February 18, 2011, 12:00 pm - 1:00 pm

Director's Seminar

Shaela Jones (University of Maryland), Rigid Rotation: Coronal Conundrum or Observation Effect?

Abstract: The solar photosphere rotates differentially, with the fastest rotation rate at its equator and gradually decreasing rates with higher latitude. In contrast, the corona seems to rotate more rigidly, at something close to the equatorial photospheric rotation rate. This difference is puzzling, since logically it seems that if magnetic features in the corona are rooted in the photosphere as they appear to be, then either they must rotate at the photospheric rate or they must undergo shear and even separation from the fields below them on an ongoing basis. Such a process could have important implications for the perennial problems of coronal heating and CME initiation. It has been suggested that coronagraph measurements, on which the evidence for rigid coronal rotation is largely based, may not be reliable due to the influence of projection effects. Here I will present measurements of coronal rotation using the STEREO COR1 coronagrahs during 2007 and 2008. I will show, via comparison with tomographic reconstructions, that the relative rigidity of the rotation measured here is in fact not an observation effect due to projection.

Dr. Nicholeen Viall (NASA Goddard Space Flight Center/ORAU), SDO/AIA Light Curves and Implications for Coronal Heating

Abstract: It seems largely agreed that many coronal loops---those observed at a temperature of about 1 MK---are bundles of unresolved strands that are heated by storms of impulsive nanoflares. The nature of coronal heating in hotter loops and in the very important but largely ignored diffuse component of active regions is much less clear. Are these regions also heated impulsively, or is the heating quasi steady? The spectacular new data from the Atmospheric Imaging Assembly (AIA) telescopes on the Solar Dynamics Observatory (SDO) offer an excellent opportunity to address this question. We analyze the light curves of coronal loops and the diffuse corona in 6 different AIA channels and compare them with the predicted light curves from theoretical models. Light curves in the different AIA channels reach their peak intensities with predictable orderings as a function the nanoflare storm properties. We show that while some sets of light curves exhibit clear evidence of cooling after nanoflare storms, other cases are less straightforward to interpret. Complications arise because of line-of-sight integration through many different structures, the broadband nature of the AIA channels, and because physical properties can change substantially depending on the magnitude of the energy release. Though at first glance some light curves are complicated, they nevertheless exhibit predictable and understandable patterns which are consistent with nanoflare storm heating.

Dr. Peter Williams (NASA Goddard Space Flight Center), Analysis of Solar Convection Cells with SDO/HMI and SOHO/MDI

Abstract: Supergranulation is a component of solar convection that assists in the outward transportation of internal energy. Supergranule cells are approximately 35 Mm across, have lifetimes on the order of a day and have divergent horizontal velocities of around 300 m/s, a factor of 10 higher than their central radial components. While they have been observed using Doppler methods for around half a century, their existence is also observed in other datasets such as magnetograms and Ca II K images. These datasets clearly show the influence of supergranulation on solar magnetism and how the local field is organized by the flows of supergranule cells. The Heliospheric and Magnetic Imager (HMI) aboard SDO is making fresh observations of convection phenomena at a higher cadence and a higher resolution that should make granular features visible. Granulation and supergranulation characteristics can now be compared within the same datasets, which may lead to further understanding of any mutual influences. The temporal and spatial enhancements of HMI will also reduce the noise level within studies of convection so that more detailed studies of their characteristics may be made. We present analyses of SDO/HMI Dopplergrams that provide new estimates of convection cell sizes, lifetimes, and velocity flows, as well as the rotation rates of the convection patterns across the solar disk. Finally, we describe comparisons with previous spatial and temporal data produced by MDI.