Low-Frequency Lyα Power Spectra Observed by UVCS in a Polar Coronal Hole

Abstract

The occurrence of f-1 noise in interplanetary magnetic fields (in the 1 × 10-5 to 1 × 10-4 Hz band) and other plasma parameters has now been known for about 20 years and has been recently identified also in the photospheric magnetic fields. However, the relationship between interplanetary and solar fluctuation spectra and the identification of their sources at the Sun are problems that still need to be addressed. Moreover, interplanetary density and magnetic field power spectra show a f-2 interval at frequencies smaller that ~6 × 10-4 Hz whose source on the Sun is at present not fully understood. In this work we report on the first study of low-frequency density fluctuations in the solar corona at 2.1 Rsolar. In 2006 June the Ultraviolet Coronagraph Spectrometer (SOHO UVCS) observed over a period of about 9.2 days H Lyα intensity fluctuations at 2.1 Rsolar over a polar coronal hole. The Lyα intensity power spectra S(f) (related mainly to density fluctuations) showed a S(f)~f-2 frequency interval between 2.6 × 10-6 and 3.0 × 10-5 Hz and a S(f)~f-1 frequency interval between 3.0 × 10-5 and 1.3 × 10-4 Hz. The detection of a f-2 interval, in agreement with interplanetary density and magnetic field power spectra, has been also predicted in solar wind models as a consequence of phase-mixing mechanisms of waves propagating in coronal holes. High-latitude power spectra show a f-1 band approximately in the same frequency interval where f-1 noise has been detected in interplanetary densities, and interplanetary and photospheric magnetic fields, providing a connection between photospheric, coronal, and interplanetary f-1 noises.