VOACAP Quick Guide

The Space Weather Center

Bz vs. Planetary and Polar A & K Index

The direction of the Interplanetary Magnetic Field (Bz) is one of the most important factors if we want to assess near-term radio propagation conditions. When the Bz value turns negative, it means charged particles from the Sun will enter into the Earth's atmosphere, and will develop a geomagnetic storm.

You will find that as soon as Bz (green) turns negative, the Planetary A or Ap index (red) will increase. The Ap index is typically calculated from mid-latitude observations. Note that the effect of Bz is most prominent in the polar regions. The Polar A index (blue) is calculated from high-latitude observations.

Legend:

1. Running 30-day Bz and Planetary/Polar A index

This graph shows a running one-month development of the Bz as well Planetary and Polar A indices. The A index is calculated from K indices. The data is updated every 4 hours.

2. Running 7-day Bz and Planetary/Polar K index

This graph shows a running one-week development of the Bz as well Planetary and Polar K indices. The K index is calculated every 3 hours. The data is updated every hour.