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Innovating Airglow: Facilitating the Aeronomy Ecosystem
John Noto1, S. Kapali1, J. Riccobono1, M. Migliozzi1 R.B. Kerr2, R. Garcia2, E. Robles2, C. Brum2, J. Friedman I. Azeem3, G. Crowley3
2

1. Scientific Solutions, Inc., North Chelmsford, MA, USA 2. Arecibo Observatory 3. ASTRA, LLC, Boulder, CO, USA

ABSTRACT

Results from recently upgraded optical interferometer systems installed at the Arecibo Observatory in Arecibo, Puerto Rico are presented. These instruments autonomously and simultaneously observe optical emissions from thermospheric oxygen at two different altitudes. These new dichroic instruments, which combine a telecentric imaging system with large aperture interferometers, have resulted in a dramatic increase in instrument sensitivity and data collection cadence. The instrument control system is modernized for fully robotic operation utilizing a client-server model. Thermospheric winds and temperatures are collected every clear night of every month, with wavelength calibration, camera diagnostics (bias, dark, and flatfield calibrations), and data-taking control available from any internet connection. Neutral, meridional and zonal wind vectors typically have 1 m/s statistical errors, and neutral temperatures have statistical errors <15K in three minute exposures. Operation and analysis algorithms operate in a fully automated manner, including calculation of geophysical parameters with a data quality index. These data are made available each morning following observations. This unique Data as a Service (DaaS) methodology is being extended to a family of airglow monitoring instrumentation including all-sky imagers and meridional spectrographs.
Keywords: Thermosphere, neutral winds, Fabry Perot

1. INTRODUCTION
Recently the existing "Red-Line" Fabry-Perot (FP1) at Arecibo Observatory (AO) and Millstone Hill Observatory (MH) were upgraded to modern "Doppler Imagers" (DI) with attached "Green-Line" DI. The upgraded instruments provide F-region and E-region neutral dynamics simultaneously, with high temporal resolution, and with automated reliability. Both the red-line and green-line run for more than twenty days surrounding new moon, every month, except when either channel might be interrupted for campaign mode OH temperature measurements in the upper thermosphere.

Upgraded Doppler Imagers: ! Upgrades & automates the existing 6300е FPI at Arecibo. ! Adds a dedicated 5577е FPI. ! Will observe 6300е & 5577е automatically.

! Provides temperature and wind
data to the Madrigal database.


The primary motivation for this upgrade was to improve the current FPI sensitivity roughly 40-fold at 6300е, permitting rapid measurements of temperatures and line-of-sight winds consistent with the evolutionary timescales of artificial airglow features generated by new HF ionospheric interaction facility [Bernhardt et al., 1988a, 1988b, 1991; Mantas and Carlson, 1996]. We estimate the sensitivity improvement for 5577е measurements to be of similar magnitude. The second motivation is to provide E-region neutral dynamics data on a regular basis. A split optical path (6300е and 5577е) using the same mirror system permits simultaneous measurement of the 5577е OI greenline with the 6300е measurements. Prior to the upgrades, green-line measurements were only performed at Arecibo when a second Fabry-Perot system (FP2) was specifically configured for the measurement. Because the FP2 was commonly used for H-alpha measurements, green-line measurements of E-region neutral dynamics were seldom available from Arecibo. Given the amount of E-region and lower thermospheric research now being performed at and around Arecibo, it was time that neutral E-region dynamics and temperature became part of the regular AO data stream again. FP1 had been in operation for nearly forty years, using a photomultiplier detection system to collect airglow data nearly every month. Maintenance issues (dating to more than four years ago) with the PDP-11 based data acquisition system, coupled with the retirement of key staff, had resulted in lengthy instrument downtimes, and staccato operations. As such, the third motivation for this upgrade was the need for the provision of regular, reliable, calibrated measurements every month, through automation and remote control. This regular provision of high quality neutral dynamic data in the thermosphere is essential to the improvement of neutral wind models and to the prospect of a predictive Space Weather capability.

2. DOPPLER IMAGER OPTICS
The Arecibo Doppler Imaging FPI, shown in Figure 1 and Figure 2, is an f/8.1 system, with 15.00 cm clear aperture and a 122 cm effective focal length. The 76.2mm 6300е interference filter is 4.8е broad, in a telecentric optical configuration that relieves the requirement for image-quality filtering, Figure 2. Cosmetic filter defects are removed by flat-field calibration. The etalon system is an air-gap, with 1.056 cm spacing. This provides a 0.1879е free spectral range at 6300е, and a typical spectral resolution of 0.0211е. Five orders of the 6300е airglow emission are sampled simultaneously from a 2° field-of-view with an Andor DU934P-BV camera featuring a 1024x1024 E2V47-10 back illuminated CCD. This detector has a quantum efficiency of 95% at 6300е, with dark noise of 0.0001 es-1 pixel at -90C. Typical instrument performance produces 6300е wind vectors with accuracies of Figure 1. approximately ± 2 m/s and temperatures with the DI. accuracies of ±10K using three-minute exposure times. Exposure times can be enhanced to reduce temperature errors, or reduced to improve time resolution of wind vectors.

Optical schematic of one arm of

A twin-mirror pointing system with 0.5 degree pointing accuracy mounted beneath a clear plexiglass dome is used to point to any direction in the sky. The standard operating mode is to make measurements in the four


cardinal directions (North, South, East, and West) at a 45° zenith angle, with a fifth measurement in the zenith. Using 3-minute exposures in each direction, approximately 100 meridional and zonal wind vectors are collected each night. The Doppler Imaging FPI can be configured to automatically operate in 6300е and 5577 е observing modes (look directions, integration times) specified by the experimenter. An unlimited number of modes can be scheduled by the user each night, and for weeks in advance. Full remote operation is also integrated into the system, via internet communication. The etalon instrument function is sampled using a frequency stabilized HeNe laser transmitting at 6328.165е. Laser, CCD bias, and dark current calibrations can be easily accomplished automatically or via remote control, day or night. Similarly, flat-field calibrations can be completed automatically or by remote control. Ring image-processing proceeds by assembling multiple bias, dark, and flat-field images. Flat-field images need not be gathered each night, only at times when some aspect of the instrument configuration has been altered. Bias, dark, and laser images are collected each night. In reality, the CCD dark current is so low

Figure 2. Optical configuration for the Doppler Imagers. Light enters the instrument via a pointing head. It is then separated by a dichroic mirror into light with a wavelength greater than 6000е and light with a wavelength shorter than 6000е. The "red" light passes straight through and the "green" light is folded by 90 degrees. After passing through the appropriate etalon a series of lenses are used to image the fringe pattern telecentrically onto an interference filter. Then it passes through a field lens, collimator, and finally a camera lens is used to re-image the fringe pattern onto a CCD camera. that the dark images are dominated by the CCD bias, and dark current is negligible ­ even for exposure times exceeding five minutes. The bias, dark, and flat-field images are median-filtered in time, to remove cosmic ray-induced high-charge pixels. Each image is also passed through a 3 by 3 pixel running standard-deviation filter to remove hot or dark pixels. Airglow (sky) images and laser images are also filtered with the standarddeviation filter. The bias frames are averaged in time, and subtracted from the airglow and laser images. The dark images are also averaged in time and subtracted from the airglow and laser images. Finally, an averaged


flat-field image is normalized to unity at its highest value in arbitrary analog to digital conversion units (ADU), and divided into each airglow and laser image to account for variable response across the CCD. The flat-field removal also serves to remove image artifacts that are the consequence of cosmetic defects in the interference filter or other optical elements. Ring images processed in this manner are then analyzed for extraction of the geophysical parameters, including line-of-sight winds in each cardinal direction, meridional and zonal winds, meridional and zonal gradients, temperatures, and relative brightness. The imaged FPI ring pattern is analyzed by summing electrons in radial annuli, or radial "bins", whose radial distances (bin number) from ring-pattern center are linear in wavelength. Because the annuli mapped over square pixels consequently contain slightly different numbers of pixels, the unit of merit becomes signal/pixel. These bins are then aligned linearly to produce the 6300е Doppler spectra.

Figure 3. Typical neutral winds at Arecibo

These "bin summed" spectra are fit with a Gaussian function using the Levenberg-Marquardt non-linear least squares algorithm. The data supplied to the community uses a linear background functional form and statistical weighting in this process. The fits are then analyzed for brightness in the emission line and for background brightness by integrating under the spectra over one free spectral range. Temperatures, Figure 3, are extracted by removing the instrument function width, which is sampled by the HeNe laser calibrations. Presently, the process used to isolate the actual emission line width is to subtract the square of the laser width from the square of the measured line width, and calculate the square root of that value. Winds, Figure 4, are extracted by comparing the line center of measurements in the cardinal direction to the line center of the zenith measurements. It is assumed that vertical winds are insignificant relative to the horizontal winds and relative to the instrument spectral resolution. Line-of-sight winds are calculated by subtracting the line center bin position in a cardinal direction from the zenith center position interpolated in time to match the observation time at the cardinal point. (The zenith line center position is monitored throughout the night, and does not remain precisely constant, due either to the presence of small vertical winds, or to slight thermal instrument drift, or both.) These line-of-sight vectors are cosine-corrected to lie in the emitting layer, and use the convention of positive northward and positive eastward. Meridional [zonal] winds are calculated by summing the North-South [East-West] line-of-sight vectors and dividing by two. Each North or South [East of West] measurement creates a meridional [zonal] vector by interpolating the corresponding South or North [West of East] vector corresponding to the time of the North or South [East of West] observation. Meridional [zonal] gradients, South-to-North [West-to-East] are calculated by subtracting the North [East] line-of-sight vector from the South [West] line-of sight vector, and dividing by the distance between the two emission regions, which is established by the emission height and the observation zenith angle.


2.1 The Importance of Telecentricity
An objective lens, with a focal length of 1250mm is used to image 5 complete orders of the Fabry-Perot interference pattern onto a 76.2mm diameter narrowband interference filter, Figure 5. The 4.8е wide interference filter acts to both minimize background contamination and to order-sort the Figure 5. This raytrace demonstrates the etalon in wavelength. telecentric back end of the FPI A le n s p la c e d b e f o r e th e f ilte r is u s e d to r e n d e r th e focusing rays telecentric in the image space, Figures 5 & 6. Using a telecentric optical system, all of the rays incident on the filter are parallel to the chief ray and are incident on the filter at the same angle. This removes center-to-edge variation in the width of the fringes caused by the varying angle of incidence in a collimated system. Next is a field lens and finally a re-imaging lens system is used to image the interference pattern onto a 1024x1024 back illuminated (EEV) CCD in an Andor Ikon DU934P-BV camera system. Instrument calibration is achieved u s in g tw o different calibration sources. First a low brightness white light source consisting of a Figure 6. The small change in angle in a non-telecentric system will tungsten bulb and a increase "walk-off" or change in the width of the instrument function diffuser sheet is used to towards the edge of the FOV "flat field" the system. A frequency stabilized HeNe laser is used to measure the instrument width of the Fabry-Perot system. Flat fields and laser calibration images are collected several times nightly. Both the red and green arms are calibrated with the same HeNe laser, relying on a small amount of filter "leakage" to image the 6328е fringe pattern formed from the laser. A fiber optic feed exists between the passive optical building and the LIDAR facility which allows the calibration of the greenline etalon using the LIDAR feeder lasers, a unique capability. Determining the flat-field of the instrument is done as often as necessary. Flat fields are quite stable, as long as the instrument configuration is stable. The flat field calibration of the instrument is obtained by shining a light onto a light diffusing screen that is permanently installed above the instrument and fills the field-of-view of the instrument. An image of that uniform source of light is obtained with all components in the optical path of the instrument so that any imaging artifacts from the optical system can be removed from the data.

Telecentric

Not Telecentric

2.2 Calibration System

2.3 Software Development
SSI has developed two software packages: (a) ImageTool to operate and control its FPIs and (b) Airglow Server to analyze the resulting fringe images. For interferometer operation, ImageTool was primarily developed using Interactive Data Language (IDL); this allows rapid implementation of a graphical and user-friendly interface that is system independent. SSI has also created additional control software that can control multiple CCD cameras using a simple pull down


menu. A powerful scheduling utility was created that allows the user to create and save multiple independent observing events or modes utilizing the pointing head, filter wheel, and any selected device. ImageTool is also Internet aware and capable of remote observing sessions. Airglow Server was developed by SSI to automatically collect (either through direct internet connection or file mirroring to the Amazon EC2 cloud) airglow data collected by multiple instruments. Currently data is collected from four Fabry-Perot Interferometers and an all-sky imager. This linux-based software solution is both flexible and expandable. After the data is collected, it is sorted and analyzed using the process described above. After post processing, the wind and temperature data are uploaded to the Madrigal database and to anyone who requests a daily transfer. Data from the previous night is analyzed and added to the database by 10AM EST the following morning. Data quality is continually tested. Individual points are rejected if 1) they vary by more than 20% in spectral width or line center, 2) the background is abnormally high, 3) the fit residuals are high or 4) the spectral amplitude is less than 20% of the background. Data surviving these tests are given a quality code of 0 to 2 corresponding to "excellent" to "dubious". More information is available at http://www.neutralwinds.com.

2.4 Operational Efficacy and Performance
To date the 630nm system at AO has collected 456 nights of data since May 2012. Analysis and calculation of quality control parameters is completely automated by the "Airglow Server" that collects the data each morning from the instruments in the field and then analyzes, publishes it to the web and loads it into the Madrigal database.
Table 1. Summary of current operational parameters of facility Doppler Imagers AO Redline Wavelength [е] Emission Altitude Etalon [mm] Etalon Gap Camera Dark [e-/sec/px] QE Filter [mm] Filter FWHM [е ] FOV [deg] FSR Resolution [е ] Instrument F/# Positions Observed Int. Per [sec] Orders Observed Temperature Error Wind Error CCD Age [years] 6300 240Km 150 10.560 Andor Ikon DU934P-BV 1.0x10-4 @ -90C ~95% 76.2 4.8 2 0.1879 0.0211 8.1 5 180 5 ± 10 K ±1 m/s ~1.5 AO Greenline 5577 120Km 150 25.00 PI Pixis 1024BR 0.058@ -70C ~85% 50.8 4.0 1 0.0622 0.0078 9.5 5 180 5 N/A N/A 6


Figure 7. This figure shows the number of nights each instrument has run. The greenline system will be operational at Arecibo in the summer of 2014.

3. CONCLUSIONS
These upgraded Doppler Imagers have resulted in more precise measurements of the neutral atmosphere at Fregion altitudes. Instrument sensitivity has increase by about x30 and robotic operation has resulted in a much larger quantity of collected data. Similar instrumentation at Millstone Hill and future upgrades at additional ISR sites will result in a chain of unified neutral dynamics monitoring stations that will foster a greater understanding of the interaction between the neutral atmosphere and the ionosphere, particularly during periods of high solar activity.

4. ACKNOWLEDGMENTS
The authors gratefully acknowledge funding provided by the NSF Aeronomy program under grants AGS1100920 & AGS-0940257.


5. REFERENCES
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