Possibilities for quantification and reduction of solar velocity noise induced by active regions

Adv. Space Res. Vol. 11, No. 4, pp (4)203—(4)206, 1991 Printed in Great Britain. All rights reserved.

0273.4177/91 $0.00 + .50 Copyright © 1991 COSPAR

POSSIBILITIES FOR QUANTIFICATION AND REDUCTION OF SOLAR VELOCITY NOISE INDUCED BY ACTIVE REGIONS R. K. Ulrich*, L. Damé** and M. Martic*** * Department of Astronomy, University of California at Los Angeles, CA, U.S.A. * * Direction des Etudes de Synthèse, ONERA Meudon, Paris, France ~ Institut d’Astrophysique Spatiale d’Orsay, BP 10, 91400 Orsay, France

ABSTRACT Active regions on the solar surface induce a velocity signal in a variety of measurements of doppler shifts including the integrated sunlight measurements made by resonance cells using the NaD lines. These signals may be an important limitation on the ability of the GOLF investigation to detect solar g-modes. Although the power spectrum of this velocity signal is not well studied, it is in principal possible to develop techniques based on other properties of the spectral line to isolate the active region induced component and remove it from the observed velocity. This paper describes efforts to develop such a method and verify its utility using ground-based measurements.

INTRODUCTION The detection of solar g-modes is one of the most important goals of helioseismology because the information content in the frequencies and splittings of these modes is concentrated in the solar core in a way which is quite distinct from the information content of solar p-modes and solar neutrino fluxes. The GOLF experiment on SOHO will make a definitive effort to detect and identify the solar g-modes. If successfully executed, the limitation on the ability of the experiment to detect g-modes will come from background solar noise. The GOLF Investigation Team has placed a high priority on understanding the nature and power spectrum of the solar background noise and the report by Jiménez et al. /1/ has given an estimate based on several years of data from Izãna on Tenerife. Two sources of solar noise are anticipated to be of roughly equal strength in the critical 30 minute to 3 hour range of periods the supergranulation and the active regions. Based on a crude model which assumes that active regions change only over a period of a solar rotation, Harvey/2/ estimated that the supergranulation is the dominant source of noise. The report by Jiménez ci al. /1/ supports this conclusion because the apparent noise level in the g-mode range did not change over the interval from 1982 near the peak of the solar cycle to 1985 at solar minimum. However, even at solar minimum activity is present and could continue to contribute to the variations observed by Jiménez ci al. /1/ as a function of frequency. Hence, the relative importance of active regions and supergranulation on the basis of a correlation with the solar cycle is not yet firmly established. We emphasize this point because of our hope that it will be possible to develop a method to reduce solar noise and the active regions provide a better possibility to do this through the use of additional observational data beyond the doppler velocity. —

The GOLF approach through the use of a 4-point measurement as described by Dame ci a!. /3/ and Boumier ei a!. /4/ is based on the assumption that the doppler shift is the primary quantity needed to observe solar oscillations. The 4-point measurement allows a simultaneous calibration of the line slopes while the basic line shift is being determined. The Boumier ci a!. /4/ paper shows that this approach is effective in removing line shape changes from the basic doppler signal. Our objective in this active region study goes beyond the initial GOLF objective and attempts to identify a portion of the doppler signal as being due to active region downdraft effect line shifts and then remove these doppler shifts from the signal. This plan of dividing the observed signal into two components obviously raises the risk that it will introduce noise if done improperly. Any method of the sort we envision will be thoroughly tested on actual GOLF data before it is introduced as part of the normal data reduction process. The additional data to be JASR 11:4—N

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utilized includes maps of the location of active regions such as those provided by absolute value magnetograms (Ulrich/5/) or amplitude modulation maps from SOI/MDI as well as integrated sunlight line profiles of the NaD line for which the GOLF 4-point measurement provides the best indicator. LINE PROFILES In order to study the profile of the NaD1 line in an active region and in a quiet region, we used the doppler difference-gram map made 30 minutes prior to the line profile measurement to identify the location of an appropriate downdrafting region. The solar image was then set according to computed positioning units so that the downdrafting region fell on the entrance slit. The active region is not evident in white light and it is necessary to avoid actual sunspots where the spatial gradient of intensity would produce noise in the profile due to seeing fluctuations. Figure 1 shows a line profile observed August 21, 1989 in a downdraft region selected this way. A comparison profile on the same day in a quiet region is also shown. The change in profile shown in Figure 1 is evident and encourages us that a quantitative algorithm can be developed which uses integrated sunlight profiles to assess the fractional coverage of the solar disk with downdraft regions. Both profiles shown in Figure 1 were measured using the 75-foot Littrow spectrograph of the 150-foot solar tower telescope on Mt. Wilson. The spectrum was scanned by moving the exit slit fiber-optic pickup continuously and binning the signal every 3mA. The entire scan extended over 1.554A and required 60 seconds to complete. The profiles show are averages of 10 such scans. The spectral resolution is governed by the geometric sizes of the entrance and exit slit apertures which were both 800gm which for the 632 line/mm grating used in 4th order yields a spectral resolution of l3lmA. Beginning in September 1989 we have been using a new fiber-optic pickup with a width of 250j.im and the old exit slit of 800jim which provides a spectral resolution of 97mA. We are able to make useful measurements of the NaD line profiles with this system because the intrinsic line width is greater than our spectral resolution.

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Figure 1. This figure shows the profile of the NaD1 line in a quiet region and in an active or downdraft region. In order to carry out a quantitative study of these changes, we have parameterized the observed line profiles in terms of a polynomial centered about the point of minimum intensity. Although a non-linear fit could be carried out in terms of a more complex function, we have chosen to use a high degree polynomial in ö~X= A0 where A0 is the point of minimum intensity. The spectral region we fit is about ±0.2Abroad and contains about 65 points. Thus even a fairly high degree polynomial is well constrained and we are able to use polynomials up to degree 14. We pick the lowest degree which does not leave a systematic residual between the observed and fitted points and this is between 12 and 14 for the profiles we have studied. The parameters describing the profile are then computed from the polynomial fit rather than the actual points since this approach smooths out the point to point noise. .~ —

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We consider two line profile parameters: the average slope at the GOLF working points of ±0.095A,and an asymmetry parameter which is the velocity difference between the bisector at ±0.095Aand the point of minimum intensity defined as the point of zero slope. In the definition of the first parameter we take the average of the absolute values of the slopes at the working points divided by the intensity at each point. The line center of the NaD1 line is insensitive to the downdraft effect according to the observations of Ulrich ci a!. /6/ while the GOLF working point is intermediate in sensitivity between that of A5250 and A5237. Thus the asymmetry parameter should give us an indication of the line shift at the GOLF working points. Effectively, the point of zero slope serves as the reference wavelength for the GOLF working point velocity. Note that the asymmetry parameter is not accessible to the GOLF instrument while the slope parameter is. Our plan is to determine if the slope and asymmetry parameters are correlated in a way which would result from a variable coverage of the solar disk by active regions. Using the resolved sunlight observations to estimate the correlation between slope and asymmetry we find that in the downdraft region the slope in Figure 1 is 13.15A’ and the asymmetry parameter is 113 m/s while in the quiet region the same two parameters are I • I I 14.10A’ and 26 m/s respectively (the slope parameters include a small correction due to ,

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the change in exit slit width). During the months of June, July and August, 1990 we have used the Mt. Wilson 150-foot tower system in an integrated light mode to observe the NaD1 line profile for a period of 1 hour near local solar noon every clear day. These profile measurements use the narrower fiber-optic pickup mentioned above. Each day’s observing sequence consists of 120 spectral scans in which the fiber-optic pick .

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Fig. 2. The time dependence of the observed line asymmetry parameter. Time is given in Juban Date —2440000. Calendar dates at 0 hours UT are shown along the top of the figure.

is scanned over about 0.75A with each scan taking exactly 30 seconds. Successive scans are in opposite directions. The minimum time interval we have used is 1 minute since we wish to average over scans in opposite directions. We report here on results obtained by averaging together all 120 scans available for each day.

ESTIMATED AND OBSERVED ACTIVE REGION EFFECTS ON INTEGRATED SUNLIGHT PROFILES For reasons explained below we are not yet able to directly measure the amplitude of variations induced by the active regions using integrated sunlight. Our goal is to determine the amplitude of variation due to the active regions in all temporal frequency ranges. it is already known that active regions produce a measurable signal with periods in the range of several days due to solar rotation. As a verification of our methods we require that we should detect the variations in this period range. The data presented by Jirnenez ci al. /7/ for daily averages of the integrated sunlight velocity at the 1(1 line A7699 shows that the rotation of the active regions produces a velocity change of about 15 rn/s. During correlated intervals, the Magnetic Plage Strength index (denoted as MPSI; see iJJrich/5/ for a definition and discussion of the MPSI) changed by about 3 gauss in concert with the velocity change. We take this ratio of (15 m/s)/(3 gauss) to be a conversion factor between current changes in the MPSI and the line asymmetry parameter defined above for the NaD1 line. Figure 2 shows both the averages of the line asymmetry parameter observed for one hour each day near local solar noon together with the estimated effect based on the MPSI. It is evident that although we are close to the required level of system performance, we have not yet reduced the observational error in the measured line profiles to a small enough level to detect

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the anticipated active region effect. We can also use the line profiles in the preceding section to convert the line asymmetry changes into correlated line slope changes. Figure 3 shows both the

observed and estimated values of this parameter for the June to August, 1990 period. Again we see that system noise has prevented the detection of the active region effect. DISCUSSION This contribution represents a status report on the effort to establish ~ method for correcting the signal to be measured by the GOLF experiment for the effects of active regions. This effort has not yet succeeded. The system noise which produced the day to day variation in figures 2 and 3 is a result of vignetting of SIIIISO 7iI~i9O the beam by the housing of the spectrograph grating. In the integrated sunlight mode, — 14.3 Slop the optical system forms a pinhole camera + image of the solar disk on the grating so 14.2 that the vignetting which is not precisely controlled causes a variable obscuration of part of the sun with a consequent change in 14.1 the solar line profile. A new grating housing to eliminate this problem has been installed ..14.0 at the beginning of September, 1990. •

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Fig. 3. The time dependence of the observed line slope parameter. Time is given as in Fignrc 2.

Even when we have achieved adequate ohservational performance from the Mt. Wilson system, we will need to carry out additional analyses to compare the results directly with the measurements from the ground-based observations of Boumier ci al. /4/ and GOLF. in particular, we will need to model the line intensities at the precise positions of the GOLF observations. This will require knowledge of the limb effect shift for the NaD lines

and ephemeris velocities for both Mt. Wilson and izãna. The propagation of the active region effect through this reduction process will probably not yield a simple relationship.

REFERENCES 1. A. JimCnez, P.L. Pallé, F. Perez Hernandez, C. Régulo and T. Roca Cortés, The observed background solar velocity noise, Asir. Asirophys. 192, L7-L9 (1988). 2. J. Harvey, High-Resolution Helioseismology, in: Fuiur~Missions in Solar, He!iospheric ~ Space Plasma Physics, ed. E. R.olfe and B. Battrick, ESA SP-233, Noordwi.jk 1985, p. 199. 3. L. Dame, C. Cesarsky, P. Delache, F.L. Deubner, B. Foing, E. Fossat, C. Frölich, A. Gabriel, M. Gorisse, D. Gough, G. Grec, P. Pallé, J. Paul, T. Roca Cortés and J.L. Stenflo, Global Oscillations at low frequencies (GOLF), an investigation of the Solar Interior, Proposal submitted

to ESA and NASA in response to the Announcement of Opportunity SC1(87) 1/OSSA-1-87 for the Solar and Heliospheric Observatory, (July, 1987). 4. P. Boumier, R. Bocchia, L. Dame, M. Martic, P. Pallé, H.B. van der Raay, J.M. Robillot, and T. Roca Cortés, Preliminary Performance of a 4-point Resonance Scattering Experiment to Access Long-Period Global Oscillations from Space, this issue. 5. R.K. Ulrich, A Co-ordinated and Synergistic Analysis Strategy for Future Ground-based and Space Heioseismology, this issue. 6. R.K. Ulrich, J.E. Boyden, L. Webster and T. Shieber, Magnetically Induced Spectral Line Redshifts Full Disk Measurements, in: Seismology of the Sun & Sun-like Stars, ed. V.

Domingo and E.J. Rolfe, ESA SP-286, Noordwijk, The Netherlands 1988, p. 325. 7. A. Jiménez, P.L. Pallé, C. Régulo, T. Roca Cortés, G.R. Isaak, C.P. McLeod and H.B. van der Raay, The Radial Velocity of the Sun as a Star and the Solar Cycle, in: COSPAR Proceedings, Advances in Space Research vol. 6, no. 8, ed. editors, Pergamon Press, Oxford 1987, p. 89.