Suomi satellite brings to light a unique frontier of nighttime environmental sensing capabilities

Sources of Night Sky Brightness and Relative Magnitudes

A complete description of the nonlunar nighttime illumination sources can be found in the reviews of refs. 1 and 3. We shall limit our discussion here to the globally ubiquitous sources (namely airglow, starlight, and zodiacal light), referring to these collectively as “diffuse illumination, multisource” (or “DIM”) emissions of nighttime visible light.

The zodiacal light (11) arises from sunlight scattered by interplanetary dust in the solar system. Because this dust is concentrated along the ecliptic and increases in density with proximity to the Sun, there is a strong spatial structure to this source (ref. 3, figure 37). At high elongation angles from the Sun (i.e., near local midnight, approximately the time of the Suomi NPP overpass), this light source is minimized, with an expected 640-nm radiance range spanning roughly 0.4–1.1 × 10⁻¹⁰ W⋅cm⁻²⋅sr⁻¹⋅μm⁻¹ (from ecliptic poles to equator, respectively).

Starlight is composed mainly of contributions from the ∼8,500 visible stars and the integrated light from the ∼10¹¹ subvisible stars of the Milky Way. The brightness of the galactic source is highly variable (12), with the largest values along the galactic equator and significantly smaller values at the poles (ref. 3, table 36). Consequently the magnitude of this source varies over the course of the year, smallest in December and peaking in July. The combined starlight source produces 640-nm radiances in the range of roughly 1.8–4.0 × 10⁻¹⁰ W⋅cm⁻²⋅sr⁻¹⋅μm⁻¹. Contributions from the diffuse scatter of galactic and cosmic light by interstellar dust are two orders of magnitude lower and considered here to be negligible.

Among the DIM sources, airglow dominates in terms of its magnitude, dynamic range, and space/time variability, and so receives special attention here. Airglow refers to the self-illumination of the upper atmosphere via chemiluminescence processes. Nocturnal airglow (called “night glow”) results from the photoionization of atmospheric gases by ultraviolet (UV) sunlight. Night glow brightness ranges from 10⁻¹⁰ to 10⁻⁹ W⋅cm⁻²⋅sr⁻¹⋅μm⁻¹ (13) within the DNB sensor response when viewed from the surface and at local zenith. Whereas the OLS has not demonstrated the ability to produce useful meteorological imagery from the DIM sources, the slight red shift of the DNB sensor response (toward the significant near-infrared Meinel OH* airglow bands; see Fig. S1) makes it inherently more sensitive to these emissions.

At the surface, the airglow layer appears brightest (a factor of ∼4 larger than the zenith value) at an elevation angle of approximately 10° (the van Rhijn function, e.g., ref. 14). Airglow as seen from space has been documented from the Space Shuttle (15) and more recently from the International Space Station (ISS; http://eol.jsc.nasa.gov/Videos/CrewEarthObservationsVideos). Fig. 2, taken by astronauts on board the ISS, shows the relative brightness and distribution of several nocturnal light sources including the airglow layer.

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Fig. 2.

Earth’s limb as seen from the International Space Station on February 1, 2012, 06:11:39 UTC, over the northern Atlantic Ocean. The view is to the northeast, and shows qualitatively the relative brightness and distribution of various sources of nighttime illumination including airglow, the aurora borealis, the gibbous moon, starlight, and city lights. The lunar source is significantly brighter than the DIM sources, appearing like a star in this exposure. A solar panel from the ISS appears in the foreground. (Nikon D3s image ISS030-E-73400 courtesy of the Image Science and Analysis Laboratory, NASA Johnson Space Center).

Airglow Spectral/Space/Time Characteristics

Airglow emissions occur both as distinct bands and as continuum emission, spanning the UV through the visible and into the near infrared (16, 17). Significant contributions with respect to the DNB response include atomic oxygen at 557.7, 630, and 636 nm (altitudes between ∼250–300 km), atomic sodium at 589.0 and 589.6 nm, excited hydroxyl (OH*) radicals (500 nm out to 4.5 μm), and molecular oxygen from 761.9 nm (A-band) and 864.5 nm. The dominant OH* emission layer is geometrically thin (10–20 km), occurring around a critical air density at approximately 85 km where excited species are abundant and the favorable mechanism for energy dissipation is photon emission.

Airglow emissions are highly variable spatially, seasonally, and diurnally (18⇓–20) and track changes in solar insolation, atmospheric density, and atomic oxygen availability. This behavior includes a semiannual oscillation in maxima with amplitude variations that are lower in the tropics and higher at midlatitudes (20⇓–22). Regionally, the emissions produce complex transient banding or patch-like structures associated with planetary waves (23) and mesospheric tides (24, 25). At finer spatial scales, emission features are associated with ionospheric disturbances forced by atmospheric gravity waves (26, 27). The observed structures have been tied to tropospheric convection and seismic activity, with the latter proposed as a means for early detection of tsunamis (28).

Previous Measurements

The Orbiting Geophysical Observatory (OGO-4) provided the first global maps of the time-varying airglow distribution and intensity (19). More recent airglow monitoring sensors include the Wind Imaging Interferometer (WINDII; ref. 24), the Special Sensor UV Spectrographic Imager (SSUSI; ref. 29), the Optical Spectrograph IR imager (OSIRIS) limb-viewing camera (30), and the Thermosphere Ionosphere Mesosphere Energetics and Dynamics (TIMED) instrument (25). These sensors were designed to characterize upper atmospheric properties that influence high frequency communications and near-field sources of light contamination that impact astronomy, as opposed to meteorological applications.

Early photometer observations from OGO-4 suggested a possible meteorological utility of airglow (31), although the nonimaging and spatially coarse (∼100 km) nature of those measurements permitted only crude inference of transitions between low and high albedo features (e.g., crossing from ocean to desert). Similarly, the DMSP/SSUSI NPS (a nonimaging photometer) enlists a secondary measurement at 629.4 nm to characterize and remove background contributions from Earth’s albedo. To date, the only documented use of nonsolar/lunar sources for cloud imaging comes from surface-based camera systems (32), based on cloud extinction of down-welling airglow emissions as opposed to cloud reflectance.

Example Imagery

Fig. 1 shows VIIRS/DNB cloud imagery over the Pacific Ocean from several consecutive nighttime passes in conditions completely devoid of both sunlight and moonlight. To avoid edge-of-scan noise effects, the data were cropped at a maximum sensor zenith angle of 60°, resulting in small coverage gaps between adjacent passes at lower latitudes. City lights appear as discrete, bright features. Additional examples are published (Figs. S2–S4).

To compare the relative brightness of various illumination sources, we produced normalized distributions of DNB radiance (in W⋅cm⁻²⋅sr⁻¹) from the north/central Pacific Ocean during nighttime new moon (i.e., only DIM sources), nighttime full moon, and daytime overpasses. Fig. 3 shows that nighttime scenes illuminated by full moon are roughly 1 million times fainter than the daytime, and the new moon scenes are approximately 100 times fainter than full moon. Secondary modes in the full moon and daytime distributions correspond to lunar and solar glint (specular reflection off the ocean surface) regions, respectively.

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Fig. 3.

Distributions of Suomi NPP VIIRS DNB-measured radiances over the north/central Pacific Ocean for nighttime new moon, full moon, and daytime scenes. The secondary low-radiance modes (G) of the full moon and daytime distributions are due to moon and sun glint (respectively) off the ocean surface—a characteristic not seen in the extended diffuse light sources.

Nighttime detection of low clouds and other near-surface features in the thermal infrared is problematic because of poor thermal contrast. Here, DIM light imagery offers distinct advantages. Fig. 4 compares VIIRS DNB and M15 (10.7 μm) observations of a low cloud layer over the northern Korean Peninsula. The western and northern edges of the low cloud field are difficult to discern over land in the infrared imagery but stand out readily in the DIM light imagery. The imagery also reveals details of the low cloud structure below optically thin cirrus.

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Fig. 4.

VIIRS/DNB low-light imagery (A) of the Korean Peninsula on the night of February 23, 2012, at 1730 UTC during new moon, showing low clouds on the eastern coast and superior edge contrast over land compared with VIIRS/M15 (10.763 μm) thermal infrared imagery (B). Note in particular the western and northern edges of the cloud formation.

Fig. 5 shows a DIM light image of a convective system over the Pacific Ocean. The lightning flash (which appears as a bright segment oriented along a DNB scan line, due to the scanning nature of the sensor) and convective clouds are features that could have been detected with heritage DMSP/OLS sensor. However, in addition to these features, the DNB reveals a train of mesospheric gravity waves in the primary airglow emission, originating from the area of convection and propagating away to the east/northeast.

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Fig. 5.

DNB imagery of thunderstorms (lightning flash indicated) in the tropical central Pacific Ocean on February 22, 2012, at 1107 UTC (a zoomed-in subset of Fig. 2). Airglow waves are evident in the vicinity of the storms. The waves, which have wavelength λ ∼ 33 km, appear to propagate radially outward from the area of convection and extend far beyond the anvil cirrus.

Source Determination and Analysis

We considered the possibility that thermal emission might explain the cloud detections. Ground tests of the silicon-based DNB detectors showed no sensitivity to wavelengths > 0.91 μm, and even at this limit the levels of blackbody emission are vanishingly small—at least 10 orders of magnitude below minimum detection limits. Before the thermal infrared VIIRS bands are cooled to their 80 K operational temperature, any cross talk with the DNB would have manifested as white noise as opposed to the coherent structures observed, and the character of DNB imagery did not change as these bands were cooled. Further evidence against a thermal leak came from the simple observation that clouds in the tropics appear brighter than the surrounding clear-sky ocean background. This behavior is opposite to the emissive signature expected from cool clouds observed against a warmer underlying surface but consistent with visible light reflectance of clouds observed against a low albedo ocean background.

The imagery in Figs. 1, 4 and 5 appear similar to daytime observations, which reveals synoptic-scale patterns of migratory storm clouds, maritime boundary layer clouds, and tropical convection. One notable difference from daytime imagery, however, is the presence of broad (∼1,000 km scale) regions of diffuse brightness. These features, as noted in Fig. 1, are thought to be areas where primary airglow emissions are locally strong enough for the DNB to detect directly. Similar structures were observed on all nights but in different locations. Their scale and distribution are consistent with OGO-4 airglow observations (19).

Other characteristics of the DNB imagery speak to the unique nature of the DIM illumination. Note in Fig. 3 the smooth low-end tail structure that is unique to the new moon distribution. The secondary low-radiance modes seen in the daytime and full moon cases come primarily from the solar and lunar glint regions. The lack of such a glint mode in the new moon distribution is consistent with the extended diffuse nature of the DIM emissions. The pan-horizon illumination from airglow represents a fundamental difference from the highly directional solar/lunar sources in the sense that cloud shadows will be minimized—potentially improving cloud masks.

Concerning the airglow waves observed in Fig. 5, the measured horizontal wavelength of ∼33 km is consistent with thunderstorm-induced waves observed by the Midcourse Space Experiment (MSX) midwave-infrared (4.3 μm) limb sounder data (33, 34). Sensitivity to both primary and reflected DIM light sources provides a unique perspective on coupling between the troposphere and mesosphere. Additional examples of these waves compared against thermal infrared imagery are published (Figs. S5–S7). In all cases, the waves were not present in corresponding VIIRS thermal infrared imagery.

Research and Operational Implications

The information content of most measurements from meteorological satellites falls off markedly at night, particularly with regard to lower tropospheric clouds (owing to poor thermal contrast), which play a key role in defining Earth’s energy budget (35). Sensing based on DIM light sources could improve the nocturnal low cloud climatology. In turn, it could improve the fidelity of the sea surface temperature (SST) climate data record that is fundamental to our assessments of climate change, because SSTs are derived from nighttime observations and require a cloud screening that is inherently problematic for low clouds (36). As shown in Fig. 4, the DIM light measurements hold potential to improve nocturnal low cloud masks and, thereby, improve the uncertainty statistics associated with this key climate parameter.

Low clouds and fog also pose significant hazards to transportation by air, land, and sea. At high latitudes, detecting clouds over cold land surfaces and identifying snow and sea ice boundaries are particularly challenging tasks, especially during the winter months when sunlight is unavailable for extended periods. Although multispectral techniques (37⇓–39) in many cases overcome thermal contrast issues in nocturnal low cloud detection, these algorithms face insurmountable difficulties in the presence of overriding cirrus. DIM light imagery, with its ability to peer through optically thin cirrus layers, holds unique value for overcoming this problem.

Although the VIIRS/DNB has demonstrated the potential of DIM light, future sensors could be optimized to better exploit these signals. For example, geostationary-based sensors could provide the first continuous (24-h) visible light observing capability. Such time-resolved imagery would allow for monitoring primary airglow emissions, including mesospheric wave detection (Fig. 5) in support of tsunami warning systems. Low earth-orbiting satellites such as Suomi NPP must conduct short integration times per sample because of their motion relative to the surface, but with geostationary satellites, the observed scene is essentially fixed, allowing for much longer integration times. This staring capability would help to overcome the weaker signal due to significantly higher geostationary orbital altitudes (35,786 km), which enables imagery with spatial resolution comparable to NPP while providing more frequent updates.