Plasma and Fields of Earth

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Plasma and Fields ofEarths Inner Magnetosphere

• Pontus C. Brandt
• The Johns Hopkins University
• Applied Physics Laboratory
• Laurel, MD

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outline

1. BASIC PHYSICS OF THE INNER MAGNETOSPHERE
2. GLOBAL MEASUREMENT TECHNIQUES
3. THE RING CURRENT
4. ELECTRIC FIELDS
5. THE RADIATION BELTS
6. SATURN

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10-300 keV (H, O, He)
gt500 keV (e, H)
eV (e, H, O, He)
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magnetospheric circulation
Ionospheric outflow land in the plasmasheet where
they reach lt 1keV
Solar wind plasma enters through reconnected
field lines
Reconnection connects the solar wind with the
magnetosphere
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transport
electric drift ExB drift
The higher the energy, the stronger magnetic
drift plasmasphere E ring current E and
B radiation belts - B
corotational E-field
cross-tail E-field
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energizationadiabatic

• Two ways of thinking about adiabatic energization
• flux tubes filled with plasma convect inward
  volume decrease ? pressure and temperature
  increases
• Conservation of 1st and 2nd adiabatic invariants

total particle path length constant
convection
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energizationnon-adiabatic

• Rapid magnetic reconfiguration
• e.g. substorm dipolarizations
• 2nd then 1st adiabatic invariants are broken
• more effective for particles longer gyro periods
• Important for ring current energization

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global measurement techniques

• ionospheric flows radars (SuperDARN)
• ionospheric electron density GPS
• plasmasphere EUV imaging
• ring current ENA imaging
• radiation belts N/A

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ENA imaging
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charge exchange
ENERGETIC NEUTRAL ATOM (ENA)
ENERGETIC ION
A magnetically trapped ion captures an electron
from a neutral hydrogen atom...
creating an energetic neutral atom (ENA) that is
no longer trapped.
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ENA imaging of the ring current
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High-Energy Neutral Atom (HENA) Imager
Lead Investigator D. Mitchell, Applied Physics
Lab
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ENA inversion

• given observable data points
• Xm f(Yn)
• solve for the Yns
• ill-conditioned problem (severely under- or over
  determined system)
• find correct constraints (smoothness) to give you
  an optimum solution
• How do you know if you have the correct solution?
• Validate against in-situ measurements
• Use simulation schemes to optimize the inversion

ENA PRODUCTION
LINEARLIZED EQUATION
CONSTRAINED SOLUTION
Twomey, S. (1977), Introduction to the
Mathematics in Remote Sensing and Indirect
Measurements, Elsevier Scientific Publishing
Company. DeMajistre et al., JGR, 2004.
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validation
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PERPENDICULAR PITCH ANGLES
PARALLEL PITCH ANGLES
INVERSION VALIDATION Ion distribution and
pitch-angle components of the inversions improve
once multiple vantage points are used. In this
example a stable configuration of the main phase
ring current allowed the use of several
IMAGE/HENA images obtained from different vantage
points in the orbit to increase the accuracy of
the inversion. White lines are the
Cluster/CIS/CODIF in-situ measurements of the
perpendicular (left) and parallel (right)
pitch-angle components. Yellow and red lines are
the corresponding intensities obtained by
simulating what Cluster would have seen if it
flew through the inversion.
Brandt et al., in preparation, 2007.
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the ring current

• History A depression of the geomagnetic field
  caused by a ring of current encircling the
  Earth
• Dst index defines storm main and recovery phases
• the storm-time ring current is not a ring

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the ring currentdynamics
The ring current is highly responsive to changes
in solar-wind conditions and can completely
change morphology in a couple of hours. The
sequence of HENA images captures the transition
from the storm main phase (highly asymmetric) to
the storm recovery phase (symmetric).
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what is the ring current?

• It is not ions drifting west and electrons
  drifting east
• The ring current is pressure driven
• Consider force balance (no E-fields or inertia
  for now) and solve for Jperp
• For an isotropic pressure the perpendicular
  current is the magnetization current from
  pressure gradients

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this is the ring current

• It is not ions drifting west and electrons
  drifting east
• The ring current is pressure driven
• Consider force balance (no E-fields or inertia
  for now) and solve for Jperp
• For an isotropic pressure the perpendicular
  current is the magnetization current from
  pressure gradients

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the ring currentorigin

• IMF turns southward and convection enhances
• solar wind H enters
• ionospheric H and O to plasmasheet
• adiabatic energization increases plasma pressure
  - the ring current
• substorm dipolarization energizes O additionally

Hamilton et al., 1998.
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coupled phenomena
electric fields
radiation belts
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electric fields
Brandt et al., GRL, 2002.
ROLE OF IONOSPHERIC CONDUCTANCE Current closure
through conductance gradients formed across
terminators, in the auroral region, and in the
trough region are responsible for skewing the
electric field in the magnetosphere Wolf, 1970
Fok et al., 2001. The skewing results in a main
phase ring current centered at midnight and even
in the post-midnight sector Brandt et al., 2002
Ebihara et al., 2002.
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electric fields

• The ring current closes through the ionosphere
• Electric fields are generated in the ionosphere
  and the magnetosphere
• Plasma transport is affected in the ionosphere
  (SAPS) and the magnetosphere (plasmasphere)

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electric fieldsionosphere
Brandt et al., AGU Monogr., 2008.
The onset of the ring current correlates very
well with the onset of the SAPS flow. (a)
SuperDARN Wallops observations of the onset of a
SAPS flow channel. (b) IMAGE/HENA observations in
the 60-119 keV range.
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electric fieldsmagnetosphere

• skews ring current
• defines the plasmapause

Brandt et al., GRL, 2002.
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electric fieldsmagnetosphere

• skews ring current
• defines the plasmapause

Brandt et al., AGU Monogr., 2005 Goldstein et
al., JGR, 2005
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the radiation belts

• Why such a problem?
• Multiple loss and energization mechanisms compete
  simultaneously and their effectiveness depend on
  global propertieswe dont have those really
• Losses
• wave-particle interactions (EMIC waves)
• magnetopause loss
• Energization
• wave-particle interactions (chorus waves)
• ULF waves

Chen et al., Nature, 2007.
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the radiation beltglobal losses
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the radiation beltsglobal losses
Ukhorskiy et al., 2006.
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saturn
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DUNGEY X-LINE?
VASYLIUNAS X-LINE? Centrifugally forced
reconnetion
SATURN
EARTH
MASS LOADING?
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conclusions

• The inner magnetosphere is an intrinsically
  coupled system of plasma, fields, and currents
• The ring current carries most of the plasma
  pressure of the inner magnetosphere
• The ring current is pressure-driven and connects
  through the ionosphere, which modifies the global
  electric field
• The dynamics of the radiation belts presents a
  challenge to modern space physics, since multiple
  loss and energization mechanisms compete
  simultaneously on all scales
• Saturns magnetosphere is more corotationally
  driven than Earths, but still display similar
  phenomena such as a ring current, injections,
  there are many, many mysterious phenomena that
  we have no clue how to explain.

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extra slides
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TITAN - ENA
Titan is immersed in Saturns magnetosphere and
the magnetospheric corotating plasma clouds
interact with Titans upper atmosphere. ENA
observations by Cassini/INCA show dynamic and
asymmetric ENA emissions around Titan consistent
with finite-gyro radius effects and an
unexpectedly expanded hot corona of neutral gas.
In-situ neutral gas measurements are not
sufficiently sensitive to detect the relatively
low density of the hot neutral corona at high
altitudes. Therefore, ENA measurements provide
perhaps the only way to determine the existence,
distribution, dynamics of the hot neutral corona
of Titan and its interaction with the ambient
magnetospheric plasma of Saturn Mitchell et al.,
Science, 2005 Brandt et al., EGU, 2005 Garnier
et al., GRL, in press, 2006.
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ENA PRODUCTION
LINEARLIZED EQUATION
CONSTRAINED SOLUTION
DeMajistre et al., 2004
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2. Optimizing the inversion Detailed comparison
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2. Optimizing the inversion
(A)
(B)
(C)
(D)
Relative error contours (gray) provide a
tell-tale of what we can invert and what we
cannot invert
The constrained linear inversion technique
(Twomey, 1970) is optimized by (A) cooking up a
realistic ring current (B) then simulate what it
would like like through the imager (response
functions, noise, vantage point, etc.) (C) now
invert with particular constraints (D) simulate
the newly retrieved ring current. Compare (A)
with (C), and (B) with (D). Tweak constraints to
minimize the differences.
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the ring current
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the ring current
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the ring currentsubstorm effects

• Substorm dipolarizations observed by GOES
• H-ENA intensities slightly increasing
• O-ENA intensities dramatically increasing
• substorms pump up the oxygen ring current

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OBSERVED
Dt 21 min
Dt 35 min
SIMULATED ENA
Dt 52 min
Dt 60 min
SIMULATED O
Fok et al., manuscript in preparation, 2006.
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electric fieldsionosphere