Ferenc Varadi

Looking for Gravity-Mode Multiplets with the GOLF Experiment aboard SOHO

This paper is focused on the search for low-amplitude solar gravity modes between 150 and 400 � Hz, corresponding to low-degree, low-order modes. It presents results based on an original strategy that looks for multiplets instead of single peaks, taking into consideration our knowledge of the solar interior from acoustic modes. Five years of quasi-continuous measurements collected with the helioseismic GOLF experiment aboard the SOHO spacecraft are analyzed. We use different power spectrum estimators and calculate confidence levels for the most significant peaks. This approach allows us to look for signals with velocities down to 2 mm s � 1 ,n ot far from the limit of existing instruments aboard SOHO, amplitudes that have never been investigated up to now. We apply the method to series of 1290 days, beginning in 1996 April, near the solar cycle minimum. An automatic detection algorithm lists those peaks and multiplets that have a probability of more than 90% of not being pure noise. The detected patterns are then followed in time, considering also series of 1768 and 2034 days, partly covering the solar cycle maximum. In the analyzed frequency range, the probability of detection of the multiplets does not increase with time as for very long lifetime modes. This is partly due to the observational conditions after 1998 October and the degradation of these observational conditions near the solar maximum, since these modes have a ‘‘mixed’’ character and probably behave as acoustic modes. Several structures retain our attention because of the presence of persistent peaks along the whole time span. These features may support the idea of an increase of the rotation in the inner core. There are good arguments for thinking that complementary observations up to the solar activity minimum in 2007 will be decisive for drawing conclusions on the presence or absence of gravity modes detected aboard the SOHO satellite.

Successive Refinements in Long-Term Integrations of Planetary Orbits

We report on accurate, long-term numerical simulations of the orbits of the major planets in our solar system. The equations of motion are directly integrated by a Stormer multistep scheme, which is optimized to reduce round-off errors. The physical models are successively refined to include corrections due to general relativity and the finite size of the lunar orbit. In one case, the Earth-Moon system is resolved as two separate bodies, and the results are compared with those based on analytically averaging the lunar orbit. Through this comparison, a better analytical model is obtained. The computed orbits are in good agreement with those of previous studies for the past 5 Myr but not for earlier times. The inner planets exhibit chaotic behavior with a Lyapunov time of exponential separation of nearby orbits equal to about 4 Myr. Modeling uncertainties and chaos in the inner solar system restrict the accuracy of the computations beyond the past 50 Myr. We do not observe marked chaos in the motion of the Jovian planets in our 90 Myr integration, and we infer that the Lyapunov time for those planets is at least 30 Myr.

Comparison of Frequencies and Rotational Splittings of Solar Acoustic Modes of Low Angular Degree from Simultaneous MDI and GOLF Observations

During the years 1996 through 1998 the Michelson Doppler Imager (MDI) and the Global Oscillations at Low Frequency (GOLF) experiments on the Solar and Heliospheric Observatory (SOHO) mission have provided unique and nearly uninterrupted sequences of helioseismic observations. This paper describes the analysis carried out on power spectra from 759 days of calibrated disk-averaged velocity signals provided by these two experiments. The period investigated in this work is from 1996 May 25 to 1998 June 22. We report the results of frequency determination of low-degree (l ≤ 3) acoustic modes in the frequency range between 1.4 mHz and 3.7 mHz. Rotational splittings are also measured for nonradial modes up to 3.0 mHz. The power spectrum estimation of the signals is performed using classical Fourier analysis and the line-profile parameters of the modes are determined by means of a maximum likelihood method. All parameters have been estimated using both symmetrical and asymmetrical line profile-fitting formula. The line asymmetry parameter of all modes with frequency higher than 2.0 mHz is systematically negative and independent of l. This result is consistent with the fact that both MDI and GOLF data sets investigated in this paper are predominantly velocity signals, in agreement with previous results. A comparison of the results between the symmetric and asymmetric fits shows that there is a systematic shift in the frequencies for modes above 2.0 mHz. Below this frequency, the line width of the modes is very small and the time base of the data does not provide enough statistics to reveal an asymmetry. In general, the results show that frequency and rotational splitting values obtained from both the MDI and GOLF signals are in excellent agreement, and no significant differences exist between the two data sets within the accuracy of the measurements. Our results are consistent with a uniform rotation of the solar core at the rate of about 435 nHz and show only very small deviations of the core structure from the standard solar model.

Identification of Solar Acoustic Modes of Low Angular Degree and Low Radial Order

We present evidence for the detection of low radial order (n < 10) acoustic modes of low angular degree, l = 0-2, in the 759 day long Global Oscillations at Low Frequency and Michelson Doppler Imager time series. We used Random-Lag Singular Cross-Spectrum Analysis, which searches for simultaneous oscillatory components in two or more time series. We have determined 11 modes in the range n = 3-9, of which eight modes confirm the previous measurements by Toutain et al. and three modes of l = 0 and n = 3, 5, and 6 are reliably measured for the first time. The errors of frequency determination are also significantly reduced for several previously identified modes. New sound speed inversion results suggest that the effect of inhomogeneous initial composition of the Sun should be included in the standard solar model.

SEARCHING FOR SIGNAL IN NOISE BY RANDOM LAG SINGULAR SPECTRUM ANALYSIS

Singular spectrum analysis, a technique to detect oscillations in short and noisy time series, was first developed for geophysical applications. This work offers a generalization for long and noisy time series in astrophysical applications. The motivating problem is the detection of low-amplitude solar oscillations.

Modeling the heterogeneity in risk of progression to Alzheimer's disease across cognitive profiles in mild cognitive impairment.

IntroductionHeterogeneity in risk of conversion to Alzheimers disease (AD) among individualswith mild cognitive impairment (MCI) is well known. Novel statistical methods thatare based on partially ordered set (poset) models can be used to create modelsthat provide detailed and accurate information about performance with specificcognitive functions. This approach allows for the study of direct links betweenspecific cognitive functions and risk of conversion to AD from MCI. It also allowsfor further delineation of multi-domain amnestic MCI, in relation to specificnon-amnestic cognitive deficits, and the modeling of a range of episodic memoryfunctioning levels.MethodsFrom the Alzheimers Disease Neuroimaging Initiative (ADNI) study, conversion at24 months of 268 MCI subjects was analyzed. It was found that 101 of thosesubjects (37.7%) converted to AD within that time frame. Poset models were thenused to classify cognitive performance for MCI subjects. Respective observedconversion rates to AD were calculated for various cognitive subgroups, and byAPOE e4 allele status. These rates were then compared across subgroups.ResultsThe observed conversion rate for MCI subjects with a relatively lower functioningwith a high level of episodic memory at baseline was 61.2%. In MCI subjects whoadditionally also had relatively lower perceptual motor speed functioning and atleast one APOE e4 allele, the conversion rate was 84.2%. In contrast, the observedconversion rate was 9.8% for MCI subjects with a relatively higher episodic memoryfunctioning level and no APOE e4 allele. Relatively lower functioning withcognitive flexibility and perceptual motor speed by itself also appears to beassociated with higher conversion rates.ConclusionsAmong MCI subjects, specific baseline cognitive profiles that were derived throughposet modeling methods, are clearly associated with differential rates ofconversion to AD. More precise delineation of MCI by such cognitive functioningprofiles, including notions such as multidomain amnestic MCI, can help in gainingfurther insight into how heterogeneity arises in outcomes. Poset-based modelingmethods may be useful for providing more precise classification of cognitivesubgroups among MCI for imaging and genetics studies, and for developing moreefficient and focused cognitive test batteries.

Random-Lag Singular Cross-Spectrum Analysis.

In a previous paper (Varadi et al.), random-lag singular spectrum analysis was introduced for finding oscillations in very noisy and long time series. This work presents a generalization of the technique to search for common oscillations in two or more time series.

Latent Partially Ordered Classification Models and Normal Mixtures

Latent partially ordered sets (posets) can be employed in modeling cognitive functioning, such as in the analysis of neuropsychological (NP) and educational test data. Posets are cognitively diagnostic in the sense that classification states in these models are associated with detailed profiles of cognitive functioning. These profiles allow for deeper insight into how functioning can be affected by neurological conditions or by interventions that impact cognition or learning. Responses to NP measures or test items are used as a basis for classification. A natural and useful extension for response models that can be employed in cognitively diagnostic modeling is the implementation of nonparametric density estimation methods. For instance, an issue with NP assessment data is that complex response distributions can arise, such as for populations that are in part comprised of cognitively impaired subjects. To model such complexity, a Dirichlet process prior approach to Bayesian nonparametric density estimation for latent poset models is described. These methods are demonstrated with an analysis of NP data from a study of schizophrenia.

Erratum: “Looking for Gravity-Mode Multiplets with the GOLF Experiment aboard SOHO” (ApJ, 604, 455 [2004])

As a result of an error at the Press, the second panel of Figure 9 was repeated twice in the top row of the printed, black-andwhite version of this figure, and the first panel was omitted. This error appears in the print edition and the PDF and postscript (PS) versions available with the electronic edition of the journal, although the panels of the color figure displayed in the electronic article itself are correct. Please see below for the corrected print version of Figure 9. The Press sincerely regrets the error.

Variations in total solar and spectral irradiance as measured by the VIRGO experiment on SOHO

Abstract The Variability IRradiance Gravity Oscillation (VIRGO) experiment on SOHO has been observing total solar and spectral irradiances at 402 nm (blue channel), 500 nm (green channel), and 862 nm (red channel) since January 1996. The VIRGO observations have shown that solar active regions influence both total and spectral irradiances in a similar fashion, although the amplitude of the variations seems to be the largest for the near-UV and visible wavelengths. Comparison of the VIRGO total solar irradiance and the UARS/SUSIM Mg II h & k core-to-wing ratio shows that total irradiance started to rise in prior to UV irradiance, as represented by the Mg core-to-wing ratio. In this paper we review the most recent results on the VIRGO irradiance variations related to solar activity. We dedicate this paper to the memory of Dr. Guenther Brueckner, the late Principal Investigator of the UARS/SUSIM experiment, who will always remain in the heart and memory of the authors of this paper.