The following description is from the webpage of Niels Bohr Institue's Former Centre for Ice and Climate

The Earth reacts to the solar wind by increasing the strength of the shielding magnetic field. Therefore, higher solar activity results in stringer shielding and thus lower production of cosmogenic isotopes.

The first intuition is the solar wind will cancel Earth's magnetic field but it seems in reality it will make Earth's magnetic field stronger. How should we explain this phenomenon?

  • 2
    $\begingroup$ Stringer - - > stronger. The typo is actually on the institute's web page. $\endgroup$
    – my2cts
    Jul 8, 2019 at 20:32
  • $\begingroup$ @my2cts - hey, maybe this is an actual use of string theory! $\endgroup$
    – Jon Custer
    Jul 8, 2019 at 21:38
  • $\begingroup$ Lenz's Law and Newton's Third Law. $\endgroup$ Jul 9, 2019 at 7:40

1 Answer 1


The conservation of magnetic flux is a simple approach. During times of higher speed solar wind it is generally the case that the associated dynamic pressure is higher, which exerts a force on the bow shock formed upstream of the Earth's magnetic field. This compresses the magnetosphere on the sunward side of Earth but stretches it on the anti-sunward side. If you decrease the area through which the magnetic field is normal, the field must increase if the flux is to be conserved. Thus, on the sunward side, the magnetic field enhances and it decreases on the anti-sunward side, which induces lots of currents (think Faraday's law).

  • $\begingroup$ Thank you. If the magnetic field decreases on the anti sunside, how should we explain that the isoptopes created by cosmic ray are fewer when solar wind is stronger? $\endgroup$
    – Rikeijin
    Jul 11, 2019 at 3:58
  • $\begingroup$ @Rikeijin - The reduction in cosmic rays below a few 10s of GeV during solar maximum (i.e., energies with the highest fluxes) are why the isotope creation rates change with solar cycle/activity (e.g., see more details at physics.stackexchange.com/a/490737/59023). $\endgroup$ Jul 11, 2019 at 14:58
  • $\begingroup$ From the discussion, Forbush Decrease( britannica.com/science/Forbush-effect ) ,the magnetic field of solar flares blocking galactic cosmic rays, cause the decrease of isotopes. May I say the description of Niels Bohr Institute is flawed because it has nothing to do with Earth's magnetic field? $\endgroup$
    – Rikeijin
    Jul 11, 2019 at 23:22
  • 1
    $\begingroup$ @Rikeijin - If the part in yellow in your original question is their explanation, then yes, that is flawed. The magnetic field enhances by a few to a few 10s of percent at most in specific locations during a geomagnetic storm. The magnetic field in the solar wind can increase by factors of >20 during strong ICMEs (i.e., >2000%). The Earth's magnetic field at the crustal surface is huge compared to that in an ICME, but the magnetosphere is smaller than a GCR's gyroradii so it's really not the reason for changing the isotope levels. $\endgroup$ Jul 12, 2019 at 13:47

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