Field‐Aligned and Horizontal Currents in the Northern and Southern Hemispheres From the Swarm Satellite

Workayehu, Abiyot; Vanhamäki, Heikki; Aikio, Anita

doi:10.1029/2019ja026835

Cited by 22 publications

(35 citation statements)

References 42 publications

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“…4a, b compare well to those of previous publications (e.g., Wang et al 2008;Korth et al 2010;Milan et al 2017;He et al 2012;Coxon et al 2014;Carter et al 2016;Laundal et al 2018). Furthermore, the FACs intensity in the northern hemisphere is slightly higher than that in the southern hemisphere, which is consistent with the findings of Coxon et al (2016) and Workayehu et al (2019). Another feature seen here is that the intensity of R1 currents is stronger than for R2 currents at both dawn and Fig.…”

Section: Observationssupporting

confidence: 92%

Relationship between large-scale ionospheric field-aligned currents and electron/ion precipitations: DMSP observations

Xiong

Stolle

Alken

et al. 2020

Earth Planets Space

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In this study, we have derived field-aligned currents (FACs) from magnetometers onboard the Defense Meteorological Satellite Project (DMSP) satellites. The magnetic latitude versus local time distribution of FACs from DMSP shows comparable dependences with previous findings on the intensity and orientation of interplanetary magnetic field (IMF) By and Bz components, which confirms the reliability of DMSP FAC data set. With simultaneous measurements of precipitating particles from DMSP, we further investigate the relation between large-scale FACs and precipitating particles. Our result shows that precipitation electron and ion fluxes both increase in magnitude and extend to lower latitude for enhanced southward IMF Bz, which is similar to the behavior of FACs. Under weak northward and southward Bz conditions, the locations of the R2 current maxima, at both dusk and dawn sides and in both hemispheres, are found to be close to the maxima of the particle energy fluxes; while for the same IMF conditions, R1 currents are displaced further to the respective particle flux peaks. Largest displacement (about 3.5°) is found between the downward R1 current and ion flux peak at the dawn side. Our results suggest that there exists systematic differences in locations of electron/ion precipitation and large-scale upward/downward FACs. As outlined by the statistical mean of these two parameters, the FAC peaks enclose the particle energy flux peaks in an auroral band at both dusk and dawn sides. Our comparisons also found that particle precipitation at dawn and dusk and in both hemispheres maximizes near the mean R2 current peaks. The particle precipitation flux maxima closer to the R1 current peaks are lower in magnitude. This is opposite to the known feature that R1 currents are on average stronger than R2 currents.

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Section: Observationssupporting

confidence: 92%

Relationship between large-scale ionospheric field-aligned currents and electron/ion precipitations: DMSP observations

Xiong

Stolle

Alken

et al. 2020

Earth Planets Space

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“…The IMF B z dependence of FAC derived from GF1 compares well to those of previous publications (e.g., Wang et al 2008;Korth et al 2010;Milan et al 2017). Furthermore, the intensity of the FACs in the northern hemisphere is slightly higher than that in the southern hemisphere, which is consistent with the finding of Coxon et al (2016) derived from AMPERE data, Laundal et al (2018) and Workayehu et al (2019) derived from Swarm observations, and with Xiong et al (2020) derived from DMSP observations. These plots show that also small amplitudes near the noise level of GRACE-FO data are well accessible when they are applied in a statistical approach.…”

Section: Auroral Field-aligned Currentssupporting

confidence: 90%

Observing Earth’s magnetic environment with the GRACE-FO mission

Stolle

Michaelis

Xiong

et al. 2021

Earth Planets Space

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The Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission carries magnetometers that are dedicated to enhance the satellite’s navigation. After appropriate calibration and characterisation of artificial magnetic disturbances, these observations are valuable assets to characterise the natural variability of Earth’s magnetic field. We describe the data pre-processing, the calibration, and characterisation strategy against a high-precision magnetic field model applied to the GRACE-FO magnetic data. During times of geomagnetic quiet conditions, the mean residual to the magnetic model is around 1 nT with standard deviations below 10 nT. The mean difference to data of ESA’s Swarm mission, which is dedicated to monitor the Earth’s magnetic field, is mainly within ± 10 nT during conjunctions. The performance of GRACE-FO magnetic data is further discussed on selected scientific examples. During a magnetic storm event in August 2018, GRACE-FO reveals the local time dependence of the magnetospheric ring current signature, which is in good agreement with results from a network of ground magnetic observations. Also, derived field-aligned currents (FACs) are applied to monitor auroral FACs that compare well in amplitude and statistical behaviour for local time, hemisphere, and solar wind conditions to approved earlier findings from other missions including Swarm. On a case event, it is demonstrated that the dual-satellite constellation of GRACE-FO is most suitable to derive the persistence of auroral FACs with scale lengths of 180 km or longer. Due to a relatively larger noise level compared to dedicated magnetic missions, GRACE-FO is especially suitable for high-amplitude event studies. However, GRACE-FO is also sensitive to ionospheric signatures even below the noise level within statistical approaches. The combination with data of dedicated magnetic field missions and other missions carrying non-dedicated magnetometers greatly enhances related scientific perspectives.

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“…Hemispheric asymmetry in currents at high latitudes has been reported in several previous studies (Coxon et al, 2016; Green et al, 2009; Huang et al, 2017; Laundal et al, 2016; Milan et al, 2017; Workayehu et al, 2019). Green et al (2009) found larger average currents in the NH than SH using the cross‐track component of magnetic field data measured by Iridium satellites.…”

Section: Introductionsupporting

confidence: 60%

“…The SECS method (Amm et al, 2015; Juusola et al, 2016; Vanhamäki et al, 2020; Workayehu et al, 2019) is used to estimate FAC and horizontal currents. For estimating currents using the Swarm/SECS analysis method, we first discard magnetic data poleward of ±80° geographic latitude, where the longitudinal separation between Swarm A and Swarm C becomes too small for a reliable current estimation using this method.…”

Section: Discussionmentioning

confidence: 99%

“…Recently, Workayehu et al (2019), hereafter referred to as Paper 1, presented statistical distributions of the auroral current systems, FACs as well as the horizontal CF and DF currents, both in the NH and SH during low (Kp < 2) and high (Kp ≥ 2) geomagnetic activity conditions by using Swarm satellite data. We examined Hemispheric differences of auroral currents averaged over all local seasons in terms of the north to south ratios of integrated FACs and averaged horizontal currents.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

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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Field‐Aligned and Horizontal Currents in the Northern and Southern Hemispheres From the Swarm Satellite

Cited by 22 publications

References 42 publications

Relationship between large-scale ionospheric field-aligned currents and electron/ion precipitations: DMSP observations

Relationship between large-scale ionospheric field-aligned currents and electron/ion precipitations: DMSP observations

Observing Earth’s magnetic environment with the GRACE-FO mission

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

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