Bright Arc Research Articles

Simultaneous ground‐satellite observations of meso‐scale auroral arc undulations

We present simultaneous ground‐based and in situ measurements of a train of meso‐scale (about 100–300 km) auroral arc undulations, occurring in the postmidnight sector (∼1 MLT) between 0040 UT and 0054 UT on September 21, 2009. The undulations appeared at the auroral poleward boundary, and then moved eastward with a speed of 0.9–2.2 km s−1. Dynamic behaviors of the associated meso‐scale ionospheric plasma flows and current systems were also detected with the ground‐based magnetometer and radar measurements within the all‐sky camera field‐of‐view. During the interval of interest, simultaneous Cluster observations in the central near tail region (11–14 RE down tail) were available, and especially the ionospheric footprint of Cluster 2 (CL2) was close to the optical auroral forms. CL2 observed strong fluctuations in the in situ magnetic field with amplitude of 5–10 nT whenever a bright arc area, and its trailing adjacent area, of the auroral undulations passed its ionospheric footprint. Such in situ magnetic field changes at CL2 could be considered as a manifestation of localized upward and downward field‐aligned current sheets moving eastward at the central near‐Earth tail boundary, linked to the meso‐scale auroral undulation structures.

Magnetic conjugacy of northern and southern auroral beads

Auroral beads, i.e., azimuthally arrayed bright spots resembling a pearl necklace, have recently drawn attention as a possible precursor of auroral substorms. We used simultaneous, ground‐based, all‐sky camera observations from a geomagnetically conjugate Iceland‐Syowa Station pair to demonstrate that the auroral beads, whose wavelength is ∼30–50 km, evolve synchronously in the northern and southern hemispheres and have remarkable interhemispheric similarities. In both hemispheres: 1) they appeared almost at the same time; 2) their longitudinal wave number was similar ∼300–400, corresponding bead separation being ∼1° in longitude; 3) they started developing into a larger scale spiral form at the same time; 4) their propagation speeds and their temporal evolution were almost identical. These interhemispheric similarities provide strong evidence that there is a common driver in the magnetotail equatorial region that controls the major temporal evolution of the auroral beads; thus, the magnetosphere plays a primary role in structuring the initial brightening arc in this scale size.

Sunlight refraction in the mesosphere of Venus during the transit on June 8th, 2004

Many observers in the past gave detailed descriptions of the telescopic aspect of Venus during its extremely rare transits across the Solar disk. In particular, at the ingress and egress, the portion of the planet’s disk outside the Solar photosphere has been repeatedly perceived as outlined by a thin, bright arc (“aureole”). Those historical visual observations allowed inferring the existence of Venus’ atmosphere, the bright arc being correctly ascribed to the refraction of light by the outer layers of a dense atmosphere. On June 8th, 2004, fast photometry based on electronic imaging devices allowed the first quantitative analysis of the phenomenon. Several observers used a variety of acquisition systems to image the event – ranging from amateur-sized to professional telescopes and cameras – thus collecting for the first time a large amount of quantitative information on this atmospheric phenomenon. In this paper, after reviewing some elements brought by the historical records, we give a detailed report of the ground based observations of the 2004 transit. Besides confirming the historical descriptions, we perform the first photometric analysis of the aureole using various acquisition systems. The spatially resolved data provide measurements of the aureole flux as a function of the planetocentric latitude along the limb. A new differential refraction model of solar disk through the upper atmosphere allows us to relate the variable photometry to the latitudinal dependency of scale-height with temperature in the South polar region, as well as the latitudinal variation of the cloud-top layer altitude. We compare our measurements to recent analysis of the Venus Express VIRTIS-M, VMC and SPICAV/SOIR thermal field and aerosol distribution. Our results can be used a starting point for new, more optimized experiments during the 2012 transit event.

A multiwavelength study of the radio source G296.7−0.9: confirmation as a Galactic supernova remnant

We present a multi-wavelength study of the radio source G296.7-0.9. This source has a bilateral radio morphology, a radio spectral index of -0.5 +/- 0.1, sparse patches of linear polarisation, and thermal X-rays with a bright arc near the radio boundary. Considering these characteristics, we conclude that G296.7-0.9 is a supernova remnant (SNR). The age and morphology of the SNR in the context of its environment suggest that the source is co-located with an HII region, and that portions of the shock front have broken out into a lower density medium. We see no evidence for a neutron star or pulsar wind nebula associated with SNR G296.7-0.9.

Brightening of onset arc precedes the dipolarization onset: THEMIS observations of two events on 1 March 2008

Abstract. We present a new M-I coupling model of substorm during southward IMF based on the THEMIS observations of two events on 1 March 2008. The first event (E-1) was classified as a pseudo-breakup: brightening of the onset arc preceded the first dipolarization onset by ∼71 ± 3 s, but the breakup arcs faded within ∼5 min without substantial poleward expansion and the dipolarization stopped and reversed to thinning. The second event (E-2) was identified as a substorm: brightening of the second onset arc preceded the second dipolarization onset by ∼80 ± 3 s, leading to a full-scale expanding auroral bulge during the substorm expansion phase for ∼20 min. The Alfvén travel time from the ionosphere to the dipolarization onset region is estimated at ∼69.3 s in E-1; at ∼80.3 s in E-2, which matched well with the observed time delay of the dipolarization onset after the brightening of the onset arc, respectively in E-1 and E-2. Brightening of the onset arc precedes the dipolarization onset suggest that the onset arc brightening is caused by the intense upward field-aligned currents originating from the divergence of the Cowling electrojet in the ionosphere. The Cowling electrojet current loop (CECL) is formed to close the field-aligned currents at all times. The closure current in the Alfvén wavefront is anti-parallel to the cross-tail current. Dipolarization onset occurs when the Alfvén wavefront incident on the near-Earth plasma sheet to disrupt the cross-tail current in the dipolarization region. Slow MHD waves dominate the disruption of the cross-tail current in the dipolarization region.

Capacitive Current Interruption by HV Air-Break Disconnectors With High-Velocity Opening Auxiliary Contacts

Air-break disconnectors (DS) in a high-voltage network can interrupt small capacitive currents (e.g., occurring when disconnecting short unloaded transmission lines, current transformers, and so forth). The performance of a device with high-velocity opening auxiliary contacts attached to the DS enhancing the interrupting capability is presented. A series of laboratory experiments is carried out with the interrupted current range between 5 and 27 A. The experimental observations include voltage and current characteristics of the DS, and synchronous optical recording of the arc with a high-speed camera. Electrical and optical data are studied for fail and successful interruptions. The results show: 1) The short arcing time by the fast opening contacts limits the energy input into the arc, enhancing the probability of arc extinction; 2) the contribution to the arc energy mainly comes from the oscillations upon restrike. The energy input to the arc increases with higher breakdown voltage, larger air gap, and higher interrupted current; 3) the arc brightness is synchronous with arc current. With higher interrupted current, the overall arc is brighter and its remnants decay slower. Based on the results, potential successful approaches to increase the interruption capability are discussed.

The Wave-Like Auroral Structure around Auroral Expansion Onset

We present the direct connection between the wave-like auroral structure around the time of auroral expansion onset and the ballooning mode waves in the near-Earth magnetotail. Based on the NASA mission time history of events and macroscale interactions during substorms (THEMIS) ground-based all-sky imagers, we show that around the time of auroral expansion onset, a wave-like auroral structure first has four luminosity peaks separated by 2–3° magnetic longitude (MLON). Subsequently, the wave-like structure propagates in the azimuthal direction and an overall bright arc spans approximately 1 h magnetic local time. The wavelength is estimated to be 120–180 km. Finally, a noticeable poleward auroral expansion is observed. The ballooning mode waves are identified by two THEMIS probes in the near-Earth magnetotail. The observed wavelength of the ballooning mode waves is approximately equal to the order of the ion Larmor radius. The wavelength of 1500–3000 km in the near-Earth magnetotail is comparable with the wave-like auroral structure estimate. This study suggests that the ballooning mode waves might play a crucial role in auroral expansion onset, corresponding to the wave-like auroral structure in this study.

Energy source for auroral electrons from two proposed substorm onset processes

[1] There are two main processes in the tail that are proposed to link electron precipitation for the initial brightening auroral arc at substorm onset. These are current disruption/dipolarization (CDD) and magnetic reconnection (MR). To provide further clues in differentiating these two processes, we examine the electron energy spectrum and the associated total field-aligned energy flux during six CDD events and compare them with five MR events. Data from the Active Magnetospheric Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE), Geotail, and Time History of Events and Macroscale Ineractions during Substorms (THEMIS) missions are used in this study. It is found that (1) the total field-aligned electron energy flux from CDD sites is generally high enough to account for the energy flux required to produce bright auroral arcs at substorm onsets and (2) the total field-aligned electron energy flux from MR sites is generally a factor of one to two orders of magnitude lower than that from CDD sites. These results favor a direct link between the CDD sites and the substorm onset arcs, but they do not rule out further acceleration of auroral electrons away from these sites by other energy sources (e.g., field-aligned potential drops and kinetic Alfven waves) to produce even higher-intensity auroral arcs.

Climatology of GPS phase scintillation and HF radar backscatter for the high-latitude ionosphere under solar minimum conditions

Abstract. Maps of GPS phase scintillation at high latitudes have been constructed after the first two years of operation of the Canadian High Arctic Ionospheric Network (CHAIN) during the 2008–2009 solar minimum. CHAIN consists of ten dual-frequency receivers, configured to measure amplitude and phase scintillation from L1 GPS signals and ionospheric total electron content (TEC) from L1 and L2 GPS signals. Those ionospheric data have been mapped as a function of magnetic local time and geomagnetic latitude assuming ionospheric pierce points (IPPs) at 350 km. The mean TEC depletions are identified with the statistical high-latitude and mid-latitude troughs. Phase scintillation occurs predominantly in the nightside auroral oval and the ionospheric footprint of the cusp. The strongest phase scintillation is associated with auroral arc brightening and substorms or with perturbed cusp ionosphere. Auroral phase scintillation tends to be intermittent, localized and of short duration, while the dayside scintillation observed for individual satellites can stay continuously above a given threshold for several minutes and such scintillation patches persist over a large area of the cusp/cleft region sampled by different satellites for several hours. The seasonal variation of the phase scintillation occurrence also differs between the nightside auroral oval and the cusp. The auroral phase scintillation shows an expected semiannual oscillation with equinoctial maxima known to be associated with aurorae, while the cusp scintillation is dominated by an annual cycle maximizing in autumn-winter. These differences point to different irregularity production mechanisms: energetic electron precipitation into dynamic auroral arcs versus cusp ionospheric convection dynamics. Observations suggest anisotropy of scintillation-causing irregularities with stronger L-shell alignment of irregularities in the cusp while a significant component of field-aligned irregularities is found in the nightside auroral oval. Scintillation-causing irregularities can coexist with small-scale field-aligned irregularities resulting in HF radar backscatter. The statistical cusp and auroral oval are characterized by the occurrence of HF radar ionospheric backscatter and mean ground magnetic perturbations due to ionospheric currents.

Location of Saturn's northern infrared aurora determined from Cassini VIMS images

[1] The location of Saturn's northern infrared aurora is described in detail using 12 selected images acquired by Cassini VIMS during 2006–2008. Bright main oval arcs and prevalent polar features are displayed, which do not exhibit a preferred local time or spatial extent. The average equatorward limit of the aurora was determined using a best fit circle method. The average circle has a radius of 16.4 ± 0.2° latitude, centred 1.6° anti-sunward of Saturn's pole. The low standard deviation of the fitted circle radii (<1°) indicates the relative stability of the equatorward auroral boundary. For the first time we show that the average location of the infrared oval is similar to that of the ultraviolet oval, suggesting that the auroral emissions at different wavelengths are very likely produced by a common driver despite the distinct emission mechanisms.

Comment on “The importance of auroral features in the search for substorm onset process” by Syun-Ichi Akasofu, A. T. Y. Lui, and C.-I. Meng

Comment on “The importance of auroral features in the search for substorm onset process” by Syun-Ichi Akasofu, A. T. Y. Lui, and C.-I. Meng

THE REDSHIFT DISTRIBUTION OF GIANT ARCS IN THE SLOAN GIANT ARCS SURVEY

We measure the redshift distribution of a sample of 28 giant arcs discovered as a part of the Sloan Giant Arcs Survey (SGAS). Gemini/GMOS-North spectroscopy provides precise redshifts for 24 arcs, and "redshift desert" constraints for the remaining four. This is a direct measurement of the redshift distribution of a uniformly selected sample of bright giant arcs, which is an observable that can be used to inform efforts to predict giant arc statistics. Our primary giant arc sample has a median redshift z=1.821 and nearly two thirds of the arcs - 64% - are sources at z \gtrsim 1.4, indicating that the population of background sources that are strongly lensed into bright giant arcs resides primarily at high redshift. We also analyze the distribution of redshifts for 19 secondary strongly lensed background sources that are not visually apparent in SDSS imaging, but were identified in deeper follow-up imaging of the lensing cluster fields. Our redshift sample for the secondary sources is not spectroscopically complete, but combining it with our primary giant arc sample suggests that a large fraction of all background galaxies which are strongly lensed by foreground clusters reside at z \gtrsim 1.4. Kolmogorov-Smirnov (KS) tests indicate that our well-selected, spectroscopically complete primary giant arc redshift sample can be reproduced with a model distribution that is constructed from a combination of results from studies of strong lensing clusters in numerical simulations, and observational constraints on the galaxy luminosity function.

THE SLOAN BRIGHT ARCS SURVEY: DISCOVERY OF SEVEN NEW STRONGLY LENSED GALAXIES FROM z = 0.66-2.94

We report the discovery of seven new, very bright gravitational lens systems from our ongoing gravitational lens search, the Sloan Bright Arcs Survey (SBAS). Two of the systems are confirmed to have high source redshifts z = 2.19 and z = 2.94. Three other systems lie at intermediate redshift with z = 1.33, 1.82, 1.93 and two systems are at low redshift z = 0.66, 0.86. The lensed source galaxies in all of these systems are bright, with i-band magnitudes ranging from 19.73 to 22.06. We present the spectrum of each of the source galaxies in these systems along with estimates of the Einstein radius for each system. The foreground lens in most systems is identified by a red sequence based cluster finder as a galaxy group; one system is identified as a moderately rich cluster. In total, SBAS has now discovered 19 strong lens systems in the SDSS imaging data, 8 of which are among the highest surface brightness z ≃ 2–3 galaxies known.

Small and meso‐scale properties of a substorm onset auroral arc

We present small and meso‐scale properties of a substorm onset arc observed simultaneously by the Reimei and THEMIS satellites together with ground‐based observations by the THEMIS GBO system. The optical observations revealed the slow equatorward motion of the growth‐phase arc and the development of a much brighter onset arc poleward of it. Both arcs showed the typical particle signature of electrostatic acceleration in an inverted‐V structure together with a strong Alfvén wave acceleration signature at the poleward edge of the onset arc. Two THEMIS spacecraft encountered earthward flow bursts around the times the expanding optical aurora reached their magnetic footprints in the ionosphere. The particle and field measurements allowed for the reconstruction of the field‐aligned current system and the determination of plasma properties in the auroral source region. Auroral arc properties were extracted from the optical and particle measurements and were used to compare measured values to theoretical predictions of the electrodynamic model for the generation of auroral arcs. Good agreement could be reached for the meso‐scale arc properties. A qualitative analysis of the internal structuring of the bright onset arc suggests the operation of the tearing instability which provides a 'rope‐like' appearance due to advection of the current in the sheared flow across the arc. We also note that for the observed parameters ionospheric conductivity gradients due to electron precipitation will be unstable to the feedback instability in the ionospheric Alfvén resonator that can drive structuring in luminosity over the range of scales observed.

Ionospheric feedback instability and substorm development

We report on ground magnetic and optical observations performed during an ionospheric heating experiment at the High Frequency Active Auroral Research Program (HAARP) facility in Alaska on 29 October 2008. The experiment was aimed at generation of large‐amplitude ULF electromagnetic waves by triggering and facilitating development of the ionospheric feedback instability (IFI) in the region adjacent to a bright auroral arc. In this region the downward/return magnetic field‐aligned current decreases plasma density and enhances the electric field in the ionosphere. A combination of these two effects creates favorable conditions for the instability. The experiment occurred during a period of substorm activity, but effects from the HAARP transmitter were not sufficiently intense to be detected against the background of strong natural oscillations occurring farther north from the HAARP site. Thus the experiment did not provide concrete evidence that heating of the ionosphere with powerful HF transmitters can affect IFI development or generate intense ULF electromagnetic waves. However, during the experiment ground‐based magnetometers in Alaska and Canada detected large‐amplitude ULF waves in regions where the substorm onset auroral arcs interacted with the ionosphere. The frequencies of these waves closely matched frequencies predicted by simulations of IFI for these particular geophysical conditions. These observations support the hypothesis that geomagnetic substorms, the corresponding dynamics of discrete auroral arcs, and the ionospheric feedback instability are closely connected phenomena.

10.1007/s11478-008-1002-y

The dynamics and fine structure of auroras before and during 60 auroral breakups, including pseudobreakups and breakups at moderate and high auroral activity, have been studied using the developed method for processing television images. The IMAGE and POLAR satellite and simultaneous ground images of auroras, ground magnetic data, and measurements of IMF and solar wind plasma parameters have been analyzed. The signatures that can be precursors of breakup have been found out in the auroral dynamics and morphology in the spatial—temporal vicinity of breakup. The morphological characteristics of auroral structures have been analyzed statistically. The directions of motion of weak subvisual structures have been determined. The velocities of motion of such structures are presented. The relation of the initial auroral arc bright-ening during breakups and pseudobreakups to the beginning of magnetic activation and formation of rayed structures has been analyzed.

Dayside auroras during unusually large positive values of the IMF B z component

The characteristics of dayside auroras during the large (16-24 nT) positive values of the IMF Bz component, observed on January 14, 1988, during the interaction between the Earth's magnetosphere and the body of the interplanetary magnetic cloud, have been studied based on the optical observations on Heiss Island. A wide band of diffuse red luminosity with an intensity of 1-2 kilorayleigh (kR) was observed during 6 h in the interval 1030-1630 MLT at latitudes higher than 75° CGL. Rayed auroral arcs, the brightness of which in the 557.7 nm emission sharply increased to 3-7 kR in the postnoon sector immediately after the polarity reversal of the IMF B y component from positive to negative, were continuously registered within the band. Bright auroral arcs were observed at the equatorward edge of red luminosity. It has been found out that the red auroral intensity increases and the band equatorward boundary shifts to lower latitudes with increasing solar wind dynamic pressure. However, a direct proportional dependence of the variations in the auroral fea� tures on the dynamic pressure variations has not been found. It has been concluded that the source of bright discrete auroras is located in the region of the lowlatitude boundary layer (LLBL) on closed geomagnetic field lines. The estimated LLBL thickness is ~3 Re. It has been concluded that the intensity of the dayside red band depends on the solar wind plasma density, whereas the position of the position equatorward boundary depends on the dynamic pressure value and its variations.

THE SLOAN BRIGHT ARCS SURVEY: FOUR STRONGLY LENSED GALAXIES WITH REDSHIFT &gt; 2

We report the discovery of four very bright, strongly-lensed galaxies found via systematic searches for arcs in Sloan Digital Sky Survey Data Release 5 and 6. These were followed-up with spectroscopy and imaging data from the Astrophysical Research Consortium 3.5m telescope at Apache Point Observatory and found to have redshift $z>2.0$. With isophotal magnitudes $r = 19.2 - 20.4$ and $3\arcsec$-diameter magnitudes $r = 20.0 - 20.6$, these systems are some of the brightest and highest surface brightness lensed galaxies known in this redshift range. In addition to the magnitudes and redshifts, we present estimates of the Einstein radii, which range from $5.0 \arcsec$ to $12.7 \arcsec$, and use those to derive the enclosed masses of the lensing galaxies.

Aegaeon (Saturn LIII), a G-ring object

Aegaeon (Saturn LIII, S/2008 S1) is a small satellite of Saturn that orbits within a bright arc of material near the inner edge of Saturn’s G-ring. This object was observed in 21 images with Cassini’s Narrow-Angle Camera between June 15 (DOY 166), 2007 and February 20 (DOY 051), 2009. If Aegaeon has similar surface scattering properties as other nearby small saturnian satellites (Pallene, Methone and Anthe), then its diameter is approximately 500 m. Orbit models based on numerical integrations of the full equations of motion show that Aegaeon’s orbital motion is strongly influenced by multiple resonances with Mimas. In particular, like the G-ring arc it inhabits, Aegaeon is trapped in the 7:6 corotation eccentricity resonance with Mimas. Aegaeon, Anthe and Methone therefore form a distinctive class of objects in the Saturn system: small moons in corotation eccentricity resonances with Mimas associated with arcs of debris. Comparisons among these different ring-arc systems reveal that Aegaeon’s orbit is closer to the exact resonance than Anthe’s and Methone’s orbits are. This could indicate that Aegaeon has undergone significant orbital evolution via its interactions with the other objects in its arc, which would be consistent with the evidence that Aegaeon’s mass is much smaller relative to the total mass in its arc than Anthe’s and Methone’s masses are.

A POSSIBLE SUPERNOVA REMNANT HIGH ABOVE THE GALACTIC DISK

We present the analysis of three Suzaku observations of a bright arc in the ROSAT All-Sky Survey 1/4 keV maps at $l \approx 247\degr$, $b \approx -64\degr$. In particular, we have tested the hypothesis that the arc is the edge of a bubble blown by an extraplanar supernova. One pointing direction is near the brightest part of the arc, one is toward the interior of the hypothesized bubble, and one is toward the bubble exterior. We fit spectral models generated from 1-D hydrodynamical simulations of extraplanar supernova remnants (SNRs) to the spectra. The spectra and the size of the arc ($\mathrm{radius} \approx 5\degr$) are reasonably well explained by a model in which the arc is the bright edge of a $\sim$100,000-yr old SNR located $\sim$1--2 kpc above the disk. The agreement between the model and the observations can be improved if the metallicity of the X-ray--emitting gas is $\sim$1/3 solar, which is plausible, as the dust which sequesters some metals is unlikely to have been destroyed in the lifetime of the SNR. The width of the arc is larger than that predicted by our SNR model; this discrepancy is also seen with the Vela SNR, and may be due to the 1-D nature of our simulations. If the arc is indeed the edge of an extraplanar SNR, this work supports the idea that extraplanar supernovae contribute to the heating of the $\sim$million-degree gas in the halo.