September 12, 2014 Magnetic Disturbance - Peak Dst 0 nT Completed

On 2014 September 9 and on September 10, the National Oceanic and Atmospheric Administration, Space Weather Prediction Center (NOAA SWPC) reported the occurrence of two solar flares at a disk-centered sunspot active region on the Sun. At about the same time, a coronal mass (CME), a concentration of electrically-conducting solar-wind plasma, was ejected from the active region towards the Earth. This was followed on September 10 by an even more intense flare and energetic CME. In response to these observations, NOAA issued a warning for a magnetic storm commencing on September 12 and continuing into September 13. The first CME arrived at the Earth on 11 September at 23:47 UT (19:47 Eastern). In characteristic fashion, this was seen as a positive geomagnetic perturbation, or sudden commencement, as registered by low-latitude magnetometers operated by the Geomagnetism Program of the US Geological Survey (USGS); see the first “SC” for the San Juan magnetic observatory in the attached Figure (c). A geomagnetic sudden commencement is the result of compression of the Earth’s magnetosphere by the pressure of the solar wind. NOAA reported that the arrival of the first CME corresponded to solar wind velocity jumping from 400 km/s to about 480 km/s. The second CME arrived on 12 September at 15:57 UT (11:57 Eastern), and solar wind velocity jumped from 450 km/s to about 620 km/s. What followed over the next 24 hours was the normal main-phase of a magnetic storm: a west-ward-directed, equatorial magnetospheric electric current was activated, giving a characteristic depression in low-latitude magnetic disturbance, Figure (c). NOAA reported a Kp= 7 magnetic index, a generic measure of mid-latitude activity for a single 3-hr period, late on September 12, corresponding to a “strong” magnetic storm. It is noteworthy that at high-latitudes, Figure (a), magnetic disturbance was, as normal for a magnetic storm, much greater in amplitude. This storm also produced beautiful displays of aurora at mid-latitudes, as reported by various media outlets, including National Public Radio (link) and Sky and Telescope (link). The Sun is presently near its 11-year solar-cycle sunspot maximum, and while this particular cycle is relatively weak compared to the past few, additional storms of intensity comparable or larger than this one can be expected over the next couple years.

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Prepared on September 15, 2014
Last Updated 2014-11-24 17:27:12 by Jeffrey J. Love and E. Joshua Rigler, USGS Geomagnetism Program

For More Information:

• Northern Lights Put On Show, Thanks To Large Solar Flare - NPR

• Potent Solar Flare — Auroras Tonight? - Sky & Telescope