Non-standard solar models to resolve the solar problem fail tests imposed by , suggesting that such models are rather contrived, with resolution of the neutrino problem lying in the realm of . Using Helioseismology to Constrain Solar Models The energy radiated by the derives Jørgen Christensen-Dalsgaard from the fusion of four atoms into one atom via different paths. Teoretisk Astrofysik Center, Danmarks Grundforskningfond The reaction generates approximately 4.1 x Institut for Fysik og Astronomi, Aarhus Universitet 10-12 J (the precise amount depends on the energy lost as ) and since the PP-I PP-II PP-III receives about 1360 Wm-2 from the Sun, the total flux of neutrinos must be 1H + 1H → 2D + e+ + ve 3He + 4He → 7Be + γ 7Be + 1H → 8B + γ around 7 x 1014 m-2s-1. The cross-section 2D + 1H → 3He + γ 7Be + e- → 7Li + ve 8Be → 8B + e+ + ve for neutrino absorption is so small that vir­ 3He + 3He → 4He + 21H 7Li + 1h → 4He + 4He 8Be → 4He + 4He tually all the neutrinos escape immediately from the Sun, in contrast to the radiative Fig. 1 — The most important hydrogen-burning reactions in the Sun, the so-called PP-I, PP-II energy which reaches the solar surface and PP-III chains. The initial reaction is the fusion of two hydrogen nuclei into a through a slow diffusive process since the nucleus ; charge and lepton number conservation demands emission of a and a neu­ mean-free path of a near the solar trino. This reaction controls the overall rate of energy production. The subsequent reaction path centre is only a few mm. Thus, the neutrinos depends on the branchings between the 3He + 3He and the 3He + 4He reactions, and between in principle provide direct information about the 7Be + e- and the 7Be + 1H reactions. The PP-III branch is very rare. Neutrinos produced in nuclear reactions in the . the 1H + 1H reaction, the capture in 7Be, and the positron decay of 8B, have average Calculating an accurate estimate of the energies of 0.26 MeV, 1.06 MeV and 7.46 MeV, respectively. Neutrinos are also emitted by the neutrino detection rate is not trivial for it rare triple reaction 1H(1He-,ve)2D, and modest contributions to the energy generation and the relies on models that follow numerically the neutrino flux come from hydrogen fusion through the so-called CNO-cycle. Sun’s evolution during the conversion of hydrogen into helium (Fig. 1 gives the domi­ less than 0.02%. So the constraint that There have been several independent cal­ nant reaction paths). Results depend on the computed should fit the ob­ culations of neutrino capture rates, with re­ nuclear reaction rates and on the assumed served value essentially determines the sults depending on the assumed physics. All physics of matter, and involve simplifying number of low-energy neutrinos from the recent calculations give similar results (e.g., assumptions, the most important probably 1H(1H,e+ve)2D reaction. In contrast, the flux [1]). Fig. 2 compares neutrino event rates being to neglect hydrodynamic processes of high-energy neutrinos, especially that for measured experimentally using 37CI and that may result in a partial mixing. The out­ the 8B(,e+ve)8Be reaction, depends on the 71 Ga (see insert overleaf) with those for se­ come is called a “standard” solar model and branching ratios between the different lected standard models, computed with dif­ analogous calculations for other form chains which are in turn strongly tempera­ ferent equations of state, opacity tables, nu­ the basis for many areas of astrophysics. ture-dependent, with the fraction of the reac­ clear parameters, or ages. Although it is dif­ Clearly, any discrepancy between the ob­ tions that go through the PP-II and PP-III ficult to assign statistically meaningful error served and the predicted rates of solar neu­ chains increasing very rapidly with tempera­ bars, calculations where the relevant para­ trino detection casts doubt on this basis. ture; so therefore does the flux of high-ener­ meters were varied within reasonable limits The initial helium abundance, which can­ gy neutrinos. The table overleaf presents indicate that the error in the computed cap­ not be determined reliably from spectro­ rates for a computed using ture rate is unlikely to exceed 2 SNU for scopic observations, is treated as a free representative and up-to-date values for the detection using 37CI and 10 SNU for the parameter, being adjusted until the com­ various variables. The number of detections 71 Ga experiment [3]. So the discrepancies puted luminosity agrees with observations. is conventionally given in between theory and experiment — collec­ The computations also involve a second a Units (SNU), where one SNU corresponds to tively called the — priori unknown parameter describing con­ 10-36 captures per target atom per second. discussed in the insert are highly significant. vective heat transport near the solar sur­ face; its value Is fixed by requiring the model to give the correct radius for the Sun. The rate of neutrino detection depends strongly on the neutrino energy. In standard solar models, about 85% of the energy production comes from the PP-I chain (see Fig. 1), with the PP-III chain contributing

Jørgen Christensen-Dalsgaard has been an Asso­ ciate Professor in the Institute of Physics and Astro­ nomy, Aarhus University, DK-8000 Aarhus C, since 1984 and was appointed this year to be the Associate Director of the Theoretical Astrophysics Center, Aarhus. After graduating from Aarhus University in 1975, he received his PhD from Cambridge University in 1978 and then held postdoctoral positions at the Fig. 2 — Measured and computed neutrino event rates for the Kamiokande (left) and Ga (right) University of Liege, the National Center for Atmo­ experiments. The measured rates are indicated by error boxes and the large filled circles and spheric Research, Boulder, CO (USA), and NOR- DITA, Copenhagen. Professor Christensen-Dals­ squares show the calculated results of Table 1, where the filled squares include the effect of gaard participates in the Global Oscillations Network WIMPs simulated through a reduction in the core opacity. The crosses are based on other Group and is a Co-investigator on the SOI and GOLF standard models, differing in , opacity tables, nuclear parameters, or age. The instruments to be launched aboard SOHO. filled triangles pointing down and up are for partially and fully mixed models, respectively.

Europhys. News 25(1994) 71 One cannot exclude a “boring" solution to increased, the gradient, and Mixing might bring additional hydrogen to the neutrino problem involving modification hence the neutrino flux, is reduced. This can the solar core, allowing the of, for example, opacities and the branching be accomplished through an overall reduc­ to be generated at lower temperature and ratios in the PP chains. In particular, since tion of the opacity, e.g., by reducing the hence reducing the predicted neutrino cap­ the experiments seem consistent abundance of elements heavier than H2 and ture rates. Fig. 1 also shows results for mo­ with the neutrinos from the reaction which He. Alternatively, the efficiency of energy dels with partial and complete mixing of the dominates the energy generation, one may transport can be increased by invoking other solar interior; only the latter gives a pre­ in principle be able to reduce the capture modes of transport. dicted rate for the 37Cl experiment in accor­ rate while maintaining the solar surface One suggestion is that the solar core con­ dance with observations. luminosity. Further measurements of the tains hypothetical weakly interacting mas­ relevant nuclear reactions, particularly the sive particles (WIMPs) which transport a Conclusions somewhat uncertain 7Be(1H,γ)8B reaction, significant fraction of the luminous output in In conclusion, some experimental results are required. The possibility of remaining the solar core. I have simulated their pre­ are disturbing, notably a measured flux experimental problems must also be taken sence through a localized reduction in the which is smaller than the predicted value, in into account [4]. opacity in the solar core to give the neutrino some cases by factors of 2-3. Whereas rates shown in the table and Fig. 2. It is evi­ initial experiments were only sensitive to Non-standard models dent that the 37CI and the 71 Ga capture high-energy neutrinos coming from rare Most attempts to make the predicted neu­ rates are consistent with the measurements. reactions that contribute little to the energy trino capture rate agree with observations Note, however, that for the 37CI experiment generation and have rates which depend Involve changes in the temperature distribu­ the contribution from 8B is reduced much very strongly on the temperature of the solar tion In the solar core to reduce the central more strongly than the contribution from core, it has been difficult to achieve better temperature, and hence the flux of high- 7Be. As a result, the predicted rate for the agreement via minor adjustments to the energy neutrinos, while retaining the total Kamiokande experiment which only sees model’s parameters given that the observed energy production. The temperature gra­ neutrinos from 8B is now too low. This is solar luminosity should be reproduced. Al­ dient in the solar interior must be sufficient a general problem in attempts to match though the discrepancy is less marked, the to ensure transport of the energy from the simultaneously the 37CI and Kamiokande observed detection rate is once again less core to the surface. Therefore, if the effec­ measurements through modifying solar than the predicted rate for the more recent tive conductivity of the solar Interior is models [3]. experiments which detect neutrinos coming from the principal energy-generating reac­ tions. In general, attempts to solve the neu­ MEASUREMENTS OF THE SOLAR NEUTRINO FLUX trino problem by adjusting the solar stan­ Owing to the small neutrino cross-sec­ to the 8B neutrinos. The observed rate of dard model are essentially ad hoc since tion, measuring the flux of solar neutrinos neutrino events corresponds to a flux of they have little independent justification requires carefully designed, large detec­ 8B neutrinos of about (2.7 ± 0.5) x 1010 beyond the phenomenon being explained. tors. The first experiment used the reac­ m-2s-1, roughly half the theoretical value Hence the importance of helioseismic data tion ve + 37Cl → e- + 37Ar with a detector (see table). which may constrain models of the core consisting of a tank containing about Measurements sensitive to the neutri­ [5] and thereby test for departures from 380000 litres of C2Cl4 (a common clean­ nos from the basic 1H + 1H reaction can be standard evolution theory. ing fluid) deeply burled in the Homestake obtained from the reaction ve + 71 Ga → e- mine in South Dakota, USA, to reduce the + 71 Ge which can detect neutrinos of ener­ Solar Oscillations background from other reactions. Despite gy exceeding 0.23 MeV. Two such experi­ The Sun pulsates in many independent the large quantity of detector material, a ments have been set up: the SAGE colla­ modes of oscillation, with periods of about solar neutrino event is recorded on ave­ boration which uses a detector currently five minutes. The frequencies, which can be rage only once every second day; the consisting of almost 60 tonnes of metallic determined very precisely, depend on the average of the measured capture rates gallium, situated in southern Russia; and structure of the solar interior. Some of the over almost 20 years gives an observed the GALLEX experiment using 30 tonnes modes penetrate virtually to the solar centre capture rate of 2.2 ± 0.2 SNU. From the of gallium in the form of a gallium chloride so suitable combinations of frequencies give table we see that the dominant contribu­ solution, located in a tunnel under the information about the solar core. tion to 37CI observations comes from the Gran Sasso massive in Italy. The experi­ Although a variety of observational tech­ 8B neutrinos, despite the relatively low ments have given initial results over the niques have been used to study the oscilla­ flux, and that the computed value of about last two years, which appear to be consis­ tions of the Sun, the most extensive data 7 SNU does not agree with experiment. tent. In particular, an average over several come from measurements of the line-of- The Kamiokande II experiment has runs of the GALLEX experiment yields a sight velocity of the solar surface using the measured the scattering of neutrinos on capture rate of 87 ± 16 SNU. This is still Doppler shift of lines in the radiative spec­ . It is located in a mine in Japan below the predicted value of 128 SNU trum. Fig. 3 gives an example of a power and consists of a tank containing 3000 (see table), although by a smaller margin spectrum resulting from such observations, tonnes of water, of which 680 tonnes are than for the 37Cl and electron scattering in light integrated over the solar surface. used for neutrino detection. This experi­ experiments. Indeed, the table shows that Each oscillation mode depends on the posi­ ment records the direction of the neutri­ the rate of detection is roughly consistent tion on the solar surface as a spherical har­ nos, confirming that they do indeed come with the neutrino production from the pp monic Ylm(θ, φ) of degree / and azimuthal from the Sun; it is exclusively sensitive reactions. order m with |m| ≤ I, where θ is the angular distance from the pole and φ is the longi­ Predicted neutrino fluxes and capture rates in 37Cl and 71 Ga detectors for a tude. A mode is also described by its radial and for a model where the opacity has been reduced near order n corresponding in most cases to the the centre of the Sun. number of nodes in the radial direction. Examples of spherical harmonics are Neutrino Standard model Reduced temperature shown on the front cover. The degree I source Flux Capture rate Flux Capture rate measures, roughly speaking, the complexity 1014 m-2s-1 SNU 1014 m-2s-1 SNU of the mode on the solar surface, whereas 37Cl 71 Ga 37Cl 71Ga m is one-half the number of nodes in longi­ pp 6.01 X 10° 0.00 70.9 6.23 x 100 0.00 73.5 tude. In observations in integrated light, pep 1.43 X 1 0 -2 0.23 3.1 1.70 x 10-2 0.27 3.7 such as those shown in Fig. 3, the contri­ 7Be 4.65 X 10-1 1.12 34.0 3.03 X 10-1 0.73 22.2 butions from the higher / are largely sup­ 8B 5.13 X 10-4 5.59 12.5 1.22 x 10-1 1.33 3.0 pressed due to cancellation between re­ CNO 4.53 X 10-2 0.38 8.1 1.42 x 10-2 0.12 2.5 gions of positive and negative velocity. Total: 7.32 128.5 2.45 104.9 Hence, Fig. 3 is entirely dominated by

72 Europhys. News 25 (1994) Fig. 3 — The power spectrum of solar oscillations obtained from Doppler observations in light integrated over the disk of the Sun. The ordinate is normalized to show velocity power per frequency bin. The data were obtained by the Birmingham group from two observing stations, on Hawaii and Tenerife, and span 53 days [2]. Note the organization of the spectrum into uniformly spaced pairs of peaks (an example is indicated). Within each pair, the frequencies differ by a small amount δv which gives information about the properties of the solar core (see text). modes with I ≤ 3. The frequencies depend principally on n and I ; departures from spherical symmetry, such as rotation, intro­ duce a dependence on m, but this can be neglected in probing the overall structure of the solar interior. The individual spherical harmonics can be separated by spatially transforming obser­ vations of the velocity field as a function of θ and φ. The individual modes are then iso­ lated by carrying out a Fourier transform in time, leading to a power spectrum such as the one shown in Fig. 3. Solar oscillations the nature of the oscillations. Owing to their radius R of the Sun, is the inverse of the contain modes of all degrees from zero to comparatively high frequency, the observed time sound takes to travel across a solar about 2000, the distribution of power with modes are approximately standing sound diameter; a is determined by conditions frequency and the mean amplitude per waves. The speed of sound c increases near the surface, and enl is a small correc­ mode being roughly independent of I and m from the surface towards the centre. Assum­ tion term. If enl Is neglected, vnl ≈ vn-1 /+2 and hence corresponding to Fig. 3. Finally, ing the law, c2 ∞ T/µ where T Is and Eq. 1 predicts a strikingly simple spec­ the cyclic frequencies vnl of the modes are the temperature and the mean molecular trum: the peaks are uniformly spaced, alter­ determined through suitable fitting to the weight p is defined such that µ-1 is the num­ nating between modes of even and odd power spectrum. ber of particles per atomic mass unit. The degree, with a separation of Δv/2. This propagation of sound waves resulting from general behaviour is clearly visible in Fig. 3. Comparing observations with predictions this increase In c Is illustrated in Fig. 4: Also visible in the spectrum of Fig. 3 are The value of the frequencies as diagnos­ waves which start out from the solar surface the effects of the correction term enl: there tics of the solar Interior lies in the fact that obliquely relative to the vertical undergo is a small separation refraction as they propagate towards the they can be determined experimentally with δ vnl = vnl- vn-1 /+2 ≈ enl- en-1 /+2 (2) high precision, owing to the very long life­ solar interior, in the direction of increasing c. With decreasing degree I, the initial angle to between peaks that would otherwise coin­ time of at least some of the modes. An indi­ the vertical decreases, and hence the depth cide. A more careful analysis shows that cation of this is the sharpness of the lines in of penetration increases. Radial oscillations δvnl is mainly determined by conditions in the spectrum of Fig. 3. The most precisely (I = 0) correspond to vertically propagating the solar core. This is also indicated by the observed frequencies have been deter­ rays and hence reach the solar centre; for ray paths in Fig. 4: only In and near the zone mined with a relative error of less than 10-5. of avoidance do the paths for I = 2 differ sig­ The observations can be compared with fre­ all other modes there is a zone of avoi­ dance, whose radius rt increases with in­ nificantly from those for I = 0 so the central quencies computed quite accurately using region dominates the difference δvn0 bet­ solar models. As indicated above, given the creasing I. Thus, only low-degree modes probe the ween the frequencies of these modes. Fur­ physical properties In the Sun (including the solar core. For such modes, we have thermore, the uncertainties in the computed equation of state, the absorption coefficient frequencies resulting from the poorly known for radiation, and nuclear reaction rates) one vnl ≈ Δ v(n + I/2 + α) + enl (1) properties of the oscillations near the solar first solves the equations of where the frequency spacing Av is given surface depend on frequency but not on and evolution by following the change with degree; hence these uncertainties largely time of the chemical composition. The out­ by Δv = (2∫0R dr/c)-1 with integration over distance r to the centre up to the surface cancel In the difference in Eq. 2. The obser­ come is a table giving the variation of say ved values of δvn0 thus provide a test of , , temperature, and speed models for solving the neutrino problem. of sound with position. One then solves the equations for small-amplitude stellar oscilla­ A caveat tions obtained by linearizing the equations There is a caveat, however. Frequencies of hydrodynamics. These equations, and of stellar oscillation are largely determined the boundary conditions, define the oscilla­ by the “dynamic” properties of the , such tions as eigenfrquencies in the same way as as the speed of sound and the mass distri­ the energy levels of a hydrogen atom. The bution, which in turn depends on the distri­ energetics of the oscillations have no effect bution of density p. It can be shown that, on the computed frequencies, so the physi­ apart from the influence of the superficial cal properties of the oscillations are well layers of the Sun, the frequencies are uni­ established except close to the solar sur­ quely determined by the variation of c and ρ face, where energy exchange and inter­ with r. The temperature T is related to c and action with the turbulent convection affects p through the equation of state, but this frequencies in a manner that is difficult to Fig. 4 — Rays illustrating the propagation of also involves the chemical composition: the model (although any uncertainties can be acoustic waves in the Sun. The observed speed of sound Is given approximately by largely eliminated in the detailed analysis of oscillations result from interference between the ratio T/µ, with µ obviously depending on frequencies). such rays. In each case, the dotted line indi­ composition. From the frequencies, informa­ cates the sphere of avoidance into which the tion about temperature is therefore only The nature of oscillations modes do not penetrate. The rays going obtained in the combination T/p. To infer T, To understand the diagnostic capabilities through the centre correspond to spherically and hence the expected neutrino flux, the of the frequencies requires some insight into symmetric modes, with I = 0. present composition of the solar core must

Europhys. News 25 (1994) 73 Fig. 5 — Parameters characterizing in accord with both the neutrino measure­ the frequency separation amongst ments and the observed frequencies would low-degree modes (Eq. 4): δv is be rather contrived. the average of the frequency se­ paration δv (see Fig. 3) and s is Inverse analysis the rate of change of δv with the Helioseismology in fact provides far more order of the mode. The error box detailed constraints on the models than is corresponds to observed values, indicated by the limited set of results shown whereas the points give computed in Fig. 5. For instance, using inverse anal­ results for the solar models whose yses of the observed frequencies it is pos­ neutrino event rates are plotted in sible to determine the density and the speed Fig. 2 (symbols as in Fig. 2). of sound in most of the Sun, with a precision in c of less than 0.1%. The deviations from typical recent standard models are less than 0.5% in c [8], and less than a few percent in density: they are statistically highly signifi­ cant and indicate that corrections to the be constrained by requiring, for example, the test imposed by the frequencies sug­ models are required, but it is unlikely that that it results from solar evolution under gests that any such model (e.g., a carefully they would lead to drastic changes in the given assumptions. Thus frequency obser­ adjusted combination of WIMPs and mixing) predicted neutrino flux. vations cannot by themselves be used to predict the neutrino flux, although they can evidently rule out specific models. OTHER APPLICATIONS OF HELIOSEISMOLOGY The application to neutrino physics is rential rotation and the motions in the Results only a small, though important, corner of Sun’s outer is often assu­ Standard solar models helioseismology. Very accurate determi­ med to be responsible for the cyclic varia­ nations of solar structure will enable detai­ tion of the solar magnetic activity, yet the The most accurate measurements of fre­ led investigation of the properties of the details, or even the reality, of these dy­ quencies of low-degree modes have been solar , particularly the equation of namo processes are poorly established. made using light emission integrated over state and the opacity. This will provide fun­ Measurements of rotation will clearly help the solar surface by measuring either the damental information about the physics of in constraining the models for the genera­ Doppler velocity or the intensity. I shall con­ dense plasmas, under conditions that are tion of the magnetic field. Finally, it seems sider Doppler measurements obtained over out of reach in the laboratory. Further­ that helioseismic data will eventually un­ several years from a network of observing more, measurements of the dependence cover subtle departures from the standard stations set up by a group in Birmingham of the frequencies on azimuthal order m theory of solar evolution, possibly related [6]. When using the frequency separation allow determination of the solar internal to hydrodynamic phenomena in the solar δvnl defined in Eq. 2 for I = 0, it is conven­ angular velocity as a function of position in interior which can affect the distribution ient to parameterize the dependence of the Sun. This is of great importance in of chemical elements in the Sun. Such δvn0 on n through a least-squares fit of understanding the rotation in solar-like effects, when properly understood, would the form stars, from the generally assumed initial then have to be taken into account in com­ δvn0 = δv + s(n- 17) (3). state of rapid rotation, and will shed light putations of models of other types of stars on the origin of the with and might, for example, affect age esti­ The results can be represented in a 8v, s latitude which is observed on the solar sur­ mates of stellar clusters based on stellar diagram, as shown in Fig. 5 where the error face. Moreover, interaction between diffe- evolution calculations. box corresponds to the observed values with error bars of one standard deviation. The points show the results of applying the FUTURE PROSPECTS fit (3) to frequencies computed for a range of Observations of both solar neutrinos degree modes with substantially improved “standard” solar models that involve adjust­ and solar oscillations will undergo drama­ sensitivity. This may permit detection of ing the physics; their scatter indicates — tic improvement in the coming decade. so-called g-modes, i.e., standing internal although certainly not in a strict statistical Measurements with the Ga detectors are gravity waves. Unlike the acoustic modes sense — the uncertainty in the computed still at an early stage; the results will be­ considered in the text, the g-modes have values. While there are some discrepancies, come substantially more accurate with the large amplitudes in the solar core and the standard models give computed results continuing accumulation of data over the hence would provide precise information which are generally in good agreement with next few years. Furthermore, far more about that region. the observations. Other, independent, ob­ detailed data on the electron scattering will Additional insight into servations, notably an extensive set of come the Super Kamiokande detector will result from detailed analyses of other intensity measurements obtained from the which is expected to begin operations in types of variable stars; a very important IPHIR experiment on the Phobos Mars 1996; and the Sudbury Neutrino Observa­ contribution, particularly in the study of probe [7], confirm this conclusion. tory in Canada, with a similar planned solar-like oscillations, could be made by starting time, may test the MSW hypothe­ the STARS satellite being assessed by the Non-standard solar models sis by detecting the muon- or tau-neutrinos European Space Agency (possibly sepa­ Fig. 5 also gives results for models with a as well. rate to the Phase A studies for the next reduced core opacity and with partial mixing In addition to the networks of stations for medium-sized mission — see p. 76). Com­ of the core (the completely mixed model, Doppler observations in integrated light, bining very precise information about den­ whose 37Cl capture rate is consistent with which are still being completed, the Global sity and the speed of sound in the the measurements, falls outside the range Oscillations Network Group (GONG) pro­ entire solar interior with an improved un­ of the plot). It is evident that both these ject will establish a network of six helio­ derstanding of stellar evolution, should models are entirely inconsistent with the observatories, allowing nearly allow reliable calculation of the spectrum oscillation data. More generally, it appears continuous observations of oscillations of of neutrinos generated in the solar core. that all specific modifications to solar mo­ degree up to about 300. Even more detai­ The Sun will then essentially constitute a dels proposed to reduce the neutrino flux led data, extending to degrees of at least well-calibrated neutrino source and analy­ are ruled out by the frequencies observed. 1000, will result from the SOI-MDI instru­ sis of the results from next-generation ment on the SOHO spacecraft, to be laun­ neutrino detectors can be used directly to Although frequency measurements by ched in 1995 [Domingo V. & Schmidt R., investigate the properties of the neutrinos, themselves do not provide definite con­ Europhys. News 22 (1991) 213]. Other as reflected in their interaction with matter straints on the neutrino flux, the failure of the instruments on SOHO will study low- or magnetic fields in the solar interior. proposed non-standard models to satisfy

74 Europhys. News 25 (1994) Conclusions The conclusions are striking: although small discrepancies remain, there is gene­ One of 23 New Research Centres rally excellent agreement between the re­ The Theoretical Astrophysics Center (TAC) Is one of 23 research centres set up in 1993 sults from observed frequencies and those by a new foundation called Danmarks Grundforskningsfond (The Danish National Re­ from frequencies of standard solar models. search Foundation). They were established in 1991 after an extensive and careful evalua­ On the other hand, the proposed non-stan­ tion procedure, each with support for an initial five-year period, following a vote by the dard models with low neutrino fluxes are Danish parliament to grant additional funding for basic research. The centres cover a generally inconsistent with the oscillation broad range of topics in the humanities and the natural sciences, and are in most cases data. By allowing drastic departures from associated with one or several existing university institutes. the standard assumptions It Is still possible Apart from TAC, the physical sciences are represented by several centres including the to construct models that agree with the Aarhus Centre for Advanced Physics, based on the ASTRID storage ring which aims to observations for both neutrinos and the study basic properties of matter and its interaction with radiation, and the Centre for oscillations. Nonetheless, the helioseismic Atomic Material Physics which will make experimental and theoretical investigations of the results strongly suggest that the resolution properties of solid materials, including the growth of atomic layers on surfaces. of the neutrino problem lies in the realm of TAC is located at the The Niels Bohr Institute for Astrophysics, Physics and Geophysics, particle physics rather than astrophysics. Copenhagen, and at the Institute of Physics and Astronomy, Aarhus, with Professor Igor Future work (see insert) will hopefully re­ Novikov, Copenhagen, as Director. It aims to concentrate on studies of the origin of large- solve this major issue. scale structures In the Universe, the formation and evolution of galaxies, and the structure The most likely explanation Involves de­ and oscillations of the Sun and other stars. Although the emphasis will be on theoretical partures of the properties of the neutrinos work, close contact will be maintained with related observational activities, both within from those normally assumed. It has been Denmark and abroad. In particular, scientists at TAC take part in large international obser­ suggested that the electron-neutrinos pro­ vational projects in several areas of astrophysics. duced in the the solar core are transformed TAC is currently in the process of filling several long-term and post-doctoral positions, by the Mikheyev, Smirnov and Wolfenstein and establishing a suitable computing environment. It is expected that the Centre will fully effect (MSW; see [9] for details) Into neutri­ operational this autumn. The goal is a centre of excellence in theoretical astrophysics, nos of other types (muon- and tau-) on the working closely with groups around the world, and hence providing a major boost to astro­ way to detectors. Such a transformation is physics research In Denmark. possible if there is a finite mass difference between the three types of neutrinos, or if solar neutrinos provided the conditions in [4] Morrison D.R.O., Int. J. Mod. Phys. D 1 (1992) 281. the solar core, and hence neutrino emission, [5] Gough D.O. & Toomre J., Ann. Rev. Astron. Astro- neutrinos have a . It is phys. 29 (1991) 627. possible to choose parameters such that the can be constrained by helioseismology. [6] Elsworth Y. et al., Nature 347 (1990) 536. capture rates predicted for standard solar [1] Bahcall J.N. & Pinsonneault M.H., Rev. Mod. [7] Toutain T. & Fröhlich C., Astron. Astrophys. 257 models agree with the measurements. Phys. 64 (1992) 885. (1992) 287. [2] Claverie A. et al., Mem. Soc. Astron. Ital. 55 (1984) [8] Christensen-Dalsgaard J., Proffitt C.R. & Thomp­ The effects are too weak to be detectable 63. son M.J., Astrophys. J. 403 (1993) L75. over the scale of a laboratory on Earth. One [3] Bahcall J.N. & Bethe H.A., Phys. Rev. D 47 (1993) [9] Bahcall J.N., Neutrino Astrophysics (Cambridge way to study them lies in the analysis of 1298. University Press, 1989).

Ulysses is Proving its Worth Rudolf von Steiger from the Physikalisches Institut, Bern University, reports that a complete view of the is emerging as the Ulysses spacecraft sweeps across the Sun’s poles.

Ulysses was launched in 1990 and has Fig. 1 — Ulysses’s trajectory as it sweeps reached a heliospheric latitude of 60°S. It will across the Sun’s poles. The spacecraft scans continue to climb in latitude, reaching 80°S across a high-speed stream at this September. There follows a fast passage increasing latitudes during each rotation of from south to north, reaching 80°N in August the Sun. This stream (shown issuing from the 1995 (see Fig 1). Much of the new physics solar surface) is modelled as a co-rotating that is being discovered in this first visit to the interaction zone bounded by shock waves. third dimension of our was reviewed last month at the 28th ESLAB Sym­ posium (Friedrichshafen; 19-24 April 1994) that heralded the start of Ulysses’s race Acceleration Mechanisms Confirmed across the Sun’s south pole. A new view Owing to the declining solar activity, the of the heliosphere as a three-dimensional, polar coronal holes (CHs) have re-formed at structured, dynamic entity is beginning to high latitudes. The SWOOPS instrument with emerge, for which the well-known models of S. Bame and J. Phillips as the Principal the ecliptic will serve as the boundary condi­ Investigators (PIs) and the SWICS sensors tions of a more global view. (PIs: J. Geiss and G. Gloeckler) first detected L. Fisk outlined the questions that need the low-latitude extension of the south polar Ulysses has been completely immersed in to be answered by Ulysses in order that CH as a high-speed solar wind (SW) stream the high-speed stream from the south polar the future judgement of the mission will be in mid-1992, when Ulysses was at 13°S. CH since mid-1993 when the spacecraft was kind. They concern: the overall, three-dimen­ The stream subsequently reappeared every at 36°S. At latitudes of this , the sional structure of the solar wind; cosmic rays (i.e., about every 26 days), SW's density and pressure are found to be (both galactic and anomalous) propagating alternating with periods of low-speed SW that more uniform than those measured in the through the solar system; the properties of became shorter and shorter. The observation ecliptic plane by SWOOPS. However, sub­ the interstellar medium entering the helio­ of these repeated transitions, which arise In stantial (≈ 100 km/s) long-term speed varia­ sphere, and of the so-called pick-up ions that periods of low solar activity, was fortuitous as tions still persist. Furthermore, SWOOPS result from its ; the source and the they are Ideal for comparing the low-speed, finds at high latitudes a latitudinal gradient in acceleration mechanism of the solar wind in in-ecliptic SW with the high-speed streams the difference between the velocities of pro­ the corona. from CHs. tons and alpha-particles which is larger (of

Europhys. News 25 (1994) 75