A network of thousands of Starlink satellites is being used as a continuous sensor of the upper atmosphere by turning published orbital drag terms into a single daily "sink rate": how many meters a typical Starlink at 480 km would descend in a day without thrust. Public two-line element (TLE) fits include a drag parameter (B*); by following a fixed cohort of ~1,000 Starlinks, normalizing each to its quiet-air floor, and taking the median, a robust index of thermospheric density is produced. That index is calibrated to actual, unthrusted Planet Labs Doves (107 spacecraft) giving 35.7 m/day per unit; during the Jan 19-21 G4 geomagnetic storm (Kp 9-, Ap 144) the Doves’ decay rose 2.74× and the Starlink index rose 2.71×, agreeing to 1%. Kuiper (391) and OneWeb (651) fleets provide independent checks and altitude coverage.
The index tracked 2026 daily sink rates from a quiet 19.7 m/day (Aug. 10) to 127.6 m/day at January’s storm, showing the atmosphere “breathes” with solar UV (two-day lag) and geomagnetic storms. Multi-altitude responses ranged from 1.5× at ~350 km to ~3.0× at 555 km, diminishing by 1,200 km, matching expected heated-oxygen expansion. The public, daily measure helps calibrate orbit-prediction models for collision avoidance, but remains a relative metric: TLE fits blur over 1-3 days, Starlink attitude changes during storms lower measured peaks, and absolute densities still require external models.
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