Persistent and self-similar large-scale density fluctuations in the solar corona

Abstract

Density fluctuations of the low and midlatitude solar corona plasma are analyzed during the recent solar minimum period. Long time series of the intensity of the neutral hydrogen Lyα, 1216 Å, line have been observed with the UltraViolet Coronagraph Spectrometer/Solar and Heliospheric Observatory at 1.7 R⊙, in low-latitude streamers and in regions where the slow solar wind is accelerated. Their frequency composition is investigated by using three different techniques, namely the Fourier, the Hurst, and the phase coherence analyses. The Fourier analysis reveals the existence of low-frequency f-α power spectra in the range from ∼3 × 10-6 Hz to ∼10-4 Hz, corresponding to periods from a few hours to a few days. The coronal density fluctuations are dominated by discontinuities separating structures with a minimum characteristic timescale of about 3 hr and a corresponding spatial scale of about 3 × 10 4 km. The nonlinear analysis technique based on the structure functions shows that for large timescales the coronal density fluctuations are statistically self-affine and give rise to an average Hurst exponent 〈H〉 = 0.654 ± 0.008. This indicates that the process underlying the variability of the corona and the slow wind at coronal level is a persistent mechanism, generating correlations among the plasma density fluctuations. Finally, the analysis based on the phase coherence index shows a high degree of phase synchronization of the coronal density variations for large timescales, which shows that the solar corona is dominated by phase coherent structures. The results of the analysis suggest a coupling of the variability of the solar corona and the photospheric dynamics induced by the convection at supergranular scale. © 2009. The American Astronomical Society. All rights reserved.