Localized Origin at the Core‐Mantle Boundary of the 1969 Geomagnetic Impulse. Issue 23 (29th November 2022)

• Record Type: Journal Article
• Authors: Blangsbøll, R. M.; Finlay, C. C.; Kloss, C.
• Is Part Of: Geophysical research letters. Volume 49:Issue 23(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-11-29
• Subjects: geomagnetism -- rapid field changes -- core dynamics -- satellite observations; Geophysics -- Periodicals; Planets -- Periodicals; Lunar geology -- Periodicals; 550
• DOI: 10.1029/2022GL101070
• Languages: English
• ISSNs: 0094-8276

Abstract

Abstract: The 1969 geomagnetic impulse provided the first compelling evidence for rapid changes in Earth's core‐generated magnetic field, taking place on timescales of a few years or less. We show here it originated at the core‐mantle boundary (CMB) largely as a localized change in the field acceleration under north and central America. We find the impulse events in 1969 and 2017 involved similar amplitudes of field acceleration change with similar localized dipole structures. However in 1969 the acceleration change pattern was north‐south rather than east‐west oriented, and it propagated poleward rather than westward. Moreover, there was a distinctive local surge in the CMB secular variation leading up to the 1969 event. We propose the 1969 impulse resulted from hydromagnetic waves arriving at the CMB that were triggered by a convective burst near to the core surface; this event involved localized flux expulsion and inward propagating waves. Plain Language Summary: The 1969 geomagnetic impulse, an abrupt change in the core‐generated geomagnetic field that occurred within just a few years, was crucial in convincing researchers that rapid dynamics are taking place in Earth's core. Here we reanalyze ground and satellite magnetic data collected around the time of this event, using modern field modeling and analysis techniques. We find the impulse originated at the core‐mantle boundary (CMB) primarily as a change in the pattern of the field acceleration under north and Central America. We report evidence for a related localized increase in the amplitude of the first time derivative of the field, the secular variation, under north and Central America leading up to 1969. We also find that the field acceleration pattern moved poleward during the event. We propose these features can be understood if the impulse was triggered by convection close to the CMB that pushed field lines outwards across the boundary and triggered waves in the magnetic field which traveled inward into the core as columnar disturbances. Key Points: The 1969 impulse originated under central and North America as a north‐south oriented localized dipole of field acceleration change It was proceeded by a localized surge in secular variation and field acceleration patterns moved poleward during the event We suggest it was caused by a burst of vigorous convection close to the core surface which triggered flux expulsion and hydromagnetic waves