Experiment 4: Study of solar spectrum

Objective :

To study the solar spectrum and identify some of the prominent spectral lines in the spectrum. You will also use the spectra to compute the column density of Hydrogen and Calcium atoms in the Solar atmosphere.

Introduction :

In this experiment you will study the solar spectrum taken from space and on Earth. You will identify some prominent spectral lines and determine their equivalent width. This will be used to estimate the column density of corresponding ions or atoms contributing to these lines. Finally these will be used to estimate the relative abundance of some elements in the solar atmosphere.

You will be provided with a data file, solar_data.txt, which contains solar spectral flux received in space in units of $Watt/(m^2 nm)$. The flux is defined as the radiation energy received from sun per unit area per unit wavelength range. The wavelength is given in units of "nanometer". Also provided (below) are the high resolution graphs of CaII - H and K lines and Hydrogen Balmer line. Further you are also given a reference set of spectral lines, ReferenceSpectralLines.txt, and the relationship between the spectral line width and column density of atoms in stellar atmospheres.

Calcium II spectral lines :

Calcium.png


Hydrogen Balmer lines :
Balmer.png


General curve of growth for the sun :
GrowthCurve_Sun.jpeg


  • Step 1: Plot the solar spectral data set to see the solar spectrum using any plotting tool of your choice (free tools). A graph of the data is also provided.

  • Step 2: Use the black body intensity formula to compute the solar flux at a few representative wavelengths. Compare this with the solar spectrum data. Recall that the Sun's surface temperature is 5780 K. Explain the differences that you might observe between your calculations and data. A plot with those two data sets is shown:

    SolarSpectrum.gif


  • Step 3: The graph of solar spectrum with reference lines overlaid is provided. Identify the Ca II (K), Ca II (H), He I (Hydrogen Balmer) spectral lines in the spectrum.

  • Step 4: Use the high resolution graph of the Hydrogen Balmer and Calcium II spectral lines to estimate the equivalent width of these lines. You may estimate the equivalent width W by measuring the width at half maximum (minimum).

  • Step 5: Using the curve of growth graph which relates the column density of a species of atoms to the equivalent width of the spectral lines, find the product f N. Here N is the column density of atoms which contribute to the spectral line and f is the oscillator strength, which represents the probability that the atom or ion can contribute to the spectral line under consideration.

    The oscillator strengths for Calcium lines are f = 0.68 for wavelength 393.3 nm, f = 0.33 for wavelength 396.8 nm (source NIST) and for Hydrogen Balmer line, f = 0.637 at wavelength 656.3 nm (source Carroll and Ostlie).