Solar Wind Speed
How to Read Solar Wind Data
The top chart displays the duration that certain thresholds have been met at the ACE satellite. Generally, the more bars, and the taller the bars are, the more likely it is that there will be aurora at Earth. The bottom four charts show recent solar wind as measured by the ACE satellite.
Solar Wind Components
Overview – The four measurements on this page, solar Wind Speed, Solar Wind Density, Bt, and Bz) represent the core components of near-Earth space weather. They provide the basis for estimating the short term predicted planetary K value as it is displayed in the “current KP Values” and “Current KP Real-time Chart”. These values are collected once a minute on the ACE (Advanced Composition Explorer) satellite. Any one of these factors being very strong can disturb Earth’s geomagnetic field and produce Aurora, but typically when KP is elevated and the lights are dancing, it is due to a combination of these measures being enhanced.
Wind Speed (Blue)
There is a constant stream of particles flowing from the Sun outwards into the solar system. Solar wind speed is the rate that these particles are moving as they pass the measurement station. Typical, ambient, solar wind speed is around 300 kilometers per second. During a strong high speed wind stream generated by a coronal hole wind speeds can increase to between 500 and 750 km/s. Strong CMEs will also register high wind speeds, and can sometimes be in excess of 1000 km/s. High solar wind speed on it’s own typically won’t indicate that there should be aurora, but high solar wind speed can magnify the impact of the other three factors on this chart.
Wind speed also impacts the lag between when charged particles and interplanetary shocks hit the satellite and when they arrive at Earth. Faster wind speeds carry those particles and charge faster. The lag determines how far in advance the wingKP model can predict KP values. Here’s a handy chart to help determine the lag time based on the current wind speed:
Proton Density (Green)
Density is a measure of the number of particles that are being carried in the solar wind stream. As the density increases, the force displacing the magnetosphere increases. The combination of high density and strong solar winds together can be enough to create an aurora display. The units on the graph are parts per cubic centimeter, p/cm3, and anything above 30 is very dense. 30 protons per cubic centimeters may not sound like much, but remember this is space, and there are a lot of cubic centimeters!
Bt is a measure of the strength of the magnetic field at the ACE satellite. Bt reacts to the interaction between the magnetic field of earth and the magnetic field generated by the particles being carried in the solar wind. Imagine a large cloud of particles, produced by a CME on the Sun, that carry a magnetic charge passing Earth, they will exert a magnetic force on Earth as they pass. Bt measures the strength of as it passes.
Bz is a measure of the direction of the magnetic field at ace. The field is three dimensional, so there are three components in the magnetic force, Bx, By, and Bz. Bz is the strength of the field from the sun out through the Earth, By is the strength along the parallel cutting from East to West across the equator, and Bz is the parallel from the South to North Pole. Of the three, Bz has the most impact on aurora. When Bz is north (positive on the chart) the magnetic arriving from space line up with Earth and there is little interaction. When Bz is south (negative on the chart) it is opposite Earth’s poles and there can be significant interaction – producing aurora! Thus, a negative Bz is almost required for aurora. All the other factors can be favorable, but if Bz is positive, pack up your camera and go home. The longer the Bz has been negative, and the strong it is negative the stronger the aurora will be. However, if the strength of the magnetic field is low, and the Bz is only registering -1 or -2 nT, aurora will be unlikely.