http://europlanet-ri.eu

Some thoughts on how we ‘link together’ models

Nick AchilleosLecturer, Department of Physics

University College LondonJRA3 Workshop @ UCL: 22-24 April 2010

http://europlanet-ri.eu

From the ‘UCL point of view’ Traditionally, we have hosted activities which focus on observation and modelling of

planetary atmospheres, ionospheres and auroras. Planetary environments are strongly influenced by the coupling between different

‘sub-regions’ e.g. magnetosphere-ionosphere, ionosphere-thermosphere, different layers of atmosphere.

This coupling is conveyed in many different forms. For example: - Electrical current systems flowing between magnetospheres and ionospheres - Atmospheric flows transporting energy and momentum between neighbouring

atmosperic regions, and between atmosphere and magnetosphere. - Currents flowing on the surface of a planetary magnetopause generate magnetic

fields which act to ‘compress’ and ‘shield’ planetary dipoles from the solar wind. From a technical point of view, one is often interested in investigating coupling by

modifying in some way the boundary conditions of a finite model region, in order to parametrise, in a (hopefully) simple way, this transmission of energy / momentum.

http://europlanet-ri.eu

Example 1: UCL Magnetodisc model

What is it ? A model which calculates self-consistent magnetic field structure and plasma distributions for a `disc-like’, axisymmetric, rotating magnetosphere. Used for studies of magnetospheric structure at Saturn and Jupiter (giant rapid rotators) e.g. Achilleos, Guio and Arridge (MNRAS, 2010)

http://europlanet-ri.eu

Example 1: UCL Magnetodisc model, cont’d

Boundary condition One of the important boundary conditions is the ‘shielding field’ due to magnetopause currents. This is a function of position and magnetospheric state (‘expanded’ or ‘compressed’). At the moment we use a uniform vertical field which is azimuthally-averaged formula by Alexeev et al (JGR, 2006)

Improvement would be to relax axisymmetry (non-trivial) or to at least build in a dependence of the shielding field upon altitude (distance from equatorial plane) (non-trivial but much easier).

http://europlanet-ri.eu

Example 2: GCM for Saturn (initial development by UCL, Boston Uni, Uni of Arizona)

What is it ? A hydrodynamic model which calculates solutions of Navier-Stokes type equations of horizontal momentum (winds) and heating (temperature) in the upper atmosphere (thermosphere).

3D version described by Muller-Wodarg et al (Icarus 2006)

Axisymmetric version described by Smith et al (Nature, 2007) (see figure)

http://europlanet-ri.eu

Example 1: UCL Saturn model, cont’d

Boundary condition One of the many important boundary conditions is the energy input due to auroral precipitation. Knowledge of the altitudinal dependence of heating due to this process, as well as electron flux distribution, is required (or fixed composition / ionization profile).Improvement At the moment the model uses a ‘scaled’ form of the Jovian precipitation profile by Grodent et al (JGR 2001). It would be better to have a more self-consistent input modified for the atmospheric composition and conditions at Saturn.

http://europlanet-ri.eu

Other examplesMy feeling is that these are beyond the scope of Europlanet (too advanced) but they may

indicate ‘where we are headed’ if we follow some of the more simple ideas.

Example A: A time-dependent MHD model of Jovian or Kronian plasmadisc to couple to the appropriate atmospheric GCMs, to study transient auroral response.

Example B: A time- and space-dependent model of auroral precipitation associated with the Io-Jupiter ‘current circuit’, to investigate the nature of wind systems driven by such a localised source of auroral energy input.

http://europlanet-ri.eu

What is H3+ ? A molecular ion which strongly radiates in the infrared wavelength region (3-4

micron) - transitions between ro-vibrational levels. Important element of ‘energy budget’ for ionospheres of hydrogen-rich gas giants

esp. Jupiter and Saturn. Especially auroral regions. For example:

http://europlanet-ri.eu

Aspects of H3+ emission A molecular ion which strongly radiates in the infrared wavelength region (3-4

micron) - transitions between ro-vibrational levels. Important element of ‘energy budget’ for ionospheres of hydrogen-rich gas giants

esp. Jupiter and Saturn. Especially auroral regions. For example, Jupiter’s auroral oval radiates of order 1011 - 1012 W in the UV and IR bands.

Clarke et al,Nature, 2002

http://europlanet-ri.eu

Aspects of H3+ emission Departures from LTE become important in determining H3+ emission from the

Jovian thermosphere, at altitudes above 500 km (zero-point=1 B level) (Melin et al, Icarus, 2005)

Timescales for radiative de-excitation become competitive with collisional de-excitation. By ~2000 km altitude, typical excited vibrn levels have populations <10% of value predicted by LTE !

Example of calculation from Melin et al 2005

11JRA3 Workshop @ UCL: 22-24/04/2010

‘H3PCool’ Introduction to application (source code by S. Miller, deployment over web by D. Witherick) - ‘Home page’:

‘H3PCool’ Introductory text and references, courtesy of Dugan

‘H3PCool’ Input fields for H2 density grid and Temperature grid, email address

‘H3PCool’ Information about requested run received upon ‘click’ for submission

‘H3PCool’ Email link takes user to several output files. The most basic shows the columns of density, temperature and emission per H3+ molecule in W/str. Can download as ASCII or choose to have the output files sent in an email.Important for accurate interpretation of infrared auroral emissions, in the context of diagnosing auroral/ionospheric conditions.Important input for radiative transfer models and GCMs.