NICER observation list: Target ID 452201

We will measure vertical atmospheric density profiles in the altitude range of 80-180 km, based on atmospheric occultations of the Crab nebula with NICER/XTI for a total exposure time of 26 ks. The density profiles will be much more accurate than ever obtained in X-rays, and will be critical to establish the possible density deficit at altitudes ~100 km suggested by some earlier observations and theories. Thanks to NICER's excellent throughput and time resolution, we will derive a density profile for every single occultation scan, and thus will be able to investigate seasonal variations of the density profiles. Revealing seasonal variations of the O/N density ratio is especially important to understand the cause of ionosphere's annual anomalies such as the seasonal TEC variations.