- Title
- Ground observations of high-latitude Pc3-4 ULF waves
- Creator
- Howard, T. A.; Menk, F. W.
- Relation
- Journal of Geophysical Research-Space Physics Vol. 110, Issue. A4, A04205
- Publisher Link
- http://dx.doi.org/10.1029/2004JA010417
- Publisher
- American Geophysical Union
- Resource Type
- journal article
- Date
- 2005
- Description
- A detailed study has been undertaken of Pc3-4 waves recorded on the ground with the IMAGE magnetometer array (56 degrees < Lambda < 76 degrees) during January and March 1998. We focus only on daytime events exhibiting high coherence (>0.6) across the entire station array. Most of these had well-defined wave packet appearance in time series records and a clear peak in power spectra. Their occurrence and frequency suggest the waves are generated by the upstream ion-cyclotron resonance mechanism, with no evidence of generation by the Kelvin-Helmholtz instability. For each event the amplitude, phase, coherence, ellipticity, azimuth angle, and degree of polarization across the ground array were examined. The coherence length, azimuthal wave number, and hence the apparent wave propagation velocity were thus determined, with emphasis on the precision and significance of these measurements. It was found that these daytime Pc3-4 pulsations usually have maximum amplitude near the magnetopause projection, meridional coherence lengths of order 1.5-2.0 x 10(3) km, and low azimuthal wave numbers during morning hours, averaging around -4.0 (indicating westward propagation). Over 80% of events propagated poleward and westward, with average equivalent ground velocity of 41 km/s N43 degrees W for the H component. About 24-30% of the events are higher harmonics of field line resonances. There is no evidence that the remaining events arise from cavity modes or localized modulated electron precipitation. The observations instead suggest a mechanism involving mode coupling and field-guided propagation. In this model, fast mode waves in the Pc3-4 range entering near the subsolar point propagate earthward and due to the inhomogeneity of the magnetosphere couple to the field-guided Alfven mode. At certain latitudes, standing oscillations are established at harmonics of the local resonant frequency, while at other latitudes traveling waves convey energy to low altitudes. The expected L dependence of wave power and travel time agree well with observed amplitude and phase profiles.
- Subject
- field line resonances; interplanetary magnetic-field; pc 3-4; pulsations; solar-wind; geomagnetic micropulsations; cusp latitudes; guide modes; dependent characteristics; sudden commencements; cavity; modes
- Identifier
- http://hdl.handle.net/1959.13/25050
- Identifier
- uon:262
- Identifier
- ISSN:0196-6928
- Language
- eng
- Full Text
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