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Eureka CCD Spectrophotometer

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Acknowledgments

The Eureka CCD Spectrophotometer is operated by the Physical Sciences Department of Embry-Riddle Aeronautical University with support from the Aeronomy Program of the National Science Foundation (NSF). It is designated as a Ground Based Instrument (GBI) for the TIMED satellite mission, and so has additional support from TIMED/CEDAR NASA funding.

Data Description

A CCD Spectrophotometer (CCDS) has operated at Eureka (80.22 N, 86.18 W), Canada at 139 m above mean sea level since ????????, 19??. The CCDS observes at 25 degree elevation to the geographic north (0 deg azimuth).

At the end of 2001 at 87 km above Eureka, the apex magnetic lat,lon are (88.4, -29.1) deg. The magnetic inclination and declination angles are 88.0 deg and -77.5 deg. The MLT at 0 UT is 1656 MLT. For an elevation angle of 25 deg, the 87 km hydroxyl peak is located at a latitude spacing of 1.61 degrees, or about 179 km away. Polar cap emissions are excited by polar cap drizzle electrons with average energy between about 100 and 300 eV that peak between about 200 to 250 km. For an elevation angle of 25 deg, emissions at 250 km are located at a latitude spacing of 4.31 deg or about 479 km away.

The CCDS scans the nighttime near infrared (NIR) between about 710-950 nm. Brightness is found for atomic lines and molecular bands while rotational temperatures are found for bands. Spectra are recored coninuously during each winter night of the new moon period when the solar zenith angle exceeds 100 degrees. The airglow spectra include hydroxyl Meinel (OH-M) bands (kindat=17001) that peak near 87 km, the molecular oxygen atmospheric (O2 At) (0,1) band (kindat=17002) between 765-779 nm that peaks around 93 km (higher for aurora), and atomic oxygen (OI) lines at 777.4 (17006) and 844.6 nm (17007) that are stronger during aurora and peak between 150 and 250 km. During aurora, long-lived metastable [O+] (OII) lines at 732.0 and 733.0 nm (17008) appear above 200 km if the auroral electrons are less than 0.5 keV. Average auroral energies ~0.5 keV peak around 180 km, while energies ~6 keV peak around 110 km. These larger auroral energies produce N2 First Positive Group (1PG) bands and [N2+] Meinel (N2+M) bands which swamp the OH-M bands. Results from the N2 1PG (0,1) band between 880-983 nm and the N2+M (0,1) band between 913-928 nm are stored in kindats 17004 and 17005. The O2 At (1,1) band between 860-873 nm peaks in auroral spectra between 110-180 km and is stored in kindat 17003. The temperature error bars represent relative errors, while error bars in band or line brightness represent one sigma value for random emission processes.

The temperature can determine the height of the emission layer, and thus the auroral electron mean energy, while the brightness can determine the auroral electron energy flux in the absence of clouds. These estimates of auroral electron mean energy and energy flux are done very seldom and are reported in kindat 17101.

The CCDS has a modified Czerny-Turner configuration of optical elements, and is fitted with a 0.5 m focal length spherical-mirror collimator and a 110 mm square, 1200 grooves mm-1 plane diffraction grating, blazed around 750 nm. The slit width can be adjusted between 0 and 3 mm, with a precision of ~5 um. The spectrophotometer employs a thermoelectrically (TE) cooled, low dark current (<6e/pixel-s), low read-out noise (<6e/pixel-s), scientific grade CCD detector. The CCD detector consists of 1024 x 1024 pixels, where each pixel is 24 um square. An f/1.4, 85 mm diameter clear aperture compound lens is coupled to the CCD camera. It produces a high-quality, flat-field image of spectral lines contained within a circular field of view (fov) of 12 deg. Thus, the output of the CCD is the brightness averaged over the fov at 1024 wavelengths. The wavelength spacing is non-linear and is about 0.229 nm at 720 nm and about 0.213 nm at 940 nm. Using a 0.5 mm entrance slit width gives emission line profiles with a full width at half the maximum (FWHM) of about 0.8 nm (0.9 nm at 720 nm and 0.7 nm at 940 nm). Thus, 0.8 nm is the resolution of the CCDS.

Absolute brightness calibration of the CCDS is accomplished with the aid of a blackbody source operating at 1273 K and a quartz iodide lamp (T=3100 K) illuminating a Lambertian screen. Spectra from moderately bright aurora can be recoreded using less than a 1 s CCD exposure time, but on-site system storage requirements limit automatic exposures to about 2 min, which improves the signal-to-noise ratio (S/N) for weak airglow emission features. The spectra are summed further if necessary to improve S/N. The calibrated is in R/nm, and is stored in kindat 10001 for the wavelength range of the CCDS, or in kindat 10002 when synthetic fitted brightnesses are included in a smaller wavelength range. Raw spectra are in kindat 7001.

Clouds reduce the absolute brightness, but the temperature is unaffected since it can be found from the ratio of the brightnesses from 2 lines, and several lines are used for better statistics from various rotational bands. In addition, the rotational population of the OH-M states is thermalized in the mesopause region [Sivjee and Hamwey, 1987], so the rotational temperature is also the thermal temperature. This is also true [??] for the N2 1PG bands, the N2+M bands, and the O2 At bands [REF??].

The temperature and the brightness of OH-M lines are related by a Boltzman distribution as described in equations 1 and 2 of Walterscheid and Sivjee [2001]. Up to 8 peaks from the P branch of OH-M bands are used to determine the rotational (thermal) temperature, although usually only 6 peaks are used. The temperature values depend on several molecular parameters including Einstein's radiative transition probability A(v',J) coefficients which are derived from theoretical quantum mechanical calculations. Systematic errors in these A coefficients result in temperature differences up to 40 K derived from different OH-M bands. However, studies of planetary, tidal and gravity waves require the change in temperature, or delta(Tn)/Tn, which is about the same for each band. The brightest bands are usually the (6,2) and (7,3) bands, where the (6,2) band is preferred because OH (7,3) is overlaid by the strong auroral N2 1PG (1,0) band.

The 6 rotational peaks of the P branch of the OH-M (6,2) band are located between 836-853 nm. The wavelength peaks, their angular momentum (orbit plus spin) of state J, rotational term values F(J), and Einstein's radiative transition probability A(v',J) values are:


      P line      J    Wvl(nm)     F(cm-1)     A(s-1)
       P2(2)    0.5     8382.4      129.775     0.841
       P1(2)    1.5     8399.2        0.0       0.529
       P2(3)    1.5     8415.2      176.365     0.779
       P1(3)    2.5     8430.2       65.975     0.644
       P2(4)    2.5     8452.2      253.625     0.762
       P1(4)    3.5     8465.3      158.77      0.690

The wavelengths and F(J) are from Coxon and Foster [1982], while the A(v',J) are from Mies [1974]. The peaks can appear to be shifted in the data.

References for the instrument and data processing procedures

Coxon, J. A. and S. C. Foster, Rotational analyses of hydroxyl vibration-rotation emission bands : Molecular constants for OH X2Pi, 6 <= v <= 10, Can. J. Phys., 60, 41-48, 1982.
Mies, F. H., J. Mol. Spectrosc., 53, 150, 1974.
Sivjee, G. G. and R. M. Hamwey, Temperature and chemistry of the polar mesopause OH, J. Geophys. Res., 92, 4663-4672, 1987.
Sivjee, G. G. and D. Shen, Auroral optical emissions during the solar magnetic cloud event of October 1995, J. Geophys. Res., 102, 7431-7437, 1997.
Sivjee, G. G., D. Shen, J.-H. Yee and G. J. Romick, Variations, with peak emission altitude, in auroral )2 atmospheric (1,1)/(0,1) ratio and its relation to other auroral emissions, J. Geophys. Res., 104, 28,003-28,018, 1999.
Walterscheid, R. L. and G. G. Sivjee, Zonally symmetric oscillations observed in the airglow from South Pole station, J. Geophys. Res., 106, 3645-3654, 2001.


Summary Plots for Eureka CCD Spectrophotometer [OH] at 87 km and [O2] at 93 km Airglow

NOTE: Data are only a test and will not be available later (01/22/02).

Summary plots of the brightness and rotational (ambient) temperature from airglow hydroxyl Meinel (OH-M) bands (preferably the (6,2) band) that peak around 87 km are shown as a solid line, while the O2 Atmospheric (0,1) band that peaks around 93 km is shown as a dotted line. As an example, the spectrum from 700-960 nm is shown at the end for the first data point, where the OH-M (6,2) band is between about 836-853 nm and the O2 At (0,1) band is between about 860-873 nm. Sometimes only a portion of the spectrum is plotted rather than the full range.


-Revised 12 Sep 2005 by Barbara Emery