Geomagnetic Dipole Field Research Articles

Westward Excursion of Pc1/EMIC Waves and Their Source Protons: Paradoxical Observations From Ground and Space

AbstractPc1 geomagnetic pulsations within the frequency range of 0.2–5 Hz, the ground signatures of electromagnetic ion cyclotron (EMIC) waves, are usually observed to progress westward. A plausible explanation is that their westward‐excursion speed is defined by the drift velocity of the source particles in the geomagnetic dipole field. In this study, this relationship is examined through conjugate observations from ground‐based and spaceborne instruments. The ground‐based observations, including Pc1 pulsations and auroral emissions, consistently demonstrate a westward‐excursion speed of ∼4–5 MLT per hour, which agrees with the time delay between EMIC wave observations at two equatorial spacecraft. Observations of the wave source particles, however, indicate an inconsistency between the slow westward excursion speed and the high energy of the wave source protons. This apparent inconsistency is reconciled by the trapping motion of energetic particles within magnetic dips, which is supported by the simultaneous flux enhancements of protons across a broad energy range between ∼50 and ∼100 keV. These results improve our understanding of the westward excursion of Pc1 pulsations, and also underscore the need for caution when directly linking their westward‐excursion speed to the energies of the source particles.

A Simulation Study of the Modulation of the Geomagnetic Field Configuration on the Seasonal Variation of Ionospheric Sq Currents

AbstractBased on Global Coupled Ionosphere‐Thermosphere‐Electrodynamics Model, the solution of the 3‐dimensional current in the ionospheric region, the equivalent sheet current and filed‐aligned current are examined. The simulation study enables a comprehensive analysis of the effect of the geomagnetic field configuration, especially the non‐dipole component and tilt angle, on the ionospheric electrodynamics phenomena. Different geomagnetic field configurations are specified in the present work, including realistic geomagnetic field (RGF), tilted dipole geomagnetic field (TDGF) and zero‐declination dipole geomagnetic field (ZDGF). Our simulation focuses on the seasonal variation of Sq current, primarily governed by annual and semi‐annual variation. The modulation of the tilt angle of the geomagnetic field is globally distributed, whereas the modulation of the geomagnetic anomaly is localized. At mid‐latitudes, the annual mean and semi‐annual amplitudes of the Sq current are negatively correlated with the magnetic field strength, especially shown in geomagnetic anomaly area, while there is the opposite effect in the geomagnetic conjugate regions of the opposite hemispheres. The annual variation of Sq current system is more affected by the offset of the geomagnetic latitudes and geographical latitudes. The seasonal variation of the total Sq current is also modulated by the geomagnetic field. The annual mean, the annual and semi‐annual components of the total Sq current are negatively correlated with the magnetic field strength, while the annual variation is also controlled by the tilt angle of the geomagnetic field. The solar radiation affects the semi‐annual variation of the current more strongly than the annual variation.

Superionic effect and anisotropic texture in Earth’s inner core driven by geomagnetic field

Seismological observations suggest that Earth’s inner core (IC) is heterogeneous and anisotropic. Increasing seismological observations make the understanding of the mineralogy and mechanism for the complex IC texture extremely challenging, and the driving force for the anisotropic texture remains unclear. Under IC conditions, hydrogen becomes highly diffusive like liquid in the hexagonal-close-packed (hcp) solid Fe lattice, which is known as the superionic state. Here, we reveal that H-ion diffusion in superionic Fe-H alloy is anisotropic with the lowest barrier energy along the c-axis. In the presence of an external electric field, the alignment of the Fe-H lattice with the c-axis pointing to the field direction is energetically favorable. Due to this effect, Fe-H alloys are aligned with the c-axis parallel to the equatorial plane by the diffusion of the north–south dipole geomagnetic field into the inner core. The aligned texture driven by the geomagnetic field presents significant seismic anisotropy, which explains the anisotropic seismic velocities in the IC, suggesting a strong coupling between the IC structure and geomagnetic field.

Plate motion and a dipolar geomagnetic field at 3.25 Ga

The paleomagnetic record is an archive of Earth's geophysical history, informing reconstructions of ancient plate motions and probing the core via the geodynamo. We report a robust 3.25-billion-year-old (Ga) paleomagnetic pole from the East Pilbara Craton, Western Australia. Together with previous results from the East Pilbara between 3.34 and 3.18 Ga, this pole enables the oldest reconstruction of time-resolved lithospheric motions, documenting 160 My of both latitudinal drift and rotation at rates of at least 0.55°/My. Motions of this style, rate, and duration are difficult to reconcile with true polar wander or stagnant-lid geodynamics, arguing strongly for mobile-lid geodynamics by 3.25 Ga. Additionally, this pole includes the oldest documented geomagnetic reversal, reflecting a stably dipolar, core-generated Archean dynamo.

Contributions to Loss Across the Magnetopause During an Electron Dropout Event.

Dropout events are dramatic decreases in radiation belt electron populations that can occur in as little as 30 minutes. Loss to magnetopause due to a combination of magnetopause shadowing and outward radial transport plays a significant role in these events. We examine the dropout of relativistic electron populations during the October 2012 geomagnetic storm using simulated electron phase space density, evaluating the contribution of different processes to losses across the magnetopause. We compare loss contribution from outward transport calculated using a standard empirical radial diffusion model that assumes a dipolar geomagnetic field to an event-specific radial diffusion model evaluated with a non-dipolar geomagnetic field. We additionally evaluate the contribution of Shabansky type 1 particles, which bounce along magnetic field lines with local equatorial maxima, to the loss calculated during this event. We find that the empirical radial diffusion model with a dipolar background field underestimates the contribution of radial diffusion to this dropout event by up to 10% when compared to the event-specific, non-dipolar radial diffusion model. We additionally find that including Shabansky type 1 particles in the initial electron phase space density, that is, allowing some magnetic field lines distorted from the typical single-minima configuration in drift shell construction, increases the calculated loss by an average of 0.75%. This shows that the treatment of the geomagnetic field significantly impacts the calculation of electron losses to the magnetopause during dropout events, with the non-dipolar treatment of radial diffusion being essential to accurately quantify the loss of outer radiation belt populations.

Investigation of the Interaction Between Magnetosheath Reconnection and Magnetopause Reconnection Driven by Oblique Interplanetary Tangential Discontinuity Using Three‐Dimensional Global Hybrid Simulation

AbstractMagnetosheath reconnection due to the interaction of an interplanetary directional discontinuity with the bow shock and Earth's magnetosphere under an initially northward interplanetary magnetic field (IMF) has been investigated in previous simulations (e.g., Guo et al., 2018, https://doi.org/10.1029/2018ja025679). Under an initially southward IMF, the magnetosheath reconnection could interact with reconnection at the magnetopause. In this study, using three‐dimensional (3D) global‐scale hybrid simulations, we present cases with incoming tangential discontinuities (TDs) in an initially southward IMF, which possess various magnetic field rotation angles (ΔΦ) and half‐width (w), to study the effects of pre‐existing magnetopause reconnection on the formation of magnetosheath flux ropes, as well as the subsequent interaction between the magnetopause reconnection and magnetosheath reconnection, with downstreams of both Quasi‐perpendicular (Q‐) and Quasi‐parallel (Q‐||) shock examined. The initial IMF is assumed to be oblique, with a finite Bx and Bz, similar to that in Guo et al. (2018), https://doi.org/10.1029/2018ja025679 but with a southward Bz < 0. Compared with the cases with an initially northward IMF, magnetopause reconnection weakens the compression processes of the TD and leads to less frequent reconnection in the magnetosheath. The existence and the structure of magnetosheath reconnection are found to strongly depend on the parameters w and of the TD. When interacting with the magnetopause reconnection, the magnetosheath flux ropes can re‐reconnect with the geomagnetic dipole field lines, forming new structures of magnetopause flux ropes. The resulting evolution of flux rope configuration is illustrated.

Four‐Dimensional Paleomagnetic Dataset: Plio‐Pleistocene Paleodirection and Paleointensity Results From the Erebus Volcanic Province, Antarctica

AbstractA fundamental assumption in paleomagnetism is that a geocentric axial dipole (GAD) geomagnetic field structure extends to the ancient field. Global paleodirectional compilations that span 0–5‐million year support a GAD dominated field structure with minor non‐GAD contributions, however, the paleointensity data over the same period do not. In a GAD field, higher latitudes should preserve higher intensity, but the current database suggests that intensities are independent of latitude. To determine whether the seemingly “low” intensities from Antarctica reflect the ancient field, rather than low‐quality data or inadequate temporal sampling, we have conducted a new study of the paleomagnetic field in Antarctica. This study focuses on the paleomagnetic field structure over the Plio‐Pleistocene. We combine and reanalyze new and published paleodirectional and paleointensity results from the Erebus volcanic province to recover paleodirections from 98 sites that were both thermally and alternating field demagnetized and then subjected to a set of strict selection criteria and paleointensities from 26 sites from the Plio‐Pleistocene that underwent the IZZI modified Thellier‐Thellier experiment and were also subjected to a strict set of selection criteria. The paleopole (201.85°, 87.65°) and α95 (5.51°) recovered from our paleodirectional study supports the GAD hypothesis and the scatter of the virtual geomagnetic poles falls within the uncertainty of that predicted by TK03 paleosecular variation model. Our time‐averaged field strength estimate, 33.57 ± 2.71 μT, is significantly weaker than that expected from a GAD field estimated by the present field.

A paleomagnetic record of the early Matuyama chron including the R\xe9union subchron and\xa0the onset Olduvai boundary: High-resolution magnetostratigraphy and insights from transitional geomagnetic fields

We present a novel paleomagnetic record for the lower Matuyama chronozone, which includes the Réunion subchronozone and the lower Olduvai polarity reversal, from a continuous section of a 168-m-thick on-land marine succession in the southernmost part of the Boso Peninsula, central Japan. In this section, the Réunion subchronozone and the lower Olduvai reversal are observed at 38.6–44.6 m and 142.0 m, respectively. The average sedimentation rates between the lower and upper Réunion boundaries and between the upper Réunion boundary and lower Olduvai boundary are calculated as 25 cm/ky and 57 cm/ky, respectively. The virtual geomagnetic pole (VGP), observed in the Boso Peninsula, at both the upper and lower Réunion boundaries passed across the equator within a similar longitudinal band over Africa. Immediately below the upper boundary, between 43.0 and 43.5 m, the VGP settled in a cluster area around China. Relative paleointensity (RPI) values for the entire Réunion interval are generally lower than the average for the entire interval from the Réunion to the lower Olduvai subchronozone. Conversely, the VGP for the lower Olduvai reversal boundary did not pass across the equator within a narrow longitudinal band but settled in several cluster areas; i.e., the southern Indian Ocean, North America, and the southern South Pacific Ocean off South America. The VGP then moved rapidly between the clusters. The locations of VGP cluster areas in the lower Olduvai reversal seem to coincide with areas where a vertical component of the present geomagnetic non-axial dipole (NAD) field is dominant. During the reversal, the RPI declined rapidly and recovered slowly as the VGP moved rapidly between cluster areas. Our new paleomagnetic data are one of the most detailed records for those geomagnetic reversals from marine sediments, and will, therefore, help to understand the dynamics of the geomagnetic reversals.

Low geomagnetic field strength during End-Cretaceous Deccan volcanism and whole mantle convection

Knowledge about long-term variation of the geomagnetic dipole field remains in its nascent stage because of the paucity of reliable experimental data over geological periods. Here, we present the first robust experimental data from the largest Cretaceous flood basalt province on Earth, the ~65–66 Ma Deccan basalt within a thick (1250 m) unbiased stratigraphic section down to the basement, recovered from a drill hole of the Koyna Deep Scientific Drilling Project in the Western Ghats, India. Critical analysis of the result along with similar results of the Cretaceous age find that (i) the dipole moment during the end Cretaceous Deccan eruption is the lowest in whole of Cretaceous (ii) dipole moment at the onset/termination of the Cretaceous Normal Superchron is apparently lower relative to that in mid-superchron, however, such differences cannot be deciphered in shorter polarities probably because of insufficient time to develop recognizable variations (iii) inverse relation between dipole moment and reversal rate is lacking and (iv) a cause and effect relation between core-mantle boundary heat flux and low dipole moment that appears to be the principle governing factor in forming the Large Igneous Provinces on the surface of earth.

Two modes of ion-ion hybrid waves in magnetospheric plasma

This paper is concerned with the structure of ion-ion hybrid (IIH) waves in multicomponent magnetospheric plasma in the dipole geomagnetic field model. Like Alfvén waves, in IIH waves the field line can oscillate in different directions: in the azimuthal direction (the toroidal modes) and in the radial direction (the poloidal modes). Contrary to Alfvén waves, in IIH waves only the equatorial part of the field line can oscillate, since due to the heavy ions admixture the IIH waves are trapped in the resonator in the vicinity of the geomagnetic equator. The equatorial localization of the waves allow the calculation of the parallel structure of the toroidal and poloidal IIH modes analytically. It was found that the toroidal and poloidal modes have slightly different structures and that their frequencies are also slightly different. The difference between toroidal and poloidal frequencies (the polarization splitting) is small with respect to the basic harmonics of each mode. It was found that IIH modes have a considerable compressional component of the wave’s magnetic field.

Characteristics of a relative paleointensity record from loess deposits in arid central Asia and chronological implications

Abstract Loess-paleosol sequences in the arid central Asia (ACA) provide valuable material for evaluating the evolution of paleoclimate and dust circulation in the Northern Hemisphere. However, determination of loess chronology has yet remained a main obstacle for paleoclimate interpretation. In this study, analyses of relative paleomagnetic intensity (RPI) were conducted in a loess section on the northern piedmont of the Tianshan Mountains covering the period since the last interglacial. The RPI record was found to be generally comparable with other recognized RPI stacks, confirming a globally consistent variation of the geomagnetic dipole field. Based on the RPI age model, the attempt to achieve paleoclimate assessment gives rise to largely similar variations to the North Atlantic records. Consequently, these results suggest that the RPI correlation could serve as a promising way to evaluate the age control of loess in the ACA. Nevertheless, in consideration of the complicated topography and loess deposition very close to the dust source, four main factors including carbon contamination, the reliability of luminescence signals, in situ depositional processes and complexity of remanence acquisition, may possibly contort the chronological determination in the ACA loess. Thus, cautions are still indispensable for paleoclimate interpretation with any solely chronological model and more work is therefore required to achieve a comprehensive understanding of loess chronological setting in this area.

Диагностика границ аврорального овала на основе техники инверсии магнитограмм

It is shown that the convection reversal boundary is a fundamental parameter of the magnetosphere-ionosphere coupling, which determines a strong analogy between the electrostatic potential of the ionosphere and the equivalent current function in the dipole geomagnetic field approximation and the uniform ionospheric conductance. We have developed a new ground-based method for automatically diagnosing boundaries of the auroral oval using output data obtained with the magnetogram inversion technique (MIT). Using maps of the current function and field-aligned currents, calculated at the first stage of MIT with uniform ionospheric conductance, we determine the convection reversal boundary, polar cap boundary, equatorial boundary of the auroral oval, and line of maximum density of auroral electrojets. These parameters have previously been determined by a visual-manual method: analyzing maps of field-aligned and equivalent currents on the monitor screen and carrying out predetermined boundaries with the mouse — this took a very long time (weeks and months). The comparison between manually and automatically obtained boundaries has shown that the correlation coefficient between the two boundaries is, on average, 0.85, and the root-mean-square deviation does not exceed 2° in latitude. By providing an adequate accuracy for the boundary determination, the automatic method reduces the time for map processing by a factor of 2–3 (to minutes and hours), releasing a researcher from laborious visual work. The new method is implemented as one of the important modules in the updated MIT software.

Diagnostics of auroral oval boundaries on the basis of the magnetogram inversion technique

It is shown that the convection reversal boundary is a fundamental parameter of the magnetosphere-ionosphere coupling, which determines a strong analogy between the electrostatic potential of the ionosphere and the equivalent current function in the dipole geomagnetic field approximation and the uniform ionospheric conductance. We have developed a new ground-based method for automatically diagnosing boundaries of the auroral oval using output data obtained with the magnetogram inversion technique (MIT). Using maps of the current function and field-aligned currents, calculated at the first stage of MIT with uniform ionospheric conductance, we determine the convection reversal boundary, polar cap boundary, equatorial boundary of the auroral oval, and line of maximum density of auroral electrojets. These parameters have previously been determined by a visual-manual method: analyzing maps of field-aligned and equivalent currents on the monitor screen and carrying out predetermined boundaries with the mouse — this took a very long time (weeks and months). The comparison between manually and automatically obtained boundaries has shown that the correlation coefficient between the two boundaries is, on average, 0.85, and the root-mean-square deviation does not exceed 2° in latitude. By providing an adequate accuracy for the boundary determination, the automatic method reduces the time for map processing by a factor of 2–3 (to minutes and hours), releasing a researcher from laborious visual work. The new method is implemented as one of the important modules in the updated MIT software.

B-dot algorithm steady-state motion performance

Satellite attitude motion subject to the well-known B-dot magnetic control is considered. Unlike the majority of studies the present work focuses on the slowly rotating spacecraft. The attitude and the angular velocity acquired after detumbling the satellite is determined. This task is performed using two relatively simple geomagnetic field models. First the satellite is considered moving in the simplified dipole model. Asymptotically stable rotation around the axis of the maximum moment of inertia is found. This axis direction in the inertial space and the rotation rate are found. This result is then refined using the direct dipole geomagnetic field. Simple stable rotation transforms into the periodical motion, the rotation rate is also refined. Numerical analysis with the gravitational torque and the inclined dipole model verifies the analytical results.

A pure dipole analysis of the Gondwana apparent polar wander path: Paleogeographic implications in the evolution of Pangea

AbstractThe paleogeography of prebreakup Pangea at the beginning of the Atlantic Spreading has been a subject of debate for the past 50 years. Reconciling this debate involves theoretical corrections that cast doubt on available data and paleomagnetism as an effective tool for performing paleoreconstructions. This 50 year old debate focuses specifically on magnetic remanence and its ability to correctly record the inclination of the paleomagnetic field. In this paper, a selection of paleopoles was made to find the great circles containing the paleomagnetic pole and the respective sampling site. The true dipole pole (TDP) was then calculated by intersecting these great circles, effectively avoiding nondipolar contributions and inclination shallowing, in an innovative method. The great circle distance between each of these TDPs and the paleomagnetic means show the accuracy of paleomagnetic determinations in the context of a dominantly geocentric, axial, and dipolar geomagnetic field. The TDPs calculated allowed a bootstrap analysis to be performed to further consider the flattening factor that should be applied to the sedimentary‐derived paleopoles. It is argued that the application of a single theoretical correction factor for clastic sedimentary‐derived records could lead to a bias in the paleolatitude calculation and therefore to incorrect paleogeographic reconstructions. The unbiased APWP makes it necessary to slide Laurentia to the west in relation to Gondwana in a B‐type Pangea during the Upper Carboniferous, later evolving, during the Early Permian, to reach the final A‐type Pangea configuration of the Upper Permian.

Solar activity during the Holocene: the Hallstatt cycle and its consequence for grand minima and maxima

Cosmogenic isotopes provide the only quantitative proxy for analyzing the long-term solar variability over a centennial timescale. While essential progress has been achieved in both measurements and modeling of the cosmogenic proxy, uncertainties still remain in the determination of the geomagnetic dipole moment evolution. Here we improve the reconstruction of solar activity over the past nine millennia using a multi-proxy approach. We used records of the 14C and 10Be cosmogenic isotopes, current numerical models of the isotope production and transport in Earth's atmosphere, and available geomagnetic field reconstructions, including a new reconstruction relying on an updated archeo-/paleointensity database. The obtained series were analyzed using the singular spectrum analysis (SSA) method to study the millennial-scale trends. A new reconstruction of the geomagnetic dipole field moment, GMAG.9k, is built for the last nine millennia. New reconstructions of solar activity covering the last nine millennia, quantified in sunspot numbers, are presented and analyzed. A conservative list of grand minima and maxima is provided. The primary components of the reconstructed solar activity, as determined using the SSA method, are different for the series based on 14C and 10Be. These primary components can only be ascribed to long-term changes in the terrestrial system and not to the Sun. They have been removed from the reconstructed series. In contrast, the secondary SSA components of the reconstructed solar activity are found to be dominated by a common ~2400-yr quasi-periodicity, the so-called Hallstatt cycle, in both the 14C and 10Be based series. This Hallstatt cycle thus appears to be related to solar activity. Finally, we show that the grand minima and maxima occurred intermittently over the studied period, with clustering near highs and lows of the Hallstatt cycle, respectively.

Evolution of the dipole geomagnetic field. Observations and models

The works on paleomagnetic observations of the dipole geomagnetic field, its variations, and reversals in the last 3.5 billion years have been reviewed. It was noted that characteristic field variations are related to the evolution of the convection processes in the liquid core due to the effect of magnetic convection and solid core growth. Works on the geochemistry and energy budget of the Earth's core, the effect of the solid core on convection and the generation of the magnetic field, dynamo models are also considered. We con� sider how core growth affects the magnetic dipole generation and variations, as well as the possibility of mag� netic field generation up to the appearance of the solid core. We also pay attention to the fact that not only the magnetic field but also its configuration and time variations, which are caused by the convection evolution in the core on geological timescales, are important factors for the biosphere.

Antiquity of the South Atlantic Anomaly and evidence for top-down control on the geodynamo

The dramatic decay of dipole geomagnetic field intensity during the last 160 years coincides with changes in Southern Hemisphere (SH) field morphology and has motivated speculation of an impending reversal. Understanding these changes, however, has been limited by the lack of longer-term SH observations. Here we report the first archaeomagnetic curve from southern Africa (ca. 1000–1600 AD). Directions change relatively rapidly at ca. 1300 AD, whereas intensities drop sharply, at a rate greater than modern field changes in southern Africa, and to lower values. We propose that the recurrence of low field strengths reflects core flux expulsion promoted by the unusual core–mantle boundary (CMB) composition and structure beneath southern Africa defined by the African large low shear velocity province (LLSVP). Because the African LLSVP and CMB structure are ancient, this region may have been a steady site for flux expulsion, and triggering of geomagnetic reversals, for millions of years.

Geomagnetic paleointensity at ∼2.41 Ga as recorded by the Widgiemooltha Dike Swarm, Western Australia

Absolute geomagnetic paleointensity measurements were conducted on samples from six mafic dikes of the ∼2.41 Ga Widgiemooltha swarm (Western Australia). Rock magnetic analyses indicate that the paleointensity signal is carried by nearly stoichiometric pseudosingle-domain magnetite and/or low-Ti titanomagnetite. Paleointensity values were determined using the Thellier double-heating method supplemented by low-temperature demagnetizations (the LTD-Thellier method) in order to reduce the effect of magnetic remanence carried by large pseudosingle-domain and multidomain magnetite grains. Thirty-one samples from five dikes yielded successful paleointensity determinations with the mean value of 41.2±3.8 μT, which corresponds to a virtual dipole moment of 6.65±0.98 Am2. The mean and range of paleofield strength values are similar to those of the recent Earth's magnetic field and are consistent with a compositionally driven geodynamo established by the earliest Paleoproterozoic Era. The existence of a stable, dipolar geomagnetic field during the Proterozoic indicated by paleointensity and paleodirectional data from this and prior studies seems incompatible with a young age of the inner core suggested by recent models of the Earth's thermal evolution. Acquisition of reliable paleointensity estimates using the novel approaches such as the LTD-Thellier method is crucial in constraining the development of more realistic, Earth-like models of long-term geodynamo behavior.

Simulating ice core <sup>10</sup>Be on the glacial–interglacial timescale

Abstract. 10Be ice core measurements are an important tool for paleoclimate research, e.g., allowing for the reconstruction of past solar activity or changes in the geomagnetic dipole field. However, especially on multi-millennial timescales, the share of production and climate-induced variations of respective 10Be ice core records is still up for debate. Here we present the first quantitative climatological model of the 10Be ice concentration up to the glacial–interglacial timescale. The model approach is composed of (i) a coarse resolution global atmospheric transport model and (ii) a local 10Be air–firn transfer model. Extensive global-scale observational data of short-lived radionuclides as well as new polar 10Be snow-pit measurements are used for model calibration and validation. Being specifically configured for 10Be in polar ice, this tool thus allows for a straightforward investigation of production- and non-production-related modulation of this nuclide. We find that the polar 10Be ice concentration does not immediately record the globally mixed cosmogenic production signal. Using geomagnetic modulation and revised Greenland snow accumulation rate changes as model input, we simulate the observed Greenland Summit (GRIP and GISP2) 10Be ice core records over the last 75 kyr (on the GICC05modelext timescale). We show that our basic model is capable of reproducing the largest portion of the observed 10Be changes. However, model–measurement differences exhibit multi-millennial trends (differences up to 87% in case of normalized to the Holocene records) which call for closer investigation. Focusing on the (12–37) b2k (before the year AD 2000) period, mean model–measurement differences of 30% cannot be attributed to production changes. However, unconsidered climate-induced changes could likely explain the model–measurement mismatch. In fact, the 10Be ice concentration is very sensitive to snow accumulation changes. Here the reconstructed Greenland Summit (GRIP) snow accumulation rate record would require revision of +28% to solely account for the (12–37) b2k model–measurement differences.