Solar Physics

, Volume 240, Issue 2, pp 197–209 | Cite as

The Effect of Abnormal Granulation on Acoustic Wave Travel Times and Mode Frequencies

Article

Abstract

Observations indicate that in plage areas (i.e. in active regions outside sunspots) acoustic waves travel faster than in the quiet Sun, leading to shortened travel times and higher p-mode frequencies. Coupled with the 11-year variation of solar activity, this may also explain the solar cycle variation of oscillation frequencies. While it is clear that the ultimate cause of any difference between the quiet Sun and plage is the presence of magnetic fields of order 100 G in the latter, the mechanism by which the magnetic field exerts its influence has not yet been conclusively identified. One possible such mechanism is suggested by the observation that granular motions in plage areas tend to be slightly “abnormal”, dampened compared to the quiet Sun.

In this paper we consider the effect that abnormal granulation observed in active regions should have on the propagation of acoustic waves. Any such effect is found to be limited to a shallow surface layer where sound waves propagate nearly vertically. The magnetically suppressed turbulence implies higher sound speeds, leading to shorter travel times. This time shift Δτ is independent of the travel distance, while it shows a characteristic dependence on the assumed plage field strength. As a consequence of the variation of the acoustic cutoff with height, Δτ is expected to be significantly higher for higher frequency waves within the observed regime of 3 – 5 mHz. The lower group velocity near the upper reflection point further leads to an increased envelope time shift, as compared to the phase shift. p-mode frequencies in plage areas are increased by a corresponding amount, Δν/ν=νΔτ. These characteristics of the time and frequency shifts are in accordance with observations. The calculated overall amplitudes of the time and frequency shifts are comparable to, but still significantly less than (by a factor of 2 to 5), those suggested by measurements.

Keywords

Sun: faculae, plages Sun: granulation Sun: helioseismology 

Preview

Unable to display preview. Download preview PDF.

Unable to display preview. Download preview PDF.

References

  1. Braun, D.C., Fan, Y.: 1998, Astrophys. J. 508, L105. CrossRefADSGoogle Scholar
  2. Brown, T.M.: 1984, Science 226, 687. CrossRefADSGoogle Scholar
  3. Brüggen, M., Spruit, H.C.: 2000, Solar Phys. 196, 29. CrossRefADSGoogle Scholar
  4. Campbell, W.R., Roberts, B.: 1989, Astrophys. J. 338, 538. CrossRefADSGoogle Scholar
  5. Chou, D.-Y.: 2000, Solar Phys. 192, 241. CrossRefADSGoogle Scholar
  6. Cunha, M., Brüggen, M., Gough, D.O.: 1998, In: Structure and Dynamics of the Interior of the Sun and Sun-like Stars, Proc. 1st ASPE Euroconference, ESA Publ. SP-418, p. 905. Google Scholar
  7. D’Silva, S.: 1996a, Astrophys. J. 462, 519. CrossRefADSGoogle Scholar
  8. D’Silva, S.: 1996b, Astrophys. J. 469, 964. CrossRefADSGoogle Scholar
  9. Dunn, R.B., Zirker, J.B.: 1973, Solar Phys. 33, 281. ADSGoogle Scholar
  10. Erdélyi, R., Taroyan, Y.: 2001, In: Brekke, P., Fleck, B., Gurman, J.B. (eds.) IAU Symposium 203, p. 208. Google Scholar
  11. Erdélyi, R., Kerekes, A., Mole, N.: 2004, In: Lacoste, H. (ed.) SOHO 13 Waves, Oscillations and Small-Scale Transients Events in the Solar Atmosphere: Joint View from SOHO and TRACE, ESA Publ. SP-547, p. 75. Google Scholar
  12. Erdélyi, R., Kerekes, A., Mole, N.: 2005, Astron. Astrophys. 431, 1083. CrossRefADSGoogle Scholar
  13. Erdélyi, R., Taroyan, Y.A.: 1999, In: Magnetic Fields and Solar Processes, ESA Publ. SP-448, p. 81. Google Scholar
  14. Erdélyi, R., Varga, E., Zétényi, M.: 1999, In: Magnetic Fields and Solar Processes, ESA Publ. SP-448, p. 269. Google Scholar
  15. Evans, D.J., Roberts, B.: 1991, Astrophys. J. 371, 387. CrossRefADSGoogle Scholar
  16. Evans, D.J., Roberts, B.: 1992, Nature 355, 230. CrossRefADSGoogle Scholar
  17. Ghosh, P., Antia, H.M., Chitre, S.M.: 1995, Astrophys. J. 451, 851. CrossRefADSGoogle Scholar
  18. Gruzinov, A.V.: 1998, Astrophys. J. 498, 458. CrossRefADSGoogle Scholar
  19. Guenther, D.B., Demarque, P., Kim, Y.-C., Pinsonneault, M.H.: 1992, Astrophys. J. 387, 372. CrossRefADSGoogle Scholar
  20. Hindman, B., Haber, D., Toomre, J., Bogart, R.: 2000, Solar Phys. 192, 363. CrossRefADSGoogle Scholar
  21. Hughes, S.J., Rajaguru, S.P., Thompson, M.J.: 2005, Astrophys. J. 627, 1040. CrossRefADSGoogle Scholar
  22. Kosovichev, A.G., Duvall, T.L., Scherrer, P.H.: 2000, Solar Phys. 192, 159. CrossRefADSGoogle Scholar
  23. Libbrecht, K.G., Woodard, M.F.: 1990, Nature 345, 779. CrossRefADSGoogle Scholar
  24. Mędrek, M., Murawski, K., Roberts, B.: 1999, Astron. Astrophys. 349, 312. ADSGoogle Scholar
  25. Murawski, K.: 2003, In: Erdélyi, R., Forgács-Dajka, E., Petrovay, K. (eds.) Contributions on Turbulence, Waves and Instabilities in the Solar Plasma, Publ. Astron. Dept. Eötvös Univ., vol. 13, Budapest, p. 61. Google Scholar
  26. Murawski, K., Goossens, M.: 1993, Astron. Astrophys. 279, 225. ADSGoogle Scholar
  27. Murawski, K., Roberts, B.: 1993a, Astron. Astrophys. 272, 595. ADSGoogle Scholar
  28. Murawski, K., Roberts, B.: 1993b, Astron. Astrophys. 272, 601. ADSGoogle Scholar
  29. Rajaguru, S.P., Basu, S., Antia, H.M.: 2001, Astrophys. J. 563, 410. CrossRefADSGoogle Scholar
  30. Roberts, B., Campbell, W.R.: 1986, Nature 323, 603. CrossRefADSGoogle Scholar
  31. Rosenthal, C.S., Christensen-Dalsgaard, J., Nordlund, Å., Stein, R.F., Trampedach, R.: 1999, Astron. Astrophys. 351, 689. ADSGoogle Scholar
  32. Stix, M.: 2000, Solar Phys. 196, 19. CrossRefADSGoogle Scholar
  33. Taylor, G.I.: 1938, Proc. Roy. Soc. A 164, 476. ADSGoogle Scholar
  34. Title, A.M., Schrijver, C.J.: 1998, In: Cool Stars, Stellar Systems, and the Sun, ASP Conf. Ser., vol. 154, p. 345. Google Scholar
  35. Unno, W., Kondo, M., Xiong, D.-R.: 1985, Publ. Astron. Soc. Pac. 37, 235. ADSGoogle Scholar
  36. Zweibel, E., Bogdan, T.: 1986, Astrophys. J. 308, 401. CrossRefADSGoogle Scholar

Copyright information

© Springer 2007

Authors and Affiliations

  1. 1.Solar Physics and Upper Atmosphere Research Group, Department of Applied MathematicsUniversity of SheffieldSheffieldUK
  2. 2.Department of AstronomyEötvös UniversityBudapestHungary

Personalised recommendations