Characteristics of a stable arc based on FAST and MIRACLE observations

Janhunen, P.; Olsson, A.; Amm, O.; Kauristie, Kirsti

doi:10.1007/s00585-000-0152-5

Cited by 16 publications

(19 citation statements)

References 18 publications

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“…A comprehensive set of ground optical, radar, and magnetic data from the MIRACLE network provides additional useful information. In good agreement with Janhunen et al [2000], we find below that the current closure at and near the arc is achieved in the standard way, with a downward current equatorward of the arc feeding an upward current above the arc through a meridional Pedersen current. However, the application of ALADYN poleward of the arc reveals again that the upward FAC there is mostly closed in the longitudinal direction.…”

Section: Fast Observationssupporting

confidence: 87%

Auroral arc and oval electrodynamics in the Harang region

et al. 2009

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Auroral arcs are typically described in terms of an upward field‐aligned current (FAC) sheet above the arc, connected by ionospheric Pedersen current to a downward FAC sheet near the arc. On the basis of data measured by the FAST spacecraft, conjugate with ground optical observations, we present first a wide and stable winter evening arc, where this standard model does not apply. The arc is observed in the Harang region during the growth phase of a modest substorm, poleward of the convection reversal (CR) boundary. Although the magnetic field data suggest the typical configuration, the two FAC sheets appear to be decoupled near the satellite footprint: the upward FAC is fed by the westward electrojet (WEJ), while the downward FAC feeds the eastward electrojet (EEJ). The examination of the arc by the newly developed ALADYN technique confirms this peculiar current topology. For comparison, we apply ALADYN also to a second evening arc, located within the Harang region equatorward from the CR. The arc is confirmed to have the standard configuration, consistent with a former study, but substantial FAC‐EJ coupling is inferred in the auroral oval both poleward and equatorward of the arc. A key element for the topology of the current closure is the westward component of the electric field, which influences the relative location of the CR with respect to the large‐scale FAC reversal (FR) boundary. As proved by tests on synthetic data, a westward component of the electric field pushes the CR toward the FR, preventing thus the standard FAC closure, while the conductance and FAC pattern shape the CR profile. Since a westward electric field is often measured in the Harang region, the FAC‐EJ coupling is expected to be an essential ingredient there. This has important implications for the current closure in the equatorial magnetosphere and for the auroral current circuit in the WEJ region, closely related to the substorm process.

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Section: Fast Observationssupporting

confidence: 87%

Auroral arc and oval electrodynamics in the Harang region

et al. 2009

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“…10 a schematic of the FAC structure that we envisage being associated with our observations. The auroral luminosity is produced within a region of upward FACs with the arc pair being produced by filamentary FACs within this region; such an instance of parallel arcs present within a single upward FAC region has been observed by Janhunen et al (2000). A broad region of downward FAC lies poleward of this, with the FAC being carried by thermally upwelling electrons.…”

Section: Discussionmentioning

confidence: 92%

The spectral characteristics of E-region radar echoes co-located with and adjacent to visual auroral arcs

et al. 2002

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Abstract. Simultaneous all-sky camera and HF radar observations of the visual and E-region radar aurora in the westward electrojet suggest a close relationship between a pair of parallel east-west-aligned auroral arcs, separated by ∼30 km, and a region of strong radar backscatter. Poleward of this a broader region of radar backscatter is observed, though the spectral characteristics of the echoes in these two regions differ considerably. We suggest that the visual aurorae and their radar counterparts are produced in a region of upward fieldaligned current (FAC), whereas the backscatter poleward of this is associated with downward FAC. Relatively low electric fields (∼10 mV m −1 ) are observed in the vicinity of the arc system, suggesting that in this case, two-stream waves are not directly generated through the electrodynamics of the arc. Rather, the generation of irregularities is most probably associated with the gradient drift instability operating within horizontal electron density gradients produced by the filamentary nature of the arc FAC system. The observation of high Doppler shift echoes superimposed on slow background flow within the region of backscatter poleward of the visual aurora is argued to be consistent with previous suggestions that the ion-acoustic instability threshold is reduced in the presence of upwelling thermal electrons carrying downward FAC.

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“…The topology of the current flow looks quite surprising, the more so as the magnetic field signature suggests a standard evening configuration, with downward FAC south of the arc, connected by ionospheric Pedersen current to the upward FAC above the arc. The standard view on the auroral arc current system, originating with Boström [1964] and validated since then by a large variety of in situ [e.g., Sugiura , 1984], ground [e.g., Opgenoorth et al , 1990; Aikio et al , 1993], and conjugated measurements [e.g., Janhunen et al , 2000], does not hold in our case, at least in the vicinity of FAST footprint. Strong evidence supporting this conclusion is provided by an outstanding feature exhibited by the data (Figure 3), namely the close proximity of the convection and FAC reversal.…”

Section: Evaluation Of the Modelsmentioning

confidence: 71%

ALADYN: A method to investigate auroral arc electrodynamics from satellite data

et al. 2004

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[1] In the simplest representation of an auroral arc current system, the arc consists of a homogeneous block of increased conductance infinitely extended in longitudinal direction; field-aligned current (FAC) sheets that flow in and out of the ionosphere at the boundaries of the arc are connected through Pedersen current across the arc, while the electrojet (EJ) that flows along the arc as Hall current is divergence-free. To evaluate the deviation of the real arc current system from this ideal configuration, we developed the ALADYN (Auroral Arc Electrodynamics) method, based on a parametric model of the arc, that allows the derivation of the parameters by numerical fit to the experimental data. The method is illustrated with a wide, stable, winter evening arc, for which both Fast Auroral Snapshot (FAST) Explorer measurements at 3850 km altitude and ground optical data are available. We find that in order to obtain consistent results, one has to take into account, as a minimum, the ionospheric polarization, the contribution of the Hall current to the meridional closure of the FAC, and the coupling between the FAC and the EJ.

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Characteristics of a stable arc based on FAST and MIRACLE observations

Cited by 16 publications

References 18 publications

Auroral arc and oval electrodynamics in the Harang region

Auroral arc and oval electrodynamics in the Harang region

The spectral characteristics of E-region radar echoes co-located with and adjacent to visual auroral arcs

ALADYN: A method to investigate auroral arc electrodynamics from satellite data

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