Explosive magnetotail activity has long been understood in the context of its auroral manifestations. While global models have been used to interpret and understand many magnetospheric processes, the ...temporal and spatial scales of some auroral forms have been inaccessible to global modeling creating a gulf between observational and theoretical studies of these phenomena. We present here an important step toward bridging this gulf using a newly developed global magnetosphere‐ionosphere model with resolution capturing
≲ 30 km azimuthal scales in the auroral zone. In a global magnetohydrodynamic (MHD) simulation of the growth phase of a synthetic substorm, we find the self‐consistent formation and destabilization of localized magnetic field minima in the near‐Earth magnetotail. We demonstrate that this destabilization is due to ballooning‐interchange instability which drives earthward entropy bubbles with embedded magnetic fronts. Finally, we show that these bubbles create localized field‐aligned current structures that manifest in the ionosphere with properties matching observed auroral beads.
Plain Language Summary
The aurora has long been used as a window onto the magnetosphere. However, auroral observations are inherently limited in trying to reconstruct global magnetospheric dynamics from the “magnetic shadow” they cast on Earth. For this reason modeling has been used in tandem with observations to better contextualize and understand the data. Substorms, the violent reconfiguration of the magnetotail and one of the most dynamic magnetospheric phenomena, have been known to be preceded by the formation of bead‐like structures in the aurora. The processes responsible for auroral beading and their causal versus correlative role with substorm onset have remained an enduring mystery. The vast disparity between the spatial scales of auroral beads and those of the global magnetosphere has greatly complicated the use of modeling in unraveling this mystery. We show here for the first time a demonstration of the self‐consistent formation of a magnetospheric configuration that becomes unstable during the period preceding the substorm onset and that this instability manifests in the ionosphere with similar morphology to auroral beads. The global context of the model shows that the magnetospheric processes responsible for beading are not necessarily causal to onset but a consequence of the slow magnetotail reconfiguration that precedes onset.
Key Points
We present the first global magnetosphere simulation to reveal ballooning‐interchange instability of a narrow Bz minimum in the near‐Earth magnetotail
The instability is prominent during the substorm growth phase and generates earthward entropy bubbles with embedded magnetic fronts
The bubbles drive mesoscale ionospheric field‐aligned currents and auroral structures (beads) with properties matching to those observed
Kinetic aspects of energy conversion and dissipation near a dipolarization front (DF) in the magnetotail are considered using fully kinetic 3‐D particle‐in‐cell simulations. The energy conversion is ...described in terms of the pressure dilatation, as well as the double contraction of deviatoric pressure tensor and traceless strain rate tensor, also known as the Pi‐D parameter in turbulence studies. It is shown that in contrast to the fluid dissipation measure, the Joule heating rate, which cannot distinguish between ion and electron dissipation and reveals deep negative dips at the DF, the Pi‐D parameters, as kinetic analogs of the Joule heating rate, are largely positive and drastically different for ions and electrons. Further analysis of these parameters suggests that ions are heated at and ahead of the DF due to their reflection from the front, while electrons are heated at and behind the DF due to the long‐wavelength lower‐hybrid drift instability.
Plain Language Summary
We explore new measures of plasma dissipation in rapidly contracting tubes of magnetic flux and plasma on the nightside of the terrestrial magnetosphere. These contracting tubes make the stretched tail‐like magnetic field more dipolar and have sharp profiles of plasma density and magnetic field at the leading edge. Relaxation of the stretched magnetic field releases the energy, which is spent for plasma acceleration and heating. Since collisions are extremely rare, the energy dissipation processes are different for electrons and ions and hence require special quantitative measures. Here we derive such measures from massively parallel three‐dimensional particle‐in‐cell simulations of the tail plasmas and demonstrate that as expected for measures of dissipation, they are positive on average and different for ions and electrons. The new quantitative measures allow us to reveal specific physical processes responsible for energy dissipation.
Key Points
Newly derived kinetic dissipation parameters are largely positive and different for ions and electrons
Ion dissipation is dominated by ion reflection from fronts
Electron dissipation is dominated by the lower‐hybrid drift instability
We present full‐particle simulations of 2‐D magnetotail current sheet equilibria with open boundaries and zero driving. The simulations show that spontaneous formation of dipolarization fronts and ...subsequent formation of magnetic islands are possible in equilibria with an accumulation of magnetic flux at the tailward end of a sufficiently thin current sheet. These results confirm recent findings in the linear stability of the ion tearing mode, including the predicted dependence of the tail current sheet stability on the amount of accumulated magnetic flux expressed in terms of the specific destabilization parameter. The initial phase of reconnection onset associated with the front formation represents a process of slippage of magnetic field lines with frozen‐in electrons relative to the ion plasma species. This non‐MHD process characterized by different motions of ion and electron species generates a substantial charge separation electric field normal to the front.
Key Points
Spontaneous reconnection onset is possible in the magnetotail
Onset conditions are consistent with the tearing stability theory
Magnetotail reconnection starts from the formation of a dipolarization front
A high‐resolution global magnetohydrodynamic simulation is conducted with the Lyon‐Fedder‐Mobarry (LFM) model for idealized solar wind conditions. Within the simulation results high‐speed flows are ...seen throughout the magnetotail when the interplanetary magnetic field (IMF) is southward. Case study analysis of these flows shows that they have an enhancement in BZ and a decrease in density preceding a peak in the flow velocity. A careful examination of the structure within the magnetotail shows that these features are driven by bursts of magnetic reconnection. In addition to the case study, a superposed epoch analysis of flows occurring during a 90 min interval of southward IMF yields statistical properties that are in qualitative agreement with observational analysis of bursty bulk flows (BBFs). For the idealized simulation conditions, the most significant differences with the observational results are a broader velocity profile in time, which becomes narrower away from the center of the current sheet, and a larger density drop after flow passage. The peak BZ amplitude is larger than in observations and precedes the peak in the flow velocity. We conclude that the LFM simulations are reproducing the statistical features of BBFs and that they are driven by spatially and temporally localized reconnection events within the simulation domain.
Key Points
High‐resolution LFM simulations contain BBFs
BBFs have statistical properties similar to observations
BBFs are generated by reconnection within simulation
Much of plasma heating and transport from the magnetotail into the inner magnetosphere occurs in the form of mesoscale discrete injections associated with sharp dipolarizations of magnetic field ...(dipolarization fronts). In this paper we investigate the role of magnetic trapping in acceleration and transport of the plasma sheet ions into the ring current. For this purpose we use high‐resolution global magnetohydrodynamic (MHD) and three‐dimensional test‐particle simulations. It is shown that trapping, produced by sharp magnetic field gradients at the interface between dipolarizations and the ambient plasma, affects plasma sheet protons with energies above approximately 10 keV, enabling their transport across more than 10 Earth radii and acceleration by a factor of 10. Our estimates show that trapping is important to the buildup of the ring current plasma pressure of injected particles; depending on the plasma sheet temperature and energy spectrum, trapped protons can contribute between 20% and 60% of the plasma pressure. It is also shown that the acceleration process does not conserve the particle first invariant; on average protons are accelerated to higher energies compared to a purely adiabatic process. We also investigate how trapping and energization vary for deferent ions species and show that in accordance with recent observations, ion acceleration is proportional to the ion charge and is independent of its mass.
Key Points
Energetic protons can be trapped at dipolarization fronts, which enables their transport from the tail to the inner magnetosphere and violates the first invariant
Trapping is important for the buildup of ion pressure in the inner magnetosphere
Acceleration of trapped ions is proportional to ion charge and is independent of mass
To evaluate whether social contacts, support, and social strain/conflict are related to executive function and memory abilities in middle-age and older adults.
Longitudinal data on social contacts, ...support, and strain/conflict were examined in relation to executive function and memory at ages 35-85 years using data from the national Midlife in the U.S. (MIDUS) study. Age-related differences in patterns of association were also examined.
Regression analyses, controlling for age, sex, race, education, chronic health conditions, and health behaviors, revealed significant positive associations between histories of greater social contacts and support and both executive function and episodic memory, whereas declines in social contacts were negatively associated with both outcomes. Greater average reported frequency of social exchanges characterized by strain or conflict was negatively associated with executive function but not episodic memory. Patterns were generally consistent across different age groups; where differences were seen, associations were stronger in younger age group.
Positive and negative aspects of social relationships are related to cognition throughout adulthood, consistent with the hypothesis that social factors have life-long influences on cognition. Positive and negative aspects of social engagement may thus be important factors to consider in relation to efforts to promote optimal cognitive development and cognitive aging.
We investigated trends in disability among older Americans from 1988 through 2004 to test the hypothesis that more recent cohorts show increased burdens of disability.
We used data from 2 National ...Health and Nutrition Examination Surveys (1988-1994 and 1999-2004) to assess time trends in basic activities of daily living, instrumental activities, mobility, and functional limitations for adults aged 60 years and older. We assessed whether changes could be explained by sociodemographic, body weight, or behavioral factors.
With the exception of functional limitations, significant increases in each type of disability were seen over time among respondents aged 60 to 69 years, independent of sociodemographic characteristics, health status, relative weight, and health behaviors. Significantly greater increases occurred among non-Whites and persons who were obese or overweight (2 of the fastest-growing subgroups within this population). We detected no significant trends among respondents aged 70 to 79 years; in the oldest group (aged>or=80 years), time trends suggested lower prevalence of functional limitations among more recent cohorts.
Our results have significant and sobering implications: older Americans face increased disability, and society faces increased costs to meet the health care needs of these disabled Americans.
A key process in the interaction of magnetospheres with the solar wind is the explosive release of energy stored in the magnetotail. Based on observational evidence, magnetic reconnection is widely ...believed to be responsible. However, the very possibility of spontaneous reconnection in collisionless magnetotail plasmas has been questioned in kinetic theory for more than three decades. In addition, in situ observations by multispacecraft missions (e.g., THEMIS) reveal the development of buoyancy and flapping motions coexisting with reconnection. Never before have kinetic simulations reproduced all three primary modes in realistic 2‐D configurations with a finite normal magnetic field. Moreover, 3‐D simulations with closed boundaries suggest that the tail activity is dominated by buoyancy‐driven instabilities, whereas reconnection is a secondary effect strongly localized in the dawn‐dusk direction. In this paper, we use massively parallel 3‐D fully kinetic simulations with open boundaries to show that sufficiently far from the planet explosive processes in the tail are dominated by reconnection motions. These motions occur in the form of spontaneously generated dipolarization fronts accompanied by changes in magnetic topology which extend in the dawn‐dusk direction over the size of the simulation box, suggesting that reconnection onset causes a macroscale reconfiguration of the real magnetotail. In our simulations, buoyancy and flapping motions significantly disturb the primary dipolarization front but neither destroy it nor change the near 2‐D picture of the front evolution critically. Consistent with recent multiprobe observations, dipolarization fronts are also found to be the main regions of energy conversion in the magnetotail.
Key PointsMagnetotail bursts are dominated by reconnection far from the planetSpontaneous growth of reconnection motions in the form of dipolarization frontsFlapping reproduced for the first time in kinetic simulations of the magnetotail
This article updates trends from five national U.S. surveys to determine whether the prevalence of activity limitations among the older population continued to decline in the first decade of the ...twenty-first century. Findings across studies suggest that personal care and domestic activity limitations may have continued to decline for those ages 85 and older from 2000 to 2008, but generally were flat since 2000 for those ages 65-84. Modest increases were observed for the 55- to 64-year-old group approaching late life, although prevalence remained low for this age group. Inclusion of the institutional population is important for assessing trends among those ages 85 and older in particular.
Substorm‐type evolution of the Earth's magnetosphere is investigated by mining more than two decades (1995–2017) of spaceborne magnetometer data from multiple missions including the first two years ...(2016‐2017) of the Magnetospheric MultiScale mission. This investigation reveals interesting features of plasma evolution distinct from ideal magnetohydrodynamics (MHD) behavior: X‐lines, thin current sheets, and regions with the tailward gradient of the equatorial magnetic field Bz. X‐lines are found to form mainly beyond 20 RE, but for strong driving, with the solar wind electric field exceeding ∼5mV/m, they may come closer. For substorms with weaker driving, X‐lines may be preceded by redistribution of the magnetic flux in the tailward Bz gradient regions, similar to the magnetic flux release instability discovered earlier in PIC and MHD simulations as a precursor mechanism of the reconnection onset. Current sheets in the growth phase may be as thin as 0.2 RE, comparable to the thermal ions gyroradius, and at the same time, as long as 15 RE. Such an aspect ratio is inconsistent with the isotropic force balance for observed magnetic field configurations. These findings can help resolve kinetic mechanisms of substorm dipolarizations and adjust kinetic generalizations of global MHD models of the magnetosphere. They can also guide and complement microscale analysis of nonideal effects.
Plain Language Summary
The sun emits a steam of charged particles called the solar wind that flows past the Earth interacting with the planet's dipole magnetic field. This stretches the dipolar magnetic field away from the sun on the nightside of the planet storing energy in the stretched field. Once every few hours, this stretched configuration suddenly becomes more dipolar bringing particles and magnetic flux closer to the planet and powering aurora in the polar regions. During these processes, termed substorms, the gas of charged particles, protons, and electrons trapped by the dipole and known as plasma, behaves largely as a perfectly conducting fluid. However, only deviations from this ideal conducting plasma behavior can explain the substorm mechanisms. We mine two decades of spacecraft magnetometer data from multiple missions to form swarms of thousands of synthetic probes. They help reveal effects of nonideal plasma evolution during substorms, which cannot be captured by direct in situ observations because of their extreme paucity.
Key Points
X‐lines, including the 11 July 2017 reconnection event, are reconstructed at and beyond 20 RE, but for strong driving they can come closer
Current sheets in the growth phase may be as thin as 0.2 RE, and at the same time, as long as 15 RE, violating isotropic force balance
Bz humps form in the growth phase, and their reconfiguration may precede X‐line formation and substorm onset
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