The extended minimum of solar cycle 23 as seen by radial velocity (GOLF, GONG) and intensity (VIRGO) helioseismic instruments
D. Salabert, R.A. Garcia, P.L. Palle, S.J. Jimenez-Reyes, A. Jimenez
AAstron. Nachr. / AN , No. 88, 789 – 792 (2006) /
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The extended minimum of solar cycle 23 as seen by radial velocity(GOLF, GONG) and intensity (VIRGO) helioseismic instruments
D. Salabert , ,(cid:63) , R. A. Garc´ıa , P. L. Pall´e , , S. J. Jim´enez-Reyes , and A. Jim´enez , Instituto de Astrof´ısica de Canarias, E-38200 La Laguna, Tenerife, Spain Departamento de Astrof´ısica, Universidad de La Laguna, E-38205 La Laguna, Tenerife, Spain Laboratoire AIM, CEA/DSM-CNRS, Universit´e Paris 7 Diderot, IRFU/SAp, Centre de Saclay, F-91191 Gif-sur-Yvette,FranceThe dates of receipt and acceptance should be inserted later
Key words
Sun: activity – Sun: helioseismologyWe present an analysis of the variability of the solar oscillation spectrum during solar cycle 23 and its extended minimum.We use simultaneous observations of the low-degree solar p modes collected by the space-based, Sun-as-a-star GOLF(radial velocity) and VIRGO (intensity) instruments, and by the ground-based, multi-site network GONG. We investigatein particular the response of the p-mode eigenfrequencies to the observed peculiar deep solar minimum of surface activityof 2007–2009 as compared with the previous solar cycle 23. We study the different temporal variations of the p-modefrequencies with individual angular degrees. c (cid:13) The new millennium solar activity minimum of 2007–2009has shown the quietest Sun in almost a century with a de-layed onset of solar cycle 24. Helioseismic observations ha-ve been used to study the response of the solar oscillations tothis unusually extended minimum of solar surface activity.While the temporal variations of the p-mode frequencies areclosely correlated with solar surface activity proxies duringthe past cycles at low- (e.g., Gelly et al. 2002; Salabert etal. 2004) and higher-angular degrees (e.g., Jim´enez-Reyeset al. 2001; Salabert & Jim´enez-Reyes 2006a), significantvariations of the p-mode frequencies during the minimumof cycle 23 in contrast to the surface activity observationsover the same period have been reported (Broomhall et al.2009; Salabert et al. 2009). Furthermore, Howe et al. (2009)showed that the lack of sunspots and the low-activity levelsduring the cycle 23 minimum can be explained by a slowerthan usual jet stream associated to the production of thesunspots. These streams originating from the poles every 11years migrate slowly below the surface towards the equator.Other p-mode parameters, such as the mode power and life-time for instance, were also proven to be sensitive to the so-lar activity cycle in both Sun-as-a-star (e.g., Jim´enez-Reyeset al. 2004; Salabert et al. 2003) and spatially-resolved (e.g.,Komm et al. 2000; Salabert & Jim´enez-Reyes 2006b) ob-servations. Thus, their response to the unusually long min-imum of cycle 23 is also of great interest and is currentlyunder investigation. (cid:63)
Corresponding author: e-mail: [email protected]
Simultaneous helioseismic observations collected by threeindependent instruments are used in this work: – the space-based Global Oscillations at Low Frequency(GOLF; Gabriel et al. 1995) on board the Solar and He-liospheric Observatory (SoHO) spacecraft. GOLF is aresonant scattering spectrophotometer. It measures theDoppler wavelength shift – integrated over the solar sur-face – in the D1 and D2 Fraunhofer sodium lines at589.6 and 589.0 nm respectively; – the space-based instrument Variability of Solar IRran-diance and Gravity Oscillation (VIRGO; Fr¨ohlich et al.1995) instrument on board SoHO . VIRGO is composedof three Sun photometers (SPM) at 402 nm (blue chan-nel), 500 nm (green channel), and 862 nm (red channel); – the ground-based, multi-site instruments Global Oscil-lation Network Group (GONG; Harvey et al. 1996) iscomposed of 6 identical stations located at selected lon-gitudes around the world. The GONG instruments areMichelson Doppler Interferometers measuring in the ab-sorption line Ni I (676.8 nm).These datasets were split into contiguous 365-day and91.25-day sub-series, each series being allowed to overlapby 91.25 days and 22.8125 days respectively. The powerspectrum of each sub series was fitted to estimate the modeparameters (Salabert et al. 2007) using a standard likelihoodmaximization function (i.e. power spectrum with a χ with2 d.o.f. statistics). Each mode component is parameterizedusing a asymmetric Lorentzian profile. The temporal varia-tions of the p-mode frequencies were defined as the differ-ence between reference values (taken as the average over the c (cid:13) a r X i v : . [ a s t r o - ph . S R ] A p r
90 D. Salabert et al.: The extended minimum of solar cycle 23 as seen by GOLF, GONG, and VIRGO
96 98 00 02 04 06 08 10 − − F r e qu e n cy s h i ft s ( µ H z ) l = 096 98 00 02 04 06 08 10 204080120160200240
96 98 00 02 04 06 08 10 − −
96 98 00 02 04 06 08 10 − − R a d i o flu x ( - J / s / m / H z ) Fig. 1
Frequency shifts of the l = 0 , , and 2 solar p modes (left to right panels) extracted from the analysis of the 365-day (black dots) and the 91.25-day (blue plus signs) GOLF spectra. The associated error bars of the 365-day frequencyshifts are represented. The corresponding 10.7-cm radio flux averaged over the same 91.25-day timespan is shown as aproxy of the solar surface activity (red solid line).years 1996–1997) and the frequencies of the correspondingmodes observed at different dates. The weighted averagesof these frequency shifts were then calculated between 2200and 3300 µ Hz. Mean values of daily measurements of the10.7-cm radio flux were obtained and used as a proxy of thesolar surface activity.
A total of 5021 days of radial velocity GOLF time series(Garc´ıa et al. 2005; Ulrich et al. 2000) were analyzed. Thisdataset spans the period from 1996 April 11 to 2010 January8, with a overall duty cycle of 95.3% (see Jim´enez-Reyes etal. (2003) for the calibration method). The frequency shiftsmeasured at each angular degree ( l = 0 , , and 2) in the91.25-day and 365-day sub series are shown on Fig. 1. Thecorresponding 10.7-cm radio flux averaged over the same91.25-day timespan is also represented as a proxy of thesolar surface activity. We also analyzed 4890 days of the intensity VIRGO timeseries. This dataset starts on 1996 April 11 and ends on2009 August 30, with a duty cycle of 94.6%. The 365-dayfrequency shifts at l = 0 , , and 2 measured in the blue,green, and red VIRGO channels are represented on Fig. 2.The 365-day GOLF frequency shifts are also shown in blackfor comparison. A total of 5112 days of the integrated time series of theground-based, multi-site GONG network spanning the pe-riod from 1995 May 5 to 2009 May 4 were analyzed, withan overall duty cycle of 85.4%. Due to the spatial resolu-tion of the original GONG data, part of the power from thehigher angular degrees ( l ≥ ) are present in the integrated GONG signal. These leaks were taken into account duringthe peak-fitting by including information coming from theGONG leakage matrix. The frequency shifts at l = 0 , , and2 measured in the 365-day integrated GONG sub series arerepresented on Fig. 3. The 365-day GOLF frequency shiftsare also shown. The solar p-mode frequencies measured simultaneously byindependent space-based and ground-based helioseismic in-struments – GOLF and GONG/integrated signal in radialvelocity, and VIRGO in intensity – show similar temporalvariations during solar cycle 23 and its extended minimumof 2007–2009. Moreover, and as shown on Figs. 1, 2, and 3,different behaviors are observed amongst modes of differentangular degrees ( l = 0 , , and 2): – the l = 0 and l = 2 frequency shifts show an upturnfrom the end of 2007 while no significant surface ac-tivity was visible on the Sun. This upturn might be fol-lowed by a downturn after 2009; – the l = 1 frequency shifts follow the general decreasingtrend of the solar surface activity as clearly illustratedon Fig. 1.It is worth noticing the particular behavior of the l = 2 frequencies in the VIRGO red channel data. While, the l =2 mode is overall noisier in the VIRGO observations than inthe GOLF and GONG data, the sharp increase of the l = 2 frequency shifts after 2007 in the VIRGO red channel ob-servations is striking and does not seem to be only due to ahigher noise level. However, more work is needed to con-clude on the significance of the different sharpness of theseupturns, for example to determine if there is a depth depen-dence of the observed upturn since 2007.Nevertheless, the differences between individual angu-lar degrees might be interpreted as different geometrical re- c (cid:13)2006 WILEY-VCH Verlag GmbH&Co.KGaA, Weinheim
A total of 5021 days of radial velocity GOLF time series(Garc´ıa et al. 2005; Ulrich et al. 2000) were analyzed. Thisdataset spans the period from 1996 April 11 to 2010 January8, with a overall duty cycle of 95.3% (see Jim´enez-Reyes etal. (2003) for the calibration method). The frequency shiftsmeasured at each angular degree ( l = 0 , , and 2) in the91.25-day and 365-day sub series are shown on Fig. 1. Thecorresponding 10.7-cm radio flux averaged over the same91.25-day timespan is also represented as a proxy of thesolar surface activity. We also analyzed 4890 days of the intensity VIRGO timeseries. This dataset starts on 1996 April 11 and ends on2009 August 30, with a duty cycle of 94.6%. The 365-dayfrequency shifts at l = 0 , , and 2 measured in the blue,green, and red VIRGO channels are represented on Fig. 2.The 365-day GOLF frequency shifts are also shown in blackfor comparison. A total of 5112 days of the integrated time series of theground-based, multi-site GONG network spanning the pe-riod from 1995 May 5 to 2009 May 4 were analyzed, withan overall duty cycle of 85.4%. Due to the spatial resolu-tion of the original GONG data, part of the power from thehigher angular degrees ( l ≥ ) are present in the integrated GONG signal. These leaks were taken into account duringthe peak-fitting by including information coming from theGONG leakage matrix. The frequency shifts at l = 0 , , and2 measured in the 365-day integrated GONG sub series arerepresented on Fig. 3. The 365-day GOLF frequency shiftsare also shown. The solar p-mode frequencies measured simultaneously byindependent space-based and ground-based helioseismic in-struments – GOLF and GONG/integrated signal in radialvelocity, and VIRGO in intensity – show similar temporalvariations during solar cycle 23 and its extended minimumof 2007–2009. Moreover, and as shown on Figs. 1, 2, and 3,different behaviors are observed amongst modes of differentangular degrees ( l = 0 , , and 2): – the l = 0 and l = 2 frequency shifts show an upturnfrom the end of 2007 while no significant surface ac-tivity was visible on the Sun. This upturn might be fol-lowed by a downturn after 2009; – the l = 1 frequency shifts follow the general decreasingtrend of the solar surface activity as clearly illustratedon Fig. 1.It is worth noticing the particular behavior of the l = 2 frequencies in the VIRGO red channel data. While, the l =2 mode is overall noisier in the VIRGO observations than inthe GOLF and GONG data, the sharp increase of the l = 2 frequency shifts after 2007 in the VIRGO red channel ob-servations is striking and does not seem to be only due to ahigher noise level. However, more work is needed to con-clude on the significance of the different sharpness of theseupturns, for example to determine if there is a depth depen-dence of the observed upturn since 2007.Nevertheless, the differences between individual angu-lar degrees might be interpreted as different geometrical re- c (cid:13)2006 WILEY-VCH Verlag GmbH&Co.KGaA, Weinheim stron. Nachr. / AN (2006) 791 !" ! ! ! ! + , - ./ - ) ) µ : ; !" ! ! ! ! ! Fig. 2
Frequency shifts of the l = 0 , , and 2 solar p modes (left to right panels) extracted from the analysis of the365-day VIRGO spectra. The frequency shifts measured from the blue, green, and red VIRGO channels are representedfrom top to bottom respectively. The corresponding GOLF frequency shifts are shown for comparison (black dots). Theassociated error bars are also represented.
96 98 00 02 04 06 08 10 − − F r e qu e n cy s h i ft s ( µ H z ) l = 0
96 98 00 02 04 06 08 10 − −
96 98 00 02 04 06 08 10 − − Fig. 3
Frequency shifts of the l = 0 , , and 2 solar p modes (left to right panels) extracted from the analysis of the365-day integrated GONG spectra (red diamonds). The corresponding GOLF frequency shifts are shown for comparison(black dots). The associated error bars are also represented. c (cid:13)
92 D. Salabert et al.: The extended minimum of solar cycle 23 as seen by GOLF, GONG, and VIRGO sponses to the spatial distribution of the solar magnetic fieldbeneath the surface of the Sun. That could indicate varia-tions in the magnetic flux at high latitudes related to theonset of solar cycle 24.
We analyzed simultaneous observations of the low-degreesolar p modes collected by the space-based GOLF (radialvelocity) and VIRGO (intensity) instruments on board the
SoHO spacecraft, and by the ground-based, multi-site net-work GONG in order to investigate the response of the p-mode frequencies to the unusual deep and long minimumof solar surface activity of cycle 23 during 2007–2009. Weobserved different temporal variations of the p-mode fre-quencies between individual angular degrees. These varia-tions are identical in the GOLF, VIRGO, and GONG instru-ments. The differences between individual angular degreesmight be interpreted as different geometrical responses tothe spatial distribution of the solar magnetic field beneaththe surface of the Sun. A more detailed intercomparison be-tween several helioseismic instruments is underway.After CoRoT revealed a first stellar activity cycle in aSun-like star using asteroseismology (Garc´ıa et al. 2010),the NASA
Kepler mission (Koch et al. 2010) will reinforcethe study of stellar magnetic cycles (Karoff et al. 2009)thanks to long-duration observations (over 3 years) with avery high signal-to-noise ratio (e.g., Bedding et al. 2010;Chaplin et al. 2010). Thus, seismology will provide verydetailed inferences of the stellar structures, in particular thedepth of the convective zones. Such information will be ofkey importance to properly understand the physical mech-anisms driving the magnetic cycles and thus, will help tobetter understand and predict the dynamo processes takingplace in the Sun.
Acknowledgements.
The GOLF and VIRGO instruments on boardSoHO are a cooperative effort of many individuals, to whom weare indebted. SoHO is a project of international collaboration be-tween ESA and NASA. This work utilizes data obtained by theGlobal Oscillation Network Group (GONG) program, managed bythe National Solar Observatory, which is operated by AURA, Inc.under a cooperative agreement with the National Science Founda-tion. The data were acquired by instruments operated by the BigBear Solar Observatory, High Altitude Observatory, LearmonthSolar Observatory, Udaipur Solar Observatory, Instituto de As-trof´ısica de Canarias, and Cerro Tololo Interamerican Observa-tory. The 10.7-cm radio flux data were obtained from the NationalGeophysical Data Center. D.S. acknowledges the support from theSpanish National Research Plan (grant PNAyA2007-62650). Thiswork has been partially supported by the European Helio- and As-teroseismology Network (HELAS) and by the CNES/GOLF grantat SAp CEA-Saclay.
References
Bedding, T. R., et al.: 2010, ApJ 713, L176 Broomhall, A.-M., Chaplin, W. J., Elsworth, Y., Fletcher, S. T., &New, R.: 2009, ApJ 700, L162Chaplin, W. J., et al.: 2010, ApJ 713, L169Fr¨ohlich, C., Romero, J., Roth, H. et al.: 1995, Solar Phys. 162,101Gabriel, A. H., Grec, G., Charra, J. et al.: 1995, Solar Phys. 162,61Garc´ıa, R. A., Turck-Chi`eze, S., Boumier, P. et al.: 2005, A&A442, 385Garc´ıa, R. A., Ballot, J., Mathur, S., Salabert, D., & R´egulo, C.:2010, AN, in HELAS IV, Seismological challenges for stellarstructure, to appear in these proceedingsGelly, B., Lazrek, M., Grec, G., Ayad, A., Schmider, F. X., Renaud,C., Salabert, D., & Fossat, E.: 2002, A&A 394, 285Harvey, J. W., Hill, F., Hubbard, R. P. et al.: 1996, Science 272,1284Howe, R., Christensen-Dalsgaard, J., Hill, F., Komm, R., Schou,J., & Thompson, M. J.: 2009, ApJ 701, L87Jim´enez-Reyes, S. J., Corbard, T., Pall´e, P. L., Roca Cort´es, T., &Tomczyk, S.: 2001, A&A 379, 622Jim´enez-Reyes, S. J., Garc´ıa, R. A., Jim´enez, A. et al.: 2003,ApJ 595, 446Jim´enez-Reyes, S. J., Chaplin, W. J., Elsworth, Y., & Garc´ıa, R. A.:2004, ApJ 604, 969Karoff, C., Metcalfe, T. S., Chaplin, W. J., Elsworth, Y., Kjeldsen,H., Arentoft, T., & Buzasi, D.: 2009, MNRAS 399, 914Koch, D. G., et al.: 2010, ApJ 713, L79Komm, R. W., Howe, R., & Hill, F.: 2000, ApJ 531 1094Salabert, D., Jim´enez-Reyes, S. J., & Tomczyk, S.: 2003, A&A408, 729Salabert, D., Fossat, E., Gelly, B., Kholikov, S., Grec, G., Lazrek,M., & Schmider, F. X.: 2004, A&A 413, 1135Salabert, D., & Jim´enez-Reyes, S. J.: 2006a, in SOHO 18/GONG2006/HELAS I, Beyond the spherical Sun (ESA SP-624; ESAPublications Division), 90Salabert, D., & Jim´enez-Reyes, S. J.: 2006b, ApJ 650, 451Salabert, D., Chaplin, W. J., Elsworth, Y., New, R., & Verner,G. A.: 2007, A&A 463, 1181Salabert, D., Garc´ıa, R. A., Pall´e, P. L., & Jim´enez-Reyes, S. J.:2009, A&A 504, L1Ulrich, R. K., Garc´ıa, R. A., Robillot, J. M. et al.: 2000, A&A 364,799 c (cid:13)2006 WILEY-VCH Verlag GmbH&Co.KGaA, Weinheim