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Published May 1, 2008 | Published
Journal Article Open

Observing the Unobservable? Modeling Coronal Cavity Densities


Prominence cavities in coronal helmet streamers are readily detectable in white-light coronagraph images, yet their interpretation may be complicated by projection effects. In order to determine a cavity's density structure, it is essential to quantify the contribution of noncavity features along the line of sight. We model the coronal cavity as an axisymmetric torus that encircles the Sun at constant latitude and fit it to observations of a white-light cavity observed by the Mauna Loa Solar Observatory (MLSO) MK4 coronagraph from 2006 January 25 to 30. We demonstrate that spurious noncavity contributions (including departures from axisymmetry) are minimal enough to be incorporated in a density analysis as conservatively estimated uncertainties in the data. We calculate a radial density profile for cavity material and for the surrounding helmet streamer (which we refer to as the "cavity rim") and find that the cavity density is depleted by a maximum of 40% compared to the surrounding helmet streamer at low altitudes (1.18 R☉) but is consistently higher (double or more) than in coronal holes. We also find that the relative density depletion between cavity and surrounding helmet decreases as a function of height. We show that both increased temperature in the cavity relative to the surrounding helmet streamer and a magnetic flux rope configuration might lead to such a flattened density profile. Finally, our model provides general observational guidelines that can be used to determine when a cavity is sufficiently unobstructed to be a good candidate for plasma diagnostics.

Additional Information

© 2008. The American Astronomical Society. Received 2007 November 14; accepted 2008 January 22. We thank Joan Burkepile for internal review of this manuscript and for assistance with determining MK4 instrumental errors for the data used. We also thank David Elmore for assistance with determining MK4 errors. We thank Spiro Antiochos, Craig Deforest, Tom Holzer, Terry Kucera, and B. C. Low for helpful discussions. We would like to thank the National Science Foundation Research Experiences for Undergraduates (NSF-REU), the University of Colorado's Laboratory for Atmospheric and Space Physics (LASP), and the National Center for Atmospheric Research High Altitude Observatory (NCAR/HAO) for giving J. Fuller the opportunity to work at HAO as part of his summer 2007 NSF-REU program. We acknowledge Big Bear Solar Observatory and New Jersey Institute of Technology for the Hα data and SOHO EIT for EUV data. SOHO is a mission of international collaboration between ESA and NASA. The Mauna Loa Solar Observatory is a facility of the National Center for Atmospheric Research and is sponsored by the National Science Foundation.

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