Ronald L. Gilliland

Solar, volcanic, and CO2 forcing of recent climatic changes

The climate, as represented by the mean Northern Hemisphere temperature, has shown substantial changes within the past century. The temperature record is utilized as a means of elucidating the relative importance of anthropogenic CO2 increase, volcanic aerosols, and possible solar insolation variations in externally forcing climate changes. Solar luminosity variations, suggested by observed solar radius variations on an ≈ 80 yr time scale, allow a self-consistent explanation of the hemispheric temperature trends. Evidence for solar influences on the climate is also found on the shorter 11 and 22 yr time scales present in solar activity cycles.

Theoretical interpretation of the variability of global solar properties

Abstract As the Sun evolves through a magnetic cycle the distribution and amplitude of magnetic regions is altered. Surface phenomena (e.g., sunspots, faculae, and coronal structures) signify clearly the variations, but it is less obvious how the global, interior structure of the Sun responds. Theory suggests that the interior structure may respond directly to the magnetic fields via their associated pressure, and that convective energy transport efficiency will be perturbed. The changes of diameter and luminosity (and thus effective temperature) depend upon details of assumed mechanisms and the position in the interior at which they are applied. The p-mode oscillation frequencies can be perturbed either via changes in the mean structure, or by interaction with distributed magnetic field domains. No definitive variations of diameter, p-mode frequencies, effective temperature, neutrino flux, etc. have yet been shown to exist.

Solutions of the shallow water equations on a sphere

Abstract Various numerical solutions of the shallow water equations in two dimensions are studied in an effort to develop a computational technique applicable to hydrodynamics in spherical geometry. The equations are first cast in a form which allows periodic boundary conditions in both angular coordinates. Explicit numerical solutions using leap-frog centered differencing in time and either second, fourth, or compact fourth order centered spatial differencing are studied. The fourth order compact differencing is found to be easily adapted to spherical geometry and is superior to the second order technique. We also consider an alternating-direction implicit (ADI) scheme in an attempt to increase computational efficiency by taking larger time steps. Both analytically steady state and time dependent solutions are examined to investigate stability properties and discretization errors in time and space. Implicit methods require more computation per time step than explicit methods for solution of the shallow water equations. However, the total time for a simulation can be less with the implicit method. The ADI formalism also has advantages of importance for more physically complex problems.

A comparison of photospheric, chromospheric and coronal indices for the sun

Abstract Indices of photospheric activity as reconstructed sunspot blocking (essentially sunspot areas) are compared with the White and Livingston 1A Ca II K index and a polarization-brightness index of the corona that is proportional to mean hemispheric coronal electron density. The photospheric, chromospheric and coronal manifestations of magnetic activity associated with the solar cycle show quite similar behavior. The modulation properties of each index are remarkably similar both in terms of cycle signal-to-noise and the morphology of scatter variations with respect to cycle phase. The coronal electron density shows a more rapid rise from solar minimum to maximum than the activity indicators from lower atmospheric levels — suggesting a difference in growth rates for the large scale structures influencing the corona with respect to the smaller scale photospheric features.

The high altitude observatory-Lowell observatory solar-Stellar spectrophotometry project

We propose to study chromospheric activity on time scales of rotation and activity cycles in the Sun and solar-like stars using a high-dispersion, dedicated fiber-fed spectrograph facility currently being completed. Using CCD detectors we will record the line intensities of Ca II H and K, Hα, the Ca II IRT and other optical lines reflecting chromospheric activity. To study magnetic activity on rotational time scales we will obtain near nightly observations of about 40 stars over a 120 day time base for several seasons. We hope to extend the detection of rotation periods using rotational modulation of chromospheric emission to later spectral types and other stellar population groups than are currently available. To study stellar cycles we will obtain observations of about 200 stars once per month over a long time base. We plan to define cycle properties (amplitude, shape, morphology) as a function of stellar mass, age, rotation rate, chemical composition, etc.

Increased chromospheric activity in subgiants related to evolutionary effects

A scenario based on detailed stellar evolution computations is presented which predicts development of more intense dynamo activity with evolution of early F stars off the main sequence. On the main sequence a typical early F star rotates rapidly but has a very short convective turnover time scale; the resulting lack of rotational influence on the convection cannot support dynamo activity. The main sequence lifetime is passed in a state of inactivity with little or no rotational braking. With evolution to the subgiant domain, the convection zone deepens, the convective turnover time scale lengthens with a consequent increase of rotational influence on convection, and significant dynamo activity is expected to result. The above scenario is supported by the observations of HD 124850, an active subgiant with a surprisingly short rotation period. Indeed the mere existence of an evolved star with Prot = 7.6 days (Noyes et al. 1984) and a moderate activity level fairly well demands that such a scenario be true, since magnetic braking over less evolution has slowed many (less massive) stars of similar activity levels to much longer rotation periods.

Consequences of evolution through the giant domain for stars of 2 – 5 M⊙

Abstract Stars of 2 – 5 M ⊙ reside on the main sequence with only extremely weak and narrow surface convection zones suggesting that significant dynamo activity may not be possible. As the stars evolve off the main sequence into the giant domain the convection zone rapidly deepens allowing for the initiation of a dynamo. Evolution into the giant domain also increases the generation of acoustic flux from sub-atmospheric turbulence, the dissipation of which may contribute to chromospheric heating. We discuss the computation of convective turnover time scales for the giant configurations that are structurally quite different from main sequence envelopes. We compare the theoretical expectations for dynamo activity with observations of stellar rotation rates across the HR diagram.

The integrated sun as a (magnetic) variable star

Abstract Although the source of the solar magnetic cycle is not yet fully understood, it is clear that observed solar variations result primarily from surface manifestations of magnetic field associated active regions. Globally, and at depth in the solar interior, variations of convective efficiency following from magnetic field variations may cause luminosity and diameter changes. The waxing and waning of sunspots and faculae in the photosphere lead to luminosity variations on time scales of days to months, or longer. In the chromosphere magnetically controlled plage leads to a variation of UV flux and line emission such as Ca II H and K. The coronal structure is dominated by background magnetic fields and experiences factor of 2 variations of total mass over the solar cycle. Relative modulation characteristics are remarkably similar from the different atmospheric levels with their unique activity types.