Tuesday, June 19, 2007

GEM Workshop, Tuesday plenary session

Today's plenary session speakers were Jimmy Raeder of UNH on "Quo vadis, GGCM" and Harlan Spence of Boston University on the radiation belts and the RBSP mission.

Raeder discussed some details of how global models work in situations involving reconnection. Ideal MHD should have a zero reconnection rate, but real-world implementations will have numerical diffusion which can produce reconnection (as well as form shocks, something else that ideal MHD should not do). Numerical diffusion can also counteract the tendency of numerical dispersion (diffusion and dispersion are due to even and odd error terms, respectively, in the truncation) to drive the code unstable. If the Hall term is included, GGCMs even manage to get close to observed reconnection rates (and if they are off, the models usually get higher rates). The implication is that reconnection rates on the dayside are set by boundary conditions, not the microphysics of the reconnection region. On the nightside, average reconnection rates (over periods of an hour or more) must equal dayside reconnection rates. Substorms are the result of short-term deviations from this balance. It is not clear whether GGCMs produce genuine substorms (there are plasmoids, but some of the other signatures are absent). Tail reconnection in the GGCM models is much more fragmented than the classic substorm models. High ionospheric conductance can suppress convection.

Spence is the lead scientist on the Radiation Belt Storm Probes mission. He reviewed the history of radiation belt paradigms and science and advertised the mission. The goal is to be able to understand, and predict, the variation of relativistic particle populations in response to energy inputs from the Sun. The satellite can make in situ measurements of high energy astrophysical phenomena.

It seems that there are capacity limits on connections from the conference center. I'm posting this from my room.

No comments: