Solar eclipses present a truly unique opportunity to study the effects of a supersonic cooling shadow and its modulation of the structure and energetics of the ionosphere- thermosphere system. Atmosphere Perturbation around Eclipse Path (APEP) is a comprehensive sounding rocket campaign designed to study these eclipse dynamics. The mission name takes inspiration from serpent deity Apep from ancient Egyptian mythology. Apep is the nemesis of the Sun deity Ra and is said to have pursued him and every so often nearly consumed him, resulting in an eclipse.
Chimonas  postulated that an eclipse shadow can result in internal gravity waves which then show bow-wave patterns in Traveling Ionospheric Disturbances (TIDs) in the ionosphere. The eclipse induced ionospheric irregularities have been a topic of interest since then, and the recent total solar eclipse of 2017 provided the community with an opportunity to obtain important ground- and space-based measurements to assess the impact of the eclipse such as the generation of large scale TIDs [Coster et al 2017]. However, some of the results have caused a debate. While Zhang et al  demonstrated through ground-based TEC data that extensive bow-waves were observed directly linked to the eclipse, Mrak et al. [2018a] claimed that these TIDs were also explainable through possible modifications to existing orographic and tropospheric sources of acoustic gravity waves under conditions of a sudden reduction in temperature. In a separate study, Mrak et al [2018b] presented an argument that the irregular EUV illumination on Earth’s atmosphere in eclipsed conditions can also lead to TID like signatures that move with supersonic speed and are the result of modulation of the plasma production function. Perry et al  presented radio occultation observations from Swarm-E satellite to claim the presence of eclipse induced medium scale waves that were even present in the penumbra before totality, but their work focused only on the topside F region above 200km.
In addition to the above studies, the temperature and density gradients resulting from eclipse can seed instabilities such as the Temperature gradient instability and gradient drift instability. With the right conditions, these mechanisms can seed irregularities in scales of few meters to 100s of meters which can cause scintillation of radio signals, which cannot be captured with large-scale simulations currently used by the community. Physical processes perturbed by eclipse extend well beyond the maximum period of totality into the penumbra [Goncharenko et al. 2018], appear across vast geographical expanse [Coster et al 2017], and disrupt communication paths.
Given that solar eclipses of any type (total, annular, or partial) occur only about twice every year at some point over the Earth, opportunities to study them and their effects on the ionosphere thermosphere systems are limited due to limitations of comprehensively instrumented remote sensing sites along the path of eclipse. Even rarer is the passage of an eclipse over/near a NASA sounding rocket launch site. The upcoming passage in October 2023 of an annular eclipse almost overhead White Sands Missile Range, and the passage in April 2024 of a total solar eclipse in relative proximity of Wallops Island, presents a compelling opportunity to perform simultaneous multipoint in-situ measurements at extremely fine spatial scales supported by myriad ground based remote sensing instrumentation. This should provide crucial insight in understanding and modeling the physical processes associated with eclipses that result in ionospheric and thermospheric disturbances. The next similar opportunity from US mainland is in 2045.
The paths of the upcoming eclipses with the launch locations of White Sands Missile Range and Wallops Flight Facility pin-pointed.
Map Credits: Michala Garrison and the Scientific Visualization Studio (SVS), in collaboration with the NASA Heliophysics Activation Team (NASA HEAT), part of NASA’s Science Activation portfolio; eclipse calculations by Ernie Wright, NASA Goddard Space Flight Center
Rocket Location Marking Credit: Berit Bland/NASA
After the launch, the subpayloads get deployed at ~107 km at ~3 m/s ejection velocity. As all four move orthogonally away from the main payload, the payload flips, ejects the nosecone, deploys the booms and begins science measurements at 180 km upleg Altitude. The science measurements pause at 325 km altitude and the rocket flips to align into downleg RAM direction. The science measurements then continue until 70 km downleg, all the while the Subpayload continue moving away horizontally but in parallel parabolic arc.
While the rocket measurements are going on, the ERAU team will release high altitude balloons every 20 minutes starting 2 hours before the peak eclipse (roughly 8:30 am Local time) and continue until 2 hours after peak eclipse (12:30 pm Local time). The ground station is able to track upto 6 balloons simultaneously and lifetime of each balloon is about 2 hours. The peak altitude achieved is about 28-30 km. This will give lower atmosphere winds and temperature. Co-investigator institution Air Force Research Lab will do continuous measurements of mesosphere winds (75-110 km) as well as ionosphere measurements using multiple ionosondes. These large-scale ground based measurements will be coupled with in-situ small scale measurements to feed into the modelling studies that are being led by CU Boulder (now JHU APL) and ERAU scientists. We will also be bringing in measurements from larger science community as well as GPS TEC maps.
The above image lists the three specific science questions that the mission is trying to address. In order to do so we drive ourselves with some physical quantity measurement requirements and then select instruments that achieve those requirements for flight aboard the rockets. Furthermore, there are some mission requirements necessary to achieve the spatio-temporal variability.
APEP Mission Patch