ABSTRACT New observations of the magnetic field from 2014.7 through 2016.3288, together with the previous observations dating back to 2012 August 25, show that Voyager 1 continued to observe draped ...interstellar magnetic fields in the outer heliosheath. During this time, the direction of was nearly constant ( 3°), with no significant long-term trend. The slope of a linear least squares fit to the variation of the magnetic field strength B with time is (0.001 0.001) nT yr−1, consistent with no net change, and the average B = (0.48 0.04) nT. The new observations show a second "disturbed interval" in which B was for 267 days. This interval began with a weak shock on 2014/236 (2014.6438), contained oscillations in B with a 28 day period, and possibly ended with a pressure balanced structure or a reverse shock. It is likely this disturbed interval was associated with Sun/solar wind disturbances that impacted the heliopause and produced disturbances that propagated into the outer heliosheath. A quiet interval containing weaker less variable was observed from 2015.3700 until at least 2016.0. Unlike the previous quiet interval observed in the outer heliosheath, the direction of did not change linearly and could not be extrapolated to the center of the IBEX ribbon.
Turbulence in the Outer Heliosheath Burlaga, L. F.; Florinski, V.; Ness, N. F.
Astrophysical journal/The Astrophysical journal,
02/2018, Volume:
854, Issue:
1
Journal Article
Peer reviewed
Open access
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.
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.
The heliosphere is formed due to interaction between the solar wind (SW) and local interstellar medium (LISM). The shape and position of the heliospheric boundary, the heliopause, in space depend on ...the parameters of interacting plasma flows. The interplay between the asymmetrizing effect of the interstellar magnetic field and charge exchange between ions and neutral atoms plays an important role in the SW-LISM interaction. By performing three-dimensional, MHD plasma/kinetic neutral atom simulations, we determine the width of the outer heliosheath-the LISM plasma region affected by the presence of the heliosphere-and analyze quantitatively the distributions in front of the heliopause. It is shown that charge exchange modifies the LISM plasma to such extent that the contribution of a shock transition to the total variation of plasma parameters becomes small even if the LISM velocity exceeds the fast magnetosonic speed in the unperturbed medium. By performing adaptive mesh refinement simulations, we show that a distinct boundary layer of decreased plasma density and enhanced magnetic field should be observed on the interstellar side of the heliopause. We show that this behavior is in agreement with the plasma oscillations of increasing frequency observed by the plasma wave instrument onboard Voyager 1. We also demonstrate that Voyager observations in the inner heliosheath between the heliospheric termination shock and the heliopause are consistent with dissipation of the heliospheric magnetic field. The choice of LISM parameters in this analysis is based on the simulations that fit observations of energetic neutral atoms performed by Interstellar Boundary Explorer.
We discuss magnetic field and plasma observations of the heliosheath made by Voyager 2 (V2) during 2015, when V2 was observing the effects of near-maximum solar activity during solar cycle 24 ...following the solar minimum in 2009. The average magnetic field strength B was relatively high, 0.126 nT, as expected for solar maximum. A sector structure was observed with nearly equal maximum values of the distribution of directions of daily averages at approximately 90° and 180°, consistent with the Parker spiral magnetic field directions in the heliosheath. This structure indicates that the heliospheric current sheet (HCS) extended to high latitudes above V2 throughout most of 2015. The analysis of one sector boundary confirmed that the HCS was highly inclined (47°) with respect to the equatorial plane. The small-scale increments in B can be described by a -Gaussian distribution with q = 1.60 0.17 for daily averages and q = 1.66 0.03 for hourly averages. The magnetic flux BVRR increased at V2, indicating that there was no significant loss of magnetic energy during 2015. Two merged interaction regions and a global merged interaction region (GMIR) were observed. The flow speed increased as the GMIR moved past V2, indicating that the GMIR was still being compressed by the flow. The GMIR caused a major decrease in the >70 MeV/nuc cosmic-ray intensity. It is likely that the GMIR is causally related to a shock-like disturbance observed by Voyager 1 in the draped interstellar magnetic field beyond the heliopause.
Abstract The magnetic hump was observed from 2020.4 to ∼2022 by Burlaga et al. who offered the hypothesis that it originated at the Sun during the declining phase of the solar cycle. Voyager 1 ...observed intermittency of 1 hr increments of the magnetic field from 2013 through 2019, as well as in the magnetic hump that began at the jump pf2 at 2020.4 and ended at ∼2022. Throughout this interval, the intermittency in the components of B and the magnitude B was described by the Tsallis distribution ( q -Gaussian distribution). The q -Gaussian distributions have been observed throughout the solar wind and heliosheath. However, there was little or no intermittency in the 1 hr increments of the magnetic field observed by Voyager 1 from ∼2022.0 through day 270, 2023. During this interval Voyager 1 observed intermittency with a Gaussian distribution, which is associated with Boltzmann–Gibbs statistics. The boundary between these two regions, at ∼2022.0, coincides with the heliopause predicted by Fisk & Gloeckler. Alternatively, the Voyager 1 observations might be a solar cycle effect.
Abstract Voyager 1 has been moving through the very local interstellar medium (VLISM) for ∼1 solar cycle, from 122.58 au on 2012/DOY 238 (August 25) to 158.5 au on 2023.0. A magnetic hump was ...observed, beginning with an abrupt increase (jump) in the magnetic field strength B and proton density N by a factor of 1.35 and 1.36, respectively, in the course of ∼8 days, ending at ∼2020/DOY 147, after which it remained above the pre-jump levels for 2.6 yr, and it is still increasing. Unlike the three previous jumps, which were followed by a slow decrease in B , B in the magnetic hump continued to increase to a maximum value ∼0.56 nT at ∼2021/DOY 146.7. This paper discusses the intermittency of the magnetic field components and strength in the magnetic hump, and compares them with previous values in the VLISM. We consider the intermittency of the increments of B and its components observed on a scale of one day. The distribution function (the Tsallis distribution or q -distribution function) of the increments of hourly averages of the components of B had a Gaussian core associated with randomness in the observations, and it had two symmetric tails associated with the observed intermittency. The parameters q -dBR1, q -dBT1, and q -dBN1 increased slightly to a broad maximum with q = ∼1.4 between 2018 and 2019, and then decreased to q ∼ 1 (corresponding to no intermittency) in 2022. The Z -component of the magnetic field is within 4 ± 4 counts (0.02 nT ± 0.02 nT).
Abstract
Voyager 1 has been moving through the very local interstellar medium (VLISM) for approximately one solar cycle, from 122.58 au on 2012/DOY 238 (August 25) to 158.5 au on 2023.0. Previously, ...an abrupt increase (“jump”) in the magnetic field strength
B
and proton density
N
by a factor of 1.35 and 1.36, respectively, was observed during an interval of ∼8 days in 2020.40. After the jump,
B
continued to increase to a maximum value ∼0.56 nT at ∼2021.4 and then declined until
B
returned to the postjump value of 0.5 nT on 2021.85, 1.45 yr after the jump. The magnetic field strength declined briefly from 0.5 nT on 2021.85 to 0.47 nT on 2021.95 and then increased sporadically to 0.52 nT at 2023.0. Thus, the magnetic field strength remained strong for at least 2.6 yr. The magnetic hump and the density hump were a compression wave propagating through the VLISM. The compression wave was generated by a region with large dynamic pressure in the solar wind that propagated through the inner heliosheath and collided with the heliopause. The magnetic field strength continued to remain strong, with slow variations, until the end of our observations at 2023.0. It is suggested that the magnetic hump evolved from the large dynamic pressure, high speeds, and density observed at 1 au between ∼2015 and ∼2017.
Voyager 1 (V1) has been observing interstellar magnetic fields for more than one year beginning approximately 2012/209, when V1 crossed a current sheet, a "CS0" having the structure of a tangential ...discontinuity. The inclination of this current sheet is consistent with an interstellar magnetic field B draped on a blunt heliopause. Two other current sheets (sector boundaries) were observed at approximately 2012/167 and approximately 2011 /276 with high inclinations (99degrees + or - 10degrees and 89degrees + or - 10degrees, respectively). From 2013.0 to approximately 2013.6, the difference between the azimuthal angle lambda of B from the Parker spiral angle at the latitude 34degrees.6 of V1was lambda - lambdaP = 22degrees + or - 3degrees and the corresponding difference of the elevation angle delta was delta - delta sub(P) = 23degrees + or - 8degrees. 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 B varied smoothly in the range 0.38-0.59 nT with an average B = 0.486 + or - 0.045 after 2012/237.7. The transition from heliosheath to interstellar magnetic fields is related to a "two-step" increase in the cosmic ray intensity observed by V1 from approximately 2012.30 to approximately 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.
Launched over 35 years ago, Voyagers 1 and 2 are on an epic journey outward from the Sun to reach the boundary between the solar plasma and the much cooler interstellar medium. The boundary, called ...the heliopause, is expected to be marked by a large increase in plasma density, from about 0.002 per cubic centimeter (cm⁻³) in the outer heliosphere, to about 0.1 cm⁻³ in the interstellar medium. On 9 April 2013, the Voyager 1 plasma wave instrument began detecting locally generated electron plasma oscillations at a frequency of about 2.6 kilohertz. This oscillation frequency corresponds to an electron density of about 0.08 cm⁻³, very close to the value expected in the interstellar medium. These and other observations provide strong evidence that Voyager 1 has crossed the heliopause into the nearby interstellar plasma.
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