Stuart D. Bale

Astrophysics Experiment

Research Interests

I am interested in plasma astrophysics from the experimental point of view. Much of the universe is in the plasma state and we are just coming to appreciate the role of plasma dynamics and magnetic fields in the large-scale evolution of astrophysical systems. However, many of the fundamental processes are poorly understood and can only be studied in a limited parameter regime in the laboratory. Examples are magnetic reconnection, collisionless shocks, plasma turbulence, and solar/stellar wind generation and evolution.

My research is focused on developing experiments to understand these processes and, in particular, how microscale, kinetic phenomena, affect large-scale plasma evolution.

I am also interested in low frequency (LF) radio astronomy, at frequencies below the ionospheric cutoff (~12 MHz); these observations need to be made from space. Signatures of solar flare electrons and CMEs dominate the dynamics at these frequencies, however the large-scale structure of the radio sky below 15 MHz remains mostly unexplored.

Our group at the Space Sciences Laboratory develops, builds, and operates space-borne experiments to study in situ the plasma processes active in astrophysical, heliospheric, and magnetospheric systems. These experiments are flown on NASA and ESA spacecraft missions and are often developed as balloon and sounding rocket payloads.

Current Projects

My group is involved with NASA’s STEREO mission to study the generation and evolution of Coronal Mass Ejection (CME) phenomena. CMEs are dramatic, large-scale solar eruptions whose triggering mechanism, evolution, and role in solar physics are largely unknown; however CMEs are thought to play a role in the helicity evolution of the solar dynamo. Several of the experiments on STEREO were designed and built at the Space Sciences Lab. Our experiments will remotely sense radio emission and energetic particles from these events, as well as thermalization and shock processes in the ambient solar wind. Opportunities exist for students to be involved in data analysis on STEREO.

Magnetic reconnection is a phenomenon that allows rapid, topological reorganization of magnetic fields in a plasma; it is the primary agent in solar flare and magnetospheric dynamics and is thought to play a role in solar/stellar wind acceleration, accretion disk physics, and other astrophysical phenomena. Our experiments on the Polar, Wind, and Cluster spacecraft have made important measurements of ion and electron diffusion and plasma turbulence related to reconnection evolution. Experimental studies of electron diffusion and wave physics are continuing.

An ongoing project is the study of electric fields, wave phenomena, and particle acceleration at collisionless shocks. Shocks are ubiquitous in astrophysics and are responsible for particle (cosmic ray) acceleration, plasma heating, and mediating flow at astrophysical boundaries. Dissipation physics, particle acceleration, and the energy budget at shocks are poorly understood. Experiments on the Cluster, Wind, and Polar spacecraft are used to study these problems.

Other important problems currently accessible to experimental progress include solar wind generation and evolution, plasma radio emission mechanisms, and plasma wave-particle interactions.

Selected Publications

Bale, S. D. and F. S. Mozer, Large parallel and perpendicular electric fields on electron spatial scales in the terrestrial bow shock, Phys. Rev. Lett., 98, 205001, 2007.

Huttunen, K. E. J., S. D. Bale, T. D. Phan, M Davis, and J. T. Gosling, Wind/WAVES observations of high frequency plasma waves in solar wind reconnection exhausts, J. Geophys. Res., 112, A01102, 2007.

Sundkvist, D., A. Retinò, A. Vaivads, and S. D. Bale, Dissipation in turbulent plasma due to reconnection in thin current sheets, Phys. Rev. Lett., in press, 2007.

Bale, S. D., P. J. Kellogg, F. S. Mozer, T. S. Horbury, and H. Reme, Measurement of the electric fluctuation spectrum of magnetohydrodynamic turbulence, Phys. Rev. Lett., 94, 215002, 2005.

Bale, S. D., F. S. Mozer, and T. S. Horbury, Density-transition scale at quasi-perpendicular collisionless shocks, Phys. Rev. Lett., 91, 265004, 2003.

Mozer, F. S., S. D. Bale, T. Phan, and J. Osborne, Observations of electron diffusion regions at the subsolar magnetopause, Phys. Rev. Lett., 91, 245002, 2003.

Salem, C., D. Hubert, C. Lacombe, S. D. Bale, A. Mangeney, D. E. Larson, and R. P. Lin, Electron properties and Coulomb collisions in the solar wind at 1 AU, Astrophys. J. , 585, 1147, 2002.