Core-flow constraints on extreme archeomagnetic intensity changes

Geomagnetic intensity variations for the past 8 kyr: New archaeointensity results from Eastern China

“…The rate of change of geomagnetic intensity during this period is ∼6 μT/century or 47% per century (calculated from the lowest value at ∼2250 BCE and highest value at ∼1300 BCE). This variation is consistent with the upper bounds of ∼0.62 μT/yr suggested by Livermore et al (2014) but not as fast as the one reported by Gómez-Paccard et al (2012) with a rate that varies from ∼12 to ∼80 μT/century between 600-1300 CE in western Europe and much lower than the one reported by Shaar et al (2011) in which the intensity changes ∼30 μT in 30 yrs (910-890 BCE) in Levantine area. But a six-fold change of the field (2 × 10 22 -13 × 10 22 Am 2 ) in less than 1000 yrs is still abrupt.…”

The impact of geomagnetic spikes on the production rates of cosmogenic 14C and 10Be in the Earth's atmosphere

“…The reference B 0 (t) is the mean of the ensemble of 1000 A_FM archeomagnetic field models published by Licht et al [2013]. In order to construct B sp i (t), we adopt the optimized core flow methodology of Livermore et al [2014] and operate at Earth's core surface: for a given geomagnetic field configuration, and a given amount of available kinetic energy (specified in terms of an imposed root-mean-square velocity u rms ), this approach provides the optimal core flow u opt that generates the fastest instantaneous rate of change of geomagnetic intensity, dF∕dt, at a given site on Earth's surface (from now on the Timna-30 archeological site, with longitude = 34.95 ∘ E and latitude = 29.77 ∘ N). In order to account for our uncertain knowledge of the geomagnetic field at the core surface and to build reliable statistics, we consider, at each T i , 1000 different realizations of that field.…”

“…In order to account for our uncertain knowledge of the geomagnetic field at the core surface and to build reliable statistics, we consider, at each T i , 1000 different realizations of that field. For each realization, Gauss coefficients from spherical harmonic degree = 1 to the truncation L B =135 [Livermore et al, 2014] are specified as follows: coefficients from = 1 to = 5 are those of one member of the A_FM ensemble at epoch T i (a different member for each realization). Degrees 6 to L B are next populated following the stochastic method presented by Livermore et al [2014].…”

“…Their conclusion was that, if true, the reported occurrences of extreme intensity changes in the Near East records required a magnitude of core-surface flow 6-8 times larger than the commonly accepted value. However, Livermore et al [2014] also pointed out that an explanation for spikes may lie beyond our current perception of core dynamics and the geodynamo, and highlighted the need for further corroborative evidence of such spikes. The goal of this letter is precisely to estimate whether it is possible to detect the impact of such spikes in other records spanning the past few millennia, namely, those of the production rates of the cosmogenic radionuclides 14 C and 10 Be (see, e.g., the review by Beer et al [2012]).…”

“…Analysis of the data suggests time rates of change of geomagnetic intensity as large as several μT/yr sustained over a few decades, to contrast with the present-day maximum rate of ∼ 0.1 μT/yr. Livermore et al [2014] assessed the geophysical likelihood of these extreme events, by estimating core-surface flows able to sustain such dramatic changes. Their conclusion was that, if true, the reported occurrences of extreme intensity changes in the Near East records required a magnitude of core-surface flow 6-8 times larger than the commonly accepted value.…”