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

Nishi, Katsuki; Shiokawa, K.

doi:10.1029/2018ja025480

Cited by 4 publications

(6 citation statements)

References 37 publications

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“…The instability mechanism mentioned earlier near the dusk sector provides another potential candidate for wave excitation. Nishi et al () and Shiokawa et al () reported finger‐like auroral brightness structures at postmidnight sectors near the equatorial boundary of an auroral arc. These brightness structures were organized latitudinally with horizontal‐scale sizes ranging from 15–100 km, moved eastward with slow speeds of 150–400 m/s, and were considered as auroral fragmentation into patches.…”

Section: Resultsmentioning

confidence: 97%

Subauroral and Polar Traveling Ionospheric Disturbances During the 7–9 September 2017 Storms

et al. 2019

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This study provides new scenarios for storm time traveling ionospheric disturbance excitation and subsequent propagation at subauroral and polar latitudes. We used ground-based total electron content observations from Global Navigation Satellite System receivers combined with wide field, subauroral ionospheric plasma parameters measured with the Millstone Hill Incoherent Scatter Radar during strong September 2017 geospace storms. Observations provide the first evidence of significant influences on traveling ionospheric disturbance (TID) propagation and excitation caused by the presence of large subauroral polarization stream flow channels. Simultaneous large-and medium-scale TIDs evolved during the event in a broad subauroral and midlatitude area near dusk. Similar concurrent TIDs occurred near dawn sectors as well during a period of sustained southward Bz. Medium-scale TIDs at subauroral and midlatitudes had wave fronts aligned northwest-southeast near dusk, and northeast-southwest near dawn. These wave fronts were highly correlated with the direction of storm time large zonal plasma drift enhancements at these latitudes. At high latitudes, unexpected, predominant, and persistent storm time TIDs were identified with 2000+ km zonal wave fronts and 15% total electron content perturbation amplitudes, moving in transpolar propagation pathways from the dayside into the nightside. This propagation direction in the polar region was opposite to the normal assumption that TIDs originated in the nightside auroral region. Results suggest that significant dayside sources, such as cusp regions, can be efficient in generating transpolar TIDs during geospace storm intervals.Plain Language Summary This paper reports several new findings on the traveling ionospheric disturbances (TIDs) excited during geospace storms in 7-8 September 2017. Storm time TIDs provide pathway for momentum and energy dispersion from the solar wind-magnetosphere system to various components of the global ionosphere and thermosphere. Storm time large-scale TIDs (LSTIDs) have been identified by many studies as being initiated generally in the auroral zone where significant heating is injected, with subsequent propagation away from the source: equatorward into lower latitudes and poleward into high latitudes. Our study indicates that, during equatorward propagation, LSTIDs can encounter strong dynamic forcing at subauroral latitudes in the zonal direction. This westward velocity forcing is provided by a SAPS (subauroral polarization stream) channel and furthermore appears to be associated with the developing of medium-scale TIDs (MSTIDs). Thus, this paper provides the first causal link between these TIDs and SAPS flow channels. Concurrent LSTIDs and MSTIDs existed during the September storm in not only near dusk but also dawn sectors. In the polar cap region, conventionally anticipated poleward propagation away from the auroral zone was unexpectedly weak. In contrast, an opposite sense of transpolar propagation from the dayside into the nightside (i.e., eq...

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Section: Resultsmentioning

confidence: 97%

Subauroral and Polar Traveling Ionospheric Disturbances During the 7–9 September 2017 Storms

et al. 2019

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“…However, they focused on signatures of field‐aligned currents, and did not show electron and ion spectra and their pitch‐angle distributions in the source of the auroral arc. Nishi et al (2017, 2018) investigated electron and ion spectra and pressure variations in the source of auroral finger‐like structures in the diffuse auroral region using the Van Allen Probes and THEMIS satellites at a radial distance of 6–8 Re . However, electron and ion spectra, their pitch‐angle distributions, and field variations in the source of auroral arcs and diffuse auroras in the inner magnetosphere have not been well understood yet.…”

Section: Introductionmentioning

confidence: 99%

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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“…Using these magnetic field and plasma data, we made the following analysis procedures to obtain the phase relationship of the magnetic and plasma pressures. (1) We subtracted 10‐min running averages of the pressures data from the original data in order to extract variation components with periods of several minutes which are comparable with the time scales of finger‐like structure crossings reported by Nishi et al (, ). (2) For every 1‐hr time segments with 1,201 or 1,202 data points, we calculated power spectra of the variation components of the plasma and magnetic pressures, Δ P t h and Δ P B , respectively, and coherences and phases between these pressures using the fast Fourier transform (FFT).…”

Section: Statistical Data Set and Analysis Methodsmentioning

confidence: 99%

“…This result is consistent with the idea of the pressure‐driven instability advocated by Shiokawa et al (, ). However in the latter event (Nishi et al, ), the phase relationship of the pressure fluctuations was not systematic, differently from the THEMIS‐E conjunction event. This difference of phase relationship of plasma and magnetic pressures between THEMIS‐E (∼8 R E ) and RBSP‐A (∼5.5 R E ) associated with auroral finger‐like structures provided us a strong motivation to systematically investigate the correlation of these pressure variations in the nightside magnetosphere.…”

Section: Introductionmentioning

confidence: 91%

“…Shiokawa et al (, ) regarded auroral finger‐like structures, which we can see in diffuse auroral regions during the auroral fragmentations as manifestations of the pressure‐driven instability in the magnetosphere. Recently, Nishi et al (, ) reported two simultaneous observations of the auroral finger‐like structures using ground‐based all‐sky imagers and satellites in the magnetosphere. One used the THEMIS‐E satellite at a radial distance of ∼8 R E and the other used the RBSP‐A satellite at a radial distance of ∼5.5 R E .…”

Section: Introductionmentioning

confidence: 99%

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

Cited by 4 publications

References 37 publications

Subauroral and Polar Traveling Ionospheric Disturbances During the 7–9 September 2017 Storms

Subauroral and Polar Traveling Ionospheric Disturbances During the 7–9 September 2017 Storms

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

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

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