The Solar Chromosphere

Transcription

1 The Solar Chromosphere Han Uitenbroek National Solar Observatory/Sacramento Peak Sunspot NM, USA IUGG, Session GAiv.01, Sapporo, Japan, 2003 July 1

2 Summary The chromosphere as part of the transition between the cool photosphere and the hot corona

3 Summary The chromosphere as part of the transition between the cool photosphere and the hot corona Caution: the problem of line formation heights

4 Summary The chromosphere as part of the transition between the cool photosphere and the hot corona Caution: the problem of line formation heights Thermal structure and CO lines: is the chromosphere hot or cold, or both

5 Summary The chromosphere as part of the transition between the cool photosphere and the hot corona Caution: the problem of line formation heights Thermal structure and CO lines: is the chromosphere hot or cold, or both The dynamic chromosphere

6 Summary The chromosphere as part of the transition between the cool photosphere and the hot corona Caution: the problem of line formation heights Thermal structure and CO lines: is the chromosphere hot or cold, or both The dynamic chromosphere Conclusions

7 The chromosphere

8 The chromosphere

9 The chromosphere is highly structured

10 Photosphere Han Uitenbroek, NSO/SP The Solar Chromosphere

11 Photosphere lower Chromosphere Han Uitenbroek, NSO/SP The Solar Chromosphere

12 Photosphere lower Chromosphere upper Chromosphere Han Uitenbroek, NSO/SP The Solar Chromosphere

13 Photosphere lower Chromosphere upper Chromosphere Corona Han Uitenbroek, NSO/SP The Solar Chromosphere

14 Ca i nm IR triplet line, KPVT/NSO

15 Ca i nm IR triplet line, KPVT/NSO

16 Ca i nm IR triplet line, KPVT/NSO

17 The chromosphere: difficult to observe, difficult to model The chromosphere is highly structured in 3-D and very dynamic. Densities are low enough so that magnetic forces start to dominate over hydrodynamic forces (low β = 8πP gas /B 2 regime).

18 The chromosphere: difficult to observe, difficult to model The chromosphere is highly structured in 3-D and very dynamic. Densities are low enough so that magnetic forces start to dominate over hydrodynamic forces (low β = 8πP gas /B 2 regime). It is transparent at optical and IR wavelengths, apart from some strong lines. It becomes optically thick in the continuum only at UV and radio wavelengths.

19 The chromosphere: difficult to observe, difficult to model The chromosphere is highly structured in 3-D and very dynamic. Densities are low enough so that magnetic forces start to dominate over hydrodynamic forces (low β = 8πP gas /B 2 regime). It is transparent at optical and IR wavelengths, apart from some strong lines. It becomes optically thick in the continuum only at UV and radio wavelengths. Chromospheric densities are low enough so that radiative transition rates generally dominate over collisional rates. Therefore, Non-LTE radiative transfer is required to model spectral line formation.

20 A word of caution: line formation heights Intensity [J m 2 s 1 Hz 1 sr 1 ] Disk center Wavelength [nm]

21 A word of caution: line formation heights Intensity [J m 2 s 1 Hz 1 sr 1 ] Disk center Wavelength [nm] 1000 Height [km] Contribution function [J m 2 s 1 Hz 1 sr 1 km 1 ] Wavelength [nm]

22 Formation heights of photospheric Fe i lines Formation height [km] nm Position along slit [Mm]

23 Recovered velocities with formation height from simulation v COG [km s 1 ] v LOS [km s 1 ]

24 Recovered velocities with formation height from 1-D model v COG [km s 1 ] v LOS [km s 1 ]

25 Line formation height of the sodium D lines Average Intensity [J m 2 s 1 Hz 1 sr 1 ] disk center ray Wavelength [nm]

26 Line formation height of the sodium D lines [nm] z [km] λ[nm] Source Function [J m 2 s 1 Hz 1 sr 1 ] x [km]

27 Na i D line is decoupled from local conditions

28 Full disk O i spectrum observed with HRTS Intensity [J m 2 s 1 Hz 1 sr 1 ] Wavelength [nm] 0 Courtesy Ken Dere, NRL

29 Perhaps one of the most important chromospheric diagnostics: Ca i H and K Intensity [J m 2 s 1 Hz 1 sr 1 ] Wavelength [nm]

30 The one-dimensional hydrostatic (semi-empirical) chromosphere Quiet Sun Temperature [K] Column Mass [kg m 2 ]

31 The CO infrared vibration-rotation lines Wavelength [nm] Intensity [J s 1 m 2 Hz 1 sr 1 ] R R R 21 [13] 2 1 R R R R R R 31 [13] 7 6 R R120 [13] 5 4 R 59 [13] 4 3 R 43 [13] 5 4 R R R136 [13] 6 5 R Brightness Temperature [K] Wave number [cm 1 ]

32 It s not precisely soot, but... CO line absorption cores observed at the solar limb have brightness temperatures as low as 3700 K. 5 4 Line formation calculations indicate that these lines are very likely in LTE and that these cores form at heights that are above the temperature minimum in classical 1-D models. Energy [ev] Vibrational level It seems impossible to construct a one-dimensional model that has chromospheric temperature rise and is able to explain the dark CO line cores at the same time. and is able to explain the dark CO line cores at the same time.

33 So is the chromosphere hot or cold? Quiet Sun Temperature [K] Column Mass [kg m 2 ]

34 So is the chromosphere hot or cold? Quiet Sun CO model Temperature [K] Column Mass [kg m 2 ]

35 The chromospheric dilemma Traditional one-dimensional hydrostatic models reproduce the mean chromospheric UV emission spectrum and the shapes of strong lines in the visible fairly accurately.

36 The chromospheric dilemma Traditional one-dimensional hydrostatic models reproduce the mean chromospheric UV emission spectrum and the shapes of strong lines in the visible fairly accurately. However, they fail to decribed CO line formation and (obviously) do not deal with inhomogeneities and temporal variability.

37 The chromospheric dilemma Traditional one-dimensional hydrostatic models reproduce the mean chromospheric UV emission spectrum and the shapes of strong lines in the visible fairly accurately. However, they fail to decribed CO line formation and (obviously) do not deal with inhomogeneities and temporal variability. Can they still be used to accurately describe the energy balance of the chromosphere, and in particular evaluate its radiative losses?

38 The chromospheric dilemma Traditional one-dimensional hydrostatic models reproduce the mean chromospheric UV emission spectrum and the shapes of strong lines in the visible fairly accurately. However, they fail to decribed CO line formation and (obviously) do not deal with inhomogeneities and temporal variability. Can they still be used to accurately describe the energy balance of the chromosphere, and in particular evaluate its radiative losses? Can it even describe the average properties (like temperature stratification) accurately?

39 Dynamics in the Chromosphere as observed in Ca i K

40 The new picture: a dynamic chromosphere Drive motions in 1-D atmosphere with amplitude determined from photospheric motions (Carlsson & Stein 1994). Solve radiation-hydrodynamics with Non- LTE radiative transfer where appropriate. Acoustic waves travel up, form shocks in low density chromosphere, which is heated intermittently through dissipation. This model is highly successful in reproducing the dynamic behavior of the Ca i H and K lines

41 Ca i K grains

42 Ca i K-line cooling in dynamic atmosphere t = 0 CRD Net radiative cooling per Ca atom [J s 1 ] PRD A PRD Net radiative heating per Ca atom [J s 1 ] Column Mass [kg m ]

43 Mean Ca i K-line cooling Net radiative cooling per Ca atom [J s 1 ] CRD PRD A PRD FALC, PRD Net radiative heating per Ca atom [J s 1 ] Column Mass [kg m 2 ]

44 CO lines in the dynamic Chromosphere

45 Time-dependent chemical concentrations: CO line depth Ref: Asensio Ramos, Trujillo Bueno, Carlsson, & Cernicharo 2003, ApJ 588, L61

46 Time-dependent CO formation height Ref: Asensio Ramos, Trujillo Bueno, Carlsson, & Cernicharo 2003, ApJ 588, L61

47 CO formation in 3-D hydrodynamic model R y [arcsec] Intensity [J m 2 s 1 Hz 1 sr 1 ] x [arcsec]

48 Spatially resolved CO observations 3 2 R y [arcsec] 40 0 T [K] x [arcsec] 400

49 Temporal behavior of CO lines 3 2 R14 T bright [K] v [km/s] v broad [km/s] Time [min] x [arcsec] x [arcsec] x [arcsec]

50 Three-dimensional radiation hydrodynamics Courtesy Mats Carlsson

51 Conclusions Very few spectral diagnostics are available for the chromosphere. Apart from Ca i H and K, Na i D 1,2, and H-α, β maybe the Mg i b triplet. Of course the CO lines play an important role as do UV lines.

52 Conclusions Very few spectral diagnostics are available for the chromosphere. Apart from Ca i H and K, Na i D 1,2, and H-α, β maybe the Mg i b triplet. Of course the CO lines play an important role as do UV lines. Low density conditions require Non-LTE modeling of these strong spectral lines.

53 Conclusions Very few spectral diagnostics are available for the chromosphere. Apart from Ca i H and K, Na i D 1,2, and H-α, β maybe the Mg i b triplet. Of course the CO lines play an important role as do UV lines. Low density conditions require Non-LTE modeling of these strong spectral lines. The average observed chromospheric spectrum cannot adequately be reproduced by a single one-dimensional hydrostatic model.

54 Conclusions Very few spectral diagnostics are available for the chromosphere. Apart from Ca i H and K, Na i D 1,2, and H-α, β maybe the Mg i b triplet. Of course the CO lines play an important role as do UV lines. Low density conditions require Non-LTE modeling of these strong spectral lines. The average observed chromospheric spectrum cannot adequately be reproduced by a single one-dimensional hydrostatic model. More about the relevant physics needs to be learned from forward modeling through (three-dimensional) radiation-hydrodynamic simulations that contain the effects of convection as well as of acoustic waves.

55 Thank You