R. B. Decker

Plasma flows in the heliosheath

Voyager 2 is making the first plasma measurements in the heliosheath. The radial flow speeds in the heliosheath vary between 80 and 200 km/s with an average speed of 138 km/s. The flow in the T (azimuthal) direction is fairly constant and averages about 48 km/s; the flow direction is consistent with flow away from the heliospheric nose. Flow in the N (meridional) direction is also away from the nose and averages −14 km/s. These flows suggest that the shock is blunter in the T than in the N direction, so that the heliosphere is wider than it is high. The flow in the RN plane has quasi-periodic oscillation with a period of 110 days and an amplitude of 21 km/s. The oscillation in flow angle is about 6° in the RN plane and 17° in the TN plane and may result from periodic variations of the termination shock normal direction.

An energetic-particle-mediated termination shock observed by Voyager 2

[1]xa0Voyager 2 crossed the solar wind termination shock several times during August 30 – September 1 of 2007. During the last forty days before the crossing intensities of energetic ions measured by the LECP instrument were increasing exponentially, and their spectrum showed clear evidence of unfolding at low energies. Plasma data featured a broad velocity precursor, where solar wind speed decreased from about 380 km/s far upstream to ∼300 km/s at the shock. By using plasma and energetic particle conservation laws we demonstrate that the speed decrease during Voyager 2s last ∼40 days in the solar wind could be plausibly produced by the back-pressure of energetic ions with energies of a few MeV on the upstream plasma flow. These particles propagate diffusively upstream from the termination shock, producing a shock precursor that has a characteristic lengthscale of 0.35 AU, assuming the shock has not moved substantially during that time.

Energetic ion composition in Saturn's magnetosphere revisited

[1] We present a new analysis of the composition of energetic charged particles in Saturns inner magnetosphere. This analysis is motivated by a discovery made after the Voyager 1 and 2 Saturn encounters of penetrating electrons in some total ion measurement channels of the Low Energy Charged Particle detectors. From this new analysis, we predict that there is no significant increase in the ratio of heavy energetic ions to protons inward of the orbit of the satellite Dione, as previously thought. This implies that energetic ion sputtering of the icy rings and satellites produces much less water vapor than has been assumed based on past composition estimates. For protons between about 500 keV and a few MeV, we observe a decrease in measured flux inward of about L = 6 and suggest the cause of this decrease is not well understood.

Growth and evolution of a plasmoid associated with a small, isolated substorm: IMP 8 and GEOTAIL measurements in the magnetotail

A tailward-moving plasmoid was observed at the GEOTAIL satellite at a GSM position (−73.3, 18.1, −1.1) RE on September 16, 1993, at 0417 UT, when the IMP 8 satellite was at (−37.5, −2.5, 1.7) RE at the midnight plasma sheet/lobe interface. The first indication of the plasmoid formation a few minutes after the negative bay onset of a small, localized auroral substorm was the onset of tailward beams of energetic ions and electrons at GEOTAIL well within the plasma sheet. Earthward-streaming energetic ions observed at IMP 8 a few minutes later suggest that the plasmoid evolved from within the plasma sheet to encompass the flux of nearly the entire thickness of the plasma sheet. The opposite direction of the anisotropies at IMP 8 and GEOTAIL suggest that the particle acceleration region was between X=−37.5 and −73 RE at that time. The isolated substorm associated with this plasmoid started equatorward of 67° latitude at a location which we map to near-Earth nightside plasma sheet (|X| < 15 RE) based on ground observations of a field line resonance. The active electrojet did not expand poleward until at least 10 min after the detection of the acceleration region tailward of IMP 8 and at least several minutes after the core of the plasmoid had moved tailward of GEOTAIL. These observations reinforce by means of in situ, concurrent, multipoint measurements the attitude expressed recently by several researchers that the locations of lobe reconnection and equatorial projection of electrojet intensification during substorm expansion are distinctly different from each other.

HELIOSHEATH MAGNETIC FIELD AND PLASMA OBSERVED BY VOYAGER 2 DURING 2012 IN THE RISING PHASE OF SOLAR CYCLE 24

We discuss magnetic field and plasma observations of the heliosheath made by Voyager 2 (V2) during 2012, when V2 was observing the effects of increasing solar activity following the solar minimum in 2009. The average magnetic field strength B was 0.14 nT and B reached 0.29 nT on day 249. V2 was in a unipolar region in which the magnetic polarity was directed away from the Sun along the Parker spiral 88% of the time, indicating that V2 was poleward of the heliospheric current sheet throughout most of 2012. The magnetic flux at V2 during 2012 was constant. A merged interaction region (MIR) was observed, and the flow speed increased as the MIR moved past V2. The MIR caused a decrease in the >70MeV nuc cosmic-ray intensity. The increments of B can be described by a q-Gaussian distribution with quf0a0=uf0a01.2uf0a0±uf0a00.1 for daily averages and quf0a0=uf0a01.82uf0a0±uf0a00.03 for hour averages. Eight isolated current sheets (“PBLs”) and four closely spaced pairs of current sheets were observed. The average change of B across the current sheets was a factor of ≈2, and B increased or decreased with equal probability. Magnetic holes and magnetic humps were also observed. The characteristic size of the PBLs was ≈6 RL, where RL is the Larmor radius of protons, and the characteristic sizes of the magnetic holes and humps were ≈38 RL and ≈11 RL, respectively.

Recent Particle Measurements from Voyagers 1 and 2

We summarize recent measurements made by the LECP instrument on Voyager 1 near its crossing of the heliopause and entry into the interstellar medium on or about day 238 of 2012 at 121.6 AU, and on Voyager 2 mainly during the period 2012-2014.4 characterized by large variations in the intensities and angular distributions of low-energy heliosheath ions and the reappearance of low-energy heliosheath electrons. Results from Voyager 1 not previously published include the energy dependence of ion intensity decreases prior to the heliopause crossing and a quantitative measure of the evolution of low-energy heliosheath proton pitch angle distributions that extend across the heliopause. For Voyager 2 we describe the evolution in time and with ion energy of the tangential streaming of ions directed from the nose toward the tail of the heliosheath, summarize the recovery of low-energy heliosheath ion intensities since their decline to minimum levels during 2013.0-3013.3, and discuss the effects of this intensity minimum and subsequent recovery on the ion partial pressure in the heliosheath and on its magnitude relative to that of the thermal plasma and the magnetic field.

A Residual Source of Energetic Neutral Atoms Across the Sky Obtained by the Neutral Particle Detector on board Venus Express

We have surveyed the sky for residual energetic neutral atom (ENA) signals in the energy range of 0.2–3.0 keV. Approximately three years of data obtained by the Neutral Particle Detector (NPD) on board Venus Express (VEX) from May 2006 through August 2009 have been analyzed. After applying strict viewing criteria to minimize all known signals and subtracting the UV background from the Milky Way, we find a residual energy spectral shape with a ledge/bump at around 0.5 keV and a break in the spectral slope at about 1.0 keV. When the ENA spectrum is divided by the energy dependent charge exchange cross section its slope above 1 keV has a spectral power‐law index of 1.5, which is consistent with that measured in‐situ in the inner heliosheath (HS) by the Low‐Energy Charged Particle (LECP) instrument (>40 keV) on board Voyager‐1 (V1). The ledge/bump at about 0.5 keV appears consistent with twice the plasma flow velocity obtained by the V1 measurements in the inner HS.