Auroral Measurements Research Articles

A New Proton‐Hydrogen‐Electron Transport Model for Simulating Optical Emissions From Proton Aurora and Comparison With Ground Observations

AbstractEnergetic proton precipitation from the magnetosphere plays an important role in the magnetosphere‐ionosphere‐thermosphere coupling and energy transfer. Proton precipitation causes hydrogen emissions, such as Hβ (486.1 nm), and also triggers the excitation of other emission lines such as the blue‐line (427.8 nm) and the green‐line (557.7 nm). In light of the growing availability of ground‐based proton auroral measurements in recent years, we revisit the proton auroral modeling in this study, with more focus on the application for interpreting ground observations. An accurate simulation of these optical emissions requires a comprehensive understanding of particle transport and collisions in the upper atmosphere, where the simultaneous consideration of precipitating protons, newly generated energetic hydrogen atoms, and secondary electrons is critical. For this purpose, we couple a 3D Monte‐Carlo proton transport model and an electron transport model. The integrated model framework can compute the emission rates of most major auroral emission lines/bands resulting from proton precipitation, along with self‐consistent calculation of the ionospheric electron density variations. The model results show improved agreement with ground optical observations in terms of the Hβ yield and the green‐to‐Hβ ratio compared to previous model studies. Our new model is a valuable tool for quantifying excitation and ionization due to proton aurora. It has the potential to leverage ground observations to infer precipitating conditions at high altitudes and even for studying magnetospheric activity.

Polar Cap Boundary Identification Using Redline Optical Data and DMSP Satellite Particle Data

AbstractThe location of the polar cap boundary is typically determined by using low‐orbit satellite measurements in which the boundary is identified by its unique signature of a sharp decrease in energy and particle flux poleward of the auroral oval. In principle, this decrease in precipitating particles should appear as a concomitant sharp change in auroral luminosity. Based on a few events, Blanchard et al. (1995, https://doi.org/10.1029/94JA02631) suggested that a dramatic gradient in redline aurora may also be an indicator of the polar cap boundary. In recent years, advances in capabilities and the deployment of ground‐based all‐sky imagers have ushered in a new era of auroral measurements. Auroral imaging has moved well beyond the capabilities of the instrumentation in the previous study in terms of both spatial and temporal resolution. We now have access to decades of optical data from arrays spanning a huge spatial range, enabling a fresh examination of the relationship between redline aurora, particle precipitation, and the polar cap open closed boundary. In this study, we use data from the Defense Meteorological Satellite Program satellites in conjunction with the University of Calgary's Redline Emission Geospace Observatory (630.0 nm) data to assess the viability of automated detection of the 2‐dimensional polar cap boundary. Our results exhibit good agreement between the optical and particle polar cap boundary and suggest that a luminosity in redline emission could serve as a reasonable proxy for the location of the electron poleward boundary, while providing both high temporal and spatial resolution maps of the open‐closed boundary.

A diagnosis of the plasma waves responsible for the explosive energy release of substorm onset

During geomagnetic substorms, stored magnetic and plasma thermal energies are explosively converted into plasma kinetic energy. This rapid reconfiguration of Earth’s nightside magnetosphere is manifest in the ionosphere as an auroral display that fills the sky. Progress in understanding of how substorms are initiated is hindered by a lack of quantitative analysis of the single consistent feature of onset; the rapid brightening and structuring of the most equatorward arc in the ionosphere. Here, we exploit state-of-the-art auroral measurements to construct an observational dispersion relation of waves during substorm onset. Further, we use kinetic theory of high-beta plasma to demonstrate that the shear Alfven wave dispersion relation bears remarkable similarity to the auroral dispersion relation. In contrast to prevailing theories of substorm initiation, we demonstrate that auroral beads seen during the majority of substorm onsets are likely the signature of kinetic Alfven waves driven unstable in the high-beta magnetotail.

Proton auroras during the transitional stage of substorm onset

Optical auroral measurements repeatedly reveal the existence of a transitional stage between a quiescent preexisting arc and its significant auroral expansion in a substorm onset. Such a transitional stage of substorm onset (TSSO) is characterized by a gradual intensification and the emergence of auroral beads, along the preexisting arc. However, existing studies on TSSO are limited to electron auroras which are dominant in optical luminosity. In this study, we collect 12 substorm events to investigate the proton auroral features during the late growth phase and the TSSO. Our major results include: (1) we confirm the previous notion that the onset electron auroral arc is usually located at the poleward “shoulder” of the main proton auroral band. (2) While the electron auroral arc typically intensifies by a few times or even an order of magnitude during the TSSO, the concurrent proton aurora at the same location as the electron auroral arc shows much less noticeable variations. The proton auroral variations averaged over the arc band, as well as that integrated over the entire latitudinal range, are mostly within 10% of their mean late-growth-phase levels during the TSSO. Substantial intensifications of proton auroras occur after the poleward expansion of electron auroras. Even considering the spatial spreading of proton auroras, we estimate that the variation of ion precipitation fluxes on top of the ionosphere would be typically < 30% during the TSSO. The above observations impose implications and quantitative constraints on the possible mechanisms of TSSO and substorm onset. We assert that there is no significant ion energization or large-scale magnetic field reconfiguration during the TSSO. Instead, it is likely that the underlying mechanism of the TSSO might be certain kind of ky-dominated instability wave mode. The instability wavelength is comparable to the ion gyroradius, so that the ions are demagnetized, suppressing ion flux variations with the instability. Enhancing upward electric fields in the auroral acceleration region during the TSSO may also play a partial role in weakening the ion precipitation flux variation.

A synthesis of star calibration techniques for ground‐based narrowband electron‐multiplying charge‐coupled device imagers used in auroral photometry

AbstractA technique is presented for the periodic and systematic calibration of ground‐based optical imagers. It is important to have a common system of units (Rayleighs or photon flux) for cross comparison as well as self‐comparison over time. With the advancement in technology, the sensitivity of these imagers has improved so that stars can be used for more precise calibration. Background subtraction, flat fielding, star mapping, and other common techniques are combined in deriving a calibration technique appropriate for a variety of ground‐based imager installations. Spectral (4278, 5577, and 8446 Å) ground‐based imager data with multiple fields of view (19, 47, and 180°) are processed and calibrated using the techniques developed. The calibration techniques applied result in intensity measurements in agreement between different imagers using identical spectral filtering, and the intensity at each wavelength observed is within the expected range of auroral measurements. The application of these star calibration techniques, which convert raw imager counts into units of photon flux, makes it possible to do quantitative photometry. The computed photon fluxes, in units of Rayleighs, can be used for the absolute photometry between instruments or as input parameters for auroral electron transport models.

History of extreme ultraviolet solar measurements in XX–XXI centuries: from balloons to the International Space Station as instrumental platforms

The Sun was one of the central points for observations as long as human cultures are known. After researchers observed the extension of the solar rainbow spectrum towards shorter wavelengths with increasing altitude they started measurements of the solar ultraviolet spectrum from balloons in 1934. The acquisition of spectra in the extreme ultraviolet and soft X-ray spectral regions was beginning after 1946 using photographic recordings aboard rockets. However, it took six more years to achieve good spectral resolution below about 220 nm because servomechanisms had to be developed to fairly accurately point the spectrographs to the Sun during the rocket flight. Another big step forward was achieved by the application of photoelectric recording technology and by using satellites as a platform to observe qualitatively the strong variability of the solar ultraviolet irradiance on short-, medium- and long-term scales. Until the end of the past century these measurements have been strongly impeded by the inherent degradation of the EUV instruments with time. Since there is progress solving this serious problem, measurements with sufficient radiometric accuracy allow composing the data available to the first set of ultraviolet data covering a period of eleven years. Based on the sophisticated instrumentation verified in space, future EUV measurements of the solar spectral irradiance on the International Space Station are promising accuracy levels of about 5%. Added by low-cost equipment on-line measurements will allow providing data needed in ionospheric modeling for correcting propagation delays of navigation signals from space to Earth. Adding ultraviolet airglow and auroral emission measurements the impact of space weather on the ionosphere can be studied and also be used deriving more detailed correction procedures for the evaluation of Global Navigation Satellite System (GNSS) signals.

Comment on “Oblique Double Layers: A Comparison between Terrestrial and Auroral Measurements”

A Comment on the Letter by C. Charles, R. W. Boswell, and R. Hawkins, Phys. Rev. Lett. 103, 095001 (2009).

Oblique Double Layers: A Comparison between Terrestrial and Auroral Measurements

The S3-3, POLAR, and FAST satellite auroral observations of parallel and perpendicular electric field structures have been identified as belonging to a large "U"-shaped potential structure that supports oblique electric double layers. This interpretation is verified by terrestrial laboratory measurements of a self-consistently supported three-dimensional oblique current-free double layer. Its width is a few tens of Debye lengths, its oblicity (with respect to the magnetic field) varies from 0 up to 30 degrees, and its strength is a few times the electron temperature.

Comparison and significance of auroral studies during the Swedish and Russian bilateral expedition to Spitsbergen in 1899–1900

Abstract. Results of measurements and visual observations of aurora at Spitsbergen, carried out by the joint Swedish-Russian expedition during 1899–1900, are described. Auroral observations took place during the great bilateral Arc-of-Meridian expedition, which was patronized by the Swedish Royal Family and the Russian Imperial Family. The Russian-Swedish Arc-of-Meridian measurements were closely coordinated but auroral measurements from the two sites in the Spitsbergen Archipelago were almost independent of each other. The basic auroral data for our presentation are reports of the Russian astronomer Josef Sykora and the Swedish geophysicist Jonas Westman. Both scientists used similar types of photo cameras and spectrographs, which were the best at that time and were made in Potsdam by Toepfer. Detailed descriptions of the optical devices and the system of spectral calibration are presented. A Toepfer spectrograph, possibly the one used by Westman, is still kept at IRF in Kiruna. We present a comparative analysis of auroral data from the Russian and Swedish stations on three themes: visual observations of aurora, describing features of auroral forms and giving us statistical data on aurora occurrence and the heights of aurora, photos of aurora, and auroral spectra. It is shown that the observations contain enough data to construct an auroral oval and to determine the heights of aurora. The expedition obtained the first photographic observations of the aurora in the Arctic. The auroral spectra demonstrate a high spectral resolution and show not only the main auroral emissions in the blue-green spectral range but also some weak emissions in the violet and ultraviolet region. All data are interpreted from a modern point of view. The Russian-Swedish 1899–1900 expedition carried out the first complex auroral investigations in the Arctic using optical instruments and presented well documented data and new results.

Ionospheric response to wave‐accelerated electrons at the poleward auroral boundary

The local ionospheric response to an auroral intensification at the poleward auroral boundary has been investigated using the incoherent scatter radar (ISR) and optical instrumentation at Sondrestrom, Greenland, in conjunction with space‐borne measurements by the IMAGE and FAST satellites. ISR elevation scans through the illuminated region revealed filamentary columns of enhanced plasma density, ∼5 km in latitude by ∼200 km in altitude. Column densities were typically 5 × 1011/m3 above background and often constant over a broad range of altitudes. The brightness of the O+ 732–733 nm multiplet, monitored simultaneously by a near‐infrared spectrometer, exceeded 1.2 kR during one 4‐min period (a factor of ∼4 brighter than previously reported auroral measurements). A time‐dependent model was developed to relate O+ emission intensities to O+ column densities for a given illumination time. The results suggested that the electron source was composed of kilometer‐scale flux tubes locked in the E × B flow for several minutes whose average energy varied temporally between &lt;100 eV and &gt;1 keV over their illumination lifetime. Conjugate electrons measured by the FAST satellite at 1700 km showed evidence for energization by inertial Alfvén waves. Ionization rates computed from these spectra were sufficient to account for the observed filamentary ionospheric structure. The implications of such ionization patterns for electrodynamic coupling with the magnetosphere are discussed.

Auroral Contribution to Sky Brightness for Optical Astronomy on the Antarctic Plateau

AbstractThe Antarctic Plateau holds great promise for optical astronomy. One relatively unstudied feature of the polar night sky for optical astronomical observing is the potential contamination of observations by aurorae. In this study we analyse auroral measurements at South Pole Station and show that during an average winter season, the auroral contribution to the B band sky brightness is below 21.9 B mag arcsec−2 for 50% of the observing time. In V band, the median sky brightness contribution is 20.8 mag arcsec−2 during an average winter. South Pole Station is situated within the auroral zone and experiences strong and frequent auroral activity. The Antarctic locations of Dome C and Dome A are closer to the geomagnetic pole where auroral activity is greatly reduced compared with that of South Pole Station. Calculations based on satellite measurements of electron flux above the Antarctic Plateau are used to show that at Dome C, the contribution to sky background in the B and V bands is up to 3.1 mag less than that at the South Pole. The use of notch filters to reduce the contribution from the strongest auroral emission lines and bands is also discussed. The scientific potential of an extremely large telescope located at Dome C is discussed, with reference to the effect that auroral emissions would have on particular astronomical observations.

Auroral measurements from space brought into the classroom

Auroral measurements from space brought into the classroom

Auroral ionospheric and thermospheric measurements using the incoherent scatter technique

The incoherent scatter technique has been applied since 1965 to study the ionosphere and thermosphere in different regions of the Earth. The analysis of the received signal gives access to several ionospheric parameters as a function of height: electron density, electron and ion temperatures and ion velocity. The derivation of these parameters is usually a complicated mathematical procedure that requires a non-linear regression program. A lot of research has been done in the ionospheric and atmospheric science using this technique. In this paper we describe how one derives the ion-neutral collision frequency and the ion composition parameters. It is usually difficult to retrieve these parameters with the incoherent scatter technique; as a result, in the standard data analysis procedure, an ionospheric model is used instead. However the numerical values chosen in the model have an influence on the other derived parameters. For instance the choice of a wrong ion composition leads to erroneous plasma temperatures. It is therefore important to assess by how much the standard procedure deviates from reality. For this reason we compare the ion composition and collision frequency retrieved from a sophisticated analysis scheme with the values that are derived from models under similar geophysical conditions. It also possible to derive from the observed ionospheric parameters the neutral concentrations, temperatures and winds, by using the energy and momentum equations for the ions and the neutrals. In this paper the different methods and the corresponding assumptions involved in the data analysis are discussed. We describe the influence of the frictional heating, of the vertical neutral wind and of the ionospheric perturbations on the derivation of the neutral atmospheric parameters. Our discussion of the processes involved are drawn from results obtained by Chatanika, Sondrestrom and EISCAT radars.

Application of the intensified CCD to airglow and auroral measurements

New detector technology exemplified by advanced CCD and intensified CCD (ICCD) systems have important advantages for both spectrographic and imaging research. However, to realize the full potential of this new technology, we must consider the detector and the optical system as a whole. It is frequently not enough to simply substitute an ICCD for an earlier detector; rather, to achieve optimum results, the optics must be adapted to the specific detector. Properly designed airglow spectrographs based on the ICCD detector offer the advantages of high throughput over a broad spectral range, precise wavelength stability, low noise, and compactness. Imagers having the wide field and the high sensitivity needed for airglow research are practical as well.

N( 2D) + O 2: A source of thermospheric 6300 Å emission?

The aeronomic and laboratory evidence concerning the efficiency of the reaction N( 2D) + O 2 → NO (v)) + O( 1D ) in producing thermospheric 6300 Å emission is reviewed. Dayglow and nightside auroral measurements do not offer unambiguous evidence concerning the need for an additional source. Measurements of the temporal variation and lifetime of the 6300 and 5200 Å emissions in the polar cap and cleft aurora are not consistent with the proposed mechanism. Through consideration of reaction dynamics, we conclude that the laboratory inference of a large O( 1D) yield at cryogenic temperatures is not in conflict with reports of a low O( 1D) yield at thermospheric temperatures.

An observation of ionospheric convection and auroral arc motion

An all‐sky imager operated at Sondre Stromfjord, Greenland has been used to make auroral measurements in correlation with data from the incoherent scatter radar. The auroral images were recorded on video tape for later display as time lapse sequences. Reviewing the images as movie sequences with an apparent time compression ratio of 60 allows the eye to discern features which are not apparent in the individual frames. The faint 630 nm airglow background lying poleward of the bright auroral oval arcs appears to be structured in the time lapse observations and drifting at a uniform rate. Analysis of the motion of the airglow structures shows excellent agreement with Doppler radar measurement of the ion drift velocities. Electron density profiles derived from the radar data corroborate the existence of high altitude F‐region ionization enhancements. The electron temperature in the patches was about 1000 K and no higher than outside of the patch signifying that the patches were not produced by local soft electron precipitation. According to the radar Doppler data there was a small component of the drift velocity parallel to the arc at either side of the auroral boundary. The sense of those small components was consistent with drifts driven by a convergent electric field which would also produce an up going field aligned current. The larger component of drift velocity perpendicular to the arc appeared to be continuous on either side of the arc. This drift component shows the existence of an electric field parallel to the arc, showing that the arc was not on an equipotential. During the observations the arc moved slowly in the opposite sense to the ion drift, signifying that the auroral arc motion is not driven by the electric field impressed on the atmosphere by the magnetosphere. Calculations of the expected intensity of the 630 nm emissions are in good agreement with the measured intensities of the moving patches. The observation of the motion of these airglow structures could provide a routine measurement of the ionospheric convection speeds in regions where the radar is not available.

Model calculation of hydrogen balmer emissions under various modes of proton precipitation

A theoretical model is developed to study the H α and H β volume emission rates in proton auroras. The H α and H β photon yields are deduced by using the recent experimental cross-sections. These photon yields are then coupled with the primary proton fluxes to get the altitude profiles of the emissions. The prediction of the altitude profiles are found in good agreement with the auroral measurements. The effects of various modes of proton precipitation on the emission intensities (IH α and IH β) are discussed and a comparison is made with the earlier theoretical models. It is also found that the intensity ratio IH α/IH β varies between 5 and 6.

Rocket-borne EUV-visible emission measurements

The results from auroral measurements with spectrometers and photometers are reported for three successful rocket flights with the spectra (50–640 nm) and height profiles presented. The ratios of 391.4 557.7 nm show characteristic differences for diffuse, pulsating and stable arc conditions met. The total energy content in the electromagnetic radiation measured is 0.61 mW m 2 −2 (erg cm −2 s −1) for the diffuse aurora (1980) and 0.67 mW m −2 for the stable arc (1981). The spatial distribution of selected emissions exhibit distinct differences for the auroral events observed.

Auroral photometry from the Atmosphere Explorer Satellite

Two passes of the Atmosphere Explorer C satellite over the Poker Flat Optical Observatory and the Chatanika Radar Facility provided the best auroral measurements from space and from the ground available to date for demonstrating the capability of remote sensing from space to yield quantitative auroral and ionospheric parameters. It is shown that the emission rate of the N2+ (1NG) 4278 Å band computed from intensity measurements of energetic auroral electrons tracks the same spectral feature measured remotely from the satellite over 2 decades of intensity. This provides a stringent test for the procedure adopted to correct the optical measurements for atmospheric scattering effects and verifies the absolute intensity with respect to the ground‐based photometric measurements, referenced to a laboratory calibration. In situ satellite measurements of ion densities and ground‐based radar measurements of electron density profiles provide a consistent picture of the ionospheric response to the auroral input that our model is capable of predicting while at the same time also predicting the observed optical emission rate. Modeling therefore is the link between global remote sensing optical measurements and the ionospheric response to auroral energetic particle bombardment.

Matched tandem etalon camera—MATEC—and its application to auroral observations

The matched tandem etalon camera (MATEC) offers the high resolution of the Fabry-Perot interferometer combined with the separate advantages of a flexible image format and photon array detection. In certain applications involving complex or continuum spectra, the simultaneous detection of data over a wide spectral and spatial range can result in a large increase in effective data-taking speed over that obtainable by scanning the same range. In this new type of tandem etalon device, an intercalated field lens, whose spacing and power are both critical, allows a dielectric interference filter to isolate the same order of an airspaced Fabry-Perot at all angles of transmission. In the MATEC the field of view is imaged on a photon array detector to provide simultaneous detection of all throughput wavelengths. In a system applied to auroral measurements known as the meridian etalon camera (MEC), the MATEC views a semiconical sky mirror to provide a simultaneous dimension of spatial resolution in the N-S meridian comined with the dimension of spectral resolution. Stellar features and cloud and ground reflections are much more easily identified in the MEC format than by any scanning photometer.