WG3 REPORT: Chromospheric dynamics, magnetism and heating - Jorrit Leenaarts, Mats Carlsson, Peter Gomory, Christoph Kuckein, Ada Ortiz - Science ...
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WG3 REPORT:
Chromospheric dynamics, magnetism
and heating
Jorrit Leenaarts, Mats Carlsson, Peter Gomory,
Christoph Kuckein, Ada Ortiz
June 12th 2018, Giardini Naxos, Italy
Outline
1. The planned science of WG3: the chromosphere from a
dynamic, magnetic and energetic point of view
2. Some example Observing Programs
3. Requirements on EST imposed by chromospheric science
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 739500
1. The planned science of WG3: characteristics
Brief characteristics of the chromosphere:
!
• The chromosphere is the interface between the photosphere and the
corona.
• Dynamics change from gas-pressure dominated atmosphere to magnetically
driven atmosphere.
• radiation transport changes from optically thick to optically thin
• gas state changes from neutral to ionised
• gas state changes from local thermodynamic equilibrium to non-LTE.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 739500
1. The planned science of WG3: characteristics
The chromosphere is relatively difficult to study compared with the photosphere:
!
• Evolution timescales are shorter, and changes are observed down to a timescale
of seconds.
• Chromospheric lines are often broad and deep, thus having a small photon flux.
• When the magnetic field reaches the chromosphere it fans out, i.e. becomes
volume filling due to the low density. Therefore, flux densities and polarisation
signals are weaker than in the photosphere.
• Fundamental spatial scales are expected to be small.
• Because the chromosphere is highly stratified we cannot sample the entire layer
with a single spectral line. Multiple spectral lines are needed to sample all heights.
The magnetic field is the key quantity in the physics of the chromosphere.
Understanding its physics requires inference of the magnetic field at all locations
in the chromosphere.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 739500
1. The planned science of WG3: science cases
1. Magnetic structure at supergranular scales
!
The magnetic field in the chromosphere is much less well constrained that that in the
photosphere due to the weaker polarisation signals. This is specially true outside active
regions.
!
• We want to obtain the full 3D magnetic and atmospheric structure in the quiet Sun
at super granular scale.
!
• Fibrilar topology is present in almost all chromospheric structures. Need highest
attainable spatial resolution.
!
2. Spicules and jets
!
Type I and II spicules are ubiquitous in the chromosphere. Type II are not so well
understood. Martinez-Sykora et al (2017) proposed a model of spicule acceleration by
magnetic tension forces. A crucial point is the effect of ion-neutral interactions. Spicules
are narrow, not very long, have compact acceleration sites, evolve fast and flows reach
high speeds. IFUs are needed. Their magnetic field structure is an incognita.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 739500
1. The planned science of WG3: science cases
3. Structure of small-scale chromospheric jets
!
Numerical simulations indicate occurrence of bidirectional or counterstream flows
within chromospheric jets, but have not yet been observationally confirmed. These flows
may increase turbulence and instabilities at the interface of the jet and the ambient
plasma. The fine structure of jets is still beyond current resolution limit.
!
4. Wave propagation
!
The chromosphere is pervaded by magneto-acustic waves as well as Alfvenic waves
(transverse and torsional). Many of these waves are excited in the photosphere by
granular buffeting and motions of magnetic elements. Mode conversion occurs at b=1, as
is wave reflection. Tracking propagation of Alfvenic waves requieres very high cadence
than magneto-acustic waves due to their high speeds.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 739500
1. The planned science of WG3: science cases
5. Flux emergence and reconnection events
!
Reconfigurations of the magnetic field (after reconnection) and flux emergence produce
small-scale rapid intensity variations in the chromosphere. To resolve these phenomena
high spatial and temporal resolution are required. Larger FOV preferred.
!
6. Observational determination of electric currents
!
Dissipation of electric currents has been proposed to explain the heating of the upper
solar atmosphere. We need to measure the magnitude and direction of the electric
currents to quantify the contribution of Ohmic dissipation to the overall heating. But this
is difficult to measure. The electric current is determined by
The magnetic field must be determined through inversion of spectropolarimetric
observations. But the curl operation amplifies any noise or errors made in the inference
of the magnetic field. Various trade-offs between spatial, temporal resolution, SNR and
FOV are proposed.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 739500
1. The planned science of WG3: science cases
7. Temperature structure of the solar chromosphere
!
The temperature of the chromosphere is time-varying and highly inhomogeneous. Using
the solar 4.7 microns rovibrational bands of Carbon monoxide Ayres & Rabin (1996)
found a very cool component in the chromosphere. In addition, theory predicts that the
non-magnetic solar chromosphere should be at temperatures below 1500 K (Leenaarts
et al 2011). But the location and temperature of those cool clouds is unclear.
!
8. Magnetic field determination using Ca II H & K
!
Martinez Pillet et al. (1990) measured circular polarization in the Ca II H&K lines for
quiet Sun, plage, umbra, penumbra and a flare. They obtained Stokes V signals of 5-15% in
all but the quiet Sun. Polarization measurements in these lines is then an interesting tool
to infer magnetic fields in the formation height range between Ca II 8542 and He I
10830. Data can be obtained with SPs, Fps or IFUs. The line has a very low photon flux,
thus long integration times are needed to get a high SNR.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 739500
2. Some example Observing Programs
OP 3.2.1. Magnetic field structure in the quiet Sun chromosphere
How do network and internetwork fields look and connect in the chromosphere? At
what heights is the magnetic canopy located? How does the height of the b=1 surface
vary? A supergranule will be scanned in many phot. and chrom. lines simultaneously.
NLTE inversions to get the quiet Sun 3D magnetic and atmospheric structure.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395002. Some example Observing Programs
OP 3.3.1. Type II spicule acceleration on disk
Aim: Edge of network or plage region. Acceleration sites of type II spicules at
highest spatial & temporal resolutions. Ca II K to observe acceleration sites at
highest spatial resolution. Polarimetry from phot. to mid chrom.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395002. Some example Observing Programs
OP 3.3.3. Type II spicule evolution off-limb
Focus on the evolution after launching. He I 10830 is required to measure magnetic fields
in the upper chromosphere. Coordination with space missions is desirable to trace
spicule evolution in the UV to higher layers and temperatures. Context imaging using a
broad-band imager is needed.
Scientific questions that can be addressed with this OP are: how does spicule fine
structure look and evolve? What kind of wave motions do spicules exhibit? What is the
magnetic field structure in spicules?
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395002. Some example Observing Programs
OP 3.6.3. Reconnection at different heights
Ellerman bombs have been shown to be the signal of reconnection at the photospheric
level. UV bursts appear to be the signature of reconnection in the upper chromosphere and
transition region.
Unanswered questions: What are the fundamental differences/similarities between EBs, UV
bursts, microflares, and flaring active region fibrils? Do EBs trigger reconnection higher up?
How does reconnection happen as a function of height?
Reconnection is a magnetic process, so this OP needs relatively good polarimetric
sensitivity. High cadence required. Large spectral ranges. Co-observing with space based
UV instruments is essential to see the possible counterparts at higher layers.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395002. Some example Observing Programs
OP 3.9.1. Magnetic field determination in plage including Ca II H&K
Polarization in these lines is an interesting tool to infer magnetic fields in the formation
height range between Ca II 8542 and He I 10830. Slit spectrographs, FPs and IFUs can be
used.
The line has a very low photon flux, so long integrations times are needed to get a high
SNR. Here we choose measuring the 3D magnetic field structure in plage.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395003. Requirements on EST imposed by chromospheric science
The specific characteristics of the chromosphere require:
• high signal-to-noise ratio (SNR): to measure the weak
chromospheric polarisation signals and to reach diffraction
limited observations
• the highest spatial resolution: as fundamental spatial scales are
small
• short integration times (or high temporal resolution): evolution
timescales are short, often of the order of seconds
But these requirements are mutually exclusive, so a trade-off is
necessary.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395003. Requirements on EST imposed by chromospheric science
In solar images, the power spectrum of solar features decreases
towards smaller spatial scales (unpublished work by M. van Noort).
In order to resolve the highest frequencies (i.e. the smallest scales) a
high SNR is required the larger the diameter of the telescope
becomes.
Diffraction limited observations will need
high SNR and thus large integration times
SNR is crucial when observing the chromosphere
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395003. Requirements on EST imposed by chromospheric science
Importance of simultaneous observations in multiple lines:
• The entire chromosphere cannot be sampled with a single spectral line,
as density (or opacity) changes drastically with height. Therefore we
need multiple spectral lines observed simultaneously in order to
sample all heights.
• In addition, the main goal of EST is to couple the photosphere and
chromosphere, which also requires multi-line observations that can
sample the photosphere, and the low, mid and upper chromosphere.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395003. Requirements on EST imposed by chromospheric science
Advantages of multi-
wavelength observations:
Many mentioned in
yesterday’s talk by C. Kuckein
Multi-wavelength observations
used to perform multi-line non-
LTE inversions to estimate the
structure and heating of the
chromosphere on a flux
emerging region.
Leenaarts etThis
al.,project
A&A 612, A28
is supported by the(2018)
European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395003. Requirements on EST imposed by chromospheric science
Advantages of multi-wavelength observations
Multi-wavelength
observations used to
follow a cold magnetic
bubble of newly
emerged magnetic field
on its journey upwards
to the TR
This project is supported by the European Commission´s H2020 Programme
Ortiz et al., ApJfor825, 93 (2016)
the period March 2017 – March 2021 under Grant Agreement nº 7395003. Requirements on EST imposed by chromospheric science
• The fast evolution of structures in the chromosphere requires fast,
strictly simultaneous observations.
• As a consequence, we need a light distribution system that allows for
multiple lines to be scanned fast and strictly simultaneously.
IFUs become very important for chromospheric studies.
In spite of their small FOV, they allow high cadence and a higher SNR
than other types of instruments for the same exposure time.
FPs provide a larger FOV but require a long scanning time due to the
width of chromospheric lines and the high velocities attained.
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 7395003. Requirements on EST imposed by chromospheric science
Importance of polarimetric sensitivity:
• Polarimetric sensitivities of at least 10-4 Ic are needed to detect the
weakest chromospheric signals.
• No linear polarisation is detected in chromospheric lines when
sensitivities of the order 10-3 Ic are used (e.g. Leenaarts et al. 2018,
Gosic et al. in preparation shown yesterday by Luis).
This project is supported by the European Commission´s H2020 Programme
for the period March 2017 – March 2021 under Grant Agreement nº 739500
Leenaarts et al., A&A 612, A28 (2018)You can also read
