Self-Organized Criticality in Magnetic Substorms

1. Self-Organized Criticality in Magnetic Substorms Understanding The Mechanics of Space Weather

2. Solar Weather Terms • Coronal Mass Ejections (CMEs) • Solar Wind • Magnetosphere • Magnetic Reconnection • Van Allen Radiation Belts • Ring Current • Disturbance Storm Time (DST)

3. Necessity for Understanding DST • Ring Current field capable of inducing large currents in power lines, satellites, computer chip factories etc. Source (left): http://optics.org/objects/ole_feat/ Source (right): www.physics.co.il/News.asp

4. Coronal Mass Ejections • Solar blob of electrified gas • Occurrence: • Solar Minimum: ~ 1 CME per week • Solar Maximum: 2-3 per day Source: http://www.spacescience.org/ExploringSpace/VirtualExhibit/SpaceIsNotEmpty/

5. Magnetosphere • Dipole Magnetic Field • Protects earth, deflecting solar activity • Causes particles to spiral into atmosphere at poles (aurora) Source (bottom): http://istp.gsfc.nasa.gov/istp/news/img.html Source (top): www.spaceweather.com/aurora/ images/31mar01/haug1.jpg

6. Solar Wind & Magnetosphere Source (above): http://istp.gsfc.nasa.gov/istp/news/img.html Source (left):http://www-istp.gsfc.nasa.gov/Education/wsolwind.html

7. Magnetic Reconnection • Occurs during loading phase of cycle (typically 40 minutes) • Magnetosphere tail becomes severely stretched • Magnetic field lines become aligned anti-parallel • Field cancels, energy conserved in several ways including injection of energetic particles • Plasmoid drifts off with solar wind

8. Magnetic Reconnection Source: http://istp.gsfc.nasa.gov/istp/news/img.html

9. Magnetic Reconnection Source: http://istp.gsfc.nasa.gov/istp/news/img.html

10. Van Allen Belt & Ring Current • Drift of energetic particles • Production of “Ring Current” in belt, radius 2RE (Earth Radius) • Current produces own field interacting with magnetosphere Source: http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/m0010420.html

11. DST - Description • Disturbance Storm Time index • Measures strength of ring current field based on interaction with earth’s magnetosphere • Measured in units of nanoTesla (nT)

12. DST – Scatterplot (2001) DST Index value (nT) Time (Hours)

13. DST – Probability Distribution Probability DST Index value (nT)

14. DST – Periodicities Power Spectral Density Peaks Corresponds to 27.12 days

15. Self Organized Criticality (SOC) • Statistical analysis, independent of time • Event based, critical climax and relaxing • Classic example: Avalanches in a sand pile Source: http://www.londontoday.net/photo/industry/v001/images/full/

16. Power Laws in SOC • Event Size vs Number of Events; straight line in log-log space lnN(s) = - * ln(s) N(s) = s- • Events of a bigger size should occur less frequently than events of a smaller size

17. DST – A SOC System T=0.6923 ln(number of occurrences) ln(size of event)

18. Use for SOC Model • Precursors for magnetic reconnection discovered, severity of substorm can be determined • Ability to predict length of an event based on initial size of storm?

19. Source: http://www.spacescience.org/ExploringSpace/VirtualExhibit/SpaceIsNotEmpty/