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The quiet Sun average Doppler shift of coronal lines up to 2 MK  

Luca Teriaca   Submitted: 2011-09-22 03:12

The average Doppler shift shown by spectral lines formed from thechromosphere to the corona reveals important information on the mass and energybalance of the solar atmosphere, providing an important observationalconstraint to any models of the solar corona. Previous spectroscopicobservations of vacuum ultra-violet (VUV) lines have revealed a persistentaverage wavelength shift of lines formed at temperatures up to 1 MK. At highertemperatures, the behaviour is still essentially unknown. Here we analysecombined SUMER/SoHO and EIS/Hinode observations of the quiet Sun around diskcentre to determine, for the first time, the average Doppler shift of severalspectral lines formed between 1 and 2 MK, where the largest part of the quietcoronal emission is formed. The measurements are based on a novel techniqueapplied to EIS spectra to measure the difference in Doppler shift between linesformed at different temperatures. Simultaneous wavelength-calibrated SUMERspectra allow establishing the absolute value at the reference temperature of 1MK. The average line shifts at 1 MK < T < 1.8 MK are modestly, but clearlybluer than those observed at 1 MK. By accepting an average blue shift of about(-1.8±0.6) km s-1 at 1 MK (as provided by SUMER measurements), this translatesinto a maximum Doppler shift of (-4.4±2.2) km s-1 around 1.8 MK. The measuredvalue appears to decrease to about (-1.3±2.6) km s-1 at the Fe XV formationtemperature of 2.1 MK. The measured average Doppler shift between 0.01 and 2.1MK, for which we provide a parametrisation, appears to be qualitatively androughly quantitatively consistent with what foreseen by 3-D coronal modelswhere heating is produced by dissipation of currents induced by photosphericmotions and by reconnection with emerging magnetic flux.

Authors: N. Dadashi, L. Teriaca, S.K. Solanki
Projects: Hinode/EIS,SoHO-SUMER

Publication Status: Astronomy and Astrophysics (in press)
Last Modified: 2011-09-22 13:03
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SUMER observations of the inverse Evershed eff ect in the transition region above a sunspot  

Luca Teriaca   Submitted: 2008-11-28 03:43

Aims. We analyse SUMER spectral scans of a large sunspot within active region NOAA 10923, obtained on 14-15 November 2006, to determine the morphology and dynamics of the sunspot atmosphere at di fferent heights/temperatures. Methods. The data analysed here consist of spectroheliograms in the continuum around 142.0 nm and in the Si IV 140.2 nm, O III 70.3 nm, N IV 76.5 nm, and O IV 79.0 nm spectral lines. Gaussian-fitting of the observed profiles provides line-of-sight velocity and Doppler-width maps. Results. The data show an asymmetric downflow pattern compatible with the presence of the inverse Evershed flow in a region within roughly twice the penumbral radius at transition-region temperatures up to 0.18 MK. The motions, highly inhomogeneous on small scales, seem to occur in a collar of radially directed filamentary structures, with an average width less than the 1 Mm spatial resolution of SUMER and characterised by different plasma speeds. Assuming that the flows are directed along the field lines, we deduce that such field lines are inclined by 10 to 25 degrees with respect to the solar surface.

Authors: Teriaca L., Curdt W., Solanki S.K.
Projects: SoHO-SUMER

Publication Status: Published as a Letter on A&A volume 491, Issue 2, 2008, pp L5-L8
Last Modified: 2008-11-28 11:21
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THE EVAPORATION REGIME IN A CONFINED FLARE  

Luca Teriaca   Submitted: 2006-10-31 00:06

We studied the evolution of a small eruptive flare (GOES class C1) from its onset phase using multiwavelength observations that sample the flare atmosphere from the chromosphere to the corona. The main instruments involved were the Coronal Diagnostic Spectrometer (CDS) aboard SOHO and facilities at the Dunn Solar Tower of the National Solar Observatory/Sacramento Peak. Transition Region and Coronal Explorer (TRACE) together with Reuven Ramaty High-Energy Spectroscopic Imager (RHESSI) also provided images and spectra for this flare. Hα and TRACE images display two loop systems that outline the pre-reconnection and post-reconnection magnetic field lines and their topological changes revealing that we deal with an eruptive confined flare. RHESSI data do not record any detectable emission at energies ge~25 keV, and the observed count spectrum can be well fitted with a thermal plus a non-thermal model of the photon spectrum. A non-thermal electron flux F approx 5 imes 1010~erg~cm-2 s-1 is determined. The reconstructed images show a very compact source whose peak emission moves along the photospheric magnetic inversion line during the flare. This is probably related to the motion of the reconnection site, hinting at an arcade of small loops that brightens successively. The analysis of the chromospheric spectra (Ca~{sc ii} K, He~{sc i}~D_3 and Hgamma, acquired with a four seconds temporal cadence) shows the presence of a downward velocity (between 10 and 20 km s-1) in a small region intersected by the spectrograph slit. The region is included in an area that, at the time of the maximum X-ray emission, shows upward motions at transition region (TR) and coronal levels. For the He~{sc i}~58.4 and O~{sc v}~62.97 lines, we determine a velocity of approx-40 km s-1 while for the Fe~{sc xix}~59.22 line a velocity of approx-80 km s-1 is determined with a two-component fitting. The observations are discussed in the framework of available hydrodynamic simulations and they are consistent with the scenario outlined by inlinecite{Fisher:89}. No explosive evaporation is expected for a non-thermal electron beam of the observed characteristics, and no gentle evaporation is allowed without upward chromospheric motion. It is suggested that the energy of non-thermal electrons can be dissipated to heat the high density plasma, where possibly the reconnection occurs. The consequent conductive flux drives the evaporation process in a regime that we can call sub-explosive.

Authors: Falchi, A., Teriaca, L., Maltagliati, L.
Projects:

Publication Status: Solar Physics (accepted)
Last Modified: 2006-11-29 03:00
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Abstracts by Author
The quiet Sun average Doppler shift of coronal lines up to 2~MK
SUMER observations of the inverse Evershed eff ect in the transition region above a sunspot
THE EVAPORATION REGIME IN A CONFINED FLARE

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