As seen in this image from the International Space Station, airglow is a result of processes in Earth's upper atmosphere. NASA Credit
New research using data from NASA’s Global-scale Observations on the Limb, or GOLD mission has revealed surprising behavior in the swathes of charged particles that band Earth’s equator. This may be due to GOLD's global long-term view, which is the first such type of measurement.
GOLD is in geostationary orbit. This means that it orbits Earth at the same speed as the planet and "hovers” over the same spot overhead. GOLD can monitor the same area over time for changes across longitude and latitude. This is something most satellites cannot do.
"GOLD is on a geostationary orbital satellite, so we can capture 2D-time evolution of these dynamics," stated Dr. Xuguang Cai (researcher at High Altitude Observatory in Boulder Colorado) and the lead author of a new research paper.
GOLD is focused on Earth's upper atmosphere, which stretches from 50 to 400 miles above sea level. It also includes a neutral layer known as the thermosphere and the electrically-charged particles that make up its ionosphere. The ionosphere's charge particles react to the magnetic and electric fields that run through the atmosphere and space near Earth. This is in contrast to Earth's neutral particles. Because the neutral and charged particles are mixed, any one can have an impact on the other.
This means that the ionosphere, and the upper atmosphere, are influenced by many complex factors such as space weather conditions, such as geomagnetic thunderstorms, which are driven by the Sunand earthly weather. These areas also serve as highways for many of our navigation and communications signals. The signals that pass through these regions can be affected by changes in the density or composition of the ionosphere, such as radio and GPS.
The magnetic field of Earth (represented in orange lines in this visualization) is shaped to drive charged particles (blue), away from the Equator. This creates two dense bands north and south of equator that are known as the "equatorial Ionization anomaly". Credit: NASA's Scientific Visualization Studio
GOLD observes the ionosphere from its position on a commercial communication satellite in geostationary orbit. It makes observations that span the entire hemisphere every 30 minutes. Scientists are now able to see this region from a bird's-eye perspective, which gives them new insight into its changing conditions.
Mysterious movement
Twin bands of densely charged particles located on either side the Earth's magnetic Equator are one of the most striking features of nighttime ionosphere. These bands, known as the Equatorial Ionization Anomaly or EIA, can change in size and shape depending on the conditions within the ionosphere.
They can also change their position. Scientists have used data from satellites that passed through the region to determine how the bands are shifting over time. However, it was more difficult to track short-term changes.
Scientists knew that quick changes in the bands were symmetrical before GOLD. The northern band moves north and the southern band moves south. GOLD witnessed something in November 2018 that challenged this notion: The southern band of particles drifted to the south, while the north band remained stableall within less than two hours.
NASA's GOLD mission, short for Global-scale observations of the Limb or Disk, saw an unexpected asymmetric motion within one of the charged particles that form in the Earths atmosphere at night. This observation was possible because GOLD's unique perspective (right), is unable to be made using other ground-based instruments (left), which can't detect changes in open water. The red dots represent the peak of electron band, as measured using ground-based sensors. GOLD's peak of electron band measurement is shown in the black dots. The measured peaks are visible at different points towards the end of visualization. Credit: NASA's Scientific Visualization Studio
Although scientists have observed the bands moving like this before, this event was shorter than the six to eight hours expected. This observation could not have been made by GOLD. These observations were presented in a paper that was published in the Journal of Geophysical Research: Space Physics on December 29, 2020.
These bands are symmetrically drifted because of rising air, which drags charged particles with it. Warmer pockets of air rise upwards as night falls and temperatures drop. These warmer pockets of air contain charged particles that are bound by magnetic fields lines. For pockets close to Earth's magnetic Equator, the magnetic field shape means that upward motion pushes the charged particles horizontally. This results in the symmetrical northward/southward drift of the charged particle bands.
It is not clear what caused the GOLD-observed asymmetrical drift. However, Cai believes that a combination of several factors, including ongoing chemical reactions, electric field and high-altitude winds, could be responsible.
These findings are surprising but can be used to help scientists see the inner workings of the ionosphere, and gain a better understanding of its dynamics. Scientists rely heavily upon computer models to study the atmosphere, just as meteorologists rely on computer models to predict the weather. It is impossible to observe all processes with satellites or ground-based sensors. Scientists code in the underlying physics and then compare their predictions to the observed data.
Scientists used to get this data from passing satellites or limited ground-based observations. Scientists now have a bird's eye view thanks to GOLD.
To see the interface between Earth and space, explore further NASA GOLD Mission
More information: Xuguang Cai et.al, Observation of Postsunset OI 135 nm Radiance Enhancement over South America by GOLD Mission Journal of Geophysical Research Space Physics (2020). Xuguang Cai et al., Observation Of Postsunset OI 135.6 nmradiance Enhancement Over South America By the GOLD Mission (2020). DOI: 10.1029/2020JA028108
