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We present observations of mirror mode structures in the dawn-side magnetosphere from the Magnetospheric Multiscale Mission. The observations were made during a period of relatively stable northward interplanetary magnetic field conditions. We observed magnetic troughs only with reductions of up to 90% in the magnitude of the local magnetic field. The structures were closely aligned with the local magnetic field and had sizes perpendicular to the magnetic field of the order of 1Re and ion densities up to twice that of the surrounding regions. We used ion, magnetic field, and electric field data to examine the plasma conditions inside and outside of these structures to investigate the stability and evolution of the structures and estimate the magnitude of changes in the ion distributions. Comparing the ion bulk properties to theoretical predictions, we concluded that variations in the parameters with trough depth were due to the spacecraft traversing them at different locations along their length. Our calculations of heating and cooling of the ions at different pitch angles is consistent with theory. We conclude that these troughs were likely long-lived, bistable structures that originated in the magnetosheath near the nose of the magnetopause and were captured into the magnetosphere by dual-lobe reconnection. [ABSTRACT FROM AUTHOR]

Flux transfer events (FTEs) are transient phenomena frequently observed at the Earth's magnetopause. Their usual interpretation is a flux rope moving away from the reconnection region. However, the Magnetospheric Multiscale Mission revealed that magnetic reconnection sometimes occurs inside these structures, questioning their flux rope configuration. Here we investigate 229 FTE‐type structures and find reconnection signatures inside 19% of them. We analyze their large‐scale magnetic topology using electron heat flux and find that it is significantly different across the FTE reconnecting current sheets, demonstrating that they are constituted of two magnetically disconnected structures. We also find that the interplanetary magnetic field (IMF) associated with reconnecting FTEs presents a strong By component. We discuss several formation mechanisms to explain these observations. In particular, the maximum magnetic shear model predicts that for large IMF By, two spatially distinct X lines coexist at the magnetopause. They can generate separate magnetic flux tubes that may become interlaced. Plain Language Summary: The solar wind and the Earth's magnetosphere are two gigantic magnetic structures that collide constantly over our heads, in the near‐space environment. At the boundary of their interaction (the magnetopause), the fundamental process of magnetic reconnection can occur. It is there that dynamic magnetic structures called "flux transfer events" are formed. They travel fast along the magnetopause and transport a lot of energy, from the solar wind into the magnetosphere. These structures are yet not well understood, as underlined by the recent observations made by the Magnetospheric Multiscale Mission (MMS), launched in 2015 by National Aeronautics and Space Administration. The four‐spacecraft mission, specifically designed to study the physics happening at the magnetopause, is capable of measuring right into these magnetic structures, collecting data on their particles and magnetic field properties. When analyzing MMS data, we found that 19% of the flux transfer events were not constituted of one, but two structures with very different properties. These dual magnetic structures tend to appear when the solar wind's magnetic field is oriented mainly toward the east or the west. From these observations and based on existing models of the magnetopause, we propose a scenario that allows such dual structures to form as interlaced magnetic tubes. Key Points: Nineteen percent of FTE‐type structures observed by MMS during Phases 1A and 1B present signatures of magnetic reconnection in their coreThey seem to be formed by two magnetically disconnected interlaced flux tubes and are typically observed for large IMF BySeveral formation models are discussed, including a bifurcated X line scenario that results from the maximum shear angle model [ABSTRACT FROM AUTHOR]

We present a new method for determining the main relevant features of the local magnetic field configuration, based entirely on the knowledge of the magnetic field gradient four‐spacecraft measurements. The method, named "magnetic configuration analysis" (MCA), estimates the spatial scales on which the magnetic field varies locally. While it directly derives from the well‐known magnetic directional derivative and magnetic rotational analysis procedures (Shi et al., 2005, htpps://doi.org/10.1029/2005GL022454; Shen et al., 2007, https://doi.org/10.1029/2005JA011584), MCA was specifically designed to address the actual magnetic field geometry. By applying MCA to multispacecraft data from the Magnetospheric Multiscale (MMS) satellites, we perform both case and statistical analyses of local magnetic field shape and dimensionality at very high cadence and small scales. We apply this technique to different near‐Earth environments and define a classification scheme for the type of configuration observed. While our case studies allow us to benchmark the method with those used in past works, our statistical analysis unveils the typical shape of magnetic configurations and their statistical distributions. We show that small‐scale magnetic configurations are generally elongated, displaying forms of cigar and blade shapes, but occasionally being planar in shape like thin pancakes (mostly inside current sheets). Magnetic configurations, however, rarely show isotropy in their magnetic variance. The planar nature of magnetic configurations and, most importantly, their scale lengths strongly depend on the plasma β parameter. Finally, the most invariant direction is statistically aligned with the electric current, reminiscent of the importance of electromagnetic forces in shaping the local magnetic configuration. Key Points: We define and use a four‐spacecraft method on MMS data to study the magnetic field configuration at small scales in near Earth regionsCase studies demonstrate the ability of the method to measure local magnetic configuration, benchmarking the method on previous studiesStatistically, low plasma betas are correlated with large length scales and slightly more frequent planar shapes in magnetic configurations [ABSTRACT FROM AUTHOR]