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In particle collider experiments, elementary particle interactions with large momentum transfer produce quarks and gluons (known as partons) whose evolution is governed by the strong force, as described by the theory of quantum chromodynamics (QCD) 1 . These partons subsequently emit further partons in a process that can be described as a parton shower 2 , which culminates in the formation of detectable hadrons. Studying the pattern of the parton shower is one of the key experimental tools for testing QCD. This pattern is expected to depend on the mass of the initiating parton, through a phenomenon known as the dead-cone effect, which predicts a suppression of the gluon spectrum emitted by a heavy quark of mass m Q and energy E , within a cone of angular size m Q / E around the emitter 3 . Previously, a direct observation of the dead-cone effect in QCD had not been possible, owing to the challenge of reconstructing the cascading quarks and gluons from the experimentally accessible hadrons. We report the direct observation of the QCD dead cone by using new iterative declustering techniques 4 , 5 to reconstruct the parton shower of charm quarks. This result confirms a fundamental feature of QCD. Furthermore, the measurement of a dead-cone angle constitutes a direct experimental observation of the non-zero mass of the charm quark, which is a fundamental constant in the standard model of particle physics. The direct measurement of the QCD dead cone in charm quark fragmentation is reported, using iterative declustering of jets tagged with a fully reconstructed charmed hadron.

Energetic electron precipitation (EEP) during substorms significantly affects ionospheric chemistry and lower‐ionosphere (<100 km) conductance. Two mechanisms have been proposed to explain what causes EEP: whistler‐mode wave scattering, which dominates at low latitudes (mapping to the inner magnetosphere), and magnetic field‐line curvature scattering, which dominates poleward. In this case study, we analyzed a substorm event demonstrating the dominance of curvature scattering. Using ELFIN, POES, and THEMIS observations, we show that 50–1,000 keV EEP was driven by curvature scattering, initiated by an intensification and subsequent earthward motion of the magnetotail current sheet. Using a combination of Swarm, total electron content, and ELFIN measurements, we directly show the location of EEP with energies up to ∼1 MeV, which extended from the plasmapause to the near‐Earth plasma sheet (PS). The impact of this strong substorm EEP on ionospheric ionization is also estimated and compared with precipitation of PS (<30 keV) electrons. Plain Language Summary During magnetospheric substorms, energetic electrons in the Earth's plasma sheet (PS), the night‐side magnetosphere region filled by hot plasma, precipitate to the ionosphere. Energetic electron precipitation (EEP) affects the density, temperature, and composition of the ionosphere. However, the exact process that causes such precipitation is not well understood due to observational constraints. The challenge lies in simultaneously measuring the EEP properties at the ionosphere and the plasma and wave properties in the PS. We analyze a fortuitous satellite conjunction during a substorm, during which EEP was simultaneously captured by ELFIN, Swarm, and POES at low altitudes, and THEMIS in the equatorial PS. EEP was observed to extend across a broad equatorial domain, projecting into a wide ionospheric region and encompassing the PS region and a significant portion of the inner magnetosphere. High‐energy‐resolution measurements from ELFIN reveal that the main driver of precipitation is the scattering of energetic electrons by strongly curved magnetic field lines in the PS, as opposed to the more commonly suggested scattering mechanisms associated with wave‐particle interactions. We also show that the EEP drastically altered the ionization profile of the ionosphere. Key Points We investigate the radial location of energetic (50–1,000 keV) electron precipitation (EEP) during a substorm We compare the impact of plasma sheet electron precipitation (<30 keV) and EEP (50–1,000 keV) on the altitudinal profile of ionization Our results underscore the key role of curvature scattering in energetic electron precipitation during substorms

Electromagnetic ion cyclotron (EMIC) waves can drive radiation belt depletion and Low‐Earth Orbit satellites can detect the resulting electron and proton precipitation. The ELFIN (Electron Losses and Fields InvestigatioN) CubeSats provide an excellent opportunity to study the properties of EMIC‐driven electron precipitation with much higher energy and pitch‐angle resolution than previously allowed. We collect EMIC‐driven electron precipitation events from ELFIN observations and use POES (Polar Orbiting Environmental Satellites) to search for 10s–100s keV proton precipitation nearby as a proxy of EMIC wave activity. Electron precipitation mainly occurs on localized radial scales (∼0.3 L), over 15–24 MLT and 5–8 L shells, stronger at ∼MeV energies and weaker down to ∼100–200 keV. Additionally, the observed loss cone pitch‐angle distribution agrees with quasilinear predictions at ≳250 keV (more filled loss cone with increasing energy), while additional mechanisms are needed to explain the observed low‐energy precipitation. Plain Language Summary Electromagnetic ion cyclotron (EMIC) emissions are a type of plasma wave that can be excited in the near‐Earth environment and interact with energetic electrons in the Earth's radiation belts. Through these wave‐particle interactions, electrons can be pushed into the loss cone and lost into the Earth's atmosphere (electron precipitation), where they deposit their energy by interacting with neutral atoms and cold charged particles. EMIC‐driven electron precipitation still needs to be fully characterized and understood. In this work, we use data from the Electron Losses and Fields InvestigatioN (ELFIN) CubeSats, which provide electron fluxes at high energy and pitch‐angle (look direction) resolution at ∼450 km of altitude. Our analysis reveals that precipitation is most efficient for ∼MeV electrons and is accompanied by weaker low‐energy precipitation down to ∼100–200 keV. Given the ELFIN CubeSats spin, we can also study the distribution of the precipitating electrons along different look directions (pitch‐angles). We find that the loss cone shape is well‐reproduced by quasilinear predictions of EMIC‐electron interactions at higher energies (≳250 keV), while quasilinear calculations underestimate the observed low‐energy precipitation. Key Points Energetic electron precipitation is observed by Electron Losses and Fields InvestigatioN nearby proton precipitation (a proxy for Electromagnetic ion cyclotron waves) primarily over 15–24 MLT Precipitation efficiency increases as a function of energy: weak ∼100s keV precipitation is concurrent with intense ∼MeV precipitation The observed pitch‐angle distribution shows a loss cone filling up with energy, similar to the pitch‐angle profiles from quasilinear theory

This chapter reviews fundamental properties and recent advances of diffuse and pulsating aurora. Diffuse and pulsating aurora often occurs on closed field lines and involves energetic electron precipitation by wave-particle interaction. After summarizing the definition, large-scale morphology, types of pulsation, and driving processes, we review observation techniques, occurrence, duration, altitude, evolution, small-scale structures, fast modulation, relation to high-energy precipitation, the role of ECH waves, reflected and secondary electrons, ionosphere dynamics, and simulation of wave-particle interaction. Finally we discuss open questions of diffuse and pulsating aurora.

The modulation of chorus waves on several‐second timescales in Earth's magnetosphere plays a crucial role in modulating electron precipitation intensity, leading to the formation of pulsating aurora. However, the physical mechanism underlying chorus modulation remains not fully understood. In this study, we perform self‐consistent particle‐in‐cell simulations with typical magnetospheric plasma parameters to quantify chorus modulation driven by plasma density variations and compressional magnetic field fluctuations. It is demonstrated that chorus modulation is determined by nonlinear wave‐particle interactions, in which the condition for nonlinear wave growth is highly sensitive to background plasma parameters. The resulting electron precipitation in the ∼10–200 keV energy range exhibits modulation on comparable timescales, consistent with observations of pulsating aurora. This study enhances our understanding of how variations in magnetospheric plasma parameters influence chorus wave excitation and the associated particle dynamics.

Electromagnetic ion cyclotron (EMIC) waves are commonly observed electromagnetic emissions in Earth's magnetosphere and are widely considered to efficiently scatter relativistic electrons into bounce loss cones. However, their precise scattering effects remain highly debated due to limited energy coverage and coarse resolution of previous measurements. Here, we present high‐energy‐resolution measurements of EMIC‐induced relativistic electron precipitation from the Relativistic Electron and Proton Telescope integrated little experiment‐2 (REPTile‐2) onboard the Colorado Inner Radiation Belt Experiment (CIRBE) CubeSat. A long duration >1 MeV electron precipitation event was measured by CIRBE/REPTile‐2 in both the northern and southern hemispheres on 25 April 2023. The energy versus L dispersions of these >1 MeV precipitating electrons show good agreement with minimum resonance energies of electrons interacting with He+ band EMIC waves at specific frequencies. These novel observations unveil the detailed scattering effect of EMIC waves and provide important clues regarding wave‐particle interaction processes near the equator.

Electromagnetic ion cyclotron (EMIC) waves are transverse plasma waves generated by anisotropic proton distributions with Tperp > Tpara. They are believed to play an important role in the dynamics of the ring current and potentially, of the radiation belts. Therefore it is important to know their localization in the magnetosphere and the magnetospheric and solar wind conditions which lead to their generation. Our earlier observations from three Time History of Events and Macroscale Interactions during Substorms (THEMIS) probes demonstrated that strong magnetospheric compressions associated with high solar wind dynamic pressure (Pdyn) may drive EMIC waves in the inner dayside magnetosphere, just inside the plasmapause. Previously, magnetospheric compressions were found to generate EMIC waves mainly close to the magnetopause. In this work we use an automated detection algorithm of EMIC Pc1 waves observed by THEMIS between May 2007 to December 2011 and present the occurrence rate of those waves as a function of L‐shell, magnetic local time (MLT), Pdyn, AE, and SYMH. Consistent with earlier studies we find that the dayside (sunward of the terminator) outer magnetosphere is a preferential location for EMIC activity, with the occurrence rate in this region being strongly controlled by solar wind dynamic pressure. High EMIC occurrence, preferentially at 12–15 MLT, is also associated with high AE. Our analysis of 26 magnetic storms with Dst < −50 nT showed that the storm‐time EMIC occurrence rate in the inner magnetosphere remains low (<10%). This brings into question the importance of EMIC waves in influencing energetic particle dynamics in the inner magnetosphere during disturbed geomagnetic conditions. Key Points Dayside outer magnetosphere is a preferential location for EMIC waves Dayside EMIC occurrence rate is controlled by solar wind pressure Storm‐time EMIC occurrence in the inner magnetosphere remains low

The precipitation of tens to hundreds of keV electrons from Earth's magnetosphere plays a crucial role in magnetosphere‐ionosphere coupling, primarily driven by chorus wave scattering. Most existing simulations of electron precipitation rely on test particle models that neglect particle feedback on waves. However, both theoretical and observational studies indicate that the feedback from energetic electrons significantly influences chorus wave excitation and evolution. In this study, we quantify electron precipitation driven by chorus waves using self‐consistent simulations at L = 6 with typical magnetospheric plasma parameters. Electrons in the ∼10–200 keV range are precipitated, exhibiting energy‐dispersive characteristics. The precipitation intensity reaches ∼108–109 ${10}^{8}\\!\\mathit{\\mbox{--}}\\!{10}^{9}$ keV/s/sr/cm2/MeV $\\mathrm{k}\\mathrm{e}\\mathrm{V}/\\mathrm{s}/\\mathrm{s}\\mathrm{r}/{\\mathrm{c}\\mathrm{m}}^{2}/\\mathrm{M}\\mathrm{e}\\mathrm{V}$, consistent with the typical values in observations. As a comparison, test particle simulations underestimate the precipitation intensity by nearly an order of magnitude. These results highlight the importance of self‐consistent simulations in quantifying electron precipitation and investigating wave‐particle interactions that modulate magnetospheric dynamics.

Collisionless shock waves, found in supernova remnants, interstellar, stellar, and planetary environments, and laboratories, are one of nature’s most powerful particle accelerators. This study combines in situ satellite measurements with recent theoretical developments to establish a reinforced shock acceleration model for relativistic electrons. Our model incorporates transient structures, wave-particle interactions, and variable stellar wind conditions, operating collectively in a multiscale set of processes. We show that the electron injection threshold is on the order of suprathermal range, obtainable through multiple different phenomena abundant in various plasma environments. Our analysis demonstrates that a typical shock can consistently accelerate electrons into very high (relativistic) energy ranges, refining our comprehension of shock acceleration while providing insight on the origin of electron cosmic rays. The mechanisms resulting in particle acceleration to relativistic energies in space plasmas are an open question. Here, the authors show a reinforced shock acceleration model which enables electrons to efficiently achieve relativistic energies and reveal a low electron injection threshold.

Shocks are one of nature’s most powerful particle accelerators and have been connected to relativistic electron acceleration and cosmic rays. Upstream shock observations include wave generation, wave-particle interactions and magnetic compressive structures, while at the shock and downstream, particle acceleration, magnetic reconnection and plasma jets can be observed. Here, using Magnetospheric Multiscale (MMS) we show in-situ evidence of high-speed downstream flows (jets) generated at the Earth’s bow shock as a direct consequence of shock reformation. Jets are observed downstream due to a combined effect of upstream plasma wave evolution and an ongoing reformation cycle of the bow shock. This generation process can also be applicable to planetary and astrophysical plasmas where collisionless shocks are commonly found. Several mechanisms exist for formation of jets observed in Earth’s magnetosheath. Here, the authors show evidence of high-speed downstream flows generated at the Earth’s bow shock as a direct consequence of shock reformation, which is different than the proposed mechanisms.