How To Read The WSA-Enlil Model Output

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When there is a solar storm on its way towards Earth, or when there is a geomagnetic storm watch posted (as in the watch currently in effect), we often post an image of the WSA-Enlil model output. WSA in the name is an acronym of the three researchers who contributed to the model, Drs. Wang, Sheeley and Arge. Enlil in the name comes from the Sumerian God of wind and storms.

The model output can be confusing or disorienting at first. The following is a walk through of some features which should help in understanding how to read the output. We’ll use a snapshot of the today’s output (click for a larger view)

WSA-Enlil Model Output Jan 24, 2015
Labeled WSA-Enlil Model Output Jan 24, 2015

The two main features on this image are higher than average proton density expected to arrive this evening and the large area of high speed solar wind expected to arrive Wednesday from the southern coronal hole. We’ve labeled the image, here is a quick guide:

Label “A”: This is Earth! The left two radial graphs show the Solar System from the north think of is as “from above”, The Sun is in the center. The red and blue circles on the other side of the Sun are the two satellites (Stereo ahead and Stereo behind. At the time on the snapshot, plasma density at Earth is elevated but decreasing.

Label “B”: The two sets of graphs on the right show the data over time, the yellow bar shows the point in time the radial graphs are displaying. The plasma density, or the amount of material moving through space, is shown in the upper graph. The bottom shows the speed that material is moving as it arrives at the measurement location. Label B shows Stereo A (ahead), was seeing increased plasma density during the first part of Jan 23. The space weather at this location has no impact on geomagnetic conditions at Earth. We sometimes see people excited that the levels are increasing at this remote satelite wondering if there will be aurora – probably not, sorry.

Label “C”: This small increase in density shown is responsible for this evening’s predicted geomagnetic storm. The plasma density is elevated, you can see looking at the date along the bottom of the graph that this is likely to peak towards the end of the 24 hour period.

Label “D”: The two meridional slices (the image between the circle and the graph) show a side view of the Earth and the Sun. Above the earth is what is happening in space above our North Pole, below is the South Pole. In this image, we’ve freeze-framed the video at 19:00 UTC on 1/28, next Wednesday. There is a large area of high speed wind (orange and red) south of Earth that is coming from the Large Southern Hemisphere Coronal hole shown below in AIA 211 as the dark area.

Southern Coronal Hole on 1/24/2015 in AIA 211
Southern Coronal Hole on 1/24/2015 in AIA 211

Finally, Label “E”: This graph shows the expected solar wind speed at the same three measurement locations. This shows an increase to 500 km/s. This increase is related to the high speed wind from the coronal hole reaching a little father north. If this increase in solar wind is accompanied by an increase in plasma density because of an Earth-directed eruption on the Sun’s surface, we may see an additional geomagnetic watch and again have the possibility of seeing Aurora.

The NOAA Space Weather Prediction Center makes the WSA-Enlil model output available on their site.

Happy Hunting