Auroral Arcs Research Articles

Temporal and spatial evolution of auroral electron energy spectra in a region surrounding the magnetic zenith

We present a new method for estimating the spatial and temporal evolution of the auroral electron energy spectrum at subkilometer and subsecond scales using optical and incoherent scatter radar data. This method is applied to an event on 12 December 2006 when a thin auroral arc that exhibits subkilometer structuring is observed. The energy spectrum and resultant emission rates are estimated for a 10 s period when the arc was in the field of view of the optical instrumentation. Modeled images of the observed aurora are produced using the estimated emission rates and compared with the optical observations of the aurora. We find the modeled images reproduce the structure and dynamics of the observed aurora to within the uncertainties of the models used. The brightness underestimate of about 30% can be explained by the underestimate of the energy flux from the radar measurements.

Saturn's dayside ultraviolet auroras: Evidence for morphological dependence on the direction of the upstream interplanetary magnetic field.

We examine a unique data set from seven Hubble Space Telescope (HST) “visits” that imaged Saturn's northern dayside ultraviolet emissions exhibiting usual circumpolar “auroral oval” morphologies, during which Cassini measured the interplanetary magnetic field (IMF) upstream of Saturn's bow shock over intervals of several hours. The auroras generally consist of a dawn arc extending toward noon centered near ∼15° colatitude, together with intermittent patchy forms at ∼10° colatitude and poleward thereof, located between noon and dusk. The dawn arc is a persistent feature, but exhibits variations in position, width, and intensity, which have no clear relationship with the concurrent IMF. However, the patchy postnoon auroras are found to relate to the (suitably lagged and averaged) IMF Bz, being present during all four visits with positive Bz and absent during all three visits with negative Bz. The most continuous such forms occur in the case of strongest positive Bz. These results suggest that the postnoon forms are associated with reconnection and open flux production at Saturn's magnetopause, related to the similarly interpreted bifurcated auroral arc structures previously observed in this local time sector in Cassini Ultraviolet Imaging Spectrograph data, whose details remain unresolved in these HST images. One of the intervals with negative IMF Bz however exhibits a prenoon patch of very high latitude emission extending poleward of the dawn arc to the magnetic/spin pole, suggestive of the occurrence of lobe reconnection. Overall, these data provide evidence of significant IMF dependence in the morphology of Saturn's dayside auroras.Key PointsWe examine seven cases of joint HST Saturn auroral images and Cassini IMF dataThe persistent but variable dawn arc shows no obvious IMF dependencePatchy postnoon auroras are present for northward IMF but not for southward IMF

Ionospheric feedback instability and active discrete auroral forms

AbstractWe present results from time‐dependent, three‐dimensional numerical simulations of ULF Alfvén waves generated by the ionospheric feedback instability at high latitudes. The goal of this study is to understand physical mechanisms responsible for the formation of active discrete auroral forms (curls and folds) typically observed during the substorm onset. Our simulations demonstrate that active feedback of the auroral ionosphere on magnetic field‐aligned currents carried by ULF Alfvén waves can explain wide variety of auroral structures. The main reason for this variety is a susceptibility of the electrodynamics of the coupled magnetosphere‐ionosphere system to parameters of the electric field and plasma in the ionosphere and magnetosphere of the Earth. Due to the highly nonlinear character of the magnetosphere‐ionosphere interactions, different combinations of these parameters lead to different spatial and temporal behavior of the magnetic field‐aligned currents producing aurora. One of the main conclusions from our study is that the ionosphere is responsible for the formation of small‐scale curls and folds in the discrete aurora and for the intensification of magnetic field‐aligned currents, observed during substorms. The results from our 3‐D simulations are applied for the explanation of structure of discrete auroral arcs observed during the 29 October 2013 substorm at Fort Yukon, Alaska.

Interrelationship between preonset auroral and magnetic signatures at a geomagnetically conjugate Iceland‐Syowa pair

AbstractWe present the initiation and development of Pi1‐Pi2 band pulsations in concert with spatiotemporal evolution of preonset auroral arc during an isolated auroral substorm reported by Motoba et al. (2012), and their interhemispheric similarities/dissimilarities, based on the colocated optical and magnetic field measurements at a geomagnetically conjugate Iceland‐Syowa pair. At least ~7 min prior to the auroral expansion onset, the first identifiable signature of preonset arc began with a small‐scale (~30–50 km) azimuthal beading. Interestingly, the early development of the visible arc beading for ~3–4 min was not accompanied by any ground magnetic perturbation. Then the bead‐like forms in the arc evolved into brighter, larger undulations, eventually developing into the poleward expansion. Such a preonset optical sequence was almost identical at both stations. In the transition from the beads into undulations, Pi2 pulsations were first identified clearly, followed by Pi1 pulsations a few minutes later. Whereas the Pi2 onset was coincident with the initiation of progressive increase of the preonset arc luminosity, the Pi1 onset was associated with a subsequent steep increase. The Pi2 wave cycles superimposed on the magnetic negative bay were well correlated with the preonset arc luminosity time series. These results imply the potential linkage between the ground Pi1‐Pi2 signatures in the onset region and the preonset auroral arc processes. Although the Pi1‐Pi2 features were generally similar between the two hemispheres, there was also some dissimilarity in their temporal behaviors, which could reflect a small but noteworthy interhemispheric asymmetry in the overhead conjugate preonset aurora.

Modulation of auroras by Pc5 pulsations in the dawn sector in association with reappearance of energetic particles at geosynchronous orbit

Geomagnetic Pc5 pulsations were observed in the dawn sector of the auroral zone on 17 January 1994 in association with increased energetic ion fluxes at geosynchronous orbit 10min after the Pi2 onset. The characteristic properties of auroras associated with these pulsations were studied using movies taken by an all-sky imager. It was found that a pulsating aurora (PA) can be an optical manifestation of the Pc5 waves by a strong poloidal component observed with ground-based magnetometers. Goes7 observations showed compressional pulsations with the same period which can be attributed to the influence of the finite pressure of plasma and field line curvature on the poloidally polarized Alfvén waves. These poloidal pulsations may be generated by the ion injection observed with the LANL 1989-046 satellite. Two auroral arcs were observed north of the PA with optical features characteristic for the toroidal field line resonances: strong localization across L-shells, 180° phase change across the resonance, poleward phase propagation. Thus the Pc5 oscillations split into the toroidal and poloidal mode and oscillated coherently at latitudes from 62°N to 70°N. This study provides observational evidence of polarization splitting of the Alfven oscillation spectrum. Such a polarization splitting would occur in association with the reappearance of the energetic particles at geosynchronous orbit.

Forced reconnection in the near magnetotail: Onset and energy conversion in PIC and MHD simulations

Using two‐dimensional particle‐in‐cell (PIC) together with magnetohydrodynamic (MHD) simulations of magnetotail dynamics, we investigate the evolution toward onset of reconnection and the subsequent energy transfer and conversion. In either case, reconnection onset is preceded by a driven phase, during which magnetic flux is added to the tail at the high‐latitude boundaries, followed by a relaxation phase, during which the configuration continues to respond to the driving. The boundary deformation leads to the formation of thin embedded current sheets, which are bifurcated in the near tail, converging to a single sheet farther out in the MHD simulations. The thin current sheets in the PIC simulation are carried by electrons and are associated with a strong perpendicular electrostatic field, which may provide a connection to parallel potentials and auroral arcs and an ionospheric signal even prior to the onset of reconnection. The PIC simulation very well satisfies integral entropy conservation (intrinsic to ideal MHD) during this phase, supporting ideal ballooning stability. Eventually, the current intensification leads to the onset of reconnection, the formation and ejection of a plasmoid, and a collapse of the inner tail. The earthward flow shows the characteristics of a dipolarization front: enhancement of Bz, associated with a thin vertical electron current sheet in the PIC simulation. Both MHD and PIC simulations show a dominance of energy conversion from incoming Poynting flux to outgoing enthalpy flux, resulting in heating of the inner tail. Localized Joule dissipation plays only a minor role.

GPS scintillation effects associated with polar cap patches and substorm auroral activity: direct comparison

We directly compare the relative GPS scintillation levels associated with regions of enhanced plasma irregularities called auroral arcs, polar cap patches, and auroral blobs that frequently occur in the polar ionosphere. On January 13, 2013 from Ny-Ålesund, several polar cap patches were observed to exit the polar cap into the auroral oval, and were then termed auroral blobs. This gave us an unprecedented opportunity to compare the relative scintillation levels associated with these three phenomena. The blobs were associated with the strongest phase scintillation (σϕ), followed by patches and arcs, with σϕ up to 0.6, 0.5, and 0.1 rad, respectively. Our observations indicate that most patches in the nightside polar cap have produced significant scintillations, but not all of them. Since the blobs are formed after patches merged into auroral regions, in space weather predictions of GPS scintillations, it will be important to enable predictions of patches exiting the polar cap.

Observations of GPS scintillation during an isolated auroral substorm

This paper reports simultaneous observations of ionospheric scintillation during an auroral substorm that were made using an all-sky full-color digital single-lens reflex (DSLR) camera (ASC) and a Global Positioning System (GPS) ionospheric scintillation and total electron content monitor (GISTM) in Tromsø (69.60 N, 19.20 E), Norway. On the night of November 19, 2009, a small substorm occurred in northern Scandinavia. The ASC captured its temporal evolution from the beginning of the growth phase to the end of the recovery phase. The amplitude scintillation, as monitored by the S4 index from the GISTM, did not increase in any substorm phase. By contrast, phase scintillation, as measured by the σ φ index, occurred when discrete auroral arcs appeared on the GPS signal path. In particular, the phase scintillation was significantly enhanced for a few minutes immediately after the onset of the expansion phase. During this period, bright and discrete auroral forms covered the entire sky, which implies that structured precipitation on the scale of a few kilometers to a few tens of kilometers dominated the electron density distribution in the E region. Such inhomogeneous ionization structures probably produced significant changes in the refractive index and eventually resulted in the enhancement of the phase scintillation.

RCM‐E simulation of substorm growth phase arc associated with large‐scale adiabatic convection

Substorm auroral breakup often occurs on a longitudinally elongated arc at the end of a growth phase. We present an idealized high‐resolution simulation with the Rice Convection Model‐Equilibrium (RCM‐E) to investigate how large‐scale adiabatic convection under equilibrium conditions can give rise to an auroral arc. We find that a thin arc that maps to the magnetic transition region at r ~ 8 RE emerges in the late growth phase. The simulation implies that the arc in the premidnight sector is associated with a sheet of additional region 1 sense field‐aligned current (FAC) just poleward of the main region 2 FAC, while the arc in the postmidnight sector is associated with the poleward portion of the main upward region 2 FAC. Explanations for the location and the thickness of the arc are proposed, based on the simulation.

New optical observations of auroral flames

Auroral arcs are a visible manifestation of space weather. A common dynamic observed during geomagnetically disturbed periods is the breakup of auroral arcs into a multitude of small‐scale filaments less than 1 kilometer across. These filaments can move rapidly upward along the geomagnetic field lines. Dahlgren et al. observed such “auroral flames” during a breakup event on 1 March 2011 using an advanced high‐speed optical system with sensitive low‐light detectors. The data provide new insight into the energy and flux of electrons in individual auroral flames and could help scientists better understand the dynamic processes involved in auroral breakup.

Observation and modeling of magnetospheric cold electron heating by electromagnetic ion cyclotron waves

A cold electron heating event associated with electromagnetic ion cyclotron (EMIC) waves is observed and modeled. The observational data of particles and waves are collected by the Time History of Events and Macroscale Interactions during Substorms spacecraft at magnetic local time 17.0–17.2. During this event, intense He+ band EMIC waves with the peak frequency 0.25 Hz are excited, corresponding to the observed phase space density (PSD) of distinct anisotropic ions. Meanwhile, substantial enhancements in energy flux of cold (1–10 eV) electrons are observed in the same period. The energy flux of electrons below 10 eV is increased by several to tens of times. We use a sum of kappa distribution components to fit the observed ion PSD and then calculate the wave growth rate driven by the anisotropic hot protons. The calculated result is in good agreement with the in situ observation. Then, we investigate whether the excited EMIC waves can transfer energy to cold electrons by Landau resonant absorption and yield electron heating. Using the typical Maxwellian distribution for cold electrons, we evaluate the wave damping rates resulted from the cold electrons in gyroresonance with EMIC waves. The simulating results show that the strong wave growth region in the He+ band induced by anisotropic ions corresponds to the strong wave damping region driven by cold electrons. Moreover, cold electrons can be heated efficiently at large wave normal angles. The current results provide a direct observational evidence for EMIC‐driven cold electron heating—a potential mechanism responsible for stable auroral red arc.

Simultaneous ground and satellite observations of discrete auroral arcs, substorm aurora, and Alfvénic aurora with FAST and THEMIS GBO

AbstractThree distinct types of aurora are observed during separate events, using simultaneous measurements on the ground with all sky imagers from the THEMIS Ground‐Based Observatories and in situ with the FAST auroral satellite. The three cases show discrete stable arcs, substorm aurora, and Alfvénic aurora. The field‐aligned current structure associated with each type of aurora is shown to agree with contemporary auroral models. Inverted‐V type aurora with electron energy peaks of 1–10 keV are observed, and are associated with upward current regions and discrete auroral arcs seen in the all sky imagers. Downward currents are also identified and correlated with regions without visible aurora, and Alfvénic electron precipitation is associated with mixed current regions and dynamic aurora evident in the all sky images. The substorm aurora observed also shows upward current regions and inverted‐V's associated with bright spots in the ground‐based images. The images shown in association with the substorm aurora correlate well with existing substorm models, including that of Akasofu [1964]. In this study we present these new ground and satellite conjunctions and use them to confirm existing theories of auroral acceleration mechanisms. Additionally, we compare the observations to existing substorm models and investigate differences between specific substorms and the various auroral forms associated with them.

Stability of Alfvén eigenmodes in the vicinity of auroral arc

[1] The purpose of this study is to give a theoretical suggestion to the essential question why east-west elongated auroral arc can keep its anisotropic structure for a long time. It could be related to the stability of east-westward traveling modes in the vicinity of arc, which may develop into wavy or spiral structures, whereas north-southward modes are related to splitting of arcs. Taking into account the arc-inducing field-aligned current and magnetic shears, we examine changes in the stability of Alfven eigenmodes that are coupled to perpendicular modes in the presence of convection electric field. It is demonstrated that the poleward current shear suppresses growth of the westward mode in case of the westward convection electric field. Only the poleward mode is still unstable because of the properties of feedback shear waves. It is suggested that this tends to promote (poleward) arc splitting as often observed during quiet times. We further draw a diagram of the westward mode growth rate as a function of convection electric field and current shear, evaluating critical fields for instabilities of lower Alfven harmonics. It is discovered that a switching phenomenon of fast-growing mode from fundamental to the first harmonic occurs for a high electric field regime. Our stability criterion is applied to some observed situations of auroral arc current system during pre-breakup active times.

Conjugate observations of flow diversion in the magnetotail and auroral arc extension in the ionosphere

[1] We present a case study of a plasma flow diversion event in the inner magnetosphere to illustrate the effects of near-Earth pressure evolution, field-aligned current (FAC) generation, and associated auroral evolution during conjugate observations from THEMIS satellites and All-Sky Imagers (ASIs) on 29 January 2008. At ~0831 UT, an earthward flow at THC, at X ~ 18 Re, was observed which was likely to be associated with an auroral streamer observed at the ASI station FSMI. Near-Earth satellites THD and THE, separated in the Y-direction by ~1 Re, and magnetically mapping onto the preexisting aurora arc, also observed the earthward flow but with a small Alfven transit time of ~20 s between the magnetotail and the ionosphere. As the earthward flow arrived at THD and THE, the aurora arc near the footprints of THD and THE brightened. A duskward pressure gradient developed between THD and THE in the equatorial magnetotail, corresponding to an upward FAC and consistent with the auroral brightening. At ~0834 UT, an eastward diversion of the flow was observed first at THE and later at THD, further to the East. At the head of that flow, pressure variations, coherent between THE and THD, were observed to propagate Eastward at ~400 km/s, consistent with an eastward auroral expansion near the footprints of THE and THD. Concurrent perturbations of the By component of the magnetic field signify eastward motion of a FAC pair at the two spacecraft locations. We interpret the pressure variations as the generator of the observed FAC that feeds the arc and its eastward expansion.

The optical manifestation of dispersive field‐aligned bursts in auroral breakup arcs

High‐resolution optical observations of a substorm expansion show dynamic auroral rays with surges of luminosity traveling up the magnetic field lines. Observed in ground‐based imagers, this phenomenon has been termed auroral flames, whereas the rocket signatures of the corresponding energy dispersions are more commonly known as field‐aligned bursts. In this paper, observations of auroral flames obtained at 50 frames/s with a scientific‐grade Complementary Metal Oxide Semiconductor (CMOS) sensor (30° × 30° field of view, 30 m resolution at 120 km) are used to provide insight into the nature of the precipitating electrons similar to high‐resolution particle detectors. Thanks to the large field of view and high spatial resolution of this system, it is possible to obtain a first‐order estimate of the temporal evolution in altitude of the volume emission rate from a single sensor. The measured volume emission rates are compared with the sum of modeled eigenprofiles obtained for a finite set of electron beams with varying energy provided by the TRANSCAR auroral flux tube model. The energy dispersion signatures within each auroral ray can be analyzed in detail during a fraction of a second. The evolution of energy and flux of the precipitation shows precipitation spanning over a large range of energies, with the characteristic energy dropping from 2.1 keV to 0.87 keV over 0.2 s. Oscillations at 2.4 Hz in the magnetic zenith correspond to the period of the auroral flames, and the acceleration is believed to be due to Alfvenic wave interaction with electrons above the ionosphere.

THEMIS observations of electron acceleration associated with the evolution of substorm dipolarization in the near‐Earth tail

AbstractWe present the evolution of dipolarizations in the near‐Earth tail during a substorm on 15 March 2009, based on the two‐point measurements in the nightside plasma sheet at X ~ −8.0 RE. The earthward‐moving dipolarization, the magnetic flux pileup, and the tailward‐moving dipolarization were observed. For the 30–200 keV electrons, betatron acceleration was the dominant process, which was caused by the much larger gradient of the magnetic field there during the earthward‐moving dipolarization or by a local compression of the magnetic field during the magnetic flux pileup and tailward‐moving dipolarization. These near‐perpendicular distributions for the 30–200 keV electrons are interpreted as produced by a two‐step acceleration: Electrons were first accelerated in the dipolarization fronts in the midtail or the near‐Earth tail and then were further accelerated near the tail current disruption region. For the more than 200 keV electrons, Fermi acceleration was the dominant process, which was caused by the shrinking length of magnetic field line during the tailward‐moving dipolarization. The source region of the more than 200 keV electrons may be near the tail current disruption region, but these electrons were accelerated locally. These field‐aligned electrons can precipitate into the ionosphere and form the discrete auroral arcs. Two parallel arcs were clearly observed around the substorm onset: one propagated equatorward, another propagated poleward. We suggest that the earthward‐moving dipolarization, the magnetic flux pileup, and the tailward‐moving dipolarization near the tail current disruption region can well explain the auroral evolution around the substorm onset.

Quasi-parallel electron beams and their possible application in inferring the auroral arc's root in the magnetosphere

Abstract. In this study we investigate the upgoing electron beams at the topside ionosphere and their counterpart feature, the bidirectional quasi-parallel electron beams (QPEB) in the equatorial magnetosphere, with highlight on their potential application in estimating the location of the arc's root (AR) in the magnetotail central plasma sheet (CPS). We infer from FAST data that the upgoing electron beam is often found in the equatorward vicinity of the inverted-V arc. On the premise of such a scenario, we propose a method to estimate the location of the AR from available magnetospheric measurements by assuming that the tailward boundary of the QPEB demarcates the earthward boundary of the AR. We report two events with THEMIS observations of QPEBs in the magnetotail CPS, and demonstrate how to use the QPEB features, together with the magnetic signatures of the current circuit constituted by the QPEB and arc, to estimate the earthward boundary of the AR. We find that the estimated earthward boundary of AR is situated at the periphery of a quasi-dipolar magnetosphere characterized by a strong Bz gradient. This finding is consistent with previously existing proposals on the possible AR location in the tail (e.g., Lui and Burrows, 1978; Sergeev et al., 2012).

Widths of dayside auroral arcs observed at the Chinese Yellow River Station

Due to the altitudinal extent of an auroral arc, its observed width is different at different zenith angles even when its real width is the same. For that reason, former measurements of arc widths were obtained only for arcs located close to the geomagnetic zenith direction. A method to correct arc width is proposed in the paper, which considers the altitudinal extent of auroral arc. Then, we apply this method to the auroral arcs observed at the Chinese Yellow River Station, and analyze the widths of 17,571 dayside auroral arcs. The distributions of the widths are almost the same at different zenith angles with an average width of 18.5km. Arc widths are narrower as MLT is close to midday.

Auroral Current and Electrodynamics Structure (ACES) observations of ionospheric feedback in the Alfvén resonator and model responses

The ACES‐High rocket, part of the Auroral Current and Electrodynamics Structure (ACES) mission launched from Poker Flat Research Range on 29 January 2009, obtained the first in situ measurements indicative of both of the observational characteristics associated with the ionospheric feedback instability as it flew through an auroral arc and its associated return current region. ACES‐High observed Alfvénic wave structures localized in areas of roughly 10 km near the boundaries of the return current region associated with the discrete auroral arc and increased electron density with a temperature characteristic of a cold ionosphere. This density enhancement is believed to be caused by the excavation of plasma from lower altitudes via the ponderomotive force produced by the ionospheric Alfvén resonator, as shown by Streltsov and Lotko (2008). While this density is lower than expected from simulations and other observations by as much as an order of magnitude, the ratio of the enhancement to the background density is in agreement with predictions. The observations made by ACES‐High agree with the model results by Streltsov and Lotko (2008) but show the localized wave structures only near the boundaries of the return current region and not throughout it. This can be explained by strong small‐scale magnetic field‐aligned currents that are generated by the interaction between the large‐scale downward current and the ionosphere at these boundaries. Finally, a new model, based on that by Streltsov and Marklund (2006), was run with only one downward current region and produced results very similar to the observations seen by ACES‐High.

Estimating energy spectra of electron precipitation above auroral arcs from ground‐based observations with radar and optics

In 2008, coordinated radar‐optical auroral observations were organized in Northern Scandinavia using the European Incoherent Scatter Radar (EISCAT) and the Auroral Large Imaging System (ALIS). A bright auroral arc was imaged on 5 March 2008 from four ground‐based stations, remaining stable between 18:41 and 18:44 UT and coinciding with increased electron densities. This work presents a unified inversion framework deriving the electron energy spectrum from either optical ( 1NG(0,1) emission at 4278 Å) or radar (electron density) observations. An updated forward model of the ionosphere based on a 1‐D kinetic Monte Carlo model is described, characterizing the linear system to invert. The 3‐D blue volume emission rate is first estimated with an iterative reconstruction technique. Also presented is a novel way to calculate an accurate initial guess for auroral tomography taking into account the horizontal/vertical nonuniformity of the emission region. This technique supersedes the often‐used Chapman profile as initial guess when high spatial resolution is needed. The second step, performed for the first time with ALIS optical observations, uses the forward model to retrieve from the emission rates the 2‐D latitude/longitude map of the electron energy spectrum. The same model and inversion methods are finally applied to the EISCAT electron density profiles to derive the temporal energy spectrum of precipitating electrons along the magnetic zenith. Energy spectra from radar and optics are in good agreement. Results suggest that the arc is generated by a complex energy spectrum reminiscent of dispersive Alfvén waves with two main peaks (2.5, 6 keV) and a typical latitudinal width of 7.5 km.