Calibration of Imaging Parameters for Space-Borne Airglow Photography

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provided by Springer - Publisher Connector Hozumi et al. Earth, Planets and Space (2016) 68:155 DOI 10.1186/s40623-016-0532-z

TECHNICAL REPORT Open Access Calibration of imaging parameters for space‑borne airglow photography using city light positions Yuta Hozumi1*, Akinori Saito1 and Mitsumu K. Ejiri2,3

Abstract: A new method for calibrating imaging parameters of photographs taken from the International Space Station (ISS) is presented in this report. Airglow in the mesosphere and the F-region ionosphere was captured on the limb of the Earth with a digital single-lens reflex camera from the ISS by astronauts. To utilize the photographs as scientific data, imaging parameters, such as the angle of view, exact position, and orientation of the camera, should be determined because they are not measured at the time of imaging. A new calibration method using city light positions shown in the photographs was developed to determine these imaging parameters with high accuracy suitable for airglow study. Applying the pinhole camera model, the apparent city light positions on the photograph are matched with the actual city light locations on Earth, which are derived from the global nighttime stable light map data obtained by the Defense Meteorological Satellite Program satellite. The correct imaging parameters are determined in an iterative process by matching the apparent positions on the image with the actual city light locations. We applied this calibration method to photographs taken on August 26, 2014, and confirmed that the result is correct. The precision of the calibration was evaluated by comparing the results from six different photographs with the same imaging parameters. The precisions in determining the camera position and orientation are estimated to be 2.2 km and 0.08°, respectively. The 0.08° difference in the orientation yields a 2.9-km difference at a tangential ±point of 90 km in± altitude. The airglow structures in the photographs were mapped to geographical points using the calibrated imaging parameters and compared with a simultaneous observation by the Visible and near-Infrared Spectral Imager of the Ionosphere, Mesosphere, Upper Atmosphere, and Plasmasphere mapping mission installed on the ISS. The comparison shows good agreements and supports the validity of the calibration. This calibration technique makes it possible to utilize photographs taken on low-Earth-orbit satellites in the nighttime as a reference for the airglow and aurora structures. Keywords: Camera calibration, Airglow, Space-borne imaging, International Space Station

Introduction on NASA’s Web site (“The Gateway to Astronaut Pho- The International Space Station (ISS) is a unique facility, tography of Earth 1995”). Some of the photographs cap- providing opportunities to conduct various observations tured the airglow in the mesosphere and the F-region of the Earth’s upper atmosphere. With the recent devel- ionosphere on the night side of the Earth. This space- opment of digital photography technology, a number of borne imaging covers much wider geographical area than photographs of the upper atmosphere that contain the ground-based imaging observations. Inspired by these airglow and aurora have been taken by astronauts on photographs, we carried out the Astronaut-Ionosphere, the ISS with digital single-lens reflex (DSLR) cameras. Mesosphere, upper Atmosphere, and Plasmasphere map- These photographs are open to the public and available ping (A-IMAP) campaign observation to investigate the airglow structure of the Earth’s upper atmosphere. In this observation, an astronaut took sequential photographs *Correspondence: [email protected]‑u.ac.jp of the airglow through the window of the cupola, an ISS 1 Graduate School of Science, Kyoto University, Kyoto, Japan Full list of author information is available at the end of the article module, while the ISS flew on the night side of the Earth.

© 2016 The Author(s). This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. Hozumi et al. Earth, Planets and Space (2016) 68:155 Page 2 of 10

The A-IMAP campaign is associated with the Ionosphere, Methods Mesosphere, upper Atmosphere, and Plasmasphere map- We developed a new method for calibrating imaging param- ping (IMAP) mission. Two imagers of the IMAP mission, eters using the city light positions shown in the airglow pho- one is for the airglow in visible and infrared light and tographs taken from a low-Earth-orbit (LEO) satellite. The the other is for resonant scattering from ions in extreme apparent city light positions were compared with the actual ultraviolet light, are operated almost continuously from city light positions on the Earth by applying the pinhole September 2012 to August 2015. During the operation camera model. The actual positions were determined by the period of the IMAP mission, ten experiments of the global nighttime stable light data obtained by the visible and A-IMAP campaign were carried out in 2014 and 2015. infrared sensors (OLS) on the Defense Meteorological Satel- To utilize the photographs as scientific data, the imag- lite Program (DMSP) satellite (Version 4 DMSP-OLS Night- ing parameters must be determined. The camera was fixed time Lights Time Series 2015). The resolution of the light to the ISS during the experiments, but the orientation of data is 1 km. In the pinhole camera model (e.g., Trucco and the camera was different for each experiment because the Verri 1998; Jaehne 1997), the relation between the city light camera was detached from the window of the cupola after position on Earth and that on the photograph is given by the experiment. A zoom lens was used for the A-IMAP x X campaign, and its focus length and viewing direction were     y = R Y − t, (1) adjusted in each experiment to capture clear airglow struc-  z   Z  tures while minimizing the obstruction by other ISS structures such as a window-frame, robot arms and solar panels. x u = f + Cx, (2) Therefore, the camera orientation and angle of view (AOV) z are unknown parameters and must be determined to utilize the image for airglow study. The precise camera posi- y v =−f + Cy, (3) tion along the ISS trajectory is also an unknown parameter z because the clock of the camera is not precise. To determine xyz T these imaging parameters, we developed a new calibration where is the coordinates of the city light in the technique using the city light positions shown in the pho- camera coordinate system, which is the coordinate sys- XYZ T tographs. Because observations were conducted at night, tem fixed to the camera, and is the coordi- city light positions are better reference than topographic nates of the city light in the world coordinate system. In features. The location of the camera is an unknown param- Eq. (1), t is the camera position in the world coordinates eter. Therefore, the positions of city lights were used in our and R is the rotation matrix corresponding to the orien- method, despite that the positions of stars are generally used tation of the digital camera. The matrix R transforms the to determine the field of view (FOV) of images taken in coordinates of a point to the coordinate system fixed to the nighttime from spacecrafts, rockets, and ground-based the camera. In Eqs. (2) and (3), (u, v) is the coordinates instruments. The attitude of a spacecraft can be determined of the projection point in the photograph in units of by the position of stars; however, the positions of the space- pixel and f is the focal length expressed in units of pixel, N craft cannot be determined by stars because the distant star f = 1 pix which varies as a function of AOV as tan AOV 2 , does not have a detectable parallax. The distances between 2 the spacecraft and cities are close; hence, the city light posi- where Npix is the number of horizontal pixels. Moreover, tion in the image gives rise to a detectable parallax, which is (C , C ) necessary for calibration of the camera location. x y is the coordinates of the center of the photograph θ σ ϕ This report is organized as follows. The method of cali- in units of pixel. The Euler angles , , and are used to bration is described in the next section. In Results and describe the orientation of the camera relative to the ISS. discussion section, we present the result of applying the The relation between R and the ISS orientation RISS can method to photographs taken on August 26, 2014, and be written in terms of the Euler angles as evaluate precision of the calibration. Then, we map the airglow structures in the photographs to the geographi- cos ϕ sin ϕ 0 cos σ 0 − sin σ cal coordinates using the calibrated imaging parameters R =  − sin ϕ cos ϕ 0  01 0  and discuss their vertical and horizontal structures. A  0 01 sin σ 0 cos σ  comparison with a simultaneous observation of the 630- 10 0 nm airglow emission by another instrument of the ISS/   0 cos θ sin θ RISS. IMAP mission will also be presented. The conclusion of (4)  0 − sin θ cos θ  this report is presented in the last section. Hozumi et al. Earth, Planets and Space (2016) 68:155 Page 3 of 10

Following the formulation of Brown (1971), we assume latitude 21.8° N and longitude 93.2° E over Myanmar at the radial distortion of the lens is given by 13:38 UT and flew out of the night side at latitude 49.2° S and longitude 142.5° W over the east of New Zealand at = + 2 + 4 ucorrected u 1 k1r k2r , (5) 14:12 UT. During the night path of the ISS, photographs were taken with a DSLR camera (Nikon D3s) and a lens (AI AF-s Zoom-Nikkor 17–35 mm f/2.8D IF-ED). Four = + 2 + 4 vcorrected v 1 k1r k2r , (6) photographs were taken every 11 s with an exposure time of 2.5 s. The camera was operated in Manual mode with where r is the distance from the center of the image in an f-number of 2.8 and ISO speed of 102,400. Each pho- k1 k2 units of pixel, and and are coefficients of the radial tograph has 4256 × 2382 pixels. An image of the photo- distortion. With these equations, we can convert the city graphs is shown in Fig. 1. The observation time recorded light positions in the world coordinates to projection in the metadata is 13:47:00.08 UT. This photograph is points on the image. hereafter referred to as photograph A. The Earth’s limb The camera location was assumed to be the same as the was captured in the upper part of the image. Above ISS location. Although the location of the ISS is known the Earth’s limb, two airglow layers are identified. The as a function of time, the times at which the photographs lower layer is caused by emission from the mesopause are taken are not precise. The time when the photograph region, and the upper layer is caused by emission from was captured was recorded in the metadata of the image the F-region ionosphere. On the Earth’s surface, several file in the exchangeable image file format. However, the city lights are seen in the image. City lights of the islands clock of the camera is not accurate and has a time lag of Indonesia in the center of the image and those of the with respect to Universal Time (UT) because the clock northwest coast of Australia on the right side of the image was set by hand. It is necessary to determine the time lag are recognized. The green circles indicate city light posi- so as to obtain the exact location of the camera. Because tions used for the calibration. The city lights in the mid- the orientation of the ISS is also known as a function of dle and long distance ranges from the ISS were chosen. time, the orientation of the camera can be expressed in The city lights too close to the ISS had a large apparent terms of its orientation relative to the ISS. Therefore, the size on the photograph, leading to a reduction in posi- unknown parameters are the time lag, orientation of the tioning accuracy. The positions of the selected city lights camera relative to the ISS, AOV, and the coefficients of should be widely distributed in the image to improve lens distortion k1 and k2. accuracy of the calibration. Because of the ISS’s motion The correct set of these imaging parameters was deter- during the exposure time, 2.5 s, the apparent images of mined by applying the imaging model described above. the city lights are stretched and have line-shape along Once the set of parameters is assumed, the actual city the ISS trajectory as seen in Fig. 1. This effect is appar- light location can be converted to a position on the pho- ent for the city lights #1, #2, #5, and #7 in Fig. 1. We used tograph using Eqs. (1–6). To evaluate the correctness the low/near end of the line-shaped image of these city of the assumed parameters, the mean distance between the city light positions on the photograph and projected positions were calculated. This process was iterated over the parameter space to determine the best set of parameters that minimizes the mean distance. The total number of unknown parameters is seven, i.e., one for the time lag, three for the orientation, one for the AOV, and two for the lens distortion. Each city light position on the two-dimensional plane of a photograph provides two restrict conditions. Therefore, at least four city light positions are necessary for the calibration. Prac- tically, more than four city light positions are used to determine the parameters with improved accuracy.

Results and discussion Calibration results Fig. 1 One of the photographs taken on August 26, 2014 (Photo- We applied the calibration method to the photographs graph A). The recorded time is 13:47:00.08 UT. The green circles indicate city light positions used for the calibration. The green numbers from the A-IMAP mission taken on August 26, 2014. are corresponding to the red numbers shown in Fig. 2 On the day, the ISS flew on the night side of the Earth at Hozumi et al. Earth, Planets and Space (2016) 68:155 Page 4 of 10

lights, which correspond to the positions of city lights To evaluate the precision and stability of the calibra- at the time of end of exposure time. Figure 2 shows the tion method, we applied the method to other five pho- nighttime stable lights map derived from the DMSP-OLS tographs for different city light conditions. These five data for the year 2013 (2) with the city locations indicated photographs were taken in the same night path of the ISS by the red circles. We utilized the data for the year 2013 with 2-min time interval from 13:49 to 13:57 UT using to identify the city lights positions, ignoring the variation the same camera and the same setting as for photograph between stable lights for the year 2013 and city lights on A. Photographing time recorded in the metadata for each the day August 26, 2014. City light covered by clouds is photograph is presented in Table 1 (photographs B–F). obscurely imaged from the ISS and should not be used During this period, the focal length of the lens was not for the calibration. The coverage of clouds is easily iden- changed and the camera orientation was stable relative tified by comparison between A-IMAP photographs and to the ISS, because the camera was fixed to the ISS. The DMSP-OLS data. time lag of the camera’s clock could be regarded as sta- The calibration method was applied to photograph A. ble with 0.1-s resolution in this 10-min period. Therefore, The mean distance between the city light position on the imaging parameters derived from these five photo- the photograph and projected positions was calculated graphs should be the same as those derived from photo- for every node of seven-dimensional parameter space graph A. The distance between adjacent photographing with resolutions of 0.1 s for the time lag, 0.01° for the positions for these six photographs was about 880 km, 10−10 10−17 k Euler angles and AOV, for k1, and for 2. The and the cities used in each calibration process were not parameter set minimizing the mean distance is the best used in the calibrations for the other photographs. The set. The derived parameters are listed in the first row of calibration result and their standard deviation are pre- Table 1. Figure 3 shows the parameter dependence of the sented in Table 1. The time lag was estimated in the range mean distance. The variation of the mean distance as a from −15.5 to −16.3 s with a standard deviation of 0.3 s. function of each assumed parameter is calculated, while The camera position along the ISS orbit was derived the other parameters are kept to be the best. Because from the time lag. A standard deviation of 0.3 s was cor- each parameter has a smooth curve and a sharp mini- responding to 2.2 km in the horizontal distance along mum, as shown in Fig. 3, the searching resolution of each the ISS trajectory, considering the ISS speed of 7.4 km/s. parameter for the iteration process is valid. Therefore, the calibration precision for the camera position was ±2.2 km. The standard deviations of the derived Euler angles θ, σ, and ϕ for the orientation of the camera were determined with a standard deviations within 0.08°, yielding a 2.9-km difference in the plane perpendicular to the line of sight (LOS) at the tangential point of 90 km in altitude. The 2.9 km distance was the calibration precision for the determination of the altitude profiles of the airglow on the limb. The AOV was derived in the range from 90.53° to 91.93°. The distortion coefficientsk 1 and −8 k2 were estimated in the ranges from 4.26 × 10 to × 10−8 × 10−15 −10.55 × 10−15 −0.97 and from −0.26 to , respectively. The AOV,k 1, and k2 are not independ- ent parameters. They are related to each other, and they determine the FOV and its distortion. To evaluate their combined expression, we calculated the angle between the LOS of the pixel at the center of image and that of the pixel with a distance of 1500 pixel from the center. We denote this angle by α, and the derived values of α are listed on the rightmost column of Table 1. The derived values of α were in the range from 36.71° to 36.99° with a standard deviation of 0.12°. The calibration precision Fig. 2 City lights map observed by the DMSP satellite. The red circles indicate city light positions used for the calibration. The red numbers for the FOV was ±0.12°. The calibration errors would be are corresponding to the green numbers shown in Fig. 1. The DMSP mainly attributed to the errors in determination of city image and data were processed by NOAA’s National Geophysical light position on the photograph. Because each city light Data Center, and the DMSP data were collected by US Air Force has a finite size on the photographs, the determination of Weather Agency its position yields some errors. Hozumi et al. Earth, Planets and Space (2016) 68:155 Page 5 of 10

Table 1 Imaging parameters determined by the calibration

8 15 Recorded # of city Time lag (s) θ (deg.) ψ (deg.) φ (deg.) AOV (deg.) k1 ( 10− ) k2 ( 10− ) α (deg.) time (UT) lights × ×

Photo. A 13:47:00.08 7 16.3 34.93 9.56 9.52 91.83 0.97 0.26 36.72 − − − − Photo. B 13:49:01.16 7 15.7 34.97 9.64 9.50 91.60 1.79 1.35 36.92 − − − Photo. C 13:51:02.16 7 16.3 34.93 9.62 9.46 91.93 1.20 0.98 36.73 − − − Photo. D 13:53:00.41 8 15.9 34.96 9.76 9.38 91.37 2.57 5.20 36.71 − − − Photo. E 13:55:01.32 7 15.5 35.10 9.72 9.54 91.25 2.42 4.29 36.70 − − − Photo. F 13:57:02.49 7 16.2 34.84 9.65 9.53 90.53 4.16 10.55 36.99 − − − Standard devia- 0.3 0.08 0.07 0.05 0.24 1.06 4.63 0.12 tion

The standard deviations of these derived parameters for photograph A–F are listed in the last row

Stars are also captured in the image. The orientation raw count per pixel per cd/m2 for each monochromatic of the camera can be determined from star positions. line is shown in Fig. 4. The counts were normalized by For the current case, however, stars are captured only in divided by the peak mean count of the green channel. small upper portion of the image and city lights are cap- The result showed that the sensor is sensitive to light of tured more widely in the image. The calibration precision wavelengths from 410 to 680 nm. evaluated above seems sufficient for most studies of air- The spectra of the airglow have been observed from glow. Therefore, we use only city light positions for the ground (e.g., Krassovsky et al. 1962, Broadfoot and Ken- calibration of A-IMAP observations. However, additional dall 1968) and space (Broadfoot and Bellaire 1999). The usage of star map for determination of camera orienta- responsivity of each color channel to the airglow can be tion will provide better calibration accuracy for future estimated by comparing the camera sensitivity spectra other observations when stars are distributed widely in with the airglow spectra. The red channel has sensitiv- the image. ity in the range from 570 to 680 nm. In this wavelength The calibration results are stable in the ISS path, along range, the intense emissions of the airglow are atomic which the imaging parameters are not changed. There- oxygen emission (OI) at 630 nm, sodium emissions (NaD) fore, only one photograph is needed when applying the in the range of 588.9–589.6 nm, and hydroxyl (OH) emis- method to determine the imaging parameters for sequen- sions. Therefore, the red channel had a capability to tial photographs. Even though most photographs taken measure these three emissions. The green channel had along the ISS path did not have clear city lights, because sensitivity in the range from 460 to 620 nm with a peak of clouds or the land–sea distribution under the ISS path, near the OI 557.7-nm green line emission. The OI green calibration of the imaging parameters is possible if there line dominated the green channel, although OH and NaD were more than one photograph with an enough number emissions also contributed to the green channel. In the of city lights. blue channel sensitivity range from 410 to 540 nm, no intense emission line of the airglow was identified. How- Vertical airglow structures ever, the emissions of the O2 Herzberg I, II, and II and the The airglow structures on the photographs were mapped Chamberlain system (Chamberlain 1958; Khomich et al. to geographical locations with the derived imaging 2008) could be measured in the blue channel. parameters and compared with previous studies so as to The limb profile of the raw count for photograph A was evaluate the validity of the calibration results. derived using the imaging parameters determined by the The camera had a CMOS image sensor that contains a calibration method. The imaging data were averaged over red, green, and blue color filter of the Bayer arrangement 20 pixels along the horizontal axis of the image so as to (Bayer 1976). The number of pixels for the green chan- increase the signal-to-noise ratio. Although the hori- nel was 2142 in width and 2844 in height, and that for zontal axis of the image is not perfectly identical to the the red and blue channels was 2142 in width and 1422 in Earth’s horizontal axis, the two axes are roughly parallel height. A laboratory experiment was conducted to obtain as shown in Fig. 1. Because of the Earth’s curvature, there the sensitivity spectrum for each color filter. Thirty mon- is 0.9-km difference in tangential altitude between the ochromatic lines from 400 to 700 nm with a bandpass of right and left side pixels of the 20 pixels around 90 km. 2 nm were used to measure the spectral response of the We ignored this discrepancy of the two axes in the hori- sensor. The camera was operated in manual mode with zontal 20 pixels and averaged data along the horizon- 2.5-s exposure time and 102,400 ISO speed. The mean tal axis of the image to reduce the calculation quantity. Hozumi et al. Earth, Planets and Space (2016) 68:155 Page 6 of 10

a b

c d

e f

Fig. 3 Parameter dependence of the mean distance. a–g panels show the time lag, θ, ψ, φ, AOV, k1 and k2 dependences, respectively. Each curve has a clear minimum peak

The pixels with counts more than the average count channel was found to have 102 pixels in the range from 0 by its standard deviation were removed so as to avoid to 100 km in the tangential altitude, and the red and blue contamination from stars. We calculated the imaging channels were found to have 51 pixels in the same range. parameters from those determined using photograph A Therefore, the height resolution for the green channel (Table 1). The limb profile as a function of tangential alti- was 1.0 km and those for the red and blue channels were tude is plotted in Fig. 5. The error bars indicate the stand- 2.0 km. The green and red channels had a broad peak in ard deviation of the averaged 20 pixels. The coordinates the range from 180 to 300 km. The 557.7 and 630-nm OI of the tangential point at 90 km in altitude were 13.1° emissions from the F-region ionosphere were consid- S and 130.3° E in geographical coordinates. The green ered to be the cause for the peaks in the green and red Hozumi et al. Earth, Planets and Space (2016) 68:155 Page 7 of 10

Na 589.6 nm OI 360 557.7 nm OI 630 nm 340 )] 2 1.0 320 300 0.8 280 260 0.6 240 220 0.4 200 180 0.2 160

0.0 140 Spectral Responsivility [NC/(cd/m 400 500 600 700 120 Wave Length (nm) 100 Tangential Altitude (km) Fig. 4 Sensitivity spectrum of the camera obtained from the labora- 80 tory experiment. The response of the sensor to monochromatic light was examined in 10-nm intervals. The mean raw count per pixel per 60 cd/m2 is normalized by divided by the peak mean count of the green 40 channel. The wavelengths of the major emission lines are also plotted 20 0 100 1000 10000 channels, respectively. A previous study showed that the Counts 630-nm OI airglow emission is in the altitude range of Fig. 5 Limb profiles of the three channels derived from photograph 180–350 km and the low- and midlatitude regions based A. The coordinates of the tangential point at 90 km were 12.6° S and on data of rockets (Takahashi et al. 1990) and satellites 130.5° E. The recoded time is 13:47:00.08 UT. The raw counts of the (Adachi et al. 2010). The altitude of the 630-nm emission pixels along the vertical line of the image are averaged to 10 pixels derived from ground-based imaging network with the tri- horizontally so as to increase the signal-to-noise ratio and are plotted as a function of tangential altitude. The limb profile has its tangential angulation technique was reported to be 260 10 km in ± point at 32.3°S and 124.2°E and 90 km in altitude. The error bars show middle latitude (Kubota et al. 2000). The 557.7-nm emis- the standard deviation of the averaged pixels. The green, red, and blue sion in the F-region has an intensity that is 30 % of that lines indicate the green, red, and blue channels, respectively of the 630-nm emission (Khomich et al. 2008). The vertical profiles of the F-region emission obtained from the red and green channels of the A-IMAP observation have a good agreement with these studies. All of the channels had a clear peak around an altitude of 90 km. The and/or the Chamberlain system (Khomich et al. 2008). red channel had a peak at an altitude of 86 km. The NaD The peak altitude of Herzberg I and II and the Chamber- and OH emissions in the mesopause region were possible lain system was reported to be in the altitude range from causes for the red channel peak. The green channel had 94 to 100 km (López-González et al. 1992). The vertical a peak at an altitude of 94 km. It was mainly dominated structures of the airglow captured in the three channels by the 557.7-nm OI airglow emission in the mesopause are consistent with previous studies. region. The NaD and OH airglow would also contribute to the green channel profile in this altitude range to some Horizontal airglow structures extent. The peak altitude of the emissions observed at the We derived the horizontal structure of the Equatorial mesopause altitude had a good agreement with previ- Ionization Anomaly (EIA) of the 630-nm OI emission ous observations by rockets (e.g., Packer 1961; Baker and from the red channel of the sequential photographs of Stair 1988) and satellites (e.g., Liu and Shepherd 2006). the A-IMAP campaign and compared it with a simulta- According to those studies, the typical peak altitudes neous observation of the 630-nm emission by the Vis- are 95 km for the 557.7-nm OI emission, 90 km for NaD ible and near-Infrared Spectral Imager (VISI), one of the emissions, and 86 km for OH airglow. The emission layer instruments of the ISS-IMAP mission. in the blue channel with a peak latitude of 94 km could In order to increase the signal-to-noise ratio and be attributed to the O2 emission of Herzberg I, II, and III, reduce the calculation quantity, we compressed the Hozumi et al. Earth, Planets and Space (2016) 68:155 Page 8 of 10

horizontal axis of the A-IMAP image from 2142 to 214 track was 21 km, which is determined by the 2.75 s imag- pixels by averaging data. The raw counts of the red chan- ing interval of the photographs. The horizontal resolu- nel of the photographs were averaged over 20 pixels for tion perpendicular to the track was 19 km, corresponding every 10 pixels along the horizontal axis of the image. to the size of 10 pixels of the image. The robot arms of Because the discrepancy between the horizontal axis of the ISS obstructed the airglow imaging on the right side the image and the Earth’s horizontal axis is small in the of the FOV as can be seen in Fig. 1. They cause the two range of horizontal 20 pixels, we can safely average data line-shape data gaps on the south side of the mapping of the red channel in 20 pixels along the horizontal axis image along the ISS orbit. The solar panels of the ISS also of the image. In the averaging process, the contamina- obstructed the observation on the left side of the FOV tion by stars was removed with the same method as men- until 13:49:36 UT. The data gap caused by the solar panels tioned above. For each vertical profile along the vertical can be seen on the north side of the mapping image. An axis of the A-IMAP image, the total row count of the enhancement of the total count of the red channel caused red channel in the range from 180 to 300 km in tangen- by an enhancement of the 630-nm emission can be seen tial altitude was calculated and mapped to its tangential around the latitude of 5° S. This enhancement was inter- point at 250 km in altitude. This process was applied to preted to be caused by the EIA, because the magnetic 570 photographs taken from 13:38:59 to 14:05:09 UT on latitude of the enhancement is consistent with the typical August 26, 2014. The result is presented in Fig. 6a with a magnetic latitude of the EIA (e.g., Watthanasangmechai black–red color scale. The ISS flew toward southeast and et al. 2014, 2015). The total counts of the enhancement the center LOS of the camera pointed southeast with an on the east side of the FOV were larger than those on the AOV of 92° in the low-latitude region at that time. The west side. This is because of the alignment of the LOS width of the FOV across the track was about 2100 km at with the EIA, which has a zonally extended structure. 250 km in altitude. The horizontal resolution along the Considering the center LOS of the camera pointed southeast with an AOV of 92 at the low-latitude region, the LOS on the east side of the FOV pointed east, and then an integration along the LOS yielded a large count from a 20 3 150 the EIA. The LOS on the west side of the FOV pointed south and resulted in a less count. ) 0 5 IMAP/VISI is a Visible and near-Infrared Spectral 2 Imager installed on the Exposure Facility of the Japanese

100 120 140 -20 160 180 -160 80 Experiment Module on the ISS (Sakanoi et al. 2011). It had a simultaneous observation on August 26, 2014, with 1 Rayleigh/nm the A-IMAP observation. IMAP/VISI observed several -40 Total Count (10 emission lines of the airglow, including the 630-nm OI emission with two slit-line FOVs pointing 45° forward -60 0 10 and 45° backward to nadir. From 13:40:42 to 14:08:57 UT, b it observed the 630-nm OI emission with backward FOV ) 250 A-IMAP 180-300 km 2.0 5 and an exposure time of 4.0 s in “the spectral mode.” In 200 A-IMAP 240-300 km IMAP/VISI 1.5 the spectral mode, it recorded the spectral shape in the 150 1.0 wavelength range around 630 ± 6 nm with a resolution 100 of 1.02 pixel/nm. We assumed that the maximum count 50 0.5 Rayleigh/nm is in the wavelength range containing both the 630- 0 0.0 Total Count (10 5 0 -5 -10 -15 -20 -25 nm OI emission and background emission and that the Latitude minimum count is in the range containing only the back- Fig. 6 Mapped total counts and the IMAP/VISI data. In a, total counts ground emission. By subtracting the minimum count of the red channel of the photographs in the range between tangen- from the maximum count, the 630-nm airglow emis- tial altitudes of 180 and 300 km are mapped to the tangential points sion peak could be retrieved from the observed spectral of 250 km with a black–red color scale. The 630-nm emission observed shape. The 630-nm OI emission in units of Rayleigh/nm by the IMAP/VISI is plotted in units of Rayleigh/nm with a blue–white color scale. In b, data of A-IMAP and IMAP/VISI along the green line was mapped to an altitude of 250 km. The results are in a are presented as functions of latitude. The red solid line is for the presented in Fig. 6a with a blue–white color scale. The total counts of the red channel with the summation range from 180 width of the FOV across the track was 350 km at 250 km to 300 km and the red dash line is for that with the summation range in altitude, and the resolutions across and along the track from 230 to 300 km. The blue line is for the 630-nm emission observed were 14 and 33 km, respectively. An enhancement of the by the IMAP/VISI 630-nm emission caused by the EIA enhancement was Hozumi et al. Earth, Planets and Space (2016) 68:155 Page 9 of 10

observed by IMAP/VISI in the latitude range from 0° S Conclusions to 10° S. A new calibration technique for space-borne airglow To have more detailed comparison of the data, we photography was developed and confirmed using the plotted the data as functions of latitude in Fig. 6b. The other optical measurements. Matching the apparent city total counts of the A-IMAP red channel and the IMAP/ light positions on a photograph with the actual city light VISI data along the green line shown in Fig. 6a are plot- positions derived from the DMSP-OLS stable night light ted with the red and blue lines, respectively. To examine map in the pinhole camera model, the imaging param- the effect of the vertical summation range, we show the eters can be determined. We applied the method to the latitudinal profile of the A-IMAP data with the summa- photographs taken by astronauts on the ISS on August tion range from 230 to 300 km with the red dash line in 28, 2014, and evaluated the precision and stability of the addition to that with the summation range from 180 to calibration. The precision of the derived time lag for the 300 km. The center of the EIA was located around 5.5° camera clock was 0.3 s, and that of the camera orienta- S for both observations. Although the enhancement tion was 0.08°. The precision of the FOV was 0.12° for the peak locations showed a good coincidence, the width of pixel with a distance of 1500 pixels from the center of the the enhancement observed by A-IMAP was wider than image. The calibration result is precise enough for airglow that observed by IMAP/VISI. We computed a nonlinear and aurora studies. This calibration technique makes it least-squares fit for each latitudinal profile to a function possible to utilize photographs taken on LEO satellites as z x−A f (x) = A e 2 + A + A z = 1 0 3 4 where A2 . Assuming the a reference for the airglow and aurora structures. width of EIA is twice large as the full-width half max- The raw count data of the photographs were mapped to ima, we determined the edge of the EIA for each profile. their tangential points in geographical coordinates using The southern edge for each profile is indicated with the the calibrated imaging parameters. The peak heights for vertical arrow of each line type in Fig. 6. The southern the F-region airglow, e.g., the 630 and 557.7-nm OI emis- edge of the EIA in the red channel with the summation sions, and the mesospheric airglow, such as OI, NaD, and range from 180 to 300 km was wider by about 3.3° in OH emissions, had a good agreement with those of previ- latitude or 450 km in distance than that in the IMAP/ ous studies. The EIA structures of the 630-nm OI emis- VISI observation, while the edge in the red channel with sion obtained from the red channel of the photographs the summation range from 230 to 300 km was wider by and IMAP/VISI data also had good agreement. These about 2.2° in latitude or 300 km in distance that in the agreements support the validity of the calibration method. IMAP/VISI observation. The wider enhancement structure in the limb count profile of the photographs would Abbreviations be attributed to the horizontally integrated observa- AOV: angle of view; DMSP-OLS: the visible and infrared sensors on the Defense tion path. There was about 520-km horizontal distance Meteorological Satellite Program satellite; DSLR: digital single-lens reflex; EIA: equatorial ionization anomaly; EXIF format: exchangeable image file format; between the tangential points in the altitude range from FOV: field of view; ISS: International Space Station; ISS-IMAP: International 180 and 300 km in each image. For the summation range Space Station-Ionosphere, Mesosphere upper Atmosphere, and Plasmasphere from 230 to 300 km, the horizontal distance between the mapping mission; LEO: low Earth orbit; LOS: line of sight; VISI: Visible and near- tangential points is about 330 km. It means that the limb Infrared Spectral Imager. observation integrated the structure horizontally for Authors’ contributions about 520 or 330 km depending on its summation range. YH developed the calibration method, applied the method to data, inter- preted the calibration result, and wrote the first draft of the manuscript. AS As a result, the horizontally extended shape of EIA was conceived the airglow observation with photographs taken on the Interna- observed in the A-IMAP data. The difference in the EIA tional Space Station, coordinated observations, contributed to the improve- width derived from different summation altitude ranges ment of the calibration method. ME lead the sensitivity calibration of the camera. All authors revised and improved the manuscript. All authors read and can be explained by the difference in horizontal distance approved the final manuscript. between the tangential points in each summation range. On the other hand, IMAP/VISI has a relatively verti- Author details 1 Graduate School of Science, Kyoto University, Kyoto, Japan. 2 Department cal integration path and can observe the true horizon- of Polar Science, Graduate University for Advanced Studies (SOKENDAI), Tachi- tal edge of the EIA. Therefore, it is reasonable that the kawa, Japan. 3 National Institute of Polar Research, Tachikawa, Japan. results from different observation geometries have dif- Acknowledgements ference in the EIA edge. Although there is a difference Data that support this report are from Astronaut-Ionosphere, Mesosphere, in the width of the structure caused by the observation upper Atmosphere, and Plasmasphere mapping (A-IMAP) mission and the Ion- geometry, the EIA enhancement locations from the osphere, Mesosphere, upper Atmosphere, and Plasmasphere mapping mission from the ISS (ISS-IMAP mission). We thank all the member of the A-IMAP two observations have a good agreement. This agree- and ISS-IMAP mission. We are also thankful to NOAA’s National Geophysical ment supports the validity of the proposed calibration Data Center and US Air Force Weather Agency for the provision of the global method. nighttime stable light data obtained from the visible and infrared sensors Hozumi et al. Earth, Planets and Space (2016) 68:155 Page 10 of 10

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