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A transition between the supersonic solar wind and the subsonic heliosheath was observed by Voyager 1, but the expected termination shock was not seen owing to a gap in the telemetry. Here we report observations of the magnetic field structure and dynamics of the termination shock, made by Voyager 2 on 31 August-1 September 2007 at a distance of 83.7 au from the Sun (1 au is the Earth-Sun distance). A single crossing of the shock was expected, with a boundary that was stable on a timescale of several days. But the data reveal a complex, rippled, quasi-perpendicular supercritical magnetohydrodynamic shock of moderate strength undergoing reformation on a scale of a few hours. The observed structure suggests the importance of ionized interstellar atoms ('pickup protons') at the shock.

During relatively quiet solar conditions throughout the spring and summer of 2007, the SECCHI HI2 white-light telescope on the STEREO B solar-orbiting spacecraft observed a succession of wave fronts sweeping past Earth. We have compared these heliospheric images with in situ plasma and magnetic field measurements obtained by nearEarth spacecraft, and we have found a near perfect association between the occurrence of these waves and the arrival of density enhancements at the leading edges of high-speed solar wind streams. Virtually all of the strong corotating interaction regions are accompanied by large-scale waves, and the low-density regions between them lack such waves. Because the Sun was dominated by long-lived coronal holes and recurrent solar wind streams during this interval, there is little doubt that we have been observing the compression regions that are formed at low latitude as solar rotation causes the high-speed wind from coronal holes to run into lower speed wind ahead of it. Subject headingg s: Sun: corona -Sun: coronal mass ejections (CMEs) -Sun: magnetic fields

Magnetic fields measured by Voyager 1 (V1) show that the spacecraft crossed the boundary of an unexpected region five times between days 210 and ~238 in 2012. The magnetic field strength B increased across this boundary from ≈0.2 to ≈0.4 nanotesla, and B remained near 0.4 nanotesla until at least day 270, 2012. The strong magnetic fields were associated with unusually low counting rates of >0.5 mega-electron volt per nuclear particle. The direction of B did not change significantly across any of the five boundary crossings; it was very uniform and very close to the spiral magnetic field direction, which was observed throughout the heliosheath. The observations indicate that V1 entered a region of the heliosheath (the heliosheath depletion region), rather than the interstellar medium.

We present in situ observations of magnetic turbulence in the draped interstellar magnetic field measured by Voyager 1 during an undisturbed interval from 2015.3987 to 2016.6759 confirming the existence of the turbulence observed previously from 2013.3593 to 2014.6373. The power spectral density of the turbulence was the same in both cases. The turbulence had a Kolmogorov k −5/3 spectrum in the range from k = 1.3 × 10−13 cm−1 to 4 × 10−12 cm−1. The ratio of the turbulent fluctuations to the average magnetic field strength was only 0.02, indicating that the turbulence was very weak. Extrapolating the power-law slope to lower frequencies yields an upper limit on the turbulence outer scale of 0.01 pc = 2000 au, which may be regarded as the distance at which Voyager 1 will enter the undisturbed local interstellar medium, beyond the outer heliosheath or bow wave in the upstream direction. The maximum variance of the fluctuations was in the two directions transverse to the average magnetic field in the recent interval, whereas it was parallel to the average magnetic field in the earlier interval, suggesting a transformation from turbulence with a dominant compressive component to turbulence dominated by transverse fluctuations. As the magnitude of the fluctuations was approaching that of the uncertainties of the measurements, the latter result requires confirmation by further observations.

Explorer 34 solar wind data for the period June to December, 1967 show that (a) The magnetic pressure, PB --= B'2/8zr, and thermal pressure, P~ =_ npkT, + n,kT~ + nekT~, are variable and positively correlated on a scale of ~> 2 days, but (b) changes in PB and Pe are anticorrelated on a scale 1 hr (~0.01 AU). Thus,~flynamical hydromagnetic processes (dv/dt ~ 0) must occur on the mesoscale, but the solar wind tends to be in equilibrium (PB +P~ N constant) on a smaller scale, the microscale. The 3-hr averages show that the most probable value of fl-~P~/PB is ,8'= 1.0 • which implies that the most probable state of the solar wind at 1 AU is not one of equipartition between the thermal energy and magnetic energy. The average total pressure for a given bulk speed (P (If) = Pe § PB) is essentially independent of V, implying that P is not determined by the heating or acceleration mechanisms of the solar wind; the average pressure is/5 = (2.9 _+ 1.5) • 10 -a0 dyne/cm 2.

We study the ionospheric convection that prevails during the passage of a magnetic cloud past the Earth. For the cloud studied here, January 13-15, 1988, the ionospheric convection was measured almost continually in cross sections of the polar cap and auroral zone by the Defense Meteorological Satellite Program (DMSP) polar-orbiting satellite. In turn, the conditions in the magnetic cloud are like those of a controlled laboratory experiment: First, the magnetic field changes smoothly and slowly on time scales much longer than the expected ionospheric response time, ensuring that the external.interplanetary conditions giving rise to any ionospheric flow pattern are known to unprecedented accuracy. Second, over the longer time scale of the magnetic cloud passage, the magnetic field vector rotates by over 180 ø such that the magnetic cloud divides into two intervals of northward (B• > 0) and southward (B• < 0) pointing interplanetary magnetic field (IMF) of 11 and 18 hours duration, respectively. During the former interval our observations show that (1) for strongly northward IMF (B z > 18 nT) the convection in one (the southern) hemisphere is characterized by a two-cell convection pattern confined to high latitudes Cd-75 ø) with sunward flow over the pole ("reverse" twocell convection). (2) The strength of the flows (~ 1 kin/s) is comparable to that seen under southward IMF later. O) Superimposed on this convection pattern there are clear dawn-dusk asymmetries associated with a one-cell convection component whose sense depends on the polarity of the magnetic cloud's large east-west magnetic field component. (4) Whilst the flows in the southern hemisphere are ordered into a well-defined convection pattern, the flows in the northern hemisphere are very irregular, varying on short spatial scales. When the doud's magnetic field turns southward the following observations were made: (1) The convection is characterized by a two-cell pattern extending to lower latitudes (~ 50 ø) with antisunward flow over the pole ("standard" two-cell convection). (2) There is no evident interhemisphere difference in the structure and strength of the convection. O) Superimposed dawn-dusk asymmetries in the flow pattern are observed which are only in part attributable to the east-west component of the magnetic field. (4) A dawn-dusk asymmetry in the latitude of the convection reversal boundary also exists, which is most pronounced in the northern hemisphere (up to 10 ø difference). We study the transition from a reverse to standard two-cell convection pattern and find that because of the large By component of the magnetic field inside the magnetic cloud, this transition actually takes place before the external field turns southward. It occurs when the magnetic shear angle between the subsolar magnetospheric field and the magnetic field of the magnetic cloud is ~ 70 ø. We consider the effect of the B e component of the cloud magnetic field on the size of the open field line region or polar cap, inferred from the convection pattern. We argue ...

This paper discusses results of the Explorer 34 plasma detector obtained during three magnetic storms, May 30, June 5, and June 25, 1967. Detailed comparisons with magnetic field observations on the same satellite are used to interpret the observations and to identify shocks and tangential discontinuities. A relation between the magnitude of such sudden changes in the earth's magnetic field and the solar wind parameters has been derived by Siscoe. Our observations are consistent with this relationship. The local shock speeds are derived from data obtained at the time of the sudden commencements. An example of an si+ si− pair is discussed, and in this case the negative impulse is shown to be due to a tangential discontinuity rather than a shock, whereas the positive impulse is associated with a shock. Helium observations are described that show that during the main phase of some geomagnetic storms the abundance ratio na/np shows a marked increase. In the May 30 storm it attained a value of 0.15 ± 0.02, 5½ hours after the sc, an increase of seven times over the value observed earlier.