RESULTS FROM THE GOLF INSTRUMENT ON SOHO

Transcription

1 RESULTS FROM THE GOLF INSTRUMENT ON SOHO A. H. Gabriel 1, S. Turck-Chieze 2, R. A. Garca 2, P. L. Palle 3, P. Boumier 1, S. Thiery 1, F. Baudin 1, G. Grec 4, R. K. Ulrich 5, L. Bertello 5, T. Roca Cortes 3, J.-M. Robillot 6 1 Institut d'astrophysique Spatiale, Universite Paris XI, Orsay Cedex, France 2 DAPNIA/Service d'astrophysique, CEA/Saclay, Gif sur Yvette Cedex, France 3 Instituto de Astrofsica de Canarias, La Laguna, Tenerife, Spain 4 Observatoire de la C^ote d'azur, Laboratoire Cassini, Nice, France 5 Department of Astronomy, U.C.L.A., Los Angeles, USA 6 Observatoire de l'universite Bordeaux 1, BP 89, Floirac, France ABSTRACT An 800 day series of GOLF velocity data, with uniquely high continuity and stability, oers the best ever signal to noise ratio obtained in global Sun observations. Following meticulous eorts to provide reliable calibration, these data have been used for measurements of frequencies, line-widths and power in the p- modes, which are used for inversion to give the internal sound speed, for comparison with theoretical models. A search for g-modes is at present inconclusive, but has yielded two possible candidate frequencies. The analysis available today is regarded as preliminary and more complete methods are currently in hand. With the resumption of routine observations following the SOHO recovery, it is hoped that the data can be considerably extended, enabling changes with the solar cycle to be explored, as well as an extended g-mode search. INTRODUCTION The GOLF instrument has been in operation for some 2.5 years and, at the time of the COSPAR symposium. It had shown no indication that it could not continue through to the solar maximum, until the loss of contact with the SOHO spacecraft on June At the time of writing, SOHO has been recovered and GOLF appears to be in the same condition as before the loss. For a full description of the GOLF programme prior to launch, the reader is referred to Gabriel et al.(1995), and, for a summary of the early results, to Gabriel et al.(1997), Lazrek et al.(1997) and Turck- Chieze et al.(1997). Some problems with the polarising mechanisms, experienced early in 1996, led to a decision to change the operating programme in April In the present mode, there are no moving parts, leading to an increase in the stability and reliability. On the other hand, we have been using only the blue wing of the solar absorption line prole. This leads to a small decrease in the signal to background ratio, dicult to quantify precisely, but also makes the velocity calibration more complex and lengthy. The 800 days of GOLF data, obtained following April 1996, provide a quite exceptional data-set, in terms of continuity (> 99:7%) and stability. It is this continuity and stability that leads to the interesting results already obtained and to the expectation of more important results to come, as we learn progressively how

2 In this report, we summarise some of the results obtained, referring for details to several GOLF articles, published or in press. Figure 1: Section of the p-mode spectrum obtained from 9 months of data, showing the extremely low background level. DATA REDUCTION AND CALIBRATION The aim is to process the sequence of measured intensities for two points on the blue wing of the solar prole, in order to obtain the line-of-sight component of the Sun-SOHO velocity vector. Whilst the absolute value of this velocity is not accurately required for scientic analysis, the stability of the calibration with time can be very important for some aspects of the analysis. This calibration is inuenced by numerous factors: the annual modulation in the average Sun-SOHO velocity vector, small sudden or gradual variations in GOLF temperature due to SOHO environment changes, and a gradual monotonic decrease in the GOLF sensitivity over the years. There is also a small contamination of the GOLF velocity signal, which is a function of the solar oscillations signal in intensity. The knowledge of this component will be important for some applications (e.g. studies of phase). Its evaluation is complicated by the absence of red-wing data. However, it is not clear that it would be easily resolved, even with such data, since it seems evident that the shape of the solar absorption prole itself varies with the phase of the oscillation signal. Studies of the intensity contamination are the subject of two articles by Palle et al., (1998a) and Renaud et al., (1998). The basic velocity calibration task becomes important for the analysis of low frequency p-modes or the search for g- modes, where uctuations in sensitivity can introduce spurious components into the background GOLF velocity spectrum. Three dierent techniques are being adopted within the GOLF team and these are producing broadly similar results, at least in the p-mode region. Discussions on these methods can be found in Robillot et al., (1998) and Ulrich et al., (1998). INTERNAL STRUCTURE FROM MODE FREQUENCIES Although the l = 0 p-modes penetrate to the centre of the Sun, their predominant sensitivity, as for all of the p-modes, is in the outer layers. The reverse is true for the g-modes. For this reason, the inversion of p-mode frequencies to obtain the core structure is a very ill-conditioned procedure. Although this is widely practised and is one of the GOLF objectives, it requires a very high precision in measured frequencies in

3 amplied when we try to measure the rotational splitting of modes, where the need is to nd small dierences between two close numbers. Here, the current position remains far from what is ideally required, in spite of numerous attempts, reported in the press, to deduce limits for the internal solar rotation. The inversion problem becomes greatly simplied if we can work with the g-mode frequencies. Unfortunately, we cannot yet claim that these have been truly identied from the GOLF spectrum, although there is some hope of progress in this area. THE SEARCH FOR g-modes The GOLF time-series has exceptional qualities in terms of the low level of instrumental noise and the continuous uninterrupted length. For these reasons, we believe that it oers the best chance of nding the illusive g-modes, if indeed they exist at a reasonable level. The problem resides in looking for some or many elements of a complex spectrum of modes, against a background of \noise". The noise has two distinct components, signals produced by the Sun's random velocity elds, or convection, and the noise induced by the observing instrument, its imperfections and its environment. Random instrumental noise is mainly due to photon noise. This is at on the frequency scale. In the g-mode band it is quite small compared to the solar noise, which increases with decreasing frequency (Harvey, 1985). Another component of instrumental noise arises in the Fourier spectrum from the changes with time of the overall instrument sensitivity to velocity. As indicated above, it is for this reason that a meticulous eort has been applied to the dicult problem of the precise instrument calibration. We are left, however with the random solar velocity signal as the dominant source of background, against which to nd the g-modes. In the 200 Hz region this appears as a signal of the order 3 to 5 mm:s?1 per resolution element for a 2-year time-series. Moreover, this background shows evidence of a non-gaussian distribution, with peaks above 10 mm:s?1 more abundant than anticipated. Against this, we can already conclude that the g-mode peaks, if they exist, are not larger than 5 to 8 mm:s?1. We can already draw certain conclusion from these gures. Firstly, we cannot hope to identify g-modes simply from their prominence in the Fourier spectrum. We must rely on additional qualities, related either to their predicted pattern of frequencies, or their expected very long life-times. Furthermore, if we wish validate a search procedure by simulation, it is best to use the real observed GOLF signals for the continuum background, rather that a theoretical random noise signal. An important simulation has been performed by selecting a portion of the GOLF spectrum where no specic g-modes are expected (around 230 Hz) and adding simulated g-modes in the form of pure sine-waves to the time-series, at amplitudes comparable to the above limit. The resultant harmonic analysis shows that the background noise adds in complex space to the signal in a random way that varies from case to case and, more importantly, with time. The constant signal is thus found to come and go, typically 2 to 4 times, over the time-scale of our 2-year observation. This behaviour provides a warning that our pre-conception to search for a constant aspect to the signal must be treated with caution. On the other hand, the reappearance several times at eectively the same frequency oers a new quality as a search criterion. Figures 2 and 3 show such a simulation, where seven identical sine-waves of 8 mm:s?1 have been added. Within the GOLF analysis teams, a number of methods are being pursued for the g-mode search. These can be summarised as follows. Correlation method This technique, by searching simultaneously for a large number of modes, eectively improves the signal/noise ratio by this number. It is a variation and extension of a method previously described by Frohlich

4 Figure 2: Region of the GOLF background spectrum, to which have been added seven sine-waves of equal amplitude, 0.8 cm:s?1. Figure 3: Portion of a time-frequency analysis of GOLF background, with seven added sine-waves, from the same time-series shown in Figure 2. The positions of the added sine-waves are indicated on the right. and Delache (1984) and Palle (1986). They calculated a g-mode spectrum, using the asymptotic formula with two free variables, the characteristic period P 0 and the rotational splitting. They then searched for a maximum in the cross-correlation between the observed solar spectrum and the calculated one, as a function of these two free variables. We have extended the method, by adopting a theoretical spectrum in place of the asymptotic one, and by using 4 free parameters in place of 2, in order to match the improved precision sought in the tting. Some of the simulations are convincing, but more work is required on the t criteria. Method of exact fractions This method, described by Palle et al., (1998b), is based on a search for the equal period separation of the g-modes in the asymptotic low-frequency region. Here again, simulations demonstrate the capability and some initial runs with GOLF data begin to show some promising results. Unfortunately, the existence of rotational splitting breaks down the strict application of equal periods and dilutes the expected results. Singular Spectrum Analysis method (SSA) The idea of SSA is to separate noise from oscillations by embedding the time-series in a space of delay coordinates (Veradi et al., 1998). The technique is a form of noise sub-space decomposition, and is capable of identifying a number of frequencies, amongst which might be candidates for g-modes or low-frequency

5 Search for individual high-frequency modes The rst few low-n g-modes near their high-frequency limit deviate the most from the asymptotic prediction and are well isolated in the spectrum. Some theorists (eg Kumar et al., 1997) predict these modes to be the most intense. These have been searched for in time-frequency plots, using the above described expected behaviour of individual modes. Some conrmation of suspected modes can be found by verifying that the observed frequencies and rotational splittings lie within the limits of theoretical expectation. Two candidates, and Hz emerge from these tests with frequencies within 3 Hz of those predicted by solar models (Brun et al., 1998). They have velocities of the order 5 mm:s?1 and also appear to exhibit a multiplet structure within the accepted helioseismic limits (Provost et al., 1998). The future for g-modes It should emphasised that we are far from having exhausted our eorts on the g-mode search. Each of these techniques has further development possibilities, even if we consider only the existing GOLF 800-day data-set. Now, we have also the hope of doubling (or more) the total length of data available. Supercially this might be expected to improve the signal to noise by a factor of p 2, (or 2 on a power scale). If this factor does not seem very exciting, we should also recall the above criterion for the search for individual modes on a time/frequency plot. On doubling the total duration, we expect our candidate modes to show some 6 re-appearances with time, as compared with the 3 maxima seen at present. This would be a very signicant gain in statistics and condence. MEASUREMENTS OF p-modes The stochastic excitation of p-modes, coupled with their relatively short life-times, (ranging from days to months) leads to particular problems for the determination of accurate frequencies. The application of standard Fourier analysis techniques is ill-adapted to such non-stationary oscillations and has led us to examine and develop other methods (Baudin et al., 1993, 1996). The approximately Lorentzian prole of the damped sine-wave is seen to be broken up by large-amplitude ne-scale structure, caused by the random reexcitation. The tting of such disturbed proles by Lorentzians presents a major element in the uncertainty in determining the frequencies, uncertainty which remains large compared with the severe requirements of the inversion procedures. A time-frequency plot of one of the l = 0 modes during 80 days is shown in Figure 4. This mode, which has no rotational ne-structure, shows dramatic variations in amplitude, together with apparent variation in frequency, all due to the stochastic excitation eects. Figure 4: Time/frequency analysis of a single GOLF l = 0 mode, showing uctuations in power level and apparent changes in frequency.

6 hood techniques is inverted, in order to determine the velocity of sound in the solar interior. To accomplish this, it is necessary to use also frequencies for higher l values, important in the outer layers of the Sun. Figure 5 shows such an inversion combining GOLF with higher-l data from MDI (Turck-Chieze et al., 1998). Figure 5: Sound speed squared dierence between the inversion of GOLF + MDI data and a Saclay model for the solar interior. Using theoretical predictions (eg. Nigem and Kosevichev, 1998), it is now possible to understand that the p-mode proles are in reality somewhat asymmetric, rather than true Lorentzians (Toutain et al., 1998). In this case, it can be shown that the true resonant frequency corresponds neither with the maximum nor with the median of the observed prole, but is systematically displaced to one or other side, according to the sign of the asymmetry. Work currently in hand on tting such proles to the GOLF data results in a signicant shift in the measured frequencies, and a consequent correction to the inverted internal sound velocities. A study of the correlation in time between dierent p-modes (Foglizzo et al., 1998) has shown that, for the rst year of GOLF spectra, obtained during the solar minimum, there is eectively no correlation. However, for a similar study applied to IPHIR data, during the solar maximum, some limited correlation is observed (see also Baudin et al., 1996; Gavryusev and Gavryuseva, 1997). This leads to the suggestion by Foglizzo et al.that some external exciting agent, related to the activity cycle may be contributing, such as solar ares. Recent observations by the MDI team (Kosevichev and Zharkova, 1998) of local seismic activity following a are lends some weight to this idea. Since individual p-modes are largely uncorrelated, it would be expected that the ratio of components of a rotationally split multiplet would suer similar random variations as the separate components, over similar time-scales. There are however, indications that even after these individual variations average out in several hundreds of days, there remains a signicant persistent deviation of the multiplet ratios from their expectation values (Gavryusev, Gavryuseva and Gabriel, private communication). Work is currently in hand to determine quantitatively the validity of this eect in the GOLF spectra. Such an eect, if veried, would indicate a signicant departure from rotational parity around the Sun's rotation axis. p-modes ABOVE THE CUT-OFF FREQUENCY GOLF has detected and measured the so-called \pseudo-modes" in global oscillations, above the acoustic

7 spatially-resolved oscillations, but never before in whole-sun spectra. These observations, enabled by the high quality of the GOLF data, pose some interesting questions for their interpretation. Figure 6: An average GOLF cross-power spectrum, with boxcar smoothing, showing a regular series of peaks, extending at least up to 7.5 mhz. THE GLOBAL MAGNETIC FIELD The addition of a second rotating polarising component to the GOLF conguration was designed primarily to provide a redundant back-up for the mechanisms. It also oered the possibility of achieving a secondary objective for GOLF: the determination of the mean line-of-sight magnetic eld, to a far greater precision than previously measured. The stopping of the GOLF mechanisms in April 1996 resulted in the suppression of this secondary objective. Nevertheless, the 26 days of correct operation of the mechanisms oer a chance to verify the principle and to make a brief measurement of the eld. This is reported by Garca et al., (1998b), where elds of 0.1 to 0.25 gauss are measured. No observations of p-modes in the global magnetic eld parameter were detected, within the precision possible from such a short observation. FUTURE PROSPECTS As has already been indicated, the existing 800 day data-set provides a unique and rich source of highquality data, for which the present analysis provides only a preliminary evaluation. There are many ideas for improving the analysis methods, in particular concerning the p-mode frequency measurements and the g-mode search. Now that SOHO has been recovered, and the GOLF instrument found to be apparently unharmed by the extreme conditions experienced during the loss, we anticipate the real chance of an extended observing period, up to a total of 6 years. This has obvious benets for the g-mode programme. In addition, it oers the possibility of a detailed study of the solar cycle eects and the impact, if any of high activity or solar ares on the p-mode excitations.

8 channel A exhibits from time to time some small jumps in its output, apparently associated with jumps in its HT. This is of a level scarcely detectable in the science output counting rate, but visible in the pulse height analysis output. For work of the highest precision, when searching for g-modes, this detector is often dropped from the data. The HT applied to the detector systems has been increased from time to time, as always planned, in order to compensate for the falling gain of the dynode chains. There is also observed an overall drop in the counting rate, equivalent to 30 % over the 2 years. This may be due to coatings on the entrance window, a drop in sensitivity of the lter or cell, or a fall-o in sensitivity of the PM photo-cathodes. There is no way to isolate these, without switching to redundant channel B, which would replace the detectors with unused ones. In any case, this loss is of no major concern and would leave GOLF functioning correctly at the end of 6 years, even without using channel B. ACKNOWLEDGMENTS The GOLF instrument has been built by a consortium of ve institutes : the Service d'astrophysique at Saclay, France; the Instituto de Astrofsica de Canarias in Tenerife, Spain; the Observatoire de l'universite Bordeaux in France; and the Observatoire de la C^ote d'azur at Nice, France, under the leadership of the Institut d'astrophysique Spatiale at Orsay, France. These are supported by a large number of scientic Co-Investigators, drawn from many countries. SOHO is a mission of international co-operation between ESA and NASA. REFERENCES Baudin, F., Gabriel, A. H., Gibert, D. E., 1993, Astron & Astrophys., 276, L1. Baudin, F. E., Gabriel, A. H., Gibert, D. E., Palle, P. L., Regulo, C., 1996, Astron. & Astrophys., 311, Brun, S., Turck-Chieze, S., Morel, P., 1998, Astrophys. J., 506. Foglizzo, T., Garca, R. A., Boumier, P., Charra, J., Gabriel, A. H., Grec, G., Robillot, J.-M., Roca Cortes, T., Turck-Chieze, S., Ulrich, R. K., 1997, Astron & Astrophys, Frohlich, C. and Delache, P., 1984, Mem. S. A. It., 55, 99. Gabriel, A. H., Grec, Charra, J., G., Robillot, J.-M., Roca Cortes, T., Turck-Chieze, S., Bocchia, R., Boumier, P., Cantin, M., Cespedes, E., Cougrand, B., Cretolle, J., Dame, L., Decaudin, M., Delache, P., Denis, N., Duc, R., Dzitko, H., Fossat, E., Fourmond, J.-J., Garca, R. A., Gough, D., Grivel, C., Herreros, J. M., Lagardere, H., Moalic, J.-P., Palle, P. L., Petrou, N., Sanchez, M., Ulrich, R. K., Van der Raay, H. B., 1995, Solar Phys., 162, 61. Gabriel, A. H., Charra, J., Grec, G., Robillot, J.-M., Roca Cortes, T., Turck-Chieze, S., Ulrich, R. K., Basu, S., Baudin, F., Bertello, L., Boumier, P., Charra, M., Christensen-Dalsgaard, J., Decaudin, M., Dzitko, H., Foglizzo, T., Fossat, E., Garca, R. A., Herreros, J. M., Lazrek, M., Palle, P. L., Petrou, N., Renaud, C., Regulo, C., 1997, Solar Phys., 175, 207. Garca, R. A., Palle, P. L., Turck-Chieze, S., Osaki, Y., Shibahashi, H., Jeeries, S. M., Boumier, P., Gabriel, A. H., Grec, G., Robillot, J.-M., Roca Cortes, T., Ulrich, R. K., 1998a, Astrophys. J., 504, L51 Garca, R. A., Boumier, P., Charra, J., Foglizzo, T., Gabriel, A. H., Grec, G., Regulo, C., Robillot, J.-M., Roca Cortes, T., Turck-Chieze, S., Ulrich, R. K., 1998b, Astron. Astrophys., in press. Gavryusev, V., Gavryuseva, E., 1997, Solar Phys., 172, 27. Harvey, J., 1985, in "Future missions in solar, heliospheric and space plasma physics", ed. E. Rolfe and B. Battrick, ESA SP-235, 199.

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