Sign up ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

We have the Advanced Composition Exporer (ACE), which orbits in the L1 libration point as a source for solar wind velocity and density (particles/cm^3) data. Typically, at a velocity in the range of 400 km/s, it takes about 4 days for solar wind changes to reach ACE. Measured solar wind velocities and densities tend to change from moment to moment. Official sunspot counts are obtained from Earth-based telescopes and change much more slowly.

From casual observation, there appears to be a positive correlation between solar wind velocity and density with sunspot count. Can anyone please gve me a reference that addresses this topic?

share|cite|improve this question
Have a look at and other talks/papers in Leif's page. –  anna v Feb 17 '12 at 5:09
Thank you Mark & Anna for the references. Leif does indeed have a number of solar articles that are quite enlightening. –  Michael Luciuk Feb 17 '12 at 18:16

1 Answer 1

There are broadly 2 types of solar wind:

  1. Fast

    • bulk flow speed ~ 750 km/s
    • temperature ~ 7.5e5 K
    • density ~ 3 cm^-3
    • source regions: mostly from coronal holes where the solar magnetic field extends out a long way into the solar system
  2. Slow

    • bulk flow speed ~ 450 km/s
    • temperature ~ 1.5e5 K
    • density ~ 5 cm^-3
    • source regions: near equatorial streamer belt

[There are some hints that some of the above has started to break down in the current solar cycle (slow solar wind now ~300 km/s), this is from talking to people at conferences I can't find a good reference.]

At solar minimum (few sunspots) the coronal holes are mostly confined to the sun's poles and hence not much fast solar wind makes it to Earth (or L1). Near solar maximum (many sunspots), coronal holes tend to migrate to become trans-equatorial. This exposes the planets to streams of fast solar wind. Hence there will be a correlation between sunspot number and measured wind speed.

There are also fast transient events Coronal Mass Ejections which are essentially the sun shedding magnetic complexity. They are more common when the solar magnetic field is more complex: at maximum.

See this result from, Ulysses' solar polar orbits

These results from Ulysses' illustrate the point. On the left, near solar min, the fast wind in around the poles; on the right, near max, the fast and slow winds are much more mixed up. The line colours show the magnetic field direction which is not so important for your question, focus on the radial distance of the line segments. EDIT Sorry, I linked to the magnetic field figure, not the solar wind speed one.

For references, see the introduction to "Wave Modeling of the Solar Wind" Leon Ofman here and this one too this one too by Owens and Forsyth the chapter by Hundhausen in Kivelson and Russell's Introduction to Space Physics is worth a read too if you can get hold of it.

share|cite|improve this answer

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.