VIRGO: Experiment for helioseismology and solar irradiance monitoring

Fröhlich, C.; Romero, J.; Roth, Hansjörg; Wehrli, Christoph; Andersen, B. N.; Appourchaux, T.; Domingo, V.; Telljohann, U.; Berthomieu, G.; Delache, P.; Provost, J.; Toutain, T.; Crommelynck, D.; Chevalier, André; Fichot, Alain; Däppen, Werner; Gough, D. O.; Hoeksema, J. T.; Jiménez, A.; Gómez, Maria F.; Herreros, J. M.; Cortés, T. Roca; Jones, A. R.; Pap, J. M.; Willson, R. C.

doi:10.1007/bf00733428

Cited by 271 publications

(133 citation statements)

References 22 publications

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“…4 and Appendix B for additional figures of hysteresis at each l). The frequency shifts are actually measured to be ahead of the radio flux by about 60 days in the velocity GOLF data, while a longer delay of about 90 days is observed with the data from the three solar photometers composing the Variability of Solar Irradiance and Gravity Oscillations (VIRGO; Fröhlich et al 1995) instrument on board SoHO (Salabert et al 2013). Table 2 gives the differences in the frequency shifts averaged over l = 0, 1, and 2 modes between the maximum and minimum of each solar cycle for the different frequency ranges as illustrated in Fig.…”

Section: At Different Periods Of the Cyclesmentioning

confidence: 99%

Seismic sensitivity to sub-surface solar activity from 18 yr of GOLF/SoHO observations

Salabert

García

Turck‐Chièze

2015

A&A

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Solar activity has significantly changed over the last two Schwabe cycles. After a long and deep minimum at the end of Cycle 23, the weaker activity of Cycle 24 contrasts with the previous cycles. In this work, the response of the solar acoustic oscillations to solar activity is used in order to provide insights into the structural and magnetic changes in the sub-surface layers of the Sun during this on-going unusual period of low activity. We analyze 18 yr of continuous observations of the solar acoustic oscillations collected by the Sun-as-a-star GOLF instrument on board the SoHO spacecraft. From the fitted mode frequencies, the temporal variability of the frequency shifts of the radial, dipolar, and quadrupolar modes are studied for different frequency ranges that are sensitive to different layers in the solar sub-surface interior. The low-frequency modes show nearly unchanged frequency shifts between Cycles 23 and 24, with a time evolving signature of the quasi-biennial oscillation, which is particularly visible for the quadrupole component revealing the presence of a complex magnetic structure. The modes at higher frequencies show frequency shifts that are 30% smaller during Cycle 24, which is in agreement with the decrease observed in the surface activity between Cycles 23 and 24. The analysis of 18 yr of GOLF oscillations indicates that the structural and magnetic changes responsible for the frequency shifts remained comparable between Cycle 23 and Cycle 24 in the deeper sub-surface layers below 1400 km as revealed by the low-frequency modes. The frequency shifts of the higher-frequency modes, sensitive to shallower regions, show that Cycle 24 is magnetically weaker in the upper layers of Sun.

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Section: At Different Periods Of the Cyclesmentioning

confidence: 99%

Seismic sensitivity to sub-surface solar activity from 18 yr of GOLF/SoHO observations

Salabert

García

Turck‐Chièze

2015

A&A

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“…In Table 2 the global S ph index corrected for the photon noise as computed above, as well as the local indexes, also corrected for the photon noise: S ph (n low ) and S ph (n high ) , are given for the 22 F stars and the Sun using subseries of length 5 × P rot . To compute the solar magnetic index S ph, we used the photometric data recorded by the Sun Photometers (SPM), belonging to the VIRGO (Variability of solar Irradiance and Gravity Oscillations; Fröhlich et al 1995) instrument on board the Solar and Heliospheric Observatory (SoHO). We summed the two channels (green and red) to be close to the Kepler bandwidth and rebinned the data so that they have a sampling of 30 min like the Kepler long-cadence data.…”

Section: Magnetic Indexmentioning

confidence: 99%

Magnetic activity of F stars observed byKepler

Mathur

García

Ballot

et al. 2014

A&A

162

195

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Context. The study of stellar activity is important because it can provide new constraints for dynamo models when combined with surface rotation rates and the depth of the convection zone. We know that the dynamo mechanism, which is believed to be the main process that rules the magnetic cycle of solar-like stars, results from the interaction between (differential) rotation, convection, and magnetic field. The Kepler mission has already been collecting data for a large number of stars during four years allowing us to investigate magnetic stellar cycles. Aims. We investigated the Kepler light curves to look for magnetic activity or even hints of magnetic activity cycles. Based on the photometric data we also looked for new magnetic indexes to characterise the magnetic activity of the stars. Methods. We selected a sample of 22 solar-like F stars that have a rotation period shorter than 12 days. We performed a timefrequency analysis using the Morlet wavelet yielding a magnetic proxy for our sample of stars. We computed the magnetic index S ph as the standard deviation of the whole time series and the index S ph , which is the mean of standard deviations measured in subseries of length five times the rotation period of the star. We defined new indicators, such as the contrast between high and low activity, to take into account the fact that complete magnetic cycles are not observed for all the stars. We also inferred the Rossby number of the stars and studied their stellar background. Results. This analysis shows different types of behaviour in the 22 F stars. Two stars show behaviour very similar to magnetic activity cycles. Five stars show long-lived spots or active regions suggesting the existence of active longitudes. Two stars in our sample seem to have a decreasing or increasing trend in the temporal variation of the magnetic proxies. Finally, the last group of stars shows magnetic activity (with the presence of spots) but no sign of cycle.

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“…The optical compartment containing the filters and detectors is physically separated from the rest of the VIRGO experiment, and was permanently flushed with 5.5 grade nitrogen until launch. Further details about the VIRGO experiment are given in Fröhlich et al (1995).…”

Section: Description Of the Instrumentmentioning

confidence: 99%

Correlation of spectral solar irradiance with solar activity as measured by VIRGO

Wehrli¹,

Schmutz²,

Шапиро³

2013

A&A

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Context. The variability of solar spectral irradiance (SSI) over the rotational period and its trend over the solar activity cycle are important for understanding the Sun-Earth connection as well as for observational constraints for solar models. Recently the Spectral Irradiance Monitor (SIM) experiment on the Solar Radiation and Climate Experiment (SORCE) has published an unexpected negative correlation with total solar irradiance (TSI) of the visible spectral range. It is compensated by a strong and positive variability of the near UV range. Aims. We aim to verify whether the anti-correlated SIM-trend in the visible can be confirmed by independent observations of the Variability of solar IRadiance and Gravity Oscillations (VIRGO) experiment on the SOlar and Heliospheric Observatory (SOHO) satellite. The challenge of all space experiments measuring solar irradiance are sensitivity changes of their sensors due to exposure to intense UV radiation, which are difficult to assess in orbit. Methods. We exclude the first six years prior to 2002 where one or more fast processes contributed to instrumental changes and analyse a ten-year timeseries of VIRGO sun photometer data between 2002 and 2012. The variability of SSI is correlated with the variability of the TSI, which is taken as a proxy for solar activity. Results. Observational evidence indicates that after six years only one single long-term process governs the degradation of the backup sun photometer in VIRGO which is operated once a month. This degradation can be well approximated by a linear function over ten years. The analysis of the residuals from the linear trend yield robust positive correlations of spectral irradiance at 862, 500, and 402 nm with total irradiance. In the analysis of annual averages of these data the positive correlations change into weak negative correlations, but with little statistical significance for the 862 nm and 402 nm data. At 500 nm the annual spectral data are still positively correlated with TSI. The persisting positive correlation at 500 nm is in contradiction to the SIM results.

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VIRGO: Experiment for helioseismology and solar irradiance monitoring

Cited by 271 publications

References 22 publications

Seismic sensitivity to sub-surface solar activity from 18 yr of GOLF/SoHO observations

Seismic sensitivity to sub-surface solar activity from 18 yr of GOLF/SoHO observations

Magnetic activity of F stars observed byKepler

Correlation of spectral solar irradiance with solar activity as measured by VIRGO

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