Hydrogen Lyα Intensity Oscillations Observed by theSolar and Heliospheric ObservatoryUltraviolet Coronagraph Spectrometer

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

“…The amplitude of the fluctuations (∼7%-10%) is slightly I(Lya) larger than the amplitude of statistical uncertainties (∼5%-6%), but this has no consequences for our study because, as we verified, the superposition of a Poissonian noise over a signal whose power spectrum has a slope a flattens only the highfrequency part of the spectrum, preserving its lower frequency part. Also, as already discussed by Morgan et al (2004) the contributions of UVCS instrumental effects to the shape of power spectra are negligible; we conclude that the observed Lya fluctuations are representative of real changes in the physical properties of the emitting plasma crossing the slit field of view.…”

“…Morgan et al (2004), we conclude here that the observed fluctuations of Lya intensity are mainly associated with density fluctuations. The spectral analysis of the observed intensity versus time profiles is described in the next section.…”

“…In particular, Morgan et al (2004) found, from UVCS Lya observations over a polar coronal hole at altitudes ranging between 1.5 and 2.0 R , evi-, dence for oscillations with periods of 7-8 minutes, while Ofman et al (2000) demonstrated, from UVCS pB data acquired over a coronal hole at 1.9 and 2.1 R , the presence of oscil-, lations with periods of 6-10 minutes and a coherence time of 30 minutes. These authors concluded, from observations in the ∼3 # 10 Ϫ5 to 9 # 10 Ϫ3 Hz (Ofman et al 2000) and ∼4 # 10 Ϫ4 to 4 # 10 Ϫ3 Hz (Ofman et al 2000) frequency bands, that the observed density oscillations were a signature of fast or slow magnetosonic waves.…”

“…Previous studies of oscillations in the UV Lya line intensity (Morgan et al 2004) and the polarized brightness (Ofman et al 1997(Ofman et al , 2000 fluctuations focused only on data sets with a short duration in time (less than ∼10 hr). In particular, Morgan et al (2004) found, from UVCS Lya observations over a polar coronal hole at altitudes ranging between 1.5 and 2.0 R , evi-, dence for oscillations with periods of 7-8 minutes, while Ofman et al (2000) demonstrated, from UVCS pB data acquired over a coronal hole at 1.9 and 2.1 R , the presence of oscil-, lations with periods of 6-10 minutes and a coherence time of 30 minutes.…”

Astrophysics in 2004

“…Significant oscillations with periods of 7-8 minutes were also reported in Lya 1216 Å as far out as 1.5-2.2 solar radii (Morgan et al 2004). The damping of propagating waves and loop oscillations is currently a very hot topic, because we are just at the brink of learning which the underlying physical mechanisms are: wave leakage (Cally 2003;Díaz et al 2004), resonant absorption (VanDoorsselaere et al 2004a;Ruderman 2003;Aschwanden et al 2003;Luo et al 2004), loop curvature (VanDoorsselaere et al 2004b), gravitational stratification (Mendoza-Briceño et al 2004), collisional and viscous damping (Khodachenko et al 2004), thermal conduction (DeMoortel & Hood 2004;Klimchuk et al 2004;Carbonell et al 2004), or magnetic null points (McLaughlin & Hood 2004).…”