Structure of the Limb Chromosphere and Transition Region
In collaboration with Bart De Pontieu [Lockheed Martin Solar and Astrophysics Laboratory]
Additional Sources of Funding: NASA ROSES-2007 HGI grant NNX08AL22G; ROSES-2007 Living With A Star TR&T grant NNX08AH45G to BDP and SWM; NSF ATM-0541567; NASA ROSES-2005 NNG06GC89G
Using joint observations from the Solar Optical Telescope (SOT) on the NASA/JAXA Hinode spacecraft, images of UV continuum emission from the Transition Region and Coronal Explorer (TRACE) and UV spectroscopic observations from the SUMER instrument on the Solar and Heliospheric Observatory (SOHO), we are exploring the structure of the chromosphere and transition region at the solar limb.
Previous observations have shown that the chromospheric limb consists of at least two types of fundamental structure, known as spicules. "Type I" spicules live predominantly in closed magnetic field regions, have lifetimes of 3-7 minutes, and span out from the granular magnetic network into the cell interiors. "Type II" spicules live in the magnetic core of the granular network, have 10-100 second lifetimes and are normally oriented to the limb.
While Type-I spicules are relatively well understood, their more dynamic cousins are a not. While both of these structures have been shown to carry Alfvén waves, the latter is likely to be an important component in the process that heats the chromosphere, corona and can drive the solar wind. The new observations will provide information to understand their role at the lower boundary of the heliospheric plasma.
Figure caption: Hinode/SOT observations of the chromosphere at the limb, seen in the raw (top left panel) and after the application of an unsharp masking to the image. Studying the evolution of material along a normal path (shown as white dashed line in the top two panels) via the space-t diagrams shown in the two lower panels we see two prevalent structures, the parabolic temporal trajectories typical of "Type I" spicules and the sharp linear features of "Type II" spicules.
Spectroscopic Investigation of Transient Coronal Holes
Additional Sources of Funding: NASA ROSES-2007 HGI grant NNX08AL22G.
We have investigated the occurrence of a coronal dimming using a combination of high resolution spectro-polarimetric, spectral and broadband images which span from the deep photosphere into the corona. These observations reinforce the belief that coronal dimmings, or transient coronal holes as they are also known, are locations of open magnetic flux in the corona resulting from the launch of a Coronal Mass Ejection (CME). As open magnetic regions, they must act just as coronal holes and be sources of the fast solar wind, but only temporarily.
This behavior is exemplified by the temporal evolution of several EUV emission lines observed by the Extreme-ultraviolet Imaging Spectrometer (EIS) on the NASA/JAXA Hinode spacecraft which demonstrate large plasma outflow (~40km/s) in the region and the dynamic behavior of non-thermal line widths. The latter is consistent with the dynamic growth and decay of sub-resolution Alfvénic motions in the plasma as it opens and eventually closes. Based on recent investigations these Alfvén waves are a likely driver of the fast solar wind in the open portion of the outer corona.
This work has posed an inescapable question - what impact does this source of fast wind have on the propagation and in-flight characteristics of the CME that initiates the coronal dimming in the first place?
Figure caption: Hinode/EIS observations of the corona before (left column), and after (center and right columns) the launch of a coronal mass ejection (CME). Shown in rows of the figure from top to bottom are the line intensity, intensity difference, non-thermal spectroscopic line width and their difference. The contoured region shows a reduction in intensity of 75% over the course of the event.
Time-Distance Seismology of the Solar Corona
Steven Tomczyk & Scott W. McIntosh
Additional Sources of Funding: NASA ROSES-2007 LWS TR&T grant NNX08AU30G.
The unique spectral imaging capability of HAO's Coronal Mutli-channel Polarimeter (CoMP) allows us to perform time-distance seismology of the solar corona. We have exploited the ubiquity of the Alfvénic motions observed in the corona to construct the first k-omega diagram of the region. These k-omega diagrams, and related diagnostics, have allowed us to refine the analysis presented in Tomczyk et al. (2007, Science, 317, 1192).
This new analysis has considerably improved the correspondence between the determined angle of wave propagation and the spectro-polarimetrically measured magnetic azimuth. The relationship between wave propagation angle and magnetic azimuth will be essential to resolve the Van Vleck ambiguity in the spectro-polarimetric measurements.
Using the k-&;omega diagrams we are able to measure both the phase speed and the amount of pro-grade (upward) and retro-grade (downward) Alfvén wave power in the corona. Measuring the spatial variation in the mixture of upward and downward waves is critical in identifying the physical process, or processes responsible for heating the solar corona and driving the solar wind.
The observed contrast in the upward/downward wave motions is consistent with a picture of the corona where the low-frequency magneto-convection driven Alfvénic motions suffer significant dissipation along the magnetic structures along which they propagate, possibly as a result of turbulent interference processes.
Figure caption: HAO/CoMP observations of the solar corona (top). Extracting a wave path from the Doppler velocity measurements (solid line) we can construct a time series of the plasma motion along the path (second row); the inclined striping of the wave motions indicates the phase speed. A two-dimensional Fourier spectrum of the time series yields the k-omega diagram of the waves on the path (third row) and that for all the paths found in a surrounding region (fourth row). In both cases we see that the waves present are non-dispersive and have equal phase speeds (inclination), but a different degree of power in each direction.
Tracking MHD Waves in the Solar Corona
Scott W. McIntosh & Steve Tomczyk
In collaboration with Bart De Pontieu [Lockheed Martin Solar and Astrophysics Laboratory, Palo Alto, CA]
Additional Sources of Funding: NASA ROSES-2007 HGI grant NNX08AL22G; ROSES-2007 Living With A Star TR&T grant NNX08AH45G to BDP and SWM; NSF ATM-0541567; NASA ROSES-2005 NNG06GC89G.
We have considered the problem of automatically (and robustly) isolating and extracting information about the propagating waves and oscillations observed in EUV image sequences of the solar corona. Such an approach is essential for near real-time application to the large data flow (~1Tb/day) likely from the Atmospheric Imaging Array (AIA) on the Solar Dynamics Observatory (SDO) that will launch in 2009.
Using a simple coherence / travel-time based approach detects and provides a wealth of information on transverse and longitudinal wave phenomena in the test sequences provided by the Transition Region and Coronal Explorer (TRACE). The results of the search are "pruned" (based on diagnostic errors) to minimize false-detections such that the remainder provides robust measurements of waves in the solar corona, with the calculated propagation speed allowing automated distinction between the various wave modes observed.
The automated approach taken has verified the results of several investigations reported in the literature while finding considerably more sources of oscillation, across a broad range of frequencies, to explore in the future. Perhaps the most intriguing of the new findings is the potential connection of oscillations in the corona to low-frequency (~1.5 mHz) internal gravity waves which propagate horizontally in a considerably lower region of the atmosphere.
Figure caption: Results of the coronal wave detection algorithm for the 1.5mHz filtered timeseries for the 14 July 1998 dataset from TRACE. For context we show a TRACE 171Å intensity image of the corona (panel A), the weighted signal coherence (panel B), coherent length of the wave detected (panel C), wave propagation angle (panel D), wave phase speed (panel E), and relative error of the computed wave phase speed (panel F).