HR: 13:40h
AN: GP23B-01    [Abstracts]
TI: Short-Period (3-5 Year) Signals in Magnetic Observatory Data: Internal or External
AU: * Soukhovitskaya, V
EM: vsoukhov@fas.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138
AU: Bloxham, J
EM: bloxham@eps.harvard.edu
AF: Department of Earth and Planetary Sciences, Harvard University, 20 Oxford St., Cambridge, MA 02138
AB: First differences of annual means at certain magnetic observatories (primarily in northern Europe) reveal the presence of a quasi-periodic signal (with period around 3-5 years) in the east component. These observatories share the property that the east component of the field itself is very small, so that the east component is almost orthogonal to the main field. At first sight, it would not be unreasonable to suggest that this signal is simply an harmonic of the Solar cycle: the period of the signal is correct, and its phase remains fixed over several decades. However, closer examination reveals that this explanation may not be correct. We investigate how Solar cycle modulation of much shorter-period external field variations (which are non-zero mean) may result in Solar cycle harmonics in observatory data by building a simple model of the relevant processes. This model suggests that the 3-5 year signal is not due to the Solar cycle. This work also highlights the perils of using indices to discriminate between external and internal fields on periods of several years. We investigate this further by rotating observatory data (the three orthogonal components X,Y,Z) to a new coordinate system in which Y' is zero (on average over some time interval). The new Y' component is then less affected by external fields. This provides a means to test our model. We conclude that this signal is of internal origin and suggest that it may arise from inner core oscillations.
DE: 1507 Core processes (1213, 8115)
DE: 1515 Geomagnetic induction
DE: 1545 Spatial variations: all harmonics and anomalies
DE: 2409 Current systems (2721)
DE: 2700 MAGNETOSPHERIC PHYSICS (6939)
SC: Geomagnetism and Paleomagnetism [GP]
MN: Fall Meeting 2005