Column Emission Rates Research Articles

On mechanisms for high-frequency pump-enhanced optical emissions at 557.7 and 630.0 nm from atomic oxygen in the high-latitude F-region ionosphere

Abstract. The EISCAT (European Incoherent Scatter Scientific Association) Heating facility was used to transmit powerful high-frequency (HF) electromagnetic waves into the F-region ionosphere to enhance optical emissions at 557.7 and 630.0 nm from atomic oxygen. The emissions were imaged by several stations of ALIS (Auroral Large Imaging System) in northern Sweden, and the EISCAT UHF incoherent scatter radar was used to obtain plasma parameter values. The ratio of the 557.7 to 630.0 nm column emission rates changed from I5577/I6300≈0.2 for the HF pump frequency f0=6.200 MHz ≈4.6fe to I5577/I6300≈0.5 when f0=5.423 MHz ≲4fe, where fe is the ionospheric electron gyro frequency. The observations are interpreted in terms of decreased electron heating efficiency and thereby weaker enhancement at 630.0 nm for f0=5.423 MHz ≲ 4 fe. The emissions at 557.7 nm are attributed to electron acceleration by upper hybrid waves of metre-scale wavelengths that can be excited with f0=5.423 MHz ≲ 4 fe.

Gravity wave activity in the middle atmosphere from SATI airglow observations at northern mid-latitude: Seasonal variation and comparison with tidal and planetary wave-like activity

The study investigates the gravity waves activity detected in the mesosphere/lower thermosphere (MLT) region (80–120 km) using airglow observations performed by the Spectral Airglow Temperature Imager (SATI) at the Sierra Nevada Observatory (SNO) (37.06∘N, 3.38∘W). In this analysis column emission rates of the OH Meinel (6–2) and O2 Atmospheric (0–1) bands, as well as rotational temperatures derived from these emission rates are employed for the period of 1998–2015. The long time series allows the determination of gravity waves climatology at this site. The gravity wave activity content exhibits an annual variability with a maximum during winter and a faint increase in summer. It was found that gravity waves with periods of less than 3h are more prevalent than those with periods of 3–6h throughout the year, for both column emission rates and rotational temperatures. The gravity wave activity with periods of less than 3h is larger in summer, while gravity waves with period from 3 to 6 h have maximum activity around autumn-winter. The analysis showed that the gravity waves activity surpassed that of the semidiurnal tides, although was still smaller than the planetary wave activity, especially in summer when the planetary waves appear to dominate the dynamics of the MLT region. In general, wave activity is the main source of variability in both OH and O2 airglow emissions with the planetary wave activity being the main driver of the O2 emission variability. However, the rotational temperatures are more affected by seasonal variations, especially the OH rotational temperatures, accounting for more than the 40% of the overall observed variability.

A Method to Calculate the Ionospheric LBH Dayglow Emissions for Large Field of View

AbstractLBH dayglow emissions in the ionosphere, produced by the photoelectrons impact on the nitrogen molecules, are the most prominent molecular signals in the far ultraviolet range. Imaging the LBH dayglow emissions from the space can be a powerful method to monitor the state of the upper atmosphere. According to direct excitation theory and spherical geometry, the spectral characteristics of the LBH emission are analyzed and a revised method (RAURIC) to calculate the column emission rate of the LBH dayglow emissions for large field of view is given. Two main limitations of AURIC, that are the definition of the observation azimuth angle and the treating of the solar zenith angle as a constant along a line of sight, are improved in RAURIC. The column emission rates of the LBH bands in the range of 140∼180 nm are calculated with the method. Comparisons with results from AURIC show great agreement in nadir, while RAURIC should be used in other lines of light, especially for large field of view. This work provides a solid basis for simulating the image of ionospheric LBH dayglow emissions and the data inversion technique.

An empirical determination of proton auroral far ultraviolet emission efficiencies using a new nonclimatological proton flux extrapolation method

Model‐derived electron and proton auroral FUV emission efficiencies of relevance to auroral FUV remote sensing methods are evaluated using coincident observations by Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor J/5 (SSJ/5), both on board the Defense Meteorological Satellite Program (DMSP) satellite F16. This follows earlier work by Knight et al. (2008), which reported higher than expected proton Lyman‐Birge‐Hopfield (LBH) emission efficiencies based on F16 SSUSI and SSJ/5 comparisons, and Correira et al. (2011), which suggested a downward revision in the proton LBH efficiencies from Knight et al. (2008). These proton efficiency results rely on proton extrapolation methods to supply the unmeasured proton flux above 30 keV (the upper limit of SSJ/5). Correira et al. (2011) determined that there was a bias in the proton extrapolation method used by Knight et al. (2008) that was caused by column emission rate (CER) thresholding in the coincident SSUSI and SSJ/5 sets. In the latest work, a more robust proton flux extrapolation method is introduced which does not have the problem of CER threshold dependence. The new extrapolation method uses coincident SSUSI Lyman alpha observations to constrain the extrapolated proton flux above 30 keV in such a way that unknown Lyman alpha model yield errors and SSUSI and SSJ/5 calibration errors drop out without biasing the extrapolation. With the latest extrapolation method, SSUSI‐SSJ/5 comparisons indicate that proton aurora is typically a factor of ∼4.5 more efficient per unit of energy flux in producing LBH than electron aurora.

A downward revision of a recently reported proton auroral LBH emission efficiency

[1] A significantly higher N2 Lyman-Birge-Hopfield (LBH) emission efficiency for auroral proton precipitation compared to model calculations was reported by Knight et al. (2008) based on a statistical study utilizing coincident far ultraviolet and particle data from the sensors Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor J/5 (SSJ/5) on board the DMSP satellite F16. Here, the quantity of interest from that study is the median ratio of LBH column emission rates (CERs) from SSUSI and derived from SSJ/5 spectra using monoenergetic emission yields. The median ratio was found to be 2.83 for proton aurora, suggesting the need for significant increases in currently used LBH proton/H-atom impact cross sections. A key step in their analysis was extrapolation of SSJ/5 spectra above 30 keV. Limited testing of this algorithm using NOAA Polar Orbiting Environmental Satellites Total Energy Detector and Medium Energy Proton and Electron Detector (TM) data found no significant bias. This work reports on a more detailed investigation of the algorithm's performance, also using TM data, and has uncovered a bias that reduces the median column emission rates (CER) ratio to 1.75. Within expected uncertainties, including calibration, this still calls for cross section increases but to a lesser extent. The discovered bias becomes apparent with CER thresholding that was overlooked during testing by Knight et al. (2008). Thresholding at 400 Rayleighs (R) is necessary since Knight et al. excluded CERs < 400 R in deriving their median ratio. We show that the algorithm's performance degrades with increasing energy flux of the precipitation. A method is reported for eliminating most of the bias which utilizes auroral Ly α, whose emission strength is closely coupled to spectral hardness.

Climatology of planetary wave type oscillations with periods of 2–20 days derived from O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; atmospheric and OH(6-2) airglow observations at mid-latitude with SATI

Abstract. The presence of planetary wave type oscillations at mid-latitudes in the mesosphere/lower thermosphere region has been investigated using airglow observations. The observations were taken with a Spectral Airglow Temperature Imager (SATI) installed at Sierra Nevada Observatory (37.06° N, 3.38° W) at 2900 m height. Airglow data of the column emission rate of the O2 Atmospheric (0-1) band and of the OH Meinel (6-2) band and deduced rotational temperatures from 1998 to 2007 have been used in this study. From these observations a climatology of planetary wave type oscillations at this location is inferred. It has been found that the planetary wave type oscillations of 5-day period is predominant in our data throughout the year, with activity greater than 50% during March/April and October/November months. The planetary wave type oscillations of 2-day period is predominant during both solstices, being predominant during winter solstice in O2 while a 10-day oscillation appears throughout the year with activity around 20% and with maximum activity during spring and autumn equinoxes. The 16-day oscillation has maximum occurrence during autumn-winter while its activity is almost disappeared during spring-summer. No clear seasonal dependence of the amplitude of the planetary wave type oscillations was observed in the cases considered in this study. The waves simultaneously detected in the rotational temperatures deduced from both OH and O2 emissions usually show an upward energy propagation and are affected by dissipation processes.

Wave perturbations in the MLT at high northern latitudes in winter, observed by two SATI instruments

To investigate the Mesosphere and Lower Thermosphere (MLT) region, several ground-based instruments called SATI (Spectral Airglow Temperature Imager) were designed and built to measure airglow emission and temperature in the upper mesosphere. One SATI instrument was installed at Resolute Bay (74.7°N, 94.9°W) and has monitored the polar MLT region since November, 2001. In October 2007 another SATI instrument was installed at Eureka (80.0°N, 86.3°W) at the Polar Environment Atmospheric Research Laboratory (PEARL) as part of the Canadian Network for the Detection of Atmospheric Change (CANDAC) project. SATI is a spatial scanning Fabry–Perot spectrometer measuring column emission rates for several rotational lines of OH and O 2 airglow at 87 and 94 km height. The rotational temperatures are inferred from the ratios of these lines. The measurements are divided into 12 sectors with an annular field of view. The phase differences between the sectors yield information on the horizontal atmospheric wave direction and wavelength. Horizontal perturbations of 2–8 h period have correlatively been observed and investigated at both locations. Short-periodic oscillations identified as gravity waves with periods between 2 and 8 h propagate in southward and eastward directions, but in opposite directions in some cases. The wave propagation characteristics are often different at the two locations; the relationship with the lower mean wind is considered.

Geospace imaging using Thomson scattering

We evaluate the potential of imaging for the first time, the near-earth space plasma environment seamlessly from the ionosphere through the magnetosphere by remotely sensing Thomson scattering of solar visible light by geospace electrons. Using state of the art first principles models of the magnetosphere/ionosphere system, we show that the column emission rates are weak, generally less than 10 Rayleighs, but detectable with currently available instrument technology recently deployed for heliospheric imaging. We demonstrate that distinct features such as the bow shock, magnetosheath and magnetopause are detectable in synthetic images simulated using modified solar coronagraphs and white light imagers, providing that the large background signals are properly quantified. The availability of global geospace images of the electron concentration will enable major advances in our understanding of how Earth's near-space environment responds as a coupled system to changing solar forcings. Such images are expected to play a central role in space weather assessment and forecasting, from which significant capabilities will accrue, much as the imaging of the Earth's surface and lower atmosphere has advanced understanding and forecasting of tropospheric weather.

Evidence for significantly greater N2 Lyman‐Birge‐Hopfield emission efficiencies in proton versus electron aurora based on analysis of coincident DMSP SSUSI and SSJ/5 data

The launch of the Defense Meteorological Satellite Program (DMSP) satellite F16 in 2003 provided the first opportunity to analyze extensive sets of high‐quality coincident auroral particle and FUV data obtained by the onboard sensors Special Sensor Ultraviolet Spectrographic Imager (SSUSI) and Special Sensor Auroral Particle Sensor (SSJ/5). Features of interest are Ly α (121.6 nm), Lyman‐Birge‐Hopfield short (LBHS, the SSUSI 140–150 nm channel), and Lyman‐Birge‐Hopfield long (LBHL, 165–180 nm). We report on comparisons of column emission rates (CERs) by deriving simulated SSUSI values using SSJ/5 electron and ion (treated as proton) spectra. Field‐line tracing is performed to determine the locations of coincidences. CERs are obtained by integrating the products of particle spectra and monoenergetic emission yields. A technique is reported for deriving these yields from nonmonoenergetic CERs obtained by our particle transport model. SSJ/5 ion spectra are extrapolated above 30 keV using a statistical representation based on Polar Orbiting Environmental Satellites particle data. Key quantities of interest are ratios of SSUSI to SSJ/5‐based CERs (S‐S ratios) and corresponding ratios of proton‐produced to total emission (unity for Ly α and from 0 to 1 for LBHS and LBHL). SSJ/5‐based CERs are used to derive the latter ratios. Median ratio values are determined in order to reduce the error budget to primarily calibration and model errors. The median LBH S‐S ratios increase by a factor of ∼2.5 from electron to proton aurora and support significantly higher proton LBH emission efficiencies (3 times the electron efficiencies) assuming reported calibration uncertainties. This calls for significant increases in proton and/or H‐atom LBH cross sections. In turn, FUV auroral remote‐sensing algorithms must explicitly address both electron and proton aurora.

Tidal variations of O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; Atmospheric and OH(6-2) airglow and temperature at mid-latitudes from SATI observations

Abstract. Airglow observations with a Spectral Airglow Temperature Imager (SATI), installed at the Sierra Nevada Observatory (37.06° N, 3.38° W) at 2900-m height, have been used to investigate the presence of tidal variations at mid-latitudes in the mesosphere/lower thermosphere region. Diurnal variations of the column emission rate and vertically averaged temperature of the O2 Atmospheric (0-1) band and of the OH Meinel (6-2) band from 5 years (1998-2003) of observations have been analysed. From these observations a clear tidal variation of both emission rates and rotational temperatures is inferred. It is found that the amplitude of the daily variation for both emission rates and temperatures is greater from late autumn to spring than during summer. The amplitude decreases by more than a factor of two during summer and early autumn with respect to the amplitude in the winter-spring months. Although the tidal modulations are preferentially semidiurnal in both rotational temperatures and emission rates during the whole year, during early spring the tidal modulations seem to be more consistent with a diurnal modulation in both rotational temperatures and emission rates. Moreover, the OH emission rate from late autumn to early winter has a pattern suggesting both diurnal and semidiurnal tidal modulations.

Seasonal variations of O&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; atmospheric and OH(6−2) airglowand temperature at mid-latitudes from SATI observations

Abstract. More than 3 years of airglow observations with a Spectral Airglow Temperature Imager (SATI) installed at the Sierra Nevada Observatory (37.06°N, 3.38°W) at 2900m height have been analyzed. Values of the column emission rate and vertically averaged temperature of the O2 atmospheric (0–1) band and of the OH Meinel (6–2) band from 1998 to 2002 have been presented. From these observations a clear seasonal variation of both emission rates and rotational temperatures is inferred at this latitude. It is found that the annual variation of the temperatures is larger than the semi-annual variation, while for the emission rates the amplitudes are comparable. Key words. Atmospheric composition and structure (airglow and aurora; pressure density and temperature; instruments and techniques)

Time‐dependent transport of field‐aligned bursts of electrons in flickering aurora

We have developed a time‐dependent auroral electron transport model to study emission rates caused by field‐aligned bursts of electrons (FABs) seen in flickering aurora. We simulate flickering FABs by turning on and off a downward electron intensity distribution at a given frequency. We assume this electron beam originates and is modulated at an altitude of 4000 km. We apply collisionless transport from 4000 to 600 km and solve a time‐dependent Boltzmann equation below 600 km. Because FABs have significant flux over a large energy range, dispersion has the most important effect on the resulting emission rates. We find that for a 5 Hz flickering FAB, the column emission rate varies 93% from peak to valley, whereas for 100 Hz flickering, the variation in column emission rate is only 12% from peak to valley. This variation is dependent on the frequency and source altitude. We show that with a time‐dependent transport calculation and a filtered fast photometer or imager looking in the zenith, it is possible to obtain an upper limit on the altitude from which the optical flickering originates. We also study what electron detectors on a rocket or satellite might measure in the lower ionosphere when there exists field‐aligned bursts of electrons. Velocity dispersion calculations will give source altitudes much lower than is correct if they are derived from low energy electrons (&lt;2 keV) measured at altitudes below 150 km. Our results agree with the interpretation that field‐aligned bursts are a temporal rather than spatial feature, and from this knowledge it should be possible to reconstruct the initial electron distribution function at the source altitude.

The accuracy and sensitivity of infrared differential absorption lidar measurements of hydrocarbon emissions from process units

Shell Research Ltd has been using an infrared DIAL (differential absorption lidar) facility for two and a half years to measure hydrocarbon emissions to the atmosphere from petroleum industry process units. This paper describes the procedures used for measurement, emission rate calculation and data display, and then discusses the factors that affect the accuracy and detection limits of column content and emission rate measurements under practical operating conditions.

Nightglow zenith emission rate variations in O(1S) at low latitudes from wind imaging interferometer (WINDII) observations

Volume emission rate data of the O(1S) nightglow layer measured by the wind imaging interferometer (WINDII) aboard the Upper Atmosphere Research Satellite (UARS) are vertically integrated to give equivalent zenith column emission rates. The data are restricted to tropical latitudes and categorized according to season. The migrating tidal signature is extracted by plotting zonally averaged data as a function of local time. The main features are a premidnight maximum at the equator during equinox intervals and a midnight maximum at 20°N and S. The two equinoxes are not equivalent, nor is the behavior identical in the two hemispheres. Winter periods show little or no well‐defined tidal signature. Longitudinal variations at fixed local time were also studied. These variations are larger than the tidal signature and are geographically random, showing no consistent identification with land masses. Some of these variations could be of planetary scale, while others are probably long‐wavelength gravity waves, but in any case they were seen to extend over 10° of latitude at least. Zenithal emission rate measurements exceeding 500 R occurred at the coincidence of these longitudinal wave structures and the tidal maxima. The suggestion is put forward that much of the perceived tidal variability in the tropical lower thermosphere may be explained by such wave‐tide superpositions.

Statistics of gravity waves seen in O2 nightglow over Bear Lake Observatory

Fluctuations in the O2 nightglow column emission rate and rotational temperature were measured over Bear Lake Observatory on 63 nights, 11 of them nearly coinciding with a mesosphere‐lower thermosphere coupling study (MLTCS) campaign on January 20–30, 1993. The instrument allows for simultaneous measurement at 12 points in the sky every 2.4 min. From these the frequency, the horizontal wavelength, and the propagation direction can be measured directly, and from these the horizontal phase velocity and the vertical wavelength can be derived. Frequency spectra are shown to have slopes near to the widely accepted −5/3 law. Correction to intrinsic frequency was made by using the simultaneously acquired winds from the satellite‐borne wind imaging interferometer (WINDII). The relation between frequency and horizontal wavenumber was scattered rather than clearly linear, and there was no obvious boundary suggesting severe mesospheric filtering when the vertical wavenumber was plotted against the horizontal. Zonal phase speeds were favored during the January period, but on average the waves were isotropic at speeds above 30 m/s. Absorption of westward traveling waves at speeds less than 30 m/s was evident.

Thermospheric O I 844.6‐nm emission in twilight

The thermospheric O I 844.6‐nm column emission rate was measured over Toronto, a midlatitude station, in the autumn of 1991 using an imaging Fabry‐Perot spectrometer. Twilight decay curves were measured on four clear evenings when the solar zenith angle was between 95° and 104°, giving corresponding column emission rates between 874 R and 130 R at 20° elevation angle in the azimuth of the Sun. The expected decay curves were calculated from the field line interhemisperic plasma model assuming only photoelectron impact excitation as the production mechanism with a cross section appropriate to an optically thin atmosphere. The agreement was good when the solar and geomagnetic activity levels were low to moderate, but the emission rate was overestimated during high activity periods. The comparison indicates that the photoelectron impact mechanism with a thin‐atmosphere cross section is sufficient to explain the twilight decay of the thermospheric O I 844.6‐nm emission.

Imaging Fabry–Perot spectrometer for twilight observations

An imaging Fabry-Perot spectrometer specifically designed for ground-based studies of thermospheric photochemistry in twilight has been built, tested, and successfully applied to the measurement of the atomic-oxygen O I 844.6-nm emission. It employs a narrow-band interference filter in series with a stabilized étalon with fixed-plate separation and a 512 × 512 pixel bare CCD, and it achieves a bandpass of 0.02 nm. A least-squares data-reduction technique is shown to be effective in extracting the target feature from the surrounding background continuum. Data from one evening show slant column emission rates of 600 to 130 R.

Simultaneous optical observations of long-period gravity waves during AIDA '89

Ground-based optical instrumentation supported the AIDA '89 wind measurement comparisons by describing the gravity waves affecting the 80–100 km altitude region during clear dark hours over Puerto Rico. This study tabulates the characteristics of gravity waves with fractional column emission rate amplitudes up to 30% and with periods greater than 45 min as seen in the O 2 airglow layer by MORTI, a sensor of O 2 rotational temperature and column emission rate in twelve look directions. Data from seven other sensors operating at Guanica and the Arecibo Observatory are then compared with the MORTI data to check the consistency of the entire data set with the wave parameters, primarily velocities, deduced from MORTI. Nine nights of visually distinct crests and troughs were found, one of which was dominated by an evanescent wave and the rest by internal waves. The nights of 5/6 April and 4/5 May 1989 were selected for multi-sensor comparisons. The comparisons showed substantial agreement between the MORTI characterizations and the observations by others, and most differences were attributed to complexities introduced by higher frequency components with shorter coherence distances. Nightly summaries of the O 2 rotational temperature and column emission rate are also given.

Far ultraviolet remote sensing of ionospheric emissions by Polar BEAR

A few thousand far-ultraviolet images of the ionosphere have been obtained since December 1986 by the Atmospheric/Ionospheric Remote Sensor (AIRS) aboard the Polar BEAR satellite. Fast algorithms for applying automated satellite-attitude, geometric, and photometric corrections to these images were developed, and the first results are discussed. A software package that is based on these algorithms was implemented and tested. Since no attitude data is available at night (one of the two gauges is a Sun sensor), an algorithm was devised to extrapolate daytime attitude corrections to nighttime. An additional method is offered to correct satellite roll from the measured or extrapolated one based on the dayglow limbs observed at the edges of the image. Geometric rectifications include transformations to the image from the satellite coordinate system to a reference system, and from the reference system to the geographic coordinate system. Radiometric corrections include image enhancement, background airglow subtraction, and off-nadir normalization of auroral emissions. The latter two are based on theoretical calculations of column emission rates, derived by radiative transfer integrations of volume emission rates along the look direction. Results include mapping of auroral arcs in the corrected geomagnetic system, the one most applicable to auroral studies.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Calculated ionization rates, ion densities, and airglow emission rates due to precipitating electrons in the nightside ionosphere of Mars

The calculations presented in this paper clearly establish that the electron fluxes measured by the HARP instrument, carried on board Phobos 2, could cause significant electron impact ionization and excitation in the nightside atmosphere of Mars, if these electrons actually do precipitate. The calculated peak electron densities were found to be about a factor of 2 larger than the mean observed nightside densities, indicating that if a significant fraction of the measured electrons actually precipitate, they could be the dominant mechanism responsible for maintaining the nightside ionosphere. The calculated zenith column emission rates of the O I 5577‐Å and 6300‐Å and CO Cameron band emissions, due to electron impact and dissociative recombination mechanisms, were found to be significant.