Modeling of the Atmosphere Magnetosphere Ionosphere System (MAMI)

The aurora model: AURORA

At the center of the auroral ionospheric model AURORA is an electron transport calculation. Given a spectrum and angular distribution of incident auroral electrons, this model solves a one dimensional transport equation. Electrons spiral along a field line, penetrating the atmosphere. Collisions with ambient neutral atmospheric constituents give rise to angular scattering, energy loss, and production of secondary electrons from ionization collisions. The ambient thermal electrons are heated through Coulomb collisions, neutral atoms and molecules are excited and ionized. Some of the auroral emissions are due to these directly excited atoms and molecules, others result after a chain of chemical reactions. The penetration and energy loss of the auroral electrons is very fast, and can be treated as a steady state process in the ionospheric chemistry scheme.

Solar EUV radiation also causes ionization and produces secondary electrons. These photoelectrons range in energy from thermal (i.e. a fraction of one electron Volt, eV) to more than a keV. The electron transport calculation is also applied to these photoelectrons.

Once the precipitating particles and the solar EUV radiation and photoelectrons have heated, ionized, and excited the ambient gas, this deposited energy is released through a chain of chemical reactions, radiation, and thermal conduction. Ionized atomic oxygen at altitudes above about 200 km moves up the geomagnetic field due to ambipolar diffusion. Heated ambient electrons lose their energy by conduction along the magnetic field, excitation of rotational, vibrational, and electronic states of neutrals, and by Coulomb collisions with ions. Our model combines all this with a chemical scheme involving over 50 reactions. As a result we calculate the time dependent altitude profiles of various ion species, electron density, and electron and ion temperatures. The model determines the brightness of a number of auroral emission lines and bands as well as ionospheric parameters like conductances, plasma densities, and temperatures.

This auroral model is used to study individual auroral events and determine the response of the ionosphere to the aurora. We use it to derive and test parameterizations that are suitable for large scale and global thermospheric models. Due to practical limitation in computing power, such global models cannot afford to run a detailed auroral model such as this at every grid point and time step. However, parameterizations derived from this auroral model can help large scale and global models such as AMIE and TIEGCM to ingest data from auroral observations.

One application of the AURORA model is to infer ionospheric conductances from optical observations made with the auroral imagers on the Polar satellite (launched 24 Feb 1996.) A similar analysis is possible with Dynamics Explorer (DE-1) spectral images, albeit with much lower resolution in time and space compared to the expected images from the Polar instruments UVI, VIS, and PIXIE. Once the auroral images are analysed in terms of ionospheric parameters, they can be assimilated with other data sets by AMIE and included in global ionospheric and magnetospheric simulations.

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Last updated: 3 September 96
Questions? Comments? Please send E-mail to lumm@gi.alaska.edu