ESS 7 Lecture 14 October 31, 2008 Magnetic StormsSlide Number 2The Phases of a Magnetic Storm – Sudden CommencementSlide Number 4The Dst Index Measures the Magnetic Change Caused by the Ring CurrentSlide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10A CIR Related StormStorms Increase the Fluxes of Radiation Belt ParticlesSlide Number 13Plasmapause During Halloween StormThe Biggest Storm (Li et al., 2006)Modeled DstESS 7ESS 7 Lecture 14Lecture 14 October 31, 2008October 31, 2008 Magnetic StormsMagnetic Storms• An isolated substorm is caused by a brief (30-60 min) pulse of southward IMF.• Magnetospheric storms are large, prolonged disturbances of the magnetosphere caused by variations in the solar wind.– Many storms follow coronal mass ejections.– Storms also can be caused by high speed streams (interplanetary shocks).• Storms are global disturbances of the magnetosphere while substorms tend to be localized in the magnetotail.• During storms large amounts of energy are placed in the ring current – this does not happen in substorms.• Substorms occur during storms but also at other times. Magnetic Storms Versus SubstormsThe Phases of a Magnetic Storm – Sudden Commencement• The pressure increase from the interplanetary disturbance impulsively compresses the magnetosphere.– The sudden compression rapidly increases the magnetopause current increasing the H- component of the magnetic field.– The sudden commencement can be seen in midlatitude magnetograms.– The rise time is a few minutes and corresponds to the propagation time of low frequency (MHD) waves from the magnetopause to the point of observation. – The compressive phase of the storm lasts 2 to 8 hours.– When not followed by the other phases of the storm this part is called a sudden impulse• The ring current decreases the horizontal component of the magnetic field at the Earth’s surface. (You can use a right hand rule to check this – Place your thumb along the direction of the current. Your bent figures will give the direction of the change in the magnetic field.)The Ring Current and Magnetic StormsThe Dst Index Measures the Magnetic Change Caused by the Ring Current• The disturbance storm time (Dst) index measures these differences. A stronger ring current causes Dst to decrease. • Note other currents (e.g. magnetopause currents also can contribute to Dst).Sudden CommencementMain PhaseRecovery Phase• The total energy in the Earth’s dipole magnetic field above the surface of the Earth iswhere BE is the magnetic field at the surface of the Earth and RE is one Earth radii (6371 km).• The energy in the ring current can be expressed bywhere is the change in B measured at the surface of the Earth (Dst), • This is called the Dessler-Parker-Sckopke relationship• After some corrections for the conductivity of the Earth we get that 100nT depression in B is equal to 2.8X1015J. 32034EEmagRBWμπ=kWWBBmagRCEˆ32−=ΔrThe Energy in the Ring CurrentBrΔ• Extended periods (several hours) of southward IMF lead to the main phase of the magnetic storm.– Southward IMF leads to magnetic reconnection.– Northward IMF has only minimal dayside reconnection.• The increased dayside reconnection increases the penetration of the solar wind into the magnetosphere, increased convection and ring current injection.The Solar Wind During a CME Driven Storm• As the southward component of the IMF weakens or disappears, the ring current starts to decay. This is the recovery phase of the storm.• The recovery phase has several steps.– The reduction of the southward IMF causes the reconnection rate to decrease.– The reduction of the southward IMF results in a decreasing electric field which leads to a reduction in the injection of new particles into the ring current.The Recovery Phase• The ionosphere fills the depleted flux tubes within this expanded boundary with cold ionospheric particles.• Some of the ring current particles are scattered into the ionosphere and lost.• Another loss mechanism for ring current particles is charge exchange.– Charge exchange occurs between energetic ring-current ions and cold hydrogen atoms from the ionosphere.– The result is energetic neutral atoms and cold ions.• The result of the last two processes is a gradual decrease of the ring current over several days.+ +Energetic Ring Current IonThermal Neutral AtomThermal IonEnergetic Neutral Atom(leaves the system)The Decay of the Ring CurrentEnergetic Neutral Atom Imaging• The new neutral atoms are very energetic.• They fly from the charge exchange region in straight lines.• They can be detected by special cameras and image the ring current formation and decay.• During very intense storms oxygen from the ionosphere comes out and can dominate.A CIR Related Storm• Usually the storms associated with high speed streams are smaller.• This one had a Dst of -70nT.• Note that the magnetic field oscillates.Storms Increase the Fluxes of Radiation Belt Particles• Twelve years of data showing outer radiation belt electrons. • Each increase is storm related. • Note the largest increases are in declining phase of the sunspot cycle.New Radiation Belt formed by October 2003 Magnetic Storm- SAMPEX observations• The radiation belts are centered at about 4RE and about 1.5RE.• Storms can both decrease and increase the belts.• The region between the belts is normally without particles.• It filled with particles during the Halloween storm.Plasmapause During Halloween Storm• He+ images of plasmasphere.• During the storm the plasmapause moved from 4-5RE to inside of 2RE• Plasmapause tracks well the inner edge of the radiation belts.The Biggest Storm (Li et al., 2006)• An extremely bright solar flare occurred on September 1, 1859 and a very large magnetic storm followed.• Magnetic field observations from Mumbai, India indicate that the H-component decreased by -1600nT • For smaller storms the H-component roughly gives Dst (Siscoe et al., 2006)• Time from flare to the peak in the Mumbai magnetogram gives the velocity of the shock and empirical formulas give the speed of the ICME.• B is inferred from global MHD simulations and relationship peak Bsh = 0.08 (Vmax -VSW ) where VSW is the normal solar wind.• Density uses normal shocked solar wind values.Modeled Dst• Li et al. (2006) calculation of Dst using inferred solar