Magnetospheres

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Magnetospheres and Solar System Space Physics
Anders Eriksson Swedish Institute of Space
Physics Uppsala Lecture for course Physics of
the Planetary System, Oct 3, 2008
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Outline

• Plasmas and magnetic fields
• Solar wind
• Planetary magnetic fields
• Ionospheres
• Magnetospheres
• Space weather
• Shocks

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Motion of Charged Particles

• F q (E v x B) m g
• For charged particles, gravity can usually be
  neglected (calculate yourself!)
• Charges do a spiral around B direction
• A charge in the dipole equatorial plane also
  drifts around due to B changing with r

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Plasmas and magnetic fields

• A plasma is a gas consisting of charged particles
• Usually electrons and positve ions (e.g. H)
• Characterized by electron number density ne
• Charge neutral (ni ne) on scales larger than
  the Debye length ?D (?0 K T/ne e2)1/2
• Space plasmas usually collissionless ? very good
  conductors
• As a result, the electric field in the rest frame
  of the plasma is zero E v x B 0
• One can show that this implies that if two
  plasma elements at some time are connected by a
  B-field line, then they will always be so
• Frozen-in B-field -- rather like spaghetti in
  jelly
• the spaghetti must be infinitely stretchable
• Plasma structures therefore elongated along B

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Plasma and magnetic fields
Structures align with B everywhere
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Currents and magnetic fields

• But B-fields arise from currents, dont they?
• Ampères law ?0j ?xB ...
• ... but says nothing about which is cause and
  which is effect!
• Energy densities usually determine if B or v
  decides B2/(2 ?0) compared to ?v2/2
• On large scales...
• ...the B-field and the plasma follow each other
  (spaghetti in jelly), while...
• ...the current adapts to what it has to be...
• ...as long as the plasma can support it (finite
  conductivity effects)

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The Solar Wind

• Solar mass loss 109 kg/s
• Faster but less dense over the poles (Ulysses)

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Solar wind evidence dual comet tails

• Plasma tail (a.k.a. ion tail)
• Plasma motion controlled by EvxB 0
• Newborn ions quickly get solar wind speed

• Plasma tail close to antisolar direction
• IMF slows down due to mass loading and drapes
  around comet
• Plasma tail structured by B

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The Interplanetary Magnetic Field

• Magnetic flux follows solar wind motion
• Currents can flow in the solar wind plasma
• Field from sun decays as 1/r (not 1/r3) Thanks to
  currents flowing in the plasma

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Ionospheres

• Atmospheres are ionized by radiation
• Solar UV, X
• Cosmic rays

• Layer structure due to varying...
• Radiation penetration depth
• Chemistry
• Recombination rate
• Transport

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Planetary Magnetic Fields

• Generated by interior dynamos in fluid cores
• Dipole-like, never perfect dipoles

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Magnetospheres (1a) Boundary

• A magnetic field turns protons and ions in
  different directions as F q (E v x B)
• Protons electrons moving differently ? an
  electric current

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Magnetospheres (1b) Boundary
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Magnetospheres (2) Currents
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Magnetospheres (3a) Structure Earth as the
typical example
Solar wind 5 cm-3 10 eV 400 km/s
Magnetosheath 50 cm-3 100 eV 40 km/s
Tail lobes 0.1 cm-3 2 eV 40 km/s outflow
Magnetopause
Bow shock
Radiation belts Low density MeV particles
Plasmasphere 100 cm-3 1 eV corotating
Plasma sheet 1 cm-3 1 keV
Plasmapause
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Magnetospheres (3b) Structure

• Magnetosphere structure illustrates
• Organization of plasma by magnetic fields
• Sharp gradients (magnetopause, plasmapause)
  across B
• Small gradients along B
• Cellular structure of space organization of
  plasma into different regions with very different
  characteristics and little mixing across
  boundaries
• Trapped radiation
• Outflow (ionosphere loss) along B
• Space plasma phenomena are governed by electrical
  interactions -- mass is negligible (103 kg for
  all of Earths magnetosphere)

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Magnetospheres (4) Reconnection

• For IMF Bz lt 0, magnetic flux moves from dayside
  into the geomagnetic tail
• Can be released if tail cannot carry necessary
  current

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Magnetospheres (5) Dynamics

• Magnetic flux moved to tail when IMF Bz lt 0

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Magnetospheres (6a) Aurora

• Aurora is a consequence of currents flowing along
  B in a magnetosphere

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Magnetospheres (6b) Aurora

• Aurora maps the plasma sheet boundary along B,
  giving a statistical auroral oval distribution.

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Space Weather (1a) Solar Activity

• Solar flares and coronal mass ejections increase
  solar wind density and/or speed

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Space Weather (1b) Solar Activity

• Fast solar wind much varying with solar cycle
• Density goes down at solar minimum
• Right now (Oct 2008) lowest in 50 years

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Space Weather (2) Effects
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Other Planets
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Collisionless shocks

• Bow shocks form in front of magnetospheres,
  despite there are almost no collissions
• Before hitting the shock, the solar wind flow is
  undisturbed and supersonic
• Very common phenomenon in the universe! Earths
  bow shock is the best studied.

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The Heliosphere

• The solar wind slows down at a termination shock
  before meeting the interstellar medium at the
  heliopause