An activity index for geomagnetic paleosecular variation, excursions, and reversals

Abstract

Magnetic indices provide quantitative measures of space weather phenomena that are widely used by researchers in geomagnetism. We introduce an index focused on the internally generated field that can be used to evaluate long term variations or climatology of modern and paleomagnetic secular variation, including geomagnetic excursions, polarity reversals, and changes in reversal rate. The paleosecular variation index, P i , represents instantaneous or average deviation from a geocentric axial dipole field using normalized ratios of virtual geomagnetic pole colatitude and virtual dipole moment. The activity level of the index, r Pi , provides a measure of field stability through the temporal standard deviation of P i . P i can be calculated on a global grid from geomagnetic field models to reveal large scale geographic variations in field structure. It can be determined for individual time series, or averaged at local, regional, and global scales to detect long term changes in geomagnetic activity, identify excursions, and transitional field behavior. For recent field models, P i ranges from less than 0.05 to 0.30. Conventional definitions for geomagnetic excursions are characterized by P i exceeding 0.5. Strong field intensities are associated with low P i unless they are accompanied by large deviations from axial dipole field directions. r Pi provides a measure of geomagnetic stability that is modulated by the level of PSV or frequency of excursional activity and reversal rate. We demonstrate uses of P i for paleomagnetic observations and field models and show how it could be used to assess whether numerical simulations of the geodynamo exhibit Earth-like properties.

One Hundred Thousand Years of Geomagnetic Field Evolution

“…Directional excursions are linked to clear intensity minima or decrease in paleointensities (Lund et al, ; Valet & Meynadier, ), corresponding to low values of VDM. The PSV index P i takes both field directions and intensity into account, and values exceeding 0.5 broadly agree with common definitions of excursions (Panovska & Constable, ). All these definitions are empirical, and several fundamental questions about excursions and their driving mechanism remain unanswered.…”

“…from estimates at 500 year intervals. In general, the indices are higher in the Southern Hemisphere than the Northern Hemisphere and in the Atlantic compared to Pacific Hemisphere, a characteristic already observed for present, historical (Panovska & Constable, 2017) and Holocene geomagnetic field models (Constable et al, 2016). In the regional VADM models produced by Ziegler and Constable (2015), they also found higher average ADM in the Pacific than in the Atlantic, associated with lower field variability over the past 300 ka.…”

“…The continuous variation of the paleomagnetic field is called PSV. Panovska and Constable () introduced the PSV index, P i , that can be used to measure PSV strength and variability in either a geographical or temporal context, or both. At a given location ( λ , ϕ ) and time t the nondimensional index depends on VGP latitude, λ V , and the VDM, and is scaled by the present‐day DM of DM 0 =80 ZAm 2 : …”

Global and Regional Assessments of Paleosecular Variation Activity Over the Past 100 ka

“…We used the PSV index (Panovska & Constable, 2017) to investigate the occurrence of geomagnetic field excursions. The Laschamp excursion is found globally in individual sediment records with high index values and an average duration of 1,300 years.…”

“…Panovska and Constable () proposed the PSV index as a tool for characterizing variations in the geomagnetic field and for discriminating among PSV, excursions, and reversals. The index combines the virtual geomagnetic pole (VGP) latitude λ p position and the virtual dipole moment, M , normalized to the present‐day dipole moment M 0 : …”