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day of the dalek writes:

NOAA has issued a rare G4 watch for a severe geomagnetic storm that is expected on October 10 and 11. G4 refers to NOAA's scale for the intensity of geomagnetic storms, which ranges from G1 to G5. Prior to the geomagnetic storm in May of this year that reached G5 intensity, the last G4 watch was issued in January of 2005.

This watch was issued for a coronal mass ejection (CME) that occurred around 03 UTC on October 9 and is expected to reach Earth around 12-15 UTC on October 10. An article from the Washington Post states that the speed of the CME, around 2.5 million miles per hour, is the fastest that a CME has been ejected toward Earth during the current solar cycle. From my very limited understanding of space weather, it seems that faster CMEs generate higher ram pressures against Earth's magnetosphere and can result in more severe geomagnetic storms. The x-ray brightness of solar flares gets a lot of attention, and this was an X-class flare (the highest level on the classification scale), this was an X1.8 flare whereas flares have been observed at least up to X28. The high speed of the CME, however, seems to be a factor in the potential for a severe geomagnetic storm.

One of the main questions that we can't answer until the CME gets to within about a million miles of Earth is the orientation of its magnetic field. If the CME's magnetic field is aligned in the same direction as Earth's, it will produce a less severe geomagnetic storm than if it's aligned in the opposite direction. An excellent resource for data about space weather and this CME is NOAA's space weather enthusiasts dashboard. There's a lot of data on that page that is useful if you're concerned about the possibility of viewing auroras or potential disruptions to the power grid, so hopefully some of the comments can explain a bit more about what it means. I don't know a whole lot about space weather, but I'll try to offer a cursory explanation of what I believe some of it means.

The solar visible light shows where sunspots are currently observed, whereas the LASCO C3 images are observed from satellites can be used to see CMEs when they occur. A CME will appear like an explosion outward from the sun's corona. The data is input into a model called WSA-ENLIL, which predicts the density and radial velocity (outward from the sun) of solar wind plasma. A higher plasma density or a faster radial velocity should result in a stronger geomagnetic storm. This is also useful for estimating when a CME will reach Earth. I believe the GOES magnetometer data is used to measure how much the Earth's magnetic field is compressed or stretched and can identify the onset of geomagnetic storms. The ACE MAG and SWEPAM data are satellite-derived measurements of the solar wind. In addition to showing the plasma speed, temperature, and density, the Bz and Phi variables show the orientation of the magnetic field in the solar wind. If Bz is positive, it's a northward-oriented magnetic field. However, a negative Bz indicates a southward-oriented magnetic field, the opposite direction of Earth's magnetic field, and this can result in more severe geomagnetic storms. Basically, a strongly negative Bz around -10 or even -20 would be more favorable for a strong geomagnetic storm. The aurora forecast is a short-term forecast (~30 minutes to an hour) of the probability of auroral activity over a location, though auroras may be visible near the horizon in areas equatorward of what the forecast shows.

There's a lot of data on NOAA's space weather dashboard that can be useful for anyone hoping to see the auroras. My understanding of space weather is very limited, so if anyone else has a better understanding of what the data means, please share the information in the comments. Although a G4 or even a G5 geomagnetic storm is possible, but there's still a lot of uncertainty until the CME gets very close to Earth.

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