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 variations in the solar wind.– Many storms follow coronal mass ejections.– Storms also can be caused by high speed streams (interplanetary shocks).• The impulse from the interplanetary disturbance impulsively compresses the magnetosphere.– The sudden compression rapidly increases the magnetopause current increasing the H- component of the of the magnetic field.– The sudden commencement can be seen in midlatitudemagnetograms.– 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 the ring current population• The ring current will grow and Dstwill decrease ( ) and approach a saturation level when particle sources and losses balance.• The period during which the ring current increases is the main phase.zmagRCeWWBBˆ320−=∆r• 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 hydrogen atoms. 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 Ion Thermal IonEnergetic Neutral Atom(leaves the system)Thermal Neutral Atom• 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.• 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.• Magnetic storms can be caused by high speed solar wind.• 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)• Solar variability effects human activities in three ways.– Space travelers can be exposed to potentially lethal radiation especially when carrying out activities outside of the spacecraft.– Technology both in space and on the ground can be damaged especially during some magnetic storms.– There may be a relationship between terrestrial climate and solar activity.• When high energy particles encounter atoms or molecules within the human body, ionization may occur.– Ionization can occur when the particle is stopped by an atom or molecule. The resulting radiation can ionize nearby atoms or molecules.– Bremstrahlung (radiation released by a “near” miss) can also ionize atoms or molecules.• A rad is the amount of ionizing radiation corresponding to 0.01 Joule absorbed by one kilogram of material.– The rad unit is independent of the type of radiation.– ~100 rads will cause radiation sickness (1Gy = 100 rads).– 1 Gy has a high probability of killing a cell by producing a lesion in its DNA.– 1 rad received from x-rays is less harmful than 1 rad from high energy protons.• The relative biological effectiveness (RBE) of radiation is normalized to 200 keV x-rays. – The biological damage is measured in rem (rem=dose(rad)X RBE).– Electrons, protons, neutrons and alpha particles are the most damaging because they penetrate deeply into human tissue.• The average person in the US gets ~170mr per year from radioactive elements