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They regularly appear and disappear on the Sun’s surface – sunspots indicate how active the star is. The 11-year solar cycle of the Sun includes periods known as solar maxima – during which many sunspots are visible – and solar minima – with few sunspots. Large sunspots indicate spectacular events that can be connected with enormous coronal mass ejections. Researchers are trying to more accurately predict such activity because it influences space weather, which also affects Earth. Bernhard Kliem researches the physical processes triggered by solar eruptions at the Institute of Physics and Astronomy. The Sun’s magnetic field plays a decisive role.
Non-experts may see only white and black spots unevenly distributed on a grey background, but Bernhard Kliem recognizes patterns in these spots, which reveal something about solar activity and impending solar storms. On his monitor is the star’s surface, a map of the magnetic field the researcher is studying. The spots that particularly interest the physicist look rather unspectacular. In reality, these spots indicate that dramatic processes are taking place on and in the Sun: immense electromagnetic forces that sometimes discharge in solar eruptions. Kliem researches their formation.
The researcher uses images that are mainly taken by the NASA Solar Dynamics Observatory (SDO). The space probe has been measuring the Sun’s radiation, magnetic field, and vibration in geostationary orbit since 2010. These freely accessible data enable various examinations of solar processes worldwide that are important for our “living with a star”.
“The Sun is in a so-called plasma state, the fourth state of aggregation,” Kliem explains. Its core has an unbelievable temperature of unbelievable 15.6 million degrees Kelvin and a pressure corresponding to the weight of the Cheops Pyramid on a pinhead. This is where the nuclear fusion of hydrogen nuclei into helium occurs, which releases gigantic quantities of energy. Electrons are separated from the nucleus due to the enormous heat. “The electrical forces between positive and negative particles produce a gas that behaves like a highly conductive electrical liquid,” Kliem explains the specifics of the plasma state.
The energy initially created internally is emitted in the form of light particles and is then radiated out into space from the Sun’s surface, which is relatively cold at approximately 6000 degrees. In the in-between convection zone (the Sun’s outer third), however, so-called convective energy transport dominates, in which energy is transported through flux. “The principle is the same as in a kettle,” Kliem explains. The plasma heated from below rises, cools, and sinks back towards the core, where it is reheated and rises again. “Since the plasma is electrically conductive, these fluxes lead to a dynamo effect that creates the solar magnetic field,” the scientist continues. This is the basis of his research because the magnetic field and the motion of plasma matter are coupled.
The magnetic fields primarily in the Sun’s interior sometimes expand to the surface and become visible as sunspots, usually as a pair with opposite polarity. Strong magnetic fields suppress convective energy transport. The surface temperature is a few hundred degrees lower in these areas than in the surrounding area, as evidenced by the characteristic dark spots. In contrast, it is especially hot in the corona above sunspots – i.e. in the solar atmosphere. Researchers call this phenomenon coronal heating, which ensures that the temperatures above the Sun's surface rise again, to over a million degrees – so high that the corona expands. The resulting solar wind – the outflowing of plasma and the magnetic field – fills the interplanetary space. “How exactly coronal heating works is still a mystery,” explains the physicist. Researchers know that the magnetic field and its oscillation seem to spur this process. It forms a kind of tunnel system that channels internal energy outward into the corona.
Powered by the convection inside the Sun, the magnetic field is subject to constant change. Sunspots, for example, expand to become several times greater than the diameter of Earth and then disintegrate into increasingly smaller fragments; this may take anywhere from several days to several months. Also changing are the magnetic arches that connect positively and negatively polarized sunspots and form an “active region” in the corona. The arches act as magnetic bottles. The trapped hot plasma becomes visible in UV and X-ray images. They can, however, also capture cooler plasma of “only” 10,000 degrees Kelvin, which then floats majestically as a solar prominence in the red light over the solar limb, against the immense gravity and held together by electromagnetism.
A changing magnetic field induces electric currents in plasma. These currents contribute to the energy stored in the active regions. If this exceeds a critical value, the balance of forces becomes unstable. Any small disturbance then results in the accumulation of dramatic electromagnetic forces – which convert the stored magnetic energy like in an explosion into kinetic energy – as well as the conversion of heat of up to 100 million degrees and radiation ranging from radio waves to gamma rays. The magnetic compounds in the active region are virtually torn apart by the eruption. Such solar prominences and their surrounding hot plasma are often ejected into interplanetary space at 300-3000 km/second. There they expand into plasma clouds, many times larger than the Sun and spread throughout the solar system, temporarily intensifying solar wind. This can cause serious disturbances when they hit Earth between one and three days later.
Kliem uses a method with the complicated name of magnetohydrodynamics to analyze the Sun‘s plasma, magnetic, and energy flows. A computer-based model displays the development of the magnetic field in complex calculations and allows him to draw conclusions on the ongoing plasma motions and electric currents. Kliem investigates the interaction of hot plasma and the magnetic field using numeric simulations.
“This region might be able to trigger powerful eruptions,” Kliem explains, pointing to the magnetic field map on his monitor. It shows SDO pictures from October 2014. At this time, the Sun was at a peak of activity. The Sun's surface begins to appear in slow motion – the short film shows one image per hour. The characteristic spots are visible on the left margin of the Sun, an entire sunspot group that seems like it is slowly moving to the right margin due to the Sun’s rotation. “Solar flares can occur near strong magnetic fields,” explains Kliem. The motions of individual sunspots – whether rotating or moving past each other – provide the researcher with information on the stored energy and probability of an eruption.
Kliem’s computer model simulates what happens to individual magnetic field lines in a solar magnetic field. The researcher is especially interested in a phenomenon he calls “torsion” (or “twisting”). Through the Sun’s Coriolis force, ascending and descending plasma rotates – like low and high pressure areas on Earth. This rotation twists the embedded field lines. This becomes even stronger when sunspots rotate around their own center or around each other. “This means strong electric currents,” says Kliem. He determines the threshold value for the instability of magnetic domains in his simulations, which is at about one and a half revolutions of the field lines around the axis of the bundle. If a solar prominence becomes trapped in the magnetic field, its delicate filament structures trace the path of a few field lines and, in some cases, allow for an estimate of the torsion. This often approximates the calculated threshold before an eruption. The smaller the remaining distance, the smaller the impetus – a disturbance from inside the Sun or an adjacent active region –required to trigger the eruption.
The torsion rate is not always high. Another instability must be at play in such a case, and this is what Kliem is currently researching. It is based on the force between the current through a solar prominence and the current passing between its base points inside the Sun, closing the circuit. Forces here mimic those of an electric motor. Solar prominences can lead to solar eruptions if they have risen high enough and the surrounding magnetic field can no longer hold them in the corona. There is, again, a threshold value that Kliem wants to determine exactly.
The physicist “builds” the eruptions mathematically with models, gaining insight into their development – under what conditions they reach high speeds or move in a particular direction. These properties determine the strength of the geomagnetic disturbance if the mass ejection ends up hitting Earth. These models allow the researchers to better understand the necessary conditions for the so-called solar storms of space weather. Kliem tempers expectations for a reliable prediction model, similar to the daily weather report, because the triggering disturbances from the solar interior remain unpredictable. Nobody can see into the Sun’s interior. “In the coming years, we will have to sharpen our ‘ears’ for the Sun using helioseismology, which uses earthquake research methods,” explains Kliem.
The events that the researcher simulates with the model are 150 million km away from Earth. His research, however, has a very practical aspect for our existence. Activity on the Sun ultimately determines space weather. There is an increasing interest in improving forecasting because strong solar eruptions interfere with radio communication and satellite navigation on Earth, affecting civil and military aviation and maritime transportation. They can damage satellites and, in extreme cases, even destroy transformers in extensive power grids, especially ones at high latitudes.
The plasma clouds rushing towards Earth create the alluring phenomenon of the aurora borealis. Earth’s magnetic field first acts as a protective shield, guiding the plasma around Earth. On the “night side” of Earth, the compression of the magnetotail accelerates the charged particles moving along the magnetosphere’s field lines towards Earth. Wherever the magnetotail’s flux lines penetrate the atmosphere – near the North and South Poles – the accelerated particles collide with nitrogen and oxygen molecules in the atmosphere, which ionize and trigger glowing. These energetic interactions brighten the night sky with an impressive play of colors - a very special greeting of the Sun.
Dr. Bernhard Kliem studied physics at Humboldt-Universität zu Berlin and has conducted research at the University of Potsdam since 2010. His research interests focus on solar activities and the principles of plasma physics.
Institut für Physik und Astronomie
The research project “Triggering Solar Eruptions” investigates the physical processes that lead to solar eruptions.
Participating: University of Potsdam, Institute of Physics and Astronomy
Funding: German Research Association (DFG)
Text: Heike Kampe
Translation: Susanne Voigt
Published online by: Daniela Großmann
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