Tag Archives: STEREO-A

Low Confidence in August 16, 2016 G1 Geomagnetic Watch

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Northern Lights Now – SWPC has posted a G1 geomagnetic storm watch for August 16, 2016. This means there’s the potential for aurora as Kp values could reach 5+. However, this watch comes with a caveat that it is a low confidence forecast. The forecast discussion says:

Observations from STEREO-A
revealed a solar wind speed approaching 700 km/s and Bz decreasing to -10 to -15 nT with onset, although STEREOs position differs from Earths position by about 13 degrees relative to the ecliptic.

coronal hole high speed stream is expected to become geoeffective late on 15 Aug to early on 16 Aug, although there is a chance it may pass south of the ecliptic without interacting with Earth, as WSA-Enlil suggests. At this time, confidence in the model solution is not high enough to exclude the possibility of geoeffectiveness, so the geospace forecast reflects the high speed stream influence.

So what he heck does that mean? Let’s break it down.

First the good potential news: The coronal hole is on the surface of the Sun, and as the Sun rotates, the coronal hole co-rotates. In the diagram below, this means the coronal hole high speed wind will impact the planets and satellites in a counter clockwise direction. First Earth, then B, then A then 27 days later Earth again. From the Earth’s perspective it takes 27 days for the Sun to make a complete rotation. In the diagram A and B are satellites that are designed to capture “backside” views and data from the Sun. They are called STEREO-ahead and STEREO-behind.

Current locations of STEREO Ahead and Behind
Current locations of STEREO Ahead and Behind

With these satellites, heliophysicists capture data that can be used to predict the impact of the coronal hole when it rotates towards Earth. On it’s pass by these satellites about 14 days ago, the wind stream had a strong Bz component and wind speeds of 700 km/s. Together those two factors should be enough to put on a good show.

Why the low confidence?

Take a look at the coronal hole responsible below. Notice that it is centered in the southern hemisphere of the Sun. This means it is very possible that the high speed wind will to pass to the South of Earth.

Southern Hemisphere Coronal hole imaged on August 12, 2016 by SDO
Southern Hemisphere Coronal hole imaged on August 12, 2016 by SDO

Further, the current position of the STEREO satellites puts them in a different plane than the Earth’s orbit by 13 degrees. This also means it is possible that they could be registering just the northernmost part of the CH HSS. Again, this indicates the high speed winds may go to our south.

The final comment in that discussion, “At this time, confidence in the model solution is not high enough to exclude the possibility of geoeffectiveness, so the geospace forecast reflects the high speed stream influence,” means the forecasters don’t have enough data to exclude the possibility that this may hit Earth. Keep an eye on the data! If Earth is in line for this high speed stream, and Bz stays strongly negative, aurora hunters could be in for a show. To reflect this, the NLN AuroraCast is showing the period of potential G1 storming right at the beginning of Aug 16:

G1 storming predicted in AuroraCast in the first period of August 16
G1 storming predicted in AuroraCast in the first period of August 16

Happy Hunting!