Observations Of Aurora Research Articles

Conjugate observation of optical aurora with polar satellite and ground-based cameras

Ultraviolet images from the Polar spacecraft enable observation of the optical aurora in the sun illuminated northern summer hemisphere. All sky cameras operated in Antarctica observe large portions of the southern polar hemisphere simultaneously during the dark southern winter. This opportunity permits real conjugate auroral observations for long periods and in all sectors of local time. Optical aurora conjugacy depends on the location on closed or open field lines and on the distortion of the geomagnetic field. Several case studies show a good conjugacy close to magnetic noon but sometimes non-conjugacy close to magnetic midnight. There seems to be better conjugacy during small and northward IMF, but no systematic dependence on the solar wind dynamic pressure could be found.

D0.3-0035 use of coordinated observations of aurora, structure of energetic particle dropout and magnetic data for the diagnostics of magnetospheric substorms

D0.3-0035 use of coordinated observations of aurora, structure of energetic particle dropout and magnetic data for the diagnostics of magnetospheric substorms

The middle ultraviolet‐visible spectrum of H2 excited by electron impact

The electron‐impact‐induced emission spectrum of H2 has been measured in the extended wavelength region 175–530 nm at a spectral resolution of 1.7 nm (full width at half maximum). The laboratory spectra observed in the middle ultraviolet (MUV) and visible spectral region are characterized by underlying H2 ( ) continuum emission, together with many strong lines assigned to the radiative decay of the gerade singlet states of H2, and to members of the H Balmer series resulting from dissociative excitation of H2. Our calibrated MUV spectral data, obtained at 14, 19, and 100 eV electron‐impact energies, provide absolute emission cross sections of these H2 lines and will assist in the interpretation of planned Galileo ultraviolet spectrometer observations of Jupiter's aurora in this wavelength region.

Toward a closer integration of magnetospheric research: Magnetospheric currents inferred from ground‐based magnetic data

A new approach is needed to advance magnetospheric physics in the future to achieve a much closer integration than in the past among satellite‐based researchers, ground‐based researchers, and theorists/modelers. Specifically, we must find efficient ways to combine two‐dimensional ground‐based data and single points satellite‐based data to infer three‐dimensional aspects of magnetospheric disturbances. For this particular integration purpose, we propose a new project. It is designed to determine the currents on the magnetospheric equatorial plane from the ionospheric current distribution which has become available by inverting ground‐based magnetic data from an extensive, systematic network of observations, combined with ground‐based radar measurements of ionospheric parameters, and satellite observations of auroras, electric fields, and currents. The inversion method is based on the KRM/AMIE algorithms. In the first part of the paper, we extensively review the reliability and accuracy of the KRM and AMIE algorithms and conclude that the ionospheric quantities thus obtained are accurate enough for the next step. In the second part, the ionospheric current distribution thus obtained is projected onto the equatorial plane. This process requires a close cooperation with modelers in determining an accurate configuration of the magnetospheric field lines. If we succeed in this projection, we should be able to study the changing distribution of the currents in a vast region of the magnetospheric equatorial plane for extended periods with a time resolution of about 5 min. This process requires a model of the magnetosphere for the different phases of the magnetospheric substorm. Satellite‐based observations are needed to calibrate the projection results. Agreements and disagreements thus obtained will be crucial for theoretical studies of magnetospheric plasma convection and dynamics, particularly in studying substorms. Nothing is easy in these procedures. However, unless we can overcome the associated difficulties, we may not be able to make distinct progresses. We believe that the proposed project is one way to draw the three groups closer together in advancing magnetospheric physics in the future. It is important to note that the proposed project has become possible because ground‐based space physics has made a major advance during the last decade.

A comparison of optical and coherent HF radar backscatter observations of a post-midnight aurora

Abstract. A poleward-progressing 630 nm optical feature is observed between approximately 0100 UT and 0230 UT (0400 MLT to 0530 MLT) by a meridian-scanning photometer (MSP) located at Ny Ålesund, Svalbard. Simultaneous coherent HF radar measurements indicate a region of poleward-expanding backscatter with rapid sunward plasma flow velocity along the MSP meridian. Spatial maps of the backscatter indicate a stationary backscatter feature aligned obliquely with respect to the MSP meridian, which produces an impression of poleward-expansion as the MSP progresses to later MLT. Two interpretations of the observations are possible, depending on whether the arc system is considered to move (time-dependent) or to be stationary in time and apparent motion is produced as the MSP meridian rotates underneath it (time-independent). The first interpretation is as a poleward motion of an east-west aligned auroral arc. In this case the appearance of the region of backscatter is not associated with the optical feature, though the velocities within it are enhanced when the two are co-located. The second interpretation is as a polar arc or theta aurora, common features of the polar cap under the prevailing IMF northwards conditions. In this case the backscatter appears as an approximately 150 km wide region adjacent to the optical arc. In both interpretations the luminosity of the optical feature appears related to the magnitude of the plasma flow velocity. The optical features presented here do not generate appreciable HF coherent backscatter, and are only identifiable in the backscatter data as a modification of the flow by the arc electrodynamics.

Northward interplanetary magnetic field cusp aurora and high‐latitude magnetopause reconnection

We present observations of two types of auroral forms located at different latitudes in the cusp region. Type 1 (south) auroras are located at ∼71° – 75° MLAT and occur during intervals of southward directed interplanetary magnetic field (IMF). Higher‐latitude (77°–78°) type 2 (north) auroras are associated with northward IMF (Bz > 0). Intervals are studied which are characterized by transitions from one auroral form to the other in response to IMF switches from north (clock angle <50°) to south (clock angle >90°) and vice versa. These observations are found to be consistent with the interpretation that type south auroras are a signature of low‐latitude magnetopause reconnection and that the type north auroras are associated with high‐latitude reconnection, tailward of the cusp. The latter is supported by satellite (DMSP F11) observations of particle precipitation and ionospheric convection. The simultaneous existence of type north and type south auroras is observed during intermediate IMF states (clock angle ∼60°). Observations of type north auroras may be used to study the relationship between the IMF Bx component and high‐latitude reconnection. We find that type north auroras occur in the northern hemisphere for both Bx polarities. Enhanced emission intensity of the auroral green line seems to be a feature occurring predominantly during negative Bx conditions. The enhanced green line intensity is considered to be an indication of low‐altitude particle acceleration in regions of strong field‐aligned current intensities. Such particle accelerations (green line emission) may occur predominantly in regions of upward directed IMF By‐related currents in association with the type north cusp aurora.

Intensity correction in all-sky auroral image projection transform

BY using all-sky(fish-eye)lens and highly sensitive TV camera,the effective monitoring ra-dius and time resolution of ground aurora observation have now been raised to 600 km and1/30 s(for instance,SIT-TV camera recording in NTSC format).By incorporating high-sen-sitivity imaging device(integrated from image intensifier and CCD(charge coupled device)

High‐resolution television observations of black aurora

In view of a recent revival of interest in the black aurora and related phenomena, and a striking lack of information on the phenomenon in the literature, we present high spatial and temporal resolution optical observations of the black aurora made by the University of Calgary portable auroral imager. A variety of black auroral phenomena, such as black vortices, arcs, and eastward drifting black auroral patches and arc segments, were observed in the evening and midnight sector diffuse auroral oval during a field trip to Rabbit Lake, Saskatchewan, from February 25 to March 7, 1995. Observed spatial and temporal characteristics are reviewed, with statistical data presented in the form of histograms.

A comparison of optical and coherent HF radar backscatter observations of a post-midnight aurora

A poleward-progressing 630 nm optical feature is observed between approximately 0100 UT and 0230 UT (0400 MLT to 0530 MLT) by a meridian-scanning photometer (MSP) located at Ny à lesund, Svalbard. Simultaneous coherent HF radar measurements indicate a region of poleward-expanding backscatter with rapid sunward plasma flow velocity along the MSP meridian. Spatial maps of the backscatter indicate a stationary backscatter feature aligned obliquely with respect to the MSP meridian, which produces an impression of poleward-expansion as the MSP progresses to later MLT. Two interpretations of the observations are possible, depending on whether the arc system is considered to move (time-dependent) or to be stationary in time and apparent motion is produced as the MSP meridian rotates underneath it (time-independent). The first interpretation is as a poleward motion of an east-west aligned auroral arc. In this case the appearance of the region of backscatter is not associated with the optical feature, though the velocities within it are enhanced when the two are co-located. The second interpretation is as a polar arc or theta aurora, common features of the polar cap under the prevailing IMF northwards conditions. In this case the backscatter appears as an approximately 150 km wide region adjacent to the optical arc. In both interpretations the luminosity of the optical feature appears related to the magnitude of the plasma flow velocity. The optical features presented here do not generate appreciable HF coherent backscatter, and are only identifiable in the backscatter data as a modification of the flow by the arc electrodynamics.

Radar observations of ionospheric irregularities at Syowa Station, Antarctica: a brief overview

Abstract. We briefly overview the radar observations that have been made for 30 years at Syowa Station, Antarctica for studying small-scale electron-density irregularities in the southern high-latitude E- and F-region ionosphere. Some observational results (i.e., long-term variations of radio aurora, Doppler spectra with narrow spectral widths and low Doppler velocities, and simultaneous observations of radar and optical auroras) from VHF radars capable of detecting 1.3- to 3-m scale irregularities are presented. A new 50-MHz radar system equipped with phased-antenna arrays began operation in February 1995 to observe two-dimensional behaviours of E-region irregularities. An HF radar experiment also began in February 1995 to explore decameter-scale E- and F-region irregularities in the auroral zone and polar cap. These two radars will contribute to a better understanding of the ionospheric irregularities and ionospheric physics at southern high latitudes.

Time-Resolved Observations of Jupiter's Far-Ultraviolet Aurora

Simultaneous imaging and spectroscopic observations of Jupiter's far-ultraviolet aurora covering half a jovian rotation were made on 31 May 1994. The Hubble Space Telescope Wide Field Planetary Camera 2 images revealed dramatic and rapidly changing auroral features, including discrete longitudinal structures along the auroral ovals, with variable contrast; a poleward offset in a north oval sector, showing equatorward motion near dusk; emissions polewards of the ovals, apparently co-rotating; and a bright event developing near the dawn limb. Viewing geometry effects explain the rotational intensity modulation observed by the International Ultraviolet Explorer, without intrinsic longitudinal asymmetries.

Interpretation of Auroral “Lightcurves” with Application to Jovian H+3Emissions

Photometric observations of planetary aurorae may be used to constrain the time variation and spatial distribution of auroral emissions, thereby providing insight into planetary magnetic fields and magnetospheric dynamics. The variation of the total flux observed as a function of the planet's central meridian longitude (auroral “lightcurve”) is used to estimate the parameters of a simple auroral emission model. An inverse methodology based upon the singular-value decomposition of a matrix is used to estimate the model parameters and to characterize the uniqueness of the solutions obtained. The method of analysis is applied to images of Jupiter's polar regions at a 3.4-μm wavelength acquired during the first 4 months of 1992 using ProtoCAM at NASA's IRTF. The distribution and intensity of the H+3emission in Jupiter's auroral regions are estimated from the auroral lightcurves and compared with analyses of the actual images. Northern and southern auroral intensities covary, and for episodes of more intense activity during the period of observation, intensities observed in the north polar region generally exceeded those observed in the south. Auroral intensity varies with time in a manner that suggests considerable change in charged particle precipitation in Jupiter's magnetosphere, and energy deposition in Jupiter's atmosphere, on a time scale of months.

Simultaneous satellite and ground-based observations of polar cap aurora

On August 20, 1979 (15–17 UT) the sun-aligned polar cap arcs have been observed at the Antarctic station Vostok. Simultaneously NOAA polar spacecraft detected in the polar cap a several hundred kilometers wide region of electron and proton precipitation over those arcs, and similar precipitation structure was also observed in the northern polar cap. Within this auroral precipitation regions (interpreted as a signature of the Θ-aurora) the >30 keV proton precipitation have been also found with the intensities comparable with those in the auroral zone. These observations suggest that the Θ-aurora precipitation has its origin on the closed field lines of the plasma sheet.

The equatorial boundary of the ultraviolet Jovian north aurora observed with multispectral Hubble Space Telescope images

Multispectral observations of the far ultraviolet Jovian aurora with the faint object camera (FOC) on board the Hubble Space Telescope are reported. They are used to describe and compare the morphology and the mean brightness of the H2 Lyman and Werner bands observed at 153, 125, and 130 nm. It is shown that most of the emissions are confined inside the 6 RJ O6‐GSFC auroral oval and fill a large fraction of the polar cap. During the ∼50 hours time span of the observations following a very strong aurora, no bright arc was observed along the oval mapping the 30 RJ O6 oval. Hydrogen emissions at 125 and 130 nm are observed down to 50°N and somewhat below, a region not accessible to previous FOC observations at longer wavelengths. Temporal variations are also observed on timescales of hours to days.

特1 オーロラの物理

There have been so many optical observations of aurora so far, but there have not been adequate quantitative interpretations of their causality between the source (auroral energetic particles), the resultant auroral volume emission rates, and the fields. Difficulties of these studies are in the fact that (1) observationally, we have not obtained simultaneously and quantitatively physical quantities of the source, the resultant emission and the field, and (2) theoretically, we have not fully understood the physical processes of auroral emissions. Recently we have performed some experiments which give us some physical parameters related to these problems. On the other hand, the computer simulation schemes to calculate the photoemission rates from the source particles have been developed. In this review paper, I will present some examples of the auroral observations and give a talk what kinds of theoretical works we are now investigating; Particularly following five themes are discussed. Rocket experiment, spectroscopic observation, stereoscopic observation, satellite observations and computer simulations.

Simultaneous observations of the Saturnian aurora and polar haze with the HST/FOC

Near simultaneous observations of the Saturnian H2 north ultraviolet aurora and the polar haze were made at 153 nm and 210 nm respectively with the Faint Object Camera on board the post‐COSTAR Hubble Space Telescope. The auroral observations cover a complete rotation of the planet and, when co‐added, they reveal the presence of an auroral emission near 80°N with a brightness of about 150 kR of total H2 emission. The maximum vertical optical depth at 210 nm is found to be located ∼5° equatorward of the auroral emission zone. The haze particles are presumably formed by hydrocarbon aerosols initiated by H2+ auroral production. In this case, the 3 × 1010 W of H2 emission observed with the FOC, combined with the deduced haze optical depth requires an efficiency of aerosol formation of about 7%. This result indicates that auroral production of hydrocarbon aerosols is a viable source of high‐latitude haze.

Ground and satellite observations of postdawn aurorae near the time of a sudden storm commencement

Meridian scanning photometer measurements taken in the magnetic postdawn sector at Longyearbyen, Svalbard, between 0300 and 0630 UT on December 29, 1981, are analyzed in conjunction with particle and field data retrieved during two near passes of the Dynamics Explorer 2 (DE 2) satellite. The interval included a sudden storm commencement (SSC) at 0455 UT. Pre‐SSC optical and particle measurements showed a system of arcs that are spaced at ∼1.1° intervals in magnetic latitude, embedded within the region 1 current system and span the convection reversal. The softer particle precipitation appears to have a source near the flanks of the magnetotail while the harder, more equatorward precipitation originates closer to Earth. During the SSC period the entire sky brightened, with enhanced 630.0‐nm emissions extending from the northern horizon to south of magnetic zenith; intense but spatially separated 557.7‐nm emissions dominated the southern horizon. DE 2 detected more than an order of magnitude increase and near isotropization of ring current electron fluxes, enhanced precipitation from the plasma sheet and significantly decreased auroral zone convection. Region 1/region 2 currents remained, with wavelike structures superposed. A dual timescale response to the SSC is consistent with ground and satellite measurements. On few minute travel timescales for hydromagnetic waves to pass through the system, magnetospheric particles accelerate and precipitate to increase the ionospheric conductivity. Global, field‐aligned currents change more slowly. To maintain similar field‐aligned currents with higher ionospheric conductances requires reduced electric fields. After 0520 UT the optical emissions settled into stable, but latitudinally separated, bands of 630.0‐ and 557.7‐nm emissions characteristic of cleft and plasma sheet precipitation, respectively.

High-resolution spectra of Jupiter's northern auroral ultraviolet emission with the Hubble Space Telescope

The first spectroscopic observations of planetary aurora with the Hubble Space Telescope (HST) are reported. These include spectral regions centered on the H2 Lyman and Werner bands of a region of Jupiter's northern aurora. The observations were made with the Goddard High Resolution Spectrograph (GHRS) using the Large Science Aperture as part of a campaign to study Jupiter at the time of the Ulysses flyby. The individual rotational-vibrational bands are resolved and the observed emissions are essentially all from H2. A rotational-vibrational temperature for H2 of 530 +/- 100 K is derived, a value significantly less than the 850-1100 K reported for Jovian H3(+) in the near-infrared but consistent with the temperature reported for fundamental-band quadrupole H2 emission. Comparison with the Faint Object Camera (FOC) images shows that the observed region was not one of the hot spots of the aurora. The results are interpreted in trms of electron impact excitation of H2 from secondary particles generated by primaries precipitating into Jupiter's atmsophere from the magnetosphere. In the region of the aurora observed, the homopause level is found to be significantly hotter but not necessarily higher than observed at nonauroral latitudes. The equatorial H2 dayglow spectrum was also detected; its intensity was 3.2 kR or 13% of the strength of the observed auroral emission.

Observations of optical aurora modulated by resonant Alfvén waves

Optical aurora and magnetometer data from the Canadian Auroral Network for the OPEN Unified Study (CANOPUS) array in Canada are used in the study of the modulation of optical aurora in the 5577‐Å and 4709‐Å emission lines, by resonant shear Alfvén waves, in the frequency range of 1‐4 mHz. A total of four events, carefully chosen to represent different characteristics of timing and location, are analyzed. Typical of these events, the power spectra featured discrete spectral peaks usually near 1.3, 1.9, and 3.1 mHz. Furthermore, latitudinal phase shifts of about 180° were typically observed across the latitudinal maximum of a given frequency peak. These observations point irrevocably to the field line resonance as a major factor in the modulation of precipitation and possibly in the acceleration of electrons in forming auroral arcs. Our study demonstrates that the modulation phenomena are common features occurring in the auroral oval, observable in an extensive latitude range, from 66° to 73° invariant latitude. Of the events, one shows the modulation process accompanied by an inverted V structure of electron precipitation in the evening sector near 72°. Two events are observed in the equatorward region of the auroral oval just before substorms onset, and maybe related to the energetic electron arcs which are the precursor of substorm intensification. The fourth event is seen in the morning sector at an equatorward latitude and occurs in the recovery phase of a substorm. The diversity of these modulation events allows us to further infer that resonant Alfvén waves play a direct role in controlling the luminosity variation of the optical aurora.

Spectrometric and photometric observations of low‐latitude aurorae

The properties of a low‐latitude aurora of September 22, 1982, observed with photometers and a spectrograph from McDonald Observatory, Texas, and an all‐sky mapping photometer from a site near Boulder, Colorado, are discussed. The dominant emission was [OI] 630 nm with a 630/558 nm emission ratio of 3.8. Other weaker emissions were present that are characteristic of heavy particle aurorae and can be attributed to energetic (keV) ions and neutral atoms from the ring current. The properties of this and other low‐latitude aurorae are discussed in the context of stable auroral red (SAR) arcs and type d aurorae, and of energetic ion and neutral aurorae. The emissions associated with heavy particle precipitation are characteristic of main phase conditions. The observed equatorial movement of main phase low‐latitude red aurorae is consistent with the inward convection of ring current particles and the inward movement of the plasmapause. Previous work has hypothesized that coulomb collisions of plasmaspheric electrons with ring current O+ cause the energization of the electrons exciting SAR arcs. Calculations of [OI] 630 and 558 nm emissions show that for red aurorae with 630/558 nm ratios ∼4, additional energization processes producing electrons in the energy range 10–10,000 eV are required. We propose some clarification of nomenclature for several types of aurorae occurring at low latitudes.