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| Titel |
Changes in the response of the AL Index with solar cycle and epoch within a corotating interaction region |
| VerfasserIn |
R. L. McPherron, L. Kepko, T. I. Pulkkinen, T. S. Hsu, J. W. Weygand, L. F. Bargatze |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
0992-7689
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| Digitales Dokument |
URL |
| Erschienen |
In: Annales Geophysicae ; 27, no. 8 ; Nr. 27, no. 8 (2009-08-14), S.3165-3178 |
| Datensatznummer |
250016622
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| Publikation (Nr.) |
 copernicus.org/angeo-27-3165-2009.pdf |
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| Zusammenfassung |
| We use observations in the solar wind and on the ground to study the
interaction of the solar wind and interplanetary magnetic field with Earth's
magnetosphere. We find that the type of response depends on the state of the
solar wind. Coupling functions change as the properties of the solar wind
change. We examine this behavior quantitatively with time dependent linear
prediction filters. These filters are determined from ensemble arrays of
representative events organized by some characteristic time in the event
time series. In our study we have chosen the stream interface at the center
of a corotating interaction region as the reference time. To carry out our
analysis we have identified 394 stream interfaces in the years 1995–2007.
For each interface we have selected ten-day intervals centered on the
interface and placed data for the interval in rows of an ensemble array. In
this study we use Es the rectified dawn-dusk electric field in gsm
coordinates as input and the AL index as output. A selection window of width
one day is stepped across the ensemble and for each of the nine available
windows all events in a given year (~30) are used to calculate a
system impulse response function. A change in the properties of the system
as a consequence of changes in the solar wind relative to the reference time
will appear as a change in the shape and/or the area of the response
function. The analysis shows that typically only 45% of the AL variance
is predictable in this manner when filters are constructed from a full year
of data. We find that the weakest coupling occurs around the stream
interface and the strongest well away from the interface. The interface is
the time of peak dynamic pressure and strength of the electric field. We
also find that coupling appears to be stronger during recurrent high-speed
streams in the declining phase of the solar cycle than it is around solar
maximum. These results are consistent with the previous report that both
strong driving (Es) and high dynamic pressure (Pdyn) reduce the coupling
efficiency. Although the changes appear to be statistically significant
their physical cause cannot be uniquely identified because various
properties of the solar wind vary systematically through a corotating
interaction region. It is also possible that the quality of the propagated
solar wind data depends on the state of the solar wind. Finally it is likely
that the quality of the AL index during the last solar cycle may affect the
results. Despite these limitations our results indicate that the Es-AL
coupling function is 50% stronger outside a corotating interaction region
than inside. |
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