Variability Of Solar IRradiance And Gravity Oscillations Research Articles

Temporal variation of the photometric magnetic activity for the Sun and Kepler solar-like stars

Context. The photometric time series of solar-like stars can exhibit rotational modulation, that is, brightness variations due to active regions co-rotating with the stellar surface. These signatures allow us to constrain properties of stellar rotation and magnetic activity. Aims. In this work we investigate the behavior, particularly the variability in terms of strength, of the photometric magnetic activity of Kepler solar-like stars and compare it with that of the Sun. Methods. We adopted the photometric magnetic activity proxy, Sph, which was computed with a cadence of five times the rotation period (Prot). The average Sph was taken as the mean activity level, and the standard deviation was taken as a measure of the temporal variation of the magnetic activity over the Kepler observations. We also analyzed Sun-as-a-star photometric data from VIRGO (Variability of Solar Irradiance and Gravity Oscillations). Sun-like stars were selected from a very narrow parameter space around the solar properties, according to the recent Gaia-Kepler stellar properties catalog and the latest Kepler rotation catalog. We also looked into KIC 8006161 (HD 173701), a very active metal-rich G dwarf, and we compared its magnetic activity to that of stars with similar stellar fundamental parameters. Results. We find that the amplitude of Sph variability is strongly correlated with its mean value, independent of spectral type. An equivalent relationship has previously been found for ground-based observations of chromospheric activity emission and magnetic field strength, but in this work we show that photometric Kepler data also present the same behavior. While, depending on the phase of the cycle, the Sun is among the less active stars, we find that the Sph⊙ properties are consistent with those observed in Kepler Sun-like stars. KIC 8006161 is, however, among the most active of its peers, which tend to be metal-rich. This results from an underlying relationship between Prot and metallicity and supports the following interpretation of the magnetic activity of KIC 8006161: its strong activity is a consequence of its high metallicity, which affects the depth of the convection zone and, consequently, the efficiency of the dynamo.

A multi-instrument investigation of the frequency stability of oscillations above the acoustic cut-off frequency with solar activity

ABSTRACT Below the acoustic cut-off frequency, oscillations are trapped within the solar interior and become resonant. However, signatures of oscillations persist above the acoustic cut-off frequency, and these travelling waves are known as pseudo-modes. Acoustic oscillation frequencies are known to be correlated with the solar cycle, but the pseudo-mode frequencies are predicted to vary in antiphase. We have studied the variation in pseudo-mode frequencies with time systematically through the solar cycle. We analysed Sun-as-a-star data from Variability of Solar Irradiance and Gravity Oscillations (VIRGO), and Global Oscillations at Low Frequencies (GOLF), as well as the decomposed data from Global Oscillation Network (GONG) for harmonic degrees 0 ≤ l ≤ 200. The data cover over two solar cycles (1996–2021, depending on instrument). We split them into overlapping 100-d long segments and focused on two frequency ranges, namely 5600–$6800\, \rm \mu Hz$ and 5600–$7800\, \rm \mu Hz$. The frequency shifts between segments were then obtained by fitting the cross-correlation function between the segments’ periodograms. For VIRGO and GOLF, we found no significant variation of pseudo-mode frequencies with solar activity. However, in agreement with previous studies, we found that the pseudo-mode frequency variations are in antiphase with the solar cycle for GONG data. Furthermore, the pseudo-mode frequency shifts showed a double-peak feature at their maximum, which corresponds to solar activity minimum, and is not seen in solar activity proxies. An, as yet unexplained, pseudo-periodicity in the amplitude of the variation with harmonic degree l is also observed in the GONG data.

Modeling spurious forces on the LISA spacecraft across a full solar cycle

One source of noise for the laser interferometer space antenna (LISA) will be time-varying changes of the space environment in the form of solar wind particles and photon pressure from fluctuating solar irradiance. The approximate magnitude of these effects can be estimated from the average properties of the solar wind and the solar irradiance. We use data taken by the ACE (advanced Compton explorer) satellite and the VIRGO (variability of solar iRradiance and gravity oscillations) instrument on the SOHO satellite over an entire solar cycle to calculate the forces due to solar wind and photon pressure irradiance on the LISA spacecraft. We produce a realistic model of the effects of these environmental noise sources and their variation over the expected course of the LISA mission.

Acoustic spectrum fitting for a large set of solar-like pulsators

Asteroseismology provides the means both to constrain the global properties and to probe the internal structures of stars. Asteroseismic data are now available on large numbers of solar-type stars, thanks in particular to the CoRoT and Kepler space missions, and automated data-analysis pipelines are needed to provide efficient and timely results. Here, we present an automated algorithm that is able to extract mode parameters under low signal-to-noise ratio conditions. We use a Bayesian framework to ensure the robustness of the algorithm. We discuss the efficiency of the method and test it using Variability of Solar Irradiance and Gravity Oscillations (VIRGO) Sun-as-a-star photometry data and the artificial Astero Fitting at Low Angular degree Group (asteroFLAG) Kepler ensemble. Analysis of the VIRGO data shows that it is possible to track variations of the individual mode parameters (frequency, height, width) through the solar cycle, using short time series (30 days). The present analysis also revealed a modulation of the degree l = 2 relative height through the solar cycle. Applied on asteroFLAG data, we show that the pipeline extracts accurately the central frequency and the large separation. It is also able to identify the degree of the modes in 78 per cent of stars.

The acoustic low-degree modes of the Sun measured with 14 years of continuous GOLF & VIRGO measurements

The helioseismic Global Oscillation at Low Frequency (GOLF) and the Variability of solar Irradiance and Gravity Oscillations (VIRGO) instruments onboard SoHO, have been observing the Sun continuously for the last 14 years. In this preliminary work, we characterize the acoustic modes over the entire p-mode range in both, Doppler velocity and luminosity, with a special care for the low-frequency modes taking advantage of the stability and the high duty cycle of space observations.

ON THE FLARE INDUCED HIGH-FREQUENCY GLOBAL WAVES IN THE SUN

Recently, Karoff and Kjeldsen (2008) presented evidence of strong correlation between the energy in the high-frequency part (5.3<\nu<8.3 mHz) of the acoustic spectrum of the Sun and the solar X-ray flux. They have used disk-integrated intensity observations of the Sun obtained from the VIRGO (Variability of solar IRradiance and Gravity Oscillations) instrument on board SOHO (Solar and Heliospheric Observatory) spacecraft. Similar signature of flares in velocity observations has not been confirmed till now. The study of low-degree high-frequency waves in the Sun is important for our understanding of the dynamics of the deeper solar layers. In this paper, we present the analysis of the velocity observations of the Sun obtained from the MDI (Michelson and Doppler Imager) and the GOLF (Global Oscillations at Low Frequencies) instruments on board SOHO for some major flare events of the solar cycle 23. Application of wavelet techniques to the time series of disk-integrated velocity signals from the solar surface using the full-disk Dopplergrams obtained from the MDI clearly indicates that there is enhancement of high-frequency global waves in the Sun during the flares. This signature of flares is also visible in the Fourier Power Spectrum of these velocity oscillations. On the other hand, the analysis of disk-integrated velocity observations obtained from the GOLF shows only marginal evidence of effects of flares on high-frequency oscillations.

Sources of Differences in On-Orbital Total Solar Irradiance Measurements and Description of a Proposed Laboratory Intercomparison.

There is a 5 W/m2 (about 0.35 %) difference between current on-orbit Total Solar Irradiance (TSI) measurements. On 18–20 July 2005, a workshop was held at the National Institute of Standards and Technology (NIST) in Gaithersburg, Maryland that focused on understanding possible reasons for this difference, through an examination of the instrument designs, calibration approaches, and appropriate measurement equations. The instruments studied in that workshop included the Active Cavity Radiometer Irradiance Monitor III (ACRIM III) on the Active Cavity Radiometer Irradiance Monitor SATellite (ACRIMSAT), the Total Irradiance Monitor (TIM) on the Solar Radiation and Climate Experiment (SORCE), the Variability of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and Heliospheric Observatory (SOHO), and the Earth Radiation Budget Experiment (ERBE) on the Earth Radiation Budget Satellite (ERBS). Presentations for each instrument included descriptions of its design, its measurement equation and uncertainty budget, and the methods used to assess on-orbit degradation. The workshop also included a session on satellite- and ground-based instrument comparisons and a session on laboratory-based comparisons and the application of new laboratory comparison techniques. The workshop has led to investigations of the effects of diffraction and of aperture area measurements on the differences between instruments. In addition, a laboratory-based instrument comparison is proposed that uses optical power measurements (with lasers that underfill the apertures of the TSI instruments), irradiance measurements (with lasers that overfill the apertures of the TSI instrument), and a cryogenic electrical substitution radiometer as a standard for comparing the instruments. A summary of the workshop and an overview of the proposed research efforts are presented here.

Study of the Solar Cycle Dependence of Low‐Degreep‐Modes with Michelson Doppler Imager and VIRGO

Studying variations of observational properties of low-degree solar acoustic p-modes (radial, dipole, and quadrupole) with the activity cycle is important for understanding potential effects of the cycle in the deep interior, obtaining more accurate inversions for the sound speed and rotation, and also for estimating prospective investigation of stellar activity by asteroseismology. We have analyzed changes of low-degree p-mode parameters during the current solar cycle (number 23) by using velocity and intensity oscillation data from two instruments on the Solar and Heliospheric Observatory (SOHO): the Michelson Doppler Imager (MDI) and Variability of Solar Irradiance and Gravity Oscillations (VIRGO). We show a clear dependence of mode frequency, line width, frequency splitting, and line asymmetry on the solar cycle. Moreover, we demonstrate that these properties change differently for modes of different frequency, angular degree, and order. The greatest changes are observed for sectorial modes of frequencies higher than 3 mHz, corresponding to the fact that the most significant variations of the solar structure occur in low-latitude regions close to the solar surface.

Contribution of the Solar Constant (SOLCON) program to the long‐term total solar irradiance observations

On long timescales the variation of the total solar irradiance (TSI) received by the Earth is believed to be one of the climate change drivers. Therefore accurate and time‐stable measurements of the total solar irradiance are necessary. The Solar Constant (SOLCON) instrument made TSI measurements in April 1992 and during the International Extreme Ultraviolet Hitchhiker (IEH) 3 STS 95 shuttle flight in autumn 1998. We assume that the SOLCON instrument remained stable within 0.01 % in between those measurements, and we verify this assumption as well as possible. From the SOLCON measurements we conclude the following: (1) The 1998 Space Absolute Radiometric Reference (SARR) adjustment coefficient applicable to the Active Cavity Radiometer Irradiance Monitor II (ACRIM II) during the IEH 3 period is 1.000438 with a one sigma uncertainty of 18 ppm, compared to the 1993 SARR adjustment coefficient of 1.000258. (2) The solar monitor on the Earth Radiation Budget Satellite (ERBS), with a 1993 SARR adjustment coefficient of 1.000453, has not aged within a one sigma uncertainty level of 130 ppm; and (3) the 1998 SARR adjustment coefficients for the Variability of Solar Irradiance and Gravity Oscillations (VIRGO) radiometers have been determined with a one sigma uncertainty of 10 ppm: They are 1.000025 for the Differential Absolute Radiometer left channel (DIARAD‐L) and 1.000279 for the version 1.2 data from the PMO6‐VA radiometer.

Uncertainty of total solar irradiance: an assessment of the last twenty years of space radiometry

A complete set of total solar irradiance measurements from space is available dating from November 1978, yielding a time series of more than twenty years. From measurements made by different space radiometers (HF on NIMBUS-7, ACRIM I on SMM, ACRIM II on UARS and VIRGO on SOHO) a composite record of solar total irradiance can be compiled. The corrections needed for each radiometer, mainly to compensate for degradation, are based on a detailed study of the behaviour of the VIRGO (Variability of Solar Irradiance and Gravity Oscillations) radiometers. Moreover, several errors arising from operational changes or interruptions, which influence the overall uncertainty more than corrections for degradation, have to be corrected. A composite of the corrected time series is constructed in order to estimate the long-term stability, which is much better than the estimated 1 part in 105 per annum deduced as an upper limit from VIRGO radiometry. The absolute accuracy of the data, however, is more difficult to estimate.

Observational Upper Limits to Low‐Degree Solarg‐Modes

Observations made by the Michelson Doppler Imager (MDI) and Variability of solar IRradiance and Gravity Oscillations (VIRGO) on the Solar and Heliospheric Observatory (SOHO) and by the ground-based Birmingham Solar Oscillations Network (BiSON) and Global Oscillations Network Group (GONG) have been used in a concerted effort to search for solar gravity oscillations. All spectra are dominated by solar noise in the frequency region from 100 to 1000 μHz, where g-modes are expected to be found. Several methods have been used in an effort to extract any g-mode signal present. These include (1) the correlation of data—both full-disk and imaged (with different spatial-mask properties)—collected over different time intervals from the same instrument, (2) the correlation of near-contemporaneous data from different instruments, and (3) the extraction—through the application of complex filtering techniques—of the coherent part of data collected at different heights in the solar atmosphere. The detection limit is set by the loss of coherence caused by the temporal evolution and the motion (e.g., rotation) of superficial structures. Although we cannot identify any g-mode signature, we have nevertheless set a firm upper limit to the amplitudes of the modes: at 200 μHz, they are below 10 mm s-1 in velocity, and below 0.5 parts per million in intensity. The velocity limit corresponds very approximately to a peak-to-peak vertical displacement of δR/R☉ = 2.3 × 10-8 at the solar surface. These levels which are much lower than prior claims, are consistent with theoretical predictions.

What do VIRGO results tell about the solar interior and atmosphere

The more than two years of observations from VIRGO (Variability of solar IRradiance and Gravity Oscillations) yield a unique data set covering the activity minimum and the rising part of solar cycle 23. This allows not only to determine the influence of varying activity on the solar oscillation frequencies and amplitudes, but also to study the relationship between oscillations and irradiance variability.

In-Flight Performance of the Virgo Solar Irradiance Instruments on Soho

The in-flight performance of the total and spectral irradiance instruments within VIRGO (Variability of solar IRradiance and Gravity Oscillations) on the ESA/NASA Mission SOHO (SOlar and Heliospheric observatory) is in most aspects better than expected. The behaviour during the first year of operation of the two types of radiometers and the sunphotometers together with a description of their data evaluation procedures is presented.