Signatures of Alfvén waves in the polar coronal holes as seen by EIS/Hinode

Abstract

Context. We diagnose the properties of the plume and interplume regions in a polar coronal hole and the role of waves in the acceleration of the solar wind. Aims. We attempt to detect whether Alfvén waves are present in the polar coronal holes through variations in EUV line widths. Methods. Using spectral observations performed over a polar coronal hole region with the EIS spectrometer on Hinode, we study the variation in the line width and electron density as a function of height. We use the density sensitive line pairs of Fe xii 186.88 Å and 195.119 Å and Fe xiii 203.82 Å and 202.04 Å.Results. For the polar region, the line width data show that the nonthermal line-of-sight velocity increases from 26 km s −1 at 10 above the limb to 42 km s −1 some 150 (i.e. ∼110 000 km) above the limb. The electron density shows a decrease from 3.3 × 10 9 cm −3 to 1.9 × 10 8 cm −3 over the same distance. Conclusions. These results imply that the nonthermal velocity is inversely proportional to the quadratic root of the electron density, in excellent agreement with what is predicted for undamped radially propagating linear Alfvén waves. Our data provide signatures of Alfvén waves in the polar coronal hole regions, which could be important for the acceleration of the solar wind.

Solar Coronal Plumes

“…A summary of the plume/interplume line widths can be found in Table 2 of Wilhelm (2012). Figure 11 gives an example from Banerjee et al (2009) of the increase with altitude above the limb of the line widths, in plume and interplume regions, observed by SUMER in the Si viii 1445.75Å line and by HINODE EIS in the Fe xii 195Å line. Although widths are larger in interplumes than in plumes, the difference is minor.…”

“…The interested reader can find a thorough description of the line ratio techniques in Mason and Fossi (1994): as the methods described earlier, these techniques are affected by density or temperature inhomogeneities along the LOS. Typically, ratios of the Si viii 1446 and 1440 and Si ix 342 and 345Å line intensities have been used to evaluate densities, while temperatures have been inferred from the ratios of the O vi 1730 and 1032 and of the Mg ix 706 and 750Å line intensities (e.g., Del Zanna and Bromage, 1999;Mohan et al, 2000;Wilhelm, 2006;Banerjee et al, 2009).…”

Phase relations for seismology of photospheric flux tubes

“…They were detected in the corona using the Extreme ultraviolet Imaging Telescope (EIT) aboard the Solar and Heliospheric Observatory (SOHO; Thompson et al 1999); they were also detected with the imaging telescope on board the Transition Region and Coronal Explorer (TRACE) spacecraft (Nakariakov et al 1999). More recently, such magnetohydrodynamic (MHD) waves were observed in the corona with the Coronal MultiChannel Polarimeter (CoMP; Tomczyk et al 2007), with the extreme ultraviolet imaging spectrometer (EIS) aboard Hinode (Van Doorsselaere et al 2008;Banerjee et al 2009), and with the Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA; Liu et al 2010;Morton et al 2012). Magnetohydrodynamic waves are also found in the chromosphere using the Solar Optical Telescope (SOT) aboard Hinode ) and using the Swedish Solar Telescope (SST; Jess et al 2009).…”

Energy and energy flux in axisymmetric slow and fast waves

“…Tomczyk et al 2007), with the extreme ultraviolet imaging spectrometer (EIS) aboard Hinode (e.g. Van Doorsselaere et al 2008;Banerjee et al 2009), and with the Solar Dynamics Observatory Atmospheric Imaging Assembly (SDO/AIA; e.g. Aschwanden & Schrijver 2011;Morton et al 2012b) to give but a few examples.…”