Search

We use the Magnetospheric Multiscale mission to observe a bi‐directional electron acceleration event in the electron foreshock upstream of Earth's quasi‐perpendicular collisionless bow shock. The acceleration region is associated with a decrease in wave activity, inconsistent with common electron acceleration mechanisms such as Diffusive Shock Acceleration and Stochastic Shock Drift Acceleration. We propose a two‐step acceleration process where an electron field‐aligned beam acts as a seed population further accelerated by a shrinking magnetic bottle process, with the shock acting as the magnetic mirror(s). Plain Language Summary: Collisionless shock waves are believed to be an important source of accelerating particles up to cosmic ray energies throughout our universe. In this letter, we use spacecraft data from the Magnetospheric Multiscale mission to study an energetic electron event observed at Earth's bow shock. The event displays inconsistencies with common electron acceleration mechanisms previously studied at collisionless shocks. We propose a two‐step acceleration mechanism, combining two known mechanisms for charged particle acceleration, and provide observational evidence supporting our theory. We conclude that plasma wave‐particle interactions at the shock play a crucial role in the energization of these electrons. Key Points: Using Magnetospheric Multiscale data we observe bi‐directional energetic electrons at Earth's collisionless bow shockThe observations are inconsistent with common electron acceleration mechanisms at shocksWe propose a two‐step acceleration process where a field‐aligned electron beam is further accelerated by a shrinking magnetic bottle [ABSTRACT FROM AUTHOR]

A long‐standing problem concerning dipolarization front (DF) is why energetic electrons only appear in half the DF events? By analyzing MMS measurements, here we answer this question. We find a DF structure, behind which energetic‐electron fluxes are modulated by magnetosonic waves: At wave troughs (B‐minimum) electron fluxes are high; at wave crests (B‐maximum) electron fluxes are low. This phenomenon challenges the classical theory of betatron mechanism, so we need to propose a new theory to explain it. In our theory, there exists a magnetic bottle with time‐varying belly but steady neck behind the DF. When the belly expands, a magnetic bottle is formed, and electrons are trapped; when the belly contracts, the magnetic bottle disappears, and electrons are expelled. Quantitatively, we validate the existence of this bottle and estimate its size as 2–3 RE. Our theory can explain both the presence and absence of energetic electrons behind DFs. Key Points: Energetic‐electron fluxes increase at B‐minimum but decrease at B‐maximumThis phenomenon challenges the classical theory and thus requires new explanationOur new theory includes a magnetic bottle with time‐varying belly but steady neck [ABSTRACT FROM AUTHOR]

A magnetic bottle device with optimized sizes for high-efficiency positron moderation was manufactured and attached to a positron detection system based on two gamma detectors operating in coincidence to detect positron annihilation events. The detection system's performance was tested with a 22Na calibrated source. Titanium foils with a natural isotope mixture that were 2- μ m-thick were proton irradiated to produce 48V. Experiments with a stack of up to four 48V foils showed that the magnetic bottle trap achieved a 10-fold increase in the standard positron moderator efficiency of tungsten in transmission mode. However, the measured moderation efficiency of ∼ 0.4% was found to be in huge discrepancy with the value of 1.8% obtained by Gerchow et al. for a non-optimized trap. One possible reason for this discrepancy was pointed out, namely that Gerchow et al mainly detected moderated positrons but almost no fast/unmoderated positrons, which escape the magnetic bottle trap and should also reach the positron detector used. Our experimental and simulation results showed that the ratio of moderated to fast/unmoderated positrons that reach our detection system is about 1:7. [ABSTRACT FROM AUTHOR]

We present a case study of the Magnetospheric Multiscale (MMS) observations of the Southern Hemispheric dayside magnetospheric boundaries under southward interplanetary magnetic field direction with strong By component. During this event MMS encountered several magnetic field depressions characterized by enhanced plasma beta and high fluxes of high‐energy electrons and ions at the dusk sector of the southern cusp region that resemble previous Cluster and Polar observations of cusp diamagnetic cavities. Based on the expected maximum magnetic shear model and magnetohydrodynamic simulations, we show that for the present event the diamagnetic cavity‐like structures were formed in an unusual location. Analysis of the composition measurements of ion velocity distribution functions and magnetohydrodynamics simulations show clear evidence of the creation of a new kind of magnetic bottle structures by component reconnection occurring at lower latitudes. We propose that the high‐energy particles trapped in these cavities can sometimes end up in the loss cone and leak out, providing a likely explanation for recent high‐energy particle leakage events observed in the magnetosheath. Plain Language Summary: Understanding the origin, acceleration mechanisms, and ultimately predicting the space radiation environment of high‐energy particles is critical for safe unmanned and manned space operations. In this article we present the first observations of the generation mechanism of a new kind of magnetic bottle at the high‐latitude region of the Earth's magnetic shield with high‐fluxes of high‐energy particles (> 100 keV) using data from National Aeronautics and Space Administration's Magnetosphere Multiscale mission and numerical simulations. The Magnetosphere Multiscale composition measurements of He++, O+, H+, and e− as well as sensitivity to different energies allows us to calculate the size of the magnetic bottle, which for the present event was created by magnetic reconnection about 8 RE from the spacecraft. We show the first observations of the counterstreaming O+ of ionospheric origin in the magnetic field depressions. We show that particles from these bottles can sometimes "leak out" to the magnetosheath providing a likely explanation for recent high‐energy particle leakage event observations. Key Points: MMS orbit can reach high‐latitude dayside magnetopause and associated boundariesMMS observed trapped energetic electrons, protons, He++, and O+ in magnetic field depressions likely created by low‐latitude reconnectionCounterstreaming O+ and He++ was observed in some of the depression regions [ABSTRACT FROM AUTHOR]

Abstract: The sudden enhancements of magnetic strength, named magnetic peaks (MPs), are often observed in the magnetosheath of magnetized planets. They are usually identified as flux ropes (FRs) or magnetic mirror mode structures. Previous studies of MPs are mostly on the magnetohydrodynamics (MHD) scale. In this study, an electron scale MP is reported in the Earth magnetosheath. We present a typical case with a scale of ~7 electron gyroradii and a duration of ~0.18 s. A strong magnetic disturbance and associated electrical current are detected. Electron vortex is found perpendicular to the magnetic field line and is self‐consist with the peak. We use multipoint spacecraft techniques to determine the propagation velocity of the MP structure and find that the magnetic peak does propagate relative to the plasma (ion) flow. This is very different from the magnetic mirror mode that does not propagate relative to the plasma flow. Furthermore, we developed an efficient method that can effectively distinguish “magnetic bottle like” and “FRs like” structures. The MP presented in this study is identified as magnetic bottle like type. The mechanism to generate the electron scale magnetic bottle like structure is still unclear, suggesting that new theory needs to be developed to understand such small‐scale phenomena. [ABSTRACT FROM AUTHOR]

We report on the realisation of a cyclotron trap assisted positron tungsten moderator for the conversion of positrons with a broad keV–few MeV energy spectrum to a mono-energetic eV beam with an efficiency of 1.8(2)% defined as the ratio of the slow positrons divided by the β+ activity of the radioactive source. This is an improvement of almost two orders of magnitude compared to the state of the art of tungsten moderators. The simulation validated with this measurement suggests that, using an optimised setup, even higher efficiencies are achievable. A novel method for the production of 48V high-activity thin foil positron sources based on a solid target station at a medical cyclotron was developed. This is an improvement of more than one order of magnitude with respect to standard methods.

The oscillation frequencies of charged particles in a Penning trap can serve as sensors for spectroscopy when additional field components are introduced to the magnetic and electric fields used for confinement. The presence of so-called “magnetic bottles” and specific electric anharmonicities creates calculable energy-dependences of the oscillation frequencies in the radiofrequency domain which may be used to detect the absorption or emission of photons both in the microwave and optical frequency domains. The precise electronic measurement of these oscillation frequencies therefore represents an optical sensor for spectroscopy. We discuss possible applications for precision laser and microwave spectroscopy and their role in the determination of magnetic moments and excited state lifetimes. Also, the trap-assisted measurement of radiative nuclear de-excitations in the X-ray domain is discussed. This way, the different applications range over more than 12 orders of magnitude in the detectable photon energies, from below μeV in the microwave domain to beyond MeV in the X-ray domain.

The oscillation frequencies of charged particles in a Penning trap can serve as sensors for spectroscopy when additional field components are introduced to the magnetic and electric fields used for confinement. The presence of so-called "magnetic bottles" and specific electric anharmonicities creates calculable energy-dependences of the oscillation frequencies in the radiofrequency domain which may be used to detect the absorption or emission of photons both in the microwave and optical frequency domains. The precise electronic measurement of these oscillation frequencies therefore represents an optical sensor for spectroscopy. We discuss possible applications for precision laser and microwave spectroscopy and their role in the determination of magnetic moments and excited state lifetimes. Also, the trap-assisted measurement of radiative nuclear de-excitations in the X-ray domain is discussed. This way, the different applications range over more than 12 orders of magnitude in the detectable photon energies, from below μeV in the microwave domain to beyond MeV in the X-ray domain. [ABSTRACT FROM AUTHOR]

Abstract: Auger electron spectroscopy (AES) and photoelectron spectroscopy (PES) are (with X-ray emission spectroscopy, XES) powerful analytical tools for material science and gas phase studies. However, the interpretation of Auger spectra can be very difficult due to the number and complexity of the involved processes. A deeper analysis, that allows a better understanding of relaxation processes following inner shell ionization, is possible with coincidence Auger spectroscopy. This method gives a new insight into electron correlation and allows disentangling of complex Auger electron spectra. In this paper, we present some examples related to gas phase coincidence Auger electron spectroscopy using synchrotron radiation. The detection in coincidence of an Auger electron with a threshold photoelectron presents two main advantages which are good energy resolution and high coincidence count rates. This technique has also provided new results on double Auger decay processes. A further qualitative breakthrough has been made with the development of a new experimental set-up based on a magnetic bottle time-of-flight electron spectrometer. This opens up the field of multi-electron coincidence spectroscopy and allows a most detailed analysis with characterization of all possible decay pathways following inner shell ionization. [Copyright &y& Elsevier]

The design optimization of a magnetic bottle device for high-efficiency positron moderation is described herein. The interaction between positrons and matter, including positron tracking, was performed via GEANT and COMSOL simulation. Optimization was performed for a 10 kW power supply for the excitation of two magnetic coils and were established geometry sizes of the device, namely a chamber with a radius of 100 mm and a length of 300 mm. The irradiation of a Ti foil with a thickness of 1 ÷ 10 μ m at the TR19 cyclotron for 8 h with a proton current of 10 μ A can produce 48V with an activity of 6 ÷ 60 MBq. Given such a source of fast positrons for use in the optimized magnetic bottle device, a slow positron beam intensity of 2 ÷ 10 × 104 s−1 can be achieved. [ABSTRACT FROM AUTHOR]