Introduction to Spherical Elementary Current Systems

Vanhamäki, Heikki; Juusola, Liisa

doi:10.1007/978-3-030-26732-2_2

Cited by 49 publications

(73 citation statements)

References 40 publications

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“…For each overflight, components of the magnetic field variations are fitted with the one‐dimensional (1‐D) and two‐dimensional (2‐D) SECS (e.g., Vanhamäki & Juusola, 2020). The 1‐D SECSs describe only latitudinal variations, while the 2‐D SECSs describe both latitudinal and longitudinal variations.…”

Section: Discussionmentioning

confidence: 99%

Seasonal Effect on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents

2020

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We present a statistical investigation of the seasonal effect on hemispheric asymmetry in the auroral currents during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity. Five years of magnetic data from the Swarm satellites has been analyzed by applying the spherical elementary current system (SECS) method. Bootstrap resampling has been used to remove the seasonal differences between the hemispheres in the data set. In general, the currents are larger in the Northern Hemisphere (NH) than in the Southern Hemisphere (SH). Asymmetry is larger during low than high Kp and during local winter and local autumn than local summer and local spring. For all Kp conditions together, the NH/SH ratio for FACs in winter, autumn, spring, and summer are 1.17 ± 0.05, 1.14 ± 0.05, 1.07 ± 0.04, and 1.02 ± 0.04, respectively. The largest asymmetry is observed during low Kp in local winter, when the excess in the NH currents is 21 ± 5% in FAC, 14 ± 3% in curl-free (CF) and 10 ± 3% in divergence-free (DF) current. We also find that evening sector (13-24 MLT) contributes more to the high NH/SH ratio than the morning (01-12 MLT) sector. The physical mechanisms producing the hemispheric asymmetry are not presently understood. We calculated the solar-induced ionospheric conductances during low Kp conditions from the IRI model. The model conductance NH/SH ratios are above 1 in autumn and spring, similar to the currents, but below 1 for winter, which is in contradiction with the currents. Therefore, we do not consider solar-induced conductances as the main explanation for hemispheric asymmetry.

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Section: Discussionmentioning

confidence: 99%

Seasonal Effect on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents

2020

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“…However, our measurements are non‐uniformly distributed and fewer than the number of nodes. The solution to the under‐determined set of equations is therefore highly dependent on the choice of grid (node locations), and on the way that the inverse problem is regularized (Vanhamaki & Juusola, 2020). When applying the SECS technique to EZIE data, it is critical that the grid and regularization technique are selected such that variations reflect geophysical changes and not changes in geometry, for example as the satellite moves.…”

Section: Estimating the Electrojet From Simulated Mesospheric Magnetic Field Measurementsmentioning

confidence: 99%

Electrojet Estimates From Mesospheric Magnetic Field Measurements

et al. 2021

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The first attempts to relate ground magnetic disturbances to electric currents in space were carried out more than a century ago. Birkeland (1908) presented a horizontal two-cell equivalent current system which is reminiscent of maps derived from modern magnetometer networks (Waters et al., 2015). Birkeland further proposed a three-dimensional (3D) structure of the space currents that involved magnetic field-aligned currents. This idea remained controversial until it was confirmed by early magnetometer measurements in space (Zmuda et al., 1966). We now view the 3D ionospheric current system as composed of Birkeland currents, that flow along magnetic field lines, and a horizontal current that is confined to a thin conducting layer of the ionosphere, mainly around 100-120 km altitude. The relationship between ground magnetic field observations and this 3D current system is ambiguous, and we therefore often interpret ground magnetic field observations in terms of an equivalent 2D current system. At high latitudes, the equivalent current is nearly identical with the divergence-free part of the horizontal current (e.g., Fukushima, 1976;Untiedt & Baumjohann, 1993). In this study, we use the term electrojet as synonymous with the equivalent current, although it is sometimes used to refer to specific parts of it.

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“…In comparison with high-resolution data such as total electron content (TEC) data from Global Navigation Satellite System (GNSS) satellites, it would be almost impossible to expect all the data points are apart from a center of any basis functions. The singularities could be eliminated by modifying the original SECS functions in the neighborhood of their source points as discussed by Vanhamäki and Juusola (2020). The proposed basis functions provide another practical way which enables us to obtain a reasonable estimate as smooth as the Weimer's model.…”

Section: Estimation Of Ionospheric Plasma Velocity Fieldmentioning

confidence: 99%

“…For example, if the footprint of a spacecraft comes to the neighborhood of a singular point by chance, the data from spacecraft observation cannot be compared with an SECS analysis result. Thus, in many applications of the SECS functions, it was essential to appropriately treat the singularities as discussed by Vanhamäki and Juusola (2020).…”

Section: Introductionmentioning

confidence: 99%

A framework for estimating spherical vector fields using localized basis functions and its application to SuperDARN data processing

Nakano

Hori

Seki

et al. 2020

Earth Planets Space

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A technique for estimating a plasma drift velocity distribution in the ionosphere is presented. This technique is based on a framework for representing a global vector field on a sphere by using a set of localized basis functions which is newly derived as a variant of the spherical elementary current system (SECS). A vector field on a sphere can be divided into its divergence-free (DF) component and curl-free (CF) component. The DF and CF components can then be represented by weighted sums of the DF and CF vector-valued basis functions, respectively. While the SECS basis functions have a singular point, the new basis functions do not diverge over a sphere. This property of the new basis function allows us to achieve robust prediction of the drift velocity at any point in the ionosphere. Assuming that the ionospheric plasma drift velocity has no divergence, its distribution can be represented by a weighted sum of the DF basis functions. The proposed technique estimates the ionospheric plasma drift velocity distribution from the SuperDARN data by using the DF basis functions. Since there are some wide gaps in the spatial coverage of the SuperDARN, an empirical convection model is combined with the framework based on the new basis functions. It is demonstrated that the proposed technique is useful for the estimation and modeling of the ionospheric plasma velocity distribution.

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Introduction to Spherical Elementary Current Systems

Cited by 49 publications

References 40 publications

Seasonal Effect on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents

Seasonal Effect on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents

Electrojet Estimates From Mesospheric Magnetic Field Measurements

A framework for estimating spherical vector fields using localized basis functions and its application to SuperDARN data processing

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