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Transcript of MHD Coronal Seismology with SDO/AIA UK Community Views Len Culhane, MSSL with inputs from Valery...
MHD Coronal Seismology with SDO/AIA
UK Community Views
Len Culhane, MSSLwith inputs from
Valery Nakariakov, WarwickIneke de Moortel, St Andrews
David Williams, MSSLMihalis Mathioudakis, Queen’s University
1. Transverse (Kink) Oscillations
Commonly accepted interpretation Standing fast magnetoacoustic m=1 modes of coronal loops
Seismological applications Estimation of the mean value of the magnetic field
strength (Nakariakov and Ofman, 2001) Estimation of the density stratification in the loop
(Andries et al. 2005)
Open questions Excitation Mechanisms Decay Mechanisms
Selectivity of the Excitation
Role of Nonlinear Effects
Use of Observations for Seismology
Derivation of simple analytical expressions Linking the observables with physical parameters in the loop (e.g. magnetic field strength)
3- D MHD numerical modelling in extrapolated magnetic geometry Linking the outcomes with analytical theory Testing the robustness of the modelling, in particular to the errors in extrapolation and to computational constraints Creation of seismological methods for the determination of coronal currents by MHD waves
Necessary Theoretical Developments 3- D MHD modelling Incorporation of fine structuring effects - stratification, curvature, nonlinearity in simple magnetic geometries (isolated loop, arcade, parallel loops bundle) Parametric numerical study of the dependence of observables on model parameters (period, decay time, time dependence, scaling laws)
2. Longitudinal (Slow) Waves
Commonly Accepted Interpretation: Slow magnetoacoustic perturbations, guided
by magnetic field lines. Seismological Applications: Observables (periods, wave lengths, height
evolution) depend strongly upon thermal effects
- information about the heating function. Modes are natural probes of coronal thermal
structure - simultaneous detection of mode in TRACE 171A and 195A suggested as a probe of sub-resolution structuring of the corona (King et al., 2003)Open Questions: What is their origin and driver? What determines the periodicity and
coherence of propagating waves? What is the physical mechanism for the
abrupt disappearance of the waves at a certain height
Are the waves connected with the running penumbra waves?(King et al., 2003)
Effects of transverse structuring (physical and observational).
Incorporation of a realistic TR and chromosphere in the model, including 2-D and partial ionisation effects.
Investigation of thermal over-stability of coronal plasmas.
Forward modelling of observables.
Necessary Theoretical Developments
Novel Data Analysis Techniques - Periodogram Mapping
Periodograms of the time signals from each pixel of analysed data cube are calculated.
Determine whether there is a spectral peak with an amplitude over a prescribed confidence level.
There are two types of maps: - Periodomaps: pixel colour corresponds to the detected period if its amplitude exceeds a certain threshold - Filtermaps: pixel colour corresponds to the amplitude of the detected signal, if the period is in a certain prescribed range.
Reduce a 3-D data cube to a 2-D periodogram map
Current desktop computing for 4k x 4k datacube takes 150 hours; sample duration was ~ 83 minutes at 30 sec cadence - code in IDL - using regular Fourier transform - gain at least x 10 with FFT and better code
Periodomap of a TRACE data cube from the 2nd of July 1998. The period detection level
is 95%. The period range is 2.9-3.2 mHz.
Solar-B – Role of EUV Imaging Spectrometer Solar-B launched two years before SDO
Observations will focus on individual Active Regions and Photospheric Dynamics
Spectral line wave observations with EIS will be challenging- Identify target structures for joint observations- Measure ne (ratios), vplasma (peak shifts) and vnon-thermal (line widths)- Undertake Doppler shift and broadening observations related to
wave phenomena (e.g. torsional Alfven modes)
Before SDO launch, rehearse using EIS observations of structures in campaigns with TRACE
Establish links between Solar-B observation planning and SDO operational modes for joint study of targets of interest for coronal seismology
EIS Instrument Features
Large Effective Area in 2 EUV bands: 170-210 Å and 250-290 Å– Multi-layer Mirror (15 cm dia ) and Grating
• both with optimized Mo/Si Coatings– CCD camera
• two 2048 x 1024 high QE back-illuminated CCDs
Spatial resolution → 1 arc sec pixels/2 arc sec resolution
Line spectroscopy with ~ 25 km/s per pixel sampling
Field of View : – Raster: 6 arc min×8.5 arc min – FOV centre moveable E – W by ± 15 arc min
Wide temperature coverage: log T = 4.7, 5.4, 6.0 - 7.3 K
Simultaneous observation of up to 25 lines (spectral windows)
Density Sensitive Line Ratio
Density sensitive line ratios with pairs of forbidden lines
CHIANTI is used for this estimate
Filling factor of coronal loop estimated at 2 arc sec resolution
Fe XI line ratios 182.17/188.21 and 184.80/188.21 will also be useful (Keenan et al. 2005)
Doppler Velocity and Line Width Uncertainties
Doppler velocity
Line width
Bright AR line Flare line
Photons (11 area)-1 sec Photons (11 area)-1 (10sec)-1
One- uncertainty in: - Doppler shift (v in km/s) - Non-thermal line width ( FWHM in km/s)
Values are plotted against number of detected photons in the line for: - Bright AR line (Fe XV/284 Å) - Flare line (Fe XXIV/255 Å)
Loop Cross Section and Coronal Parameters
Simulated cross-section of a loop with radius 720 km (1” Earth view). Horizontal axis is perpendicular to the observer’s line-of-sight, whilst the vertical axis extends parallel to the line-of-sight. Coloured areas: Simulated line-of-sight velocitiesContours: Line-of-sight velocity multiplied by ne
2
Calculated emission from a flux tube imaged by Solar-B EIS assuming:
– 1 arc sec spatial pixel– 23 mÅ spectral pixel– Fe XII 195 Å line registered in EIS– possible periods ~ 10 – 100 s– assume t = 1s in 1 arc sec slice at loop apex
Assume radial profile of azimuthal velocity amplitude (Ruderman et al.,1997)is
V (r) = 16 vo (r/a)2 (1 – r/a)2
vo is maximum azimuthal velocity, a is loop
radius and is measured clockwise from line-of-sight about loop axis
Also assume Bo = 50 G, ne, max = 2.109,
Te = 1.5.106 K, ne, core is x10 external
density (r > a) and loop length L = 10a
Williams et al. simulated effect on line profile of a torsional wave passing through a
coronal AR loop – effects invisible in e.g. TRACE-like passband unless high velocities
Simulated profile of the Fe XII 195 Å line in EIS B
AR loop of radius a = 350km (0.5”)
Loop cross-section is contained in a single 1” pixel
Torsional velocity amplitude maximum of 25 km.s-1.
Rest profile centre shown by vertical dotted line.
Spatially averaged, optically thin torsional broadening mimics a non-thermal velocity of approximately half the maximum amplitude of the actual azimuthal velocity.
Simulated profile of the Fe XII 195 Å line in EIS B
AR loop of radius a = 720km (1.0”)
Loop cross-section straddles two pixels, each of which detects a profile with a different centroid
For receding right-hand side (x > 0), profile has spatially averaged red-shift of ~ 12 km.s-1
Approaching left-hand side of loop shows apparent blue-shift of ~ 5 km.s-1
Discrepancy is largely due to the photon noise in these simulated lines.
Simulated Line Profiles
Coronal Seismology Requirements SummaryObservational
Observe transverse “kink” modes in coronal loops to enable seismology applications e.g. magnetic field estimates, damping scaling laws
- search for higher harmonics for density stratification information
Study propagating longitudinal waves throughout the solar atmosphere to investigate coupling of different regions of solar atmosphere and probe thermal structure
- high-cadence observations of quiescent coronal loops in a wide Te range
Observe EIT waves to check evolution of wave amplitude and speed with distance- study wave front interaction with active regions- establish height structure of disturbances- clarify role of global magnetic topology
Search for torsional Alfven modes- observe line shifts and broadenings with Solar-B EIS and target structures
at high cadence with AIA
Coronal Seismology Requirements SummaryTheoretical
Incorporate effects of e.g fine structure, stratification, curvature, in analytical theory and numerical simulations for propagation in simple loop geometries
Incorporate transverse structure and realistic transition region and chromosphere in analytical theory and numerical simulations for propagation throughout atmosphere
Examine role of magnetic topology in wave propagation – EIT waves?
Techniques and Tools
Automated detection of wave modes and oscillations• periodogram mapping search for significant periods• amplitude search within specified frequency ranges
Develop robust and automated (?) wavelet analysis techniques
Relate NLFF extrapolated magnetic fields (HMI) with AIA wave observations
Joint spectral (Solar-B) and imaging (AIA) observations → ne, vplasma, vnon-thermal
END OF TALK
EIS Sensitivity
Ion Wavelength
(A)
logT Nphotons
AR M2-Flare
Fe X 184.54 6.00 15 36
Fe XII 186.85 / 186.88 6.11 13/21 105/130
Fe XXI 187.89 7.00 - 346
Fe XI 188.23 / 188.30 6.11 41 / 15 110/47
Fe XXIV 192.04 7.30 - 4.0104
Fe XII 192.39 6.11 46 120
Ca XVII 192.82 6.70 31 1.8103
Fe XII 193.52 6.11 135 305
Fe XII 195.12 / 195.13 6.11 241/16 538/133
Fe XIII 200.02 6.20 20 113
Fe XIII 202.04 6.20 35 82
Fe XIII 203.80 / 203.83 6.20 7/20 38/114
Detected photons per 11 area of the Sun per 1 sec exposure. Ion Wavelength
(A)
logT Nphotons
AR M2-Flare
Fe XVI 251.07 6.40 - 108
Fe XXII 253.16 7.11 - 71
Fe XVII 254.87 6.60 - 109
Fe XXVI 255.10 7.30 - 3.3103
He II 256.32 4.70 16 3.6103
Si X 258.37 6.11 14 62
Fe XVI 262.98 6.40 15 437
Fe XXIII 263.76 7.20 - 1.2103
Fe XIV 264.78 6.30 20 217
Fe XIV 270.51 6.30 17 104
Fe XIV 274.20 6.30 14 76
Fe XV 284.16 6.35 111 1.5103
Spectroscopic PerformanceLong Wavelength Band
• Ne III lines near 267 Å from the NRL Ne–Mg Penning discharge source
• Gaussian profile fitting gives the FWHM values shown in the right-hand panel
57.7 mÅ58.1 mÅ
57.9 mÅ
~ 4600
Spectroscopic PerformanceShort Wavelength Band
• Mg III lines near 187 Å from the NRL Ne–Mg Penning discharge source
• Gaussian profile fitting gives the FWHM values shown in the right-hand panel
47 mÅ 47 mÅ
~ 4000
• Four slit/slot selections available
• EUV line spectroscopy - Slits - 1 arcsec 512 arcsec slit - best spectral resolution - 2 arcsec 512 arcsec slit - higher throughput
• EUV Imaging – Slots – Overlappogram; velocity information overlapped– 40 arcsec 512 arcsec slot - imaging with little overlap– 250 arcsec 512 arcsec slot - detecting transient events
Slit and Slot Interchange
EIS Field-of-View
360
512
EIS Slit
Maximum FOV for raster observation
512
900 900
Raster-scan range
Shift of FOV center with coarse-mirror motion
250 slot
40 slot
512
EIS Effective Area
Primary and Grating: Measured - flight model data usedFilters: Measured - flight entrance and rear filters CCD QE: Measured - engineering model data used
Following the instrument end-to-end calibration, analysis suggests that the above data are representative of the flight instrument