Even on a quiet day, the magnetic field has a smooth variation. This variation is disregarded when one computes the K indices.

Then the difference between the maximum and minimum (for the three-hour interval in question) is computed for both horizontal components (normally X and Y meaning (geographical) north and east, or H and D meaning magnetic north and east).

The largest of the two differences is then compared to a table, different for different observatories. For the Lovö observatory it looks like this (upper limit inclusive):

(nT) | K |
---|---|

0--6 | K=0 |

6--12 | K=1 |

12--24 | K=2 |

24--48 | K=3 |

48--84 | K=4 |

84--144 | K=5 |

144--240 | K=6 |

240--396 | K=7 |

396--600 | K=8 |

600-- | K=9 |

(This is my first table in HTML.) |

Last change 16 December 1997.