A. Jiménez

The quest for the solar g modes

Solar gravity modes (or g modes)—oscillations of the solar interior on which buoyancy acts as the restoring force—have the potential to provide unprecedented inference on the structure and dynamics of the solar core, inference that is not possible with the well-observed acoustic modes (or p modes). The relative high amplitude of the g-mode eigenfunctions in the core and the evanesence of the modes in the convection zone make the modes particularly sensitive to the physical and dynamical conditions in the core. Owing to the existence of the convection zone, the g modes have very low amplitudes at photospheric levels, which makes the modes extremely hard to detect. In this article, we review the current state of play regarding attempts to detect g modes. We review the theory of g modes, including theoretical estimation of the g-mode frequencies, amplitudes and damping rates. Then we go on to discuss the techniques that have been used to try to detect g modes. We review results in the literature, and finish by looking to the future, and the potential advances that can be made—from both data and data-analysis perspectives—to give unambiguous detections of individual g modes. The review ends by concluding that, at the time of writing, there is indeed a consensus amongst the authors that there is currently no undisputed detection of solar g modes.

Excitation and Damping of Low-Degree Solar p-Modes during Activity Cycle 23: Analysis of GOLF and VIRGO Sun Photometer Data

We have used observations made by the Global Oscillations at Low Frequency (GOLF) and the Variability of Irradiance and Gravity Oscillations Sun Photometer (VIRGO/SPM) instruments on board the ESA/NASA Solar and Heliospheric Observatory satellite to study variations in the excitation and damping of low angular degree (low-l) solar p-modes on the rising phase of activity cycle 23. Our analysis includes a correction procedure that for the first time allows GOLF data to be treated as a single homogeneous set, thereby compensating for the change of operational configuration partway through the mission. Over the range 2.5 ≤ ν ≤ 3.5 mHz, we uncover an increase in damping and decrease in mode power that is consistent with previous findings. Furthermore, an excellent level of agreement is found between the variations extracted from the GOLF and VIRGO/SPM data. We find no net long-term changes to the modal energy supply rate. However, an analysis of the residuals uncovers the presence of a quasi-periodic signature of period ≈1.5 yr (most pronounced for SPM). While it is true that several workers claim to have uncovered similar periodicities in other phenomena related to the near-surface layers of the Sun here, we are at present more inclined to attribute our finding to an artifact of the mode-fitting procedure. We also uncover a significant change in the asymmetry of mode peaks in the GOLF data, as found in previous studies of much longer data sets. These assumed that the dominant contribution to this arose from the switch in operating configuration partway through the mission (which altered the depth in the solar atmosphere sampled by the instrument). However, our preliminary analysis of data collected over the 100 day period beginning 2002 November 19—when the instrument switched back to its original configuration—suggests that this change may have a solar cycle component.

Update on g-mode research

De´partement Cassiope´e, UMR CNRS 6202, Observatoire de la Cote dAzur, BP 4229, 06304 Nice Cedex 4, FranceReceived October 2007, accepted February 2008Published online 2008Since the beginning of this century we have attended a blooming of the gravity-mode research thanks to the unprecedentedquality of the data available, either from space with SoHO, or from the ground-based networks as BiSON or GONG. Fromthe first upper limit of the gravity-mode amplitudes fixed at 1 0 mm/s at 200 µHz given by Appourchaux et al. (2000),on one hand, a peak was supposed to be a component of the l=1, n=1 mixed mode (Garc´ia et al. 2001a, b; Gabriel etal. 2002) and, on the other hand, a couple of patterns –multiplets– were attributed to gravity modes (Turck-Chie`ze et al.2004; Mathur et al. 2007). One of these patterns, found around 220 µHz, could be labeled as the l=2, n =-3 g mode, whichis expected to be the one with the highest surface amplitude (Cox and Guzik 2004). Finally, in 2007, Garc´ia et al. wereable to measure the fingertips of the dipole gravity modes loo king for their asymptotic properties. In the present paperwe present an update of the recent developments on this subject with special attention to the 220 µHz region, the dipoleasymptotic properties and the impact of the incoming g-modeobservations on the knowledge of the solar structure androtation profile.

Variation of the low-degree solar acoustic mode parameters over the solar cycle

VIRGO/SPM is a helioseismic sunphotometer on board SOHO that observes the disk-integrated sunlight irradiance at three different colors (red, green, and blue). The data obtained for SPM since the beginning of the SOHO mission, April 1996, to March 2001 have been used to study the differences of the p-mode parameters during the solar activity cycle. These time series have been divided in sub-series of 100 days, transformed to power spectra and averaged in sets of three to yield a total number of six averaged power spectra (around one per year). A new way of analyzing the power spectrum has been applied to the six power spectra of each color; it consists of fitting the whole p-mode spectrum at once with a unique background. The results for the frequencies, line widths, power, mode energy, energy rate fed in the mode and splittings along the activity cycle are found, compared and discussed.

The quasi-biennial periodicity (QBP) in velocity and intensity helioseismic observations - The seismic QBP over solar cycle 23

We looked for signatures of Quasi-Biennial Periodicity (QBP) over different phases of solar cycle by means of acoustic modes of oscillation. Low-degree p-mode frequencies are shown to be sensitive to changes in magnetic activity due to the global dynamo. Recently have been reported evidences in favor of two-year variations in p-mode frequencies. Long high-quality helioseismic data are provided by BiSON (Birmingham Solar Oscillation Network), GONG (Global Oscillation Network Group), GOLF (Global Oscillation at Low Frequency) and VIRGO (Variability of Solar IRradiance and Gravity Oscillation) instruments. We determined the solar cycle changes in p-mode frequencies for spherical degree l=0, 1, 2 with their azimuthal components in the frequency range 2.5 mHz < nu < 3.5 mHz. We found signatures of QBP at all levels of solar activity in the modes more sensitive to higher latitudes. The signal strength increases with latitude and the equatorial component seems also to be modulated by the 11-year envelope. The persistent nature of the seismic QBP is not observed in the surface activity indices, where mid-term variations are found only time to time and mainly over periods of high activity. This feature together with the latitudinal dependence provides more evidences in favor of a mechanism almost independent and different from the one that brings up to the surface the active regions. Therefore, these findings can be used to provide more constraints on dynamo models that consider a further cyclic component on top of the 11-year cycle.

Comparison of Two Fitting Methods for Ring Diagram Analysis of Very High l Solar Oscillations

A new method of fitting tridimensional power spectra of solar oscillations is described and compared with a previous one whose use has been more common. The new method fits the parameters of the Lorentzian profiles in a bidimensional k - ω diagram constructed from an azimuthal average of the tridimensional one. The horizontal velocities are then determined keeping these parameters fixed, greatly reducing the computation time. Both methods are compared for two radial orders (n = 3, 4) of a tridimensional power spectrum obtained for a region of about 15° square around solar disk center. The images used in this work correspond to a 3 day set of 1080 × 1080 pixel intensity images obtained at the Observatorio del Teide on 1994 November 8-10 with the Taiwanese Oscillation Network (TON) instrument. The results of the fitted velocities agree within the estimated errors for the two methods. The reduction of the computing time obtained with the new method makes it convenient for the ring diagram analysis.

An Estimation of the Acoustic Cutoff Frequency of the Sun Based on the Properties of the Low-Degree Pseudomodes

The acoustic cutoff frequency (νac) is an important atmospheric parameter whose estimation has in the past been based on the study of power spectra that have yielded a wide range of values between 5300 and 5700 μHz. The discovery of a solar signal well beyond the acoustic cutoff frequency (pseudomodes) might lead one to think that the determination of νac would be even more complicated because, for example, looking for a sudden drop in the power density signal could no be longer used. Contrary to what might be thought at first sight, the existence of pseudomodes helps to provide a good estimation of νac, because the frequency pattern of pseudomodes is shifted with respect to that of p-modes. In this study a bivariate analysis (coherence and phase shift) between the intensity signals of VIRGO and the velocity signal of GOLF (both instruments on board the SOHO probe) is carried out over the frequency range of p-modes and pseudomodes. The results shows clear evidence that the acoustic cutoff frequency of the Sun is close to the theoretical value of 5300 μHz; specifically, a value around 5100 μHz is found in this research.

Disk-integrated Intensity Pseudomodes Measured by SOHO/VIRGO

The high-frequency spectrum above the acoustic cutoff frequency (νac) of the solar atmosphere has been observed in velocity measurements at low and high angular degrees and reveals significant evidence for the presence of pseudomodes characterized by their equally spaced signal. In this paper we study this region of the spectra using for the first time disk-integrated intensity measurements from the VIRGO/SPM photometers on board the SOHO probe. These pseudomodes have high visibility, and whereas the frequency separation between the pseudomodes is the same, some differences are found among the three wavelengths. We have studied the evolution of the pseudomodes with the solar cycle; nevertheless, no significant variations are found. Finally, the intensity pseudomodes of VIRGO/SPM are compared with those of velocity as seen by the GOLF instrument during the same periods and with the same analytical techniques.

Extinction over the Canarian observatories: the limited influence of Saharan dust

Abstract We present a study of the atmospheric extinction over the observatories of the Canary Islands, based mainly on photometric measurements of the V -band extinction coefficient obtained during 2850 nights at the Carlsberg Automatic Meridian Circle at the Roque de los Muchachos Observatory (La Palma), and complemented with visible and infrared data obtained at the Teide Observatory (Tenerife). The median value of the extinction coefficient in the V -band is found to be 0.113 mag airmass −1 , which compares favourably with the extinction found at other first-quality astronomical sites. Long-term extinction variations due to the effects of the volcanic eruptions of El Chichon (1982) and Mt. Pinatubo (1991) are also described in this study. A detailed description of the seasonal variations provides the opportunity to study the influence of Saharan dust episodes, mostly concentrated in the summer period. This allowed us to estimate how Saharan dust affects astronomical observations. It is concluded that more than 75% of summer nights are not affected by dust, while this number rises over 90% for the rest of the year.

ON THE SOLAR ORIGIN OF THE SIGNAL AT 220.7 μHz: A POSSIBLE COMPONENT OF A g-MODE?

Gravity modes in the Sun have been the object of a long and difficult search in recent decades. Thanks to the data accumulated with the last generation of instruments (BiSON, GONG, and three helioseismic instruments aboard the Solar and Heliospheric Observatory (SOHO)), scientists have been able to find signatures of their presence. However, the individual detection of such modes remains evasive. In this article, we study the signal at 220.7 μHz which is a peak that is present in most of the helioseismic data of the last 10 years. This signal has already been identified as being a component of a g-mode candidate detected in the GOLF Doppler velocity signal. The nature of this peak is studied in particular using the VIRGO/SPM instrument aboard SOHO. First we analyze all the available instrumental data of VIRGO and SOHO (housekeeping) to reject any possible instrumental origin. No relation was found, implying that the signal has a solar origin. Using Monte Carlo simulations, we find, with more than 99% confidence level, that the signal found in VIRGO/SPM is very unlikely to be due to pure noise.