Robert Jelle Rutten (1942 - 2022)

So long, my teacher and friend. You will be hugely missed.

1D Solar Atmosphere Models in IDL: Standard Solar Model (from "The Sun", by Stix)

Stix, in his book ("The Sun: An Introduction", Springer, 2002) in Sect.2.4 introduces a standard solar model from the core to the surface defined as $\tau = 2/3$. His Table 2.4 (on p.56) lists the values of various quantities of this model versus the column mass or the height. Check the book for the details of the model. The table in ascii format is here: stix_tab.2.4.txt The columns are: $m/m_\odot$, $r/r_\odot$, $p \mathrm[Pa]$, $T \mathrm[K]$, $\rho \mathrm[kg/m^3]$, $L/L_\odot$, $X$, $\mu$, $\Gamma_1$.

Deep-learning about horizontal velocities at the solar surface

The velocity fields are of great importance for understanding dynamics and structure of the solar atmosphere. The line of sight velocities are coded in the wavelength shifts of the spectral lines, thanks to the Doppler effect, and relatively easy to measure. On the other hand, the orthogonal ("horizontal") components of the velocity vector are impossible to measure directly.

The most popular method for estimating the horizontal velocities is so-called local correlation tracking (LCT, November & Simon, 1988). It is based on comparing successive images of the solar surface in the continuum light and transforming their differences into information about the horizontal fields. However, the LCT algorithm suffers from several limitations.

In a paper by Andres Asensio Ramos and Iker S. Requerey (with a small contribution from my side) accepted by A&A and published on Arxiv some weeks ago (2017arXiv170305128A) this problem is tackled by the deep-learning approach. A deep fully convolutional neural network is trained on synthetic observations from 3D MHD simulations of the solar photosphere and then applied to the real observation with the IMaX instrument on board the SUNRISE balloon (Martinez Pillet et al, 2011; Solanki, 2010). The method is validated using simulation snapshots of the quiet sun produced with the MANCHA code that I have been developing in the last couple of years.

First observation of linear polarization in the forbidden [OI] 630.03 nm line

In a new paper (de Wijn, Socas-Navarro & Vitas, 2017, ApJ, 836, 29D) we present the first results of our observations of a sunspot and an active region using the SP/SOT instrument on board the Hinode satellite. The novelty in our observation is a trick that we used to double the standard wavelength range observed by the instrument. Thanks to that, we were able to see the sun not only in the two iron lines at 630.2 nm, but also in four other lines. One of those is particularly interesting: the forbidden ground-based line of neutral oxygen ([OI] 630.03 nm). It is one of only few oxygen lines in the solar spectrum and probably the best diagnostics of the solar oxygen abundance. For the first time ever we observed the linear polarization in this line! As an M2 (magnetic dipole) transition, it is predicted by the theory (Landi degl'Innocenti and Landi, 2004, Section 6.8) that this line produces the linear polarization signal with the opposite sign to the lines produced by E2 transitions. It is also the first time that linear polarization in M2 and nearby E2 lines is measured simultaneously, so that the flip in sign is obvious (see the left-most spectral line in the red circle in the Figure; in linear polarization it has "W" shape, while all other lines in the wavelength range have "M" shapes). This result may bring new light to the ongoing debate on the solar oxygen crisis.

  

More details of this unique observation will appear soon in a follow-up publication.

1D Solar Atmosphere Models in IDL: Penumbra by Ding & Fang (1989)

Plane-parallel atmosphere in hydrostatic equilibrium published by Ding & Fang (1989, "A semi-empirical model of sunspots penumbra", 1989A&A...225..204D). Statistical equilibrium for hydrogen model-atom with 12 levels plus continuum. The model is produced by fitting observations of penumbra in  2 lines of H and 5 lines of Ca. The observations were carried out on McMath telescope at Kitt Peak National Observatory. The observed sunspot was small, rounded and close to the disk center. The field strength in the umbra was around 1.25 kG and 560 G in the penumbra.

It is interesting to note their Fig.2 (see it below). In the deep photosphere, the temperature in this model is similar to the temperature in the model of Yun et al. (1981). However, between the optical depths -3 and -4 it becomes close to the VALC model (Vernazza et al, 1981).

Kurucz' Atlas in GNU Linux

I have just found about great effort made by Sbordone, Bonifacio & Castelli to make the Atlas code of Bob Kurucz available at GNU Linux.

There is a webpage: http://atmos.obspm.fr/ providing the code and the documentation.

1D Solar Models in IDL: Spruit's Convection Model

The semi-empirical models of the solar atmosphere rely on the observed intensities and, therefore, they cannot say much about the invisible convection zone below the surface. Spruit (1974SoPh...34..277S) constructed a 1D model of the solar convection zone using the mixing-length theory with 4 free parameters. The model is constructed so that it matches the HSRA model (Gingerich et al, 1971SoPh...18..347G) of the solar atmosphere. The model of Spruit is sometimes used to initiate the convection simulations with 3D numerical codes.

Fig 1. The HSRA model atmosphere (dashed) and Spruit's model of convective zone (solid). This is reproduced after Fig.3 of Spruit's paper.

1D Solar Atmosphere Models in IDL: Gingerich et al (HSRA)

The Harvard-Smithsonian Reference Atmosphere (HSRA) by Gingerich et al (1971, 1971SoPh...18..347G) is another widely used semi-empirical model of the idealized plane-parallel solar atmosphere in hydrostatic equilibrium. This model also includes the chromosphere where the hydrogen ionization is solved using the statistical equilibrium equations. The model is still often used as the reference solar atmosphere in 1D and its number of citations steadily grows (> 820 so far). The paper is very clearly written and it's a very recommendable reading especially if you use this model. Beside the model itself, there is several tables with computed intensities, brightness temperature, optical depths at different wavelengths, etc.

Theory vs. Observations, by Jean-Claude Pecker

A cartoon by the famous French astronomer Jean-Claude Pecker (published in the Proceedings of the 3rd European Solar Meeting, "Solar activity, April 13-15 1981, Oxford", ed. C. Jordan; a non-ADS volume).

No matter how much we pushed the frontier of the science and how much our theoretical machinery, diagnostic tools and large and expensive gadgets evolve, this is still in a way very actual. It is important not to forget that even the most sophisticated realistic 3D models of the sun are still mammals, pardon, models. On the other hand, the results of the observations should not be consider  as the real "elephant" either. They are pictures of the elephant made with imperfect instruments!

Use of "some useful atomic data"

To illustrate how the routines in the previous post can be used, I made a couple of plots.

Fig.1 The logarithmic solar abundances relative to hydrogen A (A(H) = 12) after Asplund et al (2009, 2009ARA&A..47..481A) versus the atomic number Z. The data are loaded using load_abundances.pro function.

"The Sun is miasma of incandescent plasma"

"Why Does the Sun Shine?" is a funny song on the solar physics originally written by Singer and Zaret. Here it is covered by They Might Be Giants, an alternative band from the 80's, best know for their super-mega-hit "Istanbul (Not Constantinople)". On the same album of children's music TMBG published another song - "Why Does the Sun Really Shine?", an errata to the original one ("the Sun is miasma of incandescent plasma" :) ). Both songs can be heard on YouTube:

The final days of the Utrecht astronomy

Many streets in Utrecht are named after celestial bodies or astronomical terms - Meteoorhof, Jupiterstraat, Eclipshof, Komeetstraat, Venuslaan, etc. There is also Zonstraat - a cozy street dedicated to the Sun that goes all the way through one of the most beautiful neighborhoods to the old observatory Sonnenborgh. Some people in the street keep "solar portraits" on their facades and in the windows. It's a real shame that only memories are left of astronomy in this town.

 

More on the fallout of Utrecht astronomy: 

C. U. Keller: Sterrekundig Instituut Utrecht: The Last Years

R. J. Rutter: Sterrekundig Instituut Utrecht

Nature 

Good Night and Good Luck, Utrecht Astronomers

Austerity bites in Utrecht 

Observational Signatures of the Simulated Solar Photosphere

My PhD thesis is titled "Observational Signatures of the Simulated Solar Photosphere". The very general titles as this one are rather common among the PhD theses defended at the Dutch universities. They reflect the praxis that a thesis is assembled from several papers / research projects that are not always on the same subject. My thesis contains 5 papers, the introduction, the summary (in English, Dutch and Serbo-Croatian) and the acknowledgments. The papers address the problems of the solar abundance of indium, of the formation of a activity-sensitive MnI line, of the horizontal supersonic shocks in the solar photosphere, and - in the final two chapters - problem of various diagnostics of the umbral dots. For more information, please check the thesis introduction.

The thesis in the pdf format can be downloaded from the University of Utrecht library service.

If you cite the thesis (parts that are not yet published as individual papers), please cite it as:
Vitas, N. 2011, PhD thesis, Sterrekundig Instituut Utrecht

The woodcut on the cover is from the graphic novel "The Sun" by Flemish artist Frans Masereel.