October 20, 2000

Colloquium Speaker: Donald G. Mitchell


Dr. Donald G. Mitchell is a Principal Professional Staff Physicist at the Johns Hopkins University Applied Physics Laboratory. He received a B.A. in physics from the University of Michigan in 1971, and a Ph.D. in physics in 1975 from the University of New Hampshire. Dr. Mitchell's research interests have included radio astronomy, solar flares, solar wind, and various terrestrial and planetary magnetospheric topics. He is presently focusing on the magnetospheric physics of the Earth and of Saturn. He is the Project Manager for several NASA grants and contracts, and is involved in the design of a new generation of space instrumentation for the imaging of planetary magnetospheric energetic plasmas, in particular as Instrument Scientist for the Magnetospheric Imaging Instrument on the NASA Saturn mission, Cassini, and as the Lead Investigator for the HENA instrument on the NASA IMAGE Mission. He has also designed miniaturized instrumentation for ion mass and energy analysis in space. He has served several NRC and NASA advisory committees.


Colloquium Topic: Images of the Magnetosphere

The IMAGE mission is the first of its kind. It is designed to comprehensively image a variety of emissions from the Earth's magnetosphere with sufficient time resolution to follow the dynamics associated with the development of magnetospheric storms. This paper describes initial results from the IMAGE High Energy Neutral Atom imager (HENA). Energetic neutral atoms (ENAs) are created through charge exchange interactions between the singly charged ions in hot magnetospheric plasmas and cold, neutral exospheric atoms (the hydrogen geocorona at high altitudes and the atomic oxygen exobase at low altitudes). The ENAs freely escape their source region since they are no longer magnetically trapped. HENA instrument [Mitchell et al., 2000] images at energetic neutral atom (ENA) energies between 10 and 60 keV/nucleon reveal the distribution and the evolution of energetic ion distributions as they are injected into the ring current during geomagnetic storms, drift about the Earth on both open and closed drift paths, and decay through charge exchange to pre-storm levels. Substorm ion injections are also imaged, as are the regions of low altitude, high latitude ion precipitation into the upper atmosphere. Two qualitatively different type of events are observed, one characterized by a major magnetic storm (such as the "Bastille Day" storm of July 15 and 16, 2000, Dst ­300nT), and the other by a minor storm (as for example on June 10, 2000, Dst ~-55nT). The large storms are characterized by ion injection deep into the magnetosphere (~2 RE above the surface), while the minor storm images are consistent with injection to ~6 RE above the surface. This technique, the first capability for global imaging of the Earths hot ion plasmas, is expected to play a major role in the development of a predictive space weather capability, which is becoming increasingly important as space assets increase in number and expense, and as manned space activity increases.