Voyager 1 Observations of the Interstellar Magnetic Field and the Transition from the Heliosheath

Abstract

Voyager 1 (V1) has been observing interstellar magnetic fields (ISMF) for more than one year, from 2012/209 to at least 2013.6. From 2013.0 to 2013.6 the difference between the azimuthal angle of the ISMF and the Parker spiral angle at the latitude 34.6° of V1 was (22 ± 3)° and the corresponding difference of the elevation angle was (0 ± 8)°. During 2012 the deviation from the Parker spiral angle was somewhat smaller. The interstellar magnetic field has a West to East polarity, opposite to the direction of planetary motions. The magnitude of the ISMF varied smoothly in the range 0.38 nT to 0.59 nT with an average strength 0.49nT. The strongest interstellar fields were observed behind a shock at 2012/297 that was preceded by 2.2 KHz plasma oscillations, which implies an interstellar electron density ne = 0.05/cm−3. The ISMF was observed after V1 crossed a current sheet CS0 having the structure of a tangential discontinuity. The inclination of this current sheet is consistent with an interstellar magnetic field draped on a blunt heliopause. Two other current sheets (sector boundaries) were observed earlier in the heliosheath at 2012/167 and 2011/276 with high inclinations (99 +/−10)° and (89 ± 10)°, respectively). The transition from heliosheath to interstellar magnetic fields is related to a two-step increase in the cosmic ray intensity observed by V1 from 2012.30 to 2012.65. The first step-increase began near the end of an unusual away-polarity sector, and it reached a plateau when V1 moved into a toward-polarity sector that ended at CS0. The second step-increase began slowly after V1 crossed CS0, and it ended abruptly at 2012/237.728.