HAO 2011 Profiles In Science: Dr. Bruce Lites

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Bruce Lites is a Senior Scientist in the High Altitude Observatory of the National Center for Atmospheric Research and is a leading authority on the diagnosis of magnetism in the lower solar atmosphere. He received is PhD in Astrophysics in 1972 from the University of Colorado, Boulder. His first position at HAO began in September 1968 as a Graduate Research Assistant working with Dr. Grant Athay. His main research interest is the interpretation of measurements of the solar magnetic field in terms of structure, dynamics, and energetics of the solar atmosphere. Other research interests include applications of radiative transfer to solar problems and instrumentation for solar observations. He is a primary investigator on the HINODE mission.

Publications

(1) B. W. Lites. 2011: Hinode Observations Suggesting the Presence of a Local Small-scale Turbulent Dynamo. ApJ 737, 52.

Abstract: At the smallest scales, the atmosphere of the Sun is pervaded by an apparently turbulent magnetic field having a distribution of size scales extending well below the resolution of present-day instruments. The small-scale structure of solar magnetism might arise from disruption and shredding through the action of convective flows on the field of the large-scale solar magnetic cycle, the latter presumably arising as a result of a dynamo operating at the base of the solar convection zone. Alternatively, it might arise from a small-scale turbulent dynamo (SSD) operating as a result of vigorous turbulent convective motions very near the solar surface. It is important to distinguish between these two possibilities because the fields arising from a small-scale dynamo may affect the structure of the solar atmosphere independent of the solar magnetic cycle.

The SSD mechanism has been suggested by numerical magnetoconvection simulations, but so far there has been little observational evidence confirming or refuting its presence. The high resolution and excellent polarimetric sensitivity of the Hinode Spectro-Polarimeter permitted observations that suggest the SSD indeed is operating in the solar atmosphere. Analysis of 45 Hinode Spectro-Polarimeter maps of both quiet regions and regions showing moderate solar activity provide the basis for this study. By examining the unsigned apparent flux density from the weakest flux regions with the same quantity averaged over the entire map including stronger fields, no significant correlation of the weak flux on the total amount of flux was found. Furthermore, even when the net field of a map shows a significant dominance of one magnetic polarity, the weakest component of the field shows nearly equal amounts of positive and negative magnetic flux. Both of these indicators strongly point to a local dynamo action rather than dispersal of magnetic fields from shredding of the larger scale structures of the solar magnetic cycle.

Figure caption: The longitudinal apparent flux density displayed as a grey scale from a Hinode "deep magnetogram mode" observation taken on 15 September 2007. Here the stronger field elements (and their immediate surroundings affected by instrumental scattering) are set to uniform gray, so only the weakest component of the quiet Sun magnetic field is seen. This weakest component of the field is most likely to arise from a small-scale turbulent dynamo, and a comparison of its properties with those of all the flux within a map allows one to explore the possibility of operation of a small-scale turbulent dynamo as the mechanism for generation of these small-scale fields.