Solar cycle variations in the annual mean values of the geomagnetic components of observatory data.

Yukutake, Takesi; Cain, J. C.

doi:10.5636/jgg.39.19

Cited by 7 publications

(6 citation statements)

References 13 publications

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“…Some long‐period variations of the external field that we have been unable to recover may have been mapped into variations of the internal field. The remaining misfit between our models and the observatory annual mean data is likely partly due to non‐core signal (besides the dipolar external/induced fields), for example, due to S q current systems that have not been averaged out [ Gavoret et al ., ; Yukatake and Cain , ; Schmucker , ].…”

Section: Results and Discussion Of The Cov‐obs Family Of Geomagnetic mentioning

confidence: 99%

“…Some longperiod variations of the external field that we have been unable to recover may have been mapped into variations of the internal field. The remaining misfit between our models and the observatory annual mean data is likely partly due to non-core signal (besides the dipolar external/induced fields), for example, due to S q current systems that have not been averaged out [Gavoret et al, 1986;Yukatake and Cain, 1987;Schmucker, 1991]. [40] The dispersion in the ensemble of predictions increases toward earlier times due to fewer or less reliable data (see all components at Kakioka and especially Z at Sitka): the a priori information then controls the statistics of the field models.…”

Section: Evidence For Inter-annual Secular Variation Changesmentioning

confidence: 99%

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Stochastic modeling of the Earth's magnetic field: Inversion for covariances over the observatory era

Gillet

Jault

Finlay

et al. 2013

Geochem Geophys Geosyst

114

182

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[1] Inferring the core dynamics responsible for the observed geomagnetic secular variation requires knowledge of the magnetic field at the core-mantle boundary together with its associated model covariances. However, most currently available field models have been built using regularization conditions, which force the expansions in the spatial and time domains to converge but also hinder the calculation of reliable second-order statistics. To tackle this issue, we propose a stochastic approach that integrates, through time covariance functions, some prior information on the time evolution of the geomagnetic field. We consider the time series of spherical harmonic coefficients as realizations of a continuous and differentiable stochastic process. Our specific choice of process, such that it is not twice differentiable, mainly relies on two properties of magnetic observatory records (time spectra, existence of geomagnetic jerks). In addition, the required characteristic times for the low degree coefficients are obtained from available models of the magnetic field and its secular variation based on satellite data. We construct the new family COV-OBS of field models spanning the observatory and satellite era of 1840-2010. These models include the external dipole and permit sharper time changes of the internal field compared to previous regularized reconstructions. The a posteriori covariance matrix displays correlations in both space and time, which should be accounted for through the secular variation error model in core flow inversions and geomagnetic data assimilation studies.

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Section: Results and Discussion Of The Cov‐obs Family Of Geomagnetic mentioning

confidence: 99%

Section: Evidence For Inter-annual Secular Variation Changesmentioning

confidence: 99%

Stochastic modeling of the Earth's magnetic field: Inversion for covariances over the observatory era

Gillet

Jault

Finlay

et al. 2013

Geochem Geophys Geosyst

114

182

View full text Add to dashboard Cite

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“…On an average more than 95% of the field measured on the ground is of internal origin. However, MI currents can create magnetic disturbances of up to a few percent of the ground magnetic field during magnetic storms [see, e.g., Yukutake and Cain , 1987].…”

Section: Introductionmentioning

confidence: 99%

Response of the polar magnetic field intensity to the exceptionally high solar wind streams in 2003

Lukianova¹,

Мурсула²,

Kozlovsky³

2012

Geophysical Research Letters

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The exceptionally high solar wind stream activity in 2003 caused a record intensity in the auroral electrojet currents, leading to a major reduction of the horizontal field at auroral latitudes and to a notable strengthening of the vertical geomagnetic field in the polar cap. This strengthening is clearly visible in the observatory annual values as a significant deflection in the corresponding secular variation. A similar but weaker deflection also occurs during the strongest high speed stream years of the earlier solar cycles, e.g., in 1983 and 1994. We also found that, in addition to the disturbed times, the westward electrojet was often enhanced even during the most quiet times of the strongest high speed stream years. The quiet time level was more disturbed in 2003 than in other high speed stream years, when an exceptionally clear signal was seen in the polar cap Z intensity even in the annual mean curve in this year. We exclude other current systems like the ring current or the DPY current as possible explanations.

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“…It was recognized in the past (Yukutake and Cain, 1987) that observatory annual means are affected by the external field which varies with the solar cycle.…”

Section: Introductionmentioning

confidence: 99%

Contributions of the external field to the observatory annual means and a proposal for their corrections

et al. 2007

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In this study we separate, interpret and explain magnetospheric and ionospheric signals present in the observatory annual means. The data from 46 European geomagnetic observatories collected over 42 years are used. To characterise the various field components, we use predictions from latest magnetic field models. The core field and its secular variation are described by the CM4 model, and the magnetospheric contributions are successfully removed by parameterising the POMME model with the Dst index. We regard the remaining signal as being caused by ionospheric currents. The annual averages of the Sq variation estimated by the CM4 model are subtracted from the residuals. A remaining variation in anti-phase with the magnetic activity index Ap finally can be removed with a function properly scaled by Ap and Dst. We offer an objective procedure to suppress the external field contributions in the annual means to an uncertainty level of ±2 nT. Except for the ionospheric currents, this could be achieved by applying recent magnetic field models, which shows that the quality of present day models is sufficient to correct observatory data for average external field contributions. Understanding the signal contained in the annual means is a prerequisite for obtaining reliable and physically meaningful results when such data are used in studies of the core field and its secular variation.

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Solar cycle variations in the annual mean values of the geomagnetic components of observatory data.

Cited by 7 publications

References 13 publications

Stochastic modeling of the Earth's magnetic field: Inversion for covariances over the observatory era

Stochastic modeling of the Earth's magnetic field: Inversion for covariances over the observatory era

Response of the polar magnetic field intensity to the exceptionally high solar wind streams in 2003

Contributions of the external field to the observatory annual means and a proposal for their corrections

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