Discrete Aurora Research Articles

ULF waves and discrete aurora

We present results from the numerical study of ULF waves generated by the ionospheric feedback instability in density cavities. The goal of the study is to explain several spectral features of ULF waves detected on the ground in close vicinity of intense discrete auroral arcs. These features include (1) localization of the waves packages across the ambient magnetic field, (2) variation of the wave frequency in relatively small amplitude waves, and (3) presence of several discrete harmonics in the spectrum of the relatively large amplitude waves. Time‐dependent, two‐dimensional simulations based on the two‐fluid MHD model performed in the dipole, axisymmetrical geometry of the ambient magnetic field with realistic parameters of the plasma density in the ionosphere and the magnetosphere demonstrate that the ionospheric feedback instability inside the density cavity indeed provides a good, quantitative explanation of these features of ULF waves observed at high latitudes during substorm onsets.

A magnetohydrodynamic simulation study of Kronian field‐aligned currents and auroras

Magnetohydrodynamic simulations of the interaction of Saturn's magnetosphere with the solar wind indicate that Kelvin‐Helmholtz (K‐H) waves can form on the dayside magnetopause when the interplanetary magnetic field (IMF) is northward. Dayside magnetic reconnection occurs at Saturn for northward IMF, and the combination of K‐H waves and reconnection caused enhanced vorticity in Saturn's magnetosphere. We have used a very high resolution version (Δx = 0.1 RS) of our simulation code to study the consequences of the vortices and reconnection for the generation of field‐aligned currents (FACs) and auroras in Saturn's ionosphere. We found three bands of alternating FACs toward and away from the dawn side of the ionosphere and two sets on the dusk side. The K‐H waves generated a series of toward and away currents along the dayside magnetopause. In the ionosphere they appear as a series of spots of up and down currents. The K‐H field‐aligned currents are adjacent to nearly continuous currents located from 16:00 LT, to around past midnight, and to about 07:00 LT. The largest currents densities (j∥ > 5 × 10−8 A m−2) are found at the highest latitudes. They map to the magnetopause and to the near‐Saturn tail region. We used the away current density and the Knight relationship to estimate the energy flux related to discrete auroras and obtained ∼1 mW m−2 in the region with the strongest currents. This gives approximately 7 GW for the auroral UV emitted power. We found a region of enhanced thermal energy flux in the region where cusp auroras are observed.

Fine structure of aurora

Fine structure is present in most types of aurora, but much of it has previously not been possible to study properly because of instrument limitations. However, recent advances in optical instrumentation have provided considerable improvements in temporal and spatial resolution. Optical measurement systems are able to use a higher resolution than other types of ground-based instruments used in auroral studies. New results have been obtained regarding, for example, elemental structures in discrete auroras, generation of flickering aurora, generation of Alfvén waves in shear regions, dynamic rayed aurora, fine structure of diffuse auroras and fine structure of auroral curls. Outstanding questions are highlighted and recommendations for future research are given. The importance of a coordinated infrastructure for ionospheric research, simultaneous measurements on different scales, optical calibration facilities and the development of time-dependent high-resolution models is stressed.

Spectral observations of FUV auroral arcs and comparison with inverted‐V precipitating electrons

This paper presents first simultaneous observations of far ultraviolet (FUV) spectra of discrete and diffuse auroras, together with precipitating electrons measured on the same spacecraft, to emphasize the importance of high‐resolution FUV images for accurate estimation of precipitating energy flux in the auroral region. An FUV spectrograph image with ∼2 km × 3 km resolution show small‐scale features were embedded in the auroral arcs. Comparison of peak energies of inverted‐V events with the corresponding FUV spectra shows that the observed long band N2 Lyman‐Birge‐Hopfield (LBH) emission (long LBH band (LBHL): 160.0–171.5 nm) varied more sensitively to the peak energies compared to the short band. Comparison of the inverted‐V structures and their energy fluxes with the LBHL irradiance for ∼10 km × 10 km regions show they are well correlated for peak energy >2 keV. When the data are averaged over a larger area (70 km × ∼140 km), on the other hand, the LBHL irradiance becomes less bright for the corresponding electron energy flux due to the contribution from the low‐intensity background diffuse aurora produced by secondary electrons. This study demonstrates a reliable relationship between precipitating electron energy flux and LBHL intensity is obtained only if the precipitating region and FUV intensity are locally matched with a scale of less than 10 km corresponding to the size of discrete auroras.

Ground-based observations of auroras under the sunlit conditions

The results of the ground-based optical observations of sunlit auroras, performed at Lovozero and Apatity observatories on April 10 (event 1) and April 27, 2007(event 2), are presented. The observations were performed in the (OI) 557.7 nm emission, using a new equipment based on a Fabry-Perot interferometer connected to a PhotonMAX CCD camera. During event 1, the observations were performed in the Harang discontinuity region at a low magnetic disturbance. It has been indicated that an auroral arc was located in the polar part of the eastward electrojet, and the arc position coincides with the equatorward boundary of structured precipitation (b2e). During event 2, auroras were observed within the average statistical boundaries of the auroral oval and the region of structured precipitation under the conditions of rather high geomagnetic activity. However, during the period of low geomagnetic activity, discrete auroras were registered at a geomagnetic latitude of ~64° on that day, which is 3°—4° equatorward of the structured precipitation region. Such a low latitudinal position of auroras can be explained by the effect of a high solar wind velocity, which was ~580 km/s during the period of observations.

10.1007/s11478-008-2004-5

The optical observations on Heiss Island and the ion drift measurements on the DMSP F8 satellite were used to study the aurora characteristics and ionospheric convection before and after SC registered at 2330 UT on January 13, 1988. It has been indicated that two zones of luminosity can be distinguished in morning-time auroras during the quiet period before SC: the soft zone with auroral arcs and the harder diffuse auroral zone (equatorward of the first zone). After SC, a gradual smooth activation of auroras in both zones was followed (4–5 min later) by a more abrupt intensification of diffuse luminosity and by the appearance of numerous bright discrete auroras throughout the sky. In the diffuse auroral zone, the variations in the luminosity intensity with a period of 6–7 min were observed after SC. Auroral and geomagnetic field pulsations are closely correlated. During the quiet period before SC, sunward convection was concentrated in the soft precipitation region in the form of jets located in the vicinity of auroral arcs. After SC, considerable sunward convection was observed in the diffuse auroral zone. Peaks of the upward ion drift velocity were registered in the vicinity of auroral arcs.

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.

Diffuse, monoenergetic, and broadband aurora: The global precipitation budget

We have developed an auroral precipitation model which separately categorizes the discrete aurora and both the electron and ion diffuse aurora. The discrete aurora includes acceleration by two distinct physical mechanisms, namely, quasi‐static electric fields, producing monoenergetic peaks, and dispersive Alfvén waves, producing broadband electron acceleration. The new model is not merely finer in magnetic latitude (MLAT) and magnetic local time (MLT) resolution than previous models but is parameterized by solar wind driving instead of Kp and is based on functional fits to the solar wind coupling function which best predicts auroral power. Each of the four auroral types in each MLAT and MLT bin is separately fitted, a departure from the traditional compilation of a handful of discrete models, each assigned to represent a Kp (or other activity index) range. The variation of any of these four types of aurora at any local time can be predicted on the basis of the specific solar wind history of an epoch. This approach permits perhaps the first comprehensive comparison of the hemispheric contribution of each type of aurora. It turns out that the diffuse aurora is surprisingly dominant, constituting 84% of the energy flux into the ionosphere during conditions of low solar wind driving (63% electrons, 21% ions). The diffuse aurora is far from quiescent, tripling in power dissipation from our low to high solar wind–driving conditions. Even under the latter condition, the diffuse aurora contains 71% of the hemispheric energy flux (57% electrons, 14% ions). The monoenergetic aurora contributes more energy flux (10% quiet, 15% active) than does broadband acceleration signatures (6% quiet, 13% active). However, the broadband aurora rises fastest with activity, increasing by a factor of 8.0 from low to high driving. Moreover, this most dynamic auroral type contributes very high number fluxes, even exceeding monoenergetic aurora under active conditions (28% of hemispheric precipitation versus 21%). Thus, dynamic ionospheric heating and ion outflow is likely heavily affected by the wave aurora. Although energy flux peaks on the nightside, number flux peaks on the dayside. The cusp, as previously reported, is much better defined by ions than electrons. Hence, the ion number flux peak is confined, corresponding to the cusp, while the region with high electron number flux is broad (a cleft, corresponding to the boundary layers, including the closed low‐latitude boundary layer).

Correction to “Wave dispersion and the discrete aurora: New constraints derived from high-speed imagery”

Correction to “Wave dispersion and the discrete aurora: New constraints derived from high-speed imagery”

First direct optical observations of plasma flows using afterglow of [formula omitted] in discrete aurora

Imaging of active structured aurora in the forbidden O + ion line at 732.0 nm provides a possibility of direct observation of plasma drifts in the topside ionosphere. The metastable O + 2P state has a radiative lifetime of 5 s, so the oxygen ions can be detected after the precipitation creating them has ceased. The decay time of the O + emission is studied and modelled with a time-dependent electron transport and ion chemistry model. Four examples are given of O + afterglow observed with the multi-spectral imager, auroral structure and kinetics (ASK), which was located near Tromsø, Norway, in 2006. Estimates are given of drift velocities resulting from the analysis of the afterglow motions. Bulk plasma velocities of 340 and 720 m/s directed eastwards were found for two afterglowing arc filaments, corresponding to southward electric fields of 18 and 40 mV/m, respectively.

Wave dispersion and the discrete aurora: New constraints derived from high‐speed imagery

An analysis of multiscale observations of a substorm auroral breakup are presented which clarify the role of wave dispersion in the formation of elemental (<100 m) auroral structure. At coarse resolution (all‐sky white light camera, 1 frame/s), observations fit the established substorm morphology—namely, arc brightening, formation of spatial distortions, and breakup into multiple “rayed” structures. At fine‐scale resolution (electron multiplying charge‐coupled device [EMCCD] camera, 9‐degree field of view, prompt emission filter, 30 frames/s), an entirely different type of coherence is observed. The “arc,” as identified at lower resolution, is observed to be a dynamic structure composed of bifurcating elemental arcs that propagate outward from the center of an “arc packet.” This dynamic process is well captured in time‐brightness histories (keograms) along a cut bisecting the structure. The observations are interpreted with respect to theoretical predictions for inertial Alfvén wave dispersion. Specifically, the arc packets are interpreted as the B⊥ projection of the parallel electric field within the Alfvén resonant cone. Prebreakup observations are found to be qualitatively consistent with this model. However, some difficulties are encountered for the more active postbreakup period. The article includes a discussion of perspective considerations in interpreting small‐scale auroral features; in particular, it is shown that the “rayed” appearance of discrete breakup aurora is, in fact, a consequence of sharply kinked sheets viewed obliquely.

Lightning‐induced lower‐hybrid turbulence and trapped Extremely Low Frequency (ELF) electromagnetic waves observed in deep equatorial plasma density depletions during intense magnetic storms

During the early phase of the intense magnetic storm of 7–11 November 2004, the DEMETER satellite encountered large‐scale equatorial plasma density depletions with density decreases of two or three orders of magnitude. Wave measurements carried out inside these depletions show the occurrence of broadband and localized lower‐hybrid turbulence triggered by whistlers propagating from thunderstorm lightning occurring below the orbit path. High‐sample‐rate waveforms reveal that this lower‐hybrid turbulence can evolve into localized large‐amplitude quasi‐monochromatic wave packets similar to lower‐hybrid structures that were, up to now, only observed in the auroral regions, usually on high‐latitude magnetic field lines associated with discrete aurora. These equatorial structures have typical amplitudes of up to 20 mV/m and durations of ∼20–30 ms. Simultaneous thermal ion measurements show that these bursts are often correlated with small‐scale density depletions of 5–10%. Although the lower‐hybrid structures are less intense than those observed in the auroral zone and although their energy source is different, our observations lend support to the idea that the formation of lower‐hybrid structures is an universal mechanism operating in inhomogeneous magnetized space plasmas in the presence of VLF whistler mode turbulence. Besides the lower‐hybrid turbulence, another interesting feature is the occurrence of strong narrowband electromagnetic ELF emissions with amplitudes of a few millivolts per meter at frequencies below the proton gyrofrequency. They are continuously observed throughout the entire depletion. These emissions occur not only within the depletions but also, although less intense, outside of them over a large latitudinal range. They are tentatively identified as magnetospheric line radiations (MLRs) commonly observed during magnetically disturbed periods. Similar events were observed on 15 May 2005 and on 24 August 2005 during two other intense magnetic storms.

Response of the Geomagnetic Activity Indices to the Solar Wind Parameters

This study attempts to show how the geomagnetic indices, AU, AL and Dst, respond to the interplanetary parameters, more specifically, the solar wind electric field VBz during southward interplanetary magnetic field (IMF) period. The AU index does not seem to respond linearly to the variation of southward IMF. Only a noticeable correlation between the AU and VBz is shown during summer, when the ionospheric conductivity associated with the solar EUV radiation is high. It is highly likely that the effect of electric field on the eastward electrojet intensification is only noticeable whenever the ionospheric conductivity is significantly enhanced during summer. Thus, one should be very cautious in employing the AU as a convection index during other seasons. The AL index shows a significantly high correlation with VBz regardless of season. Considering that the auroral electrojet is the combined result of electric field and ionospheric conductivity, the intensification of these two quantities seems to occur concurrently during southward IMF period. This suggests that the AL index behaves more like a convection index rather than a substorm index as far as hourly mean AL index is concerned. Contrary to the AU index, the AL index does not register the maximum value during summer for a given level of VBz. It has something to do with the findings that discrete auroras are suppressed in sunlight hemisphere (Newell et al. 1996), thus reducing the ionospheric conductivity during summer. As expected, the Dst index tends to become more negative as VBz gets intensified. However, the Dst index (nT) is less than or equal to 15VBz(mV/m) + 50(Bz < 0). It indicates that VBz determines the lower limit of the storm size, while another factor(s), possibly substorm, seems to get further involved in intensifying storms. Although it has not been examined in this study, the duration of southward IMF would also be a factor to be considered in determining the size of a storm.

Variations in the geomagnetic field and auroras during the main phase of a large magnetic storm of November 20, 2003

The substorm characteristics during the main phase of a large magnetic storm of November 20, 2003, are studied based on the data of TV observations of auroras and auroral absorption at Tixie Bay station and at the global network of magnetic stations. The contribution of auroral particles, responsible for the emission of discrete auroras, has been estimated based on an analysis of the spatial-time variations in the auroral luminosity intensity. This contribution accounted for ∼40% of the total luminous flux, which is approximately twice as large as was previously observed in substorm disturbances. Responses of the solar wind and IMF parameters in substorms and variations in the magnetic indices, characterizing geomagnetic activity in the northern polar cap and ring current (PCN, ASY-H and SYM-H), have been detected. The spatial-time distribution of the equivalent ionospheric currents has been constructed, and the total value of these currents along the meridian has been determined based on the [Popov et al., 2001] method and using the IMAGE magnetic data. It has been obtained that the maximal total equivalent ionospheric current in the premidnight sector (∼2000 MLT) leads the minimal value of the SYM-H index by ∼1.5 h.

Features of morning-time auroras during SC

The optical observations on Heiss Island and the ion drift measurements on the DMSP F8 satellite were used to study the aurora characteristics and ionospheric convection before and after SC registered at 2330 UT on January 13, 1988. It has been indicated that two zones of luminosity can be distinguished in morning-time auroras during the quiet period before SC: the soft zone with auroral arcs and the harder diffuse auroral zone (equatorward of the first zone). After SC, a gradual smooth activation of auroras in both zones was followed (4–5 min later) by a more abrupt intensification of diffuse luminosity and by the appearance of numerous bright discrete auroras throughout the sky. In the diffuse auroral zone, the variations in the luminosity intensity with a period of 6–7 min were observed after SC. Auroral and geomagnetic field pulsations are closely correlated. During the quiet period before SC, sunward convection was concentrated in the soft precipitation region in the form of jets located in the vicinity of auroral arcs. After SC, considerable sunward convection was observed in the diffuse auroral zone. Peaks of the upward ion drift velocity were registered in the vicinity of auroral arcs.

Observations of structured optical emissions and particle precipitation equatorward of the traditional auroral oval

High‐sensitivity all‐sky images from south‐central Alaska show the common occurrence of slow‐moving, faint optical emissions at 557.7 and 427.8 nm well equatorward of the discrete aurora. These emissions appear over a continuum of forms ranging from homogenous longitudinal bands to bands with irregular structure on their poleward edge to widely distributed arrays of vortex‐like curls and widely spaced spots on the order of 10 km in diameter. These forms appear to correspond to various stages in the temporal evolution of nearly corotating precipitation regions populated by particles injected from more distant areas of the magnetosphere and may exhibit morphological control by an instability operating on cold plasma near the plasmapause. Although these phenomena are most common in the evening hours and typically persist for hours at a time, one case demonstrates that the features can remain throughout the night on occasion. These faint optical features appear to be colocated with regions of enhanced background counts in DMSP particle measurements and to be associated with the ring current/outer radiation belt region equatorward of subauroral polarization streams (SAPS). The features can occur in conjunction with HF and MF radio frequency absorption, at levels which are often too low to show up on routine absorption instruments such as the riometer. This has practical implications on the operation and interpretation of ionospheric interaction experiments carried out in the subauroral region.

Magnetospheric convection at Saturn as a function of IMF BZ

We have used three dimensional magnetohydrodynamic simulations to investigate the influence of the interplanetary magnetic field (IMF) on Saturn's magnetosphere for cases with no IMF, northward and southward IMF. The subsolar magnetopause and bow shock positions are sensitive to changes in the solar wind dynamic pressure but insensitive to changes in the IMF. Without an IMF vortices were generated near dawn where the corotating flows and solar wind are opposite. For BZ &lt; 0 the vorticity results from the interaction of flows driven by cusp reconnection and corotation. For BZ &gt; 0 vortices were generated in the morning and evening where the flows from tail reconnection are opposite to the solar wind and corotation. We used the energy flux to the ionosphere and upward field‐aligned currents (FACs) as proxies for diffuse and discrete aurora. Strong upward FACs extend to the morning sector when BZ &gt; 0. The strongest FACs are generated in the flow vortices.

Auroral Plasma Acceleration Above Martian Magnetic Anomalies

Aurora is caused by the precipitation of energetic particles into a planetary atmosphere, the light intensity being roughly proportional to the precipitating particle energy flux. From auroral research in the terrestrial magnetosphere it is known that bright auroral displays, discrete aurora, result from an enhanced energy deposition caused by downward accelerated electrons. The process is commonly referred to as the auroral acceleration process. Discrete aurora is the visual manifestation of the structuring inherent in a highly magnetized plasma. A strong magnetic field limits the transverse (to the magnetic field) mobility of charged particles, effectively guiding the particle energy flux along magnetic field lines.The typical, slanted arc structure of the Earth’s discrete aurora not only visualizes the inclination of the Earth’s magnetic field, but also illustrates the confinement of the auroral acceleration process. The terrestrial magnetic field guides and confines the acceleration processes such that the preferred acceleration of particles is frequently along the magnetic field lines. Field-aligned plasma acceleration is therefore also the signature of strongly magnetized plasma.This paper discusses plasma acceleration characteristics in the night-side cavity of Mars. The acceleration is typical for strongly magnetized plasmas – field-aligned acceleration of ions and electrons. The observations map to regions at Mars of what appears to be sufficient magnetization to support magnetic field-aligned plasma acceleration – the localized crustal magnetizations at Mars (Acuña et al., 1999). Our findings are based on data from the ASPERA-3 experiment on ESA’s Mars Express, covering 57 orbits traversing the night-side/eclipse of Mars. There are indeed strong similarities between Mars and the Earth regarding the accelerated electron and ion distributions. Specifically acceleration above Mars near local midnight and acceleration above discrete aurora at the Earth – characterized by nearly monoenergetic downgoing electrons in conjunction with nearly monoenergetic upgoing ions. We describe a number of characteristic features in the accelerated plasma: The “inverted V” energy-time distribution, beam vs temperature distribution, altitude distribution, local time distribution and connection with magnetic anomalies. We also compute the electron energy flux and find that the energy flux is sufficient to cause weak to medium strong (up to several tens of kR 557.7 nm emissions) aurora at Mars.Monoenergetic counterstreaming accelerated ions and electrons is the signature of field-aligned electric currents and electric field acceleration. The topic is reasonably well understood in terrestrial magnetospheric physics, although some controversy still remains on details and the cause-effect relationships. We present a potential cause-effect relationship leading to auroral plasma acceleration in the nightside cavity of Mars – the downward acceleration of electrons supposedly manifesting itself as discrete aurora above Mars.

Auroral hot-ion dynamo model with finite gyroradii

Discrete auroras have (1) narrow size s≲30km in at least one dimension (e.g., north-south) and (2) often rapid variation of forms, especially where the size is extremely small, s⩽1km. These points mesh with spatial and temporal features observed at several Earth radii in earthward flows (bursts) of hot plasma along high-latitude geomagnetic field lines. The flows (include PSBL) usually have some filamentary structure with transverse widths of a few local gyroradii of the hot protons (kT∼1–30keV), i.e., widths that encompass auroral-arc size when scaled by magnetic field-line separation. At these widths, modest density gradients (δn∼0.01–0.1cm−3) lead to charge separation by differential mirroring of hot protons and electrons and large perpendicular electric fields. Thermal escape of ionspheric electrons into positive charge layer builds up magnetic field-aligned potential difference that accelerates hot electrons from negative charge layer into the ionosphere within auroral arc thickness. As a corollary, the model delineates a mechanism for charge-driven plasma instabilities.

Plasma Acceleration Above Martian Magnetic Anomalies

Auroras are caused by accelerated charged particles precipitating along magnetic field lines into a planetary atmosphere, the auroral brightness being roughly proportional to the precipitating particle energy flux. The Analyzer of Space Plasma and Energetic Atoms experiment on the Mars Express spacecraft has made a detailed study of acceleration processes on the nightside of Mars. We observed accelerated electrons and ions in the deep nightside high-altitude region of Mars that map geographically to interface/cleft regions associated with martian crustal magnetization regions. By integrating electron and ion acceleration energy down to the upper atmosphere, we saw energy fluxes in the range of 1 to 50 milliwatts per square meter per second. These conditions are similar to those producing bright discrete auroras above Earth. Discrete auroras at Mars are therefore expected to be associated with plasma acceleration in diverging magnetic flux tubes above crustal magnetization regions, the auroras being distributed geographically in a complex pattern by the many multipole magnetic field lines extending into space.