The solar wind density is a measurement of the amount of particles in the solar wind measured in particles per cubic centimeter (p/cm3). The higher this number, the better our odds are of seeing the aurora. Low density values indicate very little particles available to interact with Earth’s magnetic field. Think of density as sticks, logs, or leaves (fuel) for a campfire. The more fuel you throw into the fire, the more there is available for the fire to burn, and the bigger, brighter, and hotter the campfire becomes. The density of the solar wind is an indicator of this “fuel” for the magnetic field to “burn” and, thus, provide for aurora displays.
For density, anything greater than or equal to 8 p/cm3 is good. Values less than this, and especially less than 4 p/cm3, mean our chances for seeing aurora activity are not that good. Ideally, we would like to see values above 10 p/cm3 and values above 20 p/cm3 are indicative of stronger geomagnetic storming. It is important to note, however, that density is just one component of chasing the aurora which must be taken into consideration along with everything else. A high density, absent one or more other favorable parameters, can mean the difference between seeing a brilliant display and seeing little to no aurora activity.
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The free version of the app gives you approximately two hours and 30 minutes of solar wind density data: the previous 90 minutes plus the next hour. The subscription version lets you go back and look at solar wind density data for the past 36 hours in 6-hour increments.