Articles | Volume 41, issue 1
Regular paper
06 Jan 2023
Regular paper |  | 06 Jan 2023

The altitude of green OI 557.7 nm and blue N2+ 427.8 nm aurora

Daniel K. Whiter, Noora Partamies, Björn Gustavsson, and Kirsti Kauristie

We have performed a large statistical study of the peak emission altitude of green O(1D21S0) (557.7 nm) and blue N2+ 1 N (427.8 nm) aurora using observations from a network of all-sky cameras stationed across northern Finland and Sweden recorded during seven winter seasons from 2000 to 2007. Both emissions were found to typically peak at about 114 km. The distribution of blue peak altitudes is more skewed than that for the green, and the mean peak emission altitudes were 114.84 ± 0.06 and 116.55 ± 0.07 km for green and blue emissions, respectively. We compare simultaneous measurements of the two emissions in combination with auroral modelling to investigate the emission production mechanisms.

During low-energy electron precipitation (< 4 keV), when the two emissions peak above about 110 km, it is more likely for the green emission to peak below the blue emission than vice versa, with the difference between the two heights increasing with their average. Modelling has shown that under these conditions the dominant source of O(1S), the upper state of the green line, is energy transfer from excited N2 (A3Σu+), with a rate that depends on the product of the N2 and O number densities. Since both number densities decrease with higher altitude, the production of O(1S) by energy transfer from N2 peaks at lower altitude than the N2 ionisation rate, which depends on the N2 number density only. Consequently, the green aurora peaks below the blue aurora.

When the two emissions peak below about 110 km, they typically peak at very similar altitude. The dominant source of O(1S) at low altitudes must not be energy transfer from N2, since the rate of that process peaks above the N2 ionisation rate and blue emission due to quenching of the long-lived excited N2 at low altitudes. Dissociative recombination of O2+ seems most likely to be a major source at these low altitudes, but our model is unable to reproduce observations fully, suggesting there may be additional sources of O(1S) unaccounted for.

1 Introduction

The emission height of the aurora is often used in the derivation of other auroral or ionospheric parameters, such as horizontal spatial scales, velocities, or electron precipitation energies (e.g. Kaila1989; Knudsen et al.2001; Kalmoni et al.2017), or as a height of other measurements such as neutral wind and temperature (e.g. Griffin et al.2006; Kosch et al.2011; Billett et al.2020). Usually an assumption must be made about the height of the aurora. These assumptions are typically based on observations conducted using instruments and techniques which are primitive by modern standards (Störmer1916; Harang1951; Störmer1955). Often, earlier work measured the “lower border” of an auroral arc, which is dependent on the instrument sensitivity (as pointed out by Boyd et al.1971 and Whiter et al.2013), and therefore results using such methods should be treated with caution. The present study was motivated by the need to improve estimates of the auroral peak emission height and utilises tens of thousands of camera images for which a single peak emission height has been derived for each pair of coincident auroral images, enabling a large statistical study. In this work we analyse two of the brightest (and most observed) auroral emissions: the OI 557.7 nm “green line” and the N2+ 427.8 nm “blue line”.

The auroral green line is an atomic oxygen emission, from the O(1D21S0) transition, while the blue line is a feature of the N2+ First Negative (1 N) band system (B2Σu+X2Σg+). However, despite coming from species with different number density altitude profiles, it has been known for some decades that the green and blue auroral emissions have more similar height profiles than their parent species would suggest (Henriksen and Egeland1988). This similarity is thought to be because the usual primary route by which O(1S) is produced in aurora is energy transfer from electronically excited N2 in the A3Σu+ state (Henriksen1973; Sharp and Torr1979; Burns and Reid1984; Gerdjikova and Shepherd1987; Gattinger et al.1996) (we hereafter abbreviate to N2(A)). However, there are several other pathways producing O(1S) in the aurora, including dissociative recombination of O2+, electron impact excitation of atomic O, and electron impact dissociation of O2, and the balance of these (and other) pathways under different electron precipitation conditions is not yet well understood. In a companion study to this one, Partamies et al. (2022) investigate variation in the peak emission height of green OI 557.7 nm aurora with magnetic local time (MLT) and solar wind driving, over both Lapland (nightside auroral oval) and Svalbard (dayside cusp, nightside poleward edge of oval). Here we present average peak heights for both emissions over Lapland, before examining the difference in height between coincident green and blue auroral emission in combination with modelling to investigate the emission production mechanisms. We examine statistics compiled from individual time instants treated as independent measurements; the temporal evolution of auroral heights, for example, over the course of specific events, will be considered in a separate study.

Section 2 describes the imaging instrumentation used in this work and briefly summarises the method of determining auroral peak emission heights developed by Whiter et al. (2013). Observational results are presented in Sect. 3. Section 4 explains how the green and blue line emissions are modelled, and a detailed comparison of model results with observations is presented in Sect. 5.

Table 1Locations (geographic latitude and longitude) of the camera stations used in this work.

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2 Instrumentation and image analysis

This work uses images from five ground-based all-sky cameras stationed across northern Finland and Sweden, which form part of the Magnetometers–Ionospheric Radar–All-sky Cameras Large Experiment (MIRACLE) instrument network (Syrjäsuo et al.1998). The geographic locations of the stations used in this work are listed in Table 1. The fields of view of all of the cameras overlap, allowing for determination of the auroral peak emission height using any pair of stations. The cameras operate automatically throughout the winter, producing many tens of thousands of images per station per year. From 1996 to 2007, MIRACLE primarily used charge-coupled device (CCD) detectors equipped with an image intensifier (ICCD cameras) and narrow-passband interference filters to select specific auroral emissions, mounted within a filter wheel. This work has used images of OI 557.7 nm emission (green), which is the most dominant auroral emission, and N2+ 1 N 427.8 nm emission (blue), recorded with the ICCD cameras. Green images are available for the full time period of ICCD observations, whereas blue images are only available from September 2000 onwards. Only images recorded when the solar zenith angle at the camera station was larger than 101 and the IL index (a local analogue of the AL index computed from MIRACLE magnetometer data; Kauristie et al.1996) was less than 50 nT are used in this study, to exclude conditions of sunrise and sunset and to ensure that there was auroral activity.

All MIRACLE ICCD camera images recorded between 2000 and 2007 (the period when at least two mainland stations were available and both green and blue emissions were measured) have been analysed using two fully automated independent methods for computing the auroral peak emission height. These methods have been described in detail by Whiter et al. (2013). Both methods analyse a pair of coincident images from different stations with overlapping fields of view. One of the methods (the “field line method”) involves projecting all-sky images onto many geomagnetic field lines, while the other method (the “horizontal plane method”) involves projecting the images onto planes parallel to the Earth's surface at different heights. Since the aurora is typically taller than it is wide, in general the field line method gives a more accurate measure of the peak emission height, and all height data shown in this work were obtained using the field line method with a precision of 0.5 km. However, to ensure measurements are reliable, only image pairs for which the two methods agree to within 20 km are included in the statistical analysis. Of these, there are 57 907 simultaneous measurements of the height of both green and blue emissions from the same station pair (since the cameras use filter wheels to select each individual emission, there is approximately 2 s between the beginning of the green exposure and the beginning of the following blue exposure, which is comparable to the exposure time and short enough to consider the images to be simultaneous). The uncertainty of each individual height measurement is difficult to determine and varies with type of aurora and geometrical properties of the aurora but is estimated to be ± 1–10 km. Importantly, Whiter et al. (2013) showed that the field line method is not biased towards high or low altitudes, so the uncertainty is not systematic. A degradation of the instruments' image intensifiers would lead to a decrease in the minimum observable radiance, but the methods used here to find the peak emission height do not depend on absolute intensity, and therefore it is possible to combine images from different instruments without significant bias.

Figure 1Histograms of observed peak emission heights for the green 557.7 nm emission (grey shading) and blue 427.8 nm emission (black outline).


Figure 2The peaks of the histograms as shown in Fig. 1, together with Gaussian curves fitted to the histograms for the 557.7 nm emission (green) and 427.8 nm emission (blue). The vertical green and blue lines mark the peaks of the Gaussians.


3 Observational results

Figure 1 shows histograms of measured peak emission heights of green 557.7 nm aurora (shaded grey) and blue 427.8 nm aurora (black line), with a bin width of 1 km. Only measurements at times when height estimates of both emissions are available are included in the histograms, in order to allow for a fair comparison. Both distributions peak close to 114 km and have very similar tails towards lower peak emission heights. However, the distribution of green heights has a steeper tail towards greater peak emission heights than the distribution of blue heights. These histograms show that the blue line is almost twice as likely (factor 1.8) to peak at 140 km as the green line, although it is rare for either of these emissions to peak at such high altitudes. The median peak emission heights for green and blue aurora are 114.0 and 115.0 km respectively (to nearest 0.5 km), and the corresponding mean heights are 114.84 ± 0.06 km and 116.55 ± 0.07 km, suggesting that the blue line emission peaks slightly above the green line emission. In order to find the maxima of the distributions, a Gaussian function was fitted to each of the histograms plotted in Fig. 1 within the range 105.0–121.5 km (centred roughly on the peak of the distributions). Figure 2 shows the two histograms within this range, together with the fitted Gaussians in green (for 557.7 nm emission) and blue (for 427.8 nm emission). The coloured vertical lines within this figure show the maxima of the Gaussian functions, at 113.8 km for green aurora and 113.5 km for blue aurora. These results show that green and blue aurora peak at very similar altitudes, on average, but that it is more likely for blue to peak at high altitudes (above about 130 km) than green, contrary to a common misconception that blue peaks below green.

Figure 3Two-dimensional histogram of simultaneous green (557.7 nm) and blue (427.8 nm) peak emission heights. The shading represents the number of observations in each histogram bin, according to the colour bar scale shown on the right of the figure. The dashed black line indicates where the two peak emission heights are equal.


In order to compare the peak emission heights of coincident green and blue aurora, a two-dimensional histogram was created, shown in Fig. 3. The bin width is 1 km for both green aurora (ordinate) and blue aurora (abscissa), and each 1 km× 1 km square is shaded according to the colour scale on the right of the figure to indicate the number of simultaneous height measurements of green and blue aurora within the ranges of that bin. The dashed diagonal line indicates where the two emissions peak at the same height (y=x line). The bulk of the distribution between 105 and 120 km closely follows this line, indicating that in general the two emissions peak at approximately the same altitude, as expected from the results presented above. At greater heights the majority of the distribution is below the dashed line, indicating that when the blue emission peaks at relatively high altitude, the green emission peaks below it.

Figure 4Two-dimensional histogram of auroral peak emission height observations binned according to the difference between green and blue heights (ordinate, positive values indicating green height above blue height) and mean (i.e. midpoint) of the simultaneous green and blue heights (abscissa). The shading represents the number of observations in each bin, normalised to the total number of observations in each column, according to the colour bar scale. The mean height difference for each column is shown as a white plus symbol.


This result can be seen more clearly by plotting the difference in height between the green and blue emissions against the mean height (i.e. midpoint between the green and blue heights). Figure 4 shows a two-dimensional histogram with the height difference (positive values indicating that the green emission peaks above the blue emission) on the ordinate axis and the mean height on the abscissa axis. The histogram bins have a width of 2 km in each direction, and the colour scale indicates the number of image pairs in each bin, normalised by the total number of image pairs in each column of the histogram. The white plus symbols show the mean height difference for each column of the histogram. As expected from the results shown in Fig. 3, the height difference is close to zero for mean heights up to about 120 km. As the mean height increases beyond 120 km, the average height difference becomes more negative, indicating that the blue emission peaks further above the green emission. The mean height was above 120 km in 34 % of our observations.

3.1 Resonance scattering of sunlight by N2+

In sunlit conditions, N2+ 1 N emission is produced through resonance absorption of solar photons by N2+ molecules in the ground state (Bates1949; Broadfoot1967). The resonance scattering contribution to the total blue line intensity can be significant and typically occurs at higher altitude than the auroral emission produced by electron impact excitation (Jokiaho et al.2009). The auroral emission is prompt and produced at the point of ionisation and therefore is related to the altitude profile of the neutral N2 density. Blue line emission originating through resonance scattering is instead related to the N2+ ion density and so is enhanced by soft electron precipitation producing ground-state N2+ above the shadow height and may also be influenced by ion transport.

To investigate whether the observed high peak altitude of the blue line could be caused by resonance scattering of sunlight, histograms were created for the solar zenith angle and MLT at the time of the observations. Data were selected where the blue line peaked above 130 km and at least 10 km above the coincident green line peak emission height, consisting of 8.7 % of the total height measurements. Histograms were made separately for this selected data and all other unselected data, shown in Fig. 5 (solar zenith angle) and Fig. 6 (MLT). The solar zenith angle and MLT were calculated for the central latitude and longitude of the aurora, defined to be on the field lines used in the calculation of the peak emission height, rather than for the observation stations, to improve accuracy.

Figure 5Histogram of the solar zenith angle at the time of the peak emission height measurements. The black line shows selected data (blue height above 130 km and at least 10 km above the green height) using the right ordinate axis, and the grey shading shows all remaining unselected data using the left ordinate axis.


Figure 6Histogram of the magnetic local time of the peak emission height measurements. The black line shows selected data (blue height above 130 km and at least 10 km above the green height) using the right ordinate axis, and the grey shading shows all remaining unselected data using the left ordinate axis.


These histograms show that the high blue peak emission heights do not preferentially occur for low solar zenith angles or close to dawn or dusk and therefore are not likely to be caused by resonance scattering of sunlight (or by daylight contamination on the horizon). The selected data are more centred around midnight MLT than the unselected data, but the two histograms shown in Fig. 6 are not substantially different. See the companion paper by Partamies et al. (2022) for further discussion of the variation in height with MLT.

4 Auroral modelling

We use the Southampton ionospheric model as a tool to understand the observational results presented in Sect. 3. This model is a coupled electron transport (Lummerzheim and Lilensten1994) and time-dependent ion chemistry model (Lanchester et al.2001) which calculates production and loss rates for various ion and neutral species as well as brightnesses of specific auroral emissions (Lanchester and Gustavsson2012; Whiter et al.2012). The inputs to the model are an arbitrary electron precipitation spectrum and density profiles for the major neutrals (O, N2, and O2) taken from the NRLMSIS 2.0 model (Emmert et al.2021), hereafter referred to as MSIS. In this work we use Gaussian- or Maxwellian-shaped electron precipitation spectra, as has been standard in auroral physics (e.g. Strickland et al.1993; Tanaka et al.2006). As input to MSIS we use median conditions during the observations presented in Sect. 3, of Ap =17, F10.7 =129, 81 d average F10.7 =129, and a time of 21:30 UT at Kilpisjärvi (69.02 N, 20.79 E).

The brightness of the blue line is calculated directly from the production rate of N2+ through electron impact ionisation of neutral N2. Since the emission is prompt, its brightness can be calculated directly from the ionisation–excitation rate. The lifetime of the N2+ 1 N(0,1) band is 270 ns, as obtained from the Einstein coefficient given by Gilmore et al. (1992) (note this is long compared to the lifetime of the overall N2+ 1 N band system but is appropriate for the 427.8 nm emission), which is too fast for quenching to have a significant effect on the peak emission height. Using quenching rate coefficients for the N2+ B2Σu+ state from Valk et al. (2010), together with N2 and O2 densities from the model and the quenching equation given by Vallance Jones and Gattinger (1978), we estimate less than 1 % of the blue emission is quenched at 90 km altitude.

Green and Stolarski (1972)LeClair and McConkey (1993)Sheehan and St.-Maurice (2004)Kella et al. (1997)Gattinger et al. (1985)Piper (1982)Gattinger et al. (1996)

Table 2Reactions producing O(1S) included in the Southampton ionospheric model.

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The 557.7 nm green line is long-lived, and therefore quenching is important, and so its intensity is calculated from the number density of the O(1S) upper state with an Einstein coefficient of 1.26 s−1 from Wiese et al. (1996). The reactions that produce O(1S) included in the model are listed in Table 2, together with references for their cross sections, rate coefficients, and branching ratios, where relevant. Reactions (R1) and (R2) represent auroral electron impact on O and O2, respectively. Reaction (R3) is dissociative recombination of O2+, and Reaction (R4) is an O2+ loss reaction with atomic N. Reactions (R5) and (R6) represent energy transfer to atomic O from N2 and O2, respectively. The model includes losses of O(1S) through quenching by O and O2 and losses through emission in 557.7 nm (lower state O(1D)) and 297.2 nm (lower state O(3P)). Further discussion of the rates relevant for the green line emission is included in the following section.

Figure 7Peak emission heights of 557.7 nm aurora (green) and 427.8 nm aurora (blue) as a function of characteristic electron precipitation energy for Gaussian electron spectra, obtained using the Southampton ionospheric model. The difference in these heights is shown in black, using the right ordinate axis.


Figure 8Peak emission heights of 557.7 nm aurora (green) and 427.8 nm aurora (blue) as a function of characteristic electron precipitation energy for Maxwellian electron spectra, obtained using the Southampton ionospheric model. The difference in these heights is shown in black, using the right ordinate axis.


5 Model results and comparison with observations

Figures 7 and 8 show the modelled peak emission heights of the 557.7 nm (green) and 427.8 nm (blue) lines as a function of characteristic electron precipitation energy with a Gaussian (Fig. 7) or Maxwellian (Fig. 8) electron spectrum. The difference in these heights (blue height subtracted from green height) is also shown in black, according to the right ordinate axis in each figure. To determine these relationships, the model was run independently at different characteristic energies with an electron energy flux of 20 mW m−2 and at each energy was run for a period of 12 s in order to reach a reasonably steady state. Qualitatively, the model results resemble the observations presented in Sect. 3, in that the green and blue emissions peak at about the same height across much of the electron energy range tested, and at low energies, when the emissions peak at higher altitudes, the blue emission peaks above the green emission (more negative values on the right ordinate axis).

Figure 9Observed and modelled difference in height between the green 557.7 nm and blue 427.8 nm emissions as a function of their mean height. Observations are shown as plus symbols. Grey symbols show the average height difference over all observations with the same mean height (to the nearest 0.25 km). Black symbols show the same but where observations have been binned into 2 km wide bins of mean height (reproduced from Fig. 4). The blue and red curves show the model results for Gaussian- and Maxwellian-shaped electron precipitation spectra, respectively, where the model is run for 12 s (steady state, solid curves) or for 3 s (dashed curves).


The model also does a good job of quantitatively reproducing the observations. For a quantitative comparison we return to the examination of how the height difference between the two emissions varies as a function of mean peak emission height, introduced earlier in Fig. 4 and continued here in Fig. 9, to avoid the need to estimate the electron precipitation energy during the observations. Since the peak emission heights are measured in the camera data to the nearest 0.5 km, the mean of the coincident green and blue peak heights always has a value divisible by 0.25 km. The average observed height differences calculated for possible values of the mean height (i.e. every 0.25 km) are shown as grey plus symbols in Fig. 9. The average height difference calculated for 2 km wide bins of mean height, as shown as white plus symbols in Fig. 4, is reproduced with black plus symbols in Fig. 9. These black points show less scatter than the grey points since more observations are included in the calculation of each one. The solid blue and red curves represent the model results described above for Gaussian and Maxwellian spectra, respectively, and are essentially the same as the black curves shown in Figs. 7 and 8 with the abscissae transformed from electron precipitation energy to mean height. The dashed blue (Gaussian) and red (Maxwellian) curves show the results of running the model for only 3 s instead of 12 s. We consider the results separately for low- and high-energy precipitation below.

Figure 10Modelled volume emission rate height profiles of the 427.8 nm blue line (solid blue) and 557.7 nm green line (solid green) using a 900 eV Gaussian electron precipitation spectrum. The horizontal lines mark the peaks of the volume emission rate profiles. Also shown are the production rates of O(1S) by energy transfer from N2(A) (dotted green) and electron impact on O (dashed green) and the production rate of N2(A) (dotted black).


5.1 Low-energy electron precipitation

To understand the observations and model results we examined the altitude profiles of the relevant reaction rates and number densities. Figure 10 shows the modelled volume emission rate altitude profiles of the 557.7 and 427.8 nm emissions as solid green and blue lines, respectively, for a Gaussian electron precipitation spectrum at 900 eV. The production rates of O(1S) by the two dominant sources during low-energy precipitation, energy transfer from N2(A) (Reaction R5) and electron impact excitation (Reaction R1), are shown as dotted and dashed green lines, respectively, and the production rate of N2(A) is shown as a dotted black line. The peak emission heights of the two emissions are marked with horizontal lines. This figure highlights that at low electron precipitation energies, the ionisation of N2 peaks at a higher altitude than the production of O(1S) by energy transfer from N2(A), which could seem counter-intuitive given that the ionisation energy of N2 is greater than the threshold energy for excitation to N2(A). The reason is that the N2(A)+O energy transfer rate depends on the product of the atomic O number density and N2(A) number density. Since the O number density decreases to higher altitudes, this product peaks at lower altitude than the N2(A) density and N2+ production rate. Ultimately this is why the blue 427.8 nm emission peaks above the green 557.7 nm emission.

The model results using a Maxwellian electron spectrum best match the average observations above 110 km (dashed red curve in Fig. 9). However, the observations show broad variation in the height difference for high mean heights (see Fig. 4), which is reproduced by varying the model run time and electron spectrum shape. Running the model for a shorter time period means that a steady state is not achieved, in particular regarding the N2(A) and O(1S) number densities, and the spread of observed height differences may reflect the spatial and temporal dynamics of the auroral precipitation.

5.2 High-energy electron precipitation

For both Gaussian and Maxwellian electron spectra, the model produces a green peak emission height above the blue height (i.e. positive height difference in Fig. 9) for high-energy precipitation where the mean peak emission height is below about 110 km, with the height difference increasing away from the observations with increasing energy of precipitation. The median F10.7 index at times where both emissions peak below 105 km is within 0.1 standard deviations of the median F10.7 index for other times, indicating that the low-altitude emissions are primarily a result of energetic precipitation, rather than a contracted atmosphere. At these high energies, the Gaussian spectra produce model results that are more consistent with observations than those for the Maxwellian spectra, which may be an indication that more energetic precipitation (above about 4 keV) is typically more Gaussian in spectral shape, but neither spectral shape reproduces the observations well at the lowest altitudes. The mean height peaked below 110 km in 35 % of our observations.

Figure 11Modelled production rate profiles of O(1S) after 12 s of Gaussian electron precipitation at 25 keV. The reactions producing O(1S) are listed in Table 2. The rates of Reactions (R1), (R2), (R3), (R4), (R5), and (R6) are plotted by solid yellow, dotted green, solid cyan, solid blue, dashed magenta, and dashed red lines, respectively. The total production rate of O(1S) is shown by the dotted black line. The horizontal dashed line marks the peak height of the blue 427.8 nm emission.


Figure 12Modelled peak height of the production rate of O(1S) by dissociative recombination of O2+ (Reaction R3) against the blue 427.8 nm peak emission height (electron impact ionisation of N2). The dashed line marks where the two rates would be equal.


Figure 11 shows altitude profiles of the six O(1S) production mechanisms included in the model, for a Gaussian electron spectrum at 25 keV with a flux of 20 mW m−2 running for 12 s. Auroral electron impacts on O (Reaction R1) and O2 (Reaction R2) are shown as solid yellow and dotted green lines, respectively. Dissociative recombination of O2+ (Reaction R3) is shown as a solid cyan line, and Reaction (R4) between O2+ and N is shown as a solid blue line. Energy transfer from N2(A) (Reaction R5) and O2* (Reaction R6) is shown as dashed magenta and red lines, respectively. The dotted black line shows the total production rate of O(1S), and the horizontal dashed line shows the peak height of the blue 427.8 nm emission. Of the six mechanisms, dissociative recombination of O2+, electron impact on O2, and O2++N all peak at the same height as the 427.8 nm emission. Figure 12 shows the O2+ dissociative recombination rate peak height plotted against the 427.8 nm peak emission height for Gaussian electron precipitation. The dashed line represents equal rates (y=x). The two rates peak at very similar altitudes through the E-region ionosphere, especially below 105 km.

O'Neil et al. (1979) used rocket-based optical observations of artificial aurora with modelling to conclude that dissociative recombination of O2+ (Reaction R3) is the dominant production mechanism of O(1S) below 96 km, with energy transfer from N2(A) (Reaction R5) dominating from 100 km to 116 km (the upper altitude limit of their measurements). Our work and observations support this conclusion, except that in our model the production of O(1S) by dissociative recombination of O2+ is too slow compared to energy transfer from N2(A) at low altitudes. We currently do not have a solution to this discrepancy, but in the following subsections we consider whether the modelled Reaction (R3) is too slow, Reaction (R5) is too fast, or there is an O(1S) production mechanism missing from the Southampton ionospheric model.

5.2.1 O(1S) from dissociative recombination of O2+

For dissociative recombination of O2+ the model uses a quantum yield of O(1S) of 0.05 from Kella et al. (1997), with a temperature dependent rate coefficient from the review by Sheehan and St.-Maurice (2004). The rate coefficient is given by

(1) 1.95 × 10 - 13 ( T e / 300 ) - 0.70 m 3 s - 1 ,

where Te is the electron temperature in kelvin. This rate coefficient was measured by Mehr and Biondi (1969) and agrees exactly with that found by Alge et al. (1983). Peverall et al. (2001) found a rate 23 % larger (with the same temperature dependence), although their quantum yield of O(1S) was lower than that of Kella et al. (1997) (but with an electron energy dependence). There is little evidence in the literature for a substantially larger rate coefficient than this. Abreu et al. (1983) used observations of airglow to determine the quantum yield of O(1S) and found values between 0.09 and 0.23, with a dependence on the ratio of electron density to O density (greater yield with larger ratio) from which they inferred a dependence on the vibrational excitation of the O2+. This vibrational dependence was also found in laboratory measurements by Petrignani et al. (2005), who found quantum yields of 0.06, 0.14, and 0.21 for vibrational levels v=0, 1, and 2 respectively. The Southampton model does not track individual vibrational levels of O2+, but the O(1S) quantum yield could be increased from 0.05 to reflect the average value across an assumed vibrational distribution. Using a value of 0.2 in the model does improve the fit to observations below 110 km, although the modelled height difference is still larger than observations below 100 km.

The production rate of O(1S) through dissociative recombination (Reaction R3) could also be underestimated as a consequence of an underestimated O2+ density, either because the ionisation rate is too slow or because the O2 mixing ratio is too low. We note that the N2 and O densities were reduced in the latest version of MSIS, but Emmert et al. (2021) conclude that the O2 density is more uncertain due to an absence of suitable data in the middle thermosphere.

5.2.2 O(1S) excited by energy transfer from N2(A)

The production of O(1S) through energy transfer from N2(A) depends not only on the rate of the energy transfer itself, but also on the production and loss rates of the long-lived N2(A). Production of N2(A) is by electron impact excitation, but cascading from the B3Πg state via the First Positive transition is the dominant production mechanism, and B3Πg is itself fed by cascading from the C3Πu and W3Δu states. Since these cascade transitions are prompt, to calculate production of N2(A), the model sums electron impact excitation to the A3Σu+, B3Πg, and W3Δu states, with half of the excitation to the C3Πu state also included. For N2 electron impact excitation, the model uses the “J05M09” cross sections of Yonker and Bailey (2020), which gave a marginally better agreement with our observations than the cross sections of Itikawa (2006). N2(A) is lost through quenching by O2 and O with rates from Gattinger et al. (1996) and Piper (1982), respectively, as well as emission in the Vegard–Kaplan (A–X) band system (Einstein coefficient 0.38 s−1, from Piper1993). Quenching by O2 can indirectly lead to production of O(1S) via O2*, through Reaction R6, although O2* is also quenched. During energetic precipitation (> 10 keV), O(1S) production by energy transfer from N2(A) peaks above the N2+ 1 N (427.8 nm) emission since quenching of N2(A) by O2 at low altitudes raises the peak height of the N2(A) number density. Below the mesopause, the O(1S) production by energy transfer also falls off with the O number density.

For the N2 energy transfer reaction (R5), we use the rate coefficient (2.8×10-17m3s-1molecule-1) and O(1S) quantum yield (0.75) of Piper (1982). The larger (by 25 %) rate coefficient of Thomas and Kaufman (1985) was used by Yonker and Bailey (2020), but using that coefficient in our model would increase the dominance of energy transfer from N2(A) in the production of O(1S) at low altitudes and would therefore make the match to observations worse. Hill et al. (2000) proposed a temperature-dependent rate coefficient for Reaction (R5), although Yonker and Bailey (2020) argued against a temperature dependence. Replacing the rate from Piper (1982) with that from Hill et al. (2000) led to a marginal improvement in the match between the model and observations at high electron precipitation energies but made the match considerably worse at high altitudes for low-energy precipitation.

5.2.3 Possible pathways for producing O(1S) missing from the Southampton ionospheric model

Besides overestimated or underestimated rates, there may be sources of O(1S) completely missing from the Southampton ionospheric model which could explain the discrepancy with our observations at the lowest altitudes. We briefly consider two possibilities here, represented by


Reaction (R7) is thought to occur in the upper atmosphere (e.g. Barth1992), and the production of O(1S) is energetically allowed, but the quantum yield of O(1S) is not yet known (Fox and Hać2018). The Southampton model uses a rate coefficient for this reaction from Lin and Kaufman (1971), with all O being produced in the ground state. Similarly, electron impact dissociative ionisation of O2 in which one of the products is O+ (Reaction R8) is also included in the model (as a distinct reaction to Reaction R2) but not as a source of O(1S). No branching ratio for the production of O(1S) via Reaction (R8) is available.

Figure 13Modelled rate profiles after 12 s of Gaussian electron precipitation at 25 keV. The rates of Reactions (R3), (R5), (R7) and (R8) are plotted by solid cyan, dashed magenta, solid red, and dotted blue lines, respectively. Reactions (R3) and (R5) are production of O(1S) and are exactly as plotted in Fig. 11. Reactions (R7) and (R8) are production of O in any state.


Modelled altitude profiles of the rates of Reactions (R7) and (R8) are plotted in Fig. 13 by solid red and dotted blue lines, respectively, for a Gaussian electron precipitation spectrum at 25 keV with a flux of 20 mW m−2 running for 12 s. Reaction (R3) (solid cyan) and Reaction (R5) (dashed magenta) are reproduced from Fig. 11 for comparison. The rate of Reaction (R7) is several orders of magnitude too small to have a significant effect on the auroral peak emission altitude and peaks at higher altitude than O2+ dissociative recombination, so we discount that reaction. In contrast, the rate of Reaction (R8) is approximately twice the rate of O2+ dissociative recombination (Reaction R3) at the peak and therefore may be significant if the quantum yield of O(1S) is not too low.

Further work is required to evaluate the possible contributions of these, and other, mechanisms to the production of O(1S) in the aurora, ideally including experimental determination of the relevant rate coefficients, cross sections, and quantum yields.

6 Conclusions

The typical peak emission height of both green 557.7 nm aurora and blue 427.8 nm aurora is about 114 km (113.8 km for green and 113.5 km for blue, according to our large set of observations from Lapland in 2000–2007). When the emissions peak below this typical height they peak at a very similar height, with the green very slightly ( 1 km) above the blue, on average. When they peak at higher altitudes, the blue usually peaks above the green.

During low-energy electron precipitation, the dominant mechanism producing the O(1S) responsible for the green emission is energy transfer from N2(A). The green line peaks at lower altitude than the blue line because the production rate of O(1S) (green line) depends on the product of N2 and O number densities, which both decrease with higher altitude, whereas electron impact production of N2+ (blue line) depends on the number density of only N2. The height difference between the two emissions is sensitive to the primary electron spectrum shape as well as the duration of the precipitation but on average reaches about 10 km when the green line peaks at 145 km.

During high-energy precipitation, the dominant mechanism producing O(1S) at low altitudes must not be energy transfer from N2(A). Dissociative recombination of O2+ seems the most likely candidate, but in that case the Southampton ionospheric model underestimates the rate of this process or overestimates the rate of energy transfer from N2(A) to O(1S), or there is a combination of both. We tentatively suggest that electron impact dissociative ionisation of O2+ in which O+ is one of the fragments may be an additional source of O(1S) missing from models, although its inclusion may not be enough to bring the Southampton model in line with our observations, and other mechanisms may also be missing.

A comparison between our model results and observations suggests that the auroral electron precipitation energy spectrum is typically Gaussian when the characteristic energy is above about 4 keV but closer to Maxwellian at lower characteristic energies.

Data availability

MIRACLE ASC quicklook data are available at (Finnish Meteorological Institute2022), and full-resolution image data can be requested from FMI ( The F10.7 data were obtained from Space Weather Canada, Natural Resources Canada (, Space Weather Canada2022). The Ap data were obtained from the GFZ German Research Centre for Geosciences (, Matzka et al.2022).

Author contributions

DKW performed the data analysis. All authors contributed to the discussion of the results and the writing of the paper.

Competing interests

The contact author has declared that none of the authors has any competing interests.


Publisher’s note: Copernicus Publications remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.


The MIRACLE network is operated as an international collaboration under the leadership of the Finnish Meteorological Institute. The authors thank Mikko Syrjäsuo, Sanna Mäkinen, Jyrki Mattanen, Anneli Ketola, and Carl-Fredrik Enell for careful maintenance of the MIRACLE all-sky cameras and data flow. The authors thank Liisa Juusola for providing IL data.

Financial support

This research has been supported by the Natural Environment Research Council (grant no. NE/S015167/1), the Academy of Finland (grant nos. 128553 and 268260), and Norges Forskningsråd (CoE contracts 223252 and 287427).

Review statement

This paper was edited by Keisuke Hosokawa and reviewed by Shin-ichiro Oyama and one anonymous referee.


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Short summary
We measured the height of green and blue aurorae using thousands of camera images recorded over a 7-year period. Both colours are typically brightest at about 114 km altitude. When they peak at higher altitudes the blue aurora is usually higher than the green aurora. This information will help other studies which need an estimate of the auroral height. We used a computer model to explain our observations and to investigate how the green aurora is produced.