Development of low-cost sky-scanning Fabry-Perot interferometers for airglow and auroral studies
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We have developed new Fabry-Perot interferometers (FPIs) that are designed to measure thermospheric winds and temperatures as well as mesospheric winds through the airglow/aurora emissions at wavelengths of 630.0 nm and 557.7 nm, respectively. One FPI (FP01), possessing a large aperture etalon (diameter: 116 mm), was installed at the EISCAT Tromsø site in 2009. The other FPIs, using 70-mm diameter etalons, were installed in Thailand, Indonesia, and Australia in 2010–2011 (FP02–FP04) by the Solar-Terrestrial Environment Laboratory, and in Peru (Nazca and Jicamarca) and Alaska (Poker Flat) by Clemson University. The FPIs with 70-mm etalons are low-cost compact instruments, suitable for multipoint network observations. All of these FPIs use low-noise cooled-CCD detectors with 1024 × 1024 pixels combined with a 4-stage thermoelectric cooling system that can cool the CCD temperature down to −80°C. The large incident angle (maximum: 1.3°–1.4°) to the etalon achieved by the use of multiple orders increases the throughput of the FPIs. The airglow and aurora observations at Tromsø by FP01 show wind velocities with typical random errors ranging from 2 to 13 m s−1 and from 4 to 27 m s−1 for mesosphere (557.7 nm) and thermosphere (630.0 nm) measurements, respectively. The 630.0-nm airglow observations at Shigaraki, Japan, by FP02–FP04 and by the American FPI instruments give thermospheric wind velocities with typical random errors that vary from 2 m s−1 to more than 50 m s−1 depending on airglow intensity.
Key wordsFabry-Perot interferometer thermospheric wind small etalon cooled-CCD camera
The Earth’s oxygen airglow emissions at wavelengths of 557.7 nm and 630.0 nm have emission layers at altitudes of 90–100 km and 200–300 km, respectively. The auroral emissions in these lines also come from the mesosphere and thermosphere (e.g., Chamberlain, 1961). High-resolution interferometric measurements of the spectral profiles of these auroral and airglow emissions provide a unique opportunity to make remote sensing measurements of neutral winds and temperatures in the mesosphere and thermosphere regions. Other techniques such as that of incoherent radar systems cannot observe directly the dynamics of neutral particles in the thermosphere. Fabry-Perot and Michelson interferometers are two major techniques that have been used to measure Doppler winds and temperatures for very weak emissions from airglow and aurora. These instruments have a throughput that is large when compared with the performance of grating spectrometers. Various investigations and improvements have been reported for these interferometers, for example, studies featuring long-term (solar cycle) measurements (e.g., Hernandez and Roble, 1995; Biondi et al., 1999), satellite-borne instruments (DE2: Killeen and Roble, 1988; UARS/HRDI: Hays et al., 1993; and TIMED/TIDI: Killeen et al., 1999), use of cooled-CCD detectors (e.g., Biondi et al., 1995; Shiokawa et al., 2001, 2003), two-dimensional imaging capability (e.g., Rees et al., 1984; Nakajima et al., 1995; Ishii et al., 1997; Conde et al., 2001; Sakanoi et al., 2009; Kosch et al., 2010), and tristatic measurements of the auroral thermosphere (e.g., Aruliah et al., 2004). The recent development of robust thermoelectric-cooled CCD detectors combined with a computer control system makes it possible to fully automate the operation of these Fabry-Perot interferometers (FPIs). Using these instruments, the development of multi-point network measurements of thermospheric dynamics as advocated by Meriwether (2006) is now feasible at a reasonable cost. However, large diameter etalon FPIs are rather expensive when compared with the expense of conventional all-sky imagers and photometers. The high cost of these FPIs is mainly a result of the use of large-aperture etalons with diameters of 116–150 mm in their design to measure weak airglow and auroral emissions with high sensitivity.
In this paper we report on the characteristics and initial results of the FPIs regarding the measurements of the spectral line shapes of aurora and airglow emissions at high and low latitudes. Four of these FPIs (FP01–FP04) were developed as part of the Optical Mesosphere Thermosphere Imagers (OMTIs) which consist of more than ten airglow imagers, five FPIs, and seven airglow photometers (Shiokawa et al., 1999, 2009). Most of these FPIs use small-aperture etalons with a diameter of 70 mm and a fixed gap of 15 mm. Only one FPI (FP01) uses the typical large-aperture etalon with a diameter of 116 mm and a tunable gap, giving higher sensitivity and more flexibility in the measurements. However, the price of the 70-mm fixed-gap etalon is only 1/15 of the 116-mm tunable-gap etalon. The decrease of the sensitivity for the small-aperture FPIs is compensated by enlarging the incident angle of light to the etalon. This idea was first used for the design of MiniME: a miniaturized Fabry-Perot interferometer for portable applications (e.g., Makela et al., 2009, 2011; Meriwether et al., 2011). As a result we have constructed low-cost compact FPIs that are suitable for the multi-point network observations of thermospheric dynamics on a global scale. Here we show characteristics of these FPIs and initial results from observations at high and low latitudes.
For the other FPIs (OMTI and American) the Fabry-Perot etalon (ET116) was made by IC Optical Systems (ICOS) with a diameter of 116 mm and a gap spacing, d, of 15 mm. The etalon is stored in a sealed pressure housing. The reflectivity, R, for the OMTI etalon is 0.85 (corresponding finesse: 19.3) and for the American FPIs 0.77 (corresponding finesse: 11) and the surface flatness is λ/100. The gap spacing is stabilized by a feedback controller (CS100) with three sets of glass reference capacitors and piezoelectric transducers on Zerodur pillars. The feedback accuracies of etalon spacing for ambient temperature change are 0.75 nm/K for ET116 and 0.05 nm/K for CS100. The feedback accuracy of 0.75 nm (Δd) corresponds to the Doppler shift of a fringe caused by a wind velocity v of 15 m s−1 (Δd = υd/c, where c is the speed of light). This value is rather large, since controlling the etalon temperature with an accuracy of 1 K is difficult. By assuming a uniform wind in the FoV of the FPIs, we can monitor this temperature drift of the etalon gap, as described in Section 4.
The Fabry-Perot etalons for FP02, FP03, and FP04 were made by TecOptics with a diameter of 70 mm and a gap spacing, d, of 15 mm. The American FPI etalons were purchased from ICOS. The etalon is stored in a sealed pressure housing. The reflectivity, R, is 0.76 (corresponding finesse: 11.4) and the surface flatness is λ/100. The relatively low reflectivity compared with FP01 is selected in order to increase the transmission T of the etalon (T = 0.14 for R = 0.76 and T = 0.081 for R = 0.85), though the accuracy of the temperature measurement decreases for lower finesse. The gap spacing is fixed by optical contacting Zerodur pillars to the etalon plate surface.
The temperature of the interference filters of FP01 is controlled to be about 25C, while that of the small-etalon FPIs (FP02–FP04) is not controlled. The filter temperature changes according to the room temperature. However, the shift of the transmission wavelength of interference filters does not cause an artificial Doppler shift, because we measure the shift of the emission profile line center itself. The shift of the filter wavelength may cause attenuation of the output fringe intensity, but this effect must be small since we used filters with rather wide width (FWHM = 2.5 nm) and the typical shift of the filter transmission wavelength is 0.2–0.3 nm/10 degree. The filters we used are non-image quality. For image quality filters, the authors have seen the development of multiple reflections between the image-quality filter and the etalon surface, generating ghost fringes on the CCD image. Thus, we avoid using image-quality filters.
The detector used by all four FPIs and by the American FPIs was a cooled-CCD camera, the Hamamatsu C4742-98-26KWG2S and the Andor DU 434 detector, respectively. This model has a 4-stage thermoelectric system cooling the CCD down to −80°C. The other side of the thermoelectric cooling junction is cooled by water flow, making a water cooler and a pump necessary for these FPIs. The read-out and dark noise levels of this model CCD camera are 6 electrons (r.m.s.) and 0.0012 electrons/s/pixel, respectively. These specifications of high pixel resolution (1024 × 1024 pixels) and low noise are essential for the successful detection of the 630.0-nm nightglow emission with good ratio of signal to noise. The large incident angles to the etalon (1.3° for FP01 and 1.4° for FP02–FP04) allow for the detection of many fringes (~12 fringes for FP01 and ~15 fringes for FP02–FP04) on the CCD plane, and the successful analysis of these spectral profiles after integration around the ring center requires low noise. For the American FPIs using Andor CCD cameras, the readout and dark noise are more typically 3–4 electrons (r.m.s.) per pixel and 0.0005 electrons/s/pixel.
All of the FPI optics are calibrated by using a frequency-stabilized HeNe laser (Spectra Physics 117A-LW) at a wavelength of 632.8 nm. The laser light is used for ring centering and achieving a good focus by adjusting the location and angle of the etalon and the CCD camera. The laser light is also used to determine the instrumental broadening function which is used to estimate the temperature of the emitting species (Killeen and Hays, 1984). Using a multi-mode fiber optic cable, the laser light is passed into a square-shape scattering chamber whose interior walls have been painted by a white scattering Teflon with an opal glass on the top; this approach achieves a quasi-Lambertian surface. The sky scanner is programmed to look into the opal glass of the calibration laser source by moving to the angle coordinates of an elevation angle of 180° and the azimuth angle corresponding to the direction to the calibration chamber.
In this paper we show two examples of airglow/aurora measurements. FP01 has been in operation at Tromsø, Norway (69.6°N, 19.2°E, geomagnetic latitude: 67.1°) since January 12, 2009. It has already succeeded in measuring strong vertical winds in the lower thermosphere associated with pulsating auroras (Oyama et al., 2010). Here we show data obtained on January 24, 2009, when the sky was mostly clear. The sequence of sky scan directions was made in the order of north, south, east, west, vertical, and laser with an exposure time of 1 min for each direction. The elevation angles used for each north, south, east, and west direction is 45°. The exposure time for the laser fringe was 0.2 s. The 630.0-nm and 557.7-nm emissions were observed in succession for each direction. Including each 15-s interval for image transfer from the CCD to the computer, one sequence of observations to obtain the wind field of 630.0-nm and 557.7-nm emissions takes 13 min.
We also made a test observation of FP02, FP03, and FP04 at Shigaraki, Japan (34.8°N, 136.1°E, geomagnetic latitude: 25.4°) on July–September 2009. In this paper we show data obtained on August 17, 2009, when the sky was clear. The sky scan was made in an order of north, south, east, and west with an exposure time of 3.5 min per each direction. The elevation angles to look into north, south, east, and west are 45°. Including each 15-s interval for image transfer from the CCD to the computer, one sequence of observation to obtain the wind field of 630.0-nm emissions takes 15 min. The laser fringes are obtained at the beginning and at the end of the one-night observation. The exposure time for the laser fringe was 0.2 s in routine observation. All these observation sequences are set by a schedule file in the personal computer.
The 0.1-s laser exposure for FP02 in Fig. 4 was only for the test observation. In the routine measurements, we use an exposure time of 0.2 s for all the four FPIs of FP01– FP04. The exposure times of the laser fringes do not express the actual sensitivity of the airglow measurement of FPI, because the laser wavelength (632.8 nm) is at the edge of the transmission of the 630.0-nm filter. Even though all the 630.0-nm filters are made in the same specifications, the transmittance at 632.8 nm can be different depending on filters. The laser scattering box also can have different intensity depending on the relative location between the laser beam and the center of the optical fiber.
4. Doppler Shift Determination for the Line-of-Sight Observation of the Neutral Wind Vector
The horizontal wind velocities in Figs. 7(f) and 7(g) vary with amplitudes of 50–100 m s−1. There was aurora seen at 4–6 UT and after 22 UT as shown in the peak count in Fig. 7(d). The oscillating features seen in the peak count data are due to the spatial gradient of airglow/auroral intensity; the auroras mainly appear in the northern sky of Tromsø. The standard deviations of the wind obtained by 10 fringes were 2–13 m s−1. The standard deviations (error bars in Figs. 7(f) and 7(g)) decrease as the airglow/aurora intensity becomes larger. The fitting errors in Fig. 7(c) are mostly less than 1% of the peak counts except for occasional enhancements.
The vertical wind velocity in Fig. 7(e) is shown as the deviations from the average over a single night data. It continuously decreases from 0 UT to 24 UT. This decrease is mainly due to the etalon gap drift, since a similar decreasing trend can be seen in Figs. 7(a) and 7(b). The amplitude of the decrease is 200 m s−1. According to the feedback accuracy of the etalon (0.75 nm/K, i.e., 15 m s−1/K), the 200 m s−1 drift corresponds to a temperature change of 13 K. A large change (~100 m/s = 7 K) just after the sunset at 16–18 UT is probably due to the direct heating of the black FPI body under the transparent dome by the sunlight. This etalon temperature change can be larger than the ambient temperature change due to the greenhouse effect of the dome. The causes of the variations at other times are not clear.
The vertical wind velocity in Fig. 8(e) shows continuous decreases from 0 UT to 24 UT similar to that seen in the 557.7-nm plot in Fig. 7(e), indicating that this decrease is mainly due to the etalon gap drift seen in Figs. 8(a) and 8(b). However, there is a slight difference in the trend seen in Figs. 8(a) and 8(b) compared with that in Figs. 7(a) and 7(b) and in Fig. 8(e). This may be because of spatial non-uniformity of the horizontal wind velocity in the thermosphere. The fitting errors in Fig. 8(c) are slightly larger than that in Fig. 7(c) and are mostly less than 2% of the peak counts except for occasional enhancements.
5. American FPI Observatories with 70-mm Diameter Etalons
Three FPI observatories fitted with 70-mm diameter etalons combined with Andor cameras are currently operating in Nazca, Peru (14.97°S, 74.89°W), Jicamarca (11.96°S, 76.86°W), and Poker Flat, Alaska (65.12°N, 147.43°W). Three more are nearly operational—Urbana-Champaign (Illinois), Ann Arbor (Michigan), and Jenny Jump (New Jersey). These FPI instruments follow the same design indicated in Fig. 2, but are equipped with 31 cm focal length lenses that generate 11.6 rings on the 1024 × 1024 CCD chip with 13 micron pixels. A double axis mirror system is used to direct light from the 630.0-nm nightglow layer into the etalon. The incoming light is filtered by a 77-mm diameter 630.0-nm filter placed in front of the 70-mm diameter etalon. By rotating the skyscanner to the correct azimuth and zenith angles at intervals of ~20 minutes, light from a HeNe frequency-stabilized laser is passed into the FPI etalon for calibration of the instrument function and the etalon stability. A thermal controller maintains the etalon temperature to within 0.1°, and typically, there is no more than 10–15 m s−1 drift exhibited over ten hours of operation.
6. Summary and Discussion
In this paper we describe characteristics of the four new FPIs developed by the Solar-Terrestrial Environment Laboratory, Nagoya University, and six new FPIs developed by the Clemson University and other American institutions. We show examples of their Doppler shift measurements for both the OMTI and American FPI instruments. FP01 has a large-aperture etalon with a diameter of 116 mm. FP02-FP04 and those of Clemson University, with smaller etalons (70 mm), are low-cost compact instruments which are well suited for multipoint network observations of the thermosphere. The large incident angles (maximum: 1.3–1.4°)to the etalon increases the throughput of the FPIs by increasing the number of interference fringe available for Doppler analysis. To achieve low-noise measurement with highpixel resolution, all of these FPIs use cooled-CCD detectors with 1024 × 1024 pixels and 4-stage thermoelectric cooling system that can cool the CCD temperature down to −80°C. The standard deviations of the wind estimates obtained from the analysis of each of these 10 fringes give an overall determination of the random errors of the wind measurement for each direction. The airglow and aurora observations at Tromsø by FP01 on January 24, 2009, show wind velocities with random errors of 2–13 m s−1 and 4–27 m s−1 for mesosphere (557.7 nm) and thermosphere (630.0 nm) measurements, respectively. The 630.0-nm airglow observations at Shigaraki, Japan, by FP02–FP04 on August 17, 2009, give thermospheric wind velocities with random errors varying from 2 m s−1 to more than 50 m s−1.
To show the high throughput of the present system, we compared the throughput and transmission for FP01–FP04 and for the FP00 used at Shigaraki since October 2000 (Shiokawa et al., 2003). The maximum incident angle, the etalon diameter, and reflectivity of FP00 were θ m = 0.7°, L = 116 mm, and R = 0.85, respectively. FP01 uses a 116 mmϕ tunable-gap etalon which is identical to that of FP00. The sensitivity of the FPI optics is proportional to the throughput AΩ = 2π(1 − cos θ m )π(L/2)2 and etalon transmission T = (1 − R)/(1 + R). The values of AΩT for FP00, FP01, and FP02–FP04 are 0.00402, 0.0132, and 0.0100 (cm2 sr). Thus, FP01 and FP02–FP04 are 3.3 and 2.5 times higher in sensitivity, respectively, compared with that used at Shigaraki with 116 mm etalon, although FP02-FP04 are much smaller (70 mmϕ etalon) and considerably cheaper than FP00.
The temperature drift of the etalon gap spacing can be monitored using the observed airglow fringes as shown in Eq. (3). This drift causes systematic offset on the horizontal wind velocities, because the horizontal wind is estimated from the difference of the fringe peak location between north and south. However, this systematic offset seems to be small. For example, even if there is a large etalon gap drift that corresponds to 100 m s−1 in one hour, it causes systematic offset of only 2 m s−1 for FP01 (timing difference of 1.25 min between north and south) and 6 m s−1 for FP02–FP04 (timing difference of 3.45 min between north and south). However, this etalon gap drift directly affects the vertical wind measurements, as shown in Figs. 7(e) and 8(e). This effect may be avoided by installing a slow feedback heater around the etalon box or by considering the fact that the temperature drift is a rather slow effect compared with that of auroral vertical winds.
As shown in the observation data, the random errors of the measured winds depend significantly on the airglow intensity. The test observations of FP02–FP04 at Shigaraki were carried out on July–September 2009 at the solar minimum period during which the 630.0-nm airglow intensity is generally weak. FP02, FP03, and FP04 were installed at Chiang Mai, Thailand (18.8°, 98.9°E) on February 24, 2010, at Kototabang, Indonesia (0.2°S, 100.3°E) on June 11, 2010, and at Darwin, Australia (12.4°S, 131.0°E) on March 19, 2011, respectively. The 630.0-nm airglow intensity is generally higher at these stations compared with that at Shigaraki, since the latitudes of these stations are lower than Shigaraki. Thus, we expect to determine the winds at these three locations with better accuracy. FP02–FP04 using small-diameter etalons are compact low-cost instruments suitable for multi-point network observations. Kototabang and Chiang Mai, and Darwin and Shigaraki are two pairs of nearly geomagnetic-conjugate stations at low and middle latitudes, respectively, making new conjugate observations of thermospheric winds available. The analysis procedures necessary to estimate thermospheric temperatures from the observed 630.0-nm fringes will be described in the near future. Makela et al. (2011) described the analysis procedures applied to the data from the American FPI instruments.
We are very grateful to T. Kato of the Solar-Terrestrial Environment Laboratory, Nagoya University, for his skillful support in developing the frame body of the FP02, FP03, and FP04. The frame body of FP01 and all the four sky scanners were developed by KEO Scientific. This work was supported by the Special Funds for Education and Research (Energy Transport Processes in Geospace) from MEXT, Japan and by Grants-in-Aid for Scientific Research (20244080). One of us (JWM) acknowledges support from the National Science Foundation Aeronomy program provided by the award NSF-0634671.
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