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On the periodic variations of geomagnetic activity indices Ap and ap


Title (Dublin Core)

On the periodic variations of geomagnetic activity indices Ap and ap

Description (Dublin Core)

Yearly averages of geomagnetic activity
indices Ap for the years 1967–1984 are compared to the respective averages of
&#x03BD;<sup>2</sup>·<i>B<sub>s</sub></i>, where<i> v </i>is the solar wind
velocity and <i>B<sub>s</sub></i> 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 &#x03BD;<sup>2</sup> and <i>B<sub>s</sub></i>

separately we find that, during the declining phase of the solar cycle,
&#x03BD;<sup>2</sup> and during the ascending phase <i>B<sub>s</sub></i> 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-<i>B<sub>s</sub></i> component,
the R-M mechanism is predominant, whereas during phases when ap increases as <i>v</i>
increases the equinoctial mechanism is more likely to be effective.<br><br><b>Key words.</b> Interplanetary physics · Magnetic fields ·
Solar wind plasma · Solar wind · magnetosphere interaction</p>

Creator (Dublin Core)

H. Schreiber

Subject (Dublin Core)

Geophysics. Cosmic physics

Publisher (Dublin Core)

Copernicus Publications

Date (Dublin Core)


Type (Dublin Core)


Identifier (Dublin Core)


Source (Dublin Core)

Annales Geophysicae, Vol 16, Pp 510-517 (1998)

Language (Dublin Core)


Relation (Dublin Core)

Provenance (Dublin Core)

Journal Licence: CC BY