March 17, 2015 Magnetic Disturbance - Peak Dst -195 nT

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Space Weather Events of March 17, 2015

A magnetic storm commenced on St. Patrick’s Day, Tuesday, March 17, 2015 at 04:45 UT with the arrival at Earth of a coronal mass ejection. The geomagnetic field responded at low latitudes with a relatively strong “sudden commencement” of nearly 50 nT, followed by a “main phase” and peak in –Dst of almost 200 nT. The NOAA Kp index, a measure of mid-latitude magnetic activity, reached a value of 8, qualifying as a severe magnetic storm. More details follow.

NOAA’s Advanced Composition Explorer satellite (ACE), located on the Sun-Earth line at a distance approximately 1% of the distance from the Earth to the Sun, recorded an abrupt increase in solar-wind velocity from an ambient value near 400 km/s to about 500 km/s. Concomitant solar-wind ram pressure abruptly compressed the Earth’s magnetosphere. This generated eastward-flowing magnetopause electric currents and a positive perturbation in the horizontal component of the geomagnetic field recorded at low-latitude magnetic observatories around the world. In Figure 1 we show time series of the magnetic deviation from a quiet baseline in the northward (X), eastward (Y), and down (Z) directions, as well as the full-field (F), all obtained from the US Geological Survey magnetic observatory in Honolulu, HI. The relatively large (~50nT) sudden commencement is labeled “SC” in the figure.

After this, ACE data indicate that the solar wind’s interplanetary magnetic field (IMF) was southward directed, an orientation that allows connection of the IMF onto northward pointing geomagnetic field lines. Once this connection occurred, the magnetosphere became strongly energetically coupled with the solar wind, enhancing the westward-flowing equatorial ring current of the inner magnetosphere. This process is generally recorded at low-latitude magnetic observatories as a diminishment in northward geomagnetic field, the longitudinal average of which is the Dst index. This storm saw a maximum –Dst of 195 nT, realized on March 17 at 23:00 UT and, so, by this measure this storm was the largest of this unusually subdued solar cycle. By comparison, in the last solar cycle, several storms had maximum –Dst exceeding 300 nT, and one exceeded 400 nT. The mid-latitude magnetic activity index Kp reached a value of 8, which by NOAA space weather scales qualifies as a severe storm.

As was typical for an intense storm, geomagnetic activity was particularly intense at high latitudes. For example, at the Deadhorse observatory on the northshore of Alaska1, magnetic declination varied by about 9 degrees over the course of this storm. The storm could literally be measured using a simple compass! Aurora were seen across Alaska, Canada, and many states in the lower continental United States. The storm attracted media attention, including reports published by the Washington Post, USA Today, and Christian Science Monitor.

Last Updated 2015-06-25 15:13:16 by Jeffrey J. Love and E. Joshua Rigler

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1. The Deadhorse observatory is operated as a public-private partnership between the USGS and Schlumberger. Data from the observatory are used for directional drilling for oil and gas; the data are also freely available to the wider scientific community.