Heliosheath magnetic field and plasma observed by Voyager 2 during 2011

Abstract

We discuss magnetic field and plasma observations from Voyager 2 (V2) during 2011, when V2 was beginning to see the effects of increasing solar activity following the solar minimum in 2009. The magnetic field strength (B) profile showed large amplitude fluctuations that can be resolved into a linear increase of B with time and a sinusoidal variation of the period of 86.2 ± 0.8 days. Voyager 2 was in a unipolar region in which the magnetic polarity was directed away from the Sun along the Parker spiral 96% of the time, indicating that V2 was poleward of the heliospheric current sheet throughout most of 2011. The distribution of B was lognormal, but a Gaussian distribution was observed when the linear variation of B was subtracted from the data. The distribution of daily increments of B was a q-Gaussian distribution with q = 1.1 ± 0.1, which is less intermittent than normally observed in the heliosheath. However, the distribution of hourly increments of B was a q-Gaussian distribution with q = 1.5 ± 0.03. The density, temperature, and velocity increased linearly from the beginning of 2011 to approximately day 254. The magnetic and thermal pressure tended to increase throughout the year, but the magnetic pressure dominated most of the time. The counting rate of >70 MeV/nucleon particles increased rapidly during the first 250 days, but it leveled out during the rest of the year when B was stronger. The empirical CR-B relationship describes this behavior.

Transition from the Unipolar Region to the Sector Zone: Voyager 2, 2013 and 2014

“…The sunspot number began increasing significantly in 2010 and 2011. Burlaga et al (2014) found that the minimum latitudinal extent of the HCS from the solar equatorial plane occurred near solar minimum, at which time V2 was in a unipolar region sampling magnetic fields from the southern coronal hole during 2011, consistent with the propagation time of the solar wind from 1 to ≈90 au being approximately 1 year. The smooth sunspot number reached alocal maximum of 66.9 in 2012 February and a maximum in 2014 (making it the smallest sunspot cycle since cycle 14 in 1906) and it decreased during 2015 and 2016.…”

“…Each of the enhancements in CRI B in Figure 11(a) probably corresponds to an MIR (Burlaga et al 1993(Burlaga et al , 2014 produced by the interaction of two or more ejecta or corotating interaction regions. None of the MIRs had a strong and sustained (lasting two or more solar rotations) enhancement in B, indicating that V2 observed no GMIRs during 2013 and 2014, in contrast to the GMIR observed by V2 during 2012 .…”

Heliosheath Magnetic Field and Plasma Observed by Voyager 2 During 2012 in the Rising Phase of Solar Cycle 24

“…Solar activity during solar cycle 23 decreased to minimum value in August 2009, when the monthly mean sunspot number was zero. Burlaga et al (2014) found that the minimum latitudinal extent of the heliospheric current sheet (HCS) from the solar equatorial plane occurred near solar minimum, leaving V2 in a unipolar region sampling magnetic fields from the southern coronal hole during 2011, consistent with the propagation time of the solar wind from 1 to ≈90 au being approximately 1 year.…”

“…Solar activity and B at 1 au increased after mid-2009, so one expects to observe increasingly strong magnetic fields at V2 after a propagation time from the Sun of ≈1 year. Relatively strong magnetic fields were observed at V2 during 2011 (Burlaga et al 2014) and during 2012 (as shown in Figure 2(a)).…”

“…The galactic cosmic-ray modulation for cycle 24 began to increase in 2010 January. Burlaga et al (2014) found that B measured by V2 increased in an oscillatory pattern with a period of 86 days during 2011.…”