Abstract
In the present work we derive a Differential Emission Measure (DEM) distribution from a region dominated by spicules. We use spectral data from the Solar Ultraviolet Measurements of Emitted Radiation (SUMER) spectrometer on-board the Solar Heliospheric Observatory (SoHO) covering the entire SUMER wavelength range taken off-limb in the Northern polar coronal hole to construct this DEM distribution using the CHIANTI atomic database. This distribution is then used to study the thermal properties of the emission contributing to the 171 Å channel in the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO). From our off-limb DEM we found that the radiance in the AIA 171 Å channel is dominated by emission from the Fe ix 171.07 Å line and has sparingly little contribution from other lines. The product of the Fe ix 171.07 Å line contribution function with the off-limb DEM was found to have a maximum at logT max (K)=5.8 indicating that during spicule observations the emission in this line comes from plasma at transition region temperatures rather than coronal. For comparison, the same product with a quiet Sun and prominence DEM were found to have a maximum at logT max (K)=5.9 and logT max (K)=5.7, respectively. We point out that the interpretation of data obtained from the AIA 171 Å filter should be done with foreknowledge of the thermal nature of the observed phenomenon. For example, with an off-limb DEM we find that only 3.6 % of the plasma is above a million degrees, whereas using a quiet Sun DEM, this contribution rises to 15 %.
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Acknowledgements
We would like to thank the anonymous referee for the important suggestions and comments on this manuscript. K.V, M.M, J.G.D thank ISSI for the support of the team “Small-scale transient phenomena and their contribution to coronal heating”. Research at Armagh Observatory is grant-aided by the N. Ireland Department of Culture, Arts and Leisure. CHIANTI is a collaborative project involving NRL (USA), RAL (UK), and the Universities: College London (UK), of Cambridge (UK), George Mason (USA), and of Florence (Italy). The AIA data are courtesy of SDO (NASA) and the AIA consortium. The SUMER project is financially supported by DLR, CNES, NASA and the ESA PRODEX programme (Swiss contribution). This work was supported via grant ST/F001843/1 & ST/J00135X from the UK Science and Technology Facilities Council.
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Advances in European Solar Physics
Guest Editors: Valery M. Nakariakov, Manolis K. Georgoulis, and Stefaan Poedts
An erratum to this article is available at http://dx.doi.org/10.1007/s11207-014-0603-x.
Appendix
Appendix
As mentioned in Section 2, coronal hole off-limb data are dominated by spicules. We use EIT images (see Figure 1) to ensure that no coronal bright points are present in the SUMER field-of-view since EIT was the only available imager at the time the SUMER reference spectra were obtained. However, the EIT images do not distinctly reveal spicular structure due to some instrumental limitations: low spatial resolution and high stray-light contribution. In order to establish what we see in the EIT image we compare it with co-temporal AIA 304 Å image (see Figure 6) which has a better spatial resolution. While the off-limb EIT image shows blurred streaks merging together due to its low spatial resolution, the AIA image reveals numerous narrow spikes, i.e. spicules, confirming the abundance of spicules in off-limb coronal hole data.
AIA 304 Å (top) and EIT 304 Å (bottom) images (colour table reversed) of a coronal hole region at the South Pole taken on 14 May 2010.
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Vanninathan, K., Madjarska, M.S., Scullion, E. et al. Off-limb (Spicule) DEM Distribution from SoHO/SUMER Observations. Sol Phys 280, 425–434 (2012). https://doi.org/10.1007/s11207-012-9986-8
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DOI: https://doi.org/10.1007/s11207-012-9986-8