Spectroscopic observations of propagating disturbances in a polar coronal hole: evidence of slow magneto-acoustic waves

Abstract

Aims. We focus on detecting and studying quasi-periodic propagating features that have been interpreted in terms of both slow magneto-acoustic waves and of high-speed upflows. Methods. We analyzed long-duration spectroscopic observations of the on-disk part of the south polar coronal hole taken on 1997 February 25 by the SUMER spectrometer onboard SOHO. We calibrated the velocity with respect to the off-limb region and obtained time-distance maps in intensity, Doppler velocity, and line width. We also performed a cross-correlation analysis on different time series curves at different latitudes. We studied average spectral line profiles at the roots of propagating disturbances and along the propagating ridges, and performed a red-blue asymmetry analysis. Results. We clearly find propagating disturbances in intensity and Doppler velocity with a projected propagation speed of about 60 ± 4.8 km s −1 and a periodicity of ≈14.5 min. To our knowledge, this is the first simultaneous detection of propagating disturbances in intensity as well as in Doppler velocity in a coronal hole. During the propagation, an intensity enhancement is associated with a blueshifted Doppler velocity. These disturbances are clearly seen in intensity also at higher latitudes (i.e., closer to the limb), while disturbances in Doppler velocity become faint there. The spectral line profiles averaged along the propagating ridges are found to be symmetric, to be well fitted by a single Gaussian, and have no noticeable red-blue asymmetry. Conclusions. Based on our analysis, we interpret these disturbances in terms of propagating slow magneto-acoustic waves.

O VI 1032 Å intensity and Doppler shift oscillations above a coronal hole: Magnetosonic waves or quasi-periodic upflows?

“…The reason why the O vi Doppler shift oscillations were found to be dominant in region B or that the intensity oscillations in region A did not show a clear counterpart in Doppler shift could depend on the different inclination of the plume structures that support those waves with respect to the LOS: for nearly perpendicular plumes, we would preferentially detect intensity oscillations (as in region A); for highly tilted plumes, we would also observe Doppler shift oscillations (as in region B). This geometrical effect could tentatively explain why the amplitudes of the highly significant Doppler shifts detected in this work (∼12 km s −1 ) are larger than those usually detected at lower heights ( < ∼ 6 km s −1 ) (e.g., Gupta et al 2012). Indeed, the amplitudes of the Doppler shift oscillations we detected in this particular set of observations seem quite large, even considering the expected decrease in amplitude at higher heliocentric distances due to dissipation mechanisms such as compressive viscosity and electron thermal conduction.…”

Cut-off period for slow magnetoacoustic waves in coronal plasma structures

“…Miyamoto et al (2014) detected quasi-periodic density disturbances with the period ranging from 100 to 2000 s at 1.5−20.5 solar radii. The analysis of spectroscopic observations revealed propagating disturbances of about 14.5 min period in a coronal hole (Gupta et al 2012). Krishna Prasad et al (2012) detected 12-25 min periodicities in off-limb fan loop-structures, on-disk plume-like structures, and in the polar plume/interplume regions.…”

Statistical Study of Network Jets Observed in the Solar Transition Region: a Comparison Between Coronal Holes and Quiet-Sun Regions

“…One way, for instance, would be to investigate the possible connection between these jets and the hot intermittent upflows along plume-like structures (McIntosh et al, 2010;Tian et al, 2011;Pucci et al, 2014;Pant et al, 2015) and the blue-shift patches of lower coronal lines (Tian et al, 2008a(Tian et al, , 2009Fu et al, 2014). Moreover, one must also try to address the question of dissipation of energy by the network jets in the corona, e.g., whether they trigger compressional waves or not (Gupta et al, 2012;Uritsky et al, 2013;Jiao et al, 2015;Pant et al, 2015). This aspect can be studied in more details in order to further explore the difference in mechanisms between CH and QS for generating network jets and for processing the supplied mass and energy by these jets to the corona and solar wind.…”