Effect of Interplanetary Magnetic Field on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents During Different Seasons

Workayehu, Abiyot; Vanhamäki, Heikki; Aikio, Anita; Shepherd, Simon

doi:10.1029/2021ja029475

Cited by 13 publications

(25 citation statements)

References 44 publications

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“…Regarding the overall higher degree of symmetry exhibited by total Birkeland and divergence-free currents in each hemisphere for B z < 0 relative to B z > 0, Workayehu et al (2021) draw essentially the same conclusion using Swarm satellite measurements, but a rather different methodology based on spherical elementary currents. Regarding the overall variation of the NH/SH ratio of total Birkeland current with season, Workayehu et al (2020) also find that NH total Birkeland current is greater by about 20% during local winter.…”

Section: With Respect To Dependence On Imf Orientation Bothmentioning

confidence: 55%

Testing the mirror symmetry of Birkeland and ionospheric currents with respect to magnetic latitude, dipole tilt angle, and IMF By

Hatch

Laundal

Reistad

2022

Front. Astron. Space Sci.

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It is often assumed that on average, polar ionospheric electrodynamics in the Northern and Southern Hemispheres are mirror symmetric or antisymmetric with respect to the interplanetary magnetic field By component and the dipole tilt angle ψ. For example, one might assume that the average Birkeland current density j at magnetic latitude λ is equal to the current density at magnetic latitude −λ if the signs of By and ψ are reversed and all other parameters are equal: j(λ, By, ψ, … ) = j(−λ, −By, −ψ, … ). This is a convenient assumption for empirical models, since it effectively doubles the amount of information that a measurement made in one hemisphere contains. In this study we use the Average Magnetic field and Polar current System (AMPS) model to quantify to what extent the assumption holds for Birkeland and ionospheric currents. The AMPS model is an empirical model based on Swarm and CHAMP magnetic field measurements, with no constraints on hemispheric symmetries, and with differences in main magnetic field geometry as well as biases in data point distributions in magnetic coordinates accounted for. We show that when averaged over IMF clock angle orientation, the total ionospheric divergence-free current in each hemisphere largely satisfies the mirror symmetry assumption. The same is true for the total Birkeland current in each hemisphere except during local winter, during which the Northern Hemisphere tends to dominate. We show that this local winter asymmetry is consistent with the average winter hemispheric asymmetry in total precipitating electron current derived from Fast Auroral SnapshoT (FAST) satellite observations. We attribute this and other more subtle deviations from symmetry to differences in sunlight distribution in magnetic coordinates, as well as magnetic field strength and its influence on ionospheric conductivity. Important departures from mirror symmetry also arise for some IMF clock angle orientations, particularly those for which IMF Bz > 0, as suggested by other recent studies.

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Section: With Respect To Dependence On Imf Orientation Bothmentioning

confidence: 55%

Testing the mirror symmetry of Birkeland and ionospheric currents with respect to magnetic latitude, dipole tilt angle, and IMF By

Hatch

Laundal

Reistad

2022

Front. Astron. Space Sci.

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“…Recent studies have revealed that IMF B y effects are complex and seasonally varying, showing dependence on the dipole tilt angle. The combined dependence on IMF B y and the dipole tilt (also called the explicit B y ‐dependence) strongly modulates auroral electrojets (Friis‐Christensen et al., 2017; Holappa & Mursula, 2018; Holappa et al., 2021; Workayehu et al., 2021), electron precipitation (Holappa et al., 2020), and the size of polar cap (Reistad et al., 2020). These effects are quite significant, for example, showing variations in the AL index up to 40% for opposite values of B y .…”

Section: Discussionmentioning

confidence: 99%

“…For example, a negative IMF B y component results in larger nightside auroral intensity in the Northern Hemisphere (NH; Liou et al., 1998; Shue et al., 2001) while the effect is reversed in the southern hemisphere (Liou & Mitchell, 2019). On dayside, the postnoon auroral bright spot is brighter for negative IMF B y than for positive IMF B y and the effect is reversed in the southern hemisphere (Liou & Mitchell, 2019) Also several studies (Friis‐Christensen et al., 1972, 2017; Holappa & Mursula, 2018; Murayama et al., 1980; Smith et al., 2017; Workayehu et al., 2021; Holappa et al., 2021) have shown that there is a strong IMF B y ‐dependence in auroral currents which is not symmetric with the B y sign. This so‐called explicit B y ‐dependence is especially strong in the AL index (measuring the westward electrojet), which is about 40% stronger for B y > 0 than for B y < 0 in NH winter, or under negative tilt angle of the Earth's magnetic dipole with respect to the Sun‐Earth line.…”

Section: Introductionmentioning

confidence: 96%

Explicit IMF B_y‐Dependence of Energetic Protons and the Ring Current

Holappa

Buzulukova

2022

Geophysical Research Letters

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The interaction between solar wind, interplanetary magnetic field (IMF) and the Earth's magnetic field is dominated by the north-south (B z ) component of IMF, which is the most important driver of dayside reconnection (Dungey, 1961), and thus the energy input into the magnetosphere. The dawn-dusk (B y ) component of IMF is also known play an important role, leading, for example, to a B y -dependence of the ionospheric convection patterns (

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“…Workayehu et al. (2021) compared the distributions of the auroral current density in different seasons for four IMF clock angles, and found that the field‐aligned current density near the cusp region is quite larger for negative B y in northern summer. They suggested that the influence of the IMF B y on the auroral current systems depends on the season, which is essentially consistent with ours.…”

Section: Discussionmentioning

confidence: 99%

“…These results illustrate that the 2-nT deviation of the IMF |B y | is an average effect of the season, which implies that the IMF controlling effects on the cusp auroral intensity can be affected by the season. Workayehu et al (2021) compared the distributions of the auroral current density in different seasons for four IMF clock angles, and found that the field-aligned current density near the cusp region is quite larger for negative B y in northern summer. They suggested that the influence of the IMF B y on the auroral current systems depends on the season, which is essentially consistent with ours.…”

Section: The Interhemispheric Asymmetry and Seasonal Dependence Of Th...mentioning

confidence: 99%

A Comparative Study on the Factors Controlling the Cusp Auroral Intensity Between the Northern and Southern Hemispheres

et al. 2022

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The previous study has indicated that the interplanetary magnetic field (IMF) By and solar wind flow speed (Vsw) have crucial effects on the cusp auroral intensity in the Northern Hemisphere. However, it is not clear how these factors influence the cusp aurora in the Southern Hemisphere. Using DMSP/SUSSI observations from 2004 to 2017 during non‐storm periods, we compared the factors affecting the intensity of cusp aurora between the Northern and Southern Hemispheres by dividing the observations into weak‐ and intense‐emission events. We found that the auroral intensity in the southern cusp: (a) has a positive correlation with solar wind power input, (b) is statistically weak under southward IMF conditions, and (c) is critically controlled by the IMF By and Vsw. These results are essentially consistent with those obtained in the Northern Hemisphere. However, the IMF By effect shows apparent interhemispheric asymmetry, that is, the IMF By effect reaches the maximum at 2 nT in the Northern Hemisphere, but at −2 nT in the Southern Hemisphere. Further analysis shows that the 2‐nT deviations disappear after dividing all events by season and the IMF By distributions of intense‐emission events show clear seasonal difference. Based on these results, we suggest that the dependence of the cusp auroral intensity on the IMF By is not only controlled by −Vsw × By field, but also affected by season.

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Effect of Interplanetary Magnetic Field on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents During Different Seasons

Abstract: Effect of interplanetary magnetic field on hemispheric asymmetry in ionospheric horizontal and field-aligned currents during different seasons.

Cited by 13 publications

References 44 publications

Testing the mirror symmetry of Birkeland and ionospheric currents with respect to magnetic latitude, dipole tilt angle, and IMF By

Testing the mirror symmetry of Birkeland and ionospheric currents with respect to magnetic latitude, dipole tilt angle, and IMF By

Explicit IMF B_y‐Dependence of Energetic Protons and the Ring Current

A Comparative Study on the Factors Controlling the Cusp Auroral Intensity Between the Northern and Southern Hemispheres

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Effect of Interplanetary Magnetic Field on Hemispheric Asymmetry in Ionospheric Horizontal and Field‐Aligned Currents During Different Seasons

Abstract: Effect of interplanetary magnetic field on hemispheric asymmetry in ionospheric horizontal and field-aligned currents during different seasons.

Cited by 13 publications

References 44 publications

Testing the mirror symmetry of Birkeland and ionospheric currents with respect to magnetic latitude, dipole tilt angle, and IMF By

Testing the mirror symmetry of Birkeland and ionospheric currents with respect to magnetic latitude, dipole tilt angle, and IMF By

Explicit IMF By‐Dependence of Energetic Protons and the Ring Current

A Comparative Study on the Factors Controlling the Cusp Auroral Intensity Between the Northern and Southern Hemispheres

Contact Info

Product

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Explicit IMF B_y‐Dependence of Energetic Protons and the Ring Current