TIMED: the mission to explore the least explored regions of the atmosphere
The Earth has existed for 4.6 billion years, and yet parts of our atmosphere are still unknown to us after decades of space and centuries of atmospheric exploration. Weather balloons and ground equipment can only scan the troposphere and the stratosphere, whereas historic satellites could only scan the exosphere and upper thermosphere. This actually makes the mesosphere and lower thermosphere/ionosphere (MLTI) region the least explored region, where meteors burn and Aurora Borealis is formed. This inspired the creation of TIMED, a NASA satellite mission designed to explore this region through remote sensing of factors that change Earth’s conditions.
Overview of the mission
For this type of mission, an orbital launch platform (orbiting the Earth) would work better than a suborbital one (not in Earth orbit, touches ground bit after launch) as the MLTI region is too high for weather balloons to reach, and sounding rockets only obtain temporary readings. The TIMED mission featured a Delta II rocket carrying the spacecraft to be placed in a 388-mile circular orbit (TIMED Atmospheric Spacecraft Successfully Launched, 2021). As it is scanning from above, TIMED requires sensors that would be able to remotely sense conditions in the MLTI region from a distance.
There are four main sensors involved:
- Sounding of the Atmosphere using Emission Radiometry (SABER) — observe heat
- Global Ultraviolet Imager (GUVI ) — observe ultraviolet light
- Solar Extreme Ultraviolet Experiment (SEE) — measure energy deposit from solar radiation
- TIMED Doppler Interferometer (TIDI) to measure wind and temperature conditions
Sensor Specifications
For any mission, NASA would need to determine how accurate and precise the sensors are, especially on a mission where such readings are some of the first. The MLTI region is the area where chemical forces allow Earth’s conditions to change, such as carbon dioxide buildup, methane accumulation, and even impacting the pressure, temperature, and winds of Earth’s climate. Therefore, adequate sensing equipment would be required to detect factors causing these changes.
SABER is a very powerful radiometer, able to scan up to 112 miles of atmosphere every 58 seconds on 15 longitude bands per day, obtaining data on nitrogen, water vapor, hydrogen, carbon dioxide, and ozone.
The GUVI sensor construction and maintenance is backed by John Hopkins University Applied Physics Laboratory and The Aerospace Corporation. Similar to SABER, this sensor also has quite long range, being able to scan 300 kilometers at once. Additionally, it will scan for ultraviolet light between 110 and 180 nanometers of wavelength in the MLTI region about every 1.5 hours. However, it is more vulnerable to wear and tear compared to other sensors. In fact, it cannot scan across the spacecraft’s limb anymore, and must remain in 30 degrees of nadir (facing downwards).
SEE is also able to measure (UV) radiation similar to GUVI; however, SEE focuses on measuring energy deposits from the sun. SEE focuses on the effects and changes of extreme-UV radiation, far-UV radiation, and soft X-ray. As a powerful UV and X-Ray sensor, it can scan such radiation between 25 and 200 nanometers with a resolution of 0.4 nm; additionally, it has a field of view of 6 degrees by 12 degrees, making it able to precisely scan changes in UV and soft-X ray radiation. Wear and tear impacted, adding the constraint of only being able to operate at 3% duty cycle (ON 3% of the time, OFF 97% of the time).
The last sensor is TIDI, a wind sensor onboard TIMED to detect wind and temperature changes in the MLTI region. Components in the atmosphere such as oxygen, sodium, and oxygen-hydrogen emit tiny changes in light color; TIDI is able to measure such changes to determine variation in speed and direction of wind. Four telescopes offer a high degree of precision in determining the wavelength of light to detect variation in the wind. Like all the previous sensors, this too is powerful. TIDI measures the variation of VIS/NIR light within 60 to 300 kilometers above the surface of the Earth. This means TIDI can output 125 kilometer by 2.5-kilometer spatial resolution with being able to detect light between 550 to 900 nanometers wavelength.
How was data collected with the sensors?
The GUVI sensor is able to collect data as state variables of the daytime thermosphere temperature, oxygen, nitrogen, oxygen/nitrogen density ratios, and low-latitude nighttime electron density profiles. It also outputs energy inputs of particle energy/fluxes and also solar EUV irradiance.
This is used to infer impacts of the sun based on continued variations with respect to time of these ratios. GUVI provides horizon-to-horizon images in five selectable bands of UV light:
- 121.6 nm
- 130.4 nm
- 135.6 nm
- 140 to 150 nm (N2 Lyman-Birge-Hopefield bands)
- 165 to 180 nm (N2 Lyman-Birge-Hopefield bands)
It uses a scan mirror from a field of vision 11.78 degree TIMED MISSION 6 to an arc up to 140 degree in the cross direction in 22 seconds.
SABER collects state variables of ozone, water, carbon dioxide, oxygen, and hydrogen ratios as well as temperature, pressure and density. SABER also obtains energy inputs and outputs from cooling and heating rates, especially from oxygen, hydrogen, ozone, carbon dioxide, nitric oxide, and water. Given these ratios, SABER is able to report heat emission with the heating rates too. SABER scans up and down for every 58 seconds and collects data over an altitude of 180km. It measures CO2 15.4 µm emission based on thermal emission characteristics. SABER also measures vertical distribution of ozone and water vapor molecules which have a role in solar photon energy absorption.
SEE provides irradiance energy inputs of:
- Soft X-Ray
- Extreme UV
- Far UV light
It has 9 XUV silicon photodiodes, designed to measure full-disk solar soft X-ray spectral irradiance at fixed spectral wavelengths. Each photometer in SEE has a spectral bandpass of 5 to 10 nm with 20-degree field-of-vision in diameter. It has an additional bare X-ray ultraviolet photodiode with Acton Lyman-alpha filters for measuring important Lyman-alpha irradiance.
The TIDI sensor gathers the data in the form of horizontal vector winds, as state variables. These vector winds are able to continuously track changes in the speed and direction of wind given each of these horizontal vector winds. TIDI has four telescopes with an aperture 7.5cm, f/2/2 FOV, Fabry-Perot interferometer with a CDD detector and electronics box. TIDI can scan 125.0km x 2.5km spatial resolution of individual views at 2.5 degrees horizontal and 0.05 degree vertical. With every up and down acquisition takes about 100–200s to complete resulting profiles of 750 km along the orbital track.
Conclusion
With the TIMED mission, we are able to gain a new understanding of how the MLTI layer of our earth’s atmosphere through heat emission, UV light glow, energy deposits, iron/metal detection, and wind conditions. We know that the mesosphere is one of our least explored and known layers, but these four core sensors will help change that in the near future; we would be able to project changes of air composition, wind velocity, heat accumulation, and energy accumulation to help better predict Earth’s conditions. TIMED will be able to provide us on such data through remote sensing of these conditions and qualities in the mesosphere. Especially since carbon dioxide accumulation and ozone layer regrowth is becoming a larger concern, technologies like TIMED allow us to develop metrics, charts, graphs, and plans of carbon dioxide and ozone to aid in predicting the best solution possible.
