Finding reconnection lines and flux rope axes via local coordinates in global ion-kinetic magnetospheric simulations

Magnetic reconnection is a crucially important process for energy conversion in plasma physics, with the substorm cycle of Earth's magnetosphere and solar flares being prime examples. While 2D models have been widely applied to study reconnection, investigating reconnection in 3D is still, in many aspects, an open problem. Finding sites of magnetic reconnection in a 3D setting is not a trivial task, with several approaches, from topological skeletons to Lorentz transformations, having been proposed to tackle the issue. This work presents a complementary method for quasi-2D structures in 3D settings by noting that the magnetic field structures near reconnection lines exhibit 2D features that can be identified in a suitably chosen local coordinate system. We present applications of this method to a hybrid-Vlasov Vlasiator simulation of Earth's magnetosphere, showing the complex magnetic topologies created by reconnection for simulations dominated by quasi-2D reconnection. We also quantify the dimensionalities of magnetic field structures in the simulation to justify the use of such coordinate systems.</p

Seasonal effect on hemispheric asymmetry in ionospheric horizontal and field‐aligned currents

Abstract We present a statistical investigation of the seasonal effect on hemispheric asymmetry in the auroral currents during low (Kp &lt; 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

Field‐aligned and horizontal currents in the Northern and Southern Hemispheres from the Swarm satellite

Abstract We present statistical investigation of the high‐latitude ionospheric current systems in the Northern Hemisphere (NH) and Southern Hemisphere (SH) during low (Kp &lt; 2) and high (Kp ≥ 2) geomagnetic activity levels. Nearly 4 years of vector magnetic field measurements are analyzed from the two parallel flying Swarm A and C satellites using the spherical elementary current system method. The ionospheric horizontal and field‐aligned currents (FACs) for each auroral oval crossing are calculated. The distributions of the mean values of FACs as well as the horizontal curl‐free and divergence‐free currents in magnetic latitude and magnetic local time for each hemisphere and activity level are presented. To estimate the NH/SH current ratios for the two activity levels, we remove seasonal bias in the number of samples and in the Kp distribution by bootstrap resampling. This is done in such a manner that there are equal number of samples from each season in each Kp bin. We find that for the low activity level, the currents in the NH are stronger than in the SH by 12±4% for FAC, 9±2% for the horizontal curl‐free current, and 8±2% for the horizontal divergence‐free current. During the high activity level, the hemispheric differences are not statistically significant. This suggests that the local ionospheric conditions, such as magnetic field strength or daily variations in insolation, may be important and play a larger role during quiet than disturbed periods. This issue must be studied further

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

Abstract We present a statistical investigation of the effects of interplanetary magnetic field (IMF) on hemispheric asymmetry in auroral currents. Nearly 6 years of magnetic field measurements from Swarm A and C satellites are analyzed. Bootstrap resampling is used to remove the difference in the number of samples and IMF conditions between the local seasons and the hemispheres. Currents are stronger in Northern Hemisphere (NH) than Southern Hemisphere (SH) for IMF By⁺ in NH (By⁻ in SH) in most local seasons under both signs of IMF Bz. For By⁻ in NH (By⁺ in SH), the hemispheric difference in currents is small except in local winter when currents in NH are stronger than in SH. During By⁺ and Bz⁺ in NH (By⁻ and Bz⁺ in SH), the largest hemispheric asymmetry occurs in local winter and autumn, when the NH/SH ratio of field aligned current (FAC) is 1.18±0.09 in winter and 1.17±0.09 in autumn. During (By⁺ and Bz⁻ in NH (By⁻ and Bz⁻ in SH), the largest asymmetry is observed in local autumn with NH/SH ratio of 1.16±0.07 for FAC. We also find an explicit By effect on auroral currents in a given hemisphere: on average By⁺ in NH and By⁻ in SH causes larger currents than vice versa. The explicit By effect on divergence-free current during IMF Bz⁺ is in very good agreement with the By effect on the cross polar cap potential from the Super Dual Auroral Radar Network dynamic model except at SH equinox and NH summer

Swarm satellite and EISCAT radar observations of a plasma flow channel in the auroral oval near magnetic midnight

Abstract We present Swarm satellite and EISCAT radar observations of electrodynamical parameters in the midnight sector at high latitudes. The most striking feature is a plasma flow channel located equatorward of the polar cap boundary within the dawn convection cell. The flow channel is 1.5° wide in latitude and contains southward electric field of 150 mV/m, corresponding to eastward plasma velocities of 3,300 m/s in the F‐region ionosphere. The theoretically computed ion temperature enhancement produced by the observed ion velocity is in accordance with the measured one by the EISCAT radar. The total width of the auroral oval is about 10° in latitude. While the poleward part is electric field dominant with low conductivity and the flow channel, the equatorward part is conductivity dominant with at least five auroral arcs. The main part of the westward electrojet flows in the conductivity dominant part, but it extends to the electric field dominant part. According to Kamide and Kokubun (1996), the whole midnight sector westward electrojet is expected to be conductivity dominant, so the studied event challenges the traditional view. The flow channel is observed after substorm onset. We suggest that the observed flow channel, which is associated with a 13‐kV horizontal potential difference, accommodates increased nightside plasma flows during the substorm expansion phase as a result of reconnection in the near‐Earth magnetotail

Field-aligned and ionospheric currents by AMPERE and SuperMAG during HSS/SIR-driven storms

Abstract This study considers 28 geomagnetic storms with Dst ≤ −50 nT driven by high-speed streams (HSSs) and associated stream interaction regions (SIRs) during 2010–2017. Their impact on ionospheric horizontal and field-aligned currents (FACs) have been investigated using superposed epoch analysis of SuperMAG and AMPERE data, respectively. The zero epoch (t₀) was set to the onset of the storm main phase. Storms begin in the SIR with enhanced solar wind density and compressed southward oriented magnetic field. The integrated FAC and equivalent currents maximize 40 and 58 min after t₀, respectively, followed by a small peak in the middle of the main phase (t₀ + 4 hr), and a slightly larger peak just before the Dst minimum (t₀ + 5.3 hr). The currents are strongly driven by the solar wind, and the correlation between the Akasofu ε and integrated FAC is 0.90. The number of substorm onsets maximizes near t₀. The storms were also separated into two groups based on the solar wind dynamic pressure pdyn in the vicinity of the SIR. High pdyn storms reach solar wind velocity maxima earlier and have shorter lead times from the HSS arrival to storm onset compared with low pdyn events. The high pdyn events also have sudden storm commencements, stronger solar wind driving and ionospheric response at t₀, and are primarily responsible for the first peak in the currents after t₀. After t₀ + 2 days, the currents and number of substorm onsets become higher for low compared with high pdyn events, which may be related to higher solar wind speed

Effect of ICME-driven storms on field-aligned and ionospheric currents from AMPERE and SuperMAG

Abstract This study investigates the field-aligned currents (FACs) and ionospheric equivalent currents for interplanetary coronal mass ejection (ICME)-driven storms by considering 45 events with a minimum Dst ≤ −50 nT. The FACs and ionospheric equivalent currents are studied by applying a superposed epoch analysis to data from AMPERE and SuperMAG with the zero epoch (t₀) centered at the onset of the storm main phase. The currents and number of substorm onsets begin to increase 3 hr before t0 and maximizes about 1 hr after t₀. The currents and number of substorm onsets remain high throughout the entire storm main phase, until at t₀ + 14 hr they start to slowly relax back to quiet time conditions. The storms were separated into two groups based on the solar wind dynamic pressure pddyn around t₀. High pdyn storms are mostly driven by the sheath region ahead of the ejecta. These storms have short main phase durations and larger currents early in the main phase which maximize at t₀ + 50 min. The low pdyn group contains storms that start during the magnetic clouds (MC) and have gradually increasing currents that maximize at t₀ + 11 hr, close to the end of the storm main phase. For the first 4 hr of the storm main phase, the currents in sheath-driven storms are larger than for MC-driven storms. The Russell-McPherron effect is less important for ICME-driven storms where only 44% have a contribution, compared to 82% of high speed stream/stream interaction driven storms

An ephemeral red arc appeared at 68° MLat at a pseudo breakup during geomagnetically quiet conditions

Abstract Various subauroral optical features have been studied by analyzing data collected during periods of geomagnetic disturbances. Most events have been typically found at geomagnetic latitudes of 45–60°. In this study, however, we present a red arc event found at geomagnetic 68° north (L ≈ 7.1) in the Scandinavian sector during a period of geomagnetically quiet conditions within a short intermission between two high‐speed solar wind events. The red arc appeared to coincide with a pseudo breakup at geomagnetic 71–72°N and a rapid equatorward expansion of the polar cap. However, the red arc disappeared in approximately 7 min. Simultaneous measurements with the Swarm A/C satellites indicated the appearance of the red arc at the ionospheric trough minimum and a conspicuous enhancement of the electron temperature, suggesting the generation of the arc by heat flux. Since there are meaningful differences in the red arc features from already‐known subauroral optical features such as the stable auroral red (SAR) arc, we considered that the red arc is a new phenomenon. We suggest that the ephemeral red arc may represent the moment of SAR arc birth associated with substorm particle injection, which is generally masked by bright dynamic aurorae