FG2. GGCM Modules and Methods

Co-chairs: John Dorelli (john<dot>dorelli<at>unh<dot>edu) and Michael Shay (shay<at>udel<dot>edu)

Goals

The overarching goal of this focus group is to understand the physics of collisionless magnetic reconnection on magnetospheric length scales (100-1000 ion inertial lengths). To this end, we have identified several broad questions (and a number of specific sub-questions) to be addressed during the lifetime of the focus group:

• Q1: Can global resistive magnetohydrodynamics (MHD) codes accurately model magnetospheric reconnection?
  • Q1.1: What is the effective Lundquist number of the magnetosphere? (What is the role of anomalous resistivity? Can anomalous resistivity be accurately modeled in resistive MHD codes? What are the roles of the post-MHD terms in the Generalized Ohm's Law?)
  • Q1.2: How does the physics of reconnection depend on the ad hoc resistivity model used in global MHD codes? (How does reconnection scale with resistivity in the high Lundquist number limit? What is the effect of numerical resistivity? Can we reproduce Petschek reconnection by localizing the plasma resistivity? What is the effect of current dependent resistivity?)
  • Q1.3: How does dayside magnetopause reconnection work in global MHD codes? (Is reconnection locally controlled or externally driven? Does the Cassak-Shay formula apply to the dayside magnetopause? What can resistive MHD tell us about the generation and topology of Flux Transfer Events (FTEs)?)
  • Q1.4: How does magnetotail reconnection work in global MHD codes? (Can global resistive MHD codes accurately model magnetic storms and substorms? How do simulated storms and substorms depend on the resistivity models used in resistive MHD codes?)
• Q2: How does the physics of collisionless reconnection observed in Particle-In-Cell (PIC) simulations scale up to reality?
  • Q2.1: How does the reconnection rate scale with the electron inertial length? (Does the Hall effect render the collisionless reconnection rate independent of electron mass? Is the collisionless reconnection rate universally Alfvenic?)
  • Q2.2: How does the reconnection rate scale with the ion inertial length? (Does the Hall effect render the collisionless reconnection rate independent of the ion inertial length? What is the role of magnetic flux pileup in collisionless reconnection?)
  • Q2.3: What determines the aspect ratio of the electron diffusion region in open boundary condition PIC simultions? (Are macroscopic current sheets possible in collisionless reconnection? What determines the length of the electron diffusion region in collisionless reconnection? What is the role of secondary island formation in the determination of the length of the electron diffusion region? What impact does secondary island formation have on the reconnection rate?)
  • Q2.4: Is the Hall effect necessary to produce fast collisionless reconnection? (How does fast reconnection work in electron-positron plasmas? Is fast reconnection possible in so-called "Hall-less" hybrid codes?)
  • Q2.5: What is the role of dispersive waves in the physics of fast collisionless reconnection?
• Q3: Can we extend global resistive MHD to include microscale physics which is needed to accurately model reconnection?
  • Q3.1: What is the status of global Hall MHD modeling? (What are the most robust numerical approaches? Should we go fully implicit? Semi-implicit? What about Godunov approaches? How do we handle Adaptive Mesh Refinement (AMR)?)
  • Q3.2: What is the status of global hybrid codes? (What is the role of the Hall effect in a global 3D context? How does the reconnection rate in global hybrid codes depend on the resistivity model?)
  • Q3.3: What is the status of "embedding" approaches, in which kinetic physics is added locally to an MHD code (either via code coupling or via local modification of the equations)? (What are the most important code coupling issues? Is it even possible to couple an MHD code with a PIC code? Is the region of MHD breakdown in a global MHD code sufficiently localized to make embedding computationally feasible?)

The three questions Q1-Q3 are motivated by a currently popular approach to GGCM development known as the MHD spine approach. In the MHD spine approach, a global MHD model is used as the underlying computational "spine" of the GGCM, with non-MHD physics added (e.g., via coupling with another code) in regions of the simulation domain where the MHD approximation breaks down. While this approach seems to be yielding improvements in modeling of the inner magnetosphere (e.g., several kinetic models of the ring current are being successfully coupled to global MHD codes), the important problem of collisionless reconnection -- likely the ultimate driver of magnetospheric activity -- has received little attention in the context of GGCM development.

2008 Summer Workshop

Wednesday, June 25, 1:30-3:00

• Michael Hesse -- Michael Hesse's talk addressed question Q2.3: What determines the aspect ratio of the electron diffusion region in open BC PIC simulations? In previous studies, the electron diffusion region was identified as the region where the electron frozen flux condition is violated. That is, the electron diffusion region was identified as the region where there are significant corrections to the UxB and Hall electric fields. Such an identification seems to imply that the aspect of the electron diffusion region is larger than that found in earlier PIC simulations (which used periodic boundary conditions). Hesse pointed out, however, that particles are actually losing energy (with electrons simply drifting diamagnetically) to the electromagnetic fields throughout most of this large diffusion region. If one defines the electron diffusion region to be that region where particles gain energy from the fields (i.e., the dot product of current density and electric field is positive), then the electron diffusion region is much smaller.
• Kittipat Malakit -- Kittipat Malakit's addressed question Q2.4: Is the Hall effect necessary to produce fast reconnection? Malakit's work was motivated by recent so-called "Hall-less" hybrid simulations (in which the Hall term in Ohm's law is turned off), carried out by Homa Karimabadi, which seemed to demonstrate that fast reconnection was possible even in the absence of the Hall electric field. In his talk, Malakit provided a counterexample, demonstrating that in the case of reconnection of a double Harris sheet, turning off the Hall term effectively turns off fast reconnection (producing long Sweet-Parker-like current sheets).
• Mikhail Sitnov -- Mikhail Sitnov addressed the possible role of the ion tearing mode in producing secondary magnetic islands obsevered in open BC PIC simulations, thus potentially addressing questions Q2.1-Q2.5. Sitnov argued that in periodic BC PIC simulations, there are no "passing" electron orbits (i.e., electrons which leave the system, a population which is essential to the development of the ion tearing mode). Sitnov argued that open BC simulations allow for the existence of such passing orbits and, thus, the ion tearing mode may be responsible for secondary island generation in open BC PIC simulations. There was some debate among focus group participant about the relevance of ion tearing in the secondary island generation process.
• Christopher Russell
• Joachim Birn
• Paul Cassak

Wednesday, June 25, 3:30-5:00

• John Dorelli
• Masha Kuznetsova
• Amitava Bhattacharjee
• Vadim Roytershteyn
• Brian Sullivan