Space Weather: Magnetic Storms 31 October 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 Magnetic Storms Last week we looked at the Sun as the source of Earth’s space weather. Pressure gradients in the corona drive a H + /e - supersonic solar wind – Typical densities: ~ 5 cm -3 – Typical speeds: ~ 400 km/s – Earth’s magnetic field acts like a cavity in solar wind – Bow shock stands in front of the Earth The solar wind carries a weak magnetic field away from the Sun into interplanetary space called the interplanetary magnetic field (IMF) – Dungey (1961) argued that when the IMF has a southward component it should interact strongly with the Earth’s field to drive magnetic disturbances. – Experimental studies over intervening 50 years overwhelmingly confirm Dungey’s hypothesis: magnetic activity is always preceded by southward turning of the IMF. Overview

4 Space Weather Magnetic Storms In preparing this this presentation it seemed useful to concentrate on a very simple, but very intense magnetic storm that occurred in November ACE was at the first Lagrange point L 1 where measured the solar wind density and speed as well as the interplanetary magnetic field (IMF). The GRACE satellite was in circular polar orbit near 490 km. - An onboard accelerometer measured the atmospheric drag on the spacecraft. - From the accelerometer measurements we inferred globally-averaged mass densities in the thermosphere and its total energy content We compare interplanetary forcing and thermospheric responses with variations of the stormtime disturbance Dst index - Dst measured as N-S magnetic variations observed at 4 widely spaced stations around globe - Reported at 1-hour cadence as spatial and temporal average  B NS - Linearly proportional to energy in the ring current (Dessler-Parker –Sckopke)

5 Space Weather Magnetic Storms Sun Earth Closed Field lines Interplanetary Field lines Open Field lines Magnetic merging at dayside magnetopause Magnetopause current sheet Solar Wind Three Magnetic Topologies - IMF: two feet in solar wind - Closed: two feet on Earth - Open: one foot on Earth and one in the solar wind

6 Space Weather Magnetic Storms Earth Northern Lobe Southern Lobe Plasma Injection Plasma Ejection Magnetic Reconnection in the magnetotail Near Earth X-line (activated during substorms) Distant X-line Dayside merging site Dungey’s picture provide a rational for the existence and dynamics of the plasma sheet, the then undiscovered storage region from which auroral particles are drawn.

7 Space Weather Magnetic Storms Coronal Mass Ejections SOHO observations of a CME ejection Artistic rendition of a flare and CME Computer simulation of a CME

8 Space Weather Magnetic Storms Concrete example: consider November , 2003 Storm - X-28 class X-ray flare - Coronal mass ejection - No solar energetic particles Largest magnetic storm of last solar cycle The plots to the right show measurements from ACE at L 1 : - Solar wind speed (top) - Solar wind density (blue) and dynamic pressure (red) - IMF B Z component (bottom)

9 Space Weather Magnetic Storms The two top plots repeat ACE measurements of the solar wind density and pressure as well as IMF B Z. The bottom plot shows the magnetospheric response in the form of the Dst index which indicates the growth and decay of the stormtime ring current The symbol  VS represents the magneto- spheric electric field in the equatorial plane The storm’s main phase (negative Dst slope) began when  VS turned on. The storm’s recovery phase (positive Dst slope) began when  VS turned off.

10 Space Weather Magnetic Storms Slide shows thermosphere’s response to magnetic storm driving The trace in the top panel shows that the globally-averaged thermospheric density at 490 km increased by a factor of 6 from 5 ∙ to 3 ∙ grams/cc. The trace in the middle panel indicates that the total energy of the thermosphere rose from 6.2 ∙10 17 to 6.8 ∙10 17 (  E th ~ 6 ∙ ) Joules. Since the rise occurred in ~12 hour the average power into the thermosphere was ~ 1.4 ∙10 12 Watts. The energy of the ring current estimated via D-P-S relation E RC (Joules)  3.87 ∙ ∙ |Dst (nT)| Minimum Dst  -475 nT E RC  1.9 ∙ Joules

11 Space Weather Magnetic Storms This slide shows southern auroral ionospheric response to storm driving on November 20, 2003 False color EUV image of ionosphere acquired by NASA’s Polar satellite at an altitude of ~ 9 R E. Red indicates most intense auroral emissions. During large storms auroral particle fluxes into the ionosphere are their most intense. Based on particle and optical measurements from AF, NOAA and NASA satellites, electron and ion energy precipitation rates and can reach ~ 100 GW. This is about a factor of 10 less than the electromagnetic power needed to heat the global thermosphere. Electric and magnetic field measurements from AF and NASA satellites agree with thermospheric power estimates.

12 Space Weather Magnetic Storms In looking at the magnetic superstorm November 20, 2003 we have been exposed to a wide sampling of what the Sun can throw at us. Be warned however, it does not represent the full spectrum of consequences: - Halloween storm 2003: severe MeV particle fluxes generated in the solar flare destroyed ability of ACE to measure solar wind characteristics - March 1989 storm: brought down Hydro Quebec electric grid. AFSPC lost ~3500 space objects that it was tracking. - March 1991 storm: created a new radiation belt in about two minutes. In next weeks lecture we will turn our attention to substorms the other geomagnetic disturbance that occurs after southward turnings of the IMF. This type of event involves activations of the near-Earth X line and have much shorter lifetimes than storms, but they can have deadly consequences for satellites in geostationary orbit. Summary and Conclusions

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