be antiphase. The antiphase and in-phase relationships are observed mainly at radial distances outside 8 R E and inside 8 R E , respectively. The high occurrence region of antiphase relationship is in the dawnside during magnetically quiet times and shifts to dusk side at active times defined as Dst < −10 nT. The occurrence rates of the phase relationships do not change significantly depending on the AE and Dst indices, plasma , and IMF-B z . Based on these results, we discuss the correspondence between the phase relationships and the possible magnetohydrodynamic force balances that can produce these phase relationships.Plain Language Summary Distributions of plasma and magnetic pressures are the basic information to investigate macroscopic dynamics of the Earth's magnetosphere. Several studies have been made on magnetic and plasma pressures and macroscopic plasma instabilities in the magnetosphere. However, correlation between magnetic and plasma pressure variations has not been statistically investigated. In this paper, we analyze the statistical characteristics of the phase relationships between variations of magnetic and plasma pressures at frequencies of 4-15 mHz using 2 years of the THEMIS-E satellite data in the nightside magnetosphere. Spectral peaks with coherence greater than 0.85 between magnetic and plasma pressures for 1-hr time segments were selected. The average occurrence rates of the phase relationships are antiphase (39.75%), in-phase (0.73%), and other phases (49.83%). For the other-phase events, the phase differences are much closer to antiphase rather than to in-phase. Thus, we conclude that the two pressure variations tend to be antiphase in the nightside magnetosphere. We also investigate relation of these phase relationships with geomagnetic activities, solar wind parameters, and local times. Based on these results, we discuss the correspondence between the phase relationships and the possible magnetohydrodynamic force balan...

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Arase Observation of the Source Region of Auroral Arcs and Diffuse Auroras in the Inner Magnetosphere

et al. 2020

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Auroral arcs and diffuse auroras are common phenomena at high latitudes, though characteristics of their source plasma and fields have not been well understood. We report the first observation of fields and particles including their pitch‐angle distributions in the source region of auroral arcs and diffuse auroras, using data from the Arase satellite at L ~ 6.0–6.5. The auroral arcs appeared and expanded both poleward and equatorward at local midnight from ~0308 UT on 11 September 2018 at Nain (magnetic latitude: 66°), Canada, during the expansion phase of a substorm, while diffuse auroras covered the whole sky after 0348 UT. The top part of auroral arcs was characterized by purple/blue emissions. Bidirectional field‐aligned electrons with structured energy‐time spectra were observed in the source region of auroral arcs, while source electrons became isotropic and less structured in the diffuse auroral region afterwards. We suggest that structured bidirectional electrons at energies below a few keV were caused by upward field‐aligned potential differences (upward electric field along geomagnetic field) reaching high altitudes (~30,000 km) above Arase. The bidirectional electrons above a few keV were probably caused by Fermi acceleration associated with the observed field dipolarization. Strong electric‐field fluctuations and earthward Poynting flux were observed at the arc crossing and are probably also caused by the field dipolarization. The ions showed time‐pitch‐angle dispersion caused by mirror reflection. These results indicate a clear contrast between auroral arcs and diffuse auroras in terms of source plasma and fields and generation mechanisms of auroral arcs in the inner magnetosphere.

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Conjugate observation of auroral finger‐like structures by ground‐based all‐sky cameras and THEMIS satellites

Cited by 4 publications

References 44 publications

Magnetospheric Source Region of Auroral Finger‐like Structures Observed by the RBSP‐A Satellite

Magnetospheric Source Region of Auroral Finger‐like Structures Observed by the RBSP‐A Satellite

Statistical Study of Phase Relationship Between Magnetic and Plasma Pressures in the Near‐Earth Nightside Magnetosphere Using the THEMIS‐E Satellite

Arase Observation of the Source Region of Auroral Arcs and Diffuse Auroras in the Inner Magnetosphere

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