On Molecular Hydrogen Formation and the Magnetohydrostatic Equilibrium of Sunspots

Abstract

We have investigated the problem of sunspot magnetohydrostatic equilibrium with comprehensive IR sunspot magnetic field survey observations of the highly sensitive Fe i lines at 15650 Å and nearby OH lines. We have found that some sunspots show isothermal increases in umbral magnetic field strength which cannot be explained by the simplified sunspot model with a single-component ideal gas atmosphere assumed in previous investigations. Large sunspots universally display nonlinear increases in magnetic pressure over temperature, while small sunspots and pores display linear behavior. The formation of molecules provides a mechanism for isothermal concentration of the umbral magnetic field, and we propose that this may explain the observed rapid increase in umbral magnetic field strength relative to temperature. Existing multi-component sunspot atmospheric models predict that a significant amount of molecular hydrogen (H 2 ) exists in the sunspot umbra. The formation of H 2 can significantly alter the thermodynamic properties of the sunspot atmosphere and may play a significant role in sunspot evolution. In addition to the survey observations, we have performed detailed chemical equilibrium calculations with full consideration of radiative transfer effects to establish OH as a proxy for H 2 , and demonstrate that a significant population of H 2 exists in the coolest regions of large sunspots.

On the Collective Magnetic Field Strength and Vector Structure of Dark Umbral Cores Measured by the Hinode Spectropolarimeter

“…However, while such behavior has been observed within individual spots using the infrared Fe I lines at 1565 nm (Jaeggli, Lin, and Uitenbroek, 2012), the behavior seen here is probably a result of the influence of TiO and MgH molecular lines on the MERLIN inversions. We extract the Hinode Stokes spectra from the 29 scans with an umbra exhibiting a normalized intensity below 0.1 and a magnetic field strength above 3250 G. This subsample contains only 12 unique umbrae.…”

“…King (1934) first discovered the strong correlation between the peak magnetic field strength and the photometric intensity of a collection of sunspots; furthermore, he discussed the wavelength dependence of this correlation. Since then, the empirical relationship between the emergent intensity and the magnetic field strength has since been studied by a number of authors both within individual sunspots (Martinez Pillet and Vazquez, 1993;Solanki, Walther, and Livingston, 1993;Leonard and Choudhary, 2008;Jaeggli, Lin, and Uitenbroek, 2012) and amongst a collection of sunspots (Kopp and Rabin, 1992;Penn et al, 2003;Norton and Gilman, 2004;Schad and Penn, 2010;Rezaei, Beck, and Schmidt, 2012). Collections of sunspot umbrae observed by the SOHO/MDI (Norton and Gilman, 2004) and the KPVT/SPM (Schad and Penn, 2010) exhibit power law relationships between the minimum umbral intensity and the corresponding magnetic field strength.…”

“…The Lorentz force is decomposed into the two terms corresponding to a pressure term and the field curvature tension term, F c . As in Jaeggli, Lin, and Uitenbroek (2012), under the limited assumptions of a vertical magnetic field in a field-free, ideal background atmosphere of constant height and density, Equation (3) reduces to a simple proportionality between the squared magnetic field strength and the difference between the external and internal temperatures, i.e.…”

Nonequilibrium ionization and ambipolar diffusion in solar magnetic flux emergence processes

“…The right determination of the number of H 2 molecules is important to properly calculate their contribution to the energy due to their exothermic formation. Through observations, estimations of the fraction of H 2 have been inferred in various sunspot umbrae using the equivalent width of the OH 15652 Å line as a proxy (Jaeggli et al 2012); they find that substantial H 2 molecule formation is present. However, the question remains open whether it would be possible to get similar estimates from observations of magnetic flux emergence regions.…”

He I VECTOR MAGNETOMETRY OF FIELD-ALIGNED SUPERPENUMBRAL FIBRILS

“…The reduction of the raw FIRS spectra differs in a number of ways from previous methods (Jaeggli et al 2012). Detector properties were calibrated via dark and bias subtraction, spectral flat fields, and a nonlinearity correction, although these observations were careful to utilize only the most linear portion of the detector response curve.…”