Katia J. Pinheiro

Measurements and uncertainties of the occurrence time of the 1969, 1978, 1991, and 1999 geomagnetic jerks

Geomagnetic jerks are rapid time variations of the magnetic field at the Earths surface that are thought to be of primarily internal origin. Jerks are relevant for studies of the Earth interior: they likely give information on core dynamics and possibly on mantle electrical conductivity. In such studies a precise determination of the jerk occurrence time and its error bar at each observatory is required. We analyze the most well-known global jerks (1969, 1978, and 1991) and a possible local jerk in 1999, considering all three components of the magnetic field (X, Y, and Z). Different data sets are investigated: annual means, 12 month running averages of observatory monthly means in rotated geomagnetic dipole coordinates, and data representing the core field contribution synthesized from the CM4 time-dependent field model. The secular variation in each component of the field around the time of a jerk was modeled by two straight line segments, using both least squares and 1-norm methods. The 1969, 1978, and 1991 jerks were globally detected, while the 1999 event was only locally identified. Using this simple method enables us to calculate error bars in the jerk occurrence times and to quantify their nonsimultaneous behavior. We find that our error bars are not, in general, symmetric about the mean occurrence time and that the mean errors on the X and Z components of 1.7 years and 1.5 years are larger than that of 1.1 years on the Y component. Generally, the error bars were found to be larger in the Southern Hemisphere observatories. Our results are necessary prerequisites for further studies of the inverse problem that attempt to determine mantle electrical conductivity from variations in jerk occurrence times.

Deep magnetic field stretching in numerical dynamos

AbstractThe process of magnetic field stretching transfers kinetic energy to magnetic energy and thereby maintains dynamos against ohmic dissipation. Stretching at depth may play an important role in shaping the field morphology and in the dynamo action. Here, we analyze snapshots from self-consistent 3D numerical dynamos to unravel the nature of field-flow interactions that induces stretching secular variation of the radial magnetic field at mid-depth of the shell. We search for roots of intense flux patches identified at the outer boundary. The deep radial field structures exhibit a position shift with respect to the locations of the outer boundary patches, consistent with a mixed effect of tangent cylinder rim and plume-like dynamics. A global stretching/advection rms ratio is ∼ 1.5–3 times larger than that of poloidal/toroidal flows. In addition, local stretching is often more effective than advection, in particular at regions of significant field-aligned flow. On average at roots of high-latitude flux patches, total stretching is 1.1 times larger than total advection despite the poloidal flow being only 0.37 of the toroidal flow. Radial stretching secular variation acts as an effective dynamo mechanism at regions where laterally varying radial flow shears toroidal field lines to generate a poloidal magnetic field. Stretching at depth exhibits similar parameter dependence as that of stretching at the outer boundary, with the strongest dependence being on the magnetic Prandtl number in both cases. Our results provide insights into the underlying deep dynamo mechanisms that sustain intense magnetic flux patches at the outer boundary.