Contact Persons:
Susan Nossal (nossal@physics.wisc.edu)
Edwin Mierkiewicz (emierk@wisp.physics.wisc.edu)
L. Matthew Haffner (haffner@astro.wisc.edu)
ACKNOWLEDGMENTS:
The Wisconsin H-alpha Mapper (WHAM) Fabry-Perot Interferometer is operated
at the Kitt Peak National Observatory by the Wisconsin Galactic Astronomy
Group with support from the National Science Foundation.
The Wisconsin H-alpha Mapper (WHAM) Fabry-Perot is located at the Kitt Peak Observatory (31.98N, 111.60W; alt 2120.0 m), and has been operated remotely by the University of Wisconsin Galactic Astronomy Group since 1997 [Reynolds, 1997; Haffner et al., 2003]. The geocoronal atomic hydrogen is studied using the H-alpha (656.274 nm) and H-beta (486.1 nm) emissions. The WHAM instrument is a double-etalon Fabry-Perot coupled to a Charge Coupled Device (CCD) camera where the optics are designed to image the Fabry-Perot annular interference pattern onto the CCD. Hence, the CCD images are spectral and not spatial. The geocoronal atomic hydrogen can be observed both at H-alpha (6563 A, ~1-10 Rayleighs), and H-beta (4861 A, ~0.25-1 R).
The terrestrial H-alpha emission is primarily excited by the line center portion of the solar Lyman-beta emission and thus depends on both the solar excitation flux and the density distribution of upper atmospheric hydrogen. To first order, the column of H-alpha emissions (code 2502) observed by the Fabry-Perot comes from the sunlit portion of the geocorona above the Earth's shadow. Since density falls off with height, the shadow height (code 186) is roughly just below the peak emission height. However, a portion of the signal (~1-2 R) is also due to multiple scattering of Lyman-beta radiation below the Earth's shadow height in darkness. This contribution becomes increasingly significant for observations at higher shadow altitudes. The solar zenith angle (code 180) for all observations is below the horizon (e.g. nighttime conditions).
The semi-raw spectral images are saved in FITS (.fts) format where only a portion of the chip was used and the read noise was reduced using 4 x 4 on-chip binning [Haffner et al., 2003]. The CEDAR Database FITS files include additional header information on the observing geometry, wavelength (H-alpha set at 6564 A in the original .fits headers starting in 2001), kinst (5190) and kindat (7001 for H-alpha or 7101 for H-beta).
The kindat 17000 series files contain information from the H-alpha spectral profile (.spe) where the annular interference pattern in the .fts file is summed. Equal area annuli correspond to equal spectral intervals (measured in wavenumber, so the arbitrary pixel intensities are summed in equal area annuli to produce a spectral profile of relative emission as a function of wavenumber expressed as spectral displacement in velocity units km/s with an arbitrary zero (code 2416). The relative emission is divided by the exposure time (code 60) from the .coors file to create a relative emission rate (codes 4145 for NAN calibration files and 4146 for other files). Hot pixels due to cosmic rays, dark counts, reflections and a constant average bias have been removed. The standard deviation of the relative pixel intensity in each annuli divided by the exposure time is reported as the 'error bar' for the data point (codes -4145 and -4146). Codes 2507 and 2509 are the relative annular emission after the data have been normalized with a white light flat field (from the .dat files) divided by the exposure time to make an emission rate. The initial 31 spectral displacement points (below the arbitrary value of -50 km/s) are outside the aperture and are removed in the original .dat file and also in kindats 17000-3,17100-3.
Additional information in nightly catalog files come from the headers from the FITS files and from shadow altitude calculations. This information is listed in the nightly .shad and .coors files. The UT times in the names (codes 4142 and 4143) of the original FITS files (.fts) are from .log file UTs that are contained in companion nightly catalog files. The actual UT start times listed in the FITS headers are a few seconds later than the UT start times in the .log file and reflect when the commands were started by the WHAM instrument.
Astronomical locations are given in two coordinate systems: (1) galactic coordinates with galactic longitude b in degrees (code 193) and galactic latitude l in degrees (code 195), and (2) equatorial coordinates with right ascension (RA) in hours (code 192) and declination in degrees (code 194).
The hour angle (code 191) is defined as the difference between the Local Siderial Time and the Right Ascension of the object. The hour angle is zero when the target transits the Local Meridian, which passes through the zenith direction. Hour angles are negative when the object is rising, and are positive when the object is setting. Atmospheric extinction is minimized when zenith angles are restricted to 50 degrees or less (ZA<50), which are equivalent to elevation angles (code 142) of 40 degrees or more (el>40).
Typically, WHAM locks onto a given astronomical location for integration times (code 60) of 30 to 600 seconds, so the azimuth and elevation angles (codes 132 and 142) are initial values, while RA and DEC (codes 192 and 194) remain constant.
All observations of H-alpha column emissions from the thermosphere plus exosphere are made at night during moonless conditions. All observations reported to the CEDAR Database are also made during clear sky conditions, since even high cirrus clouds can affect the H-alpha signal. Acceptable values of the quality code 4144 are:
The regions of low galactic emission are useful for geocoronal observations because uncertainty in the retrieval of the geocoronal emission due to the presence of galactic emission is minimized. The fitting does not require removing the galactic emission in the low galactic region of observations. Geocoronal observations are also available from zenith and other directions, especially in survey runs. The analysis for survey data may include one or two Gaussians in the fit, while the cleaner low galactic emission regions are usually analyzed using two Gaussians to account for fine structure. The absolute intensities (code 2502) are calibrated using the bright H-alpha emission at (RA 20.97, DEC 44.6 or l,b 85.60 -0.72) of 800 R +/-10% from the North American Nebula (NAN). There is an additional ~5% uncertainty in the relative calibration due to night-to-night variability in the transmittance of the atmosphere. These uncertainties are larger than the standard deviation (codes -4145 and -4146) of pixel intensity in the annuli divided by the exposure time.
Locations (code 4141) used for geocoronal observations or calibrations combined with the H-alpha kindats are:
Further details of the procedure are in the references, in the generic kindat catalog records, and are available from the contact persons.
Coakley, M.M., F.L. Roesler, R.J. Reynolds, and S. Nossal (1996), Fabry-Perot/CCD annular summing spectroscopy: Study and implementation for aeronomy applications, Appl. Opt. 35, 6479-6493.
Gaustad, J.E., McCullough, P.R., Rosing, W., VanBuren, D., 2001. A robotic wide-angle H-alpha survey of the southern sky. Publications of the Astronautical Society of the Pacific, 113, 1326.
Haffner, L.M., R.J. Reynolds, S.L. Tufte, G.J. Madsen, K.P. Jaehnig, and J.W. Percival (2003), The Wisconsin H-alpha mapper northern sky survey, Astrophys. J., 149, 405-422.
Hausen, N.R., Reynolds, R.J., Haffner, L.M., Tufte, S.L., 2002. Interstellar H-alpha line profiles toward HD 93521 and the Lockman Window. Astrophysical Journal 565, 1060-1068.
Leen, T. (1979), Application of radiative transfer theory to photometric studies of astronomical objects, M.S. thesis, Univ. of Wis., Madison.
Meier, R.R., 1995. Solar Lyman-series line profiles and atomic hydrogen excitation rates, Astrophysical Journal 452, 462.
Mierkiewicz, E.J., 2002. Fabry-Perot observations of the hydrogen geocorona, Ph.D. Thesis, University of Wisconsin, Madison, WI.
Mierkiewicz, E.J., F.L. Roesler, S.M. Nossal, and R.J. Reynolds, (2006), Geocoronal hydrogen studies using Fabry-Perot interferometers, Part 1: Instrumentation, observations, and analysis, J. Atmos. Solar-Terr. Phys., 68, 1520-1552, doi:10.1016/j.jastp.2005.08.024. (Link to JASTP .pdf at: http://www.elsevier.com/wps/find/journaldescription.cws_home/211/description and to author's .pdf at: TBD)
Nossal, S., 1994. Fabry-Perot observations of geocoronal hydrogen Balmer-alpha emissions. Ph.D. Thesis, University of Wisconsin, Madison, WI.
Nossal, S.M., E.J. Mierkiewicz, F.L. Roesler, R.J. Reynolds, and J. Bishop, Geocoronal hydrogen studies using Fabry-Perot interferometers, Part 2: Long-term observations, (2006), J. Atmos. Solar-Terr. Physics, 68, 1553-1575, doi:10.1016/j.jastp.2005.08.025. (Link to JASTP .pdf at: http://www.elsevier.com/wps/find/journaldescription.cws_home/211/description and to author's .pdf at: TBD)
Nossal, S. Roesler, F.L., Coakley, M.M., Reynolds, R.J., 1997. Geocoronal hydrogen Balmer alpha line profiles obtained using Fabry-Perot annular summing spectroscopy: effective temperature results. Journal of Geophysical Research, 102, A7, 14541-14553.
Nossal, S.M., F.L. Roesler, E.J. Mierkiewicz, and R.J. Reynolds, Observations of solar cyclical variations in geocoronal H-alpha column emission intensities, GRL, 31, L06110, doi:10.1029/2003GLO18729, 2004.
Nossal, S., F.L. Roesler, J. Bishop, R.J. Reynolds, M. Haffner, S. Tufte, J. Percival, and E.J. Mierkiewicz, Geocoronal H-alpha intensity measurements using the Wisconsin H-alpha Mapper Fabry-Perot facility, J. Geophys. Res., 106, A4, 5605, 2001.
Nossal, S., F.L. Roesler, and M.M. Coakley (1998), Cascade excitation in the geocoronal hydrogen Balmer alpha line, J. Geophys. Res., 103, 381-390, 1998.
Nossal, S. R.J. Reynolds, F.L. Roesler, and F. Scherb, Solar cycle variations of geocoronal Balmer-alpha emission, J. Geophys. Res., 98, 3669-3676, 1993.
Reynolds, R.J. (1997), Ionizing the galaxy, Science, 277, 1446-1447.
Scherb, F., 1981. Hydrogen production rates from ground-based Fabry-Perot observations of comet Kohoutek. Astrophysical Journal 243, 644.
Shih, P., F.L. Roesler, and F. Scherb, Intensity variations of geocoronal Balmer alpha emission: 1. Observational results, J. Geophys. Res., 90, 477-490, 1985.
Tufte, S.L., 1997, The WHAM Spectrometer: design performance characteristics and first results. Ph.D. Thesis, University of Wisconsin, Madison.
Appendix: WHAM Geocoronal Data Description
The WHAM geocoronal data are contained in .log, .notes, .fts, .spe, .dat, .shad, and .coors files described below. Their conversion to CEDAR Database format are described in the catalog record for 5190/17000-2 (from .log and .notes), the header record for 5190/17000-2 (from .spe, .dat, .shad, .coors), and additions to and from the FITS header of the .fts files.
Ground-based remote sensing of the very faint fluorescence emissions from atomic hydrogen [H-alpha (6563 Angstrom, ~1-10 Rayleighs), and, more recently, H-beta (4861 Angstrom, ~0.25-1 Rayleigh)] is one of the primary diagnostics for studying the neutral upper atmosphere. The Fabry-Perot Spectrometer is particularly advantageous for making observations of faint, diffuse sources such as the geocorona due to the instrument's simultaneous high spectral resolution and high throughput.
The thermospheric plus exospheric H-alpha emission is primarily excited by the line center portion of the solar Lyman-beta (1026 Angstrom) flux. Most of the H-alpha emission thus comes from above the Earth's shadow, the height of which can be used to determine, to first order, the base of the column of H-alpha emission. As such, the H-alpha emission intensity is dependent upon the hydrogen density profile, the solar excitation flux, and the observational viewing geometry. The H-alpha column emission intensity observed by the Fabry-Perot is a measurement of the integrated volume emission rate along the observational line-of-sight, with the peak in the emission rate arising from just above the Earth's shadow. A portion of the signal (~1-2 Rayleighs) is also due to multiple scattering of Lyman-beta radiation below the Earth's shadow. This contribution becomes increasingly significant for observations at higher shadow altitudes.
The data included here are the Fabry-Perot Interferometer 656.3 nm H-alpha emission intensity data from the Wisconsin H-alpha Mapper (WHAM) Fabry-Perot. This instrument is located at the Kitt Peak Observatory, near Tucson, Arizona (31.98 N, 111.60 W; alt 2120.0 m), and has been operated by the galactic research group of Ron Reynolds [Haffner et al., 2003; Reynolds, 1997] at the University of Wisconsin since 1997. The WHAM instrument is a double- etalon, Fabry-Perot coupled to a CCD camera and is remotely operable from Wisconsin. WHAM's resolving power of ~25,000 [lambda/delta(lambda), or delta(lambda) ~ 0.026 nm at H-alpha] is sufficient for separation of the terrestrial emission from the Doppler-shifted galactic emission line and for retrieval of the thermospheric plus exospheric H-alpha column emission intensity. The resolution, however, is not sufficiently high to resolve the H-alpha line profile.
The WHAM data included in the CEDAR Database were taken by the WHAM astronomy group and by Edwin J. Mierkiewicz.
We include the semi-raw CCD image of the Fabry-Perot's annular interference pattern (.fts) and the corresponding spectral profile (.spe) obtained by ring summing after the bias, hot cosmic ray pixels, dark counts and reflections are removed. We also include the annular summed white light flat field (whamflat.dat), the spectral profile normalized with the white light flat field (.dat), the annular summed instrumental profile (whamip.ip), and the retrieved thermospheric plus exospheric H-alpha column emission intensity. We are using the Fabry-Perot in a spectral mode whereby the optics are designed to image the Fabry-Perot annular interference pattern onto the Charge Coupled Device camera. Hence, the images presented are spectral and not spatial.
2.1 Images (.fts)
In the case of the WHAM images presented here, only a portion of the chip was used. Read noise was reduced using 4 x 4 on-chip binning [Haffner et al., 2003]. The raw CCD spectral images are saved in a ".fts" format, where more header information is given.
2.2 Spectral Profiles (.spe)
The annular interference pattern is converted to a spectral profile using annular summing spectroscopy. This procedure is based on the property that equal area annuli correspond to equal spectral intervals (measured in wavenumber). In software, pixel intensities are summed in equal area annuli to produce a spectral profile. The area of the annuli is chosen so as to sample several times per resolution element [Coakley et al., 1996, Haffner et al., 2003].
The spectral files contain three columns. The first column gives the spectral displacement expressed in velocity units. The velocity increment between data points varies slightly across the CCD chip. "Zero velocity" is placed at an arbitrary location. ****The first 31 data points of the .spe file are outside of the aperture and so should not be used for data analysis. ****
The second column in the spectral file gives the average pixel intensity in an annulus after hot pixels due to cosmic rays, dark counts, reflections, and a constant average bias have been removed. The average pixel intensity is measured in arbitrary data units, and is divided by the exposure time to create a relative emission rate.
The third column gives a measure of the variance in pixel intensity within an annulus. The pixel intensities within each annulus are plotted and then a line is fit to these intensities. The variance (standard deviation squared) of the intensities of the pixels is calculated around this fitted line.
2.3 Spectral Profiles Corrected with a Flat Field (.dat)
Corrections are made for instrumental vignetting (truncation of the off-axis light rays) and for variation in pixel sensitivity by dividing the CCD image by a white light flat field [Haffner et al., 2003; Coakley et al., 1996; Mierkiewicz et al., 2006]. The night sky spectral profile (.spe) is divided by that obtained from the CCD image of the white light flat field (whamflat.dat). The spectral profiles that have been normalized with a white light flat field are stored in ".dat" files, with the first 31 data points of the spectral profile (.spe) removed as they are outside the aperture (see section 2.2).
In the ".dat" file, the first column gives the spectral displacement expressed in velocity units (same as ".spe" files, see section 2.2 above), the second column gives the average pixel intensity in an annulus following the white light flat field correction to the ".spe" file, and the third column gives a measure of the variance in pixel intensity within an annulus (same as ".spe" files, see section 2.2 above).
2.4 Thermospheric plus Exospheric H-alpha Column Emission Intensities
A Gaussian emission model is convolved with the instrumental profile and then fit to the H-alpha spectrum. The absolute intensity of the geocoronal H-alpha emission is retrieved by comparing its integrated area with that from the nebular calibration source. More information about the fitting procedure and the nebular calibration method are included in sections 4, 5, and 6.
2.5 Nebular Calibration (nan*)
The images and spectral profiles of nebular calibration observations are included in this database. Please see the description of the nebular calibration method in section 6.
2.6 Instrumental Profile (whamip.ip)
The instrumental profile is a measure of the Fabry-Perot's effect on the emission spectrum. A thorium 6554 Angstrom line was used to obtain a measure of the instrumental profile by observing its emission at a ring radius close to that used for making the night sky H-alpha observations [Tufte, 1997; Coakley et al., 1996; Nossal et al., 1997]. The thorium 6554 Angstrom line was chosen because it is the brightest thorium emission in the spectral vicinity of H-alpha and because it is a narrow line whose width is approximately 1/40th that of the geocoronal H-alpha emission line. The instrumental spectral profile is named "whamip.ip".
2.7 Flat field
The spectra files have been divided by the spectral profile of a white light flat field to compensate for vignetting within the instrument and variation in pixel sensitivity on the chip [Haffner et al., 2003; Coakley et al., 1996]. The spectral profile obtained by ring summing the white light flat field is called "whamflat.dat".
WHAM is used primarily for making astronomical observations of interstellar medium emissions, including an all-sky survey of interstellar H-alpha emissions [Reynolds, 1997; Haffner et al., 2003]. The terrestrial spectra present in these observations comprise a rich resource of observations in multiple viewing geometries for studying thermospheric plus exospheric atomic hydrogen H-alpha emissions. In addition, WHAM also makes observations in directions specifically designed for geocoronal studies.
All of our observations of H-alpha column emissions from the thermosphere plus exosphere are made during moonless conditions. These observations are also made during only clear sky conditions as even high cirrus clouds impact the observed H-alpha signal. We have reviewed the observing notes in an effort to remove data that might have been taken in the presence of high cirrus clouds. The winter solstice typically offers the best sky conditions for observations, as well as longer nights [Nossal et al., 2004, 2006].
3.1 Low Galactic Emission Region Observations
Low galactic emission region observations (less than about 0.25 Rayleigh at H-alpha) are routinely taken as standard reference observations during almost all nights of WHAM observations. Most were pointed toward the Lockman window [right ascension (RA) 10.90 degrees, declination (DEC) 57.93 degrees], with some also pointed toward other regions of low galactic emission such as [(RA 10.38 degrees, DEC 50.24 degrees), (RA 10.81 degrees, DEC 37.57 degrees)] [Hausen et al., 2002]. Such observations minimize uncertainty due to blending between the galactic and terrestrial H-alpha emission lines.
3.1a. Fits to the Low Galactic Emission Region Observations
For WHAM observations pointed toward low galactic emission regions of the sky, the geocoronal H-alpha emission spectral profiles were fitted using a two Gaussian model representing the two dominant fine structure transitions [3^2P_(3/2) to 2^2S_(1/2); 3^2P_(1/2) to 2^2S_(1/2); centroid wavelength in air of 6562.741 Angstroms] contributing to the geocoronal H-alpha emission. The separation of the line centers of the two Gaussians was fixed to be that of the spectral spacing between these two fine structure components (2.133 km/sec) and the ratio of their areas was fixed as 2:1, corresponding to the quantum mechanical ratio of the transition probabilities of the two dominant fine structure emissions [see, for example, Nossal et al., 1998]. The spectral location of the doublet, its total area, and the intensity of the background continuum were all free parameters. This two component geocoronal H-alpha emission model was convolved with the instrumental profile and then fit to the geocoronal H-alpha emission line profile. The model was adjusted subject to the above constraints to produce a least squares, best-fit to the data using the voigt-fit code of R.C. Woodward [personal communication, 2003].
Explicit fitting of the geocoronal H-alpha fine structure emission components arising due to cascade excitation [see Meier, 1995; Nossal et al., 1998 and references therein; Mierkiewicz et al., 2006] has not been included in the fits to the WHAM data because the resolution of the WHAM instrument is too low to resolve the geocoronal line profile. The retrieved geocoronal intensities include the cascade excitation which is estimated to be 5 +/- 3% of the total intensity based upon more recent observations by Mierkiewicz [2002], Mierkiewicz et al. [2006] and revised estimates by Meier [private communication, 2004].
Detailed fitting of the galactic emission was not included for the low galactic emission region observations because of the faintness of the galactic emission in these regions of the sky. For a case of near maximum overlap between the geocoronal H-alpha emission and the Lockman window galactic emission, the geocoronal H-alpha column emission intensity retrieved when the galactic emission was included in the fit differed by less than 4% from that retrieved when the galactic emission was not fit [Nossal et al., 2004].
The absolute intensity of the geocoronal H-alpha emission is retrieved by comparing its integrated area with that from the nebular calibration source. The difference in atmospheric extinction [Haffner et al., 2003; Shih et al., 1985; Nossal et al., 1993] between the two observations is then accounted for from their difference in slant path, using the measured zenith transmission of 0.92 +/- 3% for a clear night on Kitt Peak [Haffner et al., 2003].
3.2 Survey Observations
Most of the WHAM H-alpha observations are made in a sequence rastering over a particular block of the sky. There are up to 49 observations, each with one degree field of view, and typically in a 7x7 nested grid [Haffner et al., 2003]. Usually, observations are made over several adjacent blocks before moving to another part of the sky. However, sometimes a stray block is observed separately in order to make up for a hole left during a previous sequence of observations. These blocks are put together to make an all-sky survey of the structure of emission intensities from the interstellar medium. The majority of the WHAM H-alpha survey observations were made during 1997. Observations of H-alpha emissions have continued during subsequent years, but at a reduced level, due to the instrument being used for observations of other emission lines.
For the survey observations, the geocoronal emission was modeled as a single Gaussian, but may be re-analyzed with two Gaussians. The majority of survey observations were planned for times that would minimize overlap (i.e. maximize the Doppler shift) between the geocoronal and galactic H-alpha emission components. When overlaps did arise, the geocoronal width was sometimes held fixed in order to achieve a more accurate overall fit to the galactic emission. In such cases the accuracy of the geocoronal intensity may be compromised (see Nossal et al., 2001].
3.3 Zenith Observations
Several H-alpha zenith observations are made during many of the nights of observations. Zenith observations are useful for comparisons with past H-alpha data sets and for comparisons with radiative transfer modeling.
The zenith observations are fit using two linked Gaussians of fixed spectral spacing and relative areas corresponding to the fine structure components directly excited by in solar Lyman-beta scattering (see section 3.1a). Overlaps between the geocoronal and galactic emission components in the zenith data are handled using an iterative fitting procedure [Nossal et al., 2001].
The WHAM-NSS can be used to characterize the H-alpha emission from the galaxy that is present in the WHAM observations. The WHAM website contains the results of the survey and related publications. (See http://www.astro.wisc.edu/wham/survey).
The Integrated Survey Table ( http://www.astro.wisc.edu/wham/survey) provides total galactic H-alpha intensity measurements over the velocity range of -80 km/sec < vlsr < +80 km/sec as a function of galactic latitude and longitude. A map of this velocity-integrated galactic intensity can also be found at the above website.
The header information included in the ".fts" observational file contains the galactic latitude and longitude of the observation, its right ascension and declination, as well as the local standard of rest velocity (VLSR) for the time and direction of the observation. The VLSR velocity is the deviation of the Local Standard of Rest from the geocentric zero velocity in units of km/s. The VLSR is the approximate Doppler shift of the galactic emission from the geocoronal emission; however, this offset differs slightly from the VLSR due to galactic dynamics and a different fine structure distribution between the geocoronal emission and the galactic recombination line. There is a 2.3 km/s shift between the weighted center of the geocoronal H-alpha profile and that of the galactic recombination profile at the Local Standard of Rest.
The thermospheric plus exospheric H-alpha column emission intensities included in this database have not been corrected for scattering in the troposphere. Such corrections are typically made through use of radiative transfer modeling. We estimate tropospheric scattering adjustments using a radiative transfer model developed by Leen [1979, U WI MS thesis] (also see Shih et al., 1985, JGR, p 477]. This model uses a single scattering approximation and takes into account Rayleigh scattering by molecules, Mie scattering by aerosols, and the relatively small extinction from ozone absorption. We have chosen to include only the primary measurements because we recognize the need for an improved scattering code for the higher precision measurements.
When the thermospheric plus exospheric H-alpha column emission intensities included in [Nossal et al., 2004] were corrected for tropospheric scattering using the tropospheric scattering code of Leen [1979; Shih et al., 1985], the intensities were lowered by 14-26%, depending upon the viewing geometry. The conclusions of the paper were unchanged because the correction applied to both solar minimum and maximum conditions [Nossal et al., 2004].
The absolute intensity of the thermospheric plus exospheric H-alpha column emission is calibrated through comparisons with H-alpha emissions from standard astronomical nebular calibration sources, all of which have been tied to the North American Nebula (NAN). Nebular calibration is internally consistent and is used for calibrating all of the Wisconsin-based atmospheric, planetary, and astronomical hydrogen H-alpha observations. Nebular calibration offers long term stability and like the geocorona, nebulae are spatially extended line rather than continuum emission sources. The use of nebular calibration minimizes uncertainty due to atmospheric extinction corrections since both the geocoronal hydrogen and the nebular calibration sources are outside of the Earth's atmosphere. Corrections are made for differences in atmospheric extinction due to differences in slant path associated with different observational zenith angles. The NAN observations used to calibrate the geocoronal observations were limited to observations taken at zenith angles less than or equal to 50 degrees (ZA<50) [Nossal et al., 2006]. Details about procedures for corrections for atmospheric extinction can be found in Nossal et al. [2006] and in Mierkiewicz et al. [2006].
The patch of the North American Nebula used for the calibration is centered at right ascension 20h 57m 59s and declination +44deg 34' 50". There is about a 5% uncertainty in the relative calibration due to night-to-night variability in the transmittance of the atmosphere above Kitt Peak. The North American Nebula has been calibrated using standard stars [Scherb, 1981] and has also been checked against a blackbody source [Nossal, 1994]. The accuracy of this calibration has also been corroborated with a comparison to the Southern H-alpha Sky Survey Atlas [Gaustad et al., 2001].
The intensity of the geocoronal emission can be calculated from the following expression, based on the Beer-Lambert Law:
Igeo = Inan * (Ageo/Anan) * {exp[-tau sec(ZAnan)] / exp[-tau sec(ZAgeo)]} * [exposuretime(nan)/exposuretime(geo)]
where
A composite of North American Nebular (NAN) calibration observations were used to determine a clear sky calibration factor for each yearly season of observations. Monitoring of the nebular calibration spectra enabled us to track annual changes in the sensitivity of the WHAM instrument, and to confirm our agreement with the calibration from NAN used by WHAM astronomers. We used observations of the North American Nebula taken during northern hemisphere summer and fall because during these months the nebula is visible at zenith angles of less than 50 degrees. (Observations with ZA>50 have a grade of 'B' and were not included in the calibration calculations.)
The calibration factor associated with the nebular calibration is defined as:
Fnan = Anan /{ Inan * exp[-tau sec(ZAnan)] * exposuretime(nan) }
and has the units of Adu (arbitrary data units) * km sec-2 R-1
Nossal et al. [2006] examined low galactic emission sources in a solar cycle study during winter months at Kitt Peak when sky conditions are typically clearer. Therefore, the NAN calibrations for Kitt Peak were chosen over a year period centered on northern hemisphere winter. The following table contains the calibration factors and the number of grade 'A' spectra used to compute this factor over several winters. These (and other) NAN observations are included in the CEDAR database.
| year | 1997 | 1999-2000 | 2001 | 2003-2004 | 2005-2006 |
| Fnan | 25.5 | 23.2 | 22.7 | 22.7 | 22.3 |
| # of spectra | 11 | 9 | 4 | 5 | 15 |
Calibration values from different NAN observations differed by less than 3% from the average value during a several month period, reflecting the uncertainty in the relative calibration due to night-to-night variability. These values are consistent with the current WHAM calibration value used by the astronomers of 22.8 Adu km sec-2 R-1, which is often sited as 684.1 Adu km sec-1 R-1 for a 30 second exposure. The WHAM instrument started observations at Kitt Peak in 1997 and experienced a decrease in sensitivity (a decrease in Anan, probably after dust settled into the instrument), so the 1997 Fnan values are larger than later values.
Description of files included in this directory:
Spectral file name, right ascension, declination, shadow distance along the slant path of the observational look direction, shadow altitude to the location of the intersection of the observational look direction and the Earth's shadow, azimuth of the observation, zenith angle of the observation, azimuth of the sun's position, zenith angle of the sun, absolute value of the difference in azimuth between the observational and solar directions, H-alpha column intensity (has not been corrected for tropospheric scattering).
Spectral file name, UT time of the observation, UT date of the observation, right ascension, declination, hour angle, zenith angle (ZA), LSR velocity, exposure time, grade.
The grading of the spectra is as follows:
NAN Calibration Plots for Kitt Peak H-alpha Fabry-Perot Interferometer
The North American Nebula (NAN) is used to calibrate various H-alpha instruments. The calibration for the WHAM instrument at Kitt Peak is 800 R. 'N' is shown on the plot for those nights with this NAN calibration. The dashed line is the position of local midnight.
Aug 29-Sep 27, 1997 All data for this period Sep 28-Oct 27, 1997 All data for this period May 01-May 30, 1999 All data for this period May 31-Jun 29, 1999 All data for this period Oct 28-Nov 26, 1999 All data for this period Mar 31-Apr 29, 2000 All data for this period Apr 30-May 29, 2000 All data for this period May 01-May 30, 2001 All data for this period Oct 28-Nov 26, 2003 All data for this period Mar 31-Apr 29, 2004 All data for this period Oct 28-Nov 26, 2005 All data for this period Apr 01-Apr 30, 2006 All data for this period May 01-May 30, 2006 All data for this period May 31-Jun 29, 2006 All data for this period
Summary plots of the absolute column emission in R from thermosphere-exosphere atomic hydrogen. Most of this is from above the plotted shadow height, and so is a strong function of shadow height. The dashed line is the position of local midnight.
Jan 01-Jan 30, 1997 All data for this period Jan 31-Mar 01, 1997 All data for this period Mar 02-Mar 31, 1997 All data for this period Nov 27-Dec 26, 1999 All data for this period Jan 31-Feb 29, 2000 All data for this period Jan 01-Jan 30, 2001 All data for this period Jan 31-Mar 01, 2001 All data for this period Jan 31-Feb 29, 2004 All data for this period Mar 01-Mar 30, 2004 All data for this period Jan 31-Mar 01, 2006 All data for this period
-Revised 17 Jan 2008 by emery@ucar.edu
