Search for the Heliospheric Termination Shock (TS) and Heliosheath (HS)

Voyager 1 continues to measure the very distant Heliospheric Magnetic Field (HMF) beyond 95 AU at ~35 North latitude. The MAG instrument data covers more than a full 22 years solar magnetic cycle. The magnitude of the observed HMF is well described, on average, by Parker's Archimedean spiral structure if due account is made for time variations of the source field strength and solar wind velocity. The V1 magnetic field observations do not provide any evidence for a field increase associated with entry into a subsonic solar wind region, such as the heliosheath is expected to be, nor an exit from this regime. We see no evidence for crossing of the Termination Shock (TS) as has been reported at ~85 AU by the LECP instrument. Merged Interaction Regions are identified by an increased HMF and associated decreases in the flux of >70 MeV/nuc cosmic rays which are then followed by a flux recovery. This CR-B relationship has been identified in V1 data and studied since 1982 when V1 was at 11 AU. The variance of HMF, a direct measure of the energy**1/2 in the HMF fluctuations, shows no significant changes associated with the alleged TS crossings in 2002–2003. Thus, the absence of any HMF increase at the entry into the heliosheath appears not to be due to the onset of mesoscale turbulence as proposed by Fisk. The TS has yet to be directly observed in-situ by the V1 MAG experiment in data through 2003

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

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

Observation of Bernstein Waves Excited by Newborn Interstellar Pickup Ions in the Solar Wind

A recent examination of 1.9 s magnetic field data recorded by the Voyager 2 spacecraft in transit to Jupiter revealed several instances of strongly aliased spectra suggestive of unresolved high-frequency magnetic fluctuations at 4.4 AU. A closer examination of these intervals using the highest resolution data available revealed one clear instance of wave activity at spacecraft frame frequencies from 0.2 to 1 Hz. Using various analysis techniques, we have characterized these fluctuations as Bernstein mode waves excited by newborn interstellar pickup ions. We can find no other interpretation or source consistent with the observations, but this interpretation is not without questions. In this paper, we report a detailed analysis of the waves, including their frequency and polarization, that supports our interpretation

Observations of Energetic Ions and Electrons in the Distant Heliosphere: 2001 – 2005.0

As Voyager 1 (V1) moves closer to the heliospheric termination shock (TS), a new energetic particle population is observed: Termination Shock Particle events (TSP). Interplanetary disturbances in the form of merged interaction regions (MIRs) — identified using Voyager 2 (V2) data — have a major effect on the V1 TSP events from their onset to termination along with triggering episodic increases in higher energy ions (35 MeV H) and MeV electrons. The nature of these interactions appear to evolve as V1 moves closer to the TS

Interplanetary magnetohydrodynamics

Spacecraft such as the Pioneer, Vela, and Voyager have explored the interplanetary medium between the orbits of Mercury and Pluto. The insights derived from these missions have been successfully applied to magnetospheric, astro-solar, and cosmic ray physics. This book is an overview of these insights, using magnetohydrodynamic (MHD) flows as the framework for interpreting objects and processes observed in the interplanetary medium. Topics include various types of MHD shocks and interactions among them, tangential and rotational discontinuities, force-free field configurations, the formation of merged interaction regions associated with various types of flows, the destruction of flows, the growth of the Kelvin-Helmholtz instability and formation of a heliospheric vortex street, the development of multifractal fluctuations on various scales, and the evolution of multifractal intermittent turbulence. Students and researchers in astrophysics will value the data from these missions, which provide confirmation of many theoretical models of the interstellar medium.1. Introduction2. Large-Scale Magnetic Field3. Large-Scale Plasma4. Pressure Balance Structures5. Shocks6. Magnetic Clouds and Force-Free Magnetic Fields7. Corotating Streams and Interaction Regions8. Merged Interaction Regions9. Large-Scale Fluctuations10. Heliospheric Vortex Street

New Observations of the Heliospheric Magnetic Field from the Voyager Spacecraft

We review recent observations of variations of the heliospheric magnetic field B(t) made by Voyager 1 and 2 (V1 and V2), and we discuss the boundary conditions needed for models to explain the observations. Usually, observations from a spacecraft close to the Sun, such as ACE, WIND or Ulysses are used as input to a time-dependent model. Generally, the predicted profile B(t) can be compared directly with the observed profile only when either V1 or V2 is approximately radially aligned with a near-Sun spacecraft; this happens rarely and only for a brief time interval. The Bastille Day events illustrate this situation. In the absence of radial alignment of the spacecraft it is possible to predict the development of a global structure (a GMIR) with data from ACE or WIND, if they obtain a representative sample the flows that merge to form a GMIR. When latitudinal gradients are small and when there is statistical homogeneity in the azimuthal direction, it is possible to predict the statistical properties of the large-scale fluctuations of B(t) observed by V1 or V2 during a year or so. We illustrate this situation with observations from the recent solar maximum and the declining phase of the solar cycle. Predictions of detailed observations made by V1 and V2 under general conditions (e.g., when there is a large latitudinal gradient) require boundary conditions as a function of time on a surface, such as a Sun-centered sphere with a radius of 1 AU. These conditions can only be provided by global solar observations. We suggest the feasibility of such an approach, using V2 observations for 2005 and 2006. The prediction of observations in the heliosheath requires the solution of the 3-D boundary problem for the supersonic solar wind and propagation of solar wind through the termination shock into the heliosphere. The properties of B(t) observed in the heliosheath have not yet been predicted

Compressible Turbulence in the Heliosheath

This paper describes the multi-scale structure of the compressible "turbulence" observed by Voyager 2 in the heliosheath behind the termination shock from 2007 DOY 245.0-300.8 and in a unipolar region from 2008 DOY 2.9-75.6. The magnetic field strength is highly variable on scales from 48 s to several hours in both intervals. The amplitudes of the fluctuations were greater in the post-TS region than in the unipolar region. The multiscale structure of the increments of B is described by the q-Gaussian distribution of nonextensive statistical mechanics on all scales from 48 s to 3.4 hr in the unipolar region and 6.8 hr in the post-TS region, respectively. The amplitudes of the fluctuations of increments of B are larger in the post-TS region than in the unipolar region at all scales. Time series of the magnitude and direction of B show that the fluctuations are highly compressive. The small-scale fluctuations are a mixture of coherent structures (semi-deterministic structures) and random structures, which vary significantly from day to da

Voyager 2 plasma observations of the heliopause and interstellar medium

The solar wind blows outwards from the Sun and forms a bubble of solar material in the interstellar medium. The heliopause (HP) is the boundary that divides the hot tenuous solar wind plasma in the heliosheath from the colder, denser very local interstellar medium (VLISM). The Voyager 2 plasma experiment observed the HP crossing from the solar wind into the VLISM on 5 November 2018 at 119 au. Here we present the first measurements of plasma at and near the HP and in the VLISM. A plasma boundary region with a width of 1.5 au is observed before the HP. The plasma in the boundary region slows, heats up and is twice as dense as typical heliosheath plasma. A much thinner boundary layer begins about 0.06 au inside the HP where the radial speed decreases and the density and magnetic field increase. The HP transition occurs in less than one day. The VLISM is variable near the HP and hotter than expected. Voyager 2 observations show that the temperature is 30,000-50,000 K, whereas models and observations predicted a VLISM temperature of 15,000-30,000 K