FG: The Magnetosheath

Chairs: Steven Petrinec and Katariina Nykyri

White Paper

• Magnetosheath Focus Group White Paper: 2012-May-11 (download)

Workshop Report: Summer 2012

FG Leaders: Katariina Nykyri and Steve Petrinec

Magnetosheath FG held three sessions. Sessions had two invited speakers and eight contributed presentations allowing plenty of time for questions, discussion and planning. Research methodology of the session speakers utilized magnetosheath data from various spacecraft missions including Cluster, THEMIS, Geotail etc. The modeling efforts included global hybrid simulations, global and local MHD and Hall MHD simulations and support vector regression machine technique. Also a development of the new global scale Vlasov model with initial results was discussed. We have divided a discussion of the sessions into 1. Highlights, 2. Where We Are Headed, 3. What we Have Accomplished and 4. Concluding Remarks.

1. Highlights:

Heli Hietala showed Cluster observations of super magnetosonic magnetosheath jets and discussed their possibe generation mechanism to be related to the wavy structure of the bow shock, which could lead to formation of a secondary shock. Jets can have scale sizes 1-6 R_E and produce magnetopause perturbations, magnetospheric pulsations and short local ionospheric convection enhancements.

Nick Omidi showed observations and simulations of magnetosheath density cavities and how changes in IMF can lead to large-scale changes in magnetosheath properties: 1) Change from large cone angle to small and back to large results in cavities in time series data 2) Intrinsic cavities in quasi-parallel sheath due to shock dissipation processes 3) Cavities caused by IMF discontinuities and associated structures such as Foreshock Bubbles.

Antonius Otto used 2D MHD and Hall-MHD simulations and showed that there is a significant entropy (non-adiabatic heating) increase at the magnetopause only when magnetosheath beta is low. Entropy production in 2D reconnection occurs in i) in the diffusion region ii) sometimes through weak fast shock at the leading edge of the steady outflow region, iii) through shocks at the boundary between outflow and inflow region. However, the heating in the diffusion region is likely of small importance because of the small volume ~ ion inertia scale. Also, the nonadiabatic heating is significant in the outflow regions only for small plasma beta. Nonadiabatic heating occurs in Hall dynamics at the same rate (for the same conditions) as in MHD.

Katariina Nykyri used 15 Global MHD simulations (BATSRUS hosted at CCMC) to determine the magnetosheath properties during Parker Spiral (PS) and Ortho-Parker Spiral (OPS) IMF orientation for various solar wind plasma betas and solar wind Mach numbers. She then generated 30 local simulations at the dawn-dusk terminator at each flank corresponding to global simulations. Results showed that dawn-flank is more Kelvin-Helmholtz unstable for PS orientation which can lead into more plasma heating on dawn-side magnetopause due to reconnection and shocks generated by KHI and due to kinetic Alfven wave generation.

2. Where We Are Headed:

A) Need to identify the best method for the statistical studies of magnetosheath properties: Several speakers performed statistical studies using spacecraft data of the magnetosheath properties. Interestingly some studies found dawn-dusk density asymmetry while others did not:

• Chih-Ping Wang used Themis data and showed dependence of the angle between the magnetosheath magnetic field and velocity vectors as a function of downstream distance: 1. THEMIS statistical results show that magnetosheath Bxy becomes more aligned with the magnetosheath Vxy with increasing downtail distance. 2. The sheath density and temperature observed within 3 Re from the model magnetopause do not show significant dawn-dusk asymmetry.

• Brian Walsh selected magnetopause crossings of the THEMIS data and performeda statistical study of the magnetosheath densities at the vicinity of the magentopause. He noticed that densities were higher on the dawn side magnetosheath agreeing with the previous study by Paularena et al.

• Steven Petrinec showed statistical study of the magnetosheath properties using Geotail data. He did not observe any dawn-dusk density asymmetries, but the velocities were higher on dawn-flank magnetosheath.

• Jean Berchem showed couple of comparisons between global MHD simulation results and simultaneous observations made by Themis and Cluster in the magnetosheath. He illustrated that case studies will be needed to avoid misinterpreting results from statistical studies (e.g., orbital biases, seasonal effects) when comparing magnetosheath models with observations.

B) New Model Development:

• Arto Sandroos presented a new global Vlasov code under development at Finnish Meteorological Institute where protons are included as distribution fucntions and electrons as massless charge neutralizing fluid. The code has 10^11 cells in total and computations are targeted to run on 100 000 CPU cores. First results applied to comparison of magnetsoheath and magnetospheric densities showed a good agreement with global MHD GUMICS model results.

• Yongli Wang presented a new three-dimensional magnetopause model with a support vector regression machine (SVRM) utilizing a large database of 15089 magnetopause crossings collected from multiple spacecraft. Using SVRM technique the magnetopause shape had more structure than the previous magnetopause models: clear cusp structure is seen and the whole magnetopause location changessystematically with different dipole tilts. The error sizes in the model correlate with missing data points. The model will be put into web server for community to use.

3. What have we accomplished:

Steve Petrinec/Katariina Nykyri have written a ‘steady-state’ magnetosheath white paper (‘living document’) describing the challenges involved in empirical, theoretical and numerical magnetosheath modeling. This paper also describes the Magnetosheath Challenge. There will also be repositories for model runs and results at the CCMC and at Lockheed Martin.

4. Concluding Remarks

Magnetosheath is both a place for rich plasma physical processes and a filter between solar wind and the magnetospheric plasma and magnetic field enviroments. The magnetosheath properties are crucial determining the growth rates of various instabilities at the magnetopause determining the efficiency of solar wind mass, momentum and energy transport into the magnetosphere.

Interesting ’new’ phenomena are the super magnetosonic magnetosheath flows and there was lot of discussion on the possible generation mechanisms of these jets. A lot of discussion was also devoted on hybrid simulations of the magnetosheath. One concern was the scale size of the structures in hybrid simulations vs. the real system size: e.g how many ion inertial lengths is the magnetosheath in hybrid simulation vs. in real magnetosphere.

There was a lot of discussion of how extreme care must be taken when statistical studies are done: where data is collected, orbital biases, seasonal effects, and how data is binned can affect the conclusions of the study. In this session two speakers arrived at opposite conclusions on the density asymmetry of the magnetosheath probably due to data selection: other speaker choose THEMIS data closer to the magnetopause and other further away from the magnetopause.

The purpose of the white paper is to describe the challenges in methodology for doing magnetosheath research, collect and document various model and statistical data-analysis results into repositories and understand how differences in methodology can lead to different results in statistical and modeling studies. Many publications documenting bow-shock, magnetopause and magnetosheath modeling and analysis leave out important details. Unfortunately the devil is in the details and everything needs to be documented, so that results can be reproduced by a third party. The white paper gives also examples of header files both for observational and numerical magnetosheath modeling that can assist in this quest.