ESS 200CESS 200CLecture 18Lecture 18• 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 large, prolonged VxBzin solar wind.– Many storms follow coronal mass ejections (CMEs; during solar max).• Solar Wind has increased magnetic field (flux rope) and high speeds• Bipolar signature in Bz depends on solar cycle (+/- versus -/+)– Storms can be caused by high speed streams (during solar min).• Recur with solar rotation period, 27 days, due to coronal hole recurrence– CME or fast wind generate interplanetary shock as overtaking slow wind• Impulse from the interplanetary disturbance impulsively compresses the magnetosphere.– The sudden compression rapidly increases the magnetopause current and brings it closer to Earth increasing the H- component on the ground.– The sudden commencement can be seen in midlatitude magnetograms.– The rise time is a few minutes and corresponds to the propagation time of MHD waves from the magnetopause to the point of observations. – 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 causes decreases in the horizontal component ofthe magnetic field at the Earth’s surface. • The disturbance storm time (Dst) index measures these differences.Sudden CommencementMain PhaseRecovery Phase• 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.• The enhanced duskward electric field increases the number of particles injected into the ring current.– Stronger electric fields lead to earthward expansion of the ring current region.– Heavy ionospheric particles also are added to ring current.• The ring current will grow and Dstwill decrease ( ) and approach a saturation level when particle sources and losses balance.– Ion Losses: charge exchange, scattering into the loss cone and outward diffusion + magnetopause loss– Electron Losses: pitch angle scattering into loss cone• The period during which the ring current increases is the main phase.zmagRCeWWBBˆ320−=Δr• 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 convection boundary moves outward.– The ionosphere fills the depleted flux tubes within this expanded boundary with cold ionospheric particles.– The interaction between the two plasma populations (hot ring current and cold ionospheric) causes plasma waves which scatter the ring current particles into the loss cone. This causes a loss of ion ring current particles.– Another loss mechanism for ring current particles is charge exchange. Charge exchange occurs between energetic ring-current ions and cold(er) hydrogen atoms of the extended exosphere. The result is energetic neutral atoms and cold ions. Detectors which can detect the energetic neutral atoms are have been developed. They enable us to image the ring current in three dimensions.–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)• Images in Extreme UltraViolet(EUV) (30.4nm line) of the He+resonant absorption and scattering of sunlight provides a global snapshot of the plasmaspheric He+density.• This is produced by photoionization of exospheric He, which provides an image of the exospheric density.• This image is taken from the EUV instrument on the IMAGE satellite (Sandel et al., 2000).• Charge exchange of energetic ions with exospheric neutrals produces ENA images which “image” the charged particle density of the ring current in the energy range of the neutrals.• This image is taken from the High Energy Neutral Atom instrument on the IMAGE satellite, imaging the Oxygen between 50-200keV, the peak contributors to the energy density of the storm time ring current (Mitchell et al., 2000).• During quiet times the solar wind provides ~65% of the ring current energy density and the ionosphere only ~35%. (H+ dominant).• During small and moderate storms the ionospheric contribution becomes ~50% (H+dominant).• During intense storms (Dst<-150 nT) the ionospheric contribution increases to ~70%. (O+dominant).• The O+ dominance during intense storms is greater during solar maximum.– Increased solar EUV irradiation causes increases ionospheric and atmospheric scale heights which favors the escape of O+.– Increased heating of neutral atmosphere and increased ionization rates.• Ring current injection can be explained primarily in terms of inward transport of plasma sheet and pre-existing ring-current particles.• None of the models currently includes the ionosphere.• Diffusion has been used successfully to study the injection of radiation belt particles during a storm (see figure at the right). However, the diffusion calculations don’t seem to work for the lower energies of the ring current.New Radiation Belt formed by October 2003 Magnetic Storm• During magnetic storms precipitation of auroral particles expands toward lower latitudes.• Intense red and green-line auroral emissions are found at the equatorward most part of the expanded auroral oval.• Examples of magnetic storms:• On September 24, 1998 a strong interplanetary shock reached the Wind spacecraft 185REupstream of the Earth.– When this hit the Earth the pressure at the nose of the magnetosphere went from 2nPa to 15nPa. – The x-component of velocity was -900 km/s– The IMF initially was horizontal but after 2 hours it turned southward and a strong storm began.DstBz GSM (nT)Vx (km/s)• THEMIS recently discovered that (+/-) North-then-South polarity of CMEs results in 20 times more solar wind plasma penetrating Earth's magnetosphere ahead