Slot Region Electron Loss Timescales due to

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Slot Region Electron Loss Timescales due to
Plasmaspheric Hiss and Lightning Generated
Whistlers
Nigel P. Meredith, Richard B. Horne, and Sarah
A. Glauert
British Antarctic Survey, Cambridge, UK
REPW, Rarotonga, 9th August, 2007
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Earths Radiation Belts

• Energetic electrons (E gt 100 keV) in the Earths
  radiation belts are generally confined to two
  distinct regions.
• Inner radiation belt
• 1.2 lt L lt 2
• exhibits long term stability
• Outer radiation belt
• 3 lt L lt 7
• highly dynamic

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Outer belt
Inner belt
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The Slot Region

• The slot region (2 lt L lt3)
• lies between the two belts
• region of depleted fluxes of energetic electrons
• largely formed by the resonant pitch angle
  scattering of energetic electrons by
  plasmaspheric whistler mode waves.

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Slot region
Outer belt
Inner belt
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Importance

• Enhanced fluxes of relativistic electrons
• damage satellites
• risk to humans in space.
• Relativistic electrons can penetrate to low
  altitudes where they effect
• ionisation
• conductivity
• chemistry

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Plasmaspheric Emissions

• Broadband plasmaspheric emissions can be split
  into two categories Meredith et al., 2006a
• Plamaspheric hiss
• 100 Hz lt f lt 2 kHz
• generated by wave turbulence in space
• MR whistlers
• 2 kHz lt f lt 5 kHz
• produced by thunderstorms on Earth

f gt 2 kHz
f lt 2 kHz
f gt 2 kHz
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PADIE(Pitch Angle and energy Diffusion of Ions
and Electrons)

• We assess the importance of hiss and MR whistlers
  in the slot region and beyond using the PADIE
  code Glauert and Horne, JGR, 2005.
• Models the effects of electromagnetic waves on
  charged particles trapped in a magnetic field.
• Calculates the bounce-averaged, relativistic,
  diffusion coefficients
• D??, D?E , DEE

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PADIE(Pitch Angle and energy Diffusion of Ions
and Electrons)

• The determination of the loss timescales requires
  knowledge of
• distribution of the wave normal angles
• distribution of wave spectral intensity
• ratio of fpe/fce at the magnetic equator
• We assume an electron/proton plasma and take into
  account Landau resonance and /-5 cyclotron
  harmonic resonances.

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Distribution of Wave Normal Angles
Wave Normal Distribution

• PADIE code assumes a Gaussian distribution of
  wave normal angles.
• Plasmaspheric hiss propagates over a broad range
  of wave normal angles Parrot and Lefeuvre, JGR,
  1986.
• MR whistlers that undergo many magnetospheric
  reflections are highly oblique Thorne and Horne,
  JGR,1984.

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40
60
80
0
?(o)
Small wave normal model (?m 0o)
Medium wave normal model (?m 52o)
Large wave normal model (?m 80o)
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Global Models

• Energetic electrons in the Earths outer
  radiation belt drift around the Earth on a
  timescale of the order of hours or less.
• Energetic electron decay takes place on a
  timescale of days.
• Global models of the wave intensities and the
  ratio fpe/fce are required to estimate the loss
  timescales.

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CRRES Database

• We use 15 months of wave data from the CRRES
  satellite.
• The data are averged over
• Magnetic latitude
• Magnetic local time
• to obtain global models of the wave spectral
  intensity as function of L and magnetic activity.
• Two activity levels
• Quiet (AE lt 100 nT)
• Active (AE gt 500 nT)

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Identification of Hiss and MR Whistlers

• Other wave modes, which can occur in the
  frequency range 0.1 lt f lt 5 kHz, are removed from
  the database.
• Whistler mode chorus is excluded by using a
  criterion based on the amplitude of ECH waves
  Meredith et al., 2004.
• Fast magnetosonic waves are removed by excluding
  emissions observed within 5o of the equator.

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Fitting Procedure

• Black trace represents the globally-averaged
  spectral intensity at L 3.0 during active
  conditions.
• The PADIE code assumes the waves have a Gaussian
  distribution.

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Fitting Procedure

• Black trace represents the globally-averaged
  spectral intensity at L 3.0 during active
  conditions.
• The PADIE code assumes the waves have a Gaussian
  distribution.
• Three Gaussian distributions are required to
  obtain a good fit over the entire frequency range.

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Wave Spectral Profiles

• Wave spectral intensity maximises at low
  frequencies and decreases with increasing
  frequency.
• Bulk of the wave power is associated with
  plasmaspheric hiss.

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Wave Intensities
Quiet Conditions (AE lt 100 nT)

• Wave magnetic field intensities are determined
  assuming parallel propagation.
• We calculate approximate intensities for
  propagation at 52o and 80o using the cold plasma
  dispersion solver in the HOTRAY code.
• Hiss is typically one or two orders of magnitude
  more intense than MR whistlers.
• For Hiss intensities maximise for parallel
  propagation.

Wave Intensity (pT2)
L (RE)
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Wave Intensities
Active Conditions (AE gt 500 nT)

• Wave intensities are larger during active
  conditions.
• Plasmaspheric hiss is about a factor of 3 more
  intense during active conditions.
• Lightning generated whistlers are a factor of 2
  more intense at low L but up to a factor of 10
  more intense at large L.

Wave Intensity (pT2)
L (RE)
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Ratio of fpe/fce at Magnetic Equator

• We calculate the local ratio of fpe/fce from
  CRRES observations.
• We infer the ratio at the magnetic equator
  assuming fixed density and a dipole field.
• We average over MLT and MLAT to obtain a global
  model as a function of L and magnetic activity.
• The ratio fpe/fce increases with increasing L.
• At a given L fpe/fce is 15 smaller during
  active conditions.

fpe/fce versus L
fpe/fce
L (RE)
Quiet Conditions (AE lt 100 nT)
Quiet Conditions (AE lt 100 nT)
Active Conditions (AE gt 500 nT)
Active Conditions (AE gt 500 nT)
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Estimation of Loss Timescales

• We assume that the electron distribution
  function, f, satisfies the pitch angle diffusion
  equation

• We also assume the electron distribution
  function, f, can be factorised into
  time-dependent and pitch-angle dependent
  functions

• The precipitation lifetime, ?, is given by

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Slot Region Loss Timescales

• Slot region can become filled during
  exceptionally large storms such as the Halloween
  Storms of 2003.
• Slot region subsequently reforms.

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Slot Region Loss Timescales
SAMPEX 2-6 MeV Electrons

• Slot region can become filled during
  exceptionally large storms such as the Halloween
  Storms of 2003.
• Slot region subsequently reforms.
• Loss timescales for 2-6 MeV electrons at L 2.5
  estimated to be of the order of 2.9 4.6 days.
• The dominant loss process must be able to explain
  this decay.

slot
Baker et al., Nature, 2004
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Slot Region Loss Timescales
MR Whistlers

• Loss timescales due to MR whistlers are
  prohibitively long.

Quiet Conditions (AE lt 100 nT)
Active Conditions (AE gt 500 nT)
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Slot Region Loss Timescales
Hiss (?m 80o )

• Loss timescales due to hiss propagating at large
  wave normal angles are also prohibitively long.

Quiet Conditions (AE lt 100 nT)
Quiet Conditions (AE lt 100 nT)
Active Conditions (AE gt 500 nT)
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Slot Region Loss Timescales
Hiss (?m 52o)

• Hiss propagating at medium wave normal angles can
  lead to loss timescales of the order of 10 days
  during active conditions.

Quiet Conditions (AE lt 100 nT)
Quiet Conditions (AE lt 100 nT)
Active Conditions (AE gt 500 nT)
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Slot Region Loss Timescales
Hiss (?m 0o)

• Hiss propagating at small wave normal angles can
  lead to loss timescales of the order of 1 10
  days depending on magnetic activity.
• Hiss propagating at small or medium wave normal
  angles is responsible for the formation of the
  slot region.

Quiet Conditions (AE lt 100 nT)
Quiet Conditions (AE lt 100 nT)
Active Conditions (AE gt 500 nT)
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Loss Timescales for 2.0 lt L lt 4.0

• MR whistlers and plasmaspheric hiss propagating
  at large wave normal angles do not contribute
  significantly to radiation belt loss.

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Loss Timescales for 2.0 lt L lt 4.0

• During quiet conditions plasmaspheric hiss
  propagating at small and medium wave normal
  angles can lead to loss timescales of 1 10 days
  in the region 3.0 lt L lt 4.0.

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Loss Timescales for 2.0 lt L lt 4.0

• At 1 MeV loss timescales in the region 3.0 lt L lt
  4.0 lie in the range 3 10 days consistent with
  quiet time decay observed by the CRRES MEA
  instrument Meredith et al., JGR, 2006a.

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Loss Timescales for 2.0 lt L lt 4.0

• During active conditions loss timescales can be
  of the order of 1 day or less in the region 3.5 lt
  L lt 4.0.

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Loss Timescales for 2.0 lt L lt 4.0

• Plasmaspheric hiss could thus play an important
  role in the loss of energetic electrons in the
  inner region of the outer radiation belt during
  magnetically disturbed periods.

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Conclusions

• Plasmaspheric hiss propagating at small and
  intermediate wave normal angles is responsible
  for the generation of the slot region.
• Plasmaspheric hiss propagating at large wave
  normal angles and lightning generated whistlers
  do not contribute significantly to radiation belt
  loss.

NASA
Slot region
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Conclusions

• During quiet conditions losses due to
  plasmaspheric hiss occur on a timescale of
  several days or more and can explain the quiet
  time decay of radiation belt fluxes.
• During active conditions losses due to
  plasmaspheric hiss may occur on a timescale of 1
  day or less and may play an important role during
  geomagnetic storms.

NASA
Slot region
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Figure 1
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Figure 7
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Figure 8