Journal cover Journal topic
Annales Geophysicae An interactive open-access journal of the European Geosciences Union
Journal topic

Journal metrics

Journal metrics

  • IF value: 1.585 IF 1.585
  • IF 5-year value: 1.698 IF 5-year
    1.698
  • CiteScore value: 1.62 CiteScore
    1.62
  • SNIP value: 0.820 SNIP 0.820
  • IPP value: 1.52 IPP 1.52
  • SJR value: 0.781 SJR 0.781
  • Scimago H <br class='hide-on-tablet hide-on-mobile'>index value: 83 Scimago H
    index 83
  • h5-index value: 24 h5-index 24
Volume 16, issue 5
Ann. Geophys., 16, 510–517, 1998
https://doi.org/10.1007/s00585-998-0510-2
© European Geosciences Union 1998
Ann. Geophys., 16, 510–517, 1998
https://doi.org/10.1007/s00585-998-0510-2
© European Geosciences Union 1998

  31 May 1998

31 May 1998

On the periodic variations of geomagnetic activity indices Ap and ap

H. Schreiber H. Schreiber
  • Institut für Geophysik der Universität Göttingen, Herzberger Landstrasse 180, D-37075 Göttingen, Germany
  • Fax: +49 551 397459; e-mail: hschrei@willi.uni-geophys.gwdg.de

Abstract. Yearly averages of geomagnetic activity indices Ap for the years 1967–1984 are compared to the respective averages of ν2·Bs, where v is the solar wind velocity and Bs is the southward interplanetary magnetic field (IMF) component. The correlation of both quantities is known to be rather good. Comparing the averages of Ap with ν2 and Bs separately we find that, during the declining phase of the solar cycle, ν2 and during the ascending phase Bs have more influence on Ap. According to this observation (using Fourier spectral analysis) the semiannual and 27 days, Ap variations for the years 1932–1993 were analysed separately for years before and after sunspot minima. Only those time-intervals before sunspot minima with a significant 27-day recurrent period of the IMF sector structure and those intervals after sunspot minima with a significant 28-28.5-day recurrent period of the sector structure were used. The averaged spectra of the two Ap data sets clearly show a period of 27 days before and a period of 28–29 days after sunspot minimum. Moreover, the phase of the average semiannual wave of Ap is significantly different for the two groups of data: the Ap variation maximizes near the equinoxes during the declining phase of the sunspot cycle and near the beginning of April and October during the ascending phase of the sunspot cycle, as predicted by the Russell-McPherron (R-M) mechanism. Analysing the daily variation of ap in an analogue manner, the same equinoctial and R-M mechanisms are seen, suggesting that during phases of the solar cycle, when ap depends more on the IMF-Bs component, the R-M mechanism is predominant, whereas during phases when ap increases as v increases the equinoctial mechanism is more likely to be effective.

Key words. Interplanetary physics · Magnetic fields · Solar wind plasma · Solar wind · magnetosphere interaction

Publications Copernicus
Download
Citation