The Science Thermospheric Oxygen Density [O]: As the dominant neutral species between 250 and 500 km altitude1, neu-tral oxygen plays a vital role in the physics of the ther-mosphere. The measurements of [O] from this project will be incorporated into a forward model which will improve upon existing models in representing thermos-pheric responses to impulsive events like magnetic storms and coronal mass ejections. Bowen Fluorescence: The technique for measuring [O] via Bowen fluorescence was developed by Dr. Redgie Lancaster1,2. Bowen fluorescence is generated by solar Lyman-β emissions absorbed by thermospheric oxygen in the ground state, ultimately producing 8446 Å emissions. A diagram of the pertinent energy levels is presented in Figure 1.

Dr. Redgie Lancaster3 and others have noted that Bo-wen 8446 Å emissions are characterized by a specfic intensity ratio of 8:1 between the 8446.38 and 8446.76 Å lines of the triplet (see Figure 1) . The Scientific Solutions Bowen fluorescence SHS will easily resolve those lines, and will thus be able to distinguish Bowen emissions from the 8446 Å emissions due to excitation from photoelectron impact (which observations at Millstone Hill2 suggest is a significant portion of 8446 Å light detected there).

The basic Spatial Heterodyne Spectrometer (SHS) layout (Figure 2), when fed collimated light, pro-duces crossed wavefronts which in-terfere to create a Fizeau fringe pat-tern (an example of which is shown in Figure 3). The intensity function of this inteferogram (in the disper-sion direction of the gratings) is ef-fectively a cosine Fourier trans-form of the input spectrum. The module in Figure 4 has been designed by Scientific Solutions (SSI) to observe Bowen fluorescence using SSI’s dedicated space (and pointing head) at Millstone Hill Observatory (a few miles from SSI). The narrowband filter can be slid into and out of the optical path on its rails.

Figure 3. A sec-tion of a real He-lium-Neon laser fringe pattern.

GratingsBeamsplitterCollimatingLens

FringeImagingLenses

detector

inputaperture

Θ

Θ

optical pathdifference

crossed wavefronts

Figure 2. Schematic of basic SHS

The Instrument First Light for the Spatial Heterodyne Spectrometer to Detect Thermospheric Neutral Oxygen Density

via Bowen Fluorescence at 844.6 nm

S. Watchorn1, J. Noto1, L. Waldrop2, M. Migliozzi1 1. Scientific Solutions, Inc., North Chelmsford, MA ; 2. University of

Illinois—Urbana-Champaign, Urbana, IL

Acknowledgements 1. Lancaster, R .S ., Upper Atmospheric Oxygen Den-sity Determined by Combined Optical and Incoherent Scatter Radar Measurements, Ph.D. Dissertation, Bos-ton University, March, 1997. 2. Lancaster, R. S., L.S. Waldrop, R.B. Kerr, J. Noto, S.C. Somon, C.A. Tepley, R. Garcia, and J. Friedman, Brightness measurements of the nighttime OI 8446 Å airglow emission from the Millstone Hill and Arecibo Observatories, J. Geophys. Res., 105, 5275, 2000. 3. Lancaster, R. S., R. B. Kerr, K. Ng, J. Noto, and M. Franco, Recent Observations of the OI 8446 Å emission over Millstone Hill, Geophys. Res. Letters, 21(9), 829, 1994. This work is generously supported by a grant from NSF CEDAR, ATM 0535509. Sean Sullivan provided the mechanical drawings for the SHS.

The Millstone Hill SHS has obtained a possible sodium Fraunhofer line from sunlight observations. The alignment done to observe sodium will be largely carried over to observations of O I at 8446 Å from airglow. First results for fully analyzed spectra expected by the end of July. Observations of other airglow lines will follow, from Millstone and, possibly, Arecibo Observatory.

Conclusion and Prospects

Bowen SHS Parameters

Gratings…………2 x 2 inch; 1200 mm-1 ; 30.7º blaze Resolution Limit...0.07 Å Spectral Range….31.4 Å (line width: 5 Å) Field of View….....0.6º

Figure 4. A series of images of the Bowen fluores-cence SHS. (A.) Conceptual drawing of the unit, with labels for the various components. (B.) and (D.) Two views of the fully assembled SHS system. (C1. through C3.) Close-up views of (C1.) the SHS module (gratings and beamsplitter), (C2.) the output optical train with CCD, and (C3.) the filter box. The input vertical optical train is about 1.3 meters tall, and the output train about 0.5 meters long. Both are longer than originally conceived (as shown in A.), for best imaging and proper magnification. The filter box was originally built for a different project, hence its lar-ger-than-design size. The yellow tube pictured in (B.) and (D.) contains the sodium calibration lamp. A.

B. C1.

C2.

D.

Scientific Solutions, Inc.

Observing Sodium Calibration: The first step was to calibrate the SHS. This was done using a low-pressure sodium lamp (the yellow tube in pictures B. and D. in Figure 4). The lamp produced the interferogram shown in the top left of Figure 6. Note the two superposed fringe patterns, one for each of the two sodium lines. The spectrum recovered from this fringe pattern shows the 589.0-nm line as the brighter of the two (the small peak to the right of the 589.0-nm peak is a zero-frequency remnant). The wavelength calibration is 0.104 Å/pixel. Since the two lines are on either side of the transform’s center, the Littrow wavelength is between them, at about 589.1 nm. The 589.0-nm line, being closer to Littrow, produces the coarser of the two fringe patterns. Sky Observation: A sodium line filter was used for sky observation. Through the filter, the 589.0-nm peak Na lamp peak was visible, while the 589.6-nm peak was reduced (Figure 6, lower left). With this filter in place, diffuse sunlight produced a character-istic continuum source fringe pattern (Figure 6, top right). The resulting spectrum showed the Solar continuum in the 589 neighborhood. A candidate for the 589-nm Solar Fraunhofer line was seen in the resulting data. It was shifted about 1 Å from the line in the sodium lamp. Such a Doppler shift is not unreasonable for a Solar Fraunhofer line, and the width of the recovered spectral feature also is plausible. However, more study is necessary to match up the SHS Solar spectrum with the Solar spectrum.

First Observation: Solar Sodium The Millstone Hill SHS initially observed the 589-nm Solar sodium Fraunhofer doublet, in conjunction with the devel-opment of the Spatial Heterodyne Interferometer for Emer-gent Line Discrimination Spectroscopy (SHIELDS), a de-vice for battlespace Fraunhofer Line Discrimination analy-sis on target aerosol clouds. The analysis will help deter-mine the chemical composition of the clouds by measuring fluorescent filling-in of Fraunhofer lines in sunlight scat-tered off the clouds.

0 11 32 5

j g

3 72 51 3

j g

0 12 51 3

j g

1 3

j g

2p4 3P

3s 3D0

3p3 3P

3s 3S

Figure 1. A diagram of the atomic oxygen levels involved in the Bowen fluorescence pathway, labeled with wavelength in Angstroms. The principal transitions are emboldened.

C3.

Figure 6. Three fringe patterns, with associated recovered spectra, from different stages of the Bowen fluorescence SHS testing and first observation.

Na lamp, no filter

Na lamp, with line filter

Sun, with line filter Figure 5. Solar spectrum near 589 nm, showing sodium

Fraunhofer doublet. Data courtesy of National Solar Ob-servatory.