Alessandro Cacciani

An instrument to observe low-degree solar oscillations

We have constructed an instrument optimized to observe solar oscillations of low degree. The primary goal of this instrument, which we call LOWL, is to measure the frequency splitting of the low-degree modes in order to determine the rotation rate of the solar core. The LOWL is a Doppler imager based on a magneto-optical filter. It employs a two-beam technique to simultaneously observe solar images in opposite wings of the absorption line of potassium at 769.9 nm. This instrument is very stable against drifts in the wavelength zero-point, is insensitive to noise sources due to intensity fluctuations and image motion, and has a Doppler analyzer with no moving parts. The LOWL has been deployed at HAOs observing station on Mauna Loa, Hawaii and will operate for a period of at least two years.

Helioseismic mapping of the magnetic canopy in the solar chromosphere

We determine the three-dimensional topography of the magnetic canopy in and around active regions by mapping the propagation behavior of high-frequency acoustic waves in the solar chromosphere.

Depth and latitude dependence of the solar internal angular velocity

One of the design goals for the dedicated helioseismology observing state located at Mount Wilson Observatory was the measurement of the internal solar rotation using solar p-mode oscillations. In this paper, the first p-mode splittings obtained from Mount Wilson are reported and compared with those from several previously published studies. It is demonstrated that the present splittings agree quite well with composite frequency splittings obtained from the comparisons. The splittings suggest that the angular velocity in the solar equatorial plane is a function of depth below the photosphere. The latitudinal differential rotation pattern visible at the surface appears to persist at least throughout the solar convection zone. 43 refs.

The International Robotic Antarctic Infrared Telescope (IRAIT)

Thanks to exceptional coldness, low sky brightness and low content of water vapour of the above atmosphere Dome C, one of the three highest peaks of the large Antarctic plateau, is likely to be the best site on Earth for thermal infrared observations (2.3-300 μm) as well as for the far infrared range (30 μm-1mm). IRAIT (International Robotic Antarctic Infrared Telescope) will be the first European Infrared telescope operating at Dome C. It will be delivered to Antarctica at the end of 2006, will reach Dome C at the end of 2007 and the first winter-over operation will start in spring 2008. IRAIT will offer a unique opportunity for astronomers to test and verify the astronomical quality of the site and it will be a useful test-instrument for a new generation of Antarctic telescopes and focal plane instrumentations. We give here a general overview of the project and of the logistics and transportation options adopted to facilitate the installation of IRAIT at Dome C. We summarize the results of the electrical, electronics and networking tests and of the sky polarization measurements carried out at Dome C during the 2005-2006 summer-campaign. We also present the 25 cm optical telescope (small-IRAIT project) that will installed at Dome C during the Antarctic summer 2006-2007 and that will start observations during the 2007 Antarctic winter when a member of the IRAIT collaboration will join the Italian-French Dome C winter-over team.

Confirmation of solar cycle-dependent intermediate-degree p-mode frequency shifts

Results of intercomparisons of seven different sets of frequencies of intermediate-degree p-modes obtained at several different locations between 1981 and 1989 are presented. It is shown that the frequency shifts exhibited by all of these intermediate-degree p-modes are consistent with the intermediate-degree frequency shifts presented by Libbrecht and Woodward (1990) and also with the low-degree frequency shifts presented by Elsworth et al. (1990). It is also shown that these frequency shifts correlate with solar cycle-dependent changes in sunspot number, area, and irradiance. Unbinned and binned differences between 1984 Mount Wilson Observatory and revised 1981 South Pole frequencies are illustrated.

On the constancy of intermediate-degree p-mode frequencies during the declining phase of solar cycle 21

A comparison was made between two sets of frequencies of intermediate-degree solar p-mode oscillations obtained in late 1981 and mid-1984. Good agreement was found at the 0.02 microHz level despite the 2.6 yr interval separating the two sets of observations. In particular, a comparison was made between the frequencies of 573 modes obtained at the South Pole during December 24-25, 1981 and those of the same modes as observed at the Mount Wilson Observatory 60-ft Solar Tower during July 29-August 13, 1984. The present results are consistent with no change in intermediate-degree p-mode frequencies between late 1981 and mid-1985. 17 references.

Contribution of high-degree frequency splittings to the inversion of the solar rotation rate

We present the contribution of high degree rotational splittings to the inversion of the internal rotation rate around the equator. The extention of the input data set to l of 500, allow us to improve the resolution of the solution mainly in the outermost 15% of the solar radius. The rotational profile obtained in the regions below the surface leads to an attractive picture that could reconcile different non-seismic estimates of the “surface” rotation rate.

The source of the solar oscillations: Convective or magnetic?

The origin of solar oscillations has not yet been clearly determined. The downflows due to convective rapid cooling at the surface have been invoked as a possible source. In this paper we investigate the properties of the source as inferred from the local analysis of the intensity-velocity phase dierences. The same spatial and temporal characteristics of other observed events and their correlation with the H bright points suggests downward plasma jets related to explosive chromospheric evaporation to be another possible candidate.

Evidence for radial variations in the equatorial profile of the solar internal angular velocity

We present evidence that the solar internal angular velocity, at least as measured in the equatorial plane, shows systematic radial variations in the outer half (by radius of the solar interior. Specifically, we employ the rotationally-induced frequency splittings of both high- and intermediate-degree sectoral p-mode oscillations to demonstrate that the internal angular velocity rises inwardly from the observed spectroscopic rotation rate of the photospheric gas to a higher value that is at least equal to the observed rotation rate of sunspots, if not higher, in the outer third of the convection zone before decreasing inward of the convection zone to a value which is at least two percent below the photospheric gas rotation rate. By making the assumption that the observed splittings are sensitive to solar rotation at the midpoints of the p-mode eigenfunctions we obtain an angular velocity profile which rises from 452 nHz at the photosphere to 462 nHz at a depth of about five percent of the solar radius below the photosphere. A comparison of this inferred angular velocity profile with that obtained from a formal inversion of these splittings (which is reported elsewhere in these proceedings by Korzennik et al.) suggests that the angular velocity might actually exceed the magnetic rotation rate over much of the convection zone before decreasing inwardly toward the center of the sun.

The Magneto-Optical Filter at Kanzelhöhe

An observing station based on the Magneto-Optical Filter (MOF) technology has been installed at Kanzelhohe at the beginning of 1997. In this paper, the main characteristics of this instrument are discussed and a one day solar velocity observing run is shown.