Auroral electron fluxes and
characteristic energy of
precipitating electrons inferred by
ALIS and EISCAT
H. Lamy1, C. Simon Wedlund1, B. Gustavsson3, J. De
Keyser1, T. Sergienko2, U. Brändström2
1
Belgian Institute for Space Aeronomy, Brussels, Belgium
2
Swedish Institute of Space Physics, Kiruna, Sweden
3
EISCAT Scientific Association, Kiruna, Sweden
OUTLINE OF THE TALK
• Presentation of the whole project
• ALIS : Auroral Large Imaging System
• Observation campaign of March 2008 : coordinated
observations of ALIS and EISCAT
• Inversion of EISCAT data electron fluxes in 1D
• Auroral tomography with ALIS electron fluxes in 2D
• Conclusions & perspectives
The whole project
1) Ground-based observations (ALIS, EISCAT) spectra of
precipitating electrons and characteristic energies
2 parts
2) Spectra of precipitating electrons magnetosphereionosphere coupling (with models and s/c data during
conjunctions)
This talk : only about first part of the project
ALIS : the Auroral Large Imaging System
Skibotn : B
BASKOT
2 stations
K et 0
Gustavsson (1998)
ALIS : the Auroral Large Imaging System
• High-resolution CCD cameras (1024 x 1024)
• Average FOV ~70°
• Narrow interference filters (∆λ=40 Å) for imaging of the
green line (5577 Å), the red line (6300 Å), the blue line of
N2+ 1NG band (4278 Å) and a near-IR line (8446 Å)
• One image obtained every 5 sec (including overhead time
due to filter wheel change)
Brandström (2001)
ALIS : the Auroral Large Imaging System
FOVs of the ALIS
cameras overlap at
100 km altitude
allowing to do
auroral tomography
Gustvasson (1998)
2008 Observation campaign
• Coordinated ALIS / EISCAT observations
• Tri-static UHF EISCAT observations obtained during TNA
(Trans National Access) time
• 24 hours of observations between 03/03/08 and 11/03/08
• Nice weather and several interesting auroral phenomena
• We looked first for the most stable auroral arc night of
05/03/08 around 18h40 UT.
2008 observation campaign : EISCAT data
Bright and stable auroral arc
Beata code : alt resolution ~
1.5-3km ; time resolution ~
10 sec
2008 observation campaign : EISCAT data
2008 observation campaign : ALIS data
2008 observation campaign : ALIS data
Bright stable arc nicely visible
from 4 stations simultaneously
Inversion of EISCAT data
• Following the method used by Semeter & Kamalabadi
(2005)
• We calculate the ion production rate q from the electron
density data n given by EISCAT, assuming stationnarity and
quasi-neutrality
• We assume an E region dominated by 02+ and use the
recombination coefficient α from Walls & Dunn (1974)
Ti = Te = Tn below 160 km in the absence of anomalous
local heating ⇒ we use EISCAT data for Tn
Inversion of EISCAT data : forward model
• The link between the volume production rate of ions, q, at
an altitude z and the flux Φ of electrons with energy E is
given by
E
E
Φ
• Λ = energy dissipation function (Sergienko & Ivanov 1993)
• ρ = atmospheric mass density model (MSISE-00)
• R = electron range in air (Sergienko & Ivanov 1993)
• 35.5 eV ~ average energy lost per electron-ion pair
produced [latest version with W(E)]
Inversion of EISCAT data : forward model
Φ=108 cm-2 s-1 eV-1
Ionisation rate profiles for monoenergetic electrons
Inversion of EISCAT data : forward model
• If we discretize the spectrum of the precipitating electrons
and if we use a finite number of altitudes, we obtain a
linear matrix system
φj = number of electrons with
energies between Ej and Ej + ∆Ej
We solve this linear (non-squared) system with MEM
Results of inversion of EISCAT data
∆ : ionisation rate deduced from
EISCAT data
projection of q from
the reconstructed
electron fluxes
Results of electron fluxes
obtained with MEM inversion
Results of inversion with EISCAT data
Results of inversion with EISCAT data
Energy spectra of precipitating electrons vs time
General principle of auroral tomography
Gustvasson (1998)
• Intensity in each pixel at each station is a line integral of the
3-D auroral emission distribution.
• Discretisation in cubic-like volume elements (‘blobs’) =>
huge linear system (transfer matrix has typically 1011
elements for a spatial resolution ~ 2.5 km)
• Iterative solution : ART/MART/SIRT/…
2008 Observation campaign : ALIS data
BAST stations
Time resolution : between 10 and 20 sec
Tomographic inversion of ALIS data : initial guess
• Assumed uniform in lat/long
• Various profiles in altitude have been tested : in particular, a
Chapman profile is not sharp enough at low altitudes.
• Important for convergence of the tomographic reconstruction
Tomographic inversion of ALIS data : initial guess
Tomographic inversion of ALIS data
Vertical
cut of the
3D volume
emission
rate in
the E-W
direction
Horizontal cut
at 125 km
altitude of the
3-D blue
volume
emission rate
Vertical cut of the 3D volume emission rate
in the N-S direction
Inversion of the blue line volume emission rates
• We invert ηb, the Volume Emission Rate (VER) of the blue
line to retrieve the fluxes of precipitating electrons
cm-3 s-1
• ε : energy deposition rate
• φ : electron precipitation fluxes
• ζ= 0.256 : 1 KeV of deposited energy gives 0.256 blue
photons on average
Results from the blue line inversion
• Example for an altitude profile of VER approximately above
Tromsø
• This can be done for every lat/long cell to obtain energy
spectra of precipitating electrons in 2D.
TRANS4: a kinetic/fluid transport model
TRANS4 ionosphere model (Simon et al., 2007): 1-D Boltzmann
equation for superthermal electrons and energetic protons
Inputs
- Neutral atmosphere MSIS-90
- Electron precipitation spectra
- Emission cross sections and
reaction rates
Outputs:
- excitation, ionisation and
heating rates
- calculation of ne, ni and Te
→ EISCAT
- Emission model: red, green,
blue lines and UV emissions
altitude profiles
→ ALIS
Lummerzheim & Lilensten, 1994
Lilensten & Blelly (2002)
Simon et al. (2007)
TRANS4
• Use electron precipitation fluxes inferred from inversion of
ALIS data above Tromsø as input for TRANS4 and try to
retrieve the profile of electronic densities measured by
EISCAT
• Use electron precipitation fluxes inferred from inversion of
EISCAT data as input for TRANS4 and try to retrieve the
altitude profile of blue emission line (observed at Skibotn)
Conclusions
• Both methods (inversion from ALIS and EISCAT data) do
work pretty well up to ~ 180 km altitude
• Characteristic energy and peak of altitude deposition
profile can be accurately retrieved.
• We started preliminary comparisons between energy
spectra of precipitating electrons retrieved from EISCAT
and ALIS data. Work in progress.
• Use of the electron fluxes deduced from ALIS data as
input to TRANS4 to simulate the density profiles
observed by EISCAT and auroral emissions observed by
ALIS. Work in progress.
Perspectives
• 2D inversion of the blue line VER to obtain the distribution
of electron’s characteristic energy along the arc
• Use of the quasi-static magnetosphere-ionosphere coupling
model based on the current continuity equation (Echim et al
2007) and of the model of TD generators (Roth et al 1993)
to determine ne and Te of magnetospheric electrons as well
as the altitude zm of the generator.
• Coordinated ALIS/EISCAT observations conjugated with S/C
data below or above the acceleration region.