Manuscript prepared for Geosci. Instrum. Method. Data Syst. with version 4.2 of the LATEX class copernicus.cls. Date: 2 April 2014 Auroral spectral estimation with wide-band color mosaic CCDs Brian J. Jackel1, Craig Unick1, Mikko T. Syrjasuo¨ 2, Noora Partamies3, James A. Wild4, Emma E. Woodfield5, Ian McWhirter6, Elizabeth Kendall7, and Emma Spanswick1 1Physics and Astronomy Department, University of Calgary, Canada 2School of Electrical Engineering, Aalto University, Finland 3Finnish Meteorological Institute, Helsinki, Finland 4Department of Physics, Lancaster University, United Kingdom 5British Antarctic Survey, Cambridge, United Kingdom 6Department of Physics and Astronomy, University College London, United Kingdom 7SRI International, Menlo Park, California, USA Correspondence to: Brian Jackel [email protected] Abstract. ::::::Optical::::::aurora :::can:::be ::::::::structured::::over::a::::wide::::::range ::of::::::spatial :::and::::::::temporal:::::scales:::::with ::::::spectral:::::::features::::that:::::::depend ::on:::the:::::::energy ::of:::::::::::precipitating ::::::::particles. :::::::::Scientific ::::::studies::::::::typically :::::::combine::::data::::from::::::::multiple ::::::::::instruments :::::which:::are:::::::::::individually ::::::::optimized:::for:::::::spatial,:::::::spectral,:::or :::::::temporal:::::::::resolution.::::One::::::recent :::::::addition ::::::::combines::::::all-sky:::::optics::::with:::::color::::::mosaic:::::CCD::::::::detectors 5 :::::which:Color mosaic CCDs use a matrix of different wide-band micro-filters to:::::::::produce ::an::::::image in order to produce images with several (often three) color channels. These devices ::::::provide: are increasingly employed in auroral studies to provide time sequences of two dimensional multi-spectral::::::::::: :::::::::luminosity ::::with:::::::::::simultaneous::::::::exposure:::of :::all :::::color :::::::channels::::::::allowing::::::::::::inter-channel ::::::::::comparison ::::even :::::during:::::::periods::::with::::::rapidly:::::::varying::::::aurora.:::At::::::present:::::color::::::auroral::::::image ::::data :::are ::::::::primarily 10 luminosity maps, but the color information is typically only used for qualitative analysis. In this study a:::::::::::quantitative::::::::approach:::::based:::on we use Backus-Gilbert linear inversion was:::::::used::to::::::better :::::::::understand :::the techniques to obtain quantitative measures of effective spectral resolution ::of:::::::existing :::and::::::::proposed ::::::::::instruments.: :::Two:for multi-channel color mosaic CCDs. These techniques also allow us to explore the possibility 15 of further improvements by modifying or combining multiple detectors. We consider two spec- trally calibrated commercial ::::::::detectors color CCDs (Sony ICX285AQ and ICX429AKL) with::::::::very :::::::different :::::color :::::::mosaics:::::(RGB:::vs.::::::::CYGM):::::were:::::found:::to ::::have::::very:::::::similar :::::::spectral ::::::::resolution::: :3::::::::channels ::::with::::::::::::FWHM≈100 ::::nm; :a::::NIR::::::::blocking:::::filter ::is ::::::::important:::for:::::::::stabilizing::::::::inversion:::of ::::both ::::::::3-channel::::::::::::configurations.:::::::::Operating:::the :::::::::::ICX429AKL::in:a::::::::::::non-interlaced:::::mode::::::would :::::::improve 20 ::::::spectral:::::::::resolution :::and:::::::provide ::an:::::::::additional :::near::::::::infra-red:::::::channel.::::::::::::::Transformations::::from::::::::arbitrary 1 :::::device::::::::channels ::to:::::RGB :::are :::::easily::::::::obtained ::::::through:::::::::inversion.::::::::::::Simultaneous:::::::imaging::of::::::::multiple ::::::auroral ::::::::emissions::::may ::be:::::::achieved:::::using:a::::::single ::::color::::::camera::::with::a triple-pass:::::::::::::filter.::::::::::::Combinations ::of :::::::multiple :::::::cameras ::::with::::::simple :::::filters ::::::should ::::::provide::::∼ 50:::nm:::::::::resolution:::::across:::::most ::of:::the::::::visible ::::::::spectrum. ::::::::::::Performance ::of:::::other:::::::::instrument:::::::designs:::::could:::be::::::::explored::::and ::::::::compared:::::using::::the 25 ::::same::::::::::quantitative::::::::::frameworkin order to determine effective wavelength resolution of each device individually, together, and with additional filters. From these results we develop methods to enhance the utility of existing data sets, and propose ways to improve the next generation of low-cost color auroral imaging systems. 1 Introduction 30 Visible aurora :::::occurs:is produced when energetic charges from outer space collisionally excite at- mospheric atoms and molecules. These charges tend to move along magnetic field lines connecting the upper atmosphere to more distant regions of the magnetosphere or solar wind. Consequently, ob- serving the spatial distribution and temporal variation of aurora can provide important information about geospace topology and dynamics. Table 1. Commonly studied visible auroral wavelengths. λ [nm] source + 391.4 N2 1NG(0,0) + 427.8 N2 1NG + 470.9 N2 1NG 486.1 Hβ 1 3 557.7 O( D − P2) 630.0 O(1D ! 1S) 656.3 Hα 777.4 OI 844.6 OI 35 Additional details about source population energy distributions may be obtained if optical observa- tions are ::::::::spectrally:spectroscopically resolved. Emitted photon wavelengths can have a complicated dependence on atmospheric composition and density, but certain combinations of emissions may be used to make useful inferences about the precipitation energy (Rees and Luckey, 1974; Strickland et al., 1989). A list of the most commonly studied auroral emissions is included in Table 1, and will 40 be referred to throughout this study. ::::::Visible ::::::::emissions::::are ::::::usually::::::::::dominated :::by :::::some :::::::::::combination ::of:::the::::::557.7 :::nm:::::::(green)::::and + :::::630.0 :::nm ::::(red):::::lines,::::with:::::most ::of:::the:::::::::remainder ::::::coming:::::from ::::::::N2 1NG:::::(blue):::::lines.::::The::::total:::of ::all::::near::::::::infra-red :::::(NIR)::::::::emissions::::may:::be::::even::::::larger, :::but::::most:::::::auroral :::::::imagers :::use ::::::::detectors ::::with 2 600 400 intensity [R/nm] 200 0 400 500 600 700 800 900 1000 wavelength [nm] Fig. 1. Auroral spectrum during low activity geomagnetic and solar conditions for a beam of 1 keV monoener- 1 milliwatt=m2 getic electrons with a total energy flux of :::::::.Intensity ::::range::is :::::clipped::to::::::display spectral::::::structure,::::::: ::::630.0:::nm ::::::red-line::::peak :is:::::::::::approximately:::810:::::R/nm :::and::::557.7:::nm::::::::green-line:is:::::1190 R/nm.::::: :::::::minimal ::::::::sensitivity::::::outside:::the:::::range::of::::::visible:::::::::::wavelengths.:::::::::Substantial::::::::::ultra-violet ::::(UV):::::::::emissions 45 ::are::::::::absorbed:::by :::the :::::lower :::::::::atmosphere::::and :::can::::only::be::::::::observed:::::from :::::space.: An example synthetic auroral spectrum is shown in Figure 1. This was produced by simulating the effects of a monoenergetic 1 keV electron beam incident on a realistic atmospheric model (Strickland et al., 1999). The results are highly dependent on incident energy, with higher energies penetrating to lower altitudes and exciting different atmospheric constituents than at higher altitudes. Real pre- 50 cipitation distributions are often quasi-Maxwellian, but ::::with the characteristic and total energies that::: can vary considerably as a function of :::::::location :::and::::time:time and location. In section 2.3 we show one example of drastic variations in broad-band auroral intensity over 1-hour; similar changes can occur on time-scales of a few seconds or less. :::::Some ::::::auroral:::::::features,:::::such ::as:::::::::::pre-midnight:::::arcs, :::are :::::::::essentially:::::::constant::::over::::::::intervals:::on :::the 55 ::::order::of::::10’s::of::::::::minutes. :::::Other ::::::::::phenomena,::::such::as:::::::morning::::::sector pulsating::::::::::::::patches,::::have::::::::spatially :::::stable :::::::::luminosity ::::::::structures::::that ::::::exhibit::::::::::::quasi-periodic::::::::::fluctuations:::on ::::time :::::scales::of:::::1-10 :::::::seconds :. ::::::During:::::::::extremely :::::::dynamic::::::::intervals,::::such:::as ::::::::substorm :::::onset,::::::auroral::::::::::luminosity :::can:::::::increase:::by ::::::several :::::orders:::of :::::::::magnitude ::in::::less ::::than::a ::::::minute.:::::::Figure :8::::::shows:::an :::::::example::of::::::::multiple:::::rapid ::::::::variations ::in::::::::::broad-band ::::::auroral :::::::intensity::::::during :a::1 ::::hour ::::::period.: 60 :::::Other ::::::::significant::::::::::::contributions,:In addition to the possibility of rapid changes in auroral spectral features, there are often also significant “background” contributions such as stars or cloud-scattered moonlight:, :::can::::also::::::exhibit:::::rapid::::::::::fluctuations.:::::This:. The background spectrum is typically more of a continuum, so that useful estimates can be obtained from measurements adjacent to auroral emission lines.::::::::However,:::::errors:Errors in background subtraction::::may can be a major concern when 65 attempting spectroscopy with faint ::or::::::rapidly:::::::varying auroral emissions. The disparate requirements of spatially resolved rapid low-light spectroscopy over a wide field of 3 view cannot be achieved with any single instrument. Many auroral observatories