The Sun

There are two main reasons for studying the Sun. First, there is the practical need to understand how changes at the Sun's surface affect the flow of energy and of dangerous radiation to Earth. Second, the Sun is the only star that is close enough to study its surface in detail. What we learn about sunspots and flares on the Sun can be applied to billions of other stars in the Galaxy.

• Solar Group home page

Advanced Techonology Solar Telescope

IfA scientists are participating in a project to design and develop the next-generation solar research telescope called the Advanced Technology Solar Telescope (ATST). This instrument represents the largest single advance in ground-based solar observing since the time of Galileo! The project is being funded by the National Science Foundation (NSF).

The Science Working Group of the ATST project have recommended Haleakala as the future site of the world's largest optical solar telescope, with a final decision to be made in December 2004 based on logistical and other issues.

• More about the ATST
• NSO ATST website (National Solar Observatory)

The Imaging Vector Magnetograph

The main instrument used at Mees Observatory is the imaging vector magnetograph, which allows astronomers to measure the electric currents passing through selected regions of the Sun's surface. Electric currents are closely tied with magnetic fields on the Sun and are a key to understanding what goes on both below and above the visible surface of the Sun.The Imaging Vector Magnetograph measures the circular and linear polarization of the Zeeman components of a neutral iron absorption line in the spectrum of the Sun. These data are used to make a three-dimensional map of the magnetic field at the Sun's surface; the electric current through the surface is obtained by calculating the 'curl' of the magnetic field and applying Maxwell's equations.

Sunspots

Measurements of sunspots with the imaging vector magnetograph show that magnetic fields emerge at the solar surface already carrying electric currents and that the direction of currents' flow is systematic over space and time. This work has caused us to think of solar electric currents in a different way: as probes of the solar interior and of the magnetic dynamo that drives the 11-year sunspot cycle.

The Sun as a Star 

A fundamental mystery of the Sun is why it varies with a semi-regular period of 11 years. These changes have been detected from acoustic wave observations (helioseismology) and by sensitive measurements of the Sun's brightness. While the ultimate engine that drives these changes is almost surely magnetic, we are beginning to learn how global solar properties, like the Sun's brightness are affected by cyclical changes in the deep solar interior.

Jeffrey Kuhn is working to understand the physical mechanisms of the solar cycle. He uses data from a world network of telescopes he designed with Haosheng Lin and R. Coulter, and a space satellite experiment called the Solar Oscillations Investigator which is a part of the SOHO experiment package at the Earth-Sun Lagrange point. Much of this work involves modeling and understanding small oscillations in the Sun's shape and brightness. 

• Cyclone waves on the Sun

Flares

Eruptive flares on the Sun's surface generate high-energy particles that reach Earth in a few days. These particles can disrupt radio communication, trigger the aurorae, and produce dangerous levels of radiation at high altitudes. Observations at Mees indicate that the emergence of electric currents at the Sun's surface is likely to be important to the driving of solar flares because previously existing magnetic structures suddenly can be energized. This is analogous to turning on a light bulb: It is much faster to connect it to a current-carrying system than to start a generator. To test this idea, magnetic data from Mees are being compared with images of the solar corona taken by spacecraft such as the Japanese satellite Yohkoh and the European satellite SOHO.

The Corona

The solar corona is the highest layer in the Sun's atmosphere and the place where the solar wind originates. The distribution of gas in the corona, as revealed by X-ray images taken from satellites, is strongly affected by solar magnetic fields, but the coronal fields themselves cannot be measured directly. By extrapolating the magnetic field and current data at the Sun's surface, however, it is possible to calculate the magnetic fields in the corona and relate these to the structure, temperatures, pressures, and other physical properties of the corona observed in the spacecraft images.

Jing Li studies the physical properties of the solar corona and their variation through the solar cycle. The strength of her research is to use the large amount of spacecraft data obtained in the last 8 years by Yohkoh and the last 5 years by SOHO. These spacecraft are equipped with  instruments that provide solar coronal images in X-ray, UV and white light from the surface of the Sun to radial distances up to 60 solar radii. One of the results of this study is the identification of  solar polar rays with low latitude plasma associated with active regions, meaning that the polar rays are not really polar in nature but are only seen in projection above the solar poles. 

• Jing Li's synoptic corona studies

New Solar Telescope Designs

Jeffrey Kuhn  is working on new ideas for telescopes which can see faint objects near the bright glare of, for example, the Sun. Such a prototype telescope for solar coronal observing, called SOLARC, will soon be working on Haleakala.  He is also involved with the SPHERIS satellite project to measure solar brightness, shape and radius properties with unprecedented precision: surface temperature variations of 0.1K, shape oscillations which affect the limb at the level of 1 microarcsecond, and radius changes of smaller than a milliarcsecond

• SOLARC page
• SPHERIS page