A storm main phase can produce a rapid depletion of electron fluxes in the Earth's outer radiation belt and the pitch‐angle scattering by the electromagnetic ion cyclotron (EMIC) waves is one ...mechanism that might account for the electron losses. To efficiently scatter the bulk of the electron population, below 1–2 MeV, the EMIC waves would need to have significant power very near a heavy ion gyrofrequency. We present a wave event at the storm main phase and carefully examine the wave spectrum to identify the energy range of electrons scattered by the waves. The EMIC waves exhibit power right below the He+ gyrofrequency and we estimate that they can interact with electrons having energies as low as 400 keV producing rapid scattering at almost all pitch‐angle values on the time scales of seconds. Our statistical analysis suggests that this event is not an exception; the majority of EMIC waves can scatter electrons with energies under 2 MeV. Our results show that EMIC waves can be one of the dominant radiation belt loss mechanisms during the storm main phase.
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
Structured features on top of nominally smooth distributions of radiation-belt particles at Earth have been previously associated with particle acceleration and transport mechanisms powered ...exclusively by enhanced solar-wind activity. Although planetary rotation is considered to be important for particle acceleration at Jupiter and Saturn, the electric field produced in the inner magnetosphere by Earth's rotation can change the velocity of trapped particles by only about 1-2 kilometres per second, so rotation has been thought inconsequential for radiation-belt electrons with velocities of about 100,000 kilometres per second. Here we report that the distributions of energetic electrons across the entire spatial extent of Earth's inner radiation belt are organized in regular, highly structured and unexpected 'zebra stripes', even when the solar-wind activity is low. Modelling reveals that the patterns are produced by Earth's rotation. Radiation-belt electrons are trapped in Earth's dipole-like magnetic field, where they undergo slow longitudinal drift motion around the planet because of the gradient and curvature of the magnetic field. Earth's rotation induces global diurnal variations of magnetic and electric fields that resonantly interact with electrons whose drift period is close to 24 hours, modifying electron fluxes over a broad energy range into regular patterns composed of multiple stripes extending over the entire span of the inner radiation belt.
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
This paper addresses the question of the contribution of azimuthally localized flow channels and magnetic field dipolarizations embedded in them in the global dipolarization of the inner ...magnetosphere during substorms. We employ the high‐resolution Lyon‐Fedder‐Mobarry global magnetosphere magnetohydrodynamic model and simulate an isolated substorm event, which was observed by the geostationary satellites and by the Magnetospheric Multiscale spacecraft. The results of our simulations reveal that plasma sheet flow channels (bursty bulk flows, BBFs) and elementary dipolarizations (dipolarization fronts, DFs) occur in the growth phase of the substorm but are rare and do not penetrate to the geosynchronous orbit. The substorm onset is characterized by an abrupt increase in the occurrence and intensity of BBFs/DFs, which penetrate well earthward of the geosynchronous orbit during the expansion phase. These azimuthally localized structures are solely responsible for the global (in terms of the magnetic local time) dipolarization of the inner magnetosphere toward the end of the substorm expansion. Comparison with the geostationary satellites and Magnetospheric Multiscale data shows that the properties of the BBFs/DFs in the simulation are similar to those observed, which gives credence to the above results. Additionally, the simulation reveals many previously observed signatures of BBFs and DFs, including overshoots and oscillations around their equilibrium position, strong rebounds and vortical tailward flows, and the corresponding plasma sheet expansion and thinning.
Key Points
During substorm expansion all magnetic flux transport into the inner magnetosphere occurs via azimuthally localized earthward flows
Substorm onset is characterized by an abrupt increase in the number of such flows penetrating to the geosynchronous orbit
Properties of simulated bursty bulk flows/dipolarization fronts are similar to those observed including flux tube oscillations and rebounds
Magnetospheric substorms represent key explosive processes in the interaction of the Earth's magnetosphere with the solar wind, and their understanding and modeling are critical for space weather ...forecasting. During substorms, the magnetic field on the nightside is first stretched in the antisunward direction and then it rapidly contracts earthward bringing hot plasmas from the distant space regions into the inner magnetosphere, where they contribute to geomagnetic storms and Joule dissipation in the polar ionosphere, causing impressive splashes of aurora. Here we show for the first time that mining millions of spaceborne magnetometer data records from multiple missions allows one to reconstruct the global 3‐D picture of these stretching and dipolarization processes. Stretching results in the formation of a thin (less than the Earth's radius) and strong current sheet, which is diverted into the ionosphere during dipolarization. In the meantime, the dipolarization signal propagates further into the inner magnetosphere resulting in the accumulation of a longer lived current there, giving rise to a protogeomagnetic storm. The global 3‐D structure of the corresponding substorm currents including the substorm current wedge is reconstructed from data.
Plain Language Summary
Using several millions of historical magnetometer records and data mining techniques, we form virtual spacecraft constellations of tens of thousands of spacecraft to reconstruct the global shape of the terrestrial magnetosphere at the moments of its most dramatic reconfigurations responsible for major space weather disturbances.
Key Points
Substorm tail current sheet thinning and dipolarization are reproduced using novel data mining technique
Global 3‐D structure of substorm currents including the substorm current wedge is reconstructed from data
Substorms contribute to an accumulation of a longer‐lived thick current in the innermost part of the magnetosphere
Particle injection, a major mode of plasma transport and energization throughout the magnetosphere, has been studied for decades. Nonetheless, the physical processes that lead to the acceleration and ...transport of very energetic ions in the inner magnetosphere during injection events are still under debate. In this paper, we analyze several injection events occurring near the Van Allen Probes apogee. Our analysis shows that the highest energy of an injected ion population depends on the charge state of that population. We show that most of the helium injected is doubly ionized (He++), while oxygen charge states are consistent with the presence of both ionospheric (O+) and solar wind (O6+) source populations. Based on the findings of our data analysis and with the use of a simple model, we demonstrate that the behavior of each injection of energetic ions near the Van Allen Probes apogee (5 < L < 7 RE) is well explained by simple adiabatic or nearly adiabatic transport within flow channels from higher L (≥10 RE) with velocities at 10 RE ranging between ~200 and 2,000 km/s and falling with inward transport consistent with fixed potential drops across the flow channels. Gradient/curvature drift during transport limits the highest energy/charge observed for each injection at the Van Allen Probes. Even at the highest measured ion energies where gyroradius and scattering effects might be expected to appear, energization depends on charge state but not on ion mass.
Plain Language Summary
We examine the characteristics of sudden enhancements of very energetic ions detected by sensors on the Van Allen Probe spacecraft, as they appear near the highest altitudes reached by the spacecraft (about six Earth radii) during fast flows of particles from farther out on the nightside of the Earth (beyond 10 Earth radii). We show that the energization and transport of these particles can be explained by a very simple model, which although it does not represent the full self‐consistent physics of the system, nevertheless, reproduces the key observables including the highest energies reached by the ions and the dependence of that peak energy on the net charge carried by the ion species. The results indicate that the important factors influencing the energization of the particles are described by very basic concepts, with no need for invoking exotic processes. We suggest that this approach, because it is computationally very fast, can be used to guide the parameters chosen for much more sophisticated models whose computational complexity makes iterative application expensive.
Key Points
Energetic ion acceleration in injections is proportional to charge state and independent of mass
Peak energy of high energy ions injected inside geosynchronous can be modeled as radial flow channels parameterized by their width and flow velocity
Ion composition measurements are critical to unraveling ion energization and drifts in injections
The evolution of the radiation belts in L‐shell (L), energy (E), and equatorial pitch angle (α0) is analyzed during the calm 11‐day interval (4–15 March) following the 1 March 2013 storm. Magnetic ...Electron and Ion Spectrometer (MagEIS) observations from Van Allen Probes are interpreted alongside 1D and 3D Fokker‐Planck simulations combined with consistent event‐driven scattering modeling from whistler mode hiss waves. Three (L, E, α0) regions persist through 11 days of hiss wave scattering; the pitch angle‐dependent inner belt core (L ~ <2.2 and E < 700 keV), pitch angle homogeneous outer belt low‐energy core (L > ~5 and E~ < 100 keV), and a distinct pocket of electrons (L ~ 4.5, 5.5 and E ~ 0.7, 2 MeV). The pitch angle homogeneous outer belt is explained by the diffusion coefficients that are roughly constant for α0 ~ <60°, E > 100 keV, 3.5 < L < Lpp ~ 6. Thus, observed unidirectional flux decays can be used to estimate local pitch angle diffusion rates in that region. Top‐hat distributions are computed and observed at L ~ 3–3.5 and E = 100–300 keV.
Plain Language Summary
We study the evolution of the radiation belts during quiet geomagnetic times from satellite observations and numerical codes. We reach a global understanding of the trapped electrons variation with time, space, energy, and pitch angle (the angle of the velocity vector with the magnetic field). We exhibit three stable regions, which are less sensitive to scattering from hiss waves, while, on the other hand, hiss causes flux decay over 12 days that forms the slot region between the inner and outer belt. The existing theory explains why the outer belt electron decay is independent of pitch angle but dependent upon energy. This implies that satellite observations can reveal local pitch angle diffusion rates, themselves intimately connected with the wave properties. Thus, a connection is made between observed wave properties and observed/computed scattered electron flux, consistent with theory. Regions where the flux is pitch angle dependent are isolated in the low‐energy slot region where we show that the real shape is a smoothed version of the ideal top‐hat distribution computed from theory. The impact of this work is improved understanding of the belt evolution for space weather prediction, with a proposed event‐driven method that accurately (within ×2) predicts the electron flux decay after storms.
Key Points
Global computations of the (L, E, α0) structure of the evolving radiation belt during quiet times agree well with observations
The inner belt decay is pitch angle dependent, while the outer belt is much more homogeneous with two distinct (L, E) regions
The homogeneity of the pitch angle diffusion coefficient due to hiss waves explains the uniform outer belt decay and why 1D and 3D simulations agree
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
Energetic particle transport into the inner magnetosphere during geomagnetic storms is responsible for significant plasma pressure enhancement, which is the driver of large‐scale currents that ...control the global electrodynamics within the magnetosphere‐ionosphere system. Therefore, understanding the transport of plasma from the tail deep into the near‐Earth magnetosphere, as well as the energization processes associated with this transport, is essential for a comprehensive knowledge of the near‐Earth space environment. During the main phase of a geomagnetic storm on 17 March 2013 (minimum Dst ~ −137 nT), the Radiation Belt Storm Probes Ion Composition Experiment (RBSPICE) instrument on the Van Allen Probes observed frequent, small‐scale proton injections deep into the inner nightside magnetosphere in the region L ~ 4 – 6. Although isolated injections have been previously reported inside geosynchronous orbit, the large number of small‐scale injections observed in this event suggests that, during geomagnetic storms injections provide a robust mechanism for transporting energetic ions deep into the inner magnetosphere. In order to understand the role that these injections play in the ring current dynamics, we determine the following properties for each injection: (i) associated pressure enhancement, (ii) the time duration of this enhancement, and (iii) the lowest and highest energy channels exhibiting a sharp increase in their intensities. Based on these properties, we estimate the effect of these small‐scale injections on the pressure buildup during the storm. We find that this mode of transport could make a substantial contribution to the total energy gain in the storm time inner magnetosphere.
Key Points
We investigate ion intensities from the RBSPICE instrument for a storm event
We found multiple ion injections inside geosynchronous throughout the main phase
Their contribution to the total energy gain was found to be substantial
Our investigation of the long‐term ring current proton pressure evolution in Earth's inner magnetosphere based on Van Allen Probes data shows drastically different behavior of the low‐ and high‐ ...energy components of the ring current proton population with respect to the SYM‐H index variation. We found that while the low‐energy component of the protons (<80 keV) is strongly governed by convective timescales and is very well correlated with the absolute value of SYM‐H index, the high‐energy component (>100 keV) varies on much longer timescales and shows either no correlation or anticorrelation with the absolute value of SYM‐H index. Our study also shows that the contributions of the low‐ and high‐ energy protons to the inner magnetosphere energy content are comparable. Thus, our results conclusively demonstrate that proton dynamics, and as a result the energy budget in the inner magnetosphere, do not vary strictly on storm time timescales as those are defined by the SYM‐H index.
Key Points
We are investigating the long‐term ring current dynamics in the inner magnetosphere.
The high‐energy ring current proton dynamics is not captured by the SYM‐H index variations.
The variations of the energy budget inside GEO are not on the same timescales as the SYM‐H index variations.
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