Physics
Scientific paper
May 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agusmsm31d..02e&link_type=abstract
American Geophysical Union, Spring Meeting 2008, abstract #SM31D-02
Physics
2752 Mhd Waves And Instabilities (2149, 6050, 7836), 2788 Magnetic Storms And Substorms (7954)
Scientific paper
We present the results of a study of ULF waves in the Pc 1-2 frequency range (0.2-5 Hz) recorded by the three spacecraft of NASA's ST 5 mission, which operated in a dawn-dusk, 300 x 4500 km sun-synchronous orbit in a "pearls-on-a-string" configuration, with spacing ranging from >5000 km down to under 50 km, from March 26 through June 23, 2006. We find that regions with Pc 1-2 wave activity are not only highly localized to rather narrow L shells, as has been known for some time, but they also can appear and disappear on the time scales of ~10 s to 10 min examined by ST5. Only half of the 48 identified events were observed by all three spacecraft as they passed over similar L shells, and five events were observed by only one spacecraft. We interpret the lack of more multi-spacecraft observations as indicating that the regions of the magnetosphere that become unstable to electromagnetic ion cyclotron instabilities are often short-lived during moderate to quiet geomagnetic conditions. Wave occurrence was maximum in the daytime sector, consistent with stimulation by magnetospheric compressions. Only seven events were observed below L = 4, and only one below L = 3.6, consistent with the relatively quiet geomagnetic conditions during this interval. The temporal occurrence distribution of Pc 1-2 events was similar to that recorded at Halley, Antarctica (L = 4.56) during this same interval, in that the number and intensity of events increased during magnetospheric compressions and during the recovery phase of magnetic storms, but was reduced or absent during the main phase and early recovery phase of magnetic storms. This agreement suggests that if Pc 1-2 events occur during main phase, their nearly universal absence in ground records cannot be ascribed to ionospheric screening effects or obscuration by irregular ULF noise generated in the ionosphere. These findings add support to recent suggestions that although EMIC waves might theoretically cause rapid depletion of radiation belt electrons during the main phase of storms, such waves cannot be assumed to occur during the main phase of all storms.
Engebretson Mark J.
Le Guan
Lessard Marc R.
Otto N. J.
Posch Jennifer L.
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