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M. Opher

Publications and source records attributed to M. Opher.

At least 19 recordsLinked to original sources
arXiv2026

Prediction for IMAP: Revealing the Role of the Solar Magnetic Field in the Heliotail

The classic paradigm of the heliosphere considered the solar magnetic field as having a passive role in the heliosheath, yet Opher et al. (2015) and Drake et al. (2015) suggested that the solar magnetic field plays an active role by collimating the solar wind plasma. Previous work has suggested that high-latitude lobes observed by IBEX-Hi energetic neutral atom (ENA) maps are solely due to the fast/slow solar wind profile, yet other works have found that the solar magnetic field may play a role in organizing the lobes as well. Here, using an MHD solution with and without the solar magnetic field, we find that the fast/slow wind profile largely dictates the ENA profile up to $\sim$10-18 keV. Beyond this energy, the collimation effects are more prominent and the fast/slow solar wind profile no longer organizes the maps. The collimation creates two high latitude lobes. By 80 keV, the heliosphere with collimation yields an enhancement of ENAs of the high-latitude tail lobes compared to the low-latitude region by a factor of $\gtrsim$ 1.3, whereas the uncollimated heliosphere yields a ratio $<$ 1. Therefore, IMAP should be able to identify whether the solar magnetic field collimates the solar wind in the deep heliotail.

astro-ph.SR↗
arXiv2026

Dynamics of the Upwind Heliosphere Due to Data-Driven, Solar Wind and Magnetic Field Variations and Implications for Wave Propagation into the Very Local Interstellar Medium

We introduce an updated, time-dependent treatment to the split-tail ("croissant-like") heliosphere model with data-driven solar wind conditions at 1 au, to study the evolution of the heliosphere with solar-cycle variations in plasma speed, plasma density, and magnetic field intensity. The model produces a sub-Alfv\'enic and low beta region, not observed by the Voyagers, ~15 au ahead of the heliopause. The simulated magnetic field and radial flow depart from Voyager observations in this region, indicating that time-dependent effects alone are not sufficient to understand this regime of the heliosheath. We decompose fast and slow magnetosonic wave modes from time-dependent plasma pulse structures in the heliosheath, using a linear Riemann variable analysis, for the first time. Fast mode waves can both reflect at the heliopause and transmit into the interstellar medium, and their speeds are unaffected by the low beta plasma regime in front of the heliopause. The model reproduces the pf2 jump in the magnetic field at ~2020 in the interstellar medium and we attribute the source of pressure fronts observed by Voyager 1 in the interstellar medium, and pressure pulses observed by Voyager 2 in the heliosheath, to fast mode waves that are commonly recurring solar-cycle features. The presence of fast mode waves drive a highly variable termination shock, with average radial speeds of 6.05 au yr$^{-1}$ $\pm$ 5.37 au yr$^{-1}$ in the New Horizons direction. We find that the termination shock has a sinusoidal-like oscillatory motion in the rising phase of the solar cycle, and broad inward motions during the declining phase.

astro-ph.SR↗
arXiv2026

Signatures of Reconnection and a Split Heliospheric Tail in High-Energy Energetic Neutral Atoms

The shape of the heliosphere, regarded as comet-like since the 1960s, has recently been the subject of intense debate in the last decade. There is disagreement whether the heliospheric tail extends to $\sim$10,000 au in a comet-like shape or if it is short ($\sim$400 au) with a split. Energetic neutral atom (ENA) maps from Cassini/INCA at energies from 5.2 to 13.5 keV revealed a global structure extending from the nose to the heliospheric tail known as the Belt whose origin has remained largely unexplored. Here, we use a state-of-the-art multi-ion magnetohydrodynamic (MHD) model and a novel reconnection simulation to establish that the Belt structure is consistent with a split tail heliosphere but not with a comet-like heliosphere. In a split-tail heliosphere there is a region of low-$\beta$ (ratio of thermal to magnetic pressure) in the downwind direction close to the heliopause. Direct simulations of this region reveal that magnetic reconnection is strong and drives the energetic particles that produce the >5.2keV ENAs measured by INCA in the low latitude portion of the Belt. Since the comet-like heliosphere does not produce this low-$\beta$ region and the resultant reconnection-drive mechanism for the >5.2keV ENAs, our results indicate that the INCA observations are inconsistent with a comet-like heliosphere. Further, these simulations and analysis establish for the first time that magnetic reconnection in the complex magnetic fields, expected in astrospheres across the universe, are likely to be a source of energetic particles and radiation.

astro-ph.SR↗
arXiv2025

The Solar System's passage through the Radcliffe wave during the middle Miocene

Context. As the Solar System orbits the Milky Way, it encounters various Galactic environments, including dense regions of the interstellar medium (ISM). These encounters can compress the heliosphere, exposing parts of the Solar System to the ISM, while also increasing the influx of interstellar dust into the Solar System and Earth's atmosphere. The discovery of new Galactic structures, such as the Radcliffe wave, raises the question of whether the Sun has encountered any of them. Aims. The present study investigates the potential passage of the Solar System through the Radcliffe wave gas structure over the past 30 million years (Myr). Methods. We used a sample of 56 high-quality, young ($\leq$ 30 Myr) open clusters associated with a region of interest of the Radcliffe wave to trace its motion back and investigate a potential crossing with the Solar System's past orbit. Results. We find that the Solar System's trajectory intersected the Radcliffe wave in the Orion region. We have constrained the timing of this event to between 18.2 and 11.5 Myr ago, with the closest approach occurring between 14.8 and 12.4 Myr ago. Notably, this period coincides with the Middle Miocene climate transition on Earth, providing an interdisciplinary link with paleoclimatology. The potential impact of the crossing of the Radcliffe wave on the climate on Earth is estimated. This crossing could also lead to anomalies in radionuclide abundances, which is an important research topic in the field of geology and nuclear astrophysics.

astro-ph.GA↗
arXiv2024

Turbulence, Waves, and Taylor's Hypothesis for Heliosheath Observations

Magnetic field fluctuations measured in the heliosheath by the Voyager spacecraft are often characterized as compressible, as indicated by a strong fluctuating component parallel to the mean magnetic field. However, the interpretation of the turbulence data faces the caveat that the standard Taylor hypothesis is invalid because the solar wind flow velocity in the heliosheath becomes subsonic and slower than the fast magnetosonic speed, given the contributions from hot pickup ions in the heliosheath. We attempt to overcome this caveat by introducing a 4D frequency wavenumber spectral modeling of turbulence, which is essentially a decomposition of different wave modes following their respective dispersion relations. Isotropic Alfven and fast mode turbulence are considered to represent the heliosheath fluctuations. We also include two dispersive fast wave modes derived from a three-fluid theory. We find that (1) magnetic fluctuations in the inner heliosheath are less compressible than previously thought. An isotropic turbulence spectral model with about 1/4 in compressible fluctuation power is consistent with the observed magnetic compressibility in the heliosheath; (2) the hot pickup ion component and the relatively cold solar wind ions induce two dispersive fast magnetosonic wave branches in the perpendicular propagation limit. Pickup ion fast wave may account for the spectral bump near the proton gyrofrequency in the observable spectrum; (3) it is possible that the turbulence wavenumber spectrum is not Kolmogorov-like although the observed frequency spectrum has a -5/3 power law index, depending on the partitioning of power among the various wave modes, and this partitioning may change with wavenumber.

physics.space-ph↗
arXiv2024

Interstellar Neutral Hydrogen in the Heliosphere: New Horizons Observations in the Context of Models

Interstellar neutral (ISN) hydrogen is the most abundant species in the outer heliosheath and the very local interstellar medium (VLISM). Charge exchange collisions in the outer heliosheath result in filtration, reducing the ISN hydrogen density inside the heliosphere. Additionally, these atoms are intensively ionized close to the Sun, resulting in a substantial reduction of their density within a few au from the Sun. The products of this ionization - pickup ions (PUIs) - are detected by charged particle detectors. The Solar Wind Around Pluto (SWAP) instrument on New Horizons provides, for the first time, PUI observations from the distant heliosphere. We analyze the observations collected between 22 and 52 au from the Sun to find the ISN hydrogen density profile and compare the results with predictions from global heliosphere models. We conclude that the density profile derived from the observations is inconsistent with steady-state model predictions. This discrepancy is not explained by time variations close to the Sun and thus may be related to the temporal evolution of the outer boundaries or VLISM conditions. Furthermore, we show that the cold and hot models of ISN hydrogen distribution are not a good approximation closer to the termination shock. Therefore, we recommend a new fiduciary point based on the available New Horizons observations at 40 au from the Sun, at ecliptic direction (285.62{\deg}, 1.94{\deg}), where the ISN hydrogen density is 0.11 cm$^{-3}$. The continued operation of New Horizons should give better insight into the source of the discussed discrepancy.

astro-ph.SR↗
arXiv2022

First comparison of composite 0.52-55 keV ENA spectra observed by IBEX and Cassini/INCA with simulated ENAs inferred by proton hybrid simulations downstream of the termination shock

We present a first comparison of Energetic Neutral Atom (ENA) heliosheath measurements, remotely sensed by the Interstellar Boundary Explorer (IBEX) mission and the Ion and Neutral Camera (INCA) on the Cassini mission, with modeled ENA inferred from interstellar pickup protons that have been accelerated at the termination shock, using hybrid simulations. The observed ENA intensities are an average value over the time period from 2009 to the end of 2012, along the Voyager 2 trajectory. The hybrid simulations parameters for the solar wind, interstellar pickup ions (PUIs), and magnetic field upstream of the termination shock, where Voyager 2 crossed, are based on observations. We report an energy dependent discrepancy between observed and simulated ENA fluxes, with the observed ENA fluxes, being consistently higher than the simulated ones, and discuss possible causes of this discrepancy.

physics.space-ph↗
arXiv2021

Signature of a heliotail organized by the solar magnetic field and the role of non-ideal processes in modeled IBEX ENA maps: a comparison of the BU and Moscow MHD models

Energetic neutral atom (ENA) models typically require post-processing routines to convert the distributions of plasma and H atoms into ENA maps. Here we investigate how two different kinetic-MHD models of the heliosphere (the BU and Moscow models) manifest in modeled ENA maps using the same prescription and how they compare with Interstellar Boundary Explorer (IBEX) observations. Both MHD models treat the solar wind as a single-ion plasma for protons, which include thermal solar wind ions, pick-up ions (PUIs), and electrons. Our ENA prescription partitions the plasma into three distinct ion populations (thermal solar wind, PUIs transmitted and ones energized at the termination shock) and models the populations with Maxwellian distributions. Both kinetic-MHD heliospheric models produce a heliotail with heliosheath plasma organized by the solar magnetic field into two distinct north and south columns that become lobes of high mass flux flowing down the heliotail, though in the BU model the ISM flows between the two lobes at distances in the heliotail larger than 300 AU. While our prescription produces similar ENA maps for the two different plasma and H atom solutions at the IBEX-Hi energy range (0.5 - 6 keV), the modeled ENA maps require a scaling factor of ~2 to be in agreement with the data. This problem is present in other ENA models with the Maxwellian approximation of multiple ion species and indicates that a higher neutral density or some acceleration of PUIs in the heliosheath is required.

astro-ph.SR↗
arXiv2021

The development of a split-tail heliosphere and the role of non-ideal processes: a comparison of the BU and Moscow models

Global models of the heliosphere are critical tools used in the interpretation of heliospheric observations. There are several three-dimensional magnetohydrodynamic (MHD) heliospheric models that rely on different strategies and assumptions. Until now only one paper has compared global heliosphere models, but without magnetic field effects. We compare the results of two different MHD models, the BU and Moscow models. Both models use identical boundary conditions to compare how different numerical approaches and physical assumptions contribute to the heliospheric solution. Based on the different numerical treatments of discontinuities, the BU model allows for the presence of magnetic reconnection, while the Moscow model does not. Both models predict collimation of the solar outflow in the heliosheath by the solar magnetic field and produce a split-tail where the solar magnetic field confines the charged solar particles into distinct north and south columns that become lobes. In the BU model, the ISM flows between the two lobes at large distances due to MHD instabilities and reconnection. Reconnection in the BU model at the port flank affects the draping of the interstellar magnetic field in the immediate vicinity of the heliopause. Different draping in the models cause different ISM pressures, yielding different heliosheath thicknesses and boundary locations, with the largest effects at high latitudes. The BU model heliosheath is 15% thinner and the heliopause is 7% more inwards at the north pole relative to the Moscow model. These differences in the two plasma solutions may manifest themselves in energetic neutral atom measurements of the heliosphere.

astro-ph.SR↗
arXiv2020

Major Scientific Challenges and Opportunities in Understanding Magnetic Reconnection and Related Explosive Phenomena in Solar and Heliospheric Plasmas

Magnetic reconnection underlies many explosive phenomena in the heliosphere and in laboratory plasmas. The new research capabilities in theory/simulations, observations, and laboratory experiments provide the opportunity to solve the grand scientific challenges summarized in this whitepaper. Success will require enhanced and sustained investments from relevant funding agencies, increased interagency/international partnerships, and close collaborations of the solar, heliospheric, and laboratory plasma communities. These investments will deliver transformative progress in understanding magnetic reconnection and related explosive phenomena including space weather events.

physics.space-ph↗
arXiv2020

The Confinement of the Heliosheath Plasma by the Solar Magnetic Field as Revealed by Energetic Neutral Atom Simulations

Traditionally, the solar magnetic field has been considered to have a negligible effect in the outer regions of the heliosphere. Recent works have shown that the solar magnetic field may play a crucial role in collimating the plasma in the heliosheath. Interstellar Boundary Explorer (IBEX) observations of the heliotail indicated a latitudinal structure varying with energy in the energetic neutral atom (ENA) fluxes. At energies ~1 keV, the ENA fluxes show an enhancement at low latitudes and a deficit of ENAs near the poles. At energies >2.7 keV, ENA fluxes had a deficit within low latitudes, and lobes of higher ENA flux near the poles. This ENA structure was initially interpreted to be a result of the latitudinal profile of the solar wind during solar minimum. We extend the work of Kornbleuth et al. (2018) by using solar minimum-like conditions and the recently developed SHIELD model. The SHIELD model couples the magnetohydrodynamic (MHD) plasma solution with a kinetic description of neutral hydrogen. We show that while the latitudinal profile of the solar wind during solar minimum contributes to the lobes in ENA maps, the collimation by the solar magnetic field is important in creating and shaping the two high latitude lobes of enhanced ENA flux observed by IBEX. This is the first work to explore the effect of the changing solar magnetic field strength on ENA maps. Our findings suggest that IBEX is providing the first observational evidence of the collimation of the heliosheath plasma by the solar magnetic field.

astro-ph.SR↗
arXiv2019

Corrugated Features in Coronal-mass-ejections-driven Shocks: A Discussion on the Predisposition to Particle Acceleration

The study of the acceleration of particles is an essential element of research in the heliospheric science. Here, we discuss the predisposition to the particle acceleration around coronal mass ejections (CMEs)-driven shocks with corrugated wave-like features. We adopt these attributes on shocks formed from disturbances due to the bimodal solar wind, CME deflection, irregular CME expansion, and the ubiquitous fluctuations in the solar corona. In order to understand the role of a wavy shock in particle acceleration, we define three initial smooth shock morphologies each one associated with a fast CME. Using polar Gaussian profiles we model these shocks in the low corona. We establish the corrugated appearance on smooth shock by using combinations of wave-like functions that represent the disturbances from medium and CME piston. For both shock types, smooth and corrugated, we calculate the shock normal angles between the shock normal and the radial upstream coronal magnetic field in order to classify the quasi-parallel and quasi-perpendicular regions. We consider that corrugated shocks are predisposed to different process of particle acceleration due to irregular distributions of shock normal angles around of the shock. We suggest that disturbances due to CME irregular expansion may be a decisive factor in origin of particle acceleration. Finally, we regard that accepting these features on shocks may be the start point for investigating some questions in the sheath and shock, like downstream-jets, instabilities, shock thermalization, shock stability, and injection particle process.

astro-ph.SR↗
arXiv2017

The Twist of the Draped Interstellar Magnetic Field Ahead of the Heliopause: A Magnetic Reconnection Driven Rotational Discontinuity

Based on the difference between the orientation of the interstellar $B_{ISM}$ and the solar magnetic fields, there was an expectation that the magnetic field direction would rotate dramatically across the heliopause (HP). However, the Voyager 1 spacecraft measured very little rotation across the HP. Previously we showed that the $B_{ISM}$ twists as it approaches the HP and acquires a strong T component (East-West). Here we establish that reconnection in the eastern flank of the heliosphere is responsible for the twist. On the eastern flank the solar magnetic field has twisted into the positive N direction and reconnects with the Southward pointing component of the $B_{ISM}$. Reconnection drives a rotational discontinuity (RD) that twists the $B_{ISM}$ into the -T direction and propagates upstream in the interstellar medium towards the nose. The consequence is that the N component of $B_{ISM}$ is reduced in a finite width band upstream of the HP. Voyager 1 currently measures angles ($\delta=sin^{-1}(B_{N}/B)$) close to solar values. We present MHD simulations to support this scenario, suppressing reconnection in the nose region while allowing it in the flanks, consistent with recent ideas about reconnection suppression from diamagnetic drifts. The jump in plasma $\beta$ (the plasma to magnetic pressure) across the nose of HP is much greater than in the flanks because the heliosheath $\beta$ is greater there than in the flanks. Large-scale reconnection is therefore suppressed in the nose but not at the flanks. Simulation data suggest that $B_{ISM}$ will return to its pristine value $10-15~AU$ past the HP.

physics.space-ph↗
arXiv2017

The formation of magnetic depletions and flux annihilation due to reconnection in the heliosheath

The misalignment of the solar rotation axis and the magnetic axis of the Sun produces a periodic reversal of the Parker spiral magnetic field and the sectored solar wind. The compression of the sectors is expected to lead to reconnection in the heliosheath (HS). We present particle-in-cell simulations of the sectored HS that reflect the plasma environment along the Voyager 1 and 2 trajectories, specifically including unequal positive and negative azimuthal magnetic flux as seen in the Voyager data \citep{Burlaga03}. Reconnection proceeds on individual current sheets until islands on adjacent current layers merge. At late time bands of the dominant flux survive, separated by bands of deep magnetic field depletion. The ambient plasma pressure supports the strong magnetic pressure variation so that pressure is anti-correlated with magnetic field strength. There is little variation in the magnetic field direction across the boundaries of the magnetic depressions. At irregular intervals within the magnetic depressions are long-lived pairs of magnetic islands where the magnetic field direction reverses so that spacecraft data would reveal sharp magnetic field depressions with only occasional crossings with jumps in magnetic field direction. This is typical of the magnetic field data from the Voyager spacecraft \citep{Burlaga11,Burlaga16}. Voyager 2 data reveals that fluctuations in the density and magnetic field strength are anti-correlated in the sector zone as expected from reconnection but not in unipolar regions. The consequence of the annihilation of subdominant flux is a sharp reduction in the "number of sectors" and a loss in magnetic flux as documented from the Voyager 1 magnetic field and flow data \citep{Richardson13}.

physics.space-ph↗
arXiv2016

Using ForeCAT Deflections and Rotations to Constrain the Early Evolution of CMEs

To accurately predict the space weather effects of coronal mass ejection (CME) impacts at Earth one must know if and when a CME will impact Earth, and the CME parameters upon impact. Kay et al. (2015b) presents Forecasting a CME's Altered Trajectory (ForeCAT), a model for CME deflections based on the magnetic forces from the background solar magnetic field. Knowing the deflection and rotation of a CME enables prediction of Earth impacts, and the CME orientation upon impact. We first reconstruct the positions of the 2008 April 10 and the 2012 July 12 CMEs from the observations. The first of these CMEs exhibits significant deflection and rotation (34 degrees deflection and 58 degrees rotation), while the second shows almost no deflection or rotation (<3 degrees each). Using ForeCAT, we explore a range of initial parameters, such as the CME location and size, and find parameters that can successfully reproduce the behavior for each CME. Additionally, since the deflection depends strongly on the behavior of a CME in the low corona (Kay et al. (2015a, 2015b)), we are able to constrain the expansion and propagation of these CMEs in the low corona.

astro-ph.SR↗
arXiv2016

Probability of CME Impact on Exoplanets Orbiting M Dwarfs and Solar-Like Stars

Solar coronal mass ejections (CMEs) produce adverse space weather effects at Earth. Planets in the close habitable zone of magnetically active M dwarfs may experience more extreme space weather than at Earth, including frequent CME impacts leading to atmospheric erosion and leaving the surface exposed to extreme flare activity. Similar erosion may occur for hot Jupiters with close orbits around solar-like stars. We have developed a model, Forecasting a CME's Altered Trajectory (ForeCAT), which predicts a CME's deflection. We adapt ForeCAT to simulate CME deflections for the mid-type M dwarf V374 Peg and hot Jupiters with solar-type hosts. V374 Peg's strong magnetic fields can trap CMEs at the M dwarfs's Astrospheric Current Sheet, the location of the minimum in the background magnetic field. Solar-type CMEs behave similarly, but have much smaller deflections and do not get trapped at the Astrospheric Current Sheet. The probability of planetary impact decreases with increasing inclination of the planetary orbit with respect to the Astrospheric Current Sheet - 0.5 to 5 CME impacts per day for M dwarf exoplanets, 0.05 to 0.5 CME impacts per day for solar-type hot Jupiters. We determine the minimum planetary magnetic field necessary to shield a planet's atmosphere from the CME impacts. M dwarf exoplanets require values between tens and hundreds of Gauss. Hot Jupiters around a solar-type star, however, require a more reasonable <30 G. These values exceed the magnitude required to shield a planet from the stellar wind, suggesting CMEs may be the key driver of atmospheric losses.

astro-ph.SR↗
arXiv2015

The Heliocentric Distance Where the Deflections and Rotations of Solar Coronal Mass Ejections Occur

Understanding the trajectory of a coronal mass ejection (CME), including any deflection from a radial path, and the orientation of its magnetic field is essential for space weather predictions. Kay et al. (2015b) developed a model, Forecasting a CME's Altered Trajectory (ForeCAT), of CME deflections and rotation due to magnetic forces, not including the effects of reconnection. ForeCAT is able to reproduce the deflection of observed CMEs (Kay et al. 2015a). The deflecting CMEs tend to show a rapid increase of their angular momentum close to the Sun, followed by little to no increase at farther distances. Here we quantify the distance at which the CME deflection is "determined," which we define as the distance after which the background solar wind has negligible influence on the total deflection. We consider a wide range in CME masses and radial speeds and determine that the deflection and rotation of these CMEs can be well-described by assuming they propagate with constant angular momentum beyond 10 Rs. The assumption of constant angular momentum beyond 10 Rs yields underestimates of the total deflection at 1 AU of only 1% to 5% and underestimates of the rotation of 10%. Since the deflection from magnetic forces is determined by 10 Rs, non-magnetic forces must be responsible for any observed interplanetary deflections or rotations where the CME has increasing angular momentum.

astro-ph.SR↗