alfven waves in a polar coronal hole from hinode/eis off limb observations bemporad a. & abbo l....
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ALFVEN WAVES IN A POLAR CORONAL HOLE FROM HINODE/EIS
OFF LIMB OBSERVATIONS
Bemporad A. & Abbo L.Bemporad A. & Abbo [email protected] –– [email protected]
INAF – Turin Astronomical Observatory, INAF – Turin Astronomical Observatory, via Osservatorio 20, 10025 Pino Torinese (TO), ITALYvia Osservatorio 20, 10025 Pino Torinese (TO), ITALY
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Outline
• Introduction: off-limb line broadening
• EIS observations
• Data analysis: EIS stray light, density and non-thermal velocity profiles
• Alfvèn waves energy flux
• Conclusions
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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 Rsun.(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 Tion (usually it is assumed Tion= Te or Tion = Tmax);• 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. 2005- 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?
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EIS long duration sit & stare observations
SIT & STARESlit position:Xc = 0” ; Yc = -1162”Slit width: 1”Altitude range:from 0.05 R๏ on diskup to 0.48 R๏ off limbDuration: ~ 21.6 hoursExposure time: 500s
SPATIAL RASTERSlit position:Xc = from -46.4” to +33.6”, step by 2” ; Yc = -1162”Slit width: 2”Altitude range:from 0.05 R๏ on diskup to 0.48 R๏ off limbDuration: ~ 1 hourExposure time: 100s
512”
40”
Y = -1418”
Y = -906”
SOHO/EIT FeXII 195.12 HINODE/EISfield of view
for spatial andtemporal 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 Rsun 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
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Observed spectral linesFor this study, 10 spectral panels (32 spectral bins per panel) have been acquired centered over the following lines: OVI 184.00, FeXII 186.85, CaXVII 192.82, CaXIV 193.87, FeXII 195.12, FeXIII 202.04, HeII 256.32, FeXIV 264.78, FeXIV 274.77, FeXV 284.16 (→small data volume)Over these spectral intervals, the following Fe lines have been well detected and identified:FeX 193.71, FeXII 195.12, FeXII 193.51, FeXII 186.88, FeXIII 202.04
SPATIAL RASTERS LONG DURATION SIT & STAREIntensity maps show a continuous decrease with time close
to the limb (below ~0.25 R๏), probably related to solar rotation.
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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.
FeX
III 2
02.0
4
F
eXII
19
5.12
Average over: 1 arcsec 25 arcsecs 125 arcsecs
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EIS stray light correction (1)
Spectra acquired in the FeXII 195.12 line show the presence of two more lines at the limb at 194.82 and 195.41; both lines disappear above 100” off-limb.
IDENTIFICATION of 195.4 line is ambiguous:
• Synthetic spectra (CHIANTI v.6.0) show no theoretical or observed lines at 195.4;• Brown et al. (2008): FeX 195.394 (20” off-limb EIS spectra above an AR), Tmax=106.0;• Landi & Young (2009) FeVII 195.415 (on-disk EIS spectra above an AR), Tmax=105.6.
?
?
FeXII 195.12FeXIII 202.04FeX 193.71
Unidentified at 195.41
The intensity of the 195.41 line decays with altitude much faster than FeXII 195.12 and FeXIII 202.04 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.
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RESULT: EIS stray light negligible (< 10%) above 0.07 R๏
diskdisk
straystray lineI
FeVIII
FeVIIIlineI
The stray light contribution I(line)stray of each line is computed as:
EIS stray light correction (2)
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Density estimateThe only available lines with density sensitive ratios are FeXII 195.12, 193.51 and 186.88 (186.854+186.887). Nevertheless, densities from 195.12/186.88 and 193.51/186.88 ratios are not reliable for h > 0.1 Rsun (S XI 186.838 blend?) → ne estimated with a different technique: based on G(ne,Te) curves provided by CHIANTI (v.6.0) we derived at any altitude the (n,T) values better reproducing the FeXII 195.12, 193.51, 186.88 and FeXIII 202.04 line intensities.
3779.0
7
65.9
8 10662.110987.1)(
cmhh
hne
)(10 max1.6 FeXIITKTe
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Observed vs computed line intensity profiles
In order to verify the correctness of ne and Te 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 186.88 more uncertain because of smaller intensities and maybe SXI blend
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FeXII 195 FWHM & non-thermal velocity profiles
222
202 2
2 Iion
ionB
M
Tk
c
Non-thermal velocity has been estimated by assuming:
1)I = 1.03 pixels, FWHMI = 2.42 pixels (Brown et al. 2008)
2) Tion = 106.1 i.e. Tion = Tmax(FeXII) (Mazzotta et al. 1998).
RESULT: increases with altitude up to ~ 0.2 R๏, then decreases.
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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๏.
AB
SvF AS
2
2
4
If B×A is constant and FS is conserved:
4/12/12
4/12/12 v
Theoretical curve for undamped waves
F=8.24×105 erg cm-2s-1
Alfvèn flux decayabove 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 FS is
(Moran 2001)
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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 Rsun). 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 Rsun.
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 Rsun
3) FWHM of FeXII l195.12 line increases up to 0.2 Rsun, 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๏.