Lecture 23 Homework Problems
The Sun

1:  In the photosphere, the gas energy density dominates and can push magnetic field around until it becomes concentrated into tiny bundles of high field strength.  Given typical parameters for the photosphere: n = 1023 m-3, T = 5770 K, estimate the magnetic field strength B in these bundles.  Magnetic field strength in metric units is given in tesla (T).  [Hint: the gas can concentrate the field until b ~ 1.]

2: Let us compare doppler shifts from the Sun due to solar rotation and supergranule motions near the limb of the Sun. [see http://soi.stanford.edu/results/heliowhat.html]. The doppler shift is a simple shift in frequency due to the radial velocity difference vr between the source and observer, given by Dl = (vr / c) l0, where l0 is the original wavelength.

(a) What is the velocity of the Sun at the limb due to solar rotation? [Hint: Look up the rotation period of the Sun at the equator. From that and the solar radius, you can determine the rotational velocity.]
(b) What is the corresponding doppler shift, in wavelength, of visible light at 500 nm wavelength?
(c) The horizontal velocities due to supergranule motion are roughly 300 m/s.  What is the corresponding doppler shift at 500 nm wavelength? Comparing to (b), which signal should dominate at the limb?
(d) Why do both the rotational and supergranular velocities get very small in the center of the disk?