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
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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. It is clear stance, the period between 1640 and magnetic polarities. Within sunspot and twisted by the flows. The internal the millions of modes of oscillation ergy out of the solar core. While today that it was the standard model 1710, for which it has been deter- groups, magnetic polarities reverse rotation of the solar plasma varies are used to create a solar model. A traveling through the solar plasma, of particle physics that was incor- mined that sunspot activity was un- every 11 years, resulting in the 22- both perpendicularly to the surface
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cycle. They discovered that the rota- and flows along the internal propa- tion varies not only with heliograph- gation paths of the waves. This , NSO/NASA ic latitude, but also with time: latitu- method pinpointed the existence of
ATHAWAY dinal bands of faster and slower ro- an internal flow between the equator , MDI-SOHO/ESA/NASA
: D. H tation migrate toward the equator. and the poles that could serve as a IZON
HOTO This time-varying rotation pattern mechanism for the latitudinal trans- P exhibits a periodicity of 11 years and port of the magnetic flux and deter- : L. G HOTO occurs throughout the upper half of mine the period of the solar cycle. P the convection zone. This could indi- As I was studying local processes cate a “migrating wave,” which some in the near-surface layers, I found dynamo theories predict. complex horizontal flows – also re- Flows at a depth of 1,000 kilometers (blue arrows) converge toward magnetically active regions on the surface (red and green patches). Sunspots usually occur in pairs with opposite magnetic polarities. In the In addition, mysterious, quasi- ferred to as solar subsurface weather. course of the 11-year activity cycle, sunspot pairs not only appear at varying periodic changes in the rotational The plasma flows appear to be high- heliographic latitudes, but also change polarities (shown in yellow and blue). velocity, with a period of 1.3 years, ly organized near large, magnetical- were discovered near the tachocline. ly active regions. Away from the ac- to disentangle. However, the discov- tions. Nevertheless, it is possible to This discovery has not yet been ex- tive regions, there are meanders, jets ery of organized flows beneath extract two basic parameters from and with heliographic latitude. This mostly constant along radial lines: plained, and makes clear that we are and vortices that may be related to sunspots could help explain why the the frequencies of these modes. The plays a key role in solar dynamo the- in the equatorial regions, the solar still far from a full understanding of deep large-scale convection. sunspots persist as stable structures first is the time it takes for sound to ory. The non-uniform rotation of the matter rotates about its axis once the internal dynamics of the Sun. I also analyzed relatively small- for many weeks. travel across the star’s diameter. This Sun shears the magnetic field lines every 25 days, while at high lati- scale phenomena. An example of this Local helioseismology is still de- is a global characteristic that is close- SEARCH FOR ORIGIN and pumps energy into the magnetic tudes it takes 35 days. This is in is supergranulation, which corre- veloping, but it promises many more ly linked to the average mass density, OF SOLAR ACTIVITY field. The magnetic field lines store agreement with visual observations sponds to convection cells with a di- discoveries. One of its numerous am- and thus the star’s mass. The second this energy just like a rubber band of the solar surface. The entire radia- Traditional helioseismology views ameter of around 30,000 kilometers. bitious goals is to directly image the is related to the helium content in the that is stretched and twirled. tive zone, on the other hand, appears the Sun as a body that is completely These convective motions play an magnetic field in the solar interior. core of the star. This offers a unique Motions in the north-south direc- to rotate like a rigid body, with a pe- symmetric around its axis of rota- important role in the redistribution of This will require measuring local opportunity to accurately determine tion are also considered to be one of riod of about 27 days. This means tion. Most recently, we have begun the magnetic flux on the solar sur- anisotropies in wave propagation, as a star’s age because the helium con- the causes of the polarity reversal of that there is a strong shear in the ro- to analyze the amplitudes and the face. We were surprised to discover wave speeds differ along and across tent in its core increases with time as the global magnetic field every 11 tation of the solar matter at the tran- phases of the solar oscillations to that the pattern of supergranulation magnetic field lines. nuclear fusion reactions convert hy- years, but a lot of things remain in sition between the radiative and the study the Sun in three-dimensions. propagates in the direction of the drogen into helium. QUAKES ON the dark in this regard. One of the convective zones. It is assumed that This leads to “local” helioseismology. Sun’s rotation faster than the local The frequencies of stellar oscilla- DISTANT STARS main goals of helioseismology today the solar dynamo originates from This method is similar, in a way, to plasma flow, like a traveling wave. tions include even more information. is to map these internal mass motions this transition layer – also known as ultrasonic tomography used in medi- However, we are still a long way The successes of helioseismology They could lead to the detection of and their temporal variation to help the tachocline. cine. This method measures the time from understanding supergranula- have encouraged astronomers to also key features of stellar interiors, such solve the puzzle of the solar cycle. Recently, through helioseismologi- it takes for solar waves to propagate tion, and we need further observa- apply this method to stars. Unlike the as the boundaries of convection Significant progress has already cal analysis, researchers even suc- through the interior between any tions and refined theoretical models. Sun, stars, due to their enormous dis- zones. This would allow us to refine been made. For example, helioseis- ceeded in establishing a connection two locations on the surface. The The oldest known phenomenon on tance, appear as point-like objects. the theory of energy transport by mology revealed that, throughout between internal mass movements propagation times provide informa- the Sun, the dark spots mentioned Thus, the oscillation modes that can convection, which currently remains the convection zone, rotation is and the characteristics of the solar tion about buried inhomogeneities above, are now also one of the ob- be observed are averaged across the very approximate. In order to under- jects of helioseismology research. entire surface. The challenge for as- stand stellar structure and evolution, Sunspots are regions in which the tronomers in this young field of as- however, it is important to measure energy supply from the interior is teroseismology has been to measure oscillations on stars with differing impeded, and their temperature is oscillations with sufficient accuracy masses and ages. In principle, aster- about 1,500 degrees less than that of in order to provide useful constraints oseismology can also be used to esti- the quiet surface (photosphere) – on internal stellar structure. This on- mate internal rotation, as is done for which is why they appear black in ly became possible in recent years for the Sun. Such information could MDI-SOHO/ESA/NASA contrast. Researchers have managed sun-like stars with the aid of large, help understand stellar activity cy- to measure the three-dimensional ground-based telescopes. cles and dynamo theory. CHOU AND structure of wave-speed perturba- Since, as already mentioned, the Based on numerical simulations, I : J. S
HOTO tions below sunspots. These pertur- surface of a star cannot be observed recently showed that it is even possi- P bations are caused by a combination as a disk, we can detect only the sim- ble to determine the inclination of of magnetic and temperature anom- plest modes of stellar oscillations – the rotation axis of a star, which is alies that we have not yet been able radial, dipole and quadrupole oscilla- particularly interesting in the case of
The rotation of the solar interior varies during the 11-year cycle. Red shades indicate higher rotation velocities than average, while blue shades indicate slower rotation.
52 M AXP LANCKR ESEARCH 4/2005 4/2005 M AXP LANCKR ESEARCH 53 Firsthand KNOWLEDGE . UTLER ET AL : R. B HOTO P
The Very Large Telescope was able to measure a number of modes of oscillation on Alpha Centauri, providing information about the interior of this star. The inset shows the solar spectrum of oscillations for comparison.
binary stars and central stars of work. Our team is involved in the stars. The WIRE and MOST satellites planetary systems. development of Solar Orbiter. have already detected stellar oscilla- Helioseismologists require high- Asteroseismology, too, is entering tions based on fluctuations in light quality oscillation spectra, and aster- an exciting phase, as it benefits from intensity. ESA’s COROT satellite, oseismologists also hope to obtain the search for planets orbiting dis- which is expected to be launched in measurement data for as many stars tant stars. This is because this search 2006, will be even more powerful. as possible. We are looking into a requires the measurement of minute And additional, similar missions are promising future. NASA is taking the periodic fluctuations of the star currently being planned. lead on the next major technological around the common center of the We thus hope that asteroseismolo- step for helioseismology with the extrasolar system. In a spectrum, gy will make great progress in the Solar Dynamics Observatory (SDO), they have a similar signature as coming decades. One day, it may which is set to launch in 2008. On global pulsations. The motion of the even be possible to use optical inter- board will be the first instrument star as a whole, however, is much ferometry to spatially resolve hun- that was specifically developed for slower than the pulsations – so it is dreds of oscillation modes on a sin- Why one of Europe’s most trusted local helioseismology. One of the easy to distinguish the two. gle star, as has been done for the names in light is in the spotlight. most important scientific goals is to Sun. New discoveries depend, how- SEISMOLOGISTS RELY study the fine structure and the tem- ever, not only on new observations, ON NEW INSTRUMENTS poral evolution of magnetic regions but also on improved theoretical and subsurface flows. My team at The best measurements are currently models. ● You already know that Oriel is a leading name in light sources and instrumentation. But what you the Max Planck Institute for Solar being made at the European Southern probably don’t know is that Oriel is now part of the Newport Corporation. System Research will be helping to Observatory in the Chilean Andes,
analyze the SDO data. using the HARPS spectrograph on a RIVATE Which means that the Oriel brand as you know it is Around 2013, the European Space 3.6-meter telescope in La Silla and : P HOTO
P now stronger than ever—with a bright future of new Agency (ESA) will send Solar Orbiter the UVES spectrograph on the Very DR. LAURENT GIZON (born in products, enhanced service and technical support. on a mission. This probe will leave Large Telescope. In the coming 1970) is head the plane of the solar system, the years, even more precise spectro- of the Junior Combined with the expertise and solutions provided ecliptic, to examine the Sun’s polar graphs will be installed on several Research by Newport, Oriel is now part of an all-encompassing solution for every facet of Making, regions. Also, the Solar Orbiter data large ground-based telescopes, mak- Group “Helio- Managing and Measuring Light™. Featuring modular instruments that can be configured and will be combined with that of other ing it possible to measure star veloc- and Asteroseismology” at the Max Planck Institute for Solar System reconfigured according to your needs. telescopes that look at the Sun from ities with greater precision than ever Research in Katlenburg-Lindau. a different angle. This will permit before. Gizon studied space technology in stereoscopic analyses to probe very However, observation time on the France at SupAero and astrophysics Look through any page of Newport’s new Oriel Light Resource Catalog, and you’ll see why this deep regions of the Sun. In particu- large telescopes is limited, which is at the University of Toulouse, and chapter in the Oriel story is the brightest yet. To receive your free copy, call your local office or visit lar, it will be possible to identify, in why dedicated space telescopes are obtained his Ph.D. in physics at Stanford University, California. www.newport.com/oriel1. It makes for enlightening reading. greater detail, variations at the base an attractive solution for obtaining He was awarded this year’s JOSO of the convection zone, where the nearly uninterrupted long-term ob- prize for solar physics for his work solar dynamo is presumed to be at servation of many types of pulsating on solar supergranulation. AD-110502-EN In 2004 Spectra-Physics including Oriel Instruments, Richardson Gratings, Corion Filters Opticon Mirrors and Hilger Crystals was acquired by Newport. Newport GmbH: 54 M AXP LANCKR ESEARCH 4/2005 ©2005 Newport Corporation Guerickeweg 7 – 64291 Darmstadt Telefon: 0 61 51 / 708 -0 • Telefax: 0 61 51 / 708 -954 • e-mail: [email protected]