# Explanation of 28-Day Forecast

Bartels "musical diagrams"
The graphs on this page are one way of representing data from the last couple of months about the solar wind's interaction with Earth's geomagnetic field. This system was developed by a German scientist named Julius Bartels. The musical diagram shows the quantitative values of Kp numbers in a way that is quick and easy to grasp.

What do Kp numbers mean?
The Kp number is a system of measuring aurora strength. It goes from 0 to 9 (0 being calm, 1 very weak, all the way up to 9, which would represent a major geomagnetic storm with strong auroras visible). Anything Kp5 and above is classified as a geomagnetic storm.

The Kp-index was introduced by Bartels in 1939. The abbreviation Kp comes from the German "Kennziffer Planetarische," which translates loosely to “planetary index number,” although it is better known in English as simply the planetary index.

What do the different colored vertical bars mean?
Each vertical bar represents three hours. The height and the color of the bar reflects the intensity of the Kp. Yellow indicates an average Kp number during that time interval of 1 to 2, green represents 3 or 4, and red indicates a Kp number of 5 or more. If the Kp number was zero, there is no vertical bar. There is currently a minor glitch in the code which creates the 28-day forecast page which causes the graph to present a couple days ahead of today as if it is actual data. We are working to remedy this problem!

What is a Carrington Rotation?
Each staff of "music" represents one rotation of the sun, which takes about 27 days, with the top staff representing the upcoming rotation, the one below it the current rotation, and the third one down represents the rotation just passed.

This is called a Carrington rotation after Richard C. Carrington, who in the 1850s determined the rate at which the sun rotates by watching sunspots. He defined a fixed solar coordinate system to determine that the sun rotates exactly once every 25.38 days relative to the stars. The Earth orbits the Sun, so looking from the Earth the rotation appears to be about two days longer. Additionally, the rotation rate, as viewed from Earth, varies a little during the year because of the eccentricity of the Earth's orbit; the average value is about 27.3 days.

Carrington coordinates are heliographic, i.e. sun-centered, and measure latitude and longitude. Because it is a gas, the sun does not rotate like a solid. It actually spins faster at its equator than at its poles. The sun rotates once every 24 days at its equator, but only once every 35 near its poles.

Carrington Rotation 1 began at a seemingly arbitrary instant late on Nov. 9, 1853, when Carrington began his Greenwich photo-heliographic series. Rotations are counted from that time with the central meridian longitude decreasing from 360 to 0 during each rotation as the central meridian point rotates under the Earth. Today the zero meridian scientists use is the one that passed through the ascending node of the solar equator on the ecliptic at Greenwich mean noon on Jan. 1, 1854. Interestingly, Carrington's own zero meridian was defined to have passed the ascending node 12 hours earlier, at midnight.

Where does this data come from?
Magnetometers can give an indication of auroral activity before the light from the aurora becomes visible. An array of magnetometers around the world constantly measures the effect of the solar wind on the Earth's magnetic field.

The official planetary Kp index is derived by calculating a weighted average of K indices from the network of geomagnetic observatories. Since these observatories do not report their data in real-time, various operations centers around the globe estimate the index based on data available from their local network of observatories. The Geophysical Institute’s aurora forecast webpages use geomagnetic data compiled by the Geomagnetism Group of NOAA's National Centers for Environmental Information.