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ACE News #158 - February 6, 2013 |
Since 2008, we have engaged a real-time automated routine to discover and
alert the community to shocks in the solar wind passing by the ACE spacecraft.
We use the ACE 1-minute resolution real-time data stream tracked by the NOAA
network of receiving stations as input parameters to solve the
Rankine-Hugoniot (R-H) equations. Using solar wind, magnetic field, and
energetic particle data, our code is able to to reconstruct the compression
ratio, speed, and normal direction of each shock without the need for human
interaction. This is done within less than 10 minutes of the passage of the
shock, allowing up to one hour of warning before interaction at Earth.
The R-H equations used are based on the model of a planar shock. Shocks driven
by Interplanetary Coronal Mass Ejections are assumed to travel radially in our
model, so the wind speed measurement provided by the real time data stream is
taken to be radial. When examining the behavior of past events, we noticed
that in many instances the peak speed of the ejecta driving the shock was
greater than the speed of the shock front itself. It would appear at first
glance as if the ejecta were in the process of overtaking the shock. However,
this is resolved by examining the location of the measurement relative to the
ejecta. These spacecraft crossings occurred away from the nose, so that the
shock front only has to move fast enough to escape the flank of the ejecta
rather than precede the bulk of the material.
We analyzed 39 events which displayed this characteristic, with peak ejecta
speeds attained anywhere from 2 to 12 hours after the passage of the shock,
depending upon the standoff distance. The figure at right shows the original
measurements of the peak ejecta speed versus the speed of the shock as
determined by the R-H solutions by the red circles. This peak was chosen as
having a significant duration within 15 hours of the calculated shock. The
slowest shocks are observed as corresponding to much greater differences from
their ejecta speeds than the faster events. The black triangles depict the
peak velocity scaled by the radial component of the shock normal as determined
by the R-H solution. The dashed line represents unity. Our computed normal
directions bring the peak ejecta speed into agreement with the calculated
shock speed within a reasonable degree of error.
This process is reversible, and has been incorporated into the current version
of the automated routine, allowing us to predict the peak speed of the ejecta
driving an encountered shock. While our previous iteration of the automatic
routine provided us with one hour of warning, we now realize the potential of
weaker shocks to serve as harbingers of fast-moving ejecta even farther
downstream. Unfortunately, those events with the largest discrepancy in
propagation speed are necessarily preceded by weaker flank-side shocks. We are
therefore encountered with new challenges of resolving weak shocks portending
significant ejecta from the anomalous fluctuations in solar wind activity.
For additional information, see Paulson et al., Space Weather 10, S12002,
doi:10.1029/2012SW000855 (2012).
This item was contributed by
Kristoff Paulson, David Taylor, Charles Smith, Bernard Vasquez (UNH), and Q. Hu (UAH).
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Last modified 06 Feb 2013.