What is solar wind power?

By Sean McCloat

In our previous posts, we describe how the density, speed and magnetic field strength and direction of the solar wind are measured, what Bz is, and what those mean for the aurora. We also introduced a handy parameter called the solar wind power that combines all these measurements.  Here, we provide more detail about the solar wind power that we use at Aurorasaurus.

Solar wind power is a value that is calculated based on measurements of physical properties of the solar wind (and CMEs when they occur).  The solar wind power increases in value as the speed of the solar wind and strength of its magnetic field increases.  The extent to which the magnetic field lines point in the Bz direction (see part 3) is also quantified in the calculations by describing the interplanetary magnetic field (IMF) clock angle.  The solar wind power increases as the IMF clock angle approaches the “6 o’clock position” (that is, when Bz is approaching south).

Solar wind power is measured in watts.  These watts are the same watts that are used to rate lightbulbs: the higher the wattage, the brighter the light bulb (when comparing two bulbs with the same efficiency).  Similarly for the aurora, the higher the wattage of the solar wind power, the brighter and larger the aurora will be.  However, the solar wind power is only proportional to aurora activity, not equivalent the way wattage is for light bulbs.  Aurorasaurus graphs the strength of the solar wind power here.  based off of real-time measurements taken by ACE.  Since ACE is located at L1, it takes about an hour for the solar wind it measures to actually impact Earth.

solar wind blog post

In this way, solar wind power is useful for anticipating aurora activity by looking at the average value over the last hour. Fluctuations minute-to-minute or huge spikes in the power will not greatly affect what latitudes the aurora will be seen.

Recall the windmill analogy from Part 2, where the aurora generated by the solar wind is like a windmill that is spun by regular wind.  A few strong gusts might get the blades to move a little bit, but if there is a strong sustained wind, then the blades can start and continue to spin.  Similarly, a few small spikes in the solar wind power may not be enough to stir up sustained aurora activity.  However, a sustained strong solar wind will be enough to stir up continuous aurora.  The solar wind power parameter was developed by eminent space physicist Dr. Akasofu (who is also credited with coining the phrase “sub storm”).  You can find more information about how solar wind power affects the aurora in Dr. Akasofu’s 1981 paper. As indicated by the yellow, orange and red labels on the solar wind power plot, the stronger the wind, the further south the aurora will be visible.  Note that the the color coded indicators approximate the furthest extent and intensity of the aurora.

To be concluded: Part 4 will wrap up this series of blog posts by comparing the solar wind power to another common parameter for characterizing geomagnetic activity – the Kp index.

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Sean McCloat interned with Aurorasaurus in the summer of 2015 while pursuing his masters degree in Space Studies at the University of North Dakota with a focus on the planetary sciences and astrobiology. He helped analyze the project’s data, contributed to scientific papers, presentations, and blog posts, and became good friends with Rory, the Aurorasaurus plush doll mascot.

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