OBSERVATIONS OF THE HELIOSHEATH AND SOLAR WIND NEAR THE TERMINATION SHOCK BYVOYAGER 2

Abstract

This paper describes the principal features of 24 hr averages of the magnetic field strength variations B(t) and their relationships to the plasma and energetic particles observed prior to and after the crossing of the termination shock (TS) by Voyager 2 (V2). The solar wind (pre-TS crossing) and heliosheath (post-TS crossing) data extend from day of year (DOY) 1 through 241, 2007 and from 2007 DOY 245 through 2008 DOY 80, respectively. In the solar wind, two merged interaction regions (MIRs) were observed in which the ratio of plasma pressure to magnetic pressure in the solar wind was relatively low. Strong magnetic fields and low values of beta were also observed just prior to its crossing of the TS. The predicted correlation between peaks in the intensity of energetic particles in the solar wind when V2 crossed the heliospheric current sheet from positive to negative magnetic polarity in the solar wind was not observed. In the heliosheath, V2 observed a feature characterized by large enhancements of the density N and the proton temperature T, a small increase in speed V, and a depression in B. The distributions of 24 hr averages of B and beta were approximately log-normal in both the solar wind and the heliosheath. A unipolar region was observed for 73 days in the heliosheath, as the heliospheric current sheet moved toward the equatorial plane to latitudes lower than V2.

Observations of the magnetic field and plasma in the heliosheath by Voyager 2 from 2007.7 to 2009.4

“…It is surprising that V1 observed a large oscillation of B during this interval. Large fluctuations in B were also observed in unipolar regions by V1 from 2004V1 from .98 to 2005V1 from .3 [Burlaga et al, 2006 and by V2 from 2008.11 to 2008.56 [Burlaga et al, 2009a]. It will be interesting to determine whether V2 will observe large fluctuations in B when it becomes immersed in the unipolar magnetic fields from the southern polar coronal hole.…”

“…[19] Magnetic pressure P B = (B 2 /8p), solar wind thermal pressure P P = NkT, total pressure P BP = (P B + P P ), and b = P P /P B in the solar wind ahead of the termination shock (TS) and in heliosheath behind the TS (from DOY 245, 2007to DOY 75, 2008 are discussed by Burlaga et al [2009a]. In the heliosheath from DOY 1 − 75, 2008 they reported that P BP = 14.1 × 10 −14 dyn cm −2 , and b = 1.1.…”

Tracking the Solar Cycle Through Ibex Observations of Energetic Neutral Atom Flux Variations at the Heliospheric Poles

“…What is different is the modeled total pressure, which is higher at the poles than downwind, leading to an IHS that is about 30% thinner at the poles than their 220 au value. However, Galli et al (2016) base their poleward pressure on the flow speed observed by Voyager 2 in the heliosheath, 140 km s −1 , which originated as aslow SW (∼400 km s −1 ) prior to the TS (Burlaga et al 2009). We argue that this is not appropriate for the poles over most of the solar cycle where the heliosheath flow speed should be considerably higher.…”

“…Based on a survey of outer heliospheric models (e.g., Pogorelov et al 2007Pogorelov et al , 2013Izmodenov et al 2009;Opher et al 2009Opher et al , 2015, a proton outbound along the heliographic equator will travel roughly an additional 200 au through the IHS to the pole as compared to a proton traveling directly poleward. For a flow speed in the IHS of ∼150 km s −1 (based on Voyager 2 observations; Burlaga et al 2009), this corresponds to a travel time difference of ∼6.5 years, or more than half a solar cycle. Thus even if the individual streamlines retain memory of their preshock solar wind conditions, ENAs observed along a polar-directed LOS will reflect SW variability averaged over half a solar cycle.…”

Structure of the heliospheric current sheet from plasma convection in time-dependent heliospheric models

“…After crossing of the termination shock, the Voyagers continue observing the sector structure (Burlaga et al 2006(Burlaga et al , 2007(Burlaga et al , 2009, implying that the HCS extends into the inner heliosheath. With the distance from the termination shock increasing and the distance to the heliopause going down, the plasma flow deviates more and more from the radial flow (Richardson et al 2009).…”