VLEO Environment
The Very Low Earth Environment (VLEO) ranges from about 80km in altitude to 200km in altitude. This encompasses a portion of the mesosphere and thermosphere, which is composed mostly of atomic oxygen, molecular nitrogen, and molecular oxygen. The temperature of these regions is an example of extremes; as low as 100K at the mesopause and as high as 1200k in the thermosphere. This neutral atmosphere is entangled with the plasma that makes up the ionosphere, which has layers labeled D, E, and F.
Because of the large drag on in-situ objects and the corrosive atomic oxygen environment, a critical technique for observing this environment is Far-UltraViolet (FUV) remote sensing.
The VLEO environment experiences incredible variability: solar cycle, seasonal, impulsive events, latitudinal, longitudinal, and altitudinal. Its variability is driven from above by the physics of the sun and from below by Earth’s lower atmosphere VLEO is where these forces interact, nonlinearly.
Missions
OWLS
The Occultation Wave Limb Sounder (OWLS) is a NASA sponsored mission. The PI is Dr. Edward Thiemann (LASP) and Dr. Greer is the deputy PI. Using FUV solar occultations and EUV photometers, the suite of instruments will examine density profiles for atmospheric gravity waves and thermospheric temperature. It is expected to launch in 2027.
GOLD
The Global-scale Observations of Limb and Disk (GOLD) is a NASA mission of opportunity aboard SES-14 in geostationary orbit over the mouth of the Amazon river. The PI is Dr. Richard Eastes; Dr. Greer is a mission scientist. GOLD is an FUV spectrometer that is able to observe thermospheric and exospheric temperatures, thermospheric composition, and nighttime plasma densities.
GDC
The Geospace Dynamics Constellation (GDC) mission is a mission concept to study the coupling between the magnetosphere and the ionosphere/thermosphere (IT) system, and how that coupled system responds to external energy input. Dr. Greer is a Co-Investigator on the NEXUS Interdisciplinary Science Team, specifically tasked with coordinating the Calibration and Validation of the six instruments on the six identical satellites. The status of GDC is unclear at this time (March 2026).
ICON
The Ionospheric CONnections Explorer was a NASA mission to study the VLEO environment. The PI is Dr. Tom Immel. Dr. Greer contributed to the mission by building some of the science data pipeline.