DOY from 2004.0

Fig. 3.6.9. V1 Hourly average Heliospheric Magnetic Field (HMF) in heliographic coordinates observed by Voyager 1. Crossing of Terminal Shock (TS) occurs at day 351 of 2004. Sector boundaries, crossings of the Heliospheric Current Sheet (HCS), readily evident in 2004 but not yet seen in 2005. According to Ness et al. (2005).

In Fig. 3.6.9 the field vector is represented by the magnitude, B, and the direction by the heliographic longitude and latitude angles X and S. Readily evident in the longitude angle plot is that it displays a well defined bi-modal distribution, characteristically near 270° or 90°. These correspond to Parker's Archimedean spiral angles at this distance for fields with a sense pointing outward from or inward toward the Sun. Sudden changes in X correspond to crossings of the well known and long studied sector boundaries between Heliospheric magnetic field regions of uniform but opposite polarity in the solar wind: a manifestation of the Heliospheric current sheet. The large field values from day 352 onward are accompanied by large fluctuations of the magnitude while the longitude angle remains fixed near 270°. These field orientations and the sudden large increase in the average field magnitude indicate that the observed Terminal shock is, as expected, classified as a perpendicular shock.

There appear to be a few sector boundaries observed shortly after the Terminal shock crossing but for most of the time that Voyager 1 is in the Heliosheath, the polarity of the field remains the same. Since polarities of the fields across sector boundaries or the Heliospheric current sheet are expected to be transmitted through the Terminal shock without any field polarity reversal, this long duration of a fixed polarity is somewhat of a puzzle at the present time. One suggested explanation is that the Terminal shock may be in motion relative to Voyager 1.

An expanded time scale of field magnitude and fluctuations across the Terminal shock crossing are shown in Fig. 3.6.10. The horizontal bars on either side of the Terminal shock represent the average values during the periods indicated and show a field jump by a factor of ~3. The sudden large and sustained increase in this SD parameter, coincident with the field magnitude increase, suggests that the Heliosheath is a different astrophysical plasma regime than observed in earlier studies of the Heliospheric magnetic field. Throughout the many years prior to the Terminal shock crossing, the typical value of this SD parameter was 0.012-0.014 nT except during passage of a propagating Merged interaction regions (Burlaga et al., 2001).

Solar TS 0.20 Wind

Heliosheath e CO

e CO

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