Instruments:WHAM Appendix

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Contents 1 Appendix: WHAM Geocoronal Data Description 1.1 1. Introduction 1.2 2. Data Description 1.2.1 2.1 Images (.fts) 1.2.2 2.2 Spectral Profiles (.spe) 1.2.3 2.3 Spectral Profiles Corrected with a Flat Field (.dat) 1.2.4 2.4 Thermospheric plus Exospheric H-alpha Column Emission Intensities 1.2.5 2.5 Nebular Calibration (nan*) 1.2.6 2.6 Instrumental Profile (whamip.ip) 1.2.7 2.7 Flat field 1.3 3. WHAM Observations 1.3.1 3.1 Low Galactic Emission Region Observations 1.3.1.1 3.1a Fits to the Low Galactic Emission Region Observations 1.3.2 3.2 Survey Observations 1.3.3 3.3 Zenith Observations 1.4 4. Accounting for the Galactic Background using the WHAM Northern Sky Survey 1.5 5. Tropospheric Scattering 1.6 6. Intensity Calibration 1.7 7. README

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.

1. Introduction

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.

2. Data Description

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"..dat".

3. WHAM Observations

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].

4. Accounting for the Galactic Background using the WHAM Northern Sky Survey

The WHAM Northern Sky Survey (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.

5. Tropospheric Scattering

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].

6. Intensity Calibration

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

• Igeo = geocoronal intensity
• Inan = intensity of the North American Nebula which is:

• • (a) 800 R +/- 10% from a 1.0 degree patch from Kitt Peak Observatory
  • (b) 850 R +/- 50 R from a 0.8 degree patch from Pine Bluff Observatory
  • (c) 620 R from a 1.2 degree patch from Pine Bluff Observatory

(field-of-view of the high resolution Fabry-Perot at Pine Bluff)

• Ageo = area of the geocoronal emission spectral profile
• Anan = area of the NAN emission spectral profile
• tau = atmospheric extinction coefficient which for clear sky is:

• • (a) = 0.078 at Kitt Peak Observatory (31.98N, 111.60W; alt 2120.0 m)
  • (b) = 0.14 at Pine Bluff Observatory (43.08N, 89.68W; alt 366 m)

• ZAgeo = zenith angle of the geocoronal observation
• ZAnan = zenith angle of the NAN observation
• exposuretime(geo) = exposure time for the geocoronal observation (in sec)
• exposuretime(nan) = exposure time for the NAN observation (in sec)

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.

7. README

Description of files included in this directory:

• *.log file: contains a list of observation type, times, and exposure times. There is a log file for each night of observations.
• *.notes file: contains notes pertaining to the night of observations. These files contain information about the observing conditions and any unusual instrumental or observing issues. There is a *.notes file for only some of the nights of observations.
• *.shad file: contains the following information concerning the observational and solar look directions, the position of the Earth's shadow, and the H-alpha column intensity:
  • 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).
• *.coors file: contains the following information concerning the observation:
  • 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:
    • 1=A Excellent conditions
    • 2=A- Clouds sighted much later or earlier in the night
    • 3=B Clouds sighted within a few hours of the observation; or spectrum close to sunset or sunrise at the observatory OR ZA>50
    • -32767 Missing (consider ZA<50 as best data)

To the best of our ability, we have removed data taken when clouds were overhead.

• *.spe file: contains the spectrum (not yet corrected with a flat field)
• *.fts file: contains the fits file combined with the spectra: Fits header, CCD image, and (sometimes) spectrum
• *.dat file: contains the spectrum corrected with a white light flat field