Very Local Interstellar Medium Research Articles

Intermittency in the Magnetic Hump in the VLISM

Voyager 1 has been moving through the very local interstellar medium (VLISM) for ∼1 solar cycle, from 122.58 au on 2012/DOY 238 (August 25) to 158.5 au on 2023.0. A magnetic hump was observed, beginning with an abrupt increase (jump) in the magnetic field strength B and proton density N by a factor of 1.35 and 1.36, respectively, in the course of ∼8 days, ending at ∼2020/DOY 147, after which it remained above the pre-jump levels for 2.6 yr, and it is still increasing. Unlike the three previous jumps, which were followed by a slow decrease in B, B in the magnetic hump continued to increase to a maximum value ∼0.56 nT at ∼2021/DOY 146.7. This paper discusses the intermittency of the magnetic field components and strength in the magnetic hump, and compares them with previous values in the VLISM. We consider the intermittency of the increments of B and its components observed on a scale of one day. The distribution function (the Tsallis distribution or q-distribution function) of the increments of hourly averages of the components of B had a Gaussian core associated with randomness in the observations, and it had two symmetric tails associated with the observed intermittency. The parameters q-dBR1, q-dBT1, and q-dBN1 increased slightly to a broad maximum with q = ∼1.4 between 2018 and 2019, and then decreased to q ∼ 1 (corresponding to no intermittency) in 2022. The Z-component of the magnetic field is within 4 ± 4 counts (0.02 nT ± 0.02 nT).

Fourteen Years of Energetic Neutral Atom Observations from IBEX

The Interstellar Boundary Explorer (IBEX) has been observing the outer heliosphere and its interactions with the very local interstellar medium (VLISM) via measurements of energetic neutral atoms (ENAs) for over 14 yr. We discovered the IBEX Ribbon—a structure completely unanticipated by any prior theory or model—that almost certainly resides beyond the heliopause in the VLISM. We also characterized the other major source of heliospheric ENAs, the globally distributed flux (GDF), produced largely in the heliosheath between the termination shock and heliopause. In this study, we make three major new contributions. First, we validate, provide, and analyze the most recent 3 yr of IBEX-Hi (0.5–6 keV FWHM) data (2020–2022) for the first time. Second, we link these observations to the prior 11 yr of observations, exploring long-term variations. Finally, we provide the first IBEX team-validated Ribbon/GDF separation scheme and separated maps. Because of the uncertainty in separating different line-of-sight integrated sources, we provide not just best guess (median) maps, but also maps with upper and lower reasonable values of Ribbon and GDF fluxes, along with bounding fluxes that add the uncertainties to the upper and lower values. This allows theories and models to be compared with a range of possible values that the IBEX team believes are consistent with data. These observations, along with the reanalysis of the prior 11 yr of IBEX-Hi data, provide new insights and even further develop our detailed understanding of the heliosphere’s interaction with the local interstellar medium unlocked by IBEX.

Plasma line detected by Voyager 1 in the interstellar medium: Tips and traps for quasi-thermal noise spectroscopy

The quasi-thermal motion of plasma particles produces electrostatic fluctuations, whose voltage power spectrum induced on electric antennas reveals plasma properties. In weakly magnetised plasmas, the main feature of the spectrum is a line at the plasma frequency – proportional to the square root of the electron density – whose global shape can reveal the electron temperature, while the fine structure reveals the suprathermal electrons. Since it is based on electrostatic waves, quasi-thermal noise spectroscopy (QTN) provides in situ measurements. This method has been successfully used for more than four decades in a large variety of heliosphere environments. Very recently, it has been tentatively applied in the very local interstellar medium (VLISM) to interpret the weak line discovered on board Voyager 1 and in the context of the proposed interstellar probe mission. The present paper shows that the line is still observed in the Voyager Plasma Wave Science data, and concentrates on the main features that distinguish the plasma QTN in the VLISM from that in the heliosphere. We give several tools to interpret it in this medium and highlight the errors arising when it is interpreted without caution, as has recently been done in several publications. We show recent solar wind data, which confirm that the electric field of the QTN line in a weakly magnetised stable plasma is not aligned with the local magnetic field. We explain why the amplitude of the line does not depend on the concentration of suprathermal electrons, and why its observation with a short antenna does not require a kappa electron velocity distribution. Finally, we suggest an origin for the suprathermal electrons producing the QTN and we summarise the properties of the VLISM that could be deduced from an appropriate implementation of QTN spectroscopy on a suitably designed instrument.

Toward Interpreting the IBEX Ribbon with Mirror Diffusion in Interstellar Turbulent Magnetic Fields

We investigate the role of the magnetohydrodynamic (MHD) turbulence measured by Voyager in the very local interstellar medium (VLISM) in modeling the Interstellar Boundary Explorer ribbon. We demonstrate that the mirroring by compressible modes of MHD turbulence dominates over that by the mean magnetic field. Based on the new mirror diffusion mechanism identified by Lazarian & Xu for particles with large pitch angles in MHD turbulence, we find that the mirror diffusion can both confine pickup ions and preserve their initial pitch angles, and thus it accounts for the enhanced intensity of energetic neutral atoms that return to the heliosphere. The ribbon width is determined by both the range of pitch angles for effective turbulent mirroring and the field line wandering induced by Alfvénic modes. It in turn provides a constraint on the amplitude of magnetic fluctuations of fast modes. The field line wandering also affects the coherence of the ribbon structure across the sky. By extrapolating the magnetic energy spectrum measured by Voyager, we find that the injection scale of the turbulence in the VLISM must be less than ∼500 au for the ribbon structure to be coherent.

Interstellar Conditions Deduced from Interstellar Neutral Helium Observed by IBEX and Global Heliosphere Modeling

In situ observations of interstellar neutral (ISN) helium atoms by the IBEX-Lo instrument on board the Interstellar Boundary Explorer (IBEX) mission are used to determine the velocity and temperature of the pristine very local interstellar medium (VLISM). Most ISN helium atoms penetrating the heliosphere, known as the primary population, originate in the pristine VLISM. As the primary atoms travel through the outer heliosheath, they charge exchange with He+ ions in slowed and compressed plasma, creating the secondary population. With more than 2.4 million ISN helium atoms being sampled by IBEX during ISN seasons 2009–2020, we compare the observations with the predictions of a parameterized model of ISN helium transport in the heliosphere. We account for the filtration of ISN helium atoms at the heliospheric boundaries by charge-exchange and elastic collisions. We examine the sensitivity of the ISN helium fluxes to the interstellar conditions described by the pristine VLISM velocity, temperature, magnetic field, and composition. We show that comprehensive modeling of the filtration processes is critical for interpreting ISN helium observations, as the change in the derived VLISM conditions exceeds the statistical uncertainties when accounting for these effects. The pristine VLISM parameters found by this analysis are the flow speed (26.6 km s−1), inflow direction in ecliptic coordinates (255.°7, 5.°04), temperature (7350 K), and B − V plane inclination to the ecliptic plane (53.°7). The derived pristine VLISM He+ density is 9.7 × 10−3 cm−3. Additionally, we show a strong correlation between the interstellar plasma density and magnetic field strength deduced from these observations.

A Large Magnetic Hump in the VLISM Observed by Voyager 1 in 2020–2022

Voyager 1 has been moving through the very local interstellar medium (VLISM) for approximately one solar cycle, from 122.58 au on 2012/DOY 238 (August 25) to 158.5 au on 2023.0. Previously, an abrupt increase (“jump”) in the magnetic field strength B and proton density N by a factor of 1.35 and 1.36, respectively, was observed during an interval of ∼8 days in 2020.40. After the jump, B continued to increase to a maximum value ∼0.56 nT at ∼2021.4 and then declined until B returned to the postjump value of 0.5 nT on 2021.85, 1.45 yr after the jump. The magnetic field strength declined briefly from 0.5 nT on 2021.85 to 0.47 nT on 2021.95 and then increased sporadically to 0.52 nT at 2023.0. Thus, the magnetic field strength remained strong for at least 2.6 yr. The magnetic hump and the density hump were a compression wave propagating through the VLISM. The compression wave was generated by a region with large dynamic pressure in the solar wind that propagated through the inner heliosheath and collided with the heliopause. The magnetic field strength continued to remain strong, with slow variations, until the end of our observations at 2023.0. It is suggested that the magnetic hump evolved from the large dynamic pressure, high speeds, and density observed at 1 au between ∼2015 and ∼2017.

Turbulence and Diffusive Transport of Cosmic Rays in the Very Local Interstellar Medium

We study the transport of fast charged particles, such as galactic cosmic rays, in the very local interstellar medium (VLISM), which is currently being explored by the two Voyager space probes. Guided by the observations of magnetic fluctuations, the paper develops a simple theoretical framework for computing scattering rates and spatial diffusion coefficients that can be used to model cosmic-ray transport in the VLISM. The local interstellar magnetic turbulence is represented as a superposition of (a) Alfvénic, (b) transverse 2D, and (c) longitudinal components obeying distinctive geometry rules in the plasma frame. The model is based on the weakly nonlinear formalism where particle trajectory’s deviation from the unperturbed helix is caused primarily by guiding-center diffusion across the mean magnetic field. The transverse component plays the dominant role in perpendicular diffusion, while the longitudinal component has only a minor effect. Pitch-angle scattering is extremely weak in the VLISM, so that cosmic-ray transport can be considered essentially scatter-free on heliospheric scales. We test our theoretical model with the help of particle orbit simulations to find good agreement for perpendicular diffusion. We also find that cosmic rays disperse faster than in a conventional random walk (diffusive) process if the turbulence power spectrum contains fluctuations whose wavelength is larger than the size of the heliosphere.

Relating Energetic Ion Spectra to Energetic Neutral Atoms

Heliospheric energetic neutral atoms (ENAs) originate from energetic ions that are neutralized by charge exchange with neutral atoms in the heliosheath and very local interstellar medium (VLISM). Since neutral atoms are unaffected by electromagnetic fields, they propagate ballistically with the same speeds as parent particles. Consequently, measurements of ENA distributions allow one to remotely image the energetic ion distributions in the heliosheath and VLISM. The origin of the energetic ions that spawn ENAs is still debated, particularly at energies higher than ∼keV. In this work, we summarize five possible sources of energetic ions in the heliosheath that cover the ENA energy from a few keV to hundreds of keV. Three sources of the energetic ions are related to pickup ions (PUIs): those PUIs transmitted across the heliospheric termination shock (HTS), those reflected once or multiple times at the HTS, i.e., reflected PUIs, and those PUIs multiply reflected and further accelerated by the HTS. Two other kinds of ions that can be considered are ions transmitted from the suprathermal tail of the PUI distribution and other particles accelerated at the HTS. By way of illustration, we use these energetic particle distributions, taking account of their evolution in the heliosheath, to calculate the ENA intensities and to analyze the characteristics of ENA spectra observed at 1 au.

Are the Heliosphere, Very Local Interstellar Medium, and Local Cavity in Pressure Balance with Galactic Gravity?* *Based on observations made with the NASA/ESA Hubble Space Telescope obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS AR-09525.01A. These observations are associated with programs #12475, 12596.

The Voyager spacecraft are providing the first in situ measurements of physical properties in the outer heliosphere beyond the heliopause. These data, together with data from the IBEX and Hubble Space Telescope and physical models consistent with these data, now provide critical measurements of pressures in the heliosphere and surrounding interstellar medium. Using these data, we assemble the first comprehensive survey of total pressures inside and outside of the heliopause, in the interstellar gas surrounding the heliosphere, and in the surrounding Local Cavity to determine whether the total pressures in each region are in balance with each other and with the gravitational pressure exerted by the galaxy. We intercompare total pressures in each region that include thermal, nonthermal, plasma, ram, and magnetic pressure components. An important result is the role of dynamic (ram) pressure. Total pressure balance at the heliopause can only be maintained with a substantial contribution of dynamic pressure from the inside. Also, total pressure balance between the outer heliosphere and pristine very local interstellar medium (VLISM) and between the pristine VLISM and the Local Cavity requires large dynamic pressure contributions.

On the Injection Scale of the Turbulence in the Partially Ionized Very Local Interstellar Medium

The cascade of magnetohydrodynamic (MHD) turbulence is subject to ion–neutral collisional damping and neutral viscous damping in the partially ionized interstellar medium. By examining the damping effects in the warm and partially ionized local interstellar medium, we find that the interstellar turbulence is damped by neutral viscosity at ∼261 au and cannot account for the turbulent magnetic fluctuations detected by Voyager 1 and 2. The MHD turbulence measured by Voyager in the very local interstellar medium (VLISM) should be locally injected in the regime where ions are decoupled from neutrals for its cascade to survive the damping effects. With the imposed ion–neutral decoupling condition and the strong turbulence condition for the observed Kolmogorov magnetic energy spectrum, we find that the turbulence in the VLISM is sub-Alfvénic, and its largest possible injection scale is ∼194 au.

On the Energy Dependence of Galactic Cosmic Ray Anisotropies in the Very Local Interstellar Medium

We report on the energy dependence of Galactic cosmic rays (GCRs) in the very local interstellar medium (VLISM) as measured by the Low Energy Charged Particle (LECP) instrument on the Voyager 1 spacecraft. The LECP instrument includes a dual-ended solid-state detector particle telescope mechanically scanning through 360° across eight equally spaced angular sectors. As reported previously, LECP measurements showed a dramatic increase in GCR intensities for all sectors of the ≥211 MeV count rate (CH31) at the Voyager 1 heliopause (HP) crossing in 2012; however, since then the count rate data have demonstrated systematic episodes of intensity decrease for particles around 90° pitch angle. To shed light on the energy dependence of these GCR anisotropies over a wide range of energies, we use Voyager 1 LECP count rate and pulse height analyzer (PHA) data from ≥211 MeV channel together with lower-energy LECP channels. Our analysis shows that, while GCR anisotropies are present over a wide range of energies, there is a decreasing trend in the amplitude of second-order anisotropy with increasing energy during anisotropy episodes. A stronger pitch angle scattering at higher velocities is argued as a potential cause for this energy dependence. A possible cause for this velocity dependence arising from weak rigidity dependence of the scattering mean free path and resulting velocity-dominated scattering rate is discussed. This interpretation is consistent with a recently reported lack of corresponding GCR electron anisotropies.

Magnetic Field Observations in the Very Local Interstellar Medium by Voyagers 1 and 2

Observations of the magnetic field B in the very local interstellar medium (VLISM) were made by Voyager 1 (V1) in the northern hemisphere from 2012 to mid-2021 and by Voyager 2 (V2) in the southern hemisphere from 2018 through 2020. Near 2019.4, V2 observed an abrupt increase in B associated with a pressure front near the heliopause. During 2020, V2 observed an abrupt increase in B at a jump in B that was preceded by electron plasma oscillations and cosmic rays, indicating that it was a shock. The shock was followed by a decrease in B ending ∼50 days later. V2 observed large-scale waves in all three components of B , before and after the shock. The largest- and intermediate-amplitude waves were in the BN and BR component, respectively, indicating that the waves were predominantly transverse several au from the heliopause. It was shown previously that waves near the heliopause were predominantly longitudinal at V1 and V2. Thus, V2 observed a mode transformation process within 10 au of the heliopause in the southern hemisphere, like that observed by V1 in the northern hemisphere. The elevation and azimuthal angles observed by V1 and V2 varied linearly with increasing distance in the VLISM. Voyager 1 observed jumps in B at two shocks and a pressure front, each followed by a decrease in B in a ramp. V1 also observed a fourth jump in B, at 2020.4, but B continued to increase until at least year 2021.5. This long-lasting increase in B was not anticipated.

Interstellar probe – Destination: Universe!

The idea of an “Outer solar system probe: to be aimed away from the Sun in the plane of the ecliptic” dates from a report of the “Simpson Committee” of the Space Science Board of the National Academy of Sciences in March of 1960. After many studies and name changes, what is now known as “Interstellar Probe” has matured as a concept for making new discoveries that can be made in no other way, by going places yet to be explored. The central technical question has always been propulsion with “near-future” capabilities taken as the backdrop for defining the mission requirements. However, the real issue has always been to unite compelling science with engineering and technical reality. With that perspective in mind the Johns Hopkins University Applied Physics Laboratory (APL) has been tasked by the NASA Heliophysics Division to (re-)study the mission and provide a Technical Report to be delivered late 2021 for input to next Solar and Space Physics Decadal Survey. This “pragmatic Interstellar Probe” of the study is a mission through the outer heliosphere and to the nearby “Very Local” interstellar medium (VLISM), uses today's technology to take the first explicit step on the path of interstellar exploration, and can pave the way, scientifically, technically, and programmatically for more ambitious future journeys (and more ambitious science goals). To enforce these goals broadly-based engineering requirements include (1) readiness to launch no later than January 1, 2030; (2) capability to transmit useful scientific data from 1000 au; (3) powered by no more than 600 W (electric) at the beginning of the mission and no more than half of that at mission's end; and, (4) lifetime of no less than 50 years. To travel as far and as fast as possible with available technology, the use of the Space Launch System Block 2 (SLS B2) cargo version is enabling: carrying the spacecraft as well as a 3rd and 4th stage, solar system escape speeds of at least twice that of Voyager 1 (i.e., up to 7.2 au/yr) should be possible. We provide a top-level summary of work accomplished to date, focusing on how the science goals drive and are affected by telecommunication options, guidance and control requirements, trajectory options, and the baseline system architecture and approach.“It isn't about where we are going. It's about the journey out there."

Very Local Interstellar Medium Revealed by a Complete Solar Cycle of Interstellar Neutral Helium Observations with IBEX

Abstract The IBEX-Lo instrument on board the Interstellar Boundary Explorer (IBEX) mission samples interstellar neutral (ISN) helium atoms penetrating the heliosphere from the very local interstellar medium (VLISM). In this study, we analyze the IBEX-Lo ISN helium observations covering a complete solar cycle, from 2009 through 2020 using a comprehensive uncertainty analysis including statistical and systematic sources. We employ the Warsaw Test Particle Model to simulate ISN helium fluxes at IBEX, which are subsequently compared with the observed count rate in the three lowest energy steps of IBEX-Lo. The χ 2 analysis shows that the ISN helium flows from ecliptic λ , β = ( 255 .° 59 ± 0 .° 23 , 5 .° 14 ± 0 .° 08 ) , with speed v HP = 25.86 ± 0.21 km s−1 and temperature T HP = 7450 ±140 K at the heliopause. Accounting for gravitational attraction and elastic collisions, the ISN helium speed and temperature in the pristine VLISM far from the heliopause are v VLISM = 25.9 km s−1 and T VLISM = 6150 K, respectively. The time evolution of the ISN helium fluxes at 1 au over 12 yr suggests significant changes in the IBEX-Lo detection efficiency, higher ionization rates of ISN helium atoms in the heliosphere than assumed in the model, or an additional unaccounted for signal source in the analyzed observations. Nevertheless, we do not find any indication of the evolution of the derived parameters of ISN helium over the period analyzed. Finally, we argue that the continued operation of IBEX-Lo to overlap with the Interstellar Mapping and Acceleration Probe will be pivotal in tracking possible physical changes in the VLISM.

Magnetic Field and Plasma Density Observations of a Pressure Front by Voyager 1 during 2020 in the Very Local Interstellar Medium

Abstract Voyager 1 has been moving through the very local interstellar medium (VLISM) from the time that it crossed the heliopause on 2012/DOY 238 to 2020/DOY 292. Three notable objects in the magnetic field of the VLISM have been observed: two shocks and one pressure front. This paper reports the observation of a fourth object observed near 2020/DOY 147. There were no upstream electron plasma oscillations of the type often observed ahead of shocks, abrupt increases in energetic particles, or fluctuations in the 48 s increments of the magnetic field associated with this feature, suggesting that it was probably not a shock. This feature was associated with a relatively large increase in the magnetic field strength (B2/B1 = 1.35) and in the electron density determined by the Plasma Wave Science experiment (N2/N1 = 1.36) using a new method described in this paper. This feature appears to be a pressure front associated with a compressive wave in the VLISM. The two shocks and the two pressure fronts were associated with the four largest maxima observed in B(t) between 2012/DOY 238 and 2020/DOY 292. Each feature was associated with a jump–ramp structure. The jump–ramp structures were separated by long relatively undisturbed quiet intervals.

Slowdown and Heating of Interstellar Neutral Helium by Elastic Collisions Beyond the Heliopause.

Direct sampling of interstellar neutral (ISN) atoms close to the Sun enables studies of the very local interstellar medium (VLISM) around the heliosphere. The primary population of ISN helium atoms has, until now, been assumed to reflect the pristine VLISM conditions at the heliopause. Consequently, the atoms observed at 1 au by the Interstellar Boundary Explorer (IBEX) were used to determine the VLISM temperature and velocity relative to the Sun, without accounting for elastic collisions with other species outside the heliopause. Here, we evaluate the effect of these collisions on the primary ISN helium population. We follow trajectories of helium atoms and track their collisions with slowed plasma and interstellar hydrogen atoms ahead of the heliopause. Atoms typically collide a few times in the outer heliosheath, and only ~1.5% of the atoms are not scattered at all. We use calculated differential cross sections to randomly choose scattering angles in these collisions. We estimate that the resulting primary ISN helium atoms at the heliopause are slowed down by ~0.45 km s-1 and heated by ~1100 K compared to the pristine VLISM. The resulting velocity distribution is asymmetric and shows an extended tail in the antisunward direction. Accounting for this change in the parameters derived from IBEX observations gives the Sun's relative speed of 25.85 km s-1 and temperature of 6400 K in the pristine VLISM. Finally, this paper serves as a source of the differential cross sections for elastic collisions with helium atoms.

Waves and Turbulence in the Very Local Interstellar Medium: From Macroscales to Microscales

Abstract Voyager 1 (V1) has been exploring the heliospheric boundary layer in the very local interstellar medium (VLISM) since 2012 August. The measurements revealed a spectrum of fluctuations over a vast range of space and timescales, but the nature of these fluctuations continues to be an intriguing question. Numerous manifestations of turbulence cannot be explained using a single phenomenology. Weak shocks and compressions are the prominent features of the VLISM. We use high-resolution (48 s) measurements to perform a multiscale analysis of turbulence at V1 between the years of 2013.36 and 2019.0 (124–144 au from the Sun). On relatively large scales, wave trains of mixed compressible/transverse nature with the correlation scale in the range of 15–100 days dominate the spectrum of fluctuations. The observed magnetic field profiles are suggestive of a Burgers-like (f −2) turbulence phenomenology induced by solar activity. We demonstrate that the level of large-scale compressible fluctuations is still significant in late 2018. We analyze the turbulence down to small scales comparable to the ion inertial length and show that magnetic compressibility is always large on these scales. Besides the shock-induced turbulence measured from 2014.486, the intensity and intermittency of small-scale fluctuations have been growing smoothly since 2018.5. Our analysis suggests that local processes are contributing to the production of turbulence on small scales. We present the estimates of transport coefficients in the plasma traversed by V1. The range of scales is identified where V1 measurements are affected by the contribution from pickup ions.

Intermittency and q-Gaussian Distributions in the Magnetic Field of the Very Local Interstellar Medium (VLISM) Observed by Voyager 1 and Voyager 2

Abstract Voyager 1 (V1) observed q-Gaussian distributions of 1 hr increments of the components the magnetic field throughout the very local interstellar medium (VLISM) on scales on the order of 1 year, from 2013 through 2019, excluding the disturbed regions near two shocks. The intermittency of the components of B is described by q from the q-Gaussian distributions of 1 hr increments B as well as by the kurtosis K derived from the time series of the increments of . Farther than ∼5 au from the heliopause, Voyager 1 observed larger intermittency in the fluctuations of the predominantly transverse component B N and in the nearly longitudinal component B T of . The average values of q for the B N component and transverse B T component for these data in the VLISM are q = 1.32 ± 0.03 and q = 1.22 ± 0.03, respectively. The corresponding average values of the kurtosis are K = 7.2 and K = 4.9. Voyager 2 (V2) crossed the heliopause on 2018 November 5. Near the heliopause, in 2019, V2 observed large intermittency in 1 hr increments of the compressive component B T and smaller intermittency in B N. The Voyager 2 observations give q = 1.51 ± 0.06 and q = 1.17 ± 0.06 for the B N and B T components of B, respectively, as well as K = 7.2 and K = 12.3, respectively. The intermittency in B T and B N, observed by V2 near the heliopause was significantly larger than that observed by V1 when it was near the heliopause.

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.

Outer Heliospheric Turbulence and the Angular Broadening of Radio Sources from the Voyager Data

The properties of density turbulence are important in estimating the angular broadening of radio sources in the outer heliosphere and very local interstellar medium (VLISM). We calculate the density variance, inner scale, correlation length of the velocity, and relative density amplitude as a function of radial distance using plasma and magnetometer data from the Voyager 2 spacecraft over the period 1977 to 2018. We apply three different techniques to estimate the density turbulence amplitude, including an inner scale, using the Voyager 2 data. Related analyses compare Voyager data determined turbulence amplitudes with results from the density turbulence theories of Zank et al. [49, 50] and Bellamy et al. [6]. Theoretical predictions and observations show strong similarities: both qualitatively and quantitatively. In addition, the predicted turbulence amplitudes show radial trends similar to those observed. The numerical prediction of the scattering angle uses a turbulence spectrum that includes an inner scale. The numerical and analytic results are compared, and we find similar radial trends.