Science

The solar and heliospheric research group covers research topics that include the solar atmosphere, the space environments of the planets, and the interactions of the heliosphere with our galactic neighborhood. Many of the research topics are fundamental in nature. For example,

• What is the composition of the Sun, and what does it tell about the history of the solar system?
• How does the solar magnetic field cause the solar wind to form?
• What is the composition of the galactic environment, and what does it tell us about the history of the Galaxy?
• What are the fundamental processes leading to particle acceleration on the Sun, in the heliosphere, and elsewhere?
  

These issues are invaluable to those who work with space technology or pursue space exploration. These applications require accurate predictions of the heliospheric environment and energetic particles therein, not unlike understanding the meteorological forecast before you go hiking.

A more detailed summary of the research topics is given below, with relevant publications (to be linked at a later date) for each topic.

• The Solar System is believed to have been formed when material in the pre-solar nebular, formed by a supernova, coalesced into a disk. Gravitational instabilities gradually clumped material together forming the Sun and the planets. The Sun is composed of mostly hydrogen (74% by mass) and helium (25% by mass) with only 1% made up of other “heavier” elements. Due to the extreme temperatures in the solar atmosphere, the atoms are ionized. These ions are accelerated away from the Sun and form the solar wind. By studying the composition of the solar wind with detectors in space, we can determine what the Sun is made of and therefore the composition of the pre-solar nebula.

• The core of the Sun has temperatures near 15 million degrees Kelvin which result from the thermonuclear reactions that have kept the Sun burning for almost 5 billion years, and will enable it to shine for another 5 billion years. The temperature decreases as you move away from the center of the Sun until it reaches around 5000 degrees Kelvin in the photosphere. However, as you enter the Sun's corona, the temperature rapidly increases up to 1 million degrees Kelvin. The Sun's magnetic field is generated in the outer layers of the solar atmosphere via a dynamo, not in the center near the core. The strange increase in temperature far from the core is unusual since you would expect a steady decrease in temperature as you move away from the energy source. This strange temperature profile tells us that energy is being released in the corona, heating it up to millions of degrees Kelvin. The source of this heating has nothing to do with what is going on in the center of the Sun, but it believed to be the result of the solar magnetic field rearranging itself through “reconnection”. This “reconnection” or change in the connection between field lines releases vast amount of magnetic energy. This energy heats and ionizes particles in the corona to high temperatures.

Interplanetary space is nearly a perfect vacuum, with less than 5 particles per square centimeter near the earth. Since the Sun is dense and hot and interplanetary space is cold and a vacuum, the Sun's corona expands into interplanetary space. The expanding charged particles and magnetic field that originate in the corona flow outward to form the solar wind. This solar wind can be detected at Earth and beyond and detailed examination of its properties can give us insight into the processes at work in the corona.

• The Sun's magnetic field reaches far out into interplanetary space, far beyond Pluto, to form what is called the “Heliosphere”. The Heliosphere is the region of space that is filled with the Sun's magnetic field and therefore processes that occur within this sphere are directly affected by the magnetic field. The solar wind is made of more than just the charged particles that are accelerated out of the corona. The heliospheric magnetic field (or the heliospheric extension of the corona magnetic field) affects all charged particles traveling in space, including those that originate outside of our solar system. In general, charged particles originating outside of our solar system are deflected away when the encounter the heliospheric magnetic field. However, interstellar neutral particles can pass unimpeded through the heliospheric magnetic field. Sometimes, these particles pass close to the Sun and charge exchange with the solar wind ions becoming singly charged interstellar ions. These so-called “pickup ions” represent material that originated from outside of the solar system. Because most solar wind particle detectors that we build on Earth to fly in space can only look at charged particles, we can identify these singly charged pickup ions. By studying these pickup ions, we can learn about the composition and history of the galactic environment around the Sun.

• We mentioned above that the solar wind forms as a result of the competition between the cold vacuum of space and hot dense corona. The specific mechanisms that actually accelerate the solar wind at the corona and other processes that accelerate particles in the heliosphere have not been agreed upon widely. Shocks, plasma waves and instabilities, and magnetic reconnection have been theoretically shown to accelerate particles up to high energies. Shock waves can accelerate particles through by bouncing particles back and forth across the shock front gaining speed with each bounce. When particles oscillate in union with plasma waves, they can "surf" with the wave speed gaining energy like a surfer being pushed onto the shore. When reconnection occurs, the energy released can be imparted into the ion’s kinetic energy, thereby accelerating the ion. However, the exact mechanism and the relative importance of each phenomenon are not well understood.