Dr. Martin A. Lee Professor Space Plasma Physics/Astrophysics

Research Interests: Theoretical space physics, astrophysics, and plasma physics.

My specialty within the theory group is the behavior of energetic ions throughout the heliosphere, that cavity extending beyond the planets created by the solar wind.

Energetic ions (and electrons) are a significant component of the heliospheric plasma. Solar cosmic rays up to energies of 1 GeV/nucleon are produced in solar flares and spewed into the solar wind. Galactic cosmic rays penetrate the heliosphere from outside and provide information on its structure as they propagate to the orbit of Earth. These populations of energetic particles have been known for decades. More recently spacecraft exploring the heliosphere from Mars to its outer reaches have discovered many new populations including diffuse ions of 30-100 keV/nucleon at Earth's bow shock, energetic storm particles at shock waves moving away from the Sun, pickup ions in the comae of comets, and the anomalous cosmic rays with energies intermediate between solar and galactic cosmic rays. Many of these particles are associated with shock waves, whose behavior in the tenuous collisionless solar wind is of interest in its own right.

Using primarily analytical techniques, my students and I have contributed to the theoretical understanding of both the acceleration and propagation of most of these particle populations. A key feature of the acceleration of ions at shock waves is their confinement near the shock by hydromagnetic waves which they themselves excite. We developed the definitive theory for this coupled ion acceleration and wave excitation (Publication 1). We also developed the first theory for acceleration of ions in a turbulent wave field, including the backreaction of the ions on the waves, and applied the theory to pickup ions in the solar wind (Publication 2). We are currently embarking on a program to predict heliospheric structure based on the observed temporal variations of galactic cosmic rays (e.g., Publication 3). The ability to test new theory directly with spacecraft observations is particularly rewarding.

We collaborate actively with scientists at other institutions including the University of Maryland, High Altitude Observatory in Boulder, Colorado, Los Alamos National Laboratory, and the Max-Planck-Institute near Munich. I am a co-investigator on the ion instrument on the SOHO spacecraft. We are active participants in national and international conferences and often involved in conference organization. The Theory Group has NSF as well as NASA funding, which also supports the graduate students who participate in these activities as the basis of their doctoral or master's thesis.

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