Sun and Heliosphere
The Sun - the gaseous giant
The Sun is a large ball of gas, 1.4 million kilometer across, consisting primarily of hydrogen and helium, and taking up 99.8 percent of the mass of the solar system. At its center the temperature is 15 million degrees Celsius, hot enough to force hydrogen nuclei to combine to form helium. The energy that is released by this process is transported by radiation and gas currents (convection) to the visible surface of the Sun, where the temperature is 6000 degrees, and is then radiated out into space.
The gas currents are thought to generate magnetic fields in the interior of the Sun, causing dark spots and other features to appear on the surface. In the very extended corona, which can be observed without special equipment only during solar eclipses, the temperature again increases to over a million degrees. The hot gas in the corona remains predominantly bound to the Sun’s magnetic field, as though enclosed inside a cage. However, some of this gas escapes, streaming as the solar wind through interplanetary space with speeds of up to three million kilometers per hour.
Researchers at the Institute study the entire gamut of the Sun’s dynamic and often spectacular processes from the solar interior to the external heliosphere. The main focus of the research is the magnetic field, which plays a decisive role in these processes. Answers to the most basic questions are sought: How is the magnetic field generated? Why does it vary in an eleven-year cycle? How does the Sun’s magnetic field hold the various layers of the solar atmosphere together? How is the corona heated to several million degrees?
New fundamental knowledge about the Sun has been obtained with instruments (co-)developed by the Institute on board the space probes SOHO and STEREO. The measurements from the UV spectrometer SUMER on SOHO have led to the recognition of the decisive role of the magnetic field in dynamic processes, while STEREO allowed for the first time 3D observations of the Sun and the inner heliosphere.
The Sun - a life-giving star
The Sun’s radiation brings warmth to the Earth, without which life could not exist. Therefore, any changes on the Sun must be examined for their possible effects on the Earth’s biosphere. Using satellite-borne detectors, researchers have discovered that the total brightness of the Sun varies by about 0.1 percent during the eleven-year magnetic field cycle. Although this variation is very small, it could influence the sensitive equilibrium in the terrestrial climate.
Explosive eruptions on the Sun propel clouds of gas and magnetic fields into space that come into contact with the Earth’s magnetic field. Particularly strong solar storms can penetrate this natural shield, leading to intense auroral activity ("northern lights"), to disruptions in radio transmissions, or even to damage in communication satellites, telephone and power lines. An important research area at the Institute is the investigation of the effects on the Earth resulting from variable activity on the Sun. To this end, the scientists are greatly involved in the NASA project STEREO, in which two satellites are to observe disturbances on the Sun from different directions, allowing predictions of potentially dangerous outbursts.
Current and future projects concentrate on the investigation of the physical causes of these variations on the Sun. The telescope Sunrise, built under leadership of the MPS, observed the structure of the magnetic field in the lower layers of the solar atmosphere with unprecedented resolution during two flights in the years 2009 and 2013, carried by a balloon to a height of about 35 kilometers.
The ambitious ESA mission Solar Orbiter, which originates from our Institute’s proposal, plans to launch a spacecraft in 2017, that will approach the Sun at a distance of 28 percent of the Earth-Sun separation, in order to study the magnetic field and its effects in different layers of the solar atmosphere.