arXiv subjects

David J. McComas

Publications and source records attributed to David J. McComas.

At least 19 recordsLinked to original sources

arXiv2026

Local Interstellar Flow Parameters from the First Intersection of IMAP-Lo's Parameter Tubes

The Sun's motion through the interstellar medium creates a flow of interstellar neutral (ISN) atoms through the heliosphere. ISN He, due to its high universal abundance and relatively low ionization rate, is the most abundant of the interstellar species near 1 au and ideal for flow parameter determination. The Interstellar Boundary Explorer (IBEX) measurements of ISN He flow parameters (speed, temperature, and direction) yielded a tube in 4D parameter space -- narrow in cross-section but highly extended along one parameter axis (e.g., ecliptic longitude direction). This ``4D parameter tube'' results in large systematic uncertainties, a direct consequence of IBEX-Lo's fixed viewing orientation on the spacecraft. On the Interstellar Mapping and Acceleration Probe (IMAP), the articulation of the IMAP-Lo boresight using its pivot platform enables multiple viewing orientations of the ISN flow for significant systematic uncertainty reduction. We provide first results that definitively intersect ISN parameter tubes for elongation angles $79^\circ$, 94$^\circ$, and 109$^\circ$, resulting in precise interstellar parameters: speed $26.37 \pm 0.82$ km s$^{-1}$, ecliptic longitude direction $74.85^\circ \pm 0.96^\circ$, ecliptic latitude direction $-5.212^\circ \pm 0.035^\circ$, and temperature $7740^{+770}_{-730}$ K. The inferred flow of the Very Local Interstellar Medium is not consistent with either the Local Interstellar Cloud or the G-Cloud, but rather an intermediate state. IMAP is now positioned to study the detailed physics of this complex, nearby interstellar region. By resolving and understanding its physics, we determine how the heliosphere responds to the local interstellar flow, and how it may evolve in time

astro-ph.GA↗

arXiv2026

Review of Machine Learning Models for Solar Energetic Particle Prediction

Solar energetic particle (SEP) events have attracted increasing attention due to their significant radiation hazards for aviation, spacecraft electronics, and human missions beyond Earth's magnetosphere. From a scientific perspective, SEP events are intriguing because they arise from a set of physical processes extending from the solar surface and corona through the heliosphere, offering insight into particle acceleration and transport mechanisms that are widely applicable across astrophysics. Therefore, advancing our ability to understand and predict SEP events is essential both for deepening our knowledge of such mechanisms and for safeguarding space technologies and exploration. Traditionally, researchers have modeled SEPs using physics-based simulations and empirical methods. More recently, machine learning (ML) has emerged as a new tool for understanding and predicting SEP events. The purpose of this manuscript is to review the currently available ML models for SEP prediction, identify the datasets used for training, compare their architectures, inputs, and outputs, and, based on these insights, outline good practices and recommendations for future research.

astro-ph.SR↗

arXiv2026

Polarimeter to Unify the Corona and Heliosphere (PUNCH)

The Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission is a NASA Small Explorer to determine the cross-scale processes that unify the solar corona and heliosphere. PUNCH has two science objectives: (1) understand how coronal structures become the ambient solar wind, and (2) understand the dynamic evolution of transient structures, such as coronal mass ejections, in the young solar wind. To address these objectives, PUNCH uses a constellation of four small spacecraft in Sun-synchronous low Earth orbit, to collect linearly polarized images of the K corona and young solar wind. The four spacecraft each carry one visible-light imager in a 1+3 configuration: a single Narrow Field Imager solar coronagraph captures images of the outer corona at all position angles, and at solar elongations from 1.5 degrees (6 R$_\odot$) to 8 degrees (32 R$_\odot$); and three separate Wide Field Imager heliospheric imagers together capture views of the entire inner solar system, at solar elongations from 3 degrees (12 R$_\odot$) to 45 degrees (180 R$_\odot$) from the Sun. PUNCH images include linear-polarization data, to enable inferring the three-dimensional structure of visible features without stereoscopy. The instruments are matched in wavelength passband, support overlapping instantaneous fields of view, and are operated synchronously, to act as a single ``virtual instrument'' with a 90 degree wide field of view, centered on the Sun. PUNCH launched in March of 2025 and began science operations in June of 2025. PUNCH has an open data policy with no proprietary period, and PUNCH Science Team Meetings are open to all.

astro-ph.SR↗

arXiv2025

Ionization Rate of Interstellar Neutral Helium from New Horizons/SWAP Observations

Interstellar neutral (ISN) atoms enable studies of the physical conditions in the local interstellar medium surrounding the heliosphere. ISN helium, which is the most abundant species at 1 au, is directly observed by space missions, such as Interstellar Boundary Explorer (IBEX). However, some of these atoms are ionized by solar ultraviolet radiation before reaching 1 au, producing pickup ions (PUIs). A recent analysis of IBEX data suggests that the helium photoionization rates predicted by models are underestimated by up to 40%. The Solar Wind Around Pluto (SWAP) instrument on board New Horizons enables the study of PUIs giving complementary insight into the other side of the ionization process. Our goal is to verify this increased helium ionization by determining the ionization rate of ISN helium in the heliosphere based on the SWAP observations of helium PUIs. For this purpose, we analyze SWAP data collected between 2012 and 2022, at distances 22 to 54 au from the Sun. We develop a new method for fitting model distribution functions to the observational data using the maximum likelihood method. Our approach accounts for the spacecraft's rotation and the SWAP response function, which depends on both energy and inflow direction. We estimate SWAP's efficiency for helium relative to that for hydrogen and determine the ISN helium ionization rate. We find that the photoionization rate obtained from the SWAP observations is 43% larger than the rates predicted by models, confirming the IBEX results.

physics.space-ph↗

arXiv2025

High-Resolution Observations of Pickup Ion Mediated Shocks to 60 au

This study provides a detailed analysis of fourteen distant interplanetary shocks observed by the Solar Wind Around Pluto (SWAP) instrument onboard New Horizons. These shocks were observed with a pickup ion data cadence of approximately 30 minutes, covering a heliocentric distance range of ~52-60 au. All the shocks observed within this distance range are fast-forward shocks, and the shock compression ratios vary between ~1.2 and 1.9. The shock transition scales are generally narrow, and the SW density compressions are more pronounced compared to the previous study of seven shocks by McComas et al. (2022). A majority (64%) of these shocks have upstream sonic Mach numbers greater than one. In addition, all high-resolution measurements of distant interplanetary shocks analyzed here show that the shock transition scale is independent of the shock compression ratio. However, the shock transition scale is strongly anti-correlated with the shock speed in the upstream plasma frame, meaning that faster shocks generally yield sharper transitions.

physics.space-ph↗

arXiv2025

What defines stationarity in space plasmas

Starting from the concept of entropy defect in thermodynamics, we construct the entropy formulation of space plasmas, and then use it to develop a measure of their stationarity. In particular, we show that statistics of this entropy results in two findings that improve our understanding of stationary and nonstationary systems: (i) variations of the Boltzmann-Gibbs (BG) entropy do not exceed twice the value of the thermodynamic kappa, the parameter that provides a measure of the entropy defect in both stationary and nonstationary states, while becomes the shape parameter that labels the kappa distributions in stationary states; and (ii) the ratio of the deviation of the BG entropy with kappa scales with the kappa deviation via a power-law, while the respective exponent provides a stationarity deviation index (SDI), which measures the natural tendency of the system to depart from stationarity. We confirm the validity of these findings in three different heliospheric plasma datasets observed from three missions: (1) A solar energetic particle event, recorded by the Integrated Science Investigation of the Sun instrument onboard Parker Solar Probe; (2) Near Earth solar wind protons recorded by the Solar Wind Experiment instrument onboard WIND; and (3) Plasma protons in the heliosheath, source of energetic neutral atoms recorded by IBEX. The full strength and capability of the entropic deviation ratio and SDI can now be used by the space physics community for analyzing and characterizing the stationarity of space plasmas, as well as other researchers for analyzing any other correlated systems.

physics.space-ph↗

arXiv2024

The Theory of Thermodynamic Relativity

We introduce the theory of thermodynamic relativity, a unified framework for describing both entropies and velocities, and their respective disciplines of thermodynamics and kinematics, which share a surprisingly identical description with relativity. This is the first study to generalize relativity in a thermodynamic context, leading naturally to anisotropic and nonlinear adaptations of relativity; thermodynamic relativity constitutes a new path of generalization, as compared to the traditional passage from special to general theory based on curved spacetime. We show that entropy and velocity are characterized by three identical postulates, providing the basis of a broader framework of relativity: (1) no privileged reference frame with zero value; (2) existence of an invariant and fixed value for all reference frames; and (3) existence of stationarity. The postulates lead to a unique way of addition for entropies and for velocities, called kappa addition. The theory provides a systematic method of constructing a generalized framework of the theory of relativity, based on the kappa addition, fully consistent with both thermodynamics and kinematics. From the generality of the kappa addition, we focus on the cases corresponding to linear special relativity. Then, we show how the thermodynamic relativity leads to the addition of entropies in nonextensive thermodynamics and the addition of velocities in the isotropic special relativity of Einstein, as in two extreme cases, while intermediate cases correspond to an anisotropic adaptation of relativity. Using thermodynamic relativity for velocities, we construct the anisotropic special relativity (asymmetric Lorentz transformation, nondiagonal metric, energy momentum velocity relations). Then, we discuss the consequences of the anisotropy in known relativistic effects: (1) matter antimatter asymmetry, (2) time dilation, and (3) Doppler effect.

physics.gen-ph↗

arXiv2023

The Sun's Alfven Surface: Recent Insights and Prospects for the Polarimeter to Unify the Corona and Heliosphere (PUNCH)

The solar wind is the extension of the Sun's hot and ionized corona, and it exists in a state of continuous expansion into interplanetary space. The radial distance at which the wind's outflow speed exceeds the phase speed of Alfvenic and fast-mode magnetohydrodynamic (MHD) waves is called the Alfven radius. In one-dimensional models, this is a singular point beyond which most fluctuations in the plasma and magnetic field cannot propagate back down to the Sun. In the multi-dimensional solar wind, this point can occur at different distances along an irregularly shaped "Alfven surface." In this article, we review the properties of this surface and discuss its importance in models of solar-wind acceleration, angular-momentum transport, MHD waves and turbulence, and the geometry of magnetically closed coronal loops. We also review the results of simulations and data analysis techniques that aim to determine the location of the Alfven surface. Combined with recent perihelia of Parker Solar Probe, these studies seem to indicate that the Alfven surface spends most of its time at heliocentric distances between about 10 and 20 solar radii. It is becoming apparent that this region of the heliosphere is sufficiently turbulent that there often exist multiple (stochastic and time-dependent) crossings of the Alfven surface along any radial ray. Thus, in many contexts, it is more useful to make use of the concept of a topologically complex "Alfven zone" rather than one closed surface. This article also reviews how the Polarimeter to Unify the Corona and Heliosphere (PUNCH) mission will measure the properties of the Alfven surface and provide key constraints on theories of solar-wind acceleration.

astro-ph.SR↗

arXiv2023

Investigating the IBEX Ribbon Structure a Solar Cycle Apart

A Ribbon of enhanced energetic neutral atom (ENA) emissions was discovered by the Interstellar Boundary Explorer (IBEX) in 2009, redefining our understanding of the heliosphere boundaries and the physical processes occurring at the interstellar interface. The Ribbon signal is intertwined with that of a globally distributed flux (GDF) that spans the entire sky. To a certain extent, Ribbon separation methods enabled examining its evolution independent of the underlying GDF. Observations over a full solar cycle revealed the Ribbon's evolving nature, with intensity variations closely tracking those of the solar wind (SW) structure after a few years delay accounting for the SW-ENA recycling process. In this work, we examine the Ribbon structure, namely, its ENA fluxes, angular extent, width, and circularity properties for two years, 2009 and 2019, representative of the declining phases of two adjacent solar cycles. We find that, (i) the Ribbon ENA fluxes have recovered in the nose direction and south of it down to ~ 25° (for energies below 1.7 keV) and not at mid and high ecliptic latitudes; (ii) The Ribbon width exhibits significant variability as a function of azimuthal angle; (iii) Circularity analysis suggests that the 2019 Ribbon exhibits a statistically consistent radius with that in 2009. The Ribbon's partial recovery is aligned with the consensus of a heliosphere with its closest point being southward of the nose region. The large variability of the Ribbon width as a function of Azimuth in 2019 compared to 2009 is likely indicative of small-scale processes within the Ribbon.

astro-ph.SR↗

arXiv2022

The polytropic behavior of Solar Wind protons as observed by Ulysses spacecraft during Solar minimum

We analyze proton bulk parameters derived from Ulysses observations and investigate the polytropic behavior of solar wind protons over a wide range of heliocentric distances and latitudes. The large-scale variations of the proton density and temperature over heliocentric distance, indicate that plasma protons are governed by sub-adiabatic processes (polytropic index $γ$ < 5/3), if we assume protons with three effective kinetic degrees of freedom. From the correlation between the small-scale variations of the plasma density and temperature in selected sub-intervals, we derive a polytropic index $γ$ ~ 1.4 on average. Further examination shows that the polytropic index does not depend on the solar wind speed. This agrees with the results of previous analyses of solar wind protons at ~ 1 au. We find that the polytropic index varies slightly over the range of the heliocentric distances and heliographic latitudes explored by Ulysses. We also show that the homogeneity of the plasma and the accuracy of the polytropic model applied to the data-points vary over Ulysses orbit. We compare our results with the results of previous studies which derive the polytropic index of solar wind ions within the heliosphere using observations from various spacecraft. We finally discuss the implications of our findings in terms of heating mechanisms and the effective degrees of freedom of the plasma protons.

physics.space-ph↗

arXiv2022

Physical correlations lead to kappa distributions

The recently developed concept of "entropic defect" is important for understanding the foundations of thermodynamics in space plasma physics, and more generally, for systems with physical correlations among their particles. Using this concept, this paper derives the basic formulation of the distribution function of velocities (or kinetic energies) in space plasma particle populations. Earlier analyses have shown how the formulation of kappa distributions is interwoven with the presence of correlations among the particles' velocities. This paper shows, for the first time, that the reverse is true: the thermodynamics of particles' physical correlations are consistent only with the existence of kappa distributions.

physics.plasm-ph↗

arXiv2022

Mixing Interstellar Clouds Surrounding the Sun

On its journey through the Galaxy, the Sun passes through diverse regions of the interstellar medium. High-resolution spectroscopic measurements of interstellar absorption lines in spectra of nearby stars show absorption components from more than a dozen warm partially ionized clouds within 15 pc of the Sun. The two nearest clouds - the Local Interstellar Cloud (LIC) and Galactic (G) cloud - move toward each other. Their bulk heliocentric velocities can be compared with the interstellar neutral helium flow velocity obtained from space-based experiments. We combine recent results from Ulysses, IBEX, and STEREO observations to find a more accurate estimate of the velocity and temperature of the very local interstellar medium. We find that, contrary to the widespread viewpoint that the Sun resides inside the LIC, the locally observed velocity of the interstellar neutral helium is consistent with a linear combination of the velocities of the LIC and G cloud, but not with either of these two velocities. This finding shows that the Sun travels through a mixed-cloud interstellar medium composed of material from both these clouds. Interactions between these clouds explain the substantially higher density of the interstellar hydrogen near the Sun and toward stars located within the interaction region of these two clouds. The observed asymmetry of the interstellar helium distribution function also supports this interaction. The structure and equilibrium in this region require further studies using in situ and telescopic observations.

astro-ph.GA↗

arXiv2021

Geometry of Magnetic Fluctuations near the Sun from PSP

Solar wind magnetic fluctuations exhibit anisotropy due to the presence of a mean magnetic field in the form of the Parker spiral. Close to the Sun, direct measurements were not available until the recently launched Parker Solar Probe (PSP) mission. The nature of anisotropy and geometry of the magnetic fluctuations play a fundamental role in dissipation processes and in the transport of energetic particles in space. Using PSP data, we present measurements of geometry and anisotropy of the inner heliosphere magnetic fluctuations, from fluid to kinetic scales. The results are surprising and different from 1 au observations. We find that fluctuations evolve characteristically with size scale. However, unlike 1 au solar wind, at the outer scale, the fluctuations are dominated by wavevectors quasi-parallel to the local magnetic field. In the inertial range, average wave vectors become less field-aligned, but still remain more field aligned than near-Earth solar wind. In the dissipation range, the wavevectors become almost perpendicular to the local magnetic field in the dissipation range, to a much higher degree than those indicated by 1 au observations. We propose that this reduced degree of anisotropy in the outer scale and inertial range is due to the nature of large-scale forcing outside the solar corona.

astro-ph.SR↗

arXiv2021

Anomalous Cosmic Ray Oxygen Observations in to 0.1 au

The Integrated Science Investigation of the Sun instrument suite onboard NASA's Parker Solar Probe mission continues to measure solar energetic particles and cosmic rays closer to the Sun than ever before. Here, we present the first observations of cosmic rays into 0.1 au (21.5 solar radii), focusing specifically on oxygen from ~2018.7 to ~2021.2. Our energy spectra reveal an anomalous cosmic ray-dominated profile that is comparable to that at 1 au, across multiple solar cycle minima. The galactic cosmic ray-dominated component is similar to that of the previous solar minimum (Solar Cycle 24/25 compared to 23/24) but elevated compared to the past (Solar Cycle 20/21). The findings are generally consistent with the current trend of unusually weak solar modulation that originated during the previous solar minimum and continues today. We also find a strong radial intensity gradient: 49.4 +/- 8.0 %/au from 0.1 to 0.94 au, for energies of 6.9 to 27 MeV/nuc. This value agrees with that measured by Helios nearly 45 years ago from 0.3 to 1.0 au (48 +/- 12 %/au; 9 to 29 MeV/nuc) and is larger than predicted by models. The large ACR gradients observed close to the Sun by the Parker Solar Probe Integrated Science Investigation of the Sun instrument suite found here suggest that intermediate-scale variations in the magnetic field's structure strongly influences cosmic ray drifts, well inside 1 au.

astro-ph.SR↗

arXiv2020

Cometary ions detected by the Cassini spacecraft 6.5 au downstream of Comet 153P/Ikeya-Zhang

During March-April 2002, while between the orbits of Jupiter and Saturn, the Cassini spacecraft detected a significant enhancement in pickup proton flux. The most likely explanation for this enhancement was the addition of protons to the solar wind by the ionization of neutral hydrogen in the corona of comet 153P/Ikeya-Zhang. This comet passed relatively close to the Sun-Cassini line during that period, allowing pickup ions to be carried to Cassini by the solar wind. This pickup proton flux could have been further modulated by the passage of the interplanetary counterparts of coronal mass ejections past the comet and spacecraft. The radial distance of 6.5 Astronomical Units (au) traveled by the pickup protons, and the implied total tail length of >7.5 au make this cometary ion tail the longest yet measured.

astro-ph.EP↗

arXiv2019

Heliosheath Properties Measured from a Voyager 2 to Voyager 1 Transient

In mid-2012, a GMIR observed by Voyager 2 crossed through the heliosheath and collided with the heliopause, generating a pressure pulse that propagated into the very local interstellar medium. The effects of the transmitted wave were seen by Voyager 1 just 93 days after its own heliopause crossing. The passage of the transient was accompanied by long-lasting decreases in galactic cosmic ray intensities that occurred from ~2012.55 to ~2013.35 and ~2012.91 to ~2013.70 at Voyager 2 and Voyager 1, respectively. Omnidirectional (>20 MeV) proton-dominated measurements from each spacecraft's Cosmic Ray Subsystem reveal a remarkable similarity between these causally-related events, with a correlation coefficient of 91.2% and a time-lag of 130 days. Knowing the locations of the two spacecraft, we use the observed time-delay to calculate the GMIR's average speed through the heliosheath (inside the heliopause) as a function of temperature in the very local interstellar medium. This, combined with particle, field, and plasma observations enables us to infer previously unmeasured properties of the heliosheath, including a range of sound speeds and total effective pressures. For a nominal temperature of ~20,000 K just outside the heliopause, we find a sound speed of 314 (+/-) 32 km/s and total effective pressure of 267 (+/-) 55 fPa inside the heliopause. We compare these results with the Interstellar Boundary Explorer's data-driven models of heliosheath pressures derived from energetic neutral atom fluxes (the globally distributed flux) and present them as additional evidence that the heliosheath's dynamics are driven by suprathermal energetic processes.

physics.space-ph↗

arXiv2018

Interstellar neutral helium in the heliosphere from IBEX observations. V. Observations in IBEX-Lo ESA steps 1, 2, & 3

Direct-sampling observations of interstellar neutral (ISN) He by Interstellar Boundary Explorer (IBEX) provide valuable insight into the physical state of and processes operating in the interstellar medium ahead of the heliosphere. The ISN He atom signals are observed at the four lowest ESA steps of the IBEX-Lo sensor. The observed signal is a mixture of the primary and secondary components of ISN He and H. Previously, only data from one of the ESA steps have been used. Here, we extended the analysis to data collected in the three lowest ESA steps with the strongest ISN He signal, for the observation seasons 2009-2015. The instrument sensitivity is modeled as a linear function of the atom impact speed onto the sensor's conversion surface separately for each ESA step of the instrument. We found that the sensitivity increases from lower to higher ESA steps, but within each of the ESA steps it is a decreasing function of the atom impact speed. This result may be influenced by the hydrogen contribution, which was not included in the adopted model, but seems to exist in the signal. We conclude that the currently accepted temperature of ISN He and velocity of the Sun through the interstellar medium do not need a revision, and we sketch a plan of further data analysis aiming at investigating ISN H and a better understanding of the population of ISN He originating in the outer heliosheath.

astro-ph.SR↗

arXiv2017

Structure and Composition of Pluto's atmosphere from the New Horizons Solar Ultraviolet Occultation

The Alice instrument on NASA's New Horizons spacecraft observed an ultraviolet solar occultation by Pluto's atmosphere on 2015 July 14. The transmission vs. altitude was sensitive to the presence of N2, CH4, C2H2, C2H4, C2H6, and haze. We derived line-of-sight abundances and local number densities for the 5 molecular species, and line-of-sight optical depth and extinction coefficients for the haze. We found the following major conclusions: 1) We confirmed temperatures in Pluto's upper atmosphere that were colder than expected before the New Horizons flyby, with upper atmospheric temperatures near 65-68 K. The inferred enhanced Jeans escape rates were (3e22-7e22) N2/s and (4e25-8e25) CH4/s at the exobase (at a radius of ~2900 km, or an altitude of ~1710 km). 2) We measured CH4 abundances from 80 to 1200 km above the surface. A joint analysis of the Alice CH4 and Alice and REX N2 measurements implied a very stable lower atmosphere with a small eddy diffusion coefficient, most likely between 550 and 4000 cm2/s. Such a small eddy diffusion coefficient placed the homopause within 12 km of the surface, giving Pluto a small planetary boundary layer. The inferred CH4 surface mixing ratio was ~0.28-0.35%. 3) The abundance profiles of the C2Hx hydrocarbons (C2H2, C2H4, C2H6) were not simply exponential with altitude. We detected local maxima in line-of-sight abundance near 410 km altitude for C2H4, near 320 km for C2H2, and an inflection point or the suggestion of a local maximum at 260 km for C2H6. We also detected local minima near 200 km altitude for C2H4, near 170 km for C2H2, and an inflection point or minimum near 170-200 km for C2H6. These compared favorably with models for hydrocarbon production near 300-400 km and haze condensation near 200 km, especially for C2H2 and C2H4 (Wong et al. 2017). 4) We found haze that had an extinction coefficient approximately proportional to N2 density.

astro-ph.EP↗