Interstellar Plasma Research Articles

Kinetic and dynamic studies of the NH2+ + H2 reaction on a high-level ab initio potential energy surface.

Gas-phase ion-molecule reactions have attracted considerable attention due to their importance in the fields of interstellar chemistry, plasma chemistry, and combustion chemistry. The reaction of an amino radical cation with a hydrogen molecule is one of the crucial steps in the gas-phase formation of ammonia in the interstellar medium (ISM). The dynamics and kinetics of the NH2+ + H2 reaction are studied using the quasi-classical trajectory approach on a newly constructed ab initio potential energy surface (PES) for the ground electronic state. The PES is fitted by the fundamental invariant-neural network method, resulting in a total root mean square error (RMSE) of 0.061 kcal mol-1. Dynamics calculations show that, on one hand, the vibrational excitation of H2 largely promotes the reaction. On the other hand, the fundamental excitation of each vibrational mode of NH2+ inhibits the reaction at low collision energies which has a negligible effect at high collision energies except for the symmetric stretching mode. The relatively higher efficacy of the symmetric stretching mode than that of the asymmetric stretching mode can be rationalized by the underlying reaction mechanisms. In addition, the calculated rate coefficients of the reaction agree reasonably well with the available experimental results.

Interstellar Neutral He Parameters from Crossing Parameter Tubes with the Interstellar Mapping and Acceleration Probe Informed by 10 yr of Interstellar Boundary Explorer Observations

Abstract The Sun's motion through the interstellar medium leads to an interstellar neutral (ISN) wind through the heliosphere. Several ISN species, including He, moderately depleted by ionization are observed with pickup ions and directly imaged. Since 2009, analyzed Interstellar Boundary Explorer (IBEX) observations returned a precise 4D parameter tube associated with the bulk velocity vector and the temperature of ISN flow distribution. This 4D parameter tube is typically expressed in terms of the ISN speed, the inflow latitudinal direction, and the temperature as a function of the inflow longitudinal direction and the local flow Mach number. We have used IBEX observations and those from other spacecraft to reduce statistical parameter uncertainties: V ISN ∞ = 25.99 ± 0.18 km s−1, λ ISN ∞ = 75 .° 28 ± 0 .° 13 , β ISN ∞ = −5 .° 200 ± 0 .° 075 , and T ISN ∞ = 7496 ± 172 K. IBEX ISN viewing is restricted almost perpendicular to the Earth–Sun line, which limits observations in ecliptic longitude to ∼130° ± 30° and results in relatively small uncertainties across the IBEX parameter tube but large uncertainties along it. Operations over the last three years enabled the IBEX spin axis to drift to the maximum operational offset (7°) west of the Sun, helping to break the ISN parameter degeneracy by weakly crossing the IBEX parameter tubes: the range of possible inflow longitudes extends over the range λ ISN ∞ = 75 .° 28 − 2.21 + 2.27 and the corresponding range of other ISN parameters is V ISN ∞ = 25.99 − 1.76 + 1.86 km s−1, β ISN ∞ = −5 .° 200 − 0.085 + 0.093 , and T ISN ∞ = 7496 − 1528 + 1274 K. This enhances the full χ 2 analysis of ISN parameters through comparison with detailed models. The next-generation IBEX-Lo sensor on IMAP will be mounted on a pivot platform, enabling IMAP-Lo to follow the ISN flow over almost the entire spacecraft orbit around the Sun. A near-continuous set of 4D parameter tube orientations on IMAP will be observed for He and for O, Ne, and H that cross at varying angles to substantially reduce the ISN flow parameter uncertainties and mitigate systematic uncertainties (e.g., from ionization effects and the presence of secondary components) to derive the precise parameters of the primary and secondary local interstellar plasma flows.

МІЖЗОРЯНЕ СЕРЕДОВИЩЕ ТА ДЕКАМЕТРОВА РАДІОСПЕКТРОСКОПІЯ

Purpose: The analytical review of the main results of research in the new direction of the low-frequency radio astronomy, the interstellar medium radio spectroscopy at decameter waves, which had led to astrophysical discovery, recording of the radio recombination lines in absorption for highly excited states of interstellar carbon atoms (more than 600). Design/methodology/approach: The UTR-2 world-largest broadband radio telescope of decameter waves optimally connected with the digital correlation spectrum analyzers has been used. Continuous modernization of antenna system and devices allowed increasing the analysis band from 100 kHzto 24 MHz and a number of channels from 32 to 8192. The radio telescope and receiving equipment with appropriate software allowed to have a long efficient integration time enough for a large line series simultaneously with high resolution, noise immunity and relative sensitivity. Findings: A new type of interstellar spectral lines has been discovered and studied, the interstellar carbon radio recombination lines in absorption for the record high excited atoms with principal quantum numbers greater than 1000. The line parameters (intensity, shape, width, radial velocity) and their relation ship with the interstellar medium physical parameters have been determined. The temperature of line forming regions is about 100 K, the electron concentration up to 0.1 cm–3 and the size of a line forming region is about 10 pc. For the first time, radio recombination lines were observed in absorption. They have significant broadening and are amplified by the dielectronic-like recombination mechanism and are also the lowest frequency lines in atomic spectroscopy. Conclusions: The detected low-frequency carbon radio recombination lines and their observations have become a new highly effective tool for the cold partially ionized interstellar plasma diagnostics. Using them allows obtaining the information which is not available with the other astrophysical methods. For almost half a century of their research, a large amount of hardware-methodical and astrophysical results have been obtained including a record number of Galaxy objects, where there levant lines have been recorded. The domestic achievements have stimulated many theoretical and experimental studies in other countries, but the scientific achievements of Ukrainian scientists prove the best prospects for further development of this very important area of astronomical science. Key words: low-frequency radio astronomy; radio telescope; interstellar medium; radio recombination lines; carbon; hydrogen; spectral analyzer

The Spectrum Power of Interstellar Plasma Inhomogeneities in the Direction of Eleven Radio Pulsars

We have analyzed two-dimensional correlation functions from the dynamic spectra of 11 pulsars using the archival data of the “Radioastron” project. The time-sections of these functions were approximated by exponential functions with a power alpha . It is shown that this approximation describes the shape of the correlation function much better than the Gaussian. The temporal structure function D(Delta t) for small values of the delay Delta tis a power law with an index alpha . The spectrum power of spatial inhomogeneities of the interstellar plasma is related to the power of the structure function as n = alpha + 2. We have determined the characteristic scintillation time and the power n in the direction of 11 pulsars. In the direction of three pulsars (B0329+54, B0823+26, and B1929+10), the spectrum power of spatial inhomogeneities of the interstellar plasma turned out to be very close to the value for the Kolmogorov spectrum (n = 3.67). For other pulsars, it ranges from 3.18 to 3.86. It is shown that the measured scintillation parameters are significantly influenced by the duration of the observation session, expressed by its ratio to the characteristic scintillation time. If this parameter is less than 10, the parameter estimates may be biased: the values of alpha and the characteristic scintillation time {{t}_{{{text{scint}}}}} may decrease.

Polarization images of accretion flow around supermassive black holes: Imprints of toroidal field structure

Abstract With unprecedented angular resolution, the Event Horizon Telescope (EHT) has opened a new era of black hole studies. We have previously calculated the expected polarization images of M 87* with EHT observations in mind. There, we demonstrated that circular polarization (CP) images, as well as linear polarization (LP) maps, can convey quite useful information, such as the flow structure and magnetic field configuration around the black hole. In this paper, we make new predictions for the cases in which disk emission dominates over jet emission, bearing Sgr A* in mind. Here we set the proton-to-electron temperature ratio of the disk component to be Tp/Te ∼ 2 so as to suppress jet emission relative to emission from accretion flow. As a result, we obtain ring-like images and triple-forked images around the black hole for face-on and edge-on cases, respectively. We also find significant CP components in the images (≳10% in fraction), with both positive and negative signs, amplified through the Faraday conversion, not depending sensitively on the inclination angles. Furthermore, we find a “separatrix” in the CP images, across which the sign of CP is reversed and on which the LP flux is brightest, that can be attributed to the helical magnetic field structure in the disk. These results indicate that future full polarization EHT images are a quite useful tracer of the magnetic field structure. We also discuss to what extent we will be able to extract information regarding magnetic field configurations under the scattering in the interstellar plasma, in future EHT polarimetric observations of Sgr A*.

Deconvolving Pulsar Signals with Cyclic Spectroscopy: A Systematic Evaluation

Abstract Radio pulsar signals are significantly perturbed by their propagation through the ionized interstellar medium. In addition to the frequency-dependent pulse times of arrival due to dispersion, pulse shapes are also distorted and shifted, having been scattered by the inhomogeneous interstellar plasma, affecting pulse arrival times. Understanding the degree to which scattering affects pulsar timing is important for gravitational-wave detection with pulsar timing arrays (PTAs), which depend on the reliability of pulsars as stable clocks with an uncertainty of ∼100 ns or less over ∼10 yr or more. Scattering can be described as a convolution of the intrinsic pulse shape with an impulse response function representing the effects of multipath propagation. In previous studies, the technique of cyclic spectroscopy has been applied to pulsar signals to deconvolve the effects of scattering from the original emitted signals, increasing the overall timing precision. We present an analysis of simulated data to test the quality of deconvolution using cyclic spectroscopy over a range of parameters characterizing interstellar scattering and pulsar signal-to-noise ratio (S/N). We show that cyclic spectroscopy is most effective for high S/N and/or highly scattered pulsars. We conclude that cyclic spectroscopy could play an important role in scattering correction to distant populations of highly scattered pulsars not currently included in PTAs. For future telescopes and for current instruments such as the Green Bank Telescope upgraded with the ultrawide bandwidth receiver, cyclic spectroscopy could potentially double the number of PTA-quality pulsars.

Persistent plasma waves in interstellar space detected by Voyager 1.

In 2012, Voyager 1 became the first in situ probe of the very local interstellar medium1. The Voyager 1 Plasma Wave System has given point estimates of the plasma density spanning about 30 au of interstellar space, revealing a large-scale density gradient2,3 and turbulence4 outside the heliopause. Previous studies of the plasma density relied on the detection of discrete plasma oscillation events triggered ahead of shocks propagating outwards from the Sun and used to infer the plasma frequency and hence density5,6. We present the detection of a class of very weak, narrowband plasma wave emission in the Voyager 1 data that persists from 2017 onwards and enables the first steadily sampled measurement of the interstellar plasma density over about 10 au with an average sampling distance of 0.03 au. We find au-scale density fluctuations that trace interstellar turbulence between episodes of previously detected plasma oscillations. Possible mechanisms for the narrowband emission include thermally excited plasma oscillations and quasi-thermal noise, and could be clarified by new findings from Voyager or a future interstellar mission. The emission's persistence suggests that Voyager 1 may be able to continue tracking the interstellar plasma density in the absence of shock-generated plasma oscillation events.

Chandra large-scale mapping of the Galactic Centre: probing high-energy structures around the central molecular zone

ABSTRACT Recent observations have revealed interstellar features that apparently connect energetic activity in the central region of our Galaxy to its halo. The nature of these features, however, remains largely uncertain. We present a Chandra mapping of the central 2° × 4° field of the Galaxy, revealing a complex of X-ray-emitting threads plus plume-like structures emerging from the Galactic Centre (GC). This mapping shows that the northern plume or fountain is offset from a well-known radio lobe (or the GCL), which however may represent a foreground H ii region, and that the southern plume is well wrapped by a corresponding radio lobe recently discovered by MeerKAT. In particular, we find that a distinct X-ray thread, G0.17−0.41, is embedded well within a non-thermal radio filament, which is locally inflated. This thread with a width of ∼1.6 arcsec (FWHM) is ∼2.6 arcmin or 6 pc long at the distance of the GC and has a spectrum that can be characterized by a power law or an optically-thin thermal plasma with temperature ≳ 3 keV. The X-ray-emitting material is likely confined within a strand of magnetic field with its strength ≳ 1 mG, not unusual in such radio filaments. These morphological and spectral properties of the radio/X-ray association suggest that magnetic field re-connection is the energy source. Such re-connection events are probably common when flux tubes of antiparallel magnetic fields collide and/or become twisted in and around the diffuse X-ray plumes, representing blowout superbubbles driven by young massive stellar clusters in the GC. The understanding of the process, theoretically predicted in analog to solar flares, can have strong implications for the study of interstellar hot plasma heating, cosmic ray acceleration and turbulence.

New unexpected flow patterns in the problem of the stellar wind interaction with the interstellar medium: stationary ideal-MHD solutions

ABSTRACT The astropause (heliopause for the Sun) is the tangential discontinuity separating the stellar wind from the interstellar plasma. The global shape of the heliopause is a matter of debates. Two types of the shape are under discussion: comet-like and tube-like. In the second type, the two-jets oriented towards the stellar rotation axis are formed by the action of azimuthal component of the stellar magnetic field. We explore a simplified global astrosphere in which (1) the surrounding and moving with respect to the star circumstellar medium is fully ionized, (2) the interstellar magnetic field is neglected, (3) the radial component of the stellar magnetic field is neglected as compared with the azimuthal component, and (4) the stellar wind outflow is spherically symmetric and supersonic. We present the results of numerical 3D MHD modelling and explore how the global structure depends on the gas-dynamic Mach number of the interstellar flow, M∞, and the Alfvenic Mach number in the stellar wind. It is shown that the astropause has a tube-like shape for small values of M∞. The wings of the tube are distorted towards the tail as larger as larger the Mach number is. The new (to our knowledge) result is the reverse interstellar flow in the vicinity of the astropause in the tail. The larger the interstellar Mach number is the narrower the reverse flow is. At some values of the Mach number, the stellar wind overcomes the reverse interstellar flow and moves out in downwind. In this regime, the astropause changes its topology from tube-like to sheet-like.

The MHD Plasma Flow Near the Heliopause Stagnation Region with A View On Kinetic Consistency

Most of the representative space plasma systems in our cosmic environment, - outside of stellar interiors, - like heliospheric, interstellar, or intergalactic plasmas etc., are collision-free or, at least, only weakly collision-determined systems. Nevertheless, these plasmas consist of at least two very different particle species, namely ions and electrons, i.e. particles with very disparate masses and opposite electric charges. If in these systems concerted fluid motions are arranged by electro-magnetic or gravitational forces or by inner forces like pressure gradients, then it must be asked how this combined electron-ion system finds its common internal dynamics. In most text book literature this problem is treated by considering the plasma as a mono-fluid system in which the massive protons and the nearly massless electrons are electrically closely bound together and move as an electrically neutral couple with an identical bulk velocity. Under these conditions the well-known Bernoulli law is derived for the standard MHD. If the electron pressure, however, does compete with the energy density of the ion bulk motion, then a two-fluid situation occurs, and the resulting bulk motion of the charge-neutral plasma needs to be determined on the basis of the kinetic conditions of the two different plasma fluids. In the following we shall exactly study this specific situation.

Scintillation Arcs in Pulsar B0450–18

Abstract We report on Green Bank Telescope observations of interstellar scintillation from the pulsar B0450−18 at 340 and 825 MHz, revealing prominent arcs in the secondary spectra at both frequencies. The arcs are successfully modeled by one-dimensional angular brightness distributions, estimated by two independent techniques. The distributions do not follow the expected shape for a Kolmogorov plasma density spectrum, and their overall angular widths follow a scaling law, versus frequency, that is slightly slower than the inverse square law expected from plasma dispersion. They also exhibit discrete peaks that are the cause of the occasional reverse sub-arcs. Over a narrow (5%) frequency range these peaks do not participate in the overall spreading, having angular positions that are fixed on the sky. However, the peaks do evolve over a wider range of frequencies and cannot be traced from 825 to 340 MHz. Our conclusions add further evidence for a widespread distribution of highly localized regions of interstellar plasma with small-scale structure that is either elongated or flattened.

A Foreshock Model for Interstellar Shocks of Solar Origin: Voyager 1 and 2 Observations

Abstract The Voyager 1 (V1) and Voyager 2 (V2) spacecraft were launched in 1977 on a mission to explore the outer planets and reach the heliopause, the boundary between the hot solar plasma and the relatively cool interstellar plasma. V1 reached the heliopause on 2012 August 25, at 121.6 au, and V2 reached the heliopause on 2018 November 5, at 119.0 au. One of their remarkable discoveries was the detection of shocks propagating into the interstellar plasma from energetic solar events. These shocks are typically preceded by electron plasma oscillations excited by electron beams streaming along interstellar magnetic field lines ahead of the shocks. The frequencies of the plasma oscillations have now provided radial electron density profiles in the outer heliosphere and in the interstellar medium to radial distances of more than 145 au. The oscillations are typically preceded by bursts of high-energy ∼5–100 MeV electrons. These electron bursts are interpreted as being due to the reflection (and acceleration) of cosmic-ray electrons by the shock at the time the shock first contacts the magnetic field line that passes through the spacecraft. Relative timing between the cosmic rays reflected by the shock and the onset of the plasma oscillations allow us, for the first time, to estimate the energy, ∼20–100 eV, of the electron beams responsible for the plasma oscillations. These observations are combined into a self-consistent model called the foreshock model that describes the interaction of shocks of solar origin with the interstellar plasma.

Precision Orbital Dynamics from Interstellar Scintillation Arcs for PSR J0437–4715

Abstract Intensity scintillations of radio pulsars are known to originate from interference between waves scattered by the electron density irregularities of interstellar plasma, often leading to parabolic arcs in the two-dimensional power spectrum of the recorded dynamic spectrum. The degree of arc curvature depends on the distance to the scattering plasma and its transverse velocity with respect to the line of sight. We report the observation of annual and orbital variations in the curvature of scintillation arcs over a period of 16 yr for the bright millisecond pulsar, PSR J0437−4715. These variations are the signature of the relative transverse motions of Earth, the pulsar, and the scattering medium, which we model to obtain precise measurements of parameters of the pulsar’s binary orbit and the scattering medium itself. We observe two clear scintillation arcs in most of our >5000 observations, and we show that they originate from scattering by thin screens located at distances D 1 = 89.8 ± 0.4 pc and D 2 = 124 ± 3 pc from Earth. The best-fit scattering model we derive for the brightest arc yields the pulsar’s orbital inclination angle, i = 137.°1 ± 0.°3, and longitude of ascending node, Ω = 206.°3 ± 0.°4. Using scintillation arcs for precise astrometry and orbital dynamics can be superior to modeling variations in the diffractive scintillation timescale, because the arc curvature is independent of variations in the level of turbulence of interstellar plasma. This technique can be used in combination with pulsar timing to determine the full three-dimensional orbital geometries of binary pulsars and provides parameters essential for testing theories of gravity and constraining neutron star masses.

Viscoelastic type magnetic effects and self-gravity on the propagation of MHD waves

We take up the challenge to explain the correlation between the Jeans instability topic towards star formation within the accelerated expansion of universe and the role of torsional shear-like Alfven waves in triggering the formation of network patterns, by proposing new mathematical models for self-gravitating interstellar non ideal MHD plasmas. The diffusion of the gravitational field is included via a parabolic Einstein’s equation with the cosmological constant, whereas anomalous resistive features are described through non ideal Ohm’s laws incorporating inertia terms, to account of relaxation and retardation magnetic responses. We perform a spectral analysis to test the stability properties of the novel constitutive settings where dissipative and elastic devices act together, by emphasizing the differences with previous models. As a main result, we highlight the definition of a lower critical threshold, here called the Jeans-Einstein wavenumber, against collapse formation towards the formation of longitudinal gravito-magneto-sonic waves and transverse non gravitational Alfven waves exhibiting larger effective wavespeeds, due to the hyperbolic-parabolic diffusion of the magnetic field. Consequently shorter collisional times are allowable so, beyond the plasma-beta, another interesting key point is the definition of the Ohm number to revisit the timescale topic, towards reviewed Reynolds and Lundquist numbers able to better capture the microphysical phenomena of Magnetic Reconnection in narrow diffusion regimes.

Particle acceleration by interstellar plasma shock waves in non-uniform background magnetic field

ABSTRACTParticle acceleration by plasma shock waves is investigated for a magnetized plasma cloud propagating in a non-uniform background magnetic field by means of analytical and numerical calculations. The mechanism studied here is mainly, magnetic trapping acceleration (MTA) which is previously investigated for a cloud moving through the uniform interstellar magnetic field (IMF). In this work, the acceleration is studied for a cloud moving in an antiparallel background field with spatial variations along the direction of motion. For negative variation, the cloud moves towards an antiparallel magnetic field with an increasing intensity, the trapped particle moves to locations with higher convective electric field and therefore gains more energy over time. For positive variation, the background field decreases to zero and changes into a parallel field with an increasing intensity. It is concluded that, when the background field vanishes, the MTA mechanism ceases and the particle escapes into the space. This leads to a bouncing acceleration which further increases energy of the gyrating particle. The two processes are followed by a shock drift acceleration, where due to the background magnetic field gradient, the particle drifts along the electric field and gains energy. Although for positive variation, three different mechanisms are involved, energy gain is less than in the case of a uniform background field.

Extreme intra-hour variability of the radio source J1402+5347 discovered with Apertif

The propagation of radio waves from distant compact radio sources through turbulent interstellar plasma in our Galaxy causes these sources to twinkle, a phenomenon called interstellar scintillation. Such scintillations are a unique probe of the micro-arcsecond structure of radio sources as well as of the sub-AU-scale structure of the Galactic interstellar medium. Weak scintillations (i.e. an intensity modulation of a few percent) on timescales of a few days or longer are commonly seen at centimetre wavelengths and are thought to result from the line-of-sight integrated turbulence in the interstellar plasma of the Milky Way. So far, only three sources were known that show more extreme variations, with modulations at the level of some dozen percent on timescales shorter than an hour. This requires propagation through nearby (d ≲ 10 pc) anomalously dense (ne ∼ 102 cm−3) plasma clouds. Here we report the discovery with Apertif of a source (J1402+5347) showing extreme (∼50%) and rapid variations on a timescale of just 6.5 min in the decimetre band (1.4 GHz). The spatial scintillation pattern is highly anisotropic, with a semi-minor axis of about 20 000 km. The canonical theory of refractive scintillation constrains the scattering plasma to be within the Oort cloud. The sightline to J1402+5347, however, passes unusually close to the B3 star Alkaid (η UMa) at a distance of 32 pc. If the scintillations are associated with Alkaid, then the angular size of J1402+5347 along the minor axis of the scintels must be smaller than ≈10 μas, yielding an apparent brightness temperature for an isotropic source of ≳1014 K.

Voyager 2 Observations Near the Heliopause

This paper discusses plasma characteristics in the heliosheath region before the heliopause (HP), at the HP, and in the very local interstellar medium (VLISM). The Voyager 2 (V2) HP was a sharp boundary where the radial plasma currents went to background levels. The radial flow speeds derived from 53-85 keV (V1) and 28-43 keV (V2) ion data decreased about 2 years (8 AU) before the HP at V1 and V2. A speed decrease was not observed by the V2 plasma instrument until 160 days (1.5 AU) before the HP crossing when V2 entered the plasma boundary layer where the plasma density and 28-43 keV ion intensity increased. We determine the HP orientation based on the plasma flow and magnetic field data and show these observations are consistent with models predicting a blunt HP. Variations are observed in the currents observed in the VLISM; roll data from this region clearly show the plasma instrument observes the interstellar plasma and may be consistent with larger than expected VLISM temperatures near the HP.

Instability in a magnetised collisional plasma driven by a heat flow or a current

We solve the linearised Vlasov-Fokker-Planck (VFP) equation to show that heat flow or an electrical current in a magnetized collisional plasma is unstable to the growth of a circularly polarised transverse perturbation to a zeroth order uniform magnetic field. The Braginskii [1965 Rev Plasma Phys 1 205] transport equations exhibit the same instability in the appropriate limit. This is relevant to laser-produced plasmas, inertial fusion energy (IFE) and to dense cold interstellar plasmas.

Detailed test-particle simulations of energetic particles interacting with magnetized plasmas I. Two-component turbulence

Energetic particles interacting with interplanetary or interstellar plasmas experience transport along and across large scale magnetic fields. In particular the diffusion of particles in the perpendicular direction is difficult to understand theoretically. However, significant progress has been achieved due to the development of test-particle simulations, non-linear systematic theories, and heuristic descriptions of the transport. Non-linear analytical theories often work well but in some cases they require to incorporate a correction factor a2 in order to achieve perfect agreement with simulations. It was stated previously that a2≈1/3 but there are also cases where a2≈1 was found meaning that no correction factor is needed. It is the purpose of this paper to present comprehensive test-particle simulations for slab/2D turbulence also known as two-component turbulence. We consider a wide range of parameters and perform 48 different runs. In all cases we compare our numerical findings with analytical theories in order to determine the exact value of a2 for each case. We also discuss the physical meaning of a2 which should eventually lead to further improvements of analytical theories for perpendicular transport.

Signatures of Intermittency and Fine-scale Turbulence in the Very Local Interstellar Medium

Abstract This study presents the first Voyager 1 (V1) observation of magnetic turbulence intermittency and fine-scale structures associated with the propagation of a shock wave in the outer heliosheath. The intermittent event starts on DOY 178 in 2014 upstream of the shock wave that overtook V1 on DOY 237 at 140 au from the Sun. The distribution of magnetic field increments follows the q-Gaussian distribution with the index q ≈ 1.57 for the maximum variance component. Evidence is provided of fine-scale structures well below the Coulomb collisional scale. They undergo a turbulent cascade at scales less than 10−3 au. Observed magnetohydrodynamic fluctuations show signatures of irregular filamentary structures, sawtooth-like waveforms of mixed compressible/transverse nature, which evolve into shocklets and current sheets. Observational data challenges the interpretation of the interstellar plasma in the outer heliosheath as a featureless medium at scales below the collisional mean free path of about 1 au. These results are of importance for better understanding of turbulent processes in the outer heliosheath, including magnetic reconnection, shock-turbulence interaction, Galactic cosmic-ray transport, plasma wave generation, and radio emission.