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Periodic appearance of coronal holes and the related variation of solar wind parameters

We compared the variability of coronal hole (CH) areas (determined from daily GOES/SXI images) with solar wind (daily ACE data) and geomagnetic parameters for the time span January 25, 2005 until September 11, 2005 (minimum phase of solar cycle 23). Applying wavelet spectral analysis, a clear 9-day period is found in the CH time series. The GOES/SXI image sequence suggests that this periodic variation is caused by a triangular distribution of CHs ~ ; ; 120? ; ; apart in longitude. From solar wind parameters a 9-day periodicity was obtained as well, simultaneously with the 9-day period in the CH area time series. These findings provide strong evidence that the 9-day period in solar wind parameters, showing up as higher harmonic of the solar rotation frequency, is caused by a stable pattern of CHs on the Sun recurring for several solar rotations. Geomagnetic disturbances as quantified by the Dst index, do not match the variation of CH areas but show a better correlation with the magnetic field strength of the solar wind. A comparative analysis of daily sunspot number variations allows us to exclude that the observed solar wind variability is related to active longitudes in the photosphere. The wavelet power spectra for the whole ACE data range (~ ; ; 1998- ; ; 2006) suggest that the 9-day period is not a singular phenomenon occuring only during a specific time range during solar minumum but is also present during the maximum and decay phase of solar cycle 23.

coronal holes; solar wind; space weather

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