George H. McCabe

12C/13C ratio in ethane on titan and implications for methane's replenishment.

The (12)C/(13)C abundance ratio in ethane in the atmosphere of Titan has been measured at 822 cm(-1) from high spectral resolution ground-based observations. The value, 89(8), coincides with the telluric standard and also agrees with the ratio seen in the outer planets. It is almost identical to the result for ethane on Titan found by the composite infrared spectrometer (CIRS) on Cassini. The (12)C/(13)C ratio for ethane is higher than the ratio measured in atmospheric methane by Cassini/Huygens GCMS, 82.3(1), representing an enrichment of (12)C in the ethane that might be explained by a kinetic isotope effect of approximately 1.1 in the formation of methyl radicals. If methane is being continuously resupplied to balance photochemical destruction, then we expect the isotopic composition in the ethane product to equilibrate at close to the same (12)C/(13)C ratio as that in the supply. The telluric value of the ratio in ethane then implies that the methane reservoir is primordial.

Limb observations of the 12.32 micron solar emission line during the 1991 July total eclipse

The limb profile of the Mg I 12.32-micron emission line is determined by occultation in the July 11, 1991 total solar eclipse over Mauna Kea. It is shown that the emission peaks are very close to the 12-micron continuum limb, as predicted by recent theory for this line as a non-LTE photospheric emission. The increase in optical depth for this extreme limb-viewing situation indicates that most of the observed emission arises from above the chromospheric temperature minimum, and it is found that this emission is extended to heights well in excess of the model predictions. The line emission can be observed as high as 2000 km above the 12-micron continuum limb, whereas theory predicts it to remain observable no higher than about 500 km above the continuum limb. The substantial limb extension observed in this line is quantitatively consistent with limb extensions seen in the far-IR continuum, and it is concluded that it is indicative of departures from gravitational hydrostatic equilibrium, or spatial inhomogeneities, in the upper solar atmosphere.

Stokes polarimeter for mid-IR solar magnetic field measurements

A motorized rotating wave plate polarization analyzer has been built for modulating solar light as input to the Celeste high-resolution cryogenic grating spectrometer to record full Stokes parameter maps at 12 microns wavelength of areas around sunspot and plage regions. The instrument system was used at the McMath-Pierce (1.5 m aperture, f/54) solar telescope at Kitt Peak, where single position slit spectra are spatially 120 arc-sec long, with a double sampled spectral resolution of 0.036 cm-1. Stepper motor driven limb guiders and synchronized action of polarization wheels are under the control of the Celeste data system computer, allowing unattended programmable scanning with 2-3 arc-sec steps for several hours. The results are spatial maps of magnetic field strength and direction, strength measured directly to the level of a few hundred gauss, with a time resolution of approx. 5 minutes per slit position. In this presentation we describe the Mid-IR Stokes polarimetry instrument system, design and performance, and discuss plans for future development.

Motivation, deisgn, and development of 12-micron imaging stokes polarimeters for solar magnetic field studies

We discuss the scientific motivations, design considerations, and development status of imaging Stokes polarimeters based on the extremely Zeeman-senstive infrared emission line of MgI at 12.32 μm. The resolved Zeeman splitting exhibited by this far-infrared line enables significant progress on many problems in solar physics. These problems include vertical gradients in field strength, the magnetic structure of sunspots, the nature of magnetic energy release associated with solar flares, the measurement of horizontal currents in solar active regions, improved measurements of vertical currents, direct measurement of photospheric reconnection, the occurrence and nature of weak magnetic fields, and other problems. We discuss why 12 μm measurements are well suited to these investigations, and we comment on the capability of current instrumentation to enable these studies. Several instruments currently exist to perform 12 μm Stokes polarimetry: a Fabry-Perot system, a grating spectrometer, and the McMath-Pierce Fourier transform spectrometer (which remains crucial for many aspects). We elaborate on some design considerations and development issues, and discuss the optimal approach to be taken in future 12 μm imaging Stokes polarimetry.