Polynomial modelling of CHAMP satellite data to investigate rapid secular variation fluctuations over southern Africa during 2003 and 2004

Nahayo, E.; Kotzé, P. B.

doi:10.5047/eps.2012.03.001

Cited by 5 publications

(3 citation statements)

References 15 publications

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“…Therefore, under the condition of a constant neutral wind velocity, it can be thought that a decrease of the magnetic field intensity due to the secular variation of the Earth's magnetic field [e.g., Bloxham and Gubbins 1985;Hongre et al 1998;Korte and Constable 2011;Nahayo and Kotze 2012;Bhardwaj and Subba Rao 2013] causes a reduction of the Sq amplitude associated with the weakening of the dynamo field. However, because the height-integrated ionospheric conductivity is also affected by the intensity of the magnetic field, the dependence of the Sq amplitude on the magnetic field is more complicated.…”

Section: Introductionmentioning

confidence: 99%

Long-term variation in the upper atmosphere as seen in the geomagnetic solar quiet daily variation

et al. 2014

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Characteristics of long-term variation in the amplitude of solar quiet (Sq) geomagnetic field daily variation have been investigated using 1-h geomagnetic field data obtained from 69 geomagnetic observation stations within the period of 1947 to 2013. The Sq amplitude observed at these geomagnetic stations showed a clear dependence on the 10-to 12-year solar activity cycle and tended to be enhanced during each solar maximum phase. The Sq amplitude was the smallest around the minimum of solar cycle 23/24 in 2008 to 2009. The relationship between the solar F10.7 index and Sq amplitude was approximately linear but about 53% of geomagnetic stations showed a weak nonlinear relation to the solar F10.7 index. In order to remove the effect of solar activity seen in the long-term variation of the Sq amplitude, we calculated a linear or second-order fitting curve between the solar F10.7 index and Sq amplitude during 1947 to 2013 and examined the residual Sq amplitude, which is defined as the deviation from the fitting curve. As a result, the majority of trends in the residual Sq amplitude that passed through a trend test showed negative values over a wide region. This tendency was relatively strong in Europe, India, the eastern part of Canada, and New Zealand. The relationship between the magnetic field intensity at 100-km altitude and residual Sq amplitude showed an anti-correlation for about 71% of the geomagnetic stations. Furthermore, the residual Sq amplitude at the equatorial station (Addis Ababa) was anti-correlated with the absolute value of the magnetic field inclination. This implies movement of the equatorial electrojet due to the secular variation of the ambient magnetic field.

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Section: Introductionmentioning

confidence: 99%

Long-term variation in the upper atmosphere as seen in the geomagnetic solar quiet daily variation

et al. 2014

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“…The southern African ground survey data have been used to derive various regional main field and/or secular variations models, using the methods of polynomials for the time intervals 2004-2005) and 2005(Kotzé 2011Geese et al 2011), spherical cap harmonic functions for 1975-2000 (Kotzé 2003) and harmonic splines for 1961-2001(Geese et al 2010) and 2005(Geese et al 2011. Polynomials have also been applied to model the CHAllenging Minisatellite Payload (CHAMP) satellite data of southern Africa (Nahayo and Kotzé 2012). Comparisons of different methods have shown that considering the geomagnetic main field and its secular variation on scales on the order of several 1000 km low-degree two-dimensional polynomials provide suitable representations of the field components for many applications (Geese et al 2011).…”

Section: Introductionmentioning

confidence: 99%

Morphology of the southern African geomagnetic field derived from observatory and repeat station survey observations: 2005–2014

Kotzé

Korte

2016

Earth Planet Sp

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Geomagnetic field data from four observatories and annual field surveys between 2005 and 2015 provide a detailed description of Earth's magnetic field changes over South Africa, Namibia and Botswana on time scales of less than 1 year. The southern African area is characterized by rapid changes in the secular variation pattern and lies in close proximity to the South Atlantic Anomaly (SAA) where the geomagnetic field intensity is almost 30 % weaker than in other regions at similar latitudes around the globe. Several geomagnetic secular acceleration (SA) pulses (geomagnetic jerks) around 2007, 2010 and 2012 could be identified over the last decade in southern Africa. We present a new regional field model for declination and horizontal and vertical intensity over southern Africa (Southern African REGional (SAREG)) which is based on field survey and observatory data and covering the time interval from 2005 to 2014, i.e. including the period between 2010 and 2013 when no low Earth-orbiting vector field satellite data are available. A comparative evaluation between SAREG and global field models like CHAOS-5, the CHAMP, Orsted and SAC-C model of the Earth's magnetic field and International Geomagnetic Reference Field (IGRF-12) reveals that a simple regional field model based on a relatively dense ground network is able to provide a realistic representation of the geomagnetic field in this area. We particularly note that a global field model like CHAOS-5 does not always indicate similar short-period patterns in the field components as revealed by observatory data, while representing the general secular variation reasonably well during the time interval without near-Earth satellite vector field data. This investigation further shows the inhomogeneous occurrence and distribution of secular variation impulses in the different geomagnetic field components and at different locations in southern African.

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“…The southern African region has also been known for many years to have high spatial and temporal field variations with strong abrupt changes in the linear geomagnetic secular variation (Mandea et al, 2007;Kotzé and Korte, 2016;Kotzé, 2020). Some scholars have recently conducted studies of rapid geomagnetic time variations in southern Africa using regional models (Geese et al, 2009(Geese et al, , 2011Nahayo and Kotzé, 2012;Nahayo et al, 2015Nahayo et al, , 2018.…”

Section: Introductionmentioning

confidence: 99%

A regional geomagnetic field model over Southern Africa derived with harmonic splines from Swarm satellite and ground-based data recorded between 2014 and 2019

Nahayo

Korte

2022

Earth Planets Space

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A regional harmonic spline geomagnetic main field model, Southern Africa Core Field Model (SACFM-3), is derived from Swarm satellite and ground-based data for the southern African region, in the eastern part of the South Atlantic Anomaly (SAA) where the field intensity continues to decrease. Using SACFM-3 and the global CHAOS-6-×9 model, a detailed study was conducted to shed light on the high spatial and temporal geomagnetic field variations over Southern Africa between 2014 and 2019. The results show a steady decrease of the radial component Z in almost the entire region. In 2019, its rate of decrease in the western part of the region has reached high values, 76 nT/year and 78 nT/year at Tsumeb and Keetmanshoop magnetic observatories, respectively. For some areas in the western part of the region the radial component Z and field intensity F have decreased in strength, from 1.0 to 1.3% and from 0.9 to 1.2%, respectively, between the epochs 2014.5 and 2019.5. There is a noticeable decrease of the field intensity from the south-western coast of South Africa expanding towards the north and eastern regions. The results show that the SAA area is continuing to grow in the region. Abrupt changes in the linear secular variation in 2016 and 2017 are confirmed in the region using ground-based data, and the X component shows an abrupt change in the secular variation in 2018 at four magnetic observatories (Hermanus, Hartebeesthoek, Tsumeb and Keetmanshoop) that needs further investigation. The regional model SACFM-3 reflects to some extent these fast core field variations in the Z component at Hermanus, Hartebeesthoek and Keetmanshoop observatories. Graphical Abstract

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Polynomial modelling of CHAMP satellite data to investigate rapid secular variation fluctuations over southern Africa during 2003 and 2004

Cited by 5 publications

References 15 publications

Long-term variation in the upper atmosphere as seen in the geomagnetic solar quiet daily variation

Long-term variation in the upper atmosphere as seen in the geomagnetic solar quiet daily variation

Morphology of the southern African geomagnetic field derived from observatory and repeat station survey observations: 2005–2014

A regional geomagnetic field model over Southern Africa derived with harmonic splines from Swarm satellite and ground-based data recorded between 2014 and 2019

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