First Successful Bench Tests of NSO Solar Adaptive Optics System
Thomas Rimmele (NSO) and Richard Radick (US Air Force), working with Richard Dunn (NSO) and engineer Kit Richards, report the successful implementation of a low-order solar adaptive optics system at the R. B. Dunn Solar Telescope (DST) at NSO/Sac Peak. Adaptive Optics (AO) is a technology that permits diffraction-limited observations from large groundbased telescopes. Diffraction-limited imagery, spectroscopy, and polarimetry, performed near the diffraction limit of existing solar telescopes, will enable solar astronomers to study fundamental scientific problems such as the structure and dynamics of magnetic flux tubes, wave propagation along magnetic elements, and the generation and dissipation of small-scale magnetic fields.
The solar AO program at NSO/Sac Peak has recently achieved three major milestones. In September 1998, the control loop was closed for the first time on the bench. In November, the loop was again closed during the system's first tests at the DST with solar granulation and small pores as the wavefront sensing targets. In March 1999, the system was used to collect first AO-corrected science data using the post-focus instrumentation of the DST. The first diffraction-limited narrow-band images, magnetograms, and velocity maps of granulation, sunspots, and pores were recorded using the Universal Birefringent Filter. The diffraction limit was maintained successfully for effective exposure times of several seconds. AO-corrected images were also fed to the horizontal spectrograph at the NSO/SP Dunn Solar Telescope to measure very small-scale structure and velocity fields associated with pores and sunspots.
The current system corrects a maximum of 20 spatial (Zernike) modes of atmospheric turbulence. A correlating Shack-Hartmann wavefront sensor with 24 subapertures, capable of using solar granulation or other time-varying, low-contrast, spatially extended targets, is used to measure the wavefront aberrations. The hardware design features parallel processing using off-the-shelf components. This architecture will allow expansion of the system to more spatial modes. Drive signals for the 97 actuators of the Xinetics deformable mirror are derived from the wavefront sensor data using a modal wavefront reconstruction algorithm.
The level of atmospheric compensation achieved during the November engineering run at the DST was genuinely exciting: improved and more stable resolution and a reduction of residual jitter were both clearly evident in the corrected image. This is evident in Figure 2, which shows a pair of simultaneous corrected and uncorrected images. The servo loop was closed at an update rate of 800 Hz and a lag of about 2.0 milliseconds, resulting in a system bandwidth of about 25 Hz. This is inadequate to fully compensate the atmosphere except under the best seeing conditions. The loop bandwidth is currently limited by both the frame rate and read-out speed of the wavefront sensor camera, and to a lesser extent by the processing hardware. These bottlenecks will be alleviated through the purchase of a faster camera and improvements to the hardware. The small number of subapertures presents another performance limitation. In the face of these difficulties, the partial atmospheric compensation that the system achieved under good, but not exceptional, seeing conditions at the DST was remarkable.
NSO plans to pursue a staged adaptive optics development program, which will concentrate on the following goals:
Images and movies are available on the Internet: http://www.sunspot.noao.edu/AOWEB.
Molecular Cloudlets above Solar Granules
H. Uitenbroek (Harvard-Smithsonian Ctr. for Astrophysics) has combined high-resolution infrared spectroscopy at the NSO McMath-Pierce Telescope with sophisticated radiative transfer modeling to advance our understanding of the solar ``COmosphere"---the unexpectedly cool regions of the atmosphere in which the carbon monoxide (CO) molecule can exist. Uitenbroek took advantage of exceptional seeing conditions to acquire spectroheliograms that strongly suggest (at the limit of angular resolution) that CO exhibits an inverted granular intensity pattern: dark granules surrounded by bright intergranular lanes, rather than the bright granules and dark lanes familiar from visible continuum images. Figure 3 is an example of a high-resolution brightness temperature map derived from the core of the strong 3--2 R14 CO line at 4.67 m.
To explore this hypothesis, Uitenbroek carried out 2- and 3-dimensional radiative transfer calculations through a 3-D theoretical simulation of solar granulation by Stein and Nordlund. Figure 4 compares the derived brightness temperature in the core of the weak 7--6 R68 CO line with the adjacent infrared continuum. The brightness temperature pattern in the CO line is clearly inverted with respect to the continuum pattern. Maps of CO concentration derived from the simulation show that the 7--6 R68 line forms in a region of inverted temperature contrast caused by expansion cooling above the hot centers of granules and compression heating over intergranular lanes.
Both the observations and the models need to be extended to gain a complete understanding of the complex structure of the COmosphere. The observations will benefit from better temporal resolution and spatial coverage afforded by a large-format infrared array camera. The granular convection models do not yet extend high enough in the atmosphere to compare with the cores of strong CO lines like 3--2 R14. Also, the initial calculations assume that the concentration of CO is always in Saha chemical equilibrium, whereas previous theoretical work, as well as the detailed results of Uitenbroek's study, suggest that non-equilibrium effects may be important. The structure of the solar COmosphere is at the heart of a current lively controversy over whether the Sun, and other late-type stars, have ``full-time" chromospheres or merely heating episodes within a dominantly cool plenum.
Small-Scale Flux Tube Dynamics
Michael Sigwarth has found evidence for a strong increase in the dynamic behavior of magnetic elements when observed at high spatial and temporal resolution in the solar photosphere. The NSO/HAO Advanced Stokes Polarimeter (ASP) at the Dunn Solar Telescope was used to obtain high-resolution Stokes spectra from solar network, intra-network, and active region magnetic fields at a low noise level. Due to excellent seeing and instrumental conditions, it was possible to detect magnetic elements down to a size of ~ 150 km and to measure velocities within the magnetic flux tubes as well as in their nonmagnetic environment. A statistical analysis of Doppler shifts and asymmetries of the V spectra of FeI 630.15 and 630.25 nm was performed to obtain information on the dynamics of magnetic elements of different sizes. The analysis revealed a strong dependence of the dynamics within small-scale magnetic fields on the size of the magnetic elements as well as on the granular velocity in their vicinity. For the smallest magnetic features it was found that velocities of up to 5 kms-1 in both up- and downflows, whereas for larger elements or cluster of several flux tubes the velocities become smaller and more uniform. If averaged over all the individual profiles, there still remains a downflow exceeding 0.5 kms-1.
The observations are likely to reflect several dynamical effects. The downflows may be triggered during the formation of magnetic elements by convective collapse as well as by the stochastic interaction of evolved flux tubes with the granulation and processes during the decay of flux tubes. This may explain the broad variety in the observed flows and asymmetries of Stokes-V spectra and is supported from time series of individual magnetic features. For example, it was found that the formation of concentrated magnetic elements aligns with accelerated downflow within the magnetic field as expected for convective collapse. The properties of spatially and temporally integrated V profiles are consistent with results from FTS observations. Sigwarth's observations are in qualitative agreement with results from numerical MHD simulations.
Probing the Solar Cycle through Stellar Observations
Using the WIYN 3.5-m telescope on Kitt Peak, M. Giampapa and his colleagues, R. Radick (AFRL/NSO) and S. Baliunas (SAO), studied chromospheric emission lines in 106 Sun-like stars in the galactic cluster M67. This open cluster is an especially appropriate target of observation for the study of solar-type stars since it is approximately the same age and has the same chemical composition as the Sun. Thus, the solar-type stars in this cluster are virtually twins of our Sun. Using the Hydra multi-object spectrograph, Giampapa surveyed the strengths of the chromospheric CaII H and K emission in the many stellar counterparts of the Sun in M67. Interpreting the range of CaII emission observed in the Sun-like stars in M67 as representative of the possible amplitudes of cycle-related variability that can occur in the Sun itself, indicates that about 42% of the solar-type stars in M67 exhibit levels of activity that are either greater than that seen at solar maximum or less than that seen at solar minimum. Giampapa's analysis shows that between 10--15% of the Sun-like stars in M67 are distinguished by exceptionally quiescent levels of magnetic activity analogous to the so-called ``Maunder-minimum" episode of the Sun during A.D. 1645--1715 when visible manifestations of solar activity vanished. This period corresponded to a time of reduced average global temperatures on the Earth known as the ``Little Ice Age." About 30% of the M67 Suns are in a state of enhanced activity compared to that seen at solar maximum. It is possible that the so-called ``Medieval warm period" during A.D. 900--1200 corresponded to a time of enhanced activity on our Sun. In brief summary, these results suggest that the Sun is currently in a moderate state of activity but that excursions from contemporary activity levels can occur between 40--45% of the time.
Variability on the scale observed could have consequences for the amplitude of irradiance variability that the Sun can exhibit, and hence potentially influence global climate. Excursions in the activity itself could have an impact on the near-earth orbital environment with consequences for space operations.
In the Spring of 1999, Giampapa's team was allocated additional observing time at the WIYN telescope to monitor the solar-type stars in M67 in order to eventually obtain actual cycle periods, analogous to the 11-year solar cycle. It would then be possible to directly compare and contrast the cycle properties of the solar twins with that of the solar cycle. A parallel program of high-precision photometry is also planned, to directly measure luminosity variations in the M67 Suns, and to compare those variations with changes in activity as recorded with the WIYN observations.