Braking Region Research Articles

Energetic Particle Injection During Short Isolated Bubble as Seen in RCM Simulation and Spacecraft Observations in the Flow Braking Region

AbstractAlthough energetic particle (EP) injections are commonly thought to be formed by the flow burst intrusion from the magnetotail, important details and quantitative aspects of their transport, acceleration and flow braking need further investigation and understanding. Motivated by frequent observations of short transient EP injections being not associated with substorms, we analyze high‐resolution Rice Convection Model simulations of a short (5‐min long) localized (∼3RE width) density depletion (evacuating 90% of flux tube content) initiated at the tailward simulation boundary (∼18RE) and allowed to evolve within an otherwise typical plasma sheet environment. We note that, driven by betatron‐like acceleration, the peak EP flux at fixed energy dramatically increases in a couple of minutes when the bubble head enters the inner magnetosphere at r < 8–10 RE giving rise to a localized injection of subsequently drifting EP clouds. Here the 50–200 keV electron flux reaches values as high as #105 (cm2 s sr keV)−1, and even higher energies (up to 1 MeV) may briefly appear. Surprisingly, at a later stage of bubble penetration, after termination of bubble jet from the tail, the injection boundary of high energy (HE) particles detaches from the bubble earthward boundary while the latter continues moving inward. Time History of Events and Macroscale Interactions during Substorms multi‐spacecraft mission observations of a short bubble‐like flow burst at the spacecraft cluster located near the flow stopping point, show much similarity with simulation results but also reveal important differences between responses of HE protons and electrons attributed to the finite gyroradius effect.

Bursty Bulk Flow Turbulence as a Source of Energetic Particles to the Outer Radiation Belt

AbstractWe report observations of a Bursty Bulk Flow (BBF) penetrating close to the outer edge of the radiation belt. The turbulent BBF braking region is characterized by ion velocity fluctuations, magnetic field (B) variations, and intense electric fields (E). In this event, energetic (>100 keV) electron and ion fluxes are appreciably enhanced. Importantly, fluctuations in energetic electrons and ions suggest local energization. Using correlation distances and other observed characteristics of turbulent E, test‐particle simulations support local energization by E that favors higher‐energy electrons and leads to an enhanced energetic shoulder and tail in the electron distributions. The energetic shoulder and tail could be amplified to MeV energies by adiabatic transport into the radiation belt where |B| is higher. This analysis suggests that turbulence generated by BBFs can, in part, supply energetic particles to the outer radiation belt and that turbulence can be a significant contributor to particle acceleration.

Omega Band Magnetospheric Source Location: A Statistical Model‐Based Study

AbstractAuroral omega bands have been observed since the 1960s and their ionospheric electrodynamic properties are well established. At the same time magnetospheric source is poorly investigated leaving a room for few competing hypotheses on how the omega band forms. Here we present the statistical study of the projections of about 400 omega bands from the ionosphere to magnetosphere to investigate the location and properties of a possible omega source region in the magnetosphere. We used the Magnetometers—Ionospheric Radars—All‐sky Cameras Large Experiment all‐sky images on the list of individual omega structures (Partamies et al., 2017, http://doi.org/10.5194/angeo-35-1069-2017) which were observed in the Fennoscandian Lapland in the period of 1997–2007, and a new empirical magnetic field model (Tsyganenko & Andreeva, 2016, https://doi.org/10.1002/2016ja023217) to identify the magnetospheric equatorial projection of the observed omegas. We found that 90% of the auroral omega structures map to the radial distances of 6–14 RE from the Earth, coinciding with the bursty bulk flow braking region. An average magnetic field configuration in the vicinity of magnetospheric omega projections corresponds approximately to the transition region between tail‐ and dipole‐like magnetic configuration. Velocity estimates for omega projections reveal the dawnward as well as the radial propagation in the magnetosphere with a typical speed of up to few tens of km/s. It is shown that the source of omega structures propagates earthward in most of the events.

The ordering effects of an n-Si defect structure, induced by high fluences of ions with MeV energies

The results of studies of the structural and optical properties of silicon irradiated with light ions of MeV energies with fluences exceeding 1016 cm–2 are generalized. The structure of silicon irradiated with ions is con ventionally divided into several regions (ion path, braking, and outside the braking region), the kind of which is determined by the type of ions, their mass, energy, and temperature during irradiation. It is established that the irradiation with high fluences of light ions of MeV energies causes the formation of ordered layers in the bulk of silicon at depths up to several hundred microns, associated with defects whose properties differ from those of the matrix. It is shown that, under such irradiation conditions, the nature of the defect formation (the number and width of the revealed ordered linear structures and their location relative to the braking region of ions) depends on the mass and energy of ions, the ion beam intensity, the irradiation temperature, and the crystal properties. The effect of the ordering of defects in the form of stress lines and their propagation outside the braking region was discovered, when silicon was irradiated with ions of both hydrogen and helium. It is found that this effect depends on the irradiation intensity and occurs, only when the beam current density is less than 0.45 μA/cm2. It is established that, for silicon irradiated with helium ions in the region of ion path, characteristic is not the monocrystalline, but fragmentary structure, which has an aggregate of ordered stress lines (associated with defects) located in parallel to the braking band of helium ions, and the braking band consists of voids etched as a continuous layer and in the form of separate clusters. It is revealed that the irradiation of dis location silicon with deuterium ions leads to the movement of dislocations during the irradiation and to their crossing of the deuteron braking line due to the formation of stacking faults.

MMS Observations of Narrow-Band Quasi-Parallel Whistler Waves in the Flow Braking Region in Near-Earth Magnetotail

This paper presents an analysis of wave activity recorded by the MMS satellites during a prolonged dipolarization in the near magnetotail (XGSM ~ –17 RE). It was found that multiple narrow-band, quasi-parallel whistler wave bursts were observed behind the leading front of dipolarization, during the phase of growth of the magnetic field’s BZ component. The durations of wave bursts were ~1–15 s, and the characteristic frequencies ranged from ~0.1 to 0.8 fc (fc is the electron gyrofrequency). Based on a detailed analysis of a single wave burst, it was found that the frequency corresponding to the maximum value of a linear increment of quasi-parallel whistler waves was close in magnitude to the observed frequency, which indicates the possibility of satellite location close to the source of waves. We also demonstrated that, in the case under discussion, the electrons with pitch angles of 125°–135° and energies of ~3–12 keV made a largest contribution to the increment of whistler waves. These observations have shown that, during dipolarizations, the thermal and suprathermal electron populations are most effectively involved in resonant interaction with whistler waves.

MESSENGER Observations of Flow Braking and Flux Pileup of Dipolarizations in Mercury's Magnetotail: Evidence for Current Wedge Formation

AbstractSimilar to Earth, Mercury's magnetotail experiences frequent dipolarization of the magnetic field. These rapid (~2 s) increases in the northward component of the tail field (ΔBz ~ 30 nT) at Mercury are associated with fast sunward flows (~200 km/s) that enhance local magnetic field convection. Differences between the two magnetospheres, namely Mercury's smaller spatiotemporal scales and lack of an ionosphere, influence the dynamics of dipolarizations in these magnetotails. At Earth, the braking of fast dipolarization flows near the inner magnetosphere accumulates magnetic flux and develops the substorm current wedge. At Mercury, flow braking and flux pileup remain open topics. In this work, we develop an automated algorithm to identify dipolarizations, which allows for statistical examination of flow braking and flux pileup in Mercury's magnetotail. We find that near the inner edge of the plasma sheet, steep magnetic pressure gradients cause substantial braking of fast dipolarization flows. The dipolarization frequency and sunward flow speed decrease significantly within a region ~500 km thick located at ~900 km altitude above Mercury's local midnight surface. Due to the close proximity of the braking region to the planet, we estimate that ~10–20% of dipolarizations may reach the nightside surface of the planet. The remaining dipolarizations exhibit prolonged statistical flux pileup within the braking region similar to large‐scale dipolarization of Earth's inner magnetosphere. The existence of flow braking and flux pileup at Mercury indicates that a current wedge may form, although the limitations imposed by Mercury's magnetosphere require the braking of multiple, continuous dipolarizations for current wedge formation.

기동중 단상 결상시 3상 유도전동기의 동작 특성에 관한 연구

Induction motors cause a failure due to electrical and mechanical causes. The most significant cause of electrical failure is due to overheating due to high current. Induction motors generate high currents at start-up, but become very low as the rotational speed increases. If the current does not decrease as the rotational speed increases, it may act as an electrical stress on the winding. A single-phasing in which one phase of the three phases is suddenly opened. The generation of single-phasing can overheat the stator winding due to a sudden increase in current, if this condition persists, the winding will overheat and eventually be burned out. Further, since the rotational speed is transferred to the braking region and the rotor is restrained, the normal torque can not be ensured. Therefore, it is very important to analyze what problems occur when single-phasing exists in an induction motor.BR In this study, we analyzed the magnitude and the torque change of voltage, current, power, power factor appearing to the induction motor when single-phasing occurred from three phase power source during startup. As a result of the analysis, it was found that the current generated during single-phasing is higher than the normal start-up current, and if it is not removed, it may persist and lead to overheating. And it can be seen that normal torque can not be secured because the forward torque and the reverse torque are repeatedly operated by the restraint of the rotor.

Multiscale Currents Observed by MMS in the Flow Braking Region.

We present characteristics of current layers in the off‐equatorial near‐Earth plasma sheet boundary observed with high time‐resolution measurements from the Magnetospheric Multiscale mission during an intense substorm associated with multiple dipolarizations. The four Magnetospheric Multiscale spacecraft, separated by distances of about 50 km, were located in the southern hemisphere in the dusk portion of a substorm current wedge. They observed fast flow disturbances (up to about 500 km/s), most intense in the dawn‐dusk direction. Field‐aligned currents were observed initially within the expanding plasma sheet, where the flow and field disturbances showed the distinct pattern expected in the braking region of localized flows. Subsequently, intense thin field‐aligned current layers were detected at the inner boundary of equatorward moving flux tubes together with Earthward streaming hot ions. Intense Hall current layers were found adjacent to the field‐aligned currents. In particular, we found a Hall current structure in the vicinity of the Earthward streaming ion jet that consisted of mixed ion components, that is, hot unmagnetized ions, cold E × B drifting ions, and magnetized electrons. Our observations show that both the near‐Earth plasma jet diversion and the thin Hall current layers formed around the reconnection jet boundary are the sites where diversion of the perpendicular currents take place that contribute to the observed field‐aligned current pattern as predicted by simulations of reconnection jets. Hence, multiscale structure of flow braking is preserved in the field‐aligned currents in the off‐equatorial plasma sheet and is also translated to ionosphere to become a part of the substorm field‐aligned current system.

Numerical simulation on the multiple dipolarization fronts in the magnetotail

Using an extended MHD model including the Hall effect and finite Larmor radius effect, we reproduce multiple dipolarization fronts (DFs) associated with the interchange instability in the braking region of bursty bulk flow in the plasma sheet. Our simulations reveal that the multiple DFs produced by the interchange instability are “growing” type DFs because the maximum plasma flow speeds are behind the fronts. Both the earthward and tailward moving DFs can be produced by interchange instability in the near-Earth region. The Hall electric field is the dominant electric field component in the dip region and the DF layer. The convective and the electron pressure gradient electric field components are smaller. The sharp Bz changes in both the dip region and DF layer correspond to the oppositely directed currents that are primarily associated with electrons. The ion diamagnetic current due to the strong ion pressure gradient causes an intense downward current in the dip region, which can produce the dip ahead of the front. The energy dissipations in the dip region and DF layer are dominated by ions through the work done by the Lorentz force. Our simulation results indicate that the magnetic energy can be converted to plasmas on the DF layer, and vice versa in the dip region.

The effects of bursty bulk flows on global‐scale current systems

AbstractUsing a global magnetospheric MHD model coupled with a kinetic ring current model, we investigate the effects of magnetotail dynamics, particularly the earthward bursty bulk flows (BBFs) produced by the tail reconnection, on the global‐scale current systems. The simulation results indicate that after BBFs brake around X = −10 RE due to the dipolar “magnetic wall,” vortices are generated on the edge of the braking region and inside the inner magnetosphere. Each pair of vortex in the inner magnetosphere disturbs the westward ring current to arc radially inward as well as toward high latitudes. The resultant pressure gradient on the azimuthal direction induces region‐1 sense field‐aligned component from the ring current, which eventually is diverted into the ionosphere at high latitudes, giving rise to a pair of field‐aligned current (FAC) eddies in the ionosphere. On the edge of the flow braking region where vortices also emerge, a pair of region‐1 sense FACs arises, diverted from the cross‐tail duskward current, generating a substorm current wedge. This is again attributed to the increase of thermal pressure ahead of the bursty flows turning azimuthally. It is further found that when multiple BBFs, despite their localization, continually and rapidly impinge on the “wall,” carrying sufficient tail plasma sheet population toward the Earth, they can lead to the formation of a new ring current. These results indicate the important role that BBFs play in bridging the tail and the inner magnetosphere ring current and bring new insight into the storm‐substorm relation.

Behavior of fast earthward flow near the braking region: Hall MHD simulation

Behavior of the fast earthward flow near the braking region in the magnetotail during a substorm is investigated using the Hall MHD simulation. The results indicate that the high-speed earthward plasma flow is associated with fast reconnection in the middle tail. The fast flow is mainly confined in the range . In the region of , due to intermittent magnetic reconnection, the earthward flow exhibits a fluctuating property, i.e., the flow is localized in space and is bursty in time. The pile-up of the magnetic flux and plasma in the near-Earth region leads to formation of the fast-flow braking region or dipolarization front. After colliding into the fast-flow braking region, a part of the Earth flow bounces back, and leads to an intermittent tailward flow in the near-Earth magnetotail.

Large Eddy Simulation of Transient Flow and Inclusions Transport in Continuous Casting Mold under Different Electromagnetic Brakes

A mathematical model has been developed to analyze transient fluid flow and inclusions transport in a slab continuous casting mold, considering the effects of electromagnetic brake (EMBr) arrangement and magnetic field strength. Transient flow of molten steel in the mold is calculated by using the large eddy simulation. The electromagnetic force is incorporated into the Navier–Stokes equation. The transport of inclusion inside the mold is calculated using the Lagrangian approach based on the transient flow field. The predicted results of this model are compared with the measurements of the ultrasonic testing of the rolled steel plates and the water model experiments. The transient asymmetrical flow pattern and inclusion transport inside the mold exhibits satisfactory agreement with the corresponding measurements. With electromagnetic brake effect, the velocities around the braking region are significantly suppressed, and the recirculating flow in the lower part drops and tends to develop a plug-like flow. The EMBr arrangement has an insignificant effect on the overall removal fraction of inclusions, especially for larger inclusions. The inclusion removal rate for the flow-control mold (FCM arrangement) reduces instead compared with no EMBr, especially for smaller inclusions.

MMS Multipoint electric field observations of small‐scale magnetic holes

AbstractSmall‐scale magnetic holes (MHs), local depletions in magnetic field strength, have been observed multiple times in the Earth's magnetosphere in the bursty bulk flow (BBF) braking region. This particular subset of MHs has observed scale sizes perpendicular to the background magnetic field (B) less than the ambient ion Larmor radius (ρi). Previous observations by Time History of Events and Macroscale Interactions during Substorms (THEMIS) indicate that this subset of MHs can be supported by a current driven by the E × B drift of electrons. Ions do not participate in the E × B drift due to the small‐scale size of the electric field. While in the BBF braking region, during its commissioning phase, the Magnetospheric Multiscale (MMS) spacecraft observed a small‐scale MH. The electric field observations taken during this event suggest the presence of electron currents perpendicular to the magnetic field. These observations also suggest that these currents can evolve to smaller spatial scales.

Particle Acceleration in Collapsing Magnetic Traps with a Braking Plasma Jet

Collapsing magnetic traps (CMTs) are one proposed mechanism for generating non-thermal particle populations in solar flares. CMTs occur if an initially stretched magnetic field structure relaxes rapidly into a lower-energy configuration, which is believed to happen as a by-product of magnetic reconnection. A similar mechanism for energising particles has also been found to operate in the Earth's magnetotail. One particular feature proposed to be of importance for particle acceleration in the magnetotail is that of a braking plasma jet, i.e. a localised region of strong flow encountering stronger magnetic field which causes the jet to slow down and stop. Such a feature has not been included in previously proposed analytical models of CMTs for solar flares. In this work we incorporate a braking plasma jet into a well studied CMT model for the first time. We present results of test particle calculations in this new CMT model. We observe and characterise new types of particle behaviour caused by the magnetic structure of the jet braking region, which allows electrons to be trapped both in the braking jet region and the loop legs. We compare and contrast the behaviour of particle orbits for various parameter regimes of the underlying trap by examining particle trajectories, energy gains and the frequency with which different types of particle orbit are found for each parameter regime.

Propagation of Pi2 pulsations through the braking region in global MHD simulations

AbstractWe investigate the propagation of Pi2 period pulsations from their origin in the plasma sheet through the braking region, the region where the fast flows are slowed as they approach the inner edge of the plasma sheet. Our approach is to use both the University of California, Los Angeles (UCLA) and Lyon‐Fedder‐Mobarry (LFM) global magnetohydrodynamic (MHD) computer codes to simulate the Earth's magnetosphere during a substorm that occurred on 14 September 2004 when Pi2 pulsations were observed. We use two different MHD models in order to test the robustness of our conclusions about Pi2. The simulations are then compared with ground‐based and satellite data. We find that the propagation of the pulsations in the simulations, especially through the braking region, depends strongly on the ionospheric models used at the inner boundary of the MHD models. With respect to typical observed values, the modeled conductances are high in the UCLA model and low in the LFM model. The different conductances affect the flows, producing stronger line tying that slows the flow in the braking region more in the UCLA model than in the LFM model. Therefore, perturbations are able to propagate much more freely into the inner magnetosphere in the LFM results. However, in both models Pi2 period perturbations travel with the dipolarization front (DF) that forms at the earthward edge of the flow channel, but as the DF slows in the braking region, −8≤x≤−6 RE, the Pi2 period perturbations begin to travel ahead of it into the inner magnetosphere. This indicates that the flow channels generate compressional waves with periods that fall within the Pi2 range and that, as the flows themselves are stopped in the braking region, the compressional wave continues to propagate into the inner magnetosphere.

Whistler mode wave generation at the edges of a magnetic dip

AbstractOn 18 May 2011, the Time History of Events and Macroscale Interactions during Substorms satellite observed whistler mode waves associated with a magnetic dip behind a dipolarization front structure in the bursty bulk flow braking region. For the first time, we find that whistler mode waves are generated at the edges of magnetic dip rather than at the center (also known as “minimum‐B‐pocket”). Detailed wave analysis indicates that the waves are likely lower and upper band whistler mode chorus. We examine electron pitch angle distributions at the edges of dip and compare them with those at the center and far outside the magnetic dip. Results confirm that the positive temperature anisotropy and pancake distributions at the edges of magnetic dip provide free energy source for growth of the whistler mode waves. We also interpret the whole physical process of how whistler mode waves generate in this event.

Generation of high‐frequency electric field activity by turbulence in the Earth's magnetotail

AbstractBursty bulk flow (BBF) events, frequently observed in the magnetotail, carry significant energy and mass from the tail region at distances that are often greater than 20 RE into the near‐Earth plasma sheet at ∼10 RE where the flow is slowed and/or diverted. This region at ∼10 RE is referred to as the BBF braking region. A number of possible channels are available for the transfer or dissipation of energy in BBF events including adiabatic heating of particles, the propagation of Alfvén waves out of the BBF braking region and into the auroral region, diverted flow out of the braking region, and energy dissipation within the braking region itself. This study investigates the generation of intense high‐frequency electric field activity observed within the braking region. When present, these intense electric fields have power above the ion cyclotron frequency and almost always contain nonlinear structures such as electron phase space holes and double layers, which are often associated with field‐aligned currents. A hypothesis in which the observed high‐frequency electric field activity is generated by field‐aligned currents resulting from turbulence in the BBF braking region is considered. Although linear Alfvén waves can generate field‐aligned currents, based on theoretical calculations, the required currents are likely not the result of linear waves. Observations from the Time History of Events and Macroscale Interactions during Substorms satellites support the picture of a turbulent plasma leading to the generation of nonlinear kinetic structures. This work provides a possible mechanism for energy dissipation in turbulent plasmas.

Large‐amplitude electric fields associated with bursty bulk flow braking in the Earth's plasma sheet

AbstractWe report observations of large‐amplitude (>50 mV/m) electric fields primarily associated with bursty bulk flow events. These electric fields reach ~500 mV/m, which are some of the largest electric fields (E) observed in the magnetotail. E not only has a larger than expected component perpendicular to the magnetic field but often has an intense parallel component. High time resolution waveforms reveal nonlinear structures such as electron phase‐space holes and double layers, which suggest strong field‐aligned currents or electron beams. Further examination shows that these large‐amplitude electric fields are almost always accompanied by enhanced magnetic field fluctuations. The electric fields are enhanced both above and below the ion cyclotron frequency, whereas the magnetic field fluctuations (δB) are mostly below the ion cyclotron frequency. Analysis of the wave spectra and the Poynting flux suggest that shear Alfvén waves are participating in these events. The Alfvén waves are revealed through the |δE|/|δB| ratio and strong field‐aligned Poynting flux, sometimes reaching nearly 1 mW/m2. This value, when mapped to the low‐altitude auroral region, exceeds 1 W/m2, which is an extreme value for that region. This Alfvénic activity is accompanied by evidence of compressional modes. These observations support a hypothesis whereby intense currents or electron beams, generated by kinetic Alfvénic waves that result from a turbulent cascade in bursty bulk flow (BBF) braking region, may be an energy source for large‐amplitude electric fields. The large‐amplitude electric fields may act as a dissipation mechanism and relax the highly tangled magnetic fields that result from the turbulence. Furthermore, these observations offer strong support that Alfvénic Poynting flux from the BBF braking region can be the energy source for Alfvénic aurora.

Energetic electron bursts in the plasma sheet and their relation with BBFs

AbstractWe studied energetic electron bursts (EEBs) (40–250 keV) in the plasma sheet (PS) and their relation to bursty bulk flows (BBFs) using the data recorded by Cluster from 2001 to 2009. The EEBs in the PS can be classified into four types. Three types of EEBs are dispersionless, including EEBs accompanied with BBFs (V > 250 km/s) but without dipolarization front (DF); EEBs accompanied with both dipolarization front (DF) and BBF; and EEBs accompanied with DF and fast flow with V < 250 km/s. One type of EEB, i.e., EEBs not accompanied with BBFs and DFs, is dispersed. The energetic electrons (40–130 keV) can be easily transported earthward by BBFs due to the strong dawn‐dusk electric field embedded in BBFs. The DFs in BBFs can produce energetic electrons (40 to 250 keV). For the EEBs with DF and BBFs, the superposed epoch analyses show that the increase of energetic electron flux has two phases: gradual increase phase before DF and rapid increase phase concurrent with DF. In the PS around x = −18 RE, 60%–70% of EEBs are accompanied with BBFs, indicating that although hitherto there have been various acceleration mechanisms of energetic electrons, most of the energetic electrons in the PS are related with magnetic reconnection, and they are produced either directly by magnetic reconnection or indirectly by the DFs within BBFs. In the BBF's braking region of −12 RE < x < −10 RE, 20% of EEBs are accompanied with BBFs. The corresponding ratio between EEBs and BBFs shows a dawn‐dusk asymmetry.

Generation of Pi2 pulsations by intermittent earthward propagating dipolarization fronts: An MHD case study

Using a global magnetohydrodynamic (MHD) simulation of the magnetosphere during a disturbed interval on 14 September 2004, we have investigated fluctuations in plasma properties of the magnetotail in the Pi2 range and their relationship to dipolarization fronts (DFs). Results from the MHD simulation indicate that this event is a very active interval with variable convection and disorder in the tail on a range of scales as small as ∼1 RE. DFs are observed in the simulation at the leading edge of fast earthward flows that originate from reconnection regions that form between ∼−15 and −30 RE in the tail. Pi2 period fluctuations are identified in pressure, magnetic field, and velocity components inside −13 REfollowing each burst of DFs in the midnight sector. The fluctuations observed in the pressure appear to be generated by the successive DFs as they approach the interface between stretched tail field lines and dipolar field lines. Fluctuations in the velocity may be the result of interactions between successive DFs and are amplified directly following the passage of the DFs as they propagate earthward. Although the limited azimuthal extent of the pulsations near the plasma sheet, just inside of the braking region, makes it difficult to draw a direct comparison between the ground‐based measurements and the pulsations at −6 RE, the temporal evolution of the simulated DFs and Pi2 pulsations approximately reproduces the timing of the variations observed by satellites and ground‐based instruments. Therefore, we have been able to use the global simulation to track the bursty flows, dipolarization fronts, and associated Pi2 period fluctuations throughout the entire magnetosphere in order to understand the sources of the changes measured in the near‐Earth region.