Voyager 1 Says Goodbye
On December 16th, 2004 Voyager 1 crossed the termination shock and headed on its way out of our solar system.  This is no trivial event, not just another milestone in Voyager's and other spacecrafts' journeys.  The passage of Voyager 1 into the heliosheath and beyond gives scientists a look at a new realm.  Voyager 1, followed by Voyager 2 (most likely in 2008) will allow astronomers to better understand the solar wind, the interstellar medium, anomalous cosmic rays (ACRs) and much more. The question is, how do we know that Voyager 1 did indeed pass the termination shock on December 16 (251/2004)?  Better yet, what exactly is the termination shock?

The solar wind, an extension of the Sun's outer atmosphere, expanding outward from the Sun at supersonic speeds must eventually meet up with the local interstellar medium (ISM).  Before the solar wind can merge with the ISM, however, it has to decelerate to the speed of the ISM.  Supersonic flows do this through a shock, a place where the flow can quickly decelerate.  The place where the solar wind does this has been termed the "Termination Shock."

The location of the termination shock is governed by the dynamic pressure of the solar wind and the dynamic pressure of the local ISM.  Back of the envelope calculations can be done to dtermine an approxiamate location where these two are equal, where the termination shock is located. More precisely, the termination shock has over the years been determined to lie approxiamately 90 to 100 astronomical units (AU) from the sun (by comparison, pluto is about 40 AU from the sun).  Voyager 1 crossed the termination shock about 94 AU from the sun.  (Fisk).  The region beyond the termination shock has been dubbed the "Heliosheath", beyond which lies the bow shock of our solar system, as it interacts with local interstellar winds.

Prior to Voyager 1's crossing, there were "precursor events" that seemed to indicate that the spacecraft was just upstream of the shock.  Some took an increase in intensity of certain low-energy energetic particles as evidence that Voyager 1 crossed the termination shock in 2003, but others disagreed. (Frisk)  Their was a consensus amongst observations for a shock crossing on December 16th, however.  Magnetic field, plasma wave, and low-energy particle intensity observations all point to a shock crossing (Frisk). Voyager 2 has a functioning plasma detector, whereas Voyager 1's died prior to it's shock crossing, so when Voyager 2 crosses sometime in the near future there is hope that even more conclusive measurements can be taken.

So how is it that Voyager 1 was able to take measurements from which scientists can determine a shock crossing took place and from which the flow speed of the solar wind can be indirectly extrapolated?  Several papers in the 23 September 2005 issue of "Science" present evidence pointing to a crossing. Data from the Low-Energy Charged Particle (LECP) detector was presented by Decker et al. as evidence of a shock crossing on 351/2004.  The data is presented in the figure:

It is divided up into four time periods, A, B, C and D.  Period A (2002.58 to 2003.10, 85.3 to 87.3 AU) corresponds to an increase in intensities across the spectrum of low-energy charged particles, as well as large sunward anisotropies in those particles.  This was presented as the first indication that Voyager 1 was observing something new. (Decker et al.) Period B (2003.10 to 2004.07, 87.3 to 90.8 AU) corresponds to a drop in particle intensities.  Period C (2004.07 to 2004.96, 90.8 to 94.0 AU) corresponds to a year-long return to high intensities, followed by a quick drop and rise again.  Period D (2004.96 to 2005.50, 94.0 to 96.0 AU) begins the day prior to the shock crossing and shows a high (though steady) rate of energetic particles as well as variations in anisotropy and a decreased solar wind velocity.  All of these are cited as evidence of a shock crossing.

The data is divided into six different sets.  A, B and D trace the intensities of ions, protons and electrons respectively, of the energy ranges indicated in the figure.  Section C shows the anisotropy vector of the protons in section B.  These data sets from periods A, B and C seem to indicate activity indicative of the area near the termination shock. 

In a paper published in the Journal of Gephysical Research by Richardson et al., it is suggested that the varations in intensity and anisotropy are consistent with events that occured at Voyager 2 as well.  Further, data from several spacecraft suggest that the solar wind dynamic pressure increased to a maximum in 2003, and is most likely decreasing (Richardson et al.).  As such, the termination would have moved outward, then inward over Voyager 1 at it's crossing event in 2004.

Section E indicates when the Plasma Wave instrument measured electron plasma oscillations.  Decker et al. state that the oscillations observed on 2004/350 "is plausible evidence that the plasma oscillations were being driven by energetic electrons from the Termination Shock."  This would indicate that Voyager 1 was very near the termination shock, strengthening the evidence from a crossing on 351/2004.  Section F indicates the radial component of the solar wind velocity vector.