Atlantis over Tenerife

Japanese astronaut Sochi Noguchi took this iconic picture of the Space Shuttle Atlantis over the Atlantic Ocean and the island of Tenerife. The picure was taken from the International Space Station on Sunday, May 16 2010 at 10:28 am EDT (1428 GMT) while the Atlantis was getting ready for docking. At that time the ISS was about 350 km above the ground. This was the flight before the last one for the shuttle. In the highest resolution, the white towers of the solar telescopes at the Observatorio del Teide (IAC) are visible between the clouds. To help your eyes, I took a snapshot from the Google Maps: look for the distinctive dark patches in the NASA picture, the towers are tiny white dots just above them.

Credit: NASA/Sochi Noguchi (click for hi-res)

Credit: Google Maps (click to go to the interactive map)

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.

1D Solar Atmosphere Models in IDL: Maltby et al (MACKKL)

Here I add files containing the one-dimensional models of the solar atmosphere from the famous paper of Maltby et al (1986, 1986ApJ...306..284M). There is a quiet sun model and three models of the sunspot atmosphere (L, M, E corresponding to the late, mid and early cycle of the solar activity). For the details of these models please check the paper.

The data in the files comes directly from Tables 7, 8, 9 and 11 of Maltby et al. The variables stored in the original models are:

• geometrical height scale, 
• column mass scale, 
• optical depth at 5000 A, 
• temperature, 
• turbulent velocity, 
• number density of hydrogen, 
• electron density, 
• total pressure, 
• ratio of gas pressure to total pressure, 
• density.

Fig. The temperature stratification of the three sunspot models (remake of Fig.8 of Maltby et al)

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!

Eddington on Icarus

From the Presidential Address of Professor Arthur Eddington to Section A of the British Association in Cardiff, August, 24th, 1920 (1920Obs....43..341E, also in the book Stars & Atoms, 1927stat.book.....E):

"In ancient days two aviators procured to themselves wings. Daedalus flew safely through the middle air and was duly honoured on his landing. Icarus soared upwards to the sun till the wax melted which bound his wings and his flight ended in fiasco. The classical authorities tell us, of course, that he was only doing a stunt; but I prefer to think of him as the man who brought to light a serious constructional defect in the flying-machines of his day. So, too, in science. Cautious Daedalus will apply his theories where he feels confident they will safely go; but by his excess of caution their hidden weaknesses remain undiscovered. Icarus will strain his theories to the breaking-point till the weak joints gape. For the mere adventure? Perhaps partly, this is human nature. But if he is destined not yet to reach the sun and solve finally the riddle of its construction, we may at least hope to learn from his journey some hints to build a better machine."

Landscape with the Fall of Icarus by Pieter Bruegel the Elder (Musées royaux des Beaux-Arts de Belgique).

Diatomic partition functions in IDL

Recently I posted a set of IDL routines for the atomic partition functions (PF) based on various polynomial fits that are available in the literature. Here I do the same for the diatomic molecules.

The routines for the diatomic PF use an internal catalog of 325 diatomic molecules. The ionized molecules are separate entries. The catalog is stored as an IDL structure in catalogue_of_diatomics.sav. The structure has the following tags for every molecule:

Name	Chemical formula, e.g. 'H2', 'CO', 'H2+'
Nc	Number of constituents, always 2 for diatomics
Nuc	Number of uniq constituents, e.g. 2 for CO, 1 for H2
Constituents	Constituent atoms, e.g. ['H', 'H'], ['C', 'O']
Charge	Charge, e.g. -1, 0, +1
Code	Kurucz's code for molecules, e.g. CO '0608.00'
D0	Dissociation energy in eV
Data	List of internally available data sets for this molecule
Type	Type of data: pf for partition function, kp for chemical equilibrium constant, eint for internal energy (one entry for each dataset)
References	List of ADS references (for each dataset)

Atomic partition functions in IDL

Many books on radiative transfer and spectroscopy introduce the partition function ($U$, PF) as a statistical weight or probability that an atom is in the ionization stage $r$ at the temperature $T$. It is defined as the sum over all excitation states ($i$) of their statistical weights ($g_i$) multiplied by an
exponential factor (cf. Rutten, 2003, p.30, Eq.289):
$$U_r \equiv \sum_i g_i \mathrm{e}^{-\frac{\chi_{r, i}}{kT}},$$ where $\chi_{r, i}$ is the excitation potential of the state $i$ in the ionization stage $r$, $k$ is the Boltzmann constant) and $T$ is the temperature.

The trouble with PF is that there is an infinite number of the energy levels. No matter how small their contribution becomes (the exponential factor is decreasing with the excitation potential), PF computed after this definition goes to infinity. Farther, that means that the probability of an atom to occupy any finite state would be zero. Since this is clearly unphysical, thus there is a need to define a reliable procedure to cut off the partition function. There are many approached described in literature. However, they are usually rather non-trivial and, therefore, computing partition function even for a limited range of temperature and for a specific atomic element can be a very tedious job. The results of these computations are often published as the coefficients of the best polynomial fits for a range of temperature values. The IDL procedure that you can find below is written to load these tables from several different sources (see below) and to interpolate them for given a temperature, an atomic specie and an ionization stage.