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Highlights

Apart from their accomplishments during the Grand Tour of the giant outer planets, the Voyager spacecraft have achieved immense successes on their Interstellar Mission. The Voyager Interstellar Mission has vastly increased and will continue to make unique contributions to our understanding of the Sun, the interplanetary medium, the interstellar medium, and their mutual interactions.

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After 25 years the spacecraft are still functioning remarkably well. Five instrument teams continue to provide us with new and exciting research results: the Plasma Science experiment (PLS) measures thermal plasma the Low Energy Charged Particle experiment (LECP) detects low energy charged particles (from tens of keV to MeV) the Cosmic Ray subsystem (CRS) measures galactic and anomalous cosmic rays the magnetometer experiment (MAG) investigates interplanetary magnetic fields the Plasma Wave subsystem (PWS) observes plasma and radio waves

In addition, the Planetary Radio Astronomy and Ultraviolet Spectrometer instruments are still providing us with data, although their teams are no longer active.

We mention a few of the most outstanding Voyager Interstellar Mission highlights.

Researchers had long predicted that the solar wind speed would decrease with distance from the Sun, due to the pickup of interstellar neutrals. Voyager 2 was the first to verify this prediction. In 1999 the Ulysses spacecraft at 5 AU and Voyager 2 at 60 AU were at the same latitude and thus observed nearly the same solar wind. The PLS team used an MHD model and the Ulysses data to predict a decrease in the solar wind speed, which Voyager 2 observed eight months later.

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Solar wind speed (top panel) and heliolatitude (bottom panel) as a function of time, for Voyager 2, IMP 8, and Ulysses

Voyager 2 observed the variation in the solar wind dynamic pressure with solar cycle. Over the entire solar cycle the pressure varies by a factor of approximately 2, with the minimum occurring near solar maximum, followed by a rapid increase over the next year or two, and then decreasing slowly as solar maximum approaches. Researchers compared the Voyager 2 data with Ulysses data at high latitudes and IMP 8 data at Earth to show that this variation with solar cycle is independent of the large variations in solar wind speeds and densities at different latitudes.

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Solar wind dynamic pressure observed by Voyager 2, IMP 8, and Ulysses

By investigating so-called "pressure balanced structure," the plasma and magnetometer teams showed that, beyond 30 AU, the pressure of interstellar pickup protons is greater than that of the thermal protons. At 40 AU, the pickup ion density is 3 1 % of the solar wind density.

Models of shock waves propagating in the outer heliosphere predict that pickup ions should weaken shocks in the outer heliosphere, as has been verified by comparing shocks at 5.2 and 43 AU.

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Plot of a simulation, showing the evolution of a shock complex in heliocentric distance. Shown are velocity profiles of two forward and a reverse shock, and the weakening with distance is evident.

Voyager 2 measured the solar wind thermal proton temperature and discovered that it decreased more slowly than expected for a 5/3 adiabatic solar wind to 30 AU, increased to 50 AU, and has recently decreased again. Since the temperature and solar wind speed are correlated, models that include this dependence, together with turbulent dissipation, predict the temperature profile well.

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The thermal proton temperature observed by Voyager 2 (white), predicted by an adiabatic decrease (blue), predicted by Smith et al. in 2001 (green), and predicted by combining the Smith et al. result with a speed dependence (red)

Voyager 2 observed high speeds and low densities near solar minimum and, as solar maximum approached, recorded low speeds and high densities.

By taking into account variations of the magnetic field strength and speed with solar cycle, the theoretical Parker spiral magnetic field model agrees closely with Voyager 1's observations of the magnetic field strength.

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A plot of the interplanetary magnetic field as observed by Voyager and compared with the theoretical model developed by E.N. Parker. The dots show yearly averages of the magnetic field strength B measured by Voyager 1 from 1978 to 2001. The solid curve is Parker's model. The three local maxima and two local minima are associated with solar cycle variations in the source magnetic field strength and with both solar cycle and latitude variations in the solar wind speed. The bounding curves are for a steady solar wind speed of either 400 km/s (top) or 800 km/s (bottom). (Figure courtesy L.F. Burlaga and N.F. Ness)

Voyager observed Merged Interaction Regions (MIRs) as solar maximum approached. MIRs have relatively strong magnetic fields and often increased plasma density. In 2000, Voyager observed an MIR that, as observed in previous solar cycles, produced a step-like decrease in the cosmic ray intensity.

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The top panel shows cosmic ray intensity for particles with energies exceeding 70 MeV/N as a function of time. A large decrease in intensity is coincident with the passage of an MIR, shown in the bottom panel, where the magnetic field magnitude I plotted.

The LECP instrument on Voyager has observed the first possible in situ observation of pickup oxygen ions in the outer heliosphere.

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Panel (a) shows the LECP look directions. The arrival of pickup oxygen ions is detectable only in sector 1. Panel (b) compares the fluxes in sectors 1 and 7. The enhanced fluxes in sector 1 are evidence of pickup oxygen ions. Panel (c) shows the solar wind speed. Higher fluxes correspond to higher solar wind speeds because the pickup ion distribution is convected with the solar wind.

From 1983-1985 and 1992-1994 the LECP instrument observed that the peak proton intensity generated at Corotating Interaction Region (CIR) associated shocks decreased with helioradius according to an r-3 law. According to theory, this decrease is probably due to the weakening of the shocks as the pickup ion pressure becomes large.

Observations in the inner heliosphere tend to show that, when shocks are associated with energetic particle increases, the peak particle flux tends to coincide with the shock front. In the outer heliosphere, Voyager 2 data often shows a delay between the arrival of interplanetary shock fronts and the peaks in the energetic particle fluxes.

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The top panel shows solar wind speed and the bottom panel cosmic ray intensity. Voyager 2 data shows that, at 5 AU, the shocks and peaks in the energetic particle flux are essentially coincident, whereas, at 47 AU, the peaks in the energetic particle fluxes lag the shock fronts by about 7 days.

Voyager 2 data shows that, at 5 AU, the shocks and peaks in the energetic particle flux are essentially coincident, whereas, at 47 AU, the peaks in the energetic particle fluxes lag the shock fronts by about 7 days. Pickup ion effects cause shocks to weaken with distance so that, in the outer heliosphere, shocks may become too weak to inject particles into the shock acceleration process.

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Left: The top panel shows solar wind speed and the bottom panel cosmic ray intensity. Voyager 2 data shows that, at 5 AU, the shocks and peaks in the energetic particle flux are essentially coincident, whereas, at 47 AU, the peaks in the energetic particle fluxes lag the shock fronts by about 7 days. Right: Corresponding theoretical model of a shock accelerating particles far from the Sun and closer to the Sun. The top panel shows plasma density, the middle panel solar wind velocity, and the bottom panel cosmic ray pressure.

The Bastille Day event provided Voyager with evidence that large shocks associated with MIRs and GMIRs can persist to the outer heliosphere.

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The effects of MIRs on 0.5-17 MeV proton populations observed at Voyager 2 during the latter part of 1998 (top panel) and early in 2001 (bottom panel)

While galactic cosmic rays entering the solar system have to cross the heliosheath, anomalous cosmic rays originate at the termination shock, the inner boundary of the heliosheath. Voyager 1 data provides evidence that modulation occurs in the heliosphere and also provides an estimate for the location of the termination shock.

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Left: Recovery of ACR intensities (A, B) and GCR intensities (C) at 44 AU (yellow symbols), following the passage of a large interplanetary disturbance. The slower GCR recovery suggests that modulation is occurring in the heliosheath. Right: (top) Regression plot of ACR intensities vs. CRS intensities, showing a plateau in ACRs, whereas GCRs continue to increase; (bottom) ACR intensities in two oppositely pointing telescopes vs. GCR intensities at V!, showing a strong ACR anisotropy in early 1992 (marked by the arrow A), leading to an estimate of the location of the termination shock of 89 AU.

The Voyager plasma wave instruments have been detecting radio emissions from the outer heliosphere for 20 years at frequencies from approximately 2-3 kHz. This 20-year frequency-time spectrogram from Voyager 1 illustrates two especially intense events, in 1983-1984 and in 1992-1994. It is very likely that these emissions are produced near the heliopause by GMIRs. Thus Voyager has produced the first direct evidence of the heliopause.

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Left: A frequency-time spectrogram, showing the 1983-1984 and 1992-1994 heliospheric radio emission events. Right: A comparison of cosmic ray intensities at Earth and the intensity of the 3.11 kHZ heliospheric radio emissions detected by Voyager.

The Voyager spacecraft observed that very different levels of modulation occur in the inner and outer heliosphere.

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