VIRGO Purple Book
Version 2.2 June 1995
ISSUE SECTION / DATE CHANGES / REV. PAGE 1.0 All 7 Nov 94 N/A 1.1 All 2 2.1 6 Dec 94 Change Blue to Purple Add Scientific 2.1.5 23 Nov 94 Obj. the Sol.Phys. article Tuning after 2.2-2.2.3 8 Feb 95 C.Fröhlich's input Add input of 2.2.4 2.2 25 Nov 94 T.Toutain After input from J.Pap Add All 7 Jan 95 Diameter Variations Tuning after sections 3 26 Nov 94 C.Fröhlich's input Change name of PM06 3.1 3.1 8 Feb 95 to PMO6-V Add input of J.Romero Add data 3.1.1 23 Nov 94 from the Sol. Phys. article Add inputs 3.1.5 3.2 23 Nov 94 from Wehrli Add SPM filter profiles 3.2.1 5 Jan 94 info. (ftp server) Add Fig SPM-I Add LOI 3.2.3 3.3 31 Jan 95 detector configuration Add LOI filter 3.4 3.5 4 19 Dec 94 profile information (ftp server) Add 4.2.1 4.2.2 31 Jan 95 guiding results from Tenerife Add 5 Dec 94 lifetime variation Add data from the 13 Dec 94 Sol. Phys. article Add Roth's input Add 7 Dec 94 6 EGSE Input from A.Jimenez Add Primary Jan 95 6 reduction input from A.Jimenez Add Jan 95 2 instrumental and orbital corrections Feb 95 24 Nov 94 24 Nov 94 30 Jan 95 ISSUE SECTION / DATE CHANGES / REV. PAGE 4.2.2 3 Feb 95 Add guiding pixels' algorithm p-mode 4.2.3 16 Nov 94 identification method Add g-mode 4.2.3 25 Nov 94 identification technique and Lifetime 4.2.3 27 Jan 95 information from B.Andersen Add noise 4.2.3 5 8 Feb 95 simulation Re-ordering after 5.2 5.3 23 Jan 95 C.Fröhlich's input Add Nice inputs Add 5.4 6.1 9 Feb 95 Table 1 from Nice Add B.Andersen's input 6.2 6.2 27 Jan 95 Add par. on g-mode transmission Add 6.3 8 27 Jan 95 C.Fröhlich's input Add the data needed 8 Feb 95 7 for the flux budget Tuning after Jan 95 8 C.Fröhlich's input Add B.Andersen's Feb 95 5 input Add References Jan 95 23 Jan 95 2.1 Intro 21 Apr 95 Change VBB to VPB Remove spurious square 4.2.2 9 May 95 9 root in (3) Change Pi forSi for 4.2.2 5.2 May 95 21 compatibility in (8) Change degree to Apr 95 order in Table 1 2.2 All 15 Jun 95 Cosmetic change of the style Bullet to bullet Change of the style Formula to formula
In the course of the development of the investigation it is hoped that this document will , at any given snapshot in time, give a status of how the scientific side of the investigation is developing. By using this document we should be able to pin-point the areas where we need to concentrate our efforts; the different team members will contribute where they see their efforts are needed. Some pushing from the PI and/or ES may be attempted.
This document will certainly go through a large number of revisions, in order to keep an ordered track on the development and the status of the latest Revision, the responsibility of updating the VBB is upon the VIRGO Experiment Scientist. Currently the VBB has a formal status within the VIRGO investigation and is under configuration control.
I would like also to take this opportunity to point out the successful detection of the p modes by the Tenerife LOI. The LOI has now been working for more than 9 months. A considerable advance has been obtained for checking the integrity of the instrument, its potential problem (none found so far), the mode isolation by the optimal filters, and the extraction of the information contained in the p modes. A letter to A&A was written for summarizing the findings (Appourchaux et al, 1995).
The total irradiance is measured with active cavity radiometers (PM06-V and the Dual Irradiance Absolute Radiometer, DIARAD), the spectral irradiance by three-channel Sunphotometers (SPM) and the radiance with 12 resolution elements on the solar disk using the Luminosity Oscillations Imager (LOI).
These data will be utilized to achieve the main scientific objectives of VIRGO summarized in the following list:
Observations of the total solar irradiance will be a continuation of previous measurements from satellites, which have been performed by the radiometer HF of the Earth Radiation Budget experiment (ERB) on the NIMBUS-7 satellite from November 1978 until January 1993 (Hoyt et al 1992), by ACRIM~I on the Solar Maximum Mission satellite (SMM) from February 14, 1980 until June 1, 1989 (e.g. Willson and Hudson 1991), by ACRIM~II on the Upper Atmospheric Research Satellite (UARS) since October 1991 (Willson 1992) and by SOVA (Solar Variability) on the European Retrievable Carrier (EURECA) from August 1992 until May 1993 (Crommelynck et al 1993, Romero et al 1994).
The spectral irradiance data will resemble the data from IPHIR (Interplanetary Helioseismology with irradiance observations) on the Soviet PHOBOS Mission (Fröhlich et al 1990). As a secondary output the internal guider of the LOI component will produce time series of data proportional to the polar and equatorial solar diameters. All the data will have higher precision than any of those acquired formerly, both because of improvements to the instrumentation and because of the ideal location of the SOHO spacecraft which allows uninterrupted observation of the Sun. In addition, the spatial resolution of the LOI will allow us to deconvolve effects of different photospheric spatial variation on the solar irradiance, thereby enabling us to identify degrees and azimuthal orders of low-degree solar oscillations. The variations seen in these time series are a superposition of random and periodic phenomena that cause the whole variability of solar irradiance, both bolometric and at different wavelengths. This will be supplemented by even more detailed information from the flux budget product (128 x 128 pixels temporally averaged image of the solar disk), which will be studied in conjunction with the magnetograms provided by SOI/MDI on SOHO.
The scientific content of the gathered time series of irradiance can be studied in different ways. One way is to use it to observe and characterize solar oscillations. For the p modes the method has proven its value (e.g. Toutain and Fröhlich, 1992), and VIRGO will contribute by improving the reliability of the data to help resolve still controversial issues such as, for example, the temporal amplitude variation which is crucial for understanding in detail the excitation and damping mechanisms. But more important is the determination of the structure of the energy generating core. Of greatest interest, perhaps, is its relevance to the solar neutrino problem, for that has implications in other branches of physics. We do not even know for sure whether the neutrino problem is an issue in stellar-structure theory or is an issue in nuclear or particle physics. If it is the former, our task must surely be to investigate the wider implications in astrophysics. If it is the latter, it behoves us to determine the structure of the core - its radial stratification and its horizontal and temporal variations - as accurately as possible, in order to provide the most reliable information about the neutrino source to couple with the next generation of neutrino observations. To this end we need not only to model the variation of temperature, pressure and density, but also to try to model the related variation of chemical composition, both helium (hydrogen) abundance and the heavy-element abundances, using the direct seismic inferences. Together, the instruments on VIRGO will be able to provide extremely accurate frequencies of low-degree modes which are an essential complement to the intermediate- and high-degree data from SOI/MDI required to render it possible to carry out inversions for both the spherical and aspherical components of the stratification in the region where the thermonuclear reactions are taking place (Gough and Kosovichev, 1993).
The unambiguous detection and identification of solar gravity modes would be a real breakthrough for improving quite substantially our knowledge of the structure of the solar core. All attempts up to now have somehow failed because the amplitudes of the modes seem to be so small that the g-mode signals are buried in the noise (Fröhlich and Delache 1984). Although most of this noise is of solar origin, some observational and methodological improvements can be expected. Oscillation periods of several hours are difficult to observe from the ground and from satellites in low-Earth orbits; moreover, this range could not be explored by IPHIR owing to the noise introduced by the influence of the variability of the spacecraft pointing. This period range, however, looks promising for the search of g modes, because the modes are not as crowded in frequency as they are at lower frequencies, and the separation of the different l and m is easier. Nevertheless, the amplitudes may still be smaller than the solar noise. Probably, the only possibility to overcome this problem is similar to the heterodyne detection of a signal at a known frequency buried in noise. Another possibility may be to utilize the difference in centre-to-limb variation of the g-mode signal and the solar noise. The crucial point in the detectability of solar g modes is the surface amplitude of the modes. This again depends on the mode amplitudes below the convection zone and the amount of attenuation of the modes through the evanescent convection zone. Numerical modelling indicates that convective overshoot into the interior may excite the waves (Andersen 1994) to amplitudes that should be detectable at the surface. This result, however, may be uncertain due to the inadequate treatment of radiative damping in the interior.
We do not know the values of the frequencies of the individual modes, but we do have information from theory about the frequency pattern for different solar models, and the splitting as a functional of rotation (e.g. Berthomieu et al 1978, Provost and Berthomieu 1986) and asphericity. As several trials of this method have shown -- although with less reliable data and at lower frequencies --this a priori information alone may not be sufficient (e.g. Fröhlich and Delache, 1984). As a further ingredient the predicted relative visibility of different modes observed as intensity fluctuations and as Doppler shifts can be used. The relation between the different apparent amplitudes varies with frequency and depends also on the degree of the g mode (Berthomieu and Provost 1990), and thus the detection and unique identification of these modes may be possible only by combining data from VIRGO, GOLF and SOI/MDI, which are all on SOHO observing the Sun simultaneously. A reliable calculation of the visibilities is not available, however, because it must certainly be influenced by convection, which cannot reliably be modelled. It is important to realize that even if g modes cannot be detected the accurate measurement of low-degree p modes of low order, which VIRGO is well suited to accomplish, will augment quite substantially our diagnostic capabilities for investigating the structure of the solar core.
Solar irradiance variability can be used to investigate many physical phenomena related to convection, the effects of magnetic fields, solar activity, etc. (e.g. Fröhlich 1994). We may be interested in the phenomena causing the variation, e.g. sunspots, or we may be mainly interested in the underlying physical causes for the existence of sunspots, e.g. dynamo theory, rotation and convection. The causes of irradiance changes are crucially important for the understanding of solar and stellar evolution. Irrespective of the cause, knowledge of the possible medium- and long-term variations of the solar irradiance are equally important for the understanding of terrestrial climatic change. While the solar energy in the entire spectrum and particularly at UV wavelengths has been monitored from space for more than one and a half decades, no continuous space observations of the solar total and spectral irradiance has been made so far. Besides its climatic implications, knowledge of the amount of the solar energy flux and its variability at visible and red wavelengths is also important for solar physics. Comparison of visible and infrared solar radiation with surface manifestations of solar activity will give us a better understanding of the physical processes taking place in the photosphere. Parallel studies of the changes in the total flux and in various spectral bands will provide the first information on the spectral redistribution of the total flux variability.
These aperiodic or quasiperiodic phenomena are best studied in the time domain, where the variations may be compared directly with the signatures of solar surface intensity structures (e.g. Willson and Hudson 1988) or by multivariate spectral analysis (e.g. Fröhlich and Pap 1989). Although irradiance variations and solar oscillations are, for simplicity, currently treated independently, it is highly likely that these domains overlap physically. The influence of the changing magnetic fields during the solar cycle on the frequency of p modes and solar luminosity is one example (Kuhn and Libbrecht 1991, Bachmann and Brown, 1993). Moreover, the long-period solar oscillations may be coupled to the seemingly aperiodic or quasiperiodic variations in irradiance (which has been suggested, for example, by Wolff, 1984), either directly or by influencing surface intensity configurations. Although the solar background signal has the effect of a noise signal on the oscillation measurements, this 'noise' contains valuable information about the causative physical phenomena such as granulation, mesogranulation, supergranulation and active regions, which influence the dynamical oscillation frequencies. It will be necessary to take the modifications to the oscillation frequencies into account in order to establish what they would have been had the activity not been present, for it is those putative unmodified frequencies that are required for the structure inversions.
VIRGO SCIENCE RETURN (1) Core structure Mean values of pressure, density, temperature, chemical composition in the solar core region. Radial stratification Values for the spherically symmetric mean values of pressure, density, temperature, chemical composition as function of radius below the convection zone. Internal rotation Rotation as function of radius and latitude. Core magnetic field Investigate the possible existence of a strong core field. Internal asphericity Possible internal asphericity due to rotation or other effects. g modes and convection Study the interaction between g-modes and convection,quality of internal cavity and the upper boundary conditions, g-mode amplitude and phase coherence. Core mixing Excitation and damping Lifetime of p modes and possibly g modes (in combination with GOLF and SOI data), driving mechanisms.Visibility effects on the observability of modes. Frequency and splitting changes.
VIRGO SCIENCE RETURN (2) Solar energy budget and variability Spectral and total irradiance variation, spectral and spatial redistribution of energy, provide accurate input for terrestrial climate modelling Luminosity changes and global effects Surface temperature variation Structure and variability of large scale surface temperature inhomogeneities. Variation of p-mode amplitudes and visibility. Active region flux budget Active region influence on radiance and irradiance, study the possible energy storage in the convection zone Diameter variations Relative variation of equatorial and meridional diameter/limb darkening, possible surface asphericity. Solar noise Study global characteristics of small scale solar surface phenomena through study of the background non periodic signal.
Inversions of the sound speed from medium degree p modes indicate a sharp change near the bottom of the convection zone, the causes of which is not completely understood.
Current observational data indicate that the latitude dependant differential rotation persists throughout the convection zone and that the rotation is more rigid below the convection zone. The change in rotation curve may cause shears which can influence the oscillation modes.
From low degree p-mode analysis there are currently no indications that the core region is spinning extremely rapidly; the most probable values are one to at most four times the surface rotation rate. This is still controversial and only very low order, low degree p modes or g modes can resolve this issue convincingly.
The lifetimes of modes is generally studied from the linewidths in the power spectrum. As the form of a line may be influenced by the way the time series are analyzed this method has yielded controversial results. The same may apply to the analysis of the amplitude modulation of modes. The truly continuous observations by SOHO and new method of time series analysis will possibly resolve this issue.
Studies have been carried out by the VIRGO team to determine the visibility, that is the surface amplitude of g modes in brightness and velocity. Together with investigations of the efficiency of the convection zone in exciting g modes these results will be the basis for the strategy to search for g modes.
Since variations in the solar energy flux - persistent over long periods of time - may trigger climate changes, it is fundamental to understand the underlying physical mechanisms and thus the possibilities for a solar forcing of climate on time scales of decades to centuries. It has been assumed that the variations in the solar output are directly related to the magnetic field evolution over the solar cycle (e.g. Hudson, 1988; Kuhn, 1994; Harvey, 1994). However, sunspots, faculae and the magnetic network cannot account for all the observed variations in total solar irradiance. It has been shown that empirical models of total solar irradiance underestimate the observed changes at the maximum of both solar cycles 21 and 22 (Foukal and Lean, 1988; Pap et al., 1994). The current empirical models are developed with simple linear regression analysis that gives only an overall picture on the relation between solar irradiance and solar activity indices. Identification of the cause of the residual discrepancy is a difficult problem, since temporal changes in differential rotation in the interior of the Sun, a solar dynamo magnetic field near the base of the convective zone or large scale convective cells may all produce long-term changes in the solar luminosity (Kuhn et al., 1988).
The irradiance observations of the SOHO/VIRGO experiment and high resolution solar images, available from the ground and also from space by the SOHO/MDI instrument, will help to understand solar irradiance changes which are related to the effects near the surface. Parallel investigations of solar oscillations and variations in the solar energy flux provide additional information on the solar interior. Although solar irradiance variations and oscillations are studied separately, it is likely that these domains overlap physically. The long-period solar oscillations may be coupled to the non-periodic or quasi-periodic variations in irradiance, either directly or by influencing the causes of surface intensity configurations (Fröhlich, 1990). The ultimate goal is to understand (1) how, (2) why and (3) on what mechanism govern at different time scale the solar energy flux. From this reconstruction and prediction of the solar induced climate changes might be possible.
Wavelengths: 402 nm
Bandwidths: 5 nm
Effective bandpass The spectral profiles can be found on the ftp server:ftp.estec.esa.nl/pub/loitenerife/PurpleBook.
/optical depth: To be derived from the bandpasses
Power on detectors 402 nm: 23 uW
500 nm: 28 uW
862 nm: 30 uW
Lifetime sensitivity variation: 402 nm: 95% (IPHIR and SOVA2, 335 nm)
500 nm: 49% (IPHIR and SOVA2)
862 nm: 17% (IPHIR and SOVA2)
Temperature sensitivity (effect of the filter included)
402 nm: -710 ppm/K
500 nm: -1040 ppm/K
862 nm: -270 ppm/K
402 nm: TBD
500 nm: TBD
862 nm: TBD
Temperature sensitivity: See above
Transmission drop: 402 nm: 52%
500 nm: 7%
862 nm: 1%
UV sensitivity: No UV data available (yet!)
Surface contamination: Unknown
Angular sensitivity: Unknown
V/F converters: included in total instrumental noise
Total Instrumental Noise
Short Term (above 1 mHz) 402 nm: See Fig SPM-I
500 nm: better than at 402 nm
862 nm: better than at 402 nm
Long Term (below 1mHz) 402 nm: See Fig SPM-I
500 nm: better than at 402 nm
865 nm: better than at 402 nm
Figure SPM-I: Power spectral density of the dark-current noise of the SPM blue channel, and an LOI pixel calculated from time series of 68 h and 45 h respectively. In the 5-minute range the noise of the SPM is about 5 x 105 times smaller than the noise between the p modes of IPHIR (= 0.5 ppm2 uHz-1). For the LOI pixel it is about 1.2 x 106 times smaller than the noise expected for a 12th of the solar disk (12 time the noise power of the full disk)
Filter: 500 nm
Bandwidth: 5 nm
Effective Bandpass: The spectral profile can be found on the ftp server: ftp.estec.esa.nl/pub/loitenerife/PurpleBook. It is an ascii file called LOIAndover.dat with 2 columns (wavelength, transmission).
Effective opacity/optical depth: To be derived from the bandpass
Detector: Deep diffused silicon photodiode with 12 scientific and 4 guiding pixels as given in Figure LOI-1. The crosstalk between pixel is smaller than 10-4.
Power on detector: 0.8 mW/cm2
Temperature sensitivity: about 50-100 ppm/K
Aging of temperature sensitivity: drift of -400 ppm/K to be controlled with a ground copy of the flight detector
Temperature sensitivity: to be measured
UV sensitivity: No degradation after 1 year of solar UV (below 400 nm)
Particle sensitivity: 5% relative transmittance drop after a 7-year SOHO dose.
Surface contamination: temperature of the filter should be about 10 0C higher than the structure, and should prevent condensation of contaminants.
Angular sensitivity: Unknown
Thermal stability : 0.2'/K
Voltage sensitivity: 16' for 650 Volts
High voltage supply: -150 to 450 Volts
Servo loop aging: unknown
Guiding power Spectrum:
EW axis: 5 "/[[radical]]Hz @ 0 Hz, 3.2 "/[[radical]]Hz @ 9000 uHz, linear dependence in the power spectrum, [[sigma]]=0.4".
NS axis: 3.16 "/[[radical]]Hz @ 0 Hz, 2.23 "/[[radical]]Hz @ 9000 uHz, linear dependence in the power spectrum, [[sigma]]=0.26"
Diameter Measurements: Spectrum rather flat 3.2 "/[[radical]]Hz, [[sigma]]=0.17".
The above power spectra are based on ground-based measurements. They are essentially limited by the atmospheric seeing and its associated transparency changes. These spectra should, hopefully, be better in space.
VF converters: 1 ppm/K
Short Term (5-min range): <0.5 ppm2/uHz (ground-based measurements, limited by the atmosphere)
Dark current noise: See Fig SPM-I
Long Term: unknown. Most of the noise contribution will be due to the temperature fluctuations on the filter and detector.
Lifetime variation: 30 ppm/year
Absolute accuracy: 0.17%
Sampling rate: 1 solar total irradiance / 2 minutes
Duty cycle: 17%
Resolution: 50 ppm
30 ppm (standard deviation of the "dark irradiance" every 2 minutes)
White noise, <0.4 ppm2/uHz
Temperature dependence: TBD
The following instrumental corrections will be needed:
Please refer to the VIRGO Operation manual for the observing sequence of the PMO6-V.
Lifetime variation: 30 ppm/year
Absolute calibration: 0.15%
Sampling rate: 1 solar total irradiance / 3 minutes
Duty cycle: 11%
White noise, <0.4 ppm2/uHz
The following instrumental corrections will be needed:
The data acquisition is performed by parallel VFCs with a full scale frequency of about 320 kHz. The basic sampling period is 10 s allowing for a theoretical resolution of 0.3 ppm for one reading. Within the 10 s period the signals are integrated during 9.4 s whereas the rest of the period is used for the calibration of the VFC with reference signals (400 ms, fs-resolution: 16 bit) and the reading and resetting of the counters (twice 100 ms). This timing allows for a continuous electrical calibration and a high duty cycle of the reading (94%). The electrical calibrations are performed in turn at three points, namely zero, half full scale and full scale allowing for a first order correction of the non-linearity of the VFC during the evaluation of the data. With this system it is necessary to have as many parallel channels as signals to be measured simultaneously. 10 VFC are for the science data and 1 VFC for the HK channel.
To calibrate the VFCs and to supply internal and instrument thermistors, a set of two independent reference elements with amplifiers are implemented to supply reference voltages of 2.5V as half scale reference (HS), 3.5V as full scale reference voltage for the housekeeping channel (HK) and 4.04V for full scale reference for the science channels. This reference voltage, used for calibration of A/D channels, is indicated on "DAS Status" word on the data files HK or Sc.
Other facilities of VEOS is the execution of tests (when used with SIS), copy the received telemetry to a floppy disk.
More information about the EGSE utilities are in the EGSE User Manual (VIR-IAC-GSE/-003).
A total number of 480 blocks per day are expected. The aim of this first step is to get those 480 blocks per mission day. If some of them are wrong or missing in real time telemetry , the delayed telemetry will be used to replace them in the blocks time series. In the worst case that even retransmitted blocks are missing it would be necessary to wait to CD rom disk to have the last opportunity two get the data, if they are still missing those data are lost and zeros will be input in their places with the corresponding mission day and block of day in the headers.
Once the real definitive blocks series are obtained, the instrument files (at number of counts levels) will be generated for each instrument, obtaining complete time series for each instrument (SPM,PMO,DIARAD,LOI, engineering) with all the required information (Science and HK) necessary (TBC) for the analysis as requested by instrument CoI's.
After this level 0, the level 1 will be produced following the calibrations algorithms and known corrections supplied by instruments CoI's. During normal operation VDC will check for aberrant values in the Science or HK data, signalling when corrective actions needs to be taken.
The solar spectral irradiance S[[lambda]]0 at 1 astronomical unit (1AU) is given by:
where S[[lambda]]m is the measured spectral irradiance at the L1 point, r is the distance of the satellite to the center of the Sun, V is the satellite velocity in the frame linked to the Sun, v is the satellite velocity along the line of sight (v=V cos [[alpha]]), c is the speed of light (the correction due to V is of the order of 10-2 ppm)., [[theta]] is the off-axis angle between the line of sight and the optical axis of the LOI instrument; r is given by:
where x, y and z are the heliocentric coordinates of the satellite; v is given by:
V is given by:
And finally, the angle [[theta]] in rad can be derived from the high voltage on the Y and Z axis of the LOI instrument:
[[theta]]Z= a HZ + b HY - c (6)
[[theta]]Y= b HZ + a HY - c (7)
where HZ and HY are the high voltage on the actuators of the LOI, a, b and c are calibration factors (FM, a=0.031 '/Volt, b=-0.00367 '/Volt, c=3.56'; FS, a=0.030 '/Volt, b=-0.0017 '/Volt, c=3.7')
The current on each scientific pixel has to be corrected from 3 main effects:
where Si is the current on scientific pixel i , T is the mean temperature, in oC, of the detector as measured by the 4 temperature diodes, qT is the temperature dependence at 0 oC.
For the guiding pixel, the same remarks apply except that the changing size of the Sun should be taken into account in the first effect. And we write for the 4 guiding pixels:
where is s the temperature dependence at 0 oC for the guiding pixels. The last term is the effect of the changing size of the Sun, where Rsun is the size of the Sun with respect to the inner radius of the guiding pixel (=1.0556 after Figure LOI-1). This latter number will be refined and calibrated during the mission.
The full effects due to the limb darkening will be implemented in due time.
The total solar irradiance S0 at 1 astronomical unit (1AU) is given by:
where Sm is the measured total solar irradiance at the L1 point, ct is the rate of change of the relative difference between the measured irradiances between the nominal instrument and the back-up one.
In addition, a thermal model taking into account the radiative exchanges of the cavity with its environment must be developed for the PMO6-V radiometers. It will include the exchanges with the muffler during the open and closed states as well as the losses of the cavity to space in the open state.
Same as PMO6-V
The maximum likelihood method (MLE) give still the most reliable estimates of the p-mode parameters. When working with the data of the Tenerife LOI, we found that fitting simultaneously the 2l+1 spectra of a given l gave the less noisy estimates of the frequency and the splitting. One of the main advantage was to have a common linewidth for the 2l+1 components (Appourchaux et al, 1995).
The mode identification of p modes has been tested with the data of the Tenerife LOI and is based on the fact that for modes that stand out clearly out of the noise this is easy. The others are identified with a table of frequency (S/N<3) and their existence is checked statistically using MLE and a modified R-test. Modes, which have never been identified before, will be looked for using theoretical frequencies and the MLE.
The lifetimes of the individual p modes may be studied indirectly or directly with three different methods:
The achievement of the main scientific goals of VIRGO depends on the identification and classification of solar global p modes. Since no unambiguous measurements of these modes have been made there is no definite estimate of the amplitudes. The basic assumption of the VIRGO team is that in the region from 30-120 uHz the signal will be dominated by the influence from solar "noise" and that the g modes will not be distinguished easily. Simulations of the solar "noise" have been carried out (Andersen et al, 1994). These simulations indicate that in this range, the solar signal is comparable with the observations of ACRIM-SMM and is thus well explained by super-, meso- and granulation. The instrumental noise level of the VIRGO observations should be lower than this solar "noise" by at least the same factor.
The main strategies to look into the "noise" are the following:
where is the unperturbed flux, is the perturbation induced by changes in the equilibrium state of the photosphere, both changes of size and of thermodynamic quantities (temperature density, etc...); and is the perturbation induces by changes of the surface shape (distortion). With:
where ([[theta]],[[phi]]) are the spherical coordinates in the observer's frame (x, y, z), where z is the projection of the rotation axis of the Sun onto the plane of the sky and x is the line of sight; I (u) is the limb-darkening with u =sin [[theta]] cos [[phi]] ; D is the integration domain (LOI pixels for example); and:
where dI(u) is an equivalent limb-darkening depending on the mode eigenfunction and model of the solar photosphere (Toutain and Gouttebroze, 1993). For practical purposes, we give here below, in a table, the coefficients for dI(u) using an adiabatic approximation for the eigenfunction. Hence we write:
where the ai are given in Table 1 for a set of low-degree g modes; and in addition we also have:
Equations (13) and (15) are correct when the pulsation axis z' is also the z axis of the observer's frame. During the SOHO mission this will certainly not be the case. A small correction will be necessary due to the B angle, the P angle will be kept by the spacecraft as close to zero as possible. Therefore, the new or will be written as:
where the rotation matrices can be computed from Edmonds (1957) (See also Appourchaux and Andersen, 1990, for further details). In a first approximation, the B angle will be derived from the ephemeris. A small correction due to the position of the satellite on the halo orbit will be implemented, if necessary.
order a0 a1 a2 21 0.8544 -32.3683 106.0366 20 0.7589 -28.7309 94.6659 19 0.6698 -25.3014 83.9535 18 0.5840 -22.0435 73.7927 17 0.5039 -18.9810 64.2439 16 0.4279 -16.0947 55.2769 15 0.3565 -13.3725 46.8247 14 0.2896 -10.8102 38.9027 13 0.2259 -8.3727 31.3994 12 0.1645 -6.0507 24.2933 11 0.1078 -3.8646 17.6563 10 0.0535 -1.7864 11.4124 9 0.0013 0.2125 5.5080 8 0.0487 2.1407 -0.0668 7 0.0979 4.0498 -5.4223 6 0.1481 5.9931 10.6591
Table 1: "Limb darkening coefficients" for the lagrangian perturbation of a set of l=2 g modes.
Two different ways of computing FD have been considered, one using a Lagrangian formulation, the other an Eulerian one (Berthomieu et al 1990, Provost et al 1991, Gouttebroze et al 1994). A comparison of the two methods is being carried on. It shows an agreement between the two results. It appears that the contributions to the flux of the perturbations of source function and of the opacity of the medium almost cancel and that the contribution of the geometrical effects is not negligible. However the effect of convection is not taken into account and this should be one aim of future work.
Simulations and theoretical considerations (Andreassen et al 1992, Christensen-Dalsgaard 1980) indicate that the transmission of g modes through the convection zone depends directly on the depth of the convection and on the l values of the modes. In addition specific combinations of l values and the horizontal scales of motion in the convection zone may decrease or increase the transmission of these modes.
For the g-mode detection the comparison of the amplitudes and possibly the phases between velocity and intensity will be of crucial importance for the unique identification of the g modes. Together with the LOI data this will probably be the only way of detecting the g modes buried in the solar noise.
The 128 x 128 pixel flux budget data from MDI/SOI will be used for the studies of the influence of the photospheric manifestations of solar activity on the total and spectral irradiance, e.g. the flux budget of spots, faculae and bright network. Together with the magnetograms from MDI/SOI these images will help to better identify effects and causes of the solar irradiance variability.
On the more exotic side a catalog of cometary impacts may also be useful.
In addition it may be necessary to understand the properties of the "chromospheric" cavity. This may be important in the study of possible resonances between internal gravity modes and atmospheric gravity waves.
In summary the VIRGO would like to have the following inputs from the corona instruments:
These inputs with information about specific events are required within 3
months of the event.
Tenerife Helioseismology group "belongs" and "not belongs" to BISON Network, it depends of the point of view. Belongs in the sense that one station is held in Tenerife Observatory from 1975. This station have produced the more continuous and best data of low degree oscillations. We are free to used this data of Tenerife station under our own responsibility, in this sense, the interface with VIRGO is absolutely clear, we will cooperate as it be necessary. Not Belongs in the sense of Network. The data of BISON Network are not shared with the institutes which take part on it, so we can not access to the data of the other BISON stations, in this sense we can not do anything. If sharing of data or other kind of cooperation are requested by VIRGO team, it is necessary to contact the Birmingham BISON headquarters. The velocity data of the Tenerife station or of the BISON network (if available) will be used to study the phase difference between intensity and velocity (K line).
So far no formal contact has been taken with the GONG team but since almost any VIRGO CoI should be a GONG member, we should not have any problem to use these data. A formal request may need to be send to the GONG project in due time. The GONG data will be used to study the phase difference between intensity and velocity (Ni line).
The relation with IRIS Network is absolutely different, the data of all the stations are shared and no big problems exist. IAC as member of IRIS Network will try to make this interfacing, friendly and improve the VIRGO cooperation. The IRIS data will be used to study the phase difference between intensity and velocity (Na line).
Taiwan Oscillation Network (TON)
TON have now three stations running , Tenerife, Beijing (China) and Big Bear. All instruments works nicely and the quality of images are very high. Other stations will be held soon. The first analysis are now being performed with very nice results. Taking into account the resolution of TON (1100 x 1100 pixels). TON will be a perfect complement to GONG and MDI because none of them have that resolution. Concerning the interfacing with TON Headquarters (Dr Dean Yi Chou, Taiwan) it will be no problem. The TON project is absolutely open to cooperate with other solar experiments. Data to be used are to be defined.
The instrument should be still operational when SOHO flies. If the launch is delayed by 6 months or more, we plan to put a second LOI in Baja, California, Mexico.
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