1. Space Weather: The Satellite Drag Problem 21 November 2011
William J. Burke
Air Force Research Laboratory/Space Vehicles Directorate
Boston College Institute for Scientific Research
DMSP
C/NOFS
CRESS

2. Space Weather Course Overview
Lecture 1: Overview and Beginnings
Lecture 2: The Aurorae
Lecture 3: Basic Physics (painlessly administered)
Lecture 4: The Main Players
Lecture 5: Solar Wind Interactions with the Earth’s Magnetic Field
Lecture 6: Magnetic Storms
Lecture 7: Magnetic Substorms
Lecture 8: Magnetosphere – Ionosphere Coupling
Lecture 9 The Satellite Drag Problem
Lecture 10: Verbindung (to help make up for your rash decision not to take
Wollen Sie Deutch Sprechen?)

3. Space Weather Satellite Drag
Overview
This week we concentrate on problems associated with tracking space-objects for collision avoidance and predicting s/c re-entry.
The exercise of both critical capabilities is strongly affected by dynamics and variability of the thermosphere. Earth’s uppermost atmospheric layer extends from about 80 to 1,000 km. Like all space weather the thermosphere’s dynamics are controlled directly/indirectly by the Sun.
The zero-order structuring of the atmosphere results from solar induced photo chemistry and consequent thermodynamics.
- Structure of the atmosphere Solar EUV, UV and visible light heating
- Atmospheric forcing from below effects on re-entry predictions
- Magnetospheric forcing from above effects on orbit predictions

4. Space Weather Satellite Drag
Scope of the Problem
Air Force Space Command tracks > 12, objects in low Earth orbit (LEO).
About 10% are active payloads.
The rest are inactive payloads, rocket bodies and associated debris.
More than 4000 objects are below 700 km where aerodynamic drag is significant.
March 1989 storm: > 1500 objects lost.
Destruction of Feng yun 1C in Jan 2007 added > 2500 new trackable ( > 10 cm) objects to AFSPC catalog
Cosmos Iridium 33 collision on 10 Feb 2009 created new trackable objects in catalog.

5. Space Weather Satellite Drag
Solar Spectrum
Photospheric Spectrum
Full Solar Spectrum
Just because you can’t see it, doesn’t mean it doesn’t affect you

6. Space Weather Satellite Drag
Structure of Atmosphere
Soft X-rays and EUV from corona heat and ionize the thermosphere
Lower frequency UV maintains ozone layer
Visible and IR penetrate to ground to heat the troposphere.

7. Space Weather Satellite Drag
Predicting Re-entry
Season-longitude variability in thermospheric density observed near 400 km.
Distribution consistent with density
variability due to tides 80 to 110 km.
Re-entry predictions for unpowered vehicles degraded by atmospheric tides & planetary waves that clutter the lower thermosphere.
“Forcing from below” has become a major research focus MURI at U. Colorado

8. Space Weather Satellite Drag
S/C tracking
Newton’s laws indicate that for an orbiting satellite the downward force of gravity must be compensated by an upward centrifugal force mV2/r.
When atmospheric drag cause a s/c to fall into a lower orbit it speeds up.
If drag forces are large & unpredicted, s/c are in different places than the interrogating radar beams. They are lost! E. g. March 1989 storm

9. Space Weather Satellite Drag
Forcing from Above
Magnetic storms deposit TW of power for 10 – 12 hours into thermosphere.
Alter density structure of to increase the drag: MS aD =  Vs2 AS CD
CD: ballistic coefficient includes effects of s/c aerodynamics and materials
New technologies introduced over past decade on US and ESA satellites.

10. Space Weather Satellite Drag
Stormtime Thermosphere
Frank Marcos challenged WJB to interpret 150 days of GRACE accelerometer data that included the large storms in July and November 2004.
- Models underestimate density increases by > 100%
- Polar cap potential and - Dst follow centroid of density plots
- Thermosphere relaxes in same way when driver turns off
My Eureka moment: “On a global scale the thermosphere acts like a large driven-dissipative energy system!”

11. Space Weather Satellite Drag
Stormtime Thermosphere
Assumptions:
Independent UV & SW sources
Eth = Eth UV + Eth SW
Results:
Numerical solutions replicated Eth during multiple magnetic storms
- Introduced Dst rather than eVS
AFSPC testing: stormtime predictions errors fell from > 65% to < 15%.
AFSPC adapting operational algorithms

12. Space Weather Satellite Drag
Summary and Conclusions
The thermosphere has energy sources from above and below Atmospheric tides, planetary and gravity waves in lower thermosphere
- Solar EUV directly heats/ionizes thermosphere above 100 km On an engineering level, we have started to model stormtime dynamics
Work is continuing to analyze new information contained in data from accelerometers on CHAMP, GRACE, GOCE and SWARM.
Integrating new information with old and ongoing measurements from ground-based tracking radars.
Computer based programs that simulate responses of thermosphere to
driving from above and below are being used to help understand the local physics that underlies global approach outlined in slides 10 and 11.
Unraveling thermosphere’s stormtime dynamics continues to be a fun ride!