Adv. Polar Upper Atmos. Res., 18, 96–104, 2004
Â 2004 National Institute of Polar Research
Research note

Auroral O＋ 732/733 nm emission and its relation to ion upflow
Naoko Koizumi1*, Shoichi Okano1, Takeshi Sakanoi1,
Makoto Taguchi2 and Takehiko Aso2
1

Planetary Plasma and Atmospheric Research Center, Tohoku University, Sendai 980-8578
2
National Institute of Polar Research, Kaga 1-chome, Itabashi-ku, Tokyo 173-8515*
now at NS Solutions Corporation(Received March 3, 2004; Accepted June 10, 2004)

Abstract: Observations of auroral oxygen ion emission at 732/733 nm were made at
the Aurora station in Longyearbyen (78.2°N, 16.3°E; λ m ＝ 74.9°) using an all-sky spectrograph (ASG) during the winter season of 2000/2001. A statistical analysis showed thatthe highest occurrence of oxygen ion auroras at Longyearbyen was seen in the dayside of
the 09–12 MLT region; the intensities of these auroras were also larger than those on thenight side. To study the mechanism of ion upflow in the polar ionosphere, ASG dataobtained on December 7, 2000, was analyzed together with simultaneous ionospheric dataobtained by EISCAT Svalbard radar (ESR). Enhancements of electron temperature andion upward velocity were associated with an increase in the auroral OII intensity at themagnetic zenith. This result suggests that an ambipolar electric field associated with electron temperature enhancement caused by soft electron precipitation may be involved in themechanisms that drive ionospheric ions upward.
key words: aurora, oxygen ion, ion upflow, ESR

1. Introduction
Spectral emission lines at wavelengths of 732/733 nm were first observed by Omholt(1957) and Dufay and Moreau (1957) in low latitude aurora; later, Wallace (1959) identified
the spectral origin of these emission lines as oxygen ions. Oxygen ions in aurora are produced by the direct electron impact and ionization of oxygen atoms, 18% of which are
excited to a 2P excited state (cf. Rees et al., 1982) and undergo a transition from 2P to 2D,
corresponding to the emission at 732/733 nm.
O ＋ e → O＋(2P) ＋ 2e,
O＋(2P) → O＋(2D) ＋ hν (732/733 nm).

(1)
(2)

The 2P and 2D states of the oxygen ions are doublets and the wavelengths of the spectral lines emitted by the reaction (2) are 731.892 nm, 731.999 nm, 732.967 nm, and 733.073
nm, according to the Atomic Spectra Database Line Data published by the National Institute of Standards. The 2P state of an oxygen ion has a lifetime of about 5 s; therefore,96

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quenching dominates at altitudes of lower than 200 km. The OII 732/733 nm emission iseffectively produced by soft electron precipitation because a cross section for reaction (1)peaks at about 100 eV. The intensity of auroral OII 732/733 nm emissions is generally low
and is liable to be contaminated by the nearby N2 first positive band. This characteristic ofauroral OII 732/733 nm emissions makes observations difficult, and an instrument with ahigh wavelength resolution and a high sensitivity in this wavelength region is required.
Smith et al. (1982) observed the OII 732/733 nm aurora using a Fabry-Perot interferometerand a meridian scanning photometer at Longyearbyen in 1982. However, no systematic
observations of auroral OII 732/733 nm emissions, which are needed to describe its occurrence characteristics, have been reported so far. Since the motion of ions in the polar ionosphere is governed by the electric field, auroral OII emissions can be regarded as a visible
tracer for ion motion. The upflow of heavy ions is frequently seen in the polar ionosphere.
However, no direct comparison between ion upflow and auroral OII emissions has been previously reported.
To clarify the occurrence characteristics of the OII aurora, systematic observations
were made at the Aurora station in Longyearbyen (78.2°N, 16.3°E; λ m ＝ 74.9°) using an
all-sky spectrograph (ASG) in the winter season of 2000/2001. Furthermore, to study themechanism of ion upflow in the polar ionosphere, ASG data on December 7, 2000, wasanalyzed together with simultaneous ionospheric data obtained by EISCAT Svalbard radar(ESR).
2. Observations
2.1. Instrument
The ASG used in the present study was developed at the National Institute of Polar
Research, Japan, and installed at the Aurora station in March 2000. Details of the ASG and
its calibration have been previously reported by Taguchi et al. (2002). The ASG was initially constructed as a monochromatic all-sky imager (ASI) that employed interference filters
to select auroral spectral lines. Operation of the ASI was initiated at Syowa Station in
Antarctica in 1998. The ASI was brought back to Japan in 1999 and modified to the ASG
by replacing the interference filter with a grism as a dispersive element. The optical layoutof the ASG is shown in Fig. 1. The optical system is composed of a fast fish-eye lens ( f＝6mm, F1.4), a collimator lens system, a grism with a groove frequency of 600 g/mm, and an
imaging lens system. A slit with a width of 42 µm and a length of 20 mm is located at the
focal plane of the fish-eye lens. The slit defines a field of view of 0.38 degrees by 180
degrees. A back-thinned bare CCD (512 by 512 pixels) placed at a focal plane of the imaging lens can take images in a form such that the zenith angle and wavelength are aligned
along the rows and lines of the CCD, respectively. The CCD is cooled to a temperature
lower than −40°C using a three-stage Peltier cooler to minimize dark noise. The pixel sizeof 24 µm matches the monochromatic images produced by the slit’s width. Along the geomagnetic meridian, the CCD resolves a 180 degree field of view into approximately 500
pixels, making the spatial resolution along the meridian about 0.36 degrees. Wavelength
coverage is set to 447.1–765.7 nm. Wavelength resolution at 732/733 nm is approximately 2
nm, although this parameter varies slightly depending on the direction of the line of sight. A
sensitivity calibration of the ASG was performed at the National Institute of Polar Research

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Fig. 1.

ASG optics. The optical paths are projected on a plane that includes the optical axes of thefore optics and the focusing optics. The meridional plane is perpendicular to this plane.

Fig. 2.

Spectral image obtained by the ASG at 0756 UT on December 8, 2000.
Auroral oxygen green and red lines cover almost the whole sky. The OII
emission can be seen around the zenith. The northernmost field of view was
blocked by a neighboring building.

using a 2-m integrating sphere as a light source in March 2000. The sensitivity at 560 nm
and at zenith was measured to be 0.011 cts/R/s/pixel.
The ASG was installed at the Aurora station in Longyearbyen in March 2000. Routine
observations were started in October 2000. Observations were made with an exposure timeof 15 s every 3 min. Figure 2 shows an example of a spectral image taken at 0756 UT on
December 8, 2000.
2.2. Occurrence and intensity of the OII auroras
The dependence of the occurrence probability and the intensity of the OII auroras on
the MLT was studied using data obtained on 21 clear days during the period from October
31, 2000, through to January 20, 2001. Data obtained on days with cloudy sky conditions
were excluded from this data set. The total number of spectral images was 3157. To avoid
contamination from city lights and twilight, only data within the zenith angles of 60S to
60N were analyzed. With this field of view, the coverage in geomagnetic latitude was λ m ＝

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71.3° to λ m ＝ 78.5°, assuming that the emission height of the OII auroras was 250 km.
Since a multiplet of the OII (2P–2D) transition cannot be resolved with the ASG, signals
over 7 pixels that corresponded to 4.4 nm centered around 732.5 nm were summed and thebackground signals within this pixel bin were subtracted from the summed signal to obtain
the integrated brightness. This integrated brightness is hereafter referred to as the intensityof the OII aurora. In determining the intensities of the OII emissions, the existence of the
nearby N2 first positive and OH bands was taken into account for each spectral image. The
detection threshold for the OII emissions was 30 R. The peak intensity of the OII emissions
in each spectral image was taken as the intensity for that time. Therefore, the dependence of
the OII aurora on the zenith angle could not be evaluated. To evaluate the magnetic local
time dependence of the occurrence probability and the intensity of the OII auroras, the magnetic local time was divided into eight 3-hour sectors and the intensity was classified as lessthan 200 R, 200–300 R, 300–400 R, 400–500 R, 500–600 R, 600–700 R, or greater than
700 R. Figure 3 shows the occurrence probability of OII auroras at Longyearbyen. From
this result, the following points are of interest:
1800–0900 MLT: The occurrence frequency of the OII auroras was 15–20%, and mostof the auroras had an intensity of less than 200 R.
0900–1200 MLT: The occurrence probability of the OII auroras was more than 90%.
Among the observed auroras, 26%, 58%, and 10% had an intensity of less than 200 R,
200–500 R, and greater than 500 R, respectively.
1200–1500 MLT: The occurrence probability of OII auroras with an intensity of lessthan 200 R was 37%, while that of auroras with an intensity of greater than 200 R was 29%.
1500–1800 MLT: The occurrence probability of OII auroras with an intensity of lessthan 200 R was 35%, while that of auroras with an intensity of greater than 200 R was 9%.
These results indicate that most of the OII auroras observed at Longyearbyen appeared
mainly in the dayside and that the intensities of the auroras appearing in the dayside weregreater than those appearing in the night side. However, it should be noted that the data
used in the statistical analysis covered a limited range in geomagnetic latitude and thenightside aurora oval was not fully covered by our observations.
2.3. Simultaneous observation with ESR
During the observation period reported in this paper, the ASG and the 42-m antenna of
the ESR were simultaneously operated on December 7, 2000, and the data obtained by both
the ASG and the ESR were analyzed for relationships between the OII auroras and ionospheric conditions. Since the field of view of the 42-m antenna of the ESR was directed
toward magnetic zenith, the intensities of the OII emissions at the magnetic zenith werecompared with the ESR data. Figure 4 shows a keogram of the OII auroras (upper panel)and the time variation of the OII aurora intensity (lower panel) at magnetic zenith (zenith
angle −7.7 degrees) on December 7, 2000. Before the sky became cloudy around 0850 UT,
intensifications in the OII emissions were seen at around 0513 UT, 0630 UT, and 0745 UT.
On the other hand, Fig. 5 shows, from the upper panel to the bottom, the electron density,
electron temperature, ion temperature, and ion velocity obtained by the ESR obtained forthe same time period shown in Fig. 4. Since the sky became cloudy after 0850 UT, the ASG
and ESR data obtained during the time period of 0200–0850 UT were compared. As shownin Fig. 5, increases in the electron density extending to an altitude of 600 km were seen at

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Fig. 3.

N. Koizumi et al.

Occurrence probability of OII auroras sorted according to 3-hour magnetic local time sectors andemission intensities.

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Fig. 4.

A keogram (upper panel) and the time variation of the emission intensity at the magnetic zenith (lower
panel) of OII aurora on December 7, 2000. Intensifications of the OII auroras are indicated by thearrows. The start and end time points for the correlation analysis with the ESR data are shown by thevertical lines.

Fig. 5.

Ionospheric parameters obtained by the 42-m antenna of the ESR on December 7, 2000. OII intensifications are indicated by the arrows.

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around 0630 UT, 0745 UT and after 0800 UT. Increases in the electron temperature wereseen above 250 km at around 0510 UT, 0630 UT, and 0745 UT. On the other hand, the ion
temperature did not change, except for slight increases at around 0510 UT and 0610 UT. A
slight upward ion velocity above 450 km was seen at around 0510 UT, and a more pronounced upward ion velocity was seen at around 0630 UT and after 0750 UT. A correlation
analysis between the ionospheric parameters obtained by the ESR and the OII emission
intensity at the magnetic zenith obtained by the ASG was performed in the following manner. First, the ionospheric parameters were averaged for 50-km altitude ranges and a timeseries of each ionospheric parameter for the 50-km altitude bin was made. Then, the correlation between each ionospheric parameter and the OII emission intensity was calculated.
Data obtained between 0556 UT and 0719 UT were compared because the most pronounced enhancement of the OII aurora was seen at around 0630 UT. One point that must
be taken into account in this analysis is that the time resolutions of the ASG and ESR observations were not identical. The ASG observations were made every 3 min with a 15-s exposure, while the integration time of the ESR observations was 128 s. To ensure a completely
simultaneous comparison, only ASG data whose 15-s exposure overlapped with the 128-s
integration time of the ESR data were employed in the comparison. Figure 6 shows the correlation coefficients calculated between the intensity of the OII aurora at the magnetic

Fig. 6.

Correlation coefficients between OII intensity at the magnetic zenith and ionosphericparameters obtained by ESR. Ionospheric parameters are averaged for 50 km altitude
ranges.

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zenith and each ionospheric parameter averaged for 50-km altitude ranges. As seen in Fig.
6, correlation coefficients of greater than 0.7 were obtained for the electron temperature in
the 250–500 km range and for the ion velocity in the 400–500 km range. On the other hand,no correlations between aurora intensity and ion temperature were observed. These results
indicate that the electron temperature increased in association with the OII intensity
enhancement in the upper F region, where the OII emission occurred, and that an increasein the upward ion velocity took place at altitudes 200–300 km higher than the emission
height of the OII.
3. Discussion
The statistical analysis of the occurrence probability of the OII aurora reported in theprevious section showed that the OII auroras mainly appeared in the dayside and that theintensity of the auroras appearing in the dayside was greater than that of auroras appearing
in the night side. The precipitation of soft electrons is known to exist in the dayside cusp
region. The Aurora observatory at Longyearbyen is located just under the cusp region in thedayside. In addition, OII auroras are effectively excited by soft electron precipitation.
Therefore, our results strongly suggest that the dayside OII auroras observed at Longyearbyen were excited by soft electron precipitation in the cusp region. In addition to the dayside/nightside asymmetry in occurrence probability and intensity observed at Longyearbyen, another asymmetry around magnetic local noon was observed. As seen in Fig. 3, theoccurrence probability and the intensity of the auroras were both larger before 12 MLT.
This observation may be explained by the results obtained by Liou et al. (2001) from theUVI onboard the Polar satellite. In Plate 4 and Plate 5 of the report by Liou et al. (2001),
the average energy and number flux of the precipitating electrons are shown, respectively.
These results show that the average energy was lower and the average number flux waslarger for 09–12 MLT, compared with the averages for 12–15 MLT. This means that the soft
electron precipitation, which produces the OII 732/733 nm emissions, was more intense
before magnetic local noon; our results agree with the Polar UVI observations.
Simultaneous ASG and ESR observations of the OII auroras showed that an enhancement in OII emission intensity was associated with an increase in electron density and electron temperature in an altitude range above 200 km, where OII emissions are thought tooccur. An increase in the upward ion velocity was also associated with the OII intensity
enhancement. However, the increase in the upward ion velocity was seen at altitudes above
400 km, which is at least 200 km higher than the altitude above which the increases in electron density and electron temperature were observed. One possible mechanism for the
upward movement of ions associated with the OII intensity enhancement is proposed below.
As stated previously, OII emissions are effectively excited by the precipitation of soft
electrons. The simultaneous enhancement of both OII intensity and electron density andelectron temperature suggests that soft electron precipitation occurred at around 0630 UT.
An increase in the electron temperature in the F region and above results in an increase in
the electron scale height; thus, electron heating creates an ambipolar electric field that inturn forces ions upward. However, the collisions of ions with the ambient particles that
dominate the lower altitudes prevents the upward motion of ions. On the other hand, thehigher the altitude, the fewer the number of collisions that occur, enabling the ions to move

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upward more easily. Such a scenario for ion upflow in the F region and topside was modeled by Su et al. (1999), and ion upflows associated with enhanced electron temperature
were observed, without simultaneous observations of OII emissions, by Ogawa et al.
(2000).
4. Summary
Auroral oxygen ion emissions at 732/733 nm were observed at the Aurora station in
Longyearbyen using the ASG during the winter season of 2000/2001. The statistical analysis showed that the highest occurrence of OII auroras (94%) at Longyearbyen was seen in
the dayside of the 09–12 MLT region. Overall, 10% of the auroras exhibited an intensity ofgreater than 500 R in this time sector. On the other hand, in the night side of the 18–09
MLT, the occurrence probability was less than 20% and most of the emissions had an intensity of less than 200 R.
To study the mechanism of ion upflow in the polar ionosphere, ASG data obtained on
December 7, 2000, was analyzed together with simultaneous ionospheric data obtained by
the ESR. Enhancements in electron temperature and ion upward velocity were associatedwith an increase in the OII intensity at magnetic zenith. This result suggests that the
ambipolar electric field associated with electron temperature enhancement, caused by soft
electron precipitation, may be involved in the driving mechanism that leads to ion upflow.
The editor thanks Dr. M. Nakamura and another referee for their help in evaluating thispaper.
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