Firsthand KNOWLEDGE SOLAR SYSTEM Research The Pulse of the Sun The Sun appears to us as a calmly blazing ball of gas in the sky. In reality, it is seething inside and the entire mass vibrates like a bell. Much as geophysicists use waves caused by earthquakes to explore the structure of our planet, solar researchers use the vibrations of our star to gain insights into its interior. This is how LAURENT GIZON from the MAX PLANCK INSTITUTE FOR SOLAR SYSTEM RESEARCH in Katlenburg-Lindau studies the Sun’s magnetic field. And he wants to apply this method to distant stars, as well. n the interior of the Sun, parcels of servations of the solar surface. This Ihot gas rise to the surface, cool off is possible today thanks to a world- and descend again. Physicists call wide network of ground-based tele- this rise and fall of matter convec- scopes, and above all to the joint Eu- tion, which occurs very similarly in ropean-American space observatory a saucepan. This results in the for- SOHO, which has been recording one mation of a granular pattern that image per minute of the Sun’s sur- can be seen on the Sun. This convec- face velocity field almost without in- tion also produces sound waves that terruption since 1996. propagate through the Sun and A RICH SPECTRUM cause it to vibrate, like a clapper hits OF OSCILLATIONS a bell. Researchers working with Robert Leighton at the California In- Today we know that the five-minute stitute of Technology first discovered oscillation Leighton discovered is a this phenomenon some 40 years ago. superposition of millions of modes They registered a five-minute oscil- of oscillation of various frequencies. lation on the solar surface. The manner in which the frequencies Shortly after this observation, it and wavelengths are coupled pro- was speculated that these vibrations vides information about what kind might be caused by acoustic waves. of waves they are, and also about the German solar physicist Franz-Lud- interior structure of the Sun. Solar wig Deubner confirmed this inter- physicists study these relationships pretation with improved observa- in a power spectrum. There, the low- tions carried out in 1975, and a new ermost branch at the lowest frequen- discipline was born: helioseismology. cies corresponds to surface waves The Sun, which appears in this image from the SOHO The study of solar oscillations re- that propagate horizontally, similar satellite, looks quite peaceful. However, the arcs of light on quires long, nearly continuous ob- to those on the surface of a deep the left and right edges and the grainy surface indicate that the star that gives us life is in a constant state of turmoil. : SOHO/ESA/NASA HOTO P 48 M AXP LANCKR ESEARCH 4/2005 4/2005 M AXP LANCKR ESEARCH 49 Firsthand KNOWLEDGE SOLAR SYSTEM Research photons release a portion of their rect: contrary to what was previous- IZON WEDEN G WEDISH S , S energy in countless absorption and ly assumed, neutrinos are not mass- OYAL AURENT R re-emission events involving atoms. less. They can change “identity” on /L TOCKHOLM In the upper layers, however, which their way from the Sun to the Earth, , S ESEARCH become gradually thinner and cool- and escape detection. For the neutri- R ENTER C HYSICS OF THE P er, the behavior changes. This is no detectors on Earth, a fraction of YSTEM S OLAR S NIVERSITY where convection occurs, with gas the neutrinos is invisible. OLAR U bubbles transporting the energy. Since these initial successes, helio- S OVA FOR N One of the first triumphs of helio- seismology has made great progress. NSTITUTE FOR LBA : MPI ., I , A seismology was the finding that this Today, it is used to measure, for ex- HOTO P CIENCES outer convective envelope extends to ample, the gas motions in the solar S a depth of 0.71 solar radii, making it interior or the effect of the magnetic CHARMER ET AL possible to identify the correct model field on waves. The search for the ori- CADEMY OF A : G.B. S : G.B. of the convection zone out of many gin and variability of the solar mag- HOTO P widely differing predictions. Further- netic field is of central importance. more, the previous values for opaci- This magnetic field controls solar ties, which describe how much light activity, which varies with a period can pass through matter, had to be of about 11 years. At times of strong Cross-section of the Sun showing a mode of oscillation. The enlarged blow-out Things are particularly complex in the vicinity of sunspots. revised to reconcile the temperature activity, massive flares occur on the shows the gas flows beneath a sunspot, as identified with local helioseismology. They are exciting objects of study for helioseismologists. profile in the models with the helio- surface, and gigantic plasma clouds The Earth is depicted in the bottom right corner for size comparison. seismological data. It would not have are ejected from the outer solar at- been possible to measure opacities in mosphere, the corona, and tear number of physical parameters are laboratories on Earth, because the through the planetary system at usually low, coincided with a period inferred, such as temperature, densi- extreme conditions in the solar inte- great speed. of cold weather – the height of what ty, pressure and gas flow velocity as rior cannot be reproduced here. is now known as the Little Ice Age. FLARES PARALYZE a function of depth and heliographic Another great contribution of he- Thus, the analysis of solar magnetic ELECTRICAL NETWORKS latitude. lioseismology concerns the solar activity could have a very direct im- , MDI-SOHO/ESA/NASA : MDI-SOHO/ESA/NASA HOTO Global helioseismology can al- neutrino problem. Neutrinos are ele- Increased solar activity impacts our pact on our day-to-day lives. P ready look back on numerous suc- mentary particles that are created in planet in widely different ways. One It is not yet understood why these OSOVICHEV cesses. The standard theory of stellar great numbers by nuclear reactions or two days after a flare, a plasma eruptions occur. Physical models are : A. K HOTO structure plays a central role in mod- in the Sun’s core. They pass through cloud might collide with the Earth’s required, however, to predict “space P ern astrophysics. Stellar models not the Sun nearly unimpeded and es- magnetic field. Under certain cir- weather,” which is a subject of grow- only describe the structure of the cape into space. The standard solar cumstances, some particles can pen- ing worldwide interest, both in re- stars, but also predict how they model predicts quite accurately how etrate the magnetic cage. Then they search and in some industry sectors. evolve. These models also enable as- many of these solar neutrinos should shoot into the atmosphere and, upon It is believed that eruptions occur There are significant variations in the speed trophysicists to narrow down the age reach the Earth. Researchers were colliding with atoms and molecules, when magnetic loops above sunspots at which waves propagate below a sunspot. High wave speeds are shown in red. and chemical composition of galax- thus surprised when their instru- produce impressive northern lights. snap into a new, less energetic con- ies and the universe as a whole. The ments registered about 30 percent The high-energy particles can also figuration, releasing energy. Solar Solar waves are seen in the oscillation power Sun serves as an important test case fewer neutrinos than expected. pose a threat to astronauts, they can storms occur preferentially at the year Hale magnetic cycle. These phe- spectrum. Each branch in this diagram provides for stellar models because it is our At first, some scientists suspected damage or even completely incapac- height of the 11-year solar cycle, nomena are somehow linked to the us with a relationship between frequencies closest star and allows for very pre- that temperature and pressure in the itate satellites orbiting the Earth, and when sunspots and magnetic regions Sun’s global magnetic field. Accord- and wavelengths of solar oscillations. cise observations. Sun’s core were lower than the stan- they can interfere with radio and are larger and more numerous. At the ing to current theories, the magnetic dard model predicted. This would re- mobile communications. In extreme same time, during the course of the field originates in the outer layers of TRANSPORT OF ocean. All other branches correspond sult in the creation of fewer neutri- cases, they may cause an entire pow- cycle, sunspots preferably occur at the Sun, where convection takes SOLAR ENERGY to various overtones of the acoustic nos. This hypothesis, however, er grid to collapse. different heliographic latitudes: when place. There, matter not only rises modes of oscillation. At the longest A key issue in stellar modeling is proved to be untenable, as helioseis- There is currently a heated debate activity decreases, the spots appear and falls, but also rotates around the wavelengths, the oscillations reach how the energy in the core – where mology ultimately confirmed the about whether fluctuations in solar ever closer to the solar equator. Fur- Sun at high speeds in a well defined the deepest layers of the sun. it is released by nuclear fusion reac- standard solar model. Thus, the solu- activity will impact the climate in thermore, it is observed that sunspots way. Magnetic field lines are coupled In global helioseismology, the tions – reaches the surface. Light tion to the solar neutrino problem the long term. In Europe for in- usually occur in pairs of opposite to the solar plasma and get stretched measurements of the frequencies of particles (photons) transport the en- had to be found elsewhere.