Earth's magnetotail contains magnetic energy derived from the kinetic energy of the solar wind. Conversion of that energy back to particle energy ultimately powers Earth's auroras, heats the ...magnetospheric plasma, and energizes the Van Allen radiation belts. Where and how such electromagnetic energy conversion occurs has been unclear. Using a conjunction between eight spacecraft, we show that this conversion takes place within fronts of recently reconnected magnetic flux, predominantly at 1-to 10-electron inertial length scale, intense electrical current sheets (tens to hundreds of nanoamperes per square meter). Launched continually during intervals of geomagnetic activity, these reconnection outflow flux fronts convert ~10 to 100 gigawatts per square Earth radius of power, consistent with local magnetic flux transport, and a few times 10¹⁵ joules of magnetic energy, consistent with global magnetotail flux reduction.
Auroral substorms, dynamic phenomena that occur in the upper atmosphere at night, are caused by global reconfiguration of the magnetosphere, which releases stored solar wind energy. These storms are ...characterized by auroral brightening from dusk to midnight, followed by violent motions of distinct auroral arcs that suddenly break up, and the subsequent emergence of diffuse, pulsating auroral patches at dawn. Pulsating aurorae, which are quasiperiodic, blinking patches of light tens to hundreds of kilometres across, appear at altitudes of about 100 kilometres in the high-latitude regions of both hemispheres, and multiple patches often cover the entire sky. This auroral pulsation, with periods of several to tens of seconds, is generated by the intermittent precipitation of energetic electrons (several to tens of kiloelectronvolts) arriving from the magnetosphere and colliding with the atoms and molecules of the upper atmosphere. A possible cause of this precipitation is the interaction between magnetospheric electrons and electromagnetic waves called whistler-mode chorus waves. However, no direct observational evidence of this interaction has been obtained so far. Here we report that energetic electrons are scattered by chorus waves, resulting in their precipitation. Our observations were made in March 2017 with a magnetospheric spacecraft equipped with a high-angular-resolution electron sensor and electromagnetic field instruments. The measured quasiperiodic precipitating electron flux was sufficiently intense to generate a pulsating aurora, which was indeed simultaneously observed by a ground auroral imager.
Ion outflow formation processes in magnetic reconnection are investigated on the basis of 3‐D ion velocity distribution function observations for magnetic reconnection in the magnetotail with the ...spacecraft Geotail. Ion acceleration processes up to 40 keV can be divided into two types, and these two types consist of two‐step processes. The first type is found close to the X line in the central part of the ion‐electron decoupling region of magnetic reconnection where an intense crosstail current layer exists and electrons make high‐speed outflow jets. In this region, inflowing ions and ions making the meandering motion coexist. Ions are accelerated up to ~10 keV in the north–south direction during the inflowing process, and then they are accelerated farther by the reconnection electric field in the duskward direction. The second type is found far from the X line in the outer parts of the ion‐electron decoupling region of magnetic reconnection. In this region, the Hall current loops are formed and the inflowing Hall electrons exist near the separatrix layers. Ions are accelerated and make the dawnward motion inside the Hall current loop in the inflowing process, and then they are farther accelerated to form tailward and earthward outflows. In this region, significant amount of ions is supplied as inflows, and outflows containing ions in the wide energy range (5–40 keV) are formed. Hence, both the north–south (vertical) electric field, which is probably the Hall electric field, and the duskward reconnection electric field operate ion acceleration processes for magnetic reconnection in the magnetotail.
Key Points
Two‐step ion acceleration processes operate in magnetotail reconnection
Ions are accelerated up to 10 keV in inflowing process
Ions make the meandering motion in the immediate vicinity of the X line
The spacecraft Geotail surveyed the near‐Earth plasma sheet from XGSM = −10 to −31 RE and YGSM = −20 to +20 RE during the period from 1994 to 2022. It observed 243 magnetic reconnection events and ...785 tailward flow events under various solar wind conditions during plasma sheet residence time of over 23,000 hr. Magnetic reconnections associated with the onset of magnetospheric substorms occur mostly in the range XGSM = −23 to −31 RE. When the solar wind is intense and high substorm activities continue, magnetic reconnection can occur closer to the Earth. The YGSM locations of magnetic reconnections depend on the solar wind conditions and on previous substorm activity. Under normal solar wind conditions, magnetic reconnection occurs preferentially in the pre‐midnight plasma sheet. Under conditions with intense (weak) solar wind energy input, however, magnetic reconnection can occur in the post‐midnight (duskside) plasma sheet. Continuous substorm activity tends to shift the magnetic reconnection site duskward. The plasma sheet thinning proceeds faster under intense solar wind conditions, and the loading process that provides the preconditions for magnetic reconnection becomes shorter. When magnetic flux piles up during a prolonged period with a strongly northward‐oriented interplanetary magnetic field (IMF) Bz, the time necessary to provide the preconditions for magnetic reconnection becomes longer. Although the solar wind conditions are the primary factors that control the location and timing of magnetic reconnections, the plasma sheet conditions created by preceding substorm activity or the strongly northward IMF Bz can modify the solar wind control.
Key Points
Under normal solar wind conditions, magnetic reconnection in association with the onset of a substorm occurs in the pre‐midnight plasma sheet at XGSM = −23 to −31 RE
Intense (weak) solar wind energy input can shift the magnetic reconnection site toward the dawnside (duskside) plasma sheet
Plasma sheet thinning proceeds faster under intense solar wind energy input, although conditions in the plasma sheet can modify the solar wind control
Using the Arase and Van Allen Probes satellite observations, we investigate the nonlinear electromagnetic ion cyclotron (EMIC) rising‐tone (RT) emissions with an increase of the solar wind dynamic ...pressure in the dayside magnetosphere. We find that EMIC RT emissions are accompanied by the extended dayside uniform zone (DUZ) over |MLAT| < 25° due to the dayside magnetospheric compression by an increase in Pdyn. Using the observed plasma and magnetic field data, we modeled the threshold amplitude for the nonlinear EMIC waves and compared it with the observation. The small gradient of the ambient magnetic field strongly contributes to the reduction in the threshold amplitude of nonlinear wave growth compared to other parameters. When the threshold amplitude falls to comparable level of pre‐existing EMIC waves, EMIC RT emissions are immediately triggered, suggesting direct evidence that the DUZ is the preferred condition to cause the nonlinear EMIC RT emission in the dayside magnetosphere.
Plain Language Summary
Electromagnetic ion cyclotron (EMIC) waves play an important role in controlling the dynamics of charged particles in the inner magnetosphere. Especially, nonlinear EMIC rising‐tone (RT) emissions can cause the rapid loss of relativistic electrons and ring current ions. Here, we present direct evidence demonstrating that the distortion of the dayside magnetic field causes nonlinear EMIC RT emission in response to the intensification of the solar wind dynamic pressure. Remarkably, these nonlinear EMIC waves are generated through a reduction in the threshold wave amplitudes by the distortion of the magnetic fields, even in the absence of any significant change in the pre‐existing EMIC wave amplitude. The present result provides new insights into a triggering process of nonlinear plasma waves in the magnetosphere.
Key Points
Electromagnetic ion cyclotron (EMIC) waves with rising‐tone (RT) elements were observed in the dayside magnetosphere during an increase in the solar wind dynamic pressure
Increasing solar wind dynamic pressure extends the dayside uniform zone of the magnetic field to higher magnetic latitudes
The uniform zone leads to the reduction of the nonlinear threshold wave amplitude, which triggers nonlinear EMIC RT emissions
Using tens to hundreds of keV proton and electron flux measurements and simultaneous magnetic field measurements from three Geostationary Operational Environmental Satellites GOES‐13 (75°W), GOES‐14 ...(105°W), and GOES‐15 (135°W), we investigate proton and electron injections and their relationship to the substorm current wedge at geosynchronous altitude. Proton and electron injection processes occur only in the initial formation of the substorm current wedge, the width of which is less than 2 hr in local time in the premidnight region, for moderate substorms. Proton injections are closely related to the formation of a substorm current wedge at geosynchronous altitude, and thus the onset of a substorm, even before local dipolarization in the magnetic field. Proton injections take place only under the western upward field‐aligned currents of the current wedge. Electron injections in the energy range of <100 keV are tightly coupled with local dipolarization in the magnetic field, and these take place mostly in the central region of the current wedge, extending in the region under the western upward field‐aligned currents.
Key Points
Particle injections proceed during the initial formation of a substorm current wedge at geosynchronous altitude
Particle injections occur in the limited extent in the premidnight region, the width of which is less than 2 hr in local time
Proton injections occur even prior to local dipolarization, while electron injections are tightly coupled with local dipolarization in the magnetic field
Resonant interactions of energetic electrons with electromagnetic whistler‐mode waves (whistlers) contribute significantly to the dynamics of electron fluxes in Earth's outer radiation belt. At low ...geomagnetic latitudes, these waves are very effective in pitch angle scattering and precipitation into the ionosphere of low equatorial pitch angle, tens of keV electrons and acceleration of high equatorial pitch angle electrons to relativistic energies. Relativistic (hundreds of keV), electrons may also be precipitated by resonant interaction with whistlers, but this requires waves propagating quasi‐parallel without significant intensity decrease to high latitudes where they can resonate with higher energy low equatorial pitch angle electrons than at the equator. Wave propagation away from the equatorial source region in a non‐uniform magnetic field leads to ray divergence from the originally field‐aligned direction and efficient wave damping by Landau resonance with suprathermal electrons, reducing the wave ability to scatter electrons at high latitudes. However, wave propagation can become ducted along field‐aligned density peaks (ducts), preventing ray divergence and wave damping. Such ducting may therefore result in significant relativistic electron precipitation. We present evidence that ducted whistlers efficiently precipitate relativistic electrons. We employ simultaneous near‐equatorial and ground‐based measurements of whistlers and low‐altitude electron precipitation measurements by ELFIN CubeSat. We show that ducted waves (appearing on the ground) efficiently scatter relativistic electrons into the loss cone, contrary to non‐ducted waves (absent on the ground) precipitating only <150 keV electrons. Our results indicate that ducted whistlers may be quite significant for relativistic electron losses; they should be further studied statistically and possibly incorporated in radiation belt models.
Key Points
Near‐equatorial and ground‐based measurements of whistler‐mode waves are accompanied by relativistic electron precipitation
In the presence (absence) of ducted wave propagation, as monitored by propagation to the ground, the precipitating electron energies are above (below) 150 keV
Ducted whistler‐mode waves may play a key role in relativistic electron loss in the inner magnetosphere
Ultra‐low frequency waves interact with different particle populations all over the magnetosphere. Some interaction mechanisms are associated with certain wave modes, but is it really so and what ...about waves interaction between each other? We present a statistical analysis of Pc4 and Pc5 waves in the magnetosphere of the Earth that were observed by Arase satellite from March 2017 to December 2020. These waves were classified by polarization into toroidal, poloidal, and compressional waves. Toroidal and poloidal waves are thought to be Alfvén waves that are eigenoscillations of Earth's magnetic field lines. The former are believed to be generated by external sources, while the latter one—by internal sources. We compared spatial distribution features with well‐known case studies to reveal their nature for all three polarizations. A high inclination of Arase orbit supported a wave field‐aligned structure research. We found that toroidal waves are mostly odd harmonics and poloidal waves are both even and odd harmonics of Alfvén waves, while compressional waves were observed in a narrow equatorial region only. Different wave generation mechanisms that cause a clear difference in spatial distributions of toroidal, poloidal, and compressional waves could excite a specific wave polarization. Surprisingly, the statistics of wave polarization has a normal distribution without separate clusters. We suggest that polarization change and mode coupling processes make mixed polarization the most common type of polarization in the magnetosphere. This result raises the question of how the polarization change process affects wave‐particle interactions responsible for energy transport throughout the magnetosphere.
Plain Language Summary
Energy transport in the magnetosphere of the Earth is a complex process involving different populations of charged particles and electromagnetic waves. The waves classified as Pc4 and Pc5 have a wavelength comparable to the size of the magnetosphere and are responsible for energy transport on a large distance. We collected 46 months of Pc4 and Pc5 waves observations by Arase satellite to find a dependence of wave parameters on excitation source. We divided waves in the data set by magnetic field variation direction into radial, azimuthal, and field‐aligned waves. These three types of waves have different spatial distribution in both equatorial and meridional planes. We compared distribution features with specific energy sources using well‐known case studies. Surprisingly, wave polarization statistics did not reveal any separation of waves into clusters that could be associated with different sources or wave generation mechanisms. This fact forces us to pay more attention to wave polarization change processes and to a coupling of waves with different polarization to reveal a correct mechanism of energy exchange between the waves and charged particles in the magnetosphere.
Key Points
Transverse waves are mostly mixed polarized, without separate clusters of pure poloidal and toroidal waves
Toroidal waves are odd harmonics of field line resonance, while poloidal waves are both odd and even harmonics connected with the ion drift
Compressional waves are concentrated at the geomagnetic equator, with two essential maxima of the occurrence rate in L‐MLT distribution
Recent years have seen debate regarding the ability of electromagnetic ion cyclotron (EMIC) waves to drive EEP (energetic electron precipitation) into the Earth's atmosphere. Questions still remain ...regarding the energies and rates at which these waves are able to interact with electrons. Many studies have attempted to characterize these interactions using simulations; however, these are limited by a lack of precise information regarding the spatial scale size of EMIC activity regions. In this study we examine a fortuitous simultaneous observation of EMIC wave activity by the RBSP‐B and Arase satellites in conjunction with ground‐based observations of EEP by a subionospheric VLF network. We describe a simple method for determining the longitudinal extent of the EMIC source region based on these observations, calculating a width of 0.75 hr MLT and a drift rate of 0.67 MLT/hr. We describe how this may be applied to other similar EMIC wave events.
Plain Language Summary
The Earth is surrounded by the Van Allen radiation belts, rings of high‐energy charged particles trapped by the Earth's magnetic field. These particle populations are constantly changing, driven by forces from the Sun, Earth, and from the belts themselves. One of the most important drivers of this dynamism is the interaction between particles and electromagnetic waves. One such wave species, known as Electromagnetic Ion Cyclotron (EMIC) waves, has come under scrutiny recently due to experimental results calling into question the theoretical energy limits of their interactions with radiation belt electrons. Studying these waves and their interactions is hampered by our inability to accurately determine the size of the source region of these waves. In this study, we investigate a single EMIC wave event observed simultaneously by two separate satellites and use a network of ground‐based radio wave receivers to estimate the size of the EMIC region. We also explain how the method used in this study may be generalized to other EMIC wave events. This method will allow us to carry out statistical analysis of the size of EMIC wave regions in general, aiding future research into the impacts of these waves on the radiation belts.
Key Points
The extent of an EEP‐driving EMIC source region is estimated using conjunctions between in situ and ground‐based observations
A single EMIC wave event is observed simultaneously by conjugate RBSP‐B and Arase spacecraft and ground‐based instruments
Conjugate measurements by the AARDDVARK network are used to track the EEP from the event and estimate the extent and drift rate
The brightness of aurorae in Earth's polar region often beats with periods ranging from sub-second to a few tens of a second. Past observations showed that the beat of the aurora is composed of a ...superposition of two independent periodicities that co-exist hierarchically. However, the origin of such multiple time-scale beats in aurora remains poorly understood due to a lack of measurements with sufficiently high temporal resolution. By coordinating experiments using ultrafast auroral imagers deployed in the Arctic with the newly-launched magnetospheric satellite Arase, we succeeded in identifying an excellent agreement between the beats in aurorae and intensity modulations of natural electromagnetic waves in space called "chorus". In particular, sub-second scintillations of aurorae are precisely controlled by fine-scale chirping rhythms in chorus. The observation of this striking correlation demonstrates that resonant interaction between energetic electrons and chorus waves in magnetospheres orchestrates the complex behavior of aurora on Earth and other magnetized planets.
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