arXiv subjects

Drew L. Turner

Publications and source records attributed to Drew L. Turner.

At least 19 recordsLinked to original sources

arXiv2026

Compressive Structures in the Foreshock of Collisionless Shocks

Collisionless shocks are fundamental accelerators of energetic particles; yet, the observations of nonlinear foreshock structures, which are essential in acceleration processes, differ significantly between Interplanetary (IP) shocks and planetary bow shocks. We present a direct comparison of two high-Mach-number, quasi-parallel shocks: an IP shock observed by Solar Orbiter and the Earth's bow shock measured by the Magnetospheric Multiscale (MMS) mission during the 2024-2025 ``string-of-pearls'' campaign. We show that Foreshock Compressive Structures (FCSs) initiate upstream of both shocks at similar normalized distances ($\lesssim$50 ion inertial lengths, $d_i$) when the suprathermal ($>10$ keV) ion density exceeds $\sim$1\% of the background. However, the IP shock lacks the fully evolved, high-amplitude Short Large Amplitude Magnetic Structures (SLAMS) characteristic of the terrestrial foreshock. We demonstrate that the ``growth zone'' capable of sustaining these structures is spatially limited ($\sim$135 $d_i$), which, due to the high speed of the propagating IP shock, corresponds to a brief observational window of $<10$ s. Beyond this observational constraint, we suggest an additional physical mechanism that can inhibit foreshock maturity at IP shocks. The lack of global curvature prevents the lateral supply (``cross-talk'') of energetic ions from different shock regions. These findings suggest that while the fundamental physics of FCS initiation is unified across collisionless shocks, the achievement of full nonlinearity can be regulated by the unique shock geometry and upstream properties, while ultimately remaining observationally challenging to identify.

physics.space-ph↗

arXiv2025

Adaptive PCA-Based Outlier Detection for Multi-Feature Time Series in Space Missions

Analyzing multi-featured time series data is critical for space missions making efficient event detection, potentially onboard, essential for automatic analysis. However, limited onboard computational resources and data downlink constraints necessitate robust methods for identifying regions of interest in real time. This work presents an adaptive outlier detection algorithm based on the reconstruction error of Principal Component Analysis (PCA) for feature reduction, designed explicitly for space mission applications. The algorithm adapts dynamically to evolving data distributions by using Incremental PCA, enabling deployment without a predefined model for all possible conditions. A pre-scaling process normalizes each feature's magnitude while preserving relative variance within feature types. We demonstrate the algorithm's effectiveness in detecting space plasma events, such as distinct space environments, dayside and nightside transients phenomena, and transition layers through NASA's MMS mission observations. Additionally, we apply the method to NASA's THEMIS data, successfully identifying a dayside transient using onboard-available measurements.

cs.LG↗

arXiv2025

Universal energy limits of radiation belts in planetary and brown dwarf magnetospheric systems

Radiation belts are regions of magnetically trapped particle radiation found around all of the sufficiently magnetized planets in the Solar System and recently also observed around brown dwarfs, yet despite their ubiquity, there is not yet a general theory or model to predict the uppermost energy limits that any particular magnetospheric system's radiation belts can attain. By considering only the most fundamental loss processes, a model and corresponding theory are developed that successfully bound and explain the maximum observed energies of all documented radiation belt systems. Interestingly, this approach yields a relatively simple function for the uppermost energy limit that depends on only the surface magnetic field strength of the system. The model predicts an energy limit for all radiation belt systems that asymptotes at 7 +/- 2 TeV (for protons and electrons), offering intriguing new insight on potential sources of galactic cosmic rays. This model is also applied to an exoplanetary system, demonstrating that the planet is likely a synchrotron emitter and showcasing the model's use for identifying candidate targets for synchrotron-emitting astrophysical systems and revealing details critical to habitability at those remote worlds.

astro-ph.HE↗

arXiv2025

Energy partitioning between thermal and non-thermal electrons and ions in magnetotail reconnection

Magnetic reconnection is an explosive energy release event. It plays an important role in accelerating particles to high non-thermal energies. These particles often exhibit energy spectra characterized by a power-law distribution. However, the partitioning of energy between thermal and non-thermal components, and between ions and electrons, remains unclear. This study provides estimates of energy partition based on a statistical analysis of magnetic reconnection events in Earth's magnetotail using data from the Magnetospheric Multiscale (MMS) mission. Ions are up to ten times more energetic than electrons but have softer spectra. We found for both ions and electrons that, as the average energy of particles (temperature) increases, their energy spectra become \textit{softer} (steeper) and thus, the fraction of energy carried by the non-thermal components decreases. These results challenge existing theories of particle acceleration through magnetotail reconnection.

astro-ph.SR↗

arXiv2025

Revealing an unexpectedly low electron injection threshold via reinforced shock acceleration

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.

astro-ph.HE↗

arXiv2024

Relativistic Electron Acceleration and the 'Ankle' Spectral Feature in Earth's Magnetotail Reconnection

Electrons are accelerated to high, non-thermal energies during explosive energy-release events in space, such as magnetic reconnection. However, the properties and acceleration mechanisms of relativistic electrons directly associated with reconnection X-line are not well understood. This study utilizes Magnetospheric Multiscale (MMS) measurements to analyze the flux and spectral features of sub-relativistic to relativistic (~ 80 to 560 keV) electrons during a magnetic reconnection event in Earth's magnetotail. This event provided a unique opportunity to measure the electrons directly energized by X-line as MMS stayed in the separatrix layer, where the magnetic field directly connects to the X-line, for approximately half of the observation period. Our analysis revealed that the fluxes of relativistic electrons were clearly enhanced within the separatrix layer, and the highest flux was directed away from the X-line, which suggested that these electrons originated directly from the X-line. Spectral analysis showed that these relativistic electrons deviated from the main plasma sheet population and exhibited an "ankle" feature similar to that observed in galactic cosmic rays. The contribution of "ankle" electrons to the total electron energy density increased from 0.1% to 1% in the separatrix layer, though the spectral slopes did not exhibit clear variations. Further analysis indicated that while these relativistic electrons originated from the X-line, they experienced a non-negligible degree of scattering during transport. These findings provide clear evidence that magnetic reconnection in Earth's magnetotail can efficiently energize relativistic electrons directly at the X-line, providing new insights into the complex processes governing electron dynamics during magnetic reconnection.

astro-ph.HE↗

arXiv2024

Multi-Mission Observations of Relativistic Electrons and High-Speed Jets Linked to Shock Generated Transients

Shock-generated transients, such as hot flow anomalies (HFAs), upstream of planetary bow shocks, play a critical role in electron acceleration. Using multi-mission data from NASA's Magnetospheric Multiscale (MMS) and ESA's Cluster missions, we demonstrate the transmission of HFAs through Earth's quasi-parallel bow shock, associated with acceleration of electrons up to relativistic energies. Energetic electrons, initially accelerated upstream, are shown to remain broadly confined within the transmitted transient structures downstream, where betatron acceleration further boosts their energy due to elevated compression levels. Additionally, high-speed jets form at the compressive edges of HFAs, exhibiting a significant increase in dynamic pressure and potentially contributing to driving further localized compression. Our findings emphasize the efficiency of quasi-parallel shocks in driving particle acceleration far beyond the immediate shock transition region, expanding the acceleration region to a larger spatial domain. Finally, this study underscores the importance of multi-scale observational approach in understanding the convoluted processes behind collisionless shock physics and their broader implications.

astro-ph.HE↗

arXiv2024

Relativistic and Ultra-Relativistic Electron Bursts in Earth's Magnetotail Observed by Low-Altitude Satellites

Earth's magnetotail, a night-side region characterized by stretched magnetic field lines and strong plasma currents, is the primary site for the release of magnetic field energy and its transformation into plasma heating and kinetic energy plus charged particle acceleration during magnetic reconnection. In this study, we demonstrate that the efficiency of this acceleration can be sufficiently high to produce populations of relativistic and ultra-relativistic electrons, with energies up to several MeV, which exceeds all previous theoretical and simulation estimates. Using data from the low altitude ELFIN and CIRBE CubeSats, we show multiple events of relativistic electron bursts within the magnetotail, far poleward of the outer radiation belt. These bursts are characterized by power-law energy spectra and can be detected during even moderate substorms.

physics.space-ph↗

arXiv2023

Reimagining Heliophysics: A bold new vision for the next decade and beyond

The field of Heliophysics has a branding problem. We need an answer to the question: ``What is Heliophysics\?'', the answer to which should clearly and succinctly defines our science in a compelling way that simultaneously introduces a sense of wonder and exploration into our science and our missions. Unfortunately, recent over-reliance on space weather to define our field, as opposed to simply using it as a practical and relatable example of applied Heliophysics science, narrows the scope of what solar and space physics is and diminishes its fundamental importance. Moving forward, our community needs to be bold and unabashed in our definition of Heliophysics and its big questions. We should emphasize the general and fundamental importance and excitement of our science with a new mindset that generalizes and expands the definition of Heliophysics to include new ``frontiers'' of increasing interest to the community. Heliophysics should be unbound from its current confinement to the Sun-Earth connection and expanded to studies of the fundamental nature of space plasma physics across the solar system and greater cosmos. Finally, we need to come together as a community to advance our science by envisioning, prioritizing, and supporting -- with a unified voice -- a set of bold new missions that target compelling science questions - even if they do not explore the traditional Sun- and Earth-centric aspects of Heliophysics science. Such new, large missions to expand the frontiers and scope of Heliophysics science large missions can be the key to galvanizing the public and policymakers to support the overall Heliophysics program.

physics.space-ph↗

arXiv2023

Particle acceleration by magnetic reconnection in geospace

Particles are accelerated to very high, non-thermal energies during explosive energy-release phenomena in space, solar, and astrophysical plasma environments. While it has been established that magnetic reconnection plays an important role in the dynamics of Earth's magnetosphere, it remains unclear how magnetic reconnection can further explain particle acceleration to non-thermal energies. Here we review recent progress in our understanding of particle acceleration by magnetic reconnection in Earth's magnetosphere. With improved resolutions, recent spacecraft missions have enabled detailed studies of particle acceleration at various structures such as the diffusion region, separatrix, jets, magnetic islands (flux ropes), and dipolarization front. With the guiding-center approximation of particle motion, many studies have discussed the relative importance of the parallel electric field as well as the Fermi and betatron effects. However, in order to fully understand the particle acceleration mechanism and further compare with particle acceleration in solar and astrophysical plasma environments, there is a need for further investigation of, for example, energy partition and the precise role of turbulence.

physics.space-ph↗

arXiv2023

Multi-point Assessment of the Kinematics of Shocks (MAKOS): A Heliophysics Mission Concept Study

Collisionless shocks are fundamental processes that are ubiquitous in space plasma physics throughout the Heliosphere and most astrophysical environments. Earth's bow shock and interplanetary shocks at 1 AU offer the most readily accessible opportunities to advance our understanding of the nature of collisionless shocks via fully-instrumented, in situ observations. One major outstanding question pertains to the energy budget of collisionless shocks, particularly how exactly collisionless shocks convert incident kinetic bulk flow energy into thermalization (heating), suprathermal particle acceleration, and a variety of plasma waves, including nonlinear structures. Furthermore, it remains unknown how those energy conversion processes change for different shock orientations (e.g., quasi-parallel vs. quasi-perpendicular) and driving conditions (upstream Alfvénic and fast Mach numbers, plasma beta, etc.). Required to address these questions are multipoint observations enabling direct measurement of the necessary plasmas, energetic particles, and electric and magnetic fields and waves, all simultaneously from upstream, downstream, and at the shock transition layer with observatory separations at ion to magnetohydrodynamic (MHD) scales. Such a configuration of spacecraft with specifically-designed instruments has never been available, and this white paper describes a conceptual mission design -- MAKOS -- to address these outstanding questions and advance our knowledge of the nature of collisionless shocks.

astro-ph.IM↗

arXiv2022

Intense whistler-mode waves at foreshock transients: characteristics and regimes of wave-particle resonant interaction

Thermalization and heating of plasma flows at shocks result in unstable charged-particle distributions which generate a wide range of electromagnetic waves. These waves, in turn, can further accelerate and scatter energetic particles. Thus, the properties of the waves and their implication for wave-particle interactions are critically important for modeling energetic particle dynamics in shock environments. Whistler-mode waves, excited by the electron heat flux or a temperature anisotropy, arise naturally near shocks and foreshock transients. As a result, they can often interact with supra-thermal electrons. The low background magnetic field typical at the core of such transients and the large wave amplitudes may cause such interactions to enter the nonlinear regime. In this study, we present a statistical characterization of whistler-mode waves at foreshock transients around Earth bow shock, as they are observed under a wide range of upstream conditions. We find that a significant portion of them are sufficiently intense and coherent to warrant nonlinear treatment. Copious observations of background magnetic field gradients and intense whistler wave amplitudes suggest that phase trapping, a very effective mechanism for electron acceleration in inhomogeneous plasmas, may be the cause. We discuss the implications of our findings for electron acceleration in planetary and astrophysical shock environments.

physics.plasm-ph↗

arXiv2021

Direct Evidence for Magnetic Reflection of Heavy Ions from High Mach Number Collisionless Shocks

Strong shocks in collisionless plasmas, such as supernovae shocks and shocks driven by coronal mass ejections, are known to be a primary source of energetic particles. Due to their different mass per charge ratio, the interaction of heavy ions with the shock layer differs from that of protons, and injection of these ions into acceleration processes is a challenge. Here we show the first direct observational evidence of magnetic reflection of alpha particles from a high Mach number quasi-perpendicular shock using in-situ spacecraft measurements. The intense magnetic amplification at the shock front associated with nonstationarity modulates the trajectory of alpha particles, some of which travel back upstream as they gyrate in the enhanced magnetic field and experience further acceleration in the upstream region. Our results in particular highlight the important role of high magnetic amplification in seeding heavy ions into the energization processes at nonstationary reforming shocks.

physics.plasm-ph↗

arXiv2021

Local acceleration of protons to 100 keV in a quasi-parallel bow shock

Recent observations in the quasi-parallel bow shock by the MMS spacecraft show rapid heating and acceleration of ions up to an energy of about 100 keV. It is demonstrated that a prominent acceleration mechanism is the nonlinear interaction with a spectrum of waves produced by gradient driven instabilities, including the lower hybrid drift (LHD) instability, modified two-stream (MTS) instability and electron cyclotron drift (ECD) instability. Test-particle simulations show that the observed spectrum of waves can rapidly accelerate protons up to a few hundreds keV by the ExB mechanism. The ExB wave mechanism is related to the surfatron mechanism at shocks but through the coupling with the stochastic heating condition it produces significant acceleration on much shorter temporal and spatial scales by the interaction with bursts of waves within a cyclotron period. The results of this paper are built on the heritage of four-point measurement techniques developed for the Cluster mission and imply that the concepts of Fermi acceleration, diffusive shock acceleration, and shock drift acceleration are not needed to explain proton acceleration to hundreds keV at the Earth's bow shock.

physics.space-ph↗

arXiv2020

In situ evidence of ion acceleration between consecutive reconnection jet fronts

Processes driven by unsteady reconnection can efficiently accelerate particles in many astrophysical plasmas. An example are the reconnection jet fronts in an outflow region. We present evidence of suprathermal ion acceleration between two consecutive reconnection jet fronts observed by the Magnetospheric Multiscale mission in the terrestrial magnetotail. An earthward propagating jet is approached by a second faster jet. Between the jets, the thermal ions are mostly perpendicular to magnetic field, are trapped and are gradually accelerated in the parallel direction up to 150 keV. Observations suggest that ions are predominantly accelerated by a Fermi-like mechanism in the contracting magnetic bottle formed between the two jet fronts. The ion acceleration mechanism is presumably efficient in other environments where jet fronts produced by variable rates of reconnection are common and where the interaction of multiple jet fronts can also develop a turbulent environment, e.g. in stellar and solar eruptions.

physics.plasm-ph↗

arXiv2020

Electron energy partition across interplanetary shocks: III. Analysis

Analysis of model fit results of 15,210 electron velocity distribution functions (VDFs), observed within $\pm$2 hours of 52 interplanetary (IP) shocks by the Wind spacecraft near 1 AU, is presented as the third and final part on electron VDFs near IP shocks. The core electrons and protons dominate in the magnitude and change in the partial-to-total thermal pressure ratio, with the core electrons often gaining as much or more than the protons. Only a moderate positive correlation is observed between the electron temperature and the kinetic energy change across the shock, while weaker, if any, correlations were found with any other macroscopic shock parameter. No VDF parameter correlated with the shock normal angle. The electron VDF evolves from a narrowly peaked core with flaring suprathermal tails in the upstream to either a slightly hotter core with steeper tails or much hotter flattop core with even steeper tails downstream of the weaker and strongest shocks, respectively. Both quasi-static and fluctuating fields are examined as possible mechanisms modifying the VDF but neither is sufficient alone. For instance, flattop VDFs can be generated by nonlinear ion acoustic wave stochastic acceleration (i.e., inelastic collisions) while other work suggested they result from the combination of quasi-static and fluctuating fields. This three-part study shows that not only are these systems not thermodynamic in nature, even kinetic models may require modification to include things like inelastic collision operators to properly model electron VDF evolution across shocks or in the solar wind.

physics.space-ph↗

arXiv2019

Electron energy partition across interplanetary shocks: II. Statistics

A statistical analysis of 15,210 electron velocity distribution function (VDF) fits, observed within $\pm$2 hours of 52 interplanetary (IP) shocks by the $Wind$ spacecraft near 1 AU, is presented. This is the second in a three-part series on electron VDFs near IP shocks. The electron velocity moment statistics for the dense, low energy core, tenuous, hot halo, and field-aligned beam/strahl are a statistically significant list of values illustrated with both histograms and tabular lists for reference and baselines in future work. The beam/strahl fit results in the upstream are currently the closest thing to a proper parameterization of the beam/strahl electron velocity moments in the ambient solar wind. This work will also serve as a 1 AU baseline and reference for missions like $Parker \ Solar \ Probe$ and $Solar \ Orbiter$. The median density, temperature, beta, and temperature anisotropy values for the core(halo)[beam/strahl] components, with subscripts $ec$($eh$)[$eb$], of all fit results respectively are $n{\scriptstyle_{ec(h)[b]}}$ $\sim$ 11.3(0.36)[0.17] $cm^{-3}$, $T{\scriptstyle_{ec(h)[b], tot}}$ $\sim$ 14.6(48.4)[40.2] $eV$, $β{\scriptstyle_{ec(h)[b], tot}}$ $\sim$ 0.93(0.11)[0.05], and $\mathcal{A}{\scriptstyle_{ec(h)[b]}}$ $\sim$ 0.98(1.03)[0.93]. The nuanced details of the fitting method and data product description were published in Paper I and the detailed analysis of the results will be shown in Paper III.

physics.space-ph↗

arXiv2019

Electron energy partition across interplanetary shocks: I. Methodology and Data Product

Analysis of 15314 electron velocity distribution functions (VDFs) within $\pm$2 hours of 52 interplanetary (IP) shocks observed by the \emph{Wind} spacecraft near 1 AU are introduced. The electron VDFs are fit to the sum of three model functions for the cold dense core, hot tenuous halo, and field-aligned beam/strahl component. The best results were found by modeling the core as either a bi-kappa or a symmetric (or asymmetric) bi-self-similar velocity distribution function, while both the halo and beam/strahl components were best fit to bi-kappa velocity distribution function. This is the first statistical study to show that the core electron distribution is better fit to a self-similar velocity distribution function than a bi-Maxwellian under all conditions. The self-similar distribution deviation from a Maxwellian is a measure of inelasticity in particle scattering from waves and/or turbulence. The range of values defined by the lower and upper quartiles for the kappa exponents are $κ{\scriptstyle_{ec}}$ $\sim$ 5.40--10.2 for the core, $κ{\scriptstyle_{eh}}$ $\sim$ 3.58--5.34 for the halo, and $κ{\scriptstyle_{eb}}$ $\sim$ 3.40--5.16 for the beam/strahl. The lower-to-upper quartile range of symmetric bi-self-similar core exponents are $s{\scriptstyle_{ec}}$ $\sim$ 2.00--2.04, and asymmetric bi-self-similar core exponents are $p{\scriptstyle_{ec}}$ $\sim$ 2.20--4.00 for the parallel exponent, and $q{\scriptstyle_{ec}}$ $\sim$ 2.00--2.46 for the perpendicular exponent. The nuanced details of the fit procedure and description of resulting data product are also presented. The statistics and detailed analysis of the results are presented in Paper II and Paper III of this three-part study.

physics.space-ph↗