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

Dipankar Banerjee   Submitted: 2009-06-24 22:21

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 A & 195.119 A and Fe xiii 203.82 A & 202.04 A . Results. For the polar region, the line width data show that the nonthermal line-of-sight velocity increases from 26 km s-1 at 1000 above the limb to 42 km s-1 some 15000 (i.e. 110,000 km) above the limb. The electron density shows a decrease from 3.3 109 cm-3 to 1.9 108 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.

Authors: D. Banerjee, D. P?erez-Su?arez, and J.G. Doyle
Projects: Hinode/EIS

Publication Status: A&A Letters (in press)
Last Modified: 2009-06-25 13:25
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Propagating waves in polar coronal holes as seen by SUMER and EIS  

Dipankar Banerjee   Submitted: 2009-05-07 06:53

To study the dynamics of coronal holes and the role of waves in the acceleration of the solar wind, spectral observations were performed over polar coronal hole regions with the SUMER spectrometer on SoHO and the EIS spectrometer on Hinode. Using these observations, we aim to detect the presence of propagating waves in the corona and to study their properties. The observations analysed here consist of SUMER spectra of the Ne VIII 770 A line (T = 0.6 MK) and EIS slot images in the Fe XII 195 A line (T = 1.3 MK). Using the wavelet technique, we study line radiance oscillations at different heights from the limb in the polar coronal hole regions. We detect the presence of long period oscillations with periods of 10 to 30 min in polar coronal holes. The oscillations have an amplitude of a few percent in radiance and are not detectable in line-of-sight velocity. From the time distance maps we find evidence for propagating velocities from 75 km s-1 (Ne VIII) to 125 km s-1 (Fe XII). These velocities are subsonic and roughly in the same ratio as the respective sound speeds. We interpret the observed propagating oscillations in terms of slow magneto-acoustic waves. These waves can be important for the acceleration of the fast solar wind.

Authors: D. Banerjee, L. Teriaca, G. R. Gupta, S. Imada, G. Stenborg, S. K. Solanki
Projects: SoHO-SUMER

Publication Status: Accepted as Letter of Astronomy and Astrophysics
Last Modified: 2009-05-07 13:32
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Present and Future Observing Trends in Atmospheric Magneto-seismology  

Dipankar Banerjee   Submitted: 2007-08-09 21:39

With modern imaging and spectral instruments observing in the visible, EUV, X-ray and radio wavelengths, the detection of oscillations in the solar outer atmosphere has become a routine event. These oscillations are considered to be the signatures of a wave phenomenon and are generally interpreted in terms of magnetohydrodynamic (MHD) waves. With multi-wavelength observations from ground and space-based instruments, it has been possible to detect waves in a number of different wavelengths simultaneously and, consequently, to study their propagation properties. Observed MHD waves propagating from the lower solar atmosphere into the higher regions of the magnetized corona have the potential to provide an excellent insight into the physical processes at work at the coupling point between these different regions of the Sun. High-resolution wave observations combined with forward MHD modelling can give an unprecedented insight into the connectivity of the magnetized solar atmosphere, which further provides us with a realistic chance to reconstruct the structure of the magnetic field in the solar atmosphere. This type of solar exploration has been termed atmospheric magneto-seismology. In this review we will summarise some new trends in the observational study of waves and oscillations, discussing their origin, and their propagation through the atmosphere. In particular, we will focus on waves and oscillations in open ({it e.g.,} solar plumes) and closed ({it e.g.,} loops and prominences) magnetic structures, where there have been a number of observational highlights in the last few years. Furthermore, observations of waves in filament fibrils allied with a better characterization of their propagating and damping properties, the detection of prominence oscillations in UV lines, and the renewed interest in large-amplitude, quickly attenuated, prominence oscillations, caused by flare or explosive phenomena, will be addressed.

Authors: Banerjee, D., Erd'elyi, R., Oliver, R., and O'Shea, E.
Projects: None

Publication Status: solar Physics (in press)
Last Modified: 2007-08-10 11:00
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Abstracts by Author
Signatures of Alfv?en waves in the polar coronal holes as seen by EIS/Hinode
Propagating waves in polar coronal holes as seen by SUMER and EIS
Present and Future Observing Trends in Atmospheric Magneto-seismology

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