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MIRACLE plot showing KIL and KEV all-sky camera views mapped to 110 km altitude, STARE electron velocity vectors (black) and IMAGE ground magnetometer equivalent current vectors (dark blue) 17:36:20 UT. The color scaling corresponds to ASC pixel values (originally in the range 0±255). The red spots in the southeast are images of the Moon. Arc 2 extends through both camera views close to 70 geographic latitude. STARE shows a westward drift of about 1 km/s (corresponding to a northward electric ®eld of about 50 mV/m)
Source publication
A stable evening sector arc is studied using observations from
the FAST satellite at 1250 km altitude and the MIRACLE ground-based network,
which contains all-sky cameras, coherent radars (STARE), and magnetometers. Both
FAST and STARE observe a northward electric field region of about 200 km width
and a field magnitude of about 50 mV/m southward o...
Context in source publication
Context 1
... STARE receives an echo only when the electric ®eld is above the Farley-Buneman threshold. The threshold value depends on the E-region ion temperature but is usually about 17 mV/m. Near the boundaries of the region where STARE receives echoes we indeed see that the electric ®elds are of the order of 17 mV/m. STARE ¯ow velocity vectors are drawn in Fig. 5 only when both radars' backscatter intensity was at least 2 dB above the background ...
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Citations
... The images are provided at a 20 s cadence, with an exposure time of 1 s. In order to compare with ionospheric equivalent currents, the auroral intensity was projected to an optimal altitude near 110 km, which has been determined by Whiter Upward FAC from the ionosphere to the magnetosphere is mainly assumed to be carried by the same electron population that produces green auroras (e.g., Janhunen et al., 2000). Although green auroras only represent a portion of the energy spectrum of the precipitating electrons, observation of such auroras can be used as an indicator of upward FAC. ...
Substorms are a commonly occurring but insufficiently understood form of dynamics in the coupled magnetosphere–ionosphere system, associated with space weather disturbances and auroras. We have used principal component analysis (PCA) to characterize the spatiotemporal development of ionospheric equivalent currents as observed by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers during 28 substorm onsets identified by . Auroral observations were provided by all-sky cameras. We found that the equivalent currents can typically be described by three components: a channel of poleward equivalent current (wedgelet), a westward electrojet (WEJ) associated with an auroral arc, and a vortex. The WEJ and vortex are located at the equatorward end of the channel, which has been associated with bursty bulk flows (BBFs) by previous studies. Depending on its polarity, the vortex either indents the WEJ and arc equatorward or bulges the WEJ poleward while winding the arc into an auroral spiral. In addition, there may be a background current system associated with the large-scale convection. The dynamics of the WEJ, vortex, and channel can describe up to 95 % of the variance of the time derivative of the equivalent currents during the examined 20 min interval. Rapid geomagnetic variations at the substorm onset location, which can drive geomagnetically induced currents (GICs) in technological conductor networks, are mainly associated with the oscillations of the WEJ, which may be driven by oscillations of the transition region between dipolar and tail-like field lines in the magnetotail due to the BBF impact. The results contribute to the understanding of substorm physics and to the understanding of processes that drive intense GICs.
... The wide spread in the FAC estimates is probably associated with its different scales, since for arcs with very sharp gradients of electron density, FACs occur in narrow regions at the edges of the arc. Observations of several events using the FAST satellite showed that the FAC density associated with arcs is 1-10 μA/m 2 [16,17]. Based on the observations of the SWARM and THEMIS satellites, as well as the ASI optical camera system, a connection was found between several arcs of auroras and pairs of downward/upward FACs [18]. ...
... Due to their large amplitudes, small-scale FACs play an important role in the energy input to the upper atmosphere. In several previous studies, the FACs associated with arcs were estimated as 1-10 µA m −2 (Bythrow and Potemra, 1987;Elphic et al., 1998;Janhunen et al., 2000;Lühr et al., 2016). A larger range of current densities, varying between 4 and > 40 µA m −2 , has been observed (Aikio et al., 2002), and even more intense small-scale FACs, up to hundreds of µA m −2 , at the edges of arcs have been measured by MEO satellites (Marklund et al., 1982;Bythrow et al., 1984). ...
Swarm satellite observations are used to characterize the extreme behavior of large- and small-scale field-aligned currents (FACs) during the severe magnetic storm of September 2017. Evolutions of the current intensities and the equatorward displacement of FACs are analyzed while the satellites cross the pre-midnight, pre-noon, dusk and dawn sectors in both hemispheres. The equatorward boundaries of FACs mainly follow the dynamics of the ring current as monitored in terms of the SYM-H index. The minimum latitude of the FAC boundaries is limited to 50∘ magnetic latitude (MLat). The FAC densities are very variable and may increase dramatically, especially in the nightside ionosphere during the storm-time substorms. At the peak of substorms, the average FAC densities reach >3 µA m-2. The dawn–dusk asymmetry is manifested in the enhanced dusk-side R2 FACs in both hemispheres. In the 1 Hz data filamentary high-density structures are always observed. In the pre-noon sector, the bipolar structures (7.5 km width FACs of opposite polarities adjacent to each other) dominate, while at the other local times the upward and downward FACs tend to be latitudinally separated. The most intense small-scale FACs, up to ∼80 µA m-2, are observed just in the post-midnight sector. Simultaneous magnetic and plasma perturbations indicate that this structure is likely a current system of a mesoscale auroral arc.
... Due to their large amplitudes medium and small-scale FACs play an important role for the energy input to the upper 5 atmosphere. In several previous studies, the FACs associated with arcs were estimated as 1-10 μA/m 2 (Bythrow and Potemra, 1987;Elphic et al., 1998;Janhunen et al., 2000). Larger range of current densities, varying between 4 and >40 μA/m 2 , has been observed (Aikio et al., 2002) and even more intense small-scale FACs, up to hundred of μA/m 2 , at the edges of arcs have been measured (Marklund et al. 1982;Bythrow et al. 1984). ...
Swarm satellites observations are used to characterize the extreme behavior of large- and small-scale field-aligned currents (FACs) during the severe magnetic storm of September 2017. Evolution of the current intensities and the equatorward displacement of FACs are analyzed while the satellites cross the pre-midnight, pre-noon, dusk and dawn sectors in both hemispheres. The equatorward boundaries of FACs mainly follow the dynamics of ring current (as monitored in terms of the SYM-H index). The minimum latitude of the FAC boundaries is limited to 50° MLat, below which saturation occurs. The FAC densities are very variable and may increase dramatically, especially in the nightside ionosphere during the storm-time substorms. At the peak of substorm, the average FAC densities reach 3 μA/m², while the quite level is below 0.1 μA/m². The dawn–dusk asymmetry is manifested in the enhanced dusk-side R2 FACs in both hemispheres. Filamentary high-density structures are always observed confirming that a substantial fraction of R1/R2 FACs is composed of many small-scale currents. In the pre-noon sector, the bipolar structures (7.5 km width FACs of opposite polarities adjacent to each other) dominate, while in the post-midnight sector the upward and downward FACs tend to form more latitudinally extended structures of a certain polarity. The most intense small-scale FACs (up to ~80 μA/m²) is observed just in the post-midnight sector. Simultaneous magnetic and plasma perturbations indicate that this structure is likely a current system of a mesoscale auroral arc.
... [56] 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. ...
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. INDEX TERMS: 2407 Ionosphere: Auroral ionosphere (2704); 2411 Ionosphere: Electric fields (2712); 2409 Ionosphere: Current systems (2708); 2455 Ionosphere: Particle precipitation; KEYWORDS: auroral arc, ionospheric electric field, arc current system, auroral electrojet, field-aligned current, particle precipitation Citation: Marghitu, O., B. Klecker, G. Haerendel, and J. McFadden (2004), ALADYN: A method to investigate auroral arc electrodynamics from satellite data, J. Geophys. Res., 109, A11305, doi:10.1029/2004JA010474.
... Before this study, the inversion method has been tested with only one event, where the Fast Auroral SnapshoT (FAST) satellite flew over an auroral arc. The arc was located close to the zenith of ASC field-of-view at Kevo, while the FAST footpoint passed near Kilpisjärvi, on 3 November 1998 at 17:36 UT (Janhunen, 2001;Janhunen et al., 2000). Only green line images from the Kevo station were used in the inversion (no blue images were recorded at that time) and as the ASCs were not yet intensity calibrated an approximation of one digital unit corresponding to about 100 R was used. ...
An inversion method for reconstructing the precipitating electron energy flux from a set of multi-wavelength digital all-sky camera (ASC) images has recently been developed by tomografia. Preliminary tests suggested that the inversion is able to reconstruct the position and energy characteristics of the aurora with reasonable accuracy. This study carries out a thorough testing of the method and a few improvements for its emission physics equations. We compared the precipitating electron energy fluxes as estimated by the inversion method to the energy flux data recorded by the Defense Meteorological Satellite Program (DMSP) satellites during four passes over auroral structures. When the aurorae appear very close to the local zenith, the fluxes inverted from the blue (427.8nm) filtered ASC images or blue and green line (557.7nm) images together give the best agreement with the measured flux values. The fluxes inverted from green line images alone are clearly larger than the measured ones. Closer to the horizon the quality of the inversion results from blue images deteriorate to the level of the ones from green images. In addition to the satellite data, the precipitating electron energy fluxes were estimated from the electron density measurements by the EISCAT Svalbard Radar (ESR). These energy flux values were compared to the ones of the inversion method applied to over 100 ASC images recorded at the nearby ASC station in Longyearbyen. The energy fluxes deduced from these two types of data are in general of the same order of magnitude. In 35% of all of the blue and green image inversions the relative errors were less than 50% and in 90% of the blue and green image inversions less than 100%. This kind of systematic testing of the inversion method is the first step toward using all-sky camera images in the way in which global UV images have recently been used to estimate the energy fluxes. The advantages of ASCs, compared to the space-born imagers, are their low cost, good spatial resolution and the possibility of continuous, long-term monitoring of the auroral oval from a fixed position.
... The basis of his classification is the fact that current continuity across the arc can be maintained either by polarization electric fields or by field-aligned currents . Over the past 30 years, electrodynamic studies have concerned either " polarization " dominated arcs (de la Beau-jardì ere et al., 1977; Marklund et al., 1982 Marklund et al., , 1983; Wahlund and Opgenoorth, 1989; Janhunen et al., 2000 ) or " Birkeland current " dominated arcs (Burch et al., 1976; Aikio et al., 1993 Aikio et al., , 1995 Aikio et al., , 2002 Johnson et al., 1998). However, most of these arcs combine simultaneously strong perpendicular electric fields gradients and field-aligned currents. ...
On 12 January 2000, during a northward IMF period, two successive conjunctions occur between the CUTLASS SuperDARN radar pair and the two satellites Ørsted and FAST. This situation is used to describe and model the electrodynamic of a nightside meso-scale arc associated with a convection shear. Three field-aligned current sheets, one upward and two downward on both sides, are observed.
Based on the measurements of the parallel currents and either the conductance or the electric field profile, a model of the ionospheric current closure is developed along each satellite orbit. This model is one-dimensional, in a first attempt and a two-dimensional model is tested for the Ørsted case. These models allow one to quantify the balance between electric field gradients and ionospheric conductance gradients in the closure of the field-aligned currents.
These radar and satellite data are also combined with images from Polar-UVI, allowing for a description of the time evolution of the arc between the two satellite passes. The arc is very dynamic, in spite of quiet solar wind conditions. Periodic enhancements of the convection and of electron precipitation associated with the arc are observed, probably associated with quasi-periodic injections of particles due to reconnection in the magnetotail. Also, a northward shift and a reorganisation of the precipitation pattern are observed, together with a southward shift of the convection shear.
Key words. Ionosphere (auroral ionosphere; electric fields and currents; particle precipitation) – Magnetospheric physics (magnetosphere-ionosphere interactions)
... Generally, the downward current density was smaller and the current was more widely distributed. In previous studies, the FACs associated with arcs are often estimated as lower, from below 1 mA m À2 to about 10 mA m À2 [e.g., Brüning and Goertz, 1986;Bythrow and Potemra, 1987;Elphic et al., 1998;Janhunen et al., 2000]. However, also larger current densities have been observed, e.g., Aikio et al. [1996] discussed an upward current of 26 mA m À2 (at the ionospheric level) about 10 km wide as measured by the Viking satellite and Weimer et al. [1987] studied currents of 10-36 mA m À2 confined to regions less than 20 km wide. ...
The spatial distribution of electric fields, conductances, and currents of steadily drifting medium-scale (15–50 km) arcs in the evening sector (20–23 magnetic local time (MLT)) is obtained from European Incoherent Scatter Radar (EISCAT) and optical ground-based measurements. The current systems of stable arcs residing in the northward convection electric field region show a consistent pattern: currents flow downward on the equatorward side of the arcs, then poleward, and upward from the arcs. In one event where the arcs are located in a region of convection reversals, the current pattern is more complicated. Most of the arcs are associated with an enhanced northward-directed electric field region on the equatorward side of the arc, colocated with downward field-aligned currents (FACs) and suppressed E and F region electron densities. The width of the region of the enhanced electric field is one to four times the width of the arc. In some cases, the electron density reduction is so pronounced that the region can be described as an auroral ionospheric density cavity. The electrostatic magnetosphere–ionosphere coupling model of arcs predicts that the width L of an arc is related to the ionospheric Pedersen conductance ΣP and the “field-aligned conductance” K by . This study shows that stable medium-scale arcs in the evening sector obey this equation. A value of K = 2 × 10−8 S m−2 is obtained for 15–35 km wide arcs. It is argued that the large value of the field-aligned conductance cannot be interpreted in terms of the adiabatic theory. Possibly the high value of K results from nonadiabatic processes acting on the current-carrying electrons.
... 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. ...
Simultaneous all-sky
camera and HF radar observations of the visual and E-region radar aurora in the
west-ward 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 field-aligned 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.Key words. Ionosphere (auroral
ionosphere; ionospheric irregularities; particle precipitation)
Substorms are a commonly occurring but insufficiently understood form of dynamics in the coupled magnetosphere-ionosphere system, associated with space weather disturbances and auroras. We have used Principal Component Analysis (PCA) to characterize the spatiotemporal development of ionospheric equivalent currents as observed by the International Monitor for Auroral Geomagnetic Effects (IMAGE) magnetometers during 28 substorm onsets identified by Frey et al. (2004). Auroral observations were provided by all-sky cameras. We found that the equivalent currents can typically be described by three components: a channel of poleward equivalent current (wedgelet), a westward electrojet (WEJ) associated with an auroral arc, and a vortex. The WEJ and vortex are located at the equatorward end of the channel, which has been associated with Bursty Bulk Flows (BBFs) by previous studies. Depending on its polarity, the vortex either indents the WEJ and arc equatorward, or bulges the WEJ poleward while winding the arc into an auroral spiral. In addition, there may be a background current system associated with the large-scale convection. The dynamics of the WEJ, vortex, and channel can describe up to 95 % of the variance of the time derivative of the equivalent currents during the examined 20 min interval. Rapid geomagnetic variations at the substorm onset location, which can drive Geomagnetically Induced Currents (GIC) in technological conductor networks, are mainly associated with the oscillations of the WEJ, which may be driven by oscillations of the transition region between dipolar and tail-like field lines in the magnetotail due to the BBF impact. The results contribute to the understanding of substorm physics and to the understanding of processes that drive intense GIC.
