The Team

  1st Meeting: 31 Oct - 4 Nov 05

  Fr�jus: 27 Mar - 31 Mar 06

  2nd Meeting: 23 Apr - 27 Apr 07

  Updated on October 3, 2006




  Detection Techniques










  IAS, France
  IAC, Tenerife, S
  IoA, Cambridge, UK
  NAO of Japan
  Norwegian Space Center
  Observatoire de Nice, F
  SaP / CEA, Saclay, F
  Stanford University, USA
  University of Birmingham, UK
  World Radiation Center, CH


  ISSI, Bern, Switzerland

  The Phoebus Group   

  S EARCHING    FOR    S OLAR    G    M ODES   


Since the beginnings of global helioseismology in the late 1970s, the detection of g modes has been the quest for the Grail.  The detection of g modes would be key to the understanding of the internal structure and dynamics of the solar core, as much as the p modes are key to that of the structure of the radiative and convective zones.  There have been several claims of g-mode detections but none have been confirmed. The impact of g-mode detection will be so large that we can expect a wealth of information to be returned.  The structure and dynamics of the energy-generating core will be seen, analyzed and understood. The hydrostatic structure of the core, in particular its deepest lying layers, will be uncovered to a far higher level of accuracy and precision; while it will be possible to infer the rotational characteristics of the core, characteristics that at present we are unable to uncover, to a satisfactory level of precision, with the current p-mode data.   With this international team, we therefore wish to continue the search for g modes, and eventually find them

This quest was a major driver in the design of very precise and quiet instrumentation harboured by spacecraft such as the Solar and Heliospheric Observatory.  Aboard SOHO, there are three instruments dedicated to helioseismology all aiming at detecting g modes.  A few years after the launch of SOHO in end 1995, it was realized that g modes would not be easily detected.  In 1997, a consortium of helioseismologists was formed with the simple goal of “detecting g modes”.  Helioseismologists belonging to the SOHO consortia and to ground based networks were teaming together for that goal.  This consortium of helioseismologists was named the Phoebus group after Gaston Phébus, Comte de Foix, who wrote a book about hunting, hoping thereby to ‘catch’ a few g modes. The work focused on data analysis of SOHO instruments (VIRGO, MDI) and ground-based networks (BiSON, GONG); on probability and statistics; and on theoretical model prediction of g-mode amplitudes and frequencies.

The work of the Phoebus group over the past 7 years has enabled us to lower the detection limit previously set (Appourchaux et al, 2000) to velocity amplitudes less than a millimeter per second. The GOLF team, although not part of the Phoebus group, have also derived similar limits (Gabriel et al, 2002; Turck-Chièze et al, 2004).  It is very likely that these upper limits will not be considerably reduced, say by a factor 10, within the next few years.  With SOHO approaching its end, it is timely that we review the knowledge acquired for detecting such low amplitude signals.  It is still hoped that g modes will be detected with the available data sets, but even if they were not, the acquired knowledge will be useful for instrumentation and/or new missions planned for detecting such g modes.  For instance, one of the goals of the PICARD mission from CNES is to detect g modes by looking at the solar limb (Damé et al, 2001).  This mission, to be launched in 2007, will require the use of the techniques developed for faint signal detection.  In addition, there are other helioseismic missions that are currently being studied and specifically designed for g-mode detection such as GOLF-NG (Turck-Chièze et al, 2004). 

Last but not least, the current Cosmic Vision programme of ESA is being planned for setting the scene of what Space Science should be done in the 2015-2025 framework.  It is therefore timely to influence this programme by showing what was done, is being done, and what will be done for detecting g modes.