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ACE News #86 - Nov 15, 2004
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The biggest magnetic storm of 2004 [Dst = -383 nT according to 'near real-time' data from the World Data Center (WDC) for Geomagnetism at KyotoUniversity], and the fourth biggest of the current solar cycle, started on Nov 7, 2004 and was accurately predicted using real-time data from ACE. The Figure at right, downloaded from the University of Colorado forecast website, compares the real-time measured Dst and the forecast Dst from a recently updated prediction model [Temerin and Li, JGR, 2002] that uses ACE real-time solar-wind data (velocity, density, magnetic field; Figure at left) as the only input. Note the excellent agreement between the forecasts and measurements of these two storms, including both magnitude and timing.
A new NOAA real-time solar-wind magnetic field data-processing code has been in operation since August of 2004 (thanks to Kelvin Fedrick, Tom Detman, Ron Zwickl, Andrew Davis, and Chuck Smith), providing improved real-time measurements of the direction of the interplanetary magnetic field (IMF). The IMF direction and magnitude are the most important parameters controlling the magnitude of magnetic storms and the previous real-time algorithm for computing the IMF direction had developed problems and become less accurate, thereby reducing the accuracy of real-time predictions of magnetic storms. Note that both storms follow extended periods of negative (southward) Bz (early on Nov. 8 and 10 in the bottom panel on the left), which leads to reconnection between the IMF and the Earth s magnetic field and releases energy that powers the storm.
The Dst index is a measure of the average change in the magnetic field near the equator and a major indicator of the onset and the strength of magnetic storms. Currently the Dst index is calculated each hour by the WDC based on four ground stations near the equator and is available on the web in near real time, called real-time Dst. After a few months, the provisional Dst is produced, and about a year and a half later the final Dst becomes available.
Since the solar wind controls magnetospheric activity to a very large extent, solar wind measurements upstream of Earth near L1 can allow forecasts of geomagnetic activity about one hour in advance. Reliable forecasts of major magnetic storms are of great importance because of associated space weather effects such as power grid failures due to enhanced ground induced currents and spacecraft anomalies due to enhanced relativistic electrons associated with magnetic storms.
Comparisons of the near real-time Kyoto Dst with the final Kyoto Dst show that the near real-time Kyoto Dst is subject to substantial revision; it is found that the forecast Dst is often a more accurate representation of the `final' Dst than the `near real-time' Dst, provided the solar wind data are accurate and complete.
Contributed by Xinlin Li of University of Colorado at Boulder and Michael Temerin of University of California at Berkeley. Address comments or questions to firstname.lastname@example.org.
Last modified 15 Nov 2004, by