September 26, 2011 Magnetic Disturbance - Peak Dst -93 nT

Science Center Objects

Space Weather Events of September 26, 2011

On 24 September a coronal mass ejection (CME), a concentrated blast of electrically-conducting solar-wind plasma and tangled magnetic-field lines, left an active region on the Sun defined by a disk-centered sunspot. The CME arrived at Earth on 26 September at 12:37 UT (08:37 Eastern). In characteristic fashion, this was seen as a positive geomagnetic disturbance, or sudden commencement, as registered by low-latitude USGS magnetometers and as recorded by the USGS storm-time disturbance index Dst. This is the signature of compression of the Earth’s magnetosphere by the pressure of the solar wind. The sudden commencement reached Dst = 37 nT, placing it in approximately in the 75th percentile (75% of sudden commencements are larger). NOAA reports that the solar wind of the CME reached 650 km/s, from a background of 350 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. This storm showed two depressions, one at about 26 September 18:30 UT and one at about 27 September 00:00 UT, with an over-all maximum depth of Dst = -93 nT (70th percentile). It is noteworthy that at high-latitudes, where magnetic disturbance can be much greater in amplitude and intensity, the direction of the magnetic field at Barrow, the most northerly USGS observatory, changed by about 6.5 degrees over a period of 3 hours. Somewhat smaller variation was observed at the Deadhorse observatory near Prudhoe, AK. Such variations, large enough to be seen on an ordinary compass, would have been a severe nuisance for directional-drilling operations on the North Slope. This storm also produced beautiful displays of aurora at high latitudes. As we leave the quiet period of the last solar minimum and head into the next solar maximum, over the next 3 or 4 years we can expect a significant increase in the occurrence frequency and size of magnetic storms.

For perspective: The largest storm of the 20th century occurred on March, 1989 Dst = -574 nT. This storm induced electric currents in the Earth’s crust that found their way through ground connections into the high-voltage Canadian Hydro-Québec power grid, causing transformer failure and resulting in the loss of electric power to more than 6 million people. The same storm also damaged and disrupted the operation of satellites and it severely disrupted GPS systems and over-the-horizon radio communication systems used by the US military. The storm of March 1989, large though it was, pales in comparison to that of September 1859, the largest storm in recorded history occurred, Dst = -1600 nT (approximately). If such a storm were to occur today, the economic impact to the United States because of disrupted technological systems could exceed $1 trillion (Baker et al., 2008).

Prepared on September 27, 2011
Last Updated 2014-12-15 22:58:17 by Jeffrey J. Love & Jennifer L. Gannon