Search

This study presents a re‐evaluation of the Kullen and Janhunen (2004, https://doi.org/10.5194/angeo-22-951-2004) global northward interplanetary magnetic field (IMF) simulation, using the Grand Unified Magnetosphere–Ionosphere Coupling Simulation version 4 (GUMICS‐4), a global MHD model. We investigate the dynamic coupling between northward IMF conditions and the Earth's magnetotail and compare the results to observation‐based mechanisms for the formation of transpolar arcs. The results of this study reveal that under northward IMF conditions (and northward IMF initialization), a large closed field line region forms in the magnetotail, with similarities to transpolar arc structures observed from spacecraft data. This interpretation is supported by the simultaneous increase of closed flux measured in the magnetotail. However, the reconnection configuration differs in several respects from previously theorized magnetotail structures that have been inferred from both observations and simulations results and associated with transpolar arcs. We observe that dawn–dusk lobe regions form as a result of high‐latitude reconnection during the initialization stages, which later come into contact as the change in the IMF By component causes the magnetotail to twist. We conclude that in the GUMICS simulation, transpolar arc‐like structures are formed as a result of reconnection in the magnetotail, rather than high‐latitude reconnection or due to the mapping of the plasma sheet through a twisted magnetotail as interpreted from previous analysis of GUMICS simulations. Plain Language Summary: When the magnetic field associated with the solar wind is directed "northward," the Earth's aurora can adopt a formation where a "bar" of emission crosses the otherwise dim region that lies poleward of the usual auroral emissions. These phenomena are called "theta auroras" (due to their resemblance to the Greek letter), or "transpolar arcs," and have been observed from spacecraft observing the auroral regions. As spacecraft cannot directly observe the global configuration of magnetic field lines within planetary magnetospheres, simulations can be useful to gain insight into the possible structures that occur during different solar wind conditions. We use a global simulation to model the interaction between the solar wind and the Earth's magnetosphere (the region of space around our planet) and see that a large‐scale field line structure can form during certain northward‐directed magnetic field conditions. When we trace these field lines to the ionospheric boundary, we find that they resemble the global structures that have been observed for auroral arcs that stretch across the polar cap (also named theta aurora or transpolar arcs). We also note some key differences in their formation process from observational results. Key Points: Signatures consistent with magnetotail reconnection are observed in a simulation driven by northward interplanetary magnetic field conditionsThe simulation results in the production of a closed magnetotail structure and polar cap arcsThe above structure occurs within a magnetotail that is highly twisted such that open lobe regions cross the equatorial plane [ABSTRACT FROM AUTHOR]

One of the most significant observations associated with a sharp enhancement in solar wind dynamic pressure, PSW, is the poleward expansion of the auroral oval and the closing of the polar cap. The polar cap shrinking over a wide range of magnetic local times (MLTs), in connection with an observed increase in ionospheric convection and the transpolar potential, led to the conclusion that the nightside reconnection rate is significantly enhanced after a pressure front impact. However, this enhanced tail reconnection has never been directly measured. We demonstrate the effect of a solar wind dynamic pressure front on the polar cap closure, and for the first time, measure the enhanced reconnection rate in the magnetotail, for a case occurring during southward background Interplanetary Magnetic Field (IMF) conditions. We use Polar Ultra‐Violet Imager (UVI) measurements to detect the location of the open‐closed field line boundary, and combine them with Assimilative Mapping of Ionospheric Electrodynamics (AMIE) potentials to calculate the ionospheric electric field along the polar cap boundary, and thus evaluate the variation of the dayside/nightside reconnection rates. We find a strong response of the polar cap boundary at all available MLTs, exhibiting a significant reduction of the open flux content. We also observe an immediate response of the dayside reconnection rate, plus a phased response, delayed by ∼15–20 min, of the nightside reconnection rate. Finally, we provide comparison of the observations with the results of the Open Geospace General Circulation Model (OpenGGCM), elucidating significant agreements and disagreements. Plain Language Summary: This study provides valuable information on how the Earth's magnetosphere (the magnetized protective bubble around the Earth) is eroded by powerful explosions at the Sun. The response of the polar cap size and the reconnection rates in the magnetosphere to a solar wind high density front are investigated for an event with Interplanetary Magnetic Field orientation anti‐parallel to the Earth's magnetic field. We make a data‐based assessment, using Polar spacecraft ultraviolet images and assimilative model‐generated potentials, plus a model‐based comparison using a global magnetospheric model. An immediate response is observed at the dayside ionosphere for both the polar cap boundary and the reconnection rate mapped to the ionosphere. We also observe about 15–20 min delayed effect on various sectors of the nightside ionosphere. The comparison with the model reveals considerable discrepancies on the dayside ionosphere and significant agreements at the nightside ionosphere. Key Points: Effect of a solar wind dynamic pressure front on the open‐closed polar cap boundaryDetermination of the magnetic reconnection rate for a southward interplanetary magnetic field caseResponse of the dayside/nightside reconnection rate to a solar wind dynamic pressure front [ABSTRACT FROM AUTHOR]

A review of the impact of the cold-ion and cold-electron populations in the Earth's magnetosphere is presented. The cold populations are defined by total energy less than approximately 100 eV, i.e. in the energy range which is strongly affected by spacecraft charging and that often dominates the total plasma density. We also include the warm plasma cloak in the review, since it overlaps partially with the cold energy range and is a population that is still not well understood. The known impacts of cold ions and cold electrons that are discussed are: the source of hot magnetospheric plasma, solar-wind/magnetosphere coupling, magnetotail reconnection and substorms, Kelvin–Helmholtz instabilities on the magnetopause, chorus, hiss, electromagnetic-ion-cyclotron and ultra-low-frequency wave–particle interactions, aurora structuring and spacecraft charging. Other possible impacts are associated with refilling on open-drift trajectories, the remnant layer and plasmapause disruption. A discussion of the difficulty of cold-plasma measurements and the need for new measurement techniques that measure the full cold-ion and cold-electron distribution functions is also presented. There remain a lot of unknowns about the cold-ion and cold-electron populations, associated with their origin, properties, drivers and impacts. These populations will need to be fully understood before the magnetosphere–ionosphere system can be fully understood. • The cold-particle populations play a critical role in the Earth's magnetosphere. • This paper reviews their known and probable impacts in magnetospheric physics. • The need for new cold-plasma measurement techniques is also discussed. • The lack of understanding of the cold populations hinders magnetospheric research. [ABSTRACT FROM AUTHOR]