ALFVEN WAVES IN A POLAR CORONAL HOLE FROM HINODE/EIS OFF LIMB OBSERVATIONS Bemporad A. & Abbo L. – INAF – Turin Astronomical Observatory, via Osservatorio 20, Pino Torinese (TO), ITALY

Outline Introduction: off-limb line broadening EIS observations Data analysis: EIS stray light, density and non- thermal velocity profiles Alfvèn waves energy flux Conclusions

Introduction: past observations (O’Shea et al. 2003) Many authors reported that above the limb in both plumes and inter-plume coronal hole regions the spectral line widths increase with altitude and then start to decrease above ~ 0.1 R sun. (e.g.: Hassler et al. 1990; Banerjee et al. 1998; Lee et al. 2000; O’Shea et al. 2003). BUT impact of these results was limited:  estimate → assumption on unknown T ion (usually it is assumed T ion = T e or T ion = T max ); Other authors found: - dependence on line width with formation temperature has been found → no Alfvèn waves (e.g. O’Shea et al. 2004; Singh et al. 2003), - a plateau instead of a line width decrease (e.g. Contesse et al. 2004; Wilhelm et al stray light can affect the observed line widths in SUMER data (Dolla & Solomon 2008). The main questions I’m going to investigate are: 1.What is the EIS stray light contribution? 2.What is the behaviour observed with EIS far from the limb?

EIS long duration sit & stare observations SIT & STARE Slit position: X c = 0” ; Y c = -1162” Slit width: 1” Altitude range: from 0.05 R ๏ on disk up to 0.48 R ๏ off limb Duration: ~ 21.6 hours Exposure time: 500s SPATIAL RASTER Slit position: X c = from -46.4” to +33.6”, step by 2” ; Y c = -1162” Slit width: 2” Altitude range: from 0.05 R ๏ on disk up to 0.48 R ๏ off limb Duration: ~ 1 hour Exposure time: 100s 512” 40” Y = -1418” Y = -906” SOHO/EIT FeXII HINODE/EIS field of view for spatial and temporal rasters Between February 24-25, 2009 an off-limb study of a polar coronal hole has been performed with HINODE/EIS for the first time up to ~ 0.48 R sun above the limb. The instrument acquired, in sequence: ~ 1 hour spatial raster (context study) ~ 21.6 hours sit & stare ~ 1 hour spatial raster (context study) with the following instrumental settings

Observed spectral lines For this study, 10 spectral panels (32 spectral bins per panel) have been acquired centered over the following lines: OVI , FeXII , CaXVII , CaXIV , FeXII , FeXIII , HeII , FeXIV , FeXIV , FeXV (→small data volume) Over these spectral intervals, the following Fe lines have been well detected and identified: FeX , FeXII , FeXII , FeXII , FeXIII SPATIAL RASTERSLONG DURATION SIT & STARE Intensity maps show a continuous decrease with time close to the limb (below ~0.25 R ๏ ), probably related to solar rotation.

Averaged line profiles at different altitudes In order to derive line profiles with a good statistic data have been averaged over the whole observation interval (21.6 h). Resulting profiles show a good statistic, even up to 0.4 R ๏ above the limb. FeXIII FeXII Average over: 1 arcsec 25 arcsecs 125 arcsecs

EIS stray light correction (1) Spectra acquired in the FeXII line show the presence of two more lines at the limb at and ; both lines disappear above 100” off-limb. IDENTIFICATION of line is ambiguous: Synthetic spectra (CHIANTI v.6.0) show no theoretical or observed lines at 195.4; Brown et al. (2008): FeX (20” off-limb EIS spectra above an AR), T max = ; Landi & Young (2009) FeVII (on-disk EIS spectra above an AR), T max = ? ? FeXII FeXIII FeX Unidentified at The intensity of the line decays with altitude much faster than FeXII and FeXIII lines → we assume that this is not a coronal (FeX), but a transition region line (FeVII) → we assume that the FeVII intensity observed above the limb is solely due to EIS stray light.

RESULT: EIS stray light negligible (< 10%) above 0.07 R ๏ The stray light contribution I(line) stray of each line is computed as: EIS stray light correction (2)

Density estimate The only available lines with density sensitive ratios are FeXII , and ( ). Nevertheless, densities from / and / ratios are not reliable for h > 0.1 R sun (S XI blend?) → n e estimated with a different technique: based on G(n e,T e ) curves provided by CHIANTI (v.6.0) we derived at any altitude the (n,T) values better reproducing the FeXII , , and FeXIII line intensities.

Observed vs computed line intensity profiles In order to verify the correctness of n e and T e values, we compa- red the expected (blue line) and the observed (red diamonds) intensi- ties of the Fe lines after the stray light subtraction. Good agreement between the obser- ved and computed line intensities. Determination of FeXII more uncertain because of smaller intensities and maybe SXI blend

FeXII 195 FWHM & non-thermal velocity profiles Non-thermal velocity  has been estimated by assuming: 1)  I = 1.03 pixels, FWHM I = 2.42 pixels (Brown et al. 2008) 2) T ion = i.e. T ion = T max (FeXII) ( Mazzotta et al ). RESULT:  increases with altitude up to ~ 0.2 R ๏, then decreases. 

Interpretation: Alfvèn waves energy deposition above 0.2 R ๏ Interpretation: observed non ther- mal velocities  due to Alfvèn waves.  increase due to amplitude increase of undamped waves propagating in a den- sity stratified corona.  decrease → Alfvèn waves damping → energy deposition above 0.2 R ๏. If B×A is constant and F S is conserved: Theoretical curve for undamped waves F=8.24×10 5 erg cm -2 s -1 Alfvèn flux decay above 0.2 R ๏ RESULT: the Alfvèn energy flux decays above 0.2 R ๏. This is a signature of Alfvèn waves energy deposition in corona The Alfvèn waves energy flux F S is (Moran 2001)

SUMMARY & CONCLUSIONS: Some previous studies on variation of line profile widths found an increase followed by a decrease of line widths with altitude  interpretation: Alfvèn wave energy deposition. Nevertheless, other works found different results and interpretations. In this work we repeated the same analyses with Hinode/EIS data by performing an off-limb study in a polar coronal hole (FOV for the first time up to 1.48 R sun ). Our main results are: 1) HINODE/EIS stray light contamination is negligible above 0.07 R ๏, hence EIS stray light is not affecting line profile FWHMs above ~ 0.1 R sun. 2) By averaging over more than 20 hours, EIS data can be used to infer electron density and line profile FWHMs off-limb up to ~ 0.4 R sun 3) FWHM of FeXII l line increases up to 0.2 R sun, then decreases. This can be interpreted as a decay of Alfvèn wave energy flux above that altitude.  Alfvèn wave energy deposition above ~ 0.2 R ๏.