First results from VIRGO, the experiment for helioseismology and solar irradiance … (original) (raw)
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First Results from VIRGO, the Experiment for Helioseismology and Solar Irradiance Monitoring on SOHO
The First Results from SOHO, 1997
First results from the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) on the ESA/NASA Mission SOHO (Solar and Heliospheric Observatory) are reported. The observations started mid-January 1996 for the radiometers and sunphotometers and near the end of March for the luminosity oscillation imager. The performance of all the instruments is very good, and the time series of the first 4 -6 months are evaluated in terms of solar irradiance variability, solar background noise characteristics and p-mode oscillations. The solar irradiance is modulated by the passage of active regions across the disk, but not all of the modulation is straightforwardly explained in terms of sunspot flux blocking and facular enhancement. Helioseismic inversions of the observed p-mode frequencies are more-or-less in agreement with the latest standard solar models.
VIRGO: Experiment for helioseismology and solar irradiance monitoring
Solar Physics, 1995
The scientific objective of the VIRGO experiment (Variability of solar IRradiance and Gravity Oscillations) is to determine the characteristics of pressure and internal gravity oscillations by observing irradiance and radiance variations, to measure the solar total and spectral irradiance and to quantify their variability over periods of days to the duration of the mission. With these data helioseismological methods can be used to probe the solar interior. Certain characteristics of convection and its interaction with magnetic fields, related to, for example, activity, will be studied from the results of the irradiance monitoring and from the comparison of amplitudes and phases of the oscillations as manifest in brightness from VIRGO, in velocity from GOLF, and in both velocity and continuum intensity from SOI/MDI. The VIRGO experiment contains two different active-cavity radiometers for monitoring the solar ‘constant’, two three-channel sunphotometers (SPM) for the measurement of the spectral irradiance at 402, 500 and 862 nm, and a low-resolution imager (LOI) with 12 pixels, for the measurement of the radiance distribution over the solar disk at 500 um. In this paper the scientific objectives of VIRGO are presented, the instruments and the data acquisition and control system are described in detail, and their measured performance is given.
The First Results from SOHO, 1997
The three helioseismology instruments aboard SOHO observe solar p modes in velocity (GOLF and MDI) and in intensity (VIRGO and MDI). Time series of two months duration are compared and confirm that the instruments indeed observe the same Sun to a high degree of precision. Power spectra of 108 days are compared showing systematic differences between mode frequencies measured in intensity and in velocity. Data coverage exceeds 97% for all the instruments during this interval. The weighted mean differences (V , I) are ,0.1 ऌHz for l = 0, and ,0.16 ऌHz for l = 1.
Monthly Notices of the Royal Astronomical Society, 1999
We use the solutions to a simple, one-dimensional wave equation ± which is intended to describe the essential elements of the solar resonant acoustic cavity ± as formalistic models to which to fit low-`modes in observational helioseismic power spectra. We have analysed data collected in velocity, by the ground-based Birmingham Solar-Oscillations Network (BiSON), and in intensity, by the full-disc VIRGO Sun photometers (SPM) on board the ESA/NASA SOHO satellite.
Global p-mode oscillations throughout the complete solar cycle 23 and the beginning of cycle 24
Journal of Physics: Conference Series, 2011
The parameters of the p-mode oscillations vary with solar activity. Such temporal variations provide insights for the study of the structural and dynamical changes occurring in the Sun's interior throughout the solar cycle. We present here a complete picture of the temporal variations of the global p-mode parameters (excitation, damping, frequency, peak asymmetry, and rotational splitting) over the entire solar cycle 23 and the beginning of cycle 24 as observed by the space-based, Sun-as-a-star helioseismic GOLF and VIRGO instruments onboard SoHO.
In-Flight Performance of the Virgo Solar Irradiance Instruments on Soho
Solar Physics, 1997
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.
Solar oscillations observed in the total irradiance
Solar Physics, 1983
The total solar irradiance measurements obtained by the active-cavity radiometer on board the Solar Maximum Mission have been analyzed for evidence of global oscillations. We find that the most energetic low-degree p-mode oscillations in the five-minute band have amplitudes of a few parts per million of the total irradiance, and we positively detect modes with l = 0, I, and 2. The distribution in/differs from that of the velocity spectrum, with relatively more power at lower I values. The individual modes have narrow line widths, corresponding to values of Q greater than a few thousand, or lifetimes of at least a week. We do not detect the 160-rain oscillation in the power spectrum, and place an upper limit of 5 parts per million (99.9% confidence) on its amplitude.
MEASUREMENT OF LOW SIGNAL-TO-NOISE RATIO SOLAR p -MODES IN SPATIALLY RESOLVED HELIOSEISMIC DATA
The Astrophysical Journal, 2009
We present an adaptation of the rotation-corrected, m-averaged spectrum technique designed to observe low signal-to-noise-ratio, low-frequency solar p modes. The frequency shift of each of the 2l + 1 m spectra of a given (n, l) multiplet is chosen that maximizes the likelihood of the m-averaged spectrum. A high signal-to-noise ratio can result from combining individual low signal-to-noise-ratio, individual-m spectra, none of which would yield a strong enough peak to measure. We apply the technique to GONG and MDI data and show that it allows us to measure modes with lower frequencies than those obtained with classic peak-fitting analysis of the individual-m spectra. We measure their central frequencies, splittings, asymmetries, lifetimes, and amplitudes. The low-frequency, low-and intermediate-angular degrees rendered accessible by this new method correspond to modes that are sensitive to the deep solar interior down to the core (l ≤ 3) and to the radiative interior (4 ≤ l ≤ 35). Moreover, the low-frequency modes have deeper upper turning points, and are thus less sensitive to the turbulence and magnetic fields of the outer layers, as well as uncertainties in the nature of the external boundary condition. As a result of their longer lifetimes (narrower linewidths) at the same signal-to-noise ratio the determination of the frequencies of lower-frequency modes is more accurate, and the resulting inversions should be more precise.
Solar activity indices and their Impact on Helioshphere and GCR Modulation
Solar variability controls the structure of the heliosphere and produce changes in cosmic ray intensity. Based on the observation from Omniweb data centre for solar- interplanetary data and yearly mean count rate of cosmic ray intensity (CRI) variation data from Oulu / Moscow neutron monitors (Rc=0.80 GV & Rc=2.42 GV) during 1996-2014 . It is observed that the sun is remarkably quiet and the strength of the interplanetary magnetic field has been falling off to new low levels , reduces the GCR entering inner- heliosphere and it is high anti-correlation (-0.78) between sunspot number & GCR flux. It is also found that 10.7 cm solar radio flux, velocity of solar wind and the strength and turbulence of the interplanetary magnetic field were positive correlated with each other and inverse correlated with count rate of cosmic ray intensity.