Introduction

VIIRS DNB

Image Gallery

FAQ

Suomi NPP Site

About the Suomi NPP Satellite, VIIRS, and the Day/Night Band (DNB)

The Suomi National Polar-orbiting Partnership (NPP) mission, a joint endeavor between NOAA and NASA, is an atmospheric observation and research satellite that was launched on October 28, 2011. Five scientific instruments are aboard the Suomi NPP satellite: the Advanced Technology Microwave Sounder (ATMOS), which provides vertical profiles of temperature and water vapor; the Cross-Track Infrared Sounder (CrIS) which provides 3D observations of temperature, pressure, and moisture; the Ozone Mapping and Profiler Suite (OMPS) which measures ozone; a copy of the Clouds and the Earth's Radiant Energy System (CERES) sensor, which is measures short- and longwave radiation from the Earth; and the Visible Infrared Imaging Radiometer Suite (VIIRS), a 22-channel advanced radiometer which sees the Earth at multiple wavelengths in the visible and infrared spectrum, including a revolutionary Day-Night Band (DNB) sensor capable of seeing extremely low levels of visible light.

The VIIRS instrument represents an evolution of previous radiometers such as the Advanced Very High Resolution Radiometer (AVHRR) and the Moderate Resolution Imaging Spectroradiometer (MODIS) instruments flown on previous missions. VIIRS data, which collects information about visible and infrared energy from the Earth, can be used to compute properties of clouds and aerosols, ocean color, surface temperature, and fires as seen from space. The instrument, which continually scans the surface of the Earth, has a pixel resolution of 750m, and based on the orbital parameters of its parent satellite, gets a complete view of the Earth roughly every six orbits, which takes a little more than eight hours.

The DNB sensor, part of the VIIRS instrument, is a sensor that sees primarily visible light between 0.505 and 0.890 microns, but is much more sensitive than most traditional sensors. The DNB sensor has three 'stages' of electronic gain that allow the sensor to take measurements during the daylight, twilight, and nighttime. By selecting the high-gain mode, the DNB can 'see' in the dark, collecting light that would register below the noise threshold for most instruments.

Remote Sensing at Night: Light Sources in the Dark

During the daytime, the visible light that we see comes from the sun. From space, what a satellite sees during the daytime is the sunlight reflected by different components of the Earth's systems; reflection from the land surface, including plants, cities, snowfields, deserts, etc., as well as reflection from the ocean and from clouds.

Scientists can learn more about these features by observing how sunlight reflects off of each object — for example, the differences in how sunlight reflects from low clouds composed mostly of liquid water droplets versus how sunlight reflects from high, thin ice clouds can tell us more about the properties of each cloud. When these observations are combined with other sources of radiation (such as infrared radiation emitted by clouds and by the Earth's surface) even more can be learned about the Earth's systems. Of course, there's a catch — the bulk of the visible light available to see comes from the Sun. What happens during the nighttime, when the Sun's rays aren't available to use as an illuminating source?

Moonlight

One source of light at night is the Moon. Light from the Moon is, of course, just sunlight that has been reflected off the Moon's surface that illuminates the Earth, as if the Moon were like a mirror in space redirecting sunlight to the Earth's surface. Light from the Moon is a strong source of visible light during the night when the Moon is full (or, in other words, when the Moon in its orbit is ideally located to reflect the maximum amount of light to the Earth's surface). Scientists can use the known orbital properties of the Moon to compute the amount of light available to the surface of the Earth, but even at its fullest, the maximum amount of light from the Moon is still almost a million times less than that from the Sun.

Starlight

Another source of visible light at night comes from the stars. Consisting of light from the stars we can see from the Earth (including from the Milky Way galaxy) and the combined light from all the stars that are too distant to be seen individually from the Earth, starlight provides a small amount of visible light that can be seen by a sophisticated sensor such as the DNB. Starlight, which is approximately 1000 times dimmer than moonlight, can also be predicted by scientists using the known orbital properties of the Earth in relation to the location of the stars.

Zodiacal Light

Zodiacal light is sunlight that is scattered off of interplanetary dust in the solar system and reflected towards the Earth. This very weak source of light bathes the solar system in a faint glow at all times, and contributes to the amount of visible light that can be seen at night. Zodiacal light is approximately 1000-2000 times dimmer than moonlight.

Airglow

An interesting source of light at night is called 'airglow'. To understand airglow it helps to take a moment to understand the structure of the Earth's atmosphere, which is made of several layers. The first layer, called the troposphere, extends from the surface to an altitude of approximately 10-20km, depending on latitude and season, and contains most of the Earth's air and water vapor, and effectively all of the weather on the planet. Above the troposphere is the stratosphere, which extends from ~10km up to ~50km, and includes the ozone layer, which shields the Earth's surface from ultraviolet radiation and cosmic rays. Above the stratosphere, from 50km through 100km, is the mesosphere. Here, the density of the atmosphere is 1000 times less dense than at the surface of the Earth, and ionizing radiation can interact freely with the tenous gases of the upper atmosphere.

During the daytime, ultraviolet radiation from the Sun continually breaks apart molecules of oxygen-containing molecules. In breaking apart, these molecules acquire some of the energy from the incoming radiation. During the nighttime, these energized molecules chemically recombine, and in the process, release their acquired energy in the form of visible light. The resulting glow, which is approximately 100-1000 times dimmer than moonlight, is called 'airglow'. Airglow is easily seen from space, as the photo from the ISS above demonstrates, but predicting the strenght of the light is difficult — the variability in the photochemical processes that create airglow is not a process that is well understood by scientists yet.

Human-created light

Finally, one additional source of light is available for satellites: city lights! A sensitive instrument such as the DNB can easily see the emitted lights from human civilization on the planet, as is also seen in the photo from the ISS.

Seeing the Earth at Night: Low Clouds

All of these light sources can be used to look at the Earth — the VIIRS DNB sensor lets us see using only airglow and starlight. So what does the Earth look like at night using only airglow and starlight sources? The image here is a comparison picture, taken over the Korean peninsula from VIIRS. The image on the left was taken using the DNB sensor, while the image on the right was taken using one of the infrared sensors aboard VIIRS.

Infrared light, which is constantly being emitted by the Earth, is easily detected at night, and prior to the advent of the DNB, represented one of the few ways to 'see' clouds at night. The amount of infrared light generated depends on the temperature of the object emitting the infrared light; in the case of very low clouds, whose temperature is very close to the surface temperature of the Earth, this results in an infrared signature that is difficult to discern from the surface. In other words, certain types of clouds are very difficult to see at night using infrared emissions, and this can be seen in the comparison images. In the image, a low cloud deck starts roughly in the middle of the peninsula and extends eastward over the ocean. The temperature of this cloud deck is very nearly the same as the ground temperature, meaning both cloud and ground have nearly identical infrared signatures. And in the infrared image on the right, the cloud layer is difficult to see — one flat gray feature in a sea of flat gray features.

The low cloud deck, however, looks very different in the visible spectrum, however, since the water drops that make up the cloud reflect visible light very differently than does the soil, vegetation, and other surface feature of the ground. In the image on the left, taken from the DNB sensor, reflected starlight and airglow from space reflect off the low cloud feature, giving it a light grey hue over the much darker surface of the Earth's land and ocean features. Combined with the city lights of urban South Korea, the DNB image provides much more detail and information for locating cloud features at night.

Seeing the Earth at Night: Atmospheric Interactions

Another revolutionary way in which data from the DNB sensor is changing our understanding of the Earth's atmosphere is by looking at previously unseen interactions between the Earth's lowest layer, the troposphere, and the upper layers, including the mesosophere.

As mentioned previously, nearly all of the Earth's weather (and certainly, all of the weather we encounter) occurs in the troposphere. In the image shown here, a thunderstorm is moving across the Texas plains, just outside of Midland. Again, the image is a comparison image, with an infrared picture of the cloud on the right, while the DNB visible image is on the left. Because the thunderstorm is much colder than the surface, this time it is very easy to see the thunderstorm's signature in the infrared; a faint reflection of airglow and starlight is also visible off the cold cloud tops in the DNB image as well. Besides the prominent city lights, one other feature is clearly noticable in the DNB image: 'waves' of light appearing to radiate away from the center of the image.

What could be causing these waves? The overall structure looks similar to ripples in a pond caused by a thrown stone; here the center of the ripples is just to the west of the thunderstorm, at a location that the thunderstorm would have passed through perhaps a few hours previously. The thunderstorm over Texas is occuring in the troposphere, and consists of a large central updraft plume at the core of the thunderstorm, bringing air from the surface to the top of the troposphere, perhaps 15km up. We know very little about the small-scale details of the structures of the upper atmosphere, but we do know that the stratosphere, the layer of the atmosphere immediately above the troposphere (and our thunderstorm) is very stable; disturbances at the bottom of the stratosphere could be transmitted through to the top of the stratosphere like a wave through a spring. Once into the mesosphere, where there is a drastic decrease in temperature with height compared to the stratosphere, these disturbances would radiate outwards like waves in a pond. If we know the strength of the thunderstorm (which we can measure with radars and other other instruments) we can compute the strength of the vertical disturbances through the stratosphere; if we know the depth of the stratosphere we can compute how long it should take to those disturbances to be made known in the mesosphere, giving us indirect observations of how motions in the lower atmosphere affect the upper atmosphere.

This is all well and good, but why do we see these waves? The answer: airglow! The mesosphere is the region of the atmosphere where airglow is created, and the disturbance from the lower atmosphere is creating waves in the light-emitting upper atmosphere, creating regions of higher- and lower amount of light-emitting air. Thunderstorms are ubiquitous in the troposphere, and the disturbances they create in the upper atmosphere must be happening all the time, but thanks to airglow and the DNB, we now have the ability to see the disturbances, for the first time.

Putting it all Together

What else can we expect to learn by looking at the Earth at night using starlight and airglow? For the first time, we have the capability to leverage techniques previously only usable during the day at night, using natural light sources. Effectively, the day is now twice as long for research scientists using the DNB sensor aboard the Suomi NPP satellite.

Several things are now possible:

• Low level features like tropical clouds can be seen, which means these clouds can be masked out, making measurements of sea surface temperature more accurate, for one. The amount of low-level cloud also has an important role in understanding climate records, and nighttime observations of low cloud will improve that dataset as well.
• Snow fields and sea ice can be seen at night now as being distinct features from non-ice surface features.
• Polar winter observations are now possible in the visible - previously, no visible observations over Antarctica, for example, were available for much of the year. Starlight and airglow observations make those measurements possible.
• Understanding features of the mesosphere are also now possible - few instruments can see the mesosphere, and direct measuremnts aren't easy to take, outside of rocket launches. Seeing mesospheric structures due to changes in the light-emitting layers in the mesosphere will give us the first long-term view of how the mesosphere works.

The capabilities of the DNB will provide scientists and researchers with years of new observations and should contribute to many new breakthroughs in our understanding of the Earth and its systems, as seen from space at night.