The effects of changing solar EUV flux upon the location and structure of the dayside highlatitude t

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The effects of changing solar EUV flux upon the
location and structure of the dayside
high-latitude trough in winter Modelling
results and experimental validation M.H.
DENTON1, S.E. PRYSE1, R.W. SIMS1 AND R.L.
BALTHAZOR2.
1. Radio and Space Physics Group, University of
Wales, Aberystwyth, UK 2. Space and Atmosphere
Research Group, University of Sheffield, UK
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The dayside high-latitude trough in winter -
formation and structure

• A depletion in ionospheric electron density that
  typically occurs between regions of
• Solar EUV produced ionisation equatorwards of
  the trough minimum
• Particle precipitation produced ionisation
  polewards of the trough minimum

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Increased Solar EUV Radiation

• Thermal Expansion
• Altered Chemical Composition
• Altered Chemical Reaction Rates

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Thermal expansion driven by increased solar EUV
flux
In theory....

• Ionospheric regions are elevated in altitude
• Features within the ionosphere such as troughs
  are elevated in altitude
• Plasmasphere expands driving the plasmapause
  boundary polewards

Taken from Rees, 1989
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Altered Chemical Composition

• Solar EUV flux directly controls the density of
  neutral and ionised components of the atmosphere
  at a set altitude

MSIS - Day 355 - F10.775 - 300 km
MSIS - Day 355 - F10.7225 - 300 km
O
O
N2
HE
HE
N2
N
O2
N
O2
H
H
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Altered Electron, Ion and Neutral Temperatures
...the precise variation of electron and ion
temperatures with solar EUV flux remains
unknown! (Brace et al., 1968 Evans, 1973
Mahajan and Pandey, 1979 Denton et al., 1999)

• Typically expect Te and Ti to increase with
  increasing solar EUV flux
• Conflicting reports have shown that the plasma
  temperature can increase or decrease as solar EUV
  flux increases
• This effect is primarily dependent upon the
  charged and neutral compositions

Taken from Denton et al., 1999
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Altered Chemical Reaction Rates
Chemical loss of O
O N2 ? NO N
O O2 ? O2 O

• Chemical production and loss
• Collision frequencies
• Thermal conductivities
• Thermal diffusion coefficients

...are all temperature dependent
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The CTIP Model - A global, fully-coupled,
time-dependent model of the Earth's thermosphere,
ionosphere and plasmasphere (Developed jointly by
University of Sheffield, University College
London and SEL Boulder)

• Inputs EUV94 solar flux, electric field
  (Foster et al., 1986), particle precipitation
  (Hardy et al., 1985) etc.
• Outputs Electron, ion and neutral
  temperatures, densities and velocities in the
  vertical and horizontal directions.
• Resolution 2º latitude
• 18º longitude
• 1 scale height in altitude

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Experimental Radio Tomography
A chain of receivers in the high-Arctic receive
NIMS satellite signals on 150 and 400 MHz

• Total Electron Content (TEC) is measured along
  numerous ray paths between the satellite and
  receivers.
• This dataset is inverted in a reconstruction
  algorithm to produce a spatial image of electron
  density in two dimensions.

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Modelling Calculations and Experimental
Verification

• The present study investigates the effects of
  solar EUV flux for three activity levels on 21st
  December (Day 355)
• Changes in the neutral and ion compositions
  within the CTIP model occur in the vicinity of
  the trough as solar EUV flux is increased
• The location of the trough is modelled with the
  only change between runs being a change to the
  solar EUV input to the model (look at the effect
  of changing EUV flux in isolation from other
  geophysical changes)
• Tomographic data for similar conditions are used
  to help confirm the current model results

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Low Solar Activity - F10.775
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Low Solar Activity - F10.787
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Moderate Solar Activity - F10.7150
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Moderate Solar Activity - F10.7149
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High Solar Activity - F10.7225
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High Solar Activity - F10.7200
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Conclusions

• An increase in solar EUV flux drives the dayside
  high-latitude trough polewards. For an increase
  in F10.7 from 75 to 225, the trough minimum is
  shifted 3-4 polewards. This indicates that the
  additional solar produced ionisation is filling
  in the trough from the equatorwards edge.
• The only apparent change to modelled trough
  structure is a sharp increase in the density
  gradient on the equatorwards wall of the trough,
  again caused by the additional solar EUV
• Composition changes and related effects of
  increasing solar EUV flux appear to be of
  secondary importance to the structure of the
  dayside high-latitude trough.