Auroral Optical Emissions Research Articles

A SURVEY FOR Hα EMISSION FROM LATE L DWARFS AND T DWARFS*

ABSTRACT Recently, studies of brown dwarfs have demonstrated that they possess strong magnetic fields and have the potential to produce radio and optical auroral emissions powered by magnetospheric currents. This emission provides the only window on magnetic fields in the coolest brown dwarfs and identifying additional benchmark objects is key to constraining dynamo theory in this regime. To this end, we conducted a new red optical (6300–9700 Å) survey with the Keck telescopes looking for Hα emission from a sample of late L dwarfs and T dwarfs. Our survey gathered optical spectra for 29 targets, 18 of which did not have previous optical spectra in the literature, greatly expanding the number of moderate-resolution (R ∼ 2000) spectra available at these spectral types. Combining our sample with previous surveys, we confirm an Hα detection rate of 9.2± % for L and T dwarfs in the optical spectral range of L4–T8. This detection rate is consistent with the recently measured detection rate for auroral radio emission from Kao et al., suggesting that geometrical selection effects due to the beaming of the radio emission are small or absent. We also provide the first detection of Hα emission from 2MASS 0036+1821, previously notable as the only electron cyclotron maser radio source without a confirmed detection of Hα emission. Finally, we also establish optical standards for spectral types T3 and T4, filling in the previous gap between T2 and T5.

Auroral ion acoustic wave enhancement observed with a radar interferometer system

Abstract. Measurements of naturally enhanced ion acoustic line (NEIAL) echoes obtained with a five-antenna interferometric imaging radar system are presented. The observations were conducted with the European Incoherent SCATter (EISCAT) radar on Svalbard and the EISCAT Aperture Synthesis Imaging receivers (EASI) installed at the radar site. Four baselines of the interferometer are used in the analysis. Based on the coherence estimates derived from the measurements, we show that the enhanced backscattering region is of limited extent in the plane perpendicular to the geomagnetic field. Previously it has been argued that the enhanced backscatter region is limited in size; however, here the first unambiguous observations are presented. The size of the enhanced backscatter region is determined to be less than 900 × 500 m, and at times less than 160 m in the direction of the longest antenna separation, assuming the scattering region to have a Gaussian scattering cross section in the plane perpendicular to the geomagnetic field. Using aperture synthesis imaging methods volumetric images of the NEIAL echo are obtained showing the enhanced backscattering region to be aligned with the geomagnetic field. Although optical auroral emissions are observed outside the radar look direction, our observations are consistent with the NEIAL echo occurring on field lines with particle precipitation.

Magnetospherically driven optical and radio aurorae at the end of the stellar main sequence.

Aurorae are detected from all the magnetized planets in our Solar System, including Earth. They are powered by magnetospheric current systems that lead to the precipitation of energetic electrons into the high-latitude regions of the upper atmosphere. In the case of the gas-giant planets, these aurorae include highly polarized radio emission at kilohertz and megahertz frequencies produced by the precipitating electrons, as well as continuum and line emission in the infrared, optical, ultraviolet and X-ray parts of the spectrum, associated with the collisional excitation and heating of the hydrogen-dominated atmosphere. Here we report simultaneous radio and optical spectroscopic observations of an object at the end of the stellar main sequence, located right at the boundary between stars and brown dwarfs, from which we have detected radio and optical auroral emissions both powered by magnetospheric currents. Whereas the magnetic activity of stars like our Sun is powered by processes that occur in their lower atmospheres, these aurorae are powered by processes originating much further out in the magnetosphere of the dwarf star that couple energy into the lower atmosphere. The dissipated power is at least four orders of magnitude larger than what is produced in the Jovian magnetosphere, revealing aurorae to be a potentially ubiquitous signature of large-scale magnetospheres that can scale to luminosities far greater than those observed in our Solar System. These magnetospheric current systems may also play a part in powering some of the weather phenomena reported on brown dwarfs.

Automated determination of auroral breakup during the substorm expansion phase using all‐sky imager data

AbstractThis paper describes a novel method for quantitatively and routinely identifying auroral breakup following substorm onset using the Time History of Events and Macroscale Interactions during Substorms all‐sky imagers. Substorm onset is characterized by a brightening of the aurora that is followed by auroral poleward expansion and auroral breakup. This breakup can be identified by a sharp increase in the auroral intensity i(t) and the time derivative of auroral intensity i(t). Utilizing both i(t) and i'(t), we have developed an algorithm for identifying the time interval and spatial location of auroral breakup during the substorm expansion phase based solely on quantifiable characteristics of the optical auroral emissions. We compare the time interval determined by the algorithm to independently identified auroral onset times from three previously published studies. In each case the time interval determined by the algorithm is within error of the onset independently identified by the prior studies. We further show the utility of the algorithm by comparing the breakup intervals determined using the automated algorithm to an independent list of substorm onset times. We demonstrate that 50% of the breakup intervals characterized by the algorithm are within the uncertainty of the times identified in the list. The quantitative description and routine identification of an interval of auroral brightening during the substorm expansion phase provides a foundation for unbiased statistical analysis of the aurora and to probe the physics of the auroral substorm and identify the processes leading to auroral substorm onset.

GPS phase scintillation associated with optical auroral emissions: First statistical results from the geographic South Pole

Ionospheric irregularities affect the propagation of Global Navigation Satellite System (GNSS) signals, causing radio scintillation. Particle precipitation from the magnetosphere into the ionosphere, following solar activity, is an important production mechanism for ionospheric irregularities. Particle precipitation also causes the aurorae. However, the correlation of aurorae and GNSS scintillation events is not well established in literature. This study examines optical auroral events during 2010–2011 and reports spatial and temporal correlations with Global Positioning System (GPS) L1 phase fluctuations using instrumentation located at South Pole Station. An all‐sky imager provides a measure of optical emission intensities ([OI] 557.7 nm and 630.0 nm) at auroral latitudes during the winter months. A collocated GPS antenna and scintillation receiver facilitates superimposition of auroral images and GPS signal measurements. Correlation statistics are produced by tracking emission intensities and GPS L1 σφ indices at E and F‐region heights. This is the first time that multi‐wavelength auroral images have been compared with scintillation measurements in this way. Correlation levels of up to 74% are observed during 2–3 hour periods of discrete arc structuring. Analysis revealed that higher values of emission intensity corresponded with elevated levels of σφ. The study has yielded the first statistical evidence supporting the previously assumed relationship between the aurorae and GPS signal propagation. The probability of scintillation‐induced GPS outages is of interest for commercial and safety‐critical operations at high latitudes. Results in this paper indicate that image databases of optical auroral emissions could be used to assess the likelihood of multiple satellite scintillation activity.

Mesoscale observations of Joule heating near an auroral arc and ion-neutral collision frequency in the polar capEregion

[1] We report on the first mesoscale combined ionospheric and thermospheric observations, partly in the vicinity of an auroral arc, from Svalbard in the polar cap on 2 February 2010. The EISCAT Svalbard radar employed a novel scanning mode in order to obtain F and E region ion flows over an annular region centered on the radar. Simultaneously, a colocated Scanning Doppler Imager observed the E region neutral winds and temperatures around 110 km altitude using the 557.7 nm auroral optical emission. Combining the ion and neutral data permits the E region Joule heating to be estimated with an azimuthal spatial resolution of ∼64 km at a radius of ∼163 km from the radar. The spatial distribution of Joule heating shows significant mesoscale variation. The ion-neutral collision frequency is measured in the E region by combining all the data over the entire field of view with only weak aurora present. The estimated ion-neutral collision frequency at ∼113 km altitude is in good agreement with the MSIS atmospheric model.

Comparison of methods to determine auroral ionospheric conductances using ground-based optical and riometer data

Abstract. Ground-based images of auroral optical emissions and cosmic radio noise absorption provide information on particle precipitation which enhances ionospheric conductances. Knowledge of this conductance field is important to understand the current systems associated with auroral features. Three methods of using ground-based optical and riometer data to estimate ionospheric conductances in the aurora are compared to conductances derived from incoherent scatter radar measurements. It is shown that a method using the 557.7 nm emission intensity alone gives the best results for the Pedersen conductance whilst a method using both this intensity and cosmic noise absorption is best for the Hall conductance. A method using cosmic noise absorption alone gives reasonable performance for the Hall conductance and the Hall/Pedersen conductance ratio, but performs poorly for the Pedersen conductance. It also appears to underestimate the Hall conductance significantly during times when softer precipitation is present, for example in discrete auroral arcs. There is some indication that the methods do not degrade noticeably for angles up to ~20° off magnetic zenith.

On the importance of auroral processes in the Universe

Acceleration of charged particles in magnetic field-aligned electric potential differences at Earth and at the outer planets in the solar system is summarized and its general importance in the Universe is briefly discussed. The role of field-aligned currents, driven by parallel electric fields, in causing filamentary structure in stellar atmospheres is briefly reviewed. The differences between auroral optical emissions at various planets are summarized. The important role of field-aligned potential differences in the generation of AKR and corresponding emissions from other objects is discussed. Finally, aurora-associated processes for ejection of planetary plasma into space are briefly reviewed.

Simulations of Alfvén waves in the geomagnetic tail and their auroral signatures

Observations show that Ultra‐Low Frequency (ULF) Alfvén waves in the magnetotail have distinctive properties depending upon their location. In particular, those in the plasma sheet boundary layer (PSBL) have a larger amplitude and favor earthward propagation compared to those in the tail lobe which have the polarization of standing waves. The PSBL waves are also associated with electron acceleration and optical auroral emissions that exhibit equatorward motion. In this paper we present simulations of MHD wave coupling in the magnetotail to support an explanation for how Alfvén waves with these properties may be established. The simulations also suggest the waves should have periods from 5 min to >20 min, and produce auroral emissions in the ionosphere having a latitudinal range of 40–130 km and equatorward speed of order 1 kms–1. Field aligned currents are typically a few μAm−2.

Multi-site observations of the association between aurora and plasma convection in the cusp/polar cap during a southeastward(<i>B<sub>y</sub></i> <u>~</u> |<i>B<sub>z</sub></i>|) IMF orientation

Abstract. In a case study we demonstrate the spatiotemporal structure of aurora and plasma convection in the cusp/polar cap when the interplanetary magnetic field (IMF) Bz < 0 and By ~ | Bz | (clock angle in GSM Y - Z plane: ~ 135°). This IMF orientation elicited a response different from that corresponding to strongly northward and southward IMF. Our study of this "intermediate state" is based on a combination of ground observations of optical auroral emissions and ionospheric plasma convection. Utilizing all-sky cameras at NyAlesund, Svalbard and Heiss Island (Russian arctic), we are able to monitor the high-latitude auroral activity within the ~10:00–15:00 MLT sector. Information on plasma convection is obtained from the SuperDARN radars, with emphasis placed on line of sight observations from the radar situated in Hankasalmi, Finland (Cutlass). A central feature of the auroral observations in the cusp/polar cap region is a ~ 30-min long sequence of four brightening events, some of which consists of latitudinally and longitudinally separated forms, which are found to be associated with pulsed ionospheric flows in merging and lobe convection cells. The auroral/convection events may be separated into different forms/cells and phases, reflecting a spatiotem-poral evolution of the reconnection process on the dayside magnetopause. The initial phase consists of a brightening in the postnoon sector (~ 12:00–14:00 MLT) at ~ 73° MLAT, accompanied by a pulse of enhanced westward convection in the postnoon merging cell. Thereafter, the event evolution comprises two phenomena which occur almost simultaneously: (1) westward expansion of the auroral brightening (equatorward boundary intensification) across noon, into the ~ 10:00–12:00 MLT sector, where the plasma convection subsequently turns almost due north, in the convection throat, and where classical poleward moving auroral forms (PMAFs) are observed; and (2) auroral brightening at slightly higher latitudes (~ 75° MLAT) in the postnoon lobe cell, with expansion towards noon, giving rise to a clear cusp bifurcation. The fading phase of PMAFs is accompanied by a "patch" of enhanced (~ 1 km/s) poleward-directed merging cell convection at high latitudes (75–82° MLAT), e.g. more than 500 km poleward of the cusp equatorward boundary. The major aurora/convection events are recurring at ~ 5–10 min intervals. Key words. Magnetospheric physics (auroral phenomena; magnetopause, cusp, and boundary layers; plasma convection)

Imaging riometer observations on energetic electron precipitation at SANAE IV, Antarctica

The expansion phase of a substorm on the evening of 1 May 1997 was studied from recordings of all‐sky optical aurora and a 64‐beam imaging riometer at the Antarctic base SANAE IV (L = 4.1). Digitized all‐sky, low‐level white light images of auroral optical emissions were mapped onto the angular directivity functions of the 64‐beam imaging riometer. During the period of investigation, several optical arcs appeared and disappeared, while a persistent absorption structure appeared in the south (poleward). This and new structures changed only slowly in time. The differences in morphologies of optical emissions and absorption regions, together with temporal differences, imply that there are two categories of energetic auroral electrons: the softer electrons (<10 keV) causing optical emissions in the E and F regions of the ionosphere and the harder (>10 keV) electrons ionizing down into the D region for cosmic radio noise absorption (CRNA). Optical emissions tend to lead the appearance of cosmic radio noise absorptions by 0–60 s in a specific region of ionospheric space. Furthermore, in the all‐sky optical data, westward traveling surges (WTS) were observed coincident with a CRNA WTS and leading the CRNA WTS by ∼30–50 s. In the CRNA data the WTS was ribbon‐like, traveling westward at a speed of 2.3 km s−1. These observations suggest that the softer electrons, causing E and F region optical emissions, may be injected onto drift paths closer to Earth than the more energetic electrons, ionizing down into the D region for cosmic radio noise absorption. The harder electrons could have been accelerated by, for instance, an increasing dawn‐to‐dusk electric field for later precipitation.

Magnetotail waveguide: Fast and Alfvén waves in the plasma sheet boundary layer and lobe

Numerical simulations of MHD wave propagation and coupling in a realistic magnetotail are presented. Fast mode waves are observed to disperse and couple resonantly to Alfvén waves over a broad layer rather than on an isolated field line. Indeed, the layer is likely to be so broad as to include the entire tail lobe as well as the plasma sheet boundary layer (PSBL). It appears that small ky modes (where ky is the cross‐tail wave number) will provide the most efficient coupling as they will tend to propagate along the magnetotail field lines rather than across them and out of the tail boundaries. (Moreover, it is only small ky fast modes that will be able to penetrate the lobe.) Alfvén waves in the PSBL are shown to phase mix rapidly resulting in strong field‐aligned currents with an equatorward phase motion. These properties are in agreement with observations of optical auroral emissions. The lobe Alfvén waves do not phase mix, and are not expected to excite optical emissions. They may, however, provide a significant ponderomotive force and could account for the transport of oxygen ions from the ionosphere into the distant tail lobes.

Variations, with peak emission altitude, in auroral O2 atmospheric (1, 1)/(0, 1) ratio and its relation to other auroral emissions

Spectral distributions of auroral optical emissions, peaked at distinctly different heights in the thermosphere, show significant variations, with altitude, in the O2 atmospheric (1, 1)/(0, 1) band ratio. The latter increases with height in auroras peaked between 110 and 150 km and then gradually decreases at higher altitudes. To minimize ambiguities associated with auroral height determination needed for investigating this effect, four independent height‐assessment methods are employed. The first one is based on the incoherent scatter radar (ISR) soundings of the auroral ionization profile from which the height, where precipitating particles dissipate most of their energy, can be determined. Concurrent spectroscopic observations of the thermalized rotational distributions of auroral band emissions yield the ambient air temperature, and hence an independent assessment of the height, of the thermospheric region where these emissions peak. Changes in O/O2 and O/N2 ratios with height lead to changes in the ratios of auroral emissions, from these species, peaked at different heights. Finally, changes in collision frequency with height lead to changes in the brightness of the auroral emissions, resulting from radiatively allowed transitions relative to those produced from radiatively forbidden transitions. The four methods yield comparable values for the height of the thermospheric region where emissions, from each auroral event, peak. The observed variations in O2 atmospheric (1, 1)/(0, 1) with auroral height is compared with that expected from O (1D) + O2 excitation source and quenching by O2 and O. The effects of electron impact excitation of O2(b1∑g+, v′) and high rotational levels of the P branch of O2 atmospheric (0, 0) band on O2 atmospheric (1, 1)/(0, 1) ratio are discussed. Quantitative ratios of various auroral emissions, from O, N2, and N+2, peaked at different heights, that can provide an assessment of auroral heights where these emissions peak, are listed.

Ionospheric effects of magnetopause reconnection observed using ionospheric tomography

This paper presents key signatures in the spatial distribution of ionospheric electron density attributed to solar wind particles injected following magnetopause reconnection. Using established criteria, optical auroral emissions have been used to identify the reconnection process. A simultaneous image of electron density shows an enhanced region with a northward gradient in F‐layer peak height that may be linked to dispersion of precipitating soft ions. In addition, an E‐region enhancement at lower latitude can be associated with precipitation of pitch‐angle scattered ring current electrons on closed field lines. A narrow density enhancement extending into the high topside ionosphere may indicate the signature of a field‐aligned current. Taken together, the features enable identification in the ionosphere of the open/closed field‐line boundary.

Auroral optical emissions during the solar magnetic cloud event of October 1995

The solar magnetic cloud of October 18, 1995, swept by the Earth's magnetosphere, starting at about 2200 UT. This event triggered prolonged auroral displays in the terrestrial atmosphere. The optical emissions from these auroras were recorded with a CCD spectrograph, operating at the radar site in Sondrestromfjord, Greenland. Continuous spectroscopic measurements were made for about 10 hours, from 2200 UT on October 18, 1995, until local dawn set in at 0800 UT on October 19, 1995. These measurements show markedly different spectral characteristics from those of normal auroras observed at other times in various sectors of the auroral oval and in the polar cap region. The differences are most pronounced in the relative brightnesses of the ion and neutral line emissions from the thermospheric oxygen atoms as well as in the O2 At band emissions. Other differences between the normal auroras and the magnetic cloud induced auroras relate to the temperature of the thermospheric height regions where the N2 1PG and the N2+ M band emissions peak in these auroras. Least squares synthetic profiles fit to the rotational distributions of the N2 1PG and the N2+ M bands observed during the solar magnetic cloud event, coupled with auroral model calculations constrained to reproduce the measured emission brightnesses, indicate a higher than normal height for the emissions from the auroras associated with this event. Further evidence for the higher emission altitude is derived from the detailed comparison of the spectral distribution observed during this period with various other measurements of the relatively high‐altitude auroral emissions. All these observations point to the precipitation in the polar terrestrial atmosphere of electrons with an average energy of about 500eV ± 100eV during the interaction of the solar magnetic cloud with the near‐Earth space environment. Such an auroral electron precipitation event is unusual and occurs infrequently; it is similar to the rare type A global red aurora.

Cosmic radio noise absorption related to structures in auroral luminosity

Digitized all‐sky white light images of auroral optical emissions, recorded by a low‐light level TV system at Sanae (70.3°S, 2.4°W, L = 4.0), have been mapped onto the angular sensitivity functions of the inner four beams of a 16‐element imaging riometer. Cosmic radio waves of 5–10 m wavelengths appeared to be absorbed more strongly in directions through regions adjacent to discrete auroral arcs than through the arc regions themselves. These stronger absorptions may be due either to enhanced electron temperatures, caused by the Farley‐Buneman two‐stream instability in the presence of large electric fields around auroral arcs in the E region, or to D region absorption caused by energetic electrons precipitating along magnetic field lines passing through regions adjacent to auroral arcs. The two auroral events before local magnetic midnight reported in this paper started with high luminosity and small ionospheric absorption. The absorption increased as the event developed. Auroral luminosities and structures were changing in all four viewing directions during the event on May 10, 1992, with variations in absorptions following variations in luminosity after a delay of 30 s. The event on June 8, 1992, involved a pulsating arc structure. Changes in absorption appeared to be delayed relative to changes in luminosity that varied from 0 to 60 s in a particular viewing direction. The auroral event after local magnetic midnight on April 14, 1993, differed from the former two events in the appearance of pulsating auroral patches and in slower temporal variations, but, occasionally, changes in absorption of cosmic radio noise in the four beam directions were still delayed relative to changes in luminosity. The observed spatial and temporal differences in regions of optical emissions and ionospheric absorption should be of magnetospheric origin rather than ionospheric origin on account of the long delay times.

Coordinated measurements of auroral light intensities and Riometric radio‐wave absorption

Digitized allsky images of auroral optical emissions, recorded by a low light level TV system at Sanae (70.3°S, 2.4°W, L=4.0), have been mapped onto the angular sensitivity functions of both a broad, single‐beam riometer and narrow beams of an imaging riometer. During the rising phase of a substorm before local magnetic midnight, on 11 May, 1992, the integrated optical intensities in the inner four beam directions of the 16‐beam imaging riometer followed each other fairly closely, while the absorptions showed marked spatially inhomogeneous distributions. Regions of ionospheric absorption appeared not to coincide spatially with regions of discrete optical structures, but were adjacent to these structures. A small step of ∼0.09 dB in absorption was observed shortly after onset of the substorm on 27 July, 1979, and again on 11 May, 1992, but before the sharp increase in absorption. This step in absorption may be due to highly enhanced E‐region electron temperatures arising from strong dc electric fields connected to auroral arcs. During the event of 9 June, 1991, which started with an SSC at 00:40 UT, enhancements in absorptions followed enhancements in optical emissions repeatedly after 52 s.

Detection of very soft X-ray spectra by balloon-borne detectors

A balloon payload launched from Cold Lake, Alta., Canada, on October 14, 1982, measured the spectrum of X rays (E = 20–160 keV) with a hard and a very soft component. The latter contributes significantly only to the 20–40 keV channel; thus a "flapping motion" at the lower energy end of the composite spectrum is observed. We have simulated, using the Monte-Carlo technique, the propagation of X rays through the atmosphere and determined the e-folding energy of the very soft component, which we estimate to be 5 keV, at a particular time (10:45 UT). The electron spectrum producing this soft X-ray source agrees well with in situ satellite and rocket-borne measurements of electron spectra during active auroras. Simulataneous auroral optical emissions (5577 Å (1 Å = 10−10 m)) by a ground-based all-sky charge-coupled device camera are also available for comparison. We also determine the count rate ratio between 20–40 and 60–80 keV channels as a parameter of the variability of the intensity of the very soft component. The time-changes of this ratio and that of the auroral optical emission intensity near the vicinity of the balloon are correlated. The lack of correlation at some other times is ascribed to differences in the behavior of the low- and high-energy ends of the elecron spectrum.

Whistler duct dimensions from TV auroral images

Whistler mode VLF radiation propagates between opposite hemispheres of the earth in field aligned ducts of enhanced plasma density. Despite the fact that whistler theory is firmly based on the existence of such ducts,there is little knowledge of their cross-sectional size or shape. Mid-latitude ( L=4) ground based observations from Sanae, Antarctica, of ionospheric auroral optical emissions (imaged on a low-light level TV system) are observed in association with VLF radio emissions (recorded on a broadband VLF receiver). These optical emissions are Interpreted as arising from electrons which Interact with ducted VLF radiation via the electron cyclotron interaction. These observations provide estimates of 100 km by 400km for whistler duct cross-sections in the equatorial plane.

Aurora on Triton?

If the planetary magnetic field of Neptune lies within the range of values that have been predicted for it (or even if the field is considerably smaller), Voyager 2 will find Triton immersed within the plasmasphere of a rotationally dominated magnetosphere. We expect the ionosphere of Triton to be highly conducting, which, coupled with Triton's 40 km/sec velocity relative to the magnetospheric plasma, leads to a limiting current that produces an induced magnetosphere, as occurs with solar‐wind flow past Venus or Mars. The induced magnetosphere, analogous to that observed near Saturn's satellite Titan, serves to deviate the flowing plasma and reduce the potential across Triton. The limiting current in this case can probably be carried by the local plasma with drift speeds below the thermal speed. For these conditions, we do not expect current‐driven instabilities to occur, and any auroral emissions would probably not be detectable against the background airglow of Triton's atmosphere. However, if Triton were to have a weak intrinsic magnetic field (for example, application of Blackett's law to Triton indicates a surface field of 400 nT, although a 20–80 nT surface field on Triton would suffice), the current would be funneled into an auroral zone. This would produce a concentration of current that may trigger specific auroral acceleration processes, such as double‐layer formation, and produce auroral optical and radio emissions detectable by Voyager instruments. Thus the presence of an aurora, particularly one that is localized, will argue for the presence of an intrinsic field on Triton.