Why? -- is it produced
 

Where do the electrons come from?

• Nightside aurora:  magnetosphere
• Dayside aurora:  some from the magnetosphere and others directly from the solar wind

Figures

• Cutaway drawing of the magnetosphere:  
  This drawing shows that the magnetic field lines from most of the magnetosphere are focused down to Earth in the northern and southern high latitude regions.  The patterns of electron precipitation seen in auroral images provide information about the physical processes involving charged particles (the plasma) and electromagnetic field along the magnetic field lines.  Thus an image of the aauroral oval and polar region provide a global monitoring of activity in the magnetosphere.
• Colors (wavelengths) as function of height:  
• Lower border/altitude profile:  
  Altitude of the maximum ionization rate (points) and the altitudes at which this rate has fallen to 10% of the maximum (bars) versus energy, for ionization produced by electrons which form the discrete aurora as modeled by monoenergetic spectra of isotropic incidence [Rees 1964].
• AL and IMF plot:  
  The magnetosphere has to provide energy to the electrons that precipitate into the atmosphere to produce the aurora.  This energy comes ultimately from the sun via the solar wind and into the magnetosphere.  The ability of energy of the solar wind to enter the magnetosphere depends very strongly on the direction of the interplanetary magnetic field that is carried by the solar wind.  When this field has a northward component (Bz is positive), little energy can enter the magnetosphere and the aurora that is seen is very weak.  It is measured by an index called AL, which on the plot is normalized by the square of the velocity of the solar wind, so only the effects of the magnetic field direction are taken into accound.  However, when the magnetic field has a southward component (Bs in the plot), more and more energy enters the magnetosphere as Bs becomes larger.  Thus the direction of the weak interplanetary magnetic field acts as a valve to letting energy of the solar wind enter the magnetosphere.
• Summarizing Chart:  

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Web development:   Theresa Valentine
Last updated:  7/26/00