Cross and magnetic helicity in the outer heliosphere from Voyager 2 observations

Plasma velocity and magnetic field measurements from the Voyager 2 mission are used to study solar wind turbulence in the slow solar wind at two different heliocentric distances, 5 and 29 astronomical units, sufficiently far apart to provide information on the radial evolution of this turbulence. The magnetic helicity and the cross-helicity, which express the correlation between the plasma velocity and the magnetic field, are used to characterize the turbulence. Wave number spectra are computed by means of the Taylor hypothesis applied to time resolved single point Voyager 2 measurements. The overall picture we get is complex and difficult to interpret. A substantial decrease of the cross-helicity at smaller scales (over 1-3 hours of observation) with increasing heliocentric distance is observed. At 5 AU the only peak in the probability density of the normalized residual energy is negative, near -0.5. At 29 AU the probability density becomes doubly peaked, with a negative peak at -0.5 and a smaller peak at a positive values of about 0.7. A decrease of the cross-helicity for increasing heliocentric distance is observed, together with a reduction of the unbalance toward the magnetic energy of the energy of the fluctuations. For the smaller scales, we found that at 29 AU the normalized polarization is small and positive on average (about 0.1), it is instead zero at 5 AU. For the larger scales, the polarization is low and positive at 5 AU (average around 0.1) while it is negative (around - 0.15) at 29 AU.Comment: 14 pages 5 figures. Accepted for publication on European Journal of Mechanics B/Fluids (5/8/2015

Turbulence in the solar wind: spectra from Voyager 2 data at 5 AU

Fluctuations in the flow velocity and magnetic fields are ubiquitous in the Solar System. These fluctuations are turbulent, in the sense that they are disordered and span a broad range of scales in both space and time. The study of solar wind turbulence is motivated by a number of factors all keys to the understanding of the Solar Wind origin and thermodynamics. The solar wind spectral properties are far from uniformity and evolve with the increasing distance from the sun. Most of the available spectra of solar wind turbulence were computed at 1 astronomical unit, while accurate spectra on wide frequency ranges at larger distances are still few. In this paper we consider solar wind spectra derived from the data recorded by the Voyager 2 mission during 1979 at about 5 AU from the sun. Voyager 2 data are an incomplete time series with a voids/signal ratio that typically increases as the spacecraft moves away from the sun (45% missing data in 1979), making the analysis challenging. In order to estimate the uncertainty of the spectral slopes, different methods are tested on synthetic turbulence signals with the same gap distribution as V2 data. Spectra of all variables show a power law scaling with exponents between -2.1 and -1.1, depending on frequency subranges. Probability density functions (PDFs) and correlations indicate that the flow has a significant intermittency.Comment: 14 pages, 7 figures. Discussion improved since the previous versio

Voyager 2 solar plasma and magnetic field spectral analysis for intermediate data sparsity

The Voyager probes are the furthest, still active, spacecraft ever launched from Earth. During their 38-year trip, they have collected data regarding solar wind properties (such as the plasma velocity and magnetic field intensity). Unfortunately, a complete time evolution of the measured physical quantities is not available. The time series contains many gaps which increase in frequency and duration at larger distances. The aim of this work is to perform a spectral and statistical analysis of the solar wind plasma velocity and magnetic field using Voyager 2 data measured in 1979, when the gaps/signal ratio is of order of unity. This analysis is achieved using four different data reconstruction techniques: averages on linearly interpolated subsets, correlation of linearly interpolated data, compressed sensing spectral estimation, and maximum likelihood data reconstruction. With five frequency decades, the spectra we obtained have the largest frequency range ever computed at 5 astronomical units from the Sun; spectral exponents have been determined for all the components of the velocity and magnetic field fluctuations. Void analysis is also useful in recovering other spectral properties such as integral scales (see for instance Table 4) and, if the confidence level of the measurements is sufficiently high, the decay variation in the small scale range due, for instance, to dissipative effects.Comment: 11 pages, 7 figure

Voyager observations of magnetic field turbulence in the far heliosheath and in the local interstellar medium. Power spectra from high-resolution data.

Voyager 2 (V2) is in the heliosheath (HS) since the termination shock crossing in Aug 2007, while V1 is in the local interstellar medium (LISM) since Aug 2012. The fundamental processes at the basis of the observed solar wind's disordered fluctuations are still unclear. Open points regard the nature of compressive turbulence within the sectored and unipolar HS in proximity of the heliopause and in the LISM. Possibility that MHD waves give origin to turbulence in the LISM has been recently suggested by Zank, Du & Hunana [APJ 842,2017]. However, addressing these issues is a challenging task because of the data sparsity. We provide the first collection of magnetic field power spectra computed in consecutive periods after 2009 from 48s resolution data in the HS (V1, V2) and in the LISM (V1). A description of the fluctuations evolution with the heliocentric distance is given in terms of spectral decay law and anisotropy. In the HS, our observations are consistent with an anisotropic mainly inertial cascade in the frequency range [ 10^-4 , 5 ⋅ 10^-5 ] Hz, with spectral slopes from -1.7 to -1.9. Larger scales may be featured by wavy fluctuations leading to a f ^-1 decay for f < 10^-5 Hz. LISM spectra show a f^-1 power law in the whole observed range [ 10^-7 , 10^-2 ] Hz

The structure of magnetic turbulence in the heliosheath region observed by Voyager 2 at 106 AU

It is currently believed that the turbulent fluctuations pervade the outermost heliosphere. Turbulence, magnetic reconnection, and their link may be responsible for magnetic energy conversion in these regions. The governing mechanisms of such anisotropic and compressible magnetic turbulence in the inner heliosheath (IHS) and in the local interstellar medium (LISM) still lack a thorough description. The present literature mainly concerns large scales which are not representative of the inertial-cascade dynamics of turbulence. Moreover, lack of broadband spectral analysis makes the IHS dynamics critically understudied. Our recent study [1] shows that 48 s magnetic-field data from the Voyager mission are appropriate for a spectral analysis over a frequency range of six decades, from 5 x 10-8 Hz to 10-2 Hz. Here, focusing on the Voyager 2 observation interval from 2013.824 to 2016.0, we describe the structure of turbulence in a sector zone of the IHS. A spectral break around 7 x 10-7 Hz (magnetic structures with size l ~ 1.3 Astronomical Units) separates the energy-injection regime from the inertial-cascade regime of turbulence. A second scale is observed around 6 x 10-5 Hz (l~ 0.017 AU) and corresponds to a peak of compressibility and intermittency of fluctuations

Intermittency and cascade rate of turbulent magnetic energy in the inner heliosheath and local interstellar medium from in-situ Voyager 1 and 2 measurements between 100 AU and 140 AU

In the inner and outer heliosheath (IHS and LISM), turbulence and magnetic reconnection are believed to be the major players in the processes responsible for particle transport and magnetic energy conversion into kinetic energy and heat. State-of-art numerical simulations demonstrated that a transition to turbulence can occur in the inner heliosheath and also showed that instability and magnetic reconnection can occur in proximity of the heliopause. However, it is still unfeasible to resolve numerically scales smaller than the sector spacing, which makes it necessary to analyze in-situ data provided by the Voyager Interstellar Mission. The present study builds on our recent works [Fraternale et al ApJ 2019, Fraternale et al JPCS 2019], where the spectral properties of the energy-injection range and of the inertial-cascade regime of magnetic field fluctuations have been shown for several periods within the IHS and LISM – for heliocentric distances up to 106 AU for Voyager 2 (2017.0) and 136 AU for Voyager 1 (2016.67). In this work, we include in the analysis the data intervals 2017.0 - 2017.66 at Voyager 2 (115.2 AU, IHS) and 2017.1 - 2018.0 at Voyager 1 (140.7 AU, LISM). We investigate the properties of scale-dependent intermittency, and provide the first analysis of magnetic-energy cascade rates. In particular, the magnetic energy flux is computed both from a power spectrum-based proxy and from the third-order moments of the field’s temporal increments. In the inertial range of fluctuations, the different estimators yield values between 100 and 1000 m2/s3 in the IHS close to the heliopause, and around 0.01-0.1 m^2/s^3 in the latest LISM interval. This research is framed within the project “Turbulence as Indicator of Physical Processes at the Heliospheric Interface”, NASA’s H-GI Open Program, 18-HGIO18_2-0029

Glutathione increase by the n-butanoyl glutathione derivative (GSH-C4) inhibits viral replication and induces a predominant Th1 immune profile in old mice infected with influenza virus

During aging, glutathione (GSH) content declines and the immune system undergoes a deficiency in the induction of Th1 response. Reduced secretion of Th1 cytokines, which is associated with GSH depletion, could weaken the host defenses against viral infections. We first evaluated the concentration of GSH and cysteine in organs of old mice; then, the effect of the administration of the N-butanoyl GSH derivative (GSH-C4) on the response of aged mice infected with influenza A PR8/H1N1 virus was studied through the determination of GSH concentration in organs, lung viral titer, IgA and IgG1/IgG2a production and Th1/Th2 cytokine profile. Old mice had lower GSH than young mice in organs. Also the gene expression of endoplasmic reticulum (ER) stress markers involved in GSH metabolism and folding of proteins, i.e. Nrf2 and PDI, was reduced. Following infection, GSH content remained low and neither infection nor GSH-C4 treatment affected Nrf2 expression. In contrast, PDI expression was upregulated during infection and appeared counterbalanced by GSH-C4. Moreover, the treatment with GSH-C4 increased GSH content in organs, reduced viral replication and induced a predominant Th1 response. In conclusion, GSH-C4 treatment could be used in the elderly to contrast influenza virus infection by inducing immune response, in particular the Th1 profile

Evolution of magnetic field turbulence as observed by the Voyagers in the heliosheath and in the local interstellar medium

Voyager 1 (V1) left the heliosheath (HS) and entered the Local Interstellar Medium (LISM) in August 2012. At the same time, Voyager 2 (V2) was inside the HS and it is currently approaching the heliopause. The nature of the mainly compressive and “turbulent” fluctuations observed in the HS and in the LISM is still unclear. The presented study aims at describing the spatial and temporal evolution of turbulence in the HS and in the LISM. It shows a collection of power spectra of magnetic field fluctuations computed from consecutive periods since 2009. Unlike previous analysis, the highest resolution data (48 s) available are used to observe up to five frequency decades. Proper spectral recovery techniques applied in a previous work [Gallana et al, JGR 2016] are exploited to overcome the problem of missing data. Inside the HS, the achieved results are consistent with an anisotropic, mainly inertial, energy cascade in the frequency range [10^ -5 ,5*10^-4 ] Hz, with spectral index ranging from -1.65 (V2) to -2 (V1) and energy spectral transfer around 10^-19 erg/(cm 3 s). Anisotropy is significantly higher at V1 than at V2. In 2009 and 2010, tangential magnetic field fluctuations at V1 contain half of the fluctuating magnetic energy, which is not observed at V2. Large scales prior to the spectral break (f<10^-5 Hz) are featured by a mild spectral decay with index between -0.95 and -1.5. Observations of small scales (5*10 -4 <f<10 -2 Hz) are limited by the onboard magnetometer’s accuracy, though some kinetic effects are still visible. LISM spectra in 2013.36 - 2014.65 are in agreement with previous observations [Burlaga, Florinski & Ness ApJ Lett, 2015]. A slightly flatter spectral trend than the Kolmogorov’s is observed for the radial fluctuations at [10 -7 , 10 -6 ] Hz. However, the tangential and normal components show nearly a f -1 decay. The evolution of turbulent spectra in the LISM is investigated

Features of turbulence in proximity of the Heliopause and in the Local Interstellar Medium from Voyager 1 observations

The Voyager 1 (V1) spacecraft is providing the first and unique in loco measurements of magnetic field (B) in the Local Interstellar Medium (LISM), since the crossing of the heliopause (HP), which occurred on August 25, 2012. The presented study addresses the question of the nature of turbulence at the edge of the helioshpere. We consider magnetic field fluctuations in the HP-to-LISM transition and in the LISM from day 180, 2011 to day 100, 2017. The LISM observed so far is characterized by high-frequency (2.5-3 kHz) plasma wave events related to shocks and to anisotropy of Galactic Cosmic Rays distribution. Here, possible coexistence of pristine interstellar turbulence and waves transmitted form the heliosheath (HS) across the HP is a still unclear and debated topic [Zank Hunana, ApJ 2017]. By considering several consecutive periods, we investigate the features of magnetic field fluctuations in terms of power spectra, spectral anisotropy and spectral compressibility level, probability density functions of B-increments. Differently from previous analysis, data with the highest available resolution (48 s) are used to observe up to five frequency decades (5e−8 − 1e-2 Hz). Since about 70% of data are missing, proper spectral recovery techniques are required for the analysis [Gallana et al, JGR 2016]. LISM fluctuations are mainly compressible thus directed the mean B field (up to 75%). This is also observed in the unipolar period between 2011 and 2012 (compressibility up to 85%), when V1 was likely measuring the field inside a magnetic barrier [Pogorelov et al, ApJ, 2017]. In the LISM, the fluctuating magnetic energy cascades as a power law with spectral index α in the range [-1.35, -1.65] in the whole range of frequencies unaffected by noise in the data (f < 1e−4 Hz). Inside the heliosheath instead, a spectral break at f ≈ 1e−5 Hz is found to separate an energy-injection range (α ∈[-0.95, -1.3]) from a turbulent inertial regime (α ∈[-1.65, -1.85])

The Inertial Range of Turbulence in the Inner Heliosheath and in the Local Interstellar Medium

The governing mechanisms of magnetic field annihilation in the outer heliosphere is an intriguing topic. It is currently believed that the turbulent fluctuations pervade the inner heliosheath (IHS) and the Local Interstellar Medium (LISM). Turbulence, magnetic reconnection, or their reciprocal link may be responsible for magnetic energy conversion in the IHS.   As 1-day averaged data are typically used, the present literature mainly concerns large-scale analysis and does not describe inertial-cascade dynamics of turbulence in the IHS. Moreover, lack of spectral analysis make IHS dynamics remain critically understudied. Our group showed that 48-s MAG data from the Voyager mission are appropriate for a power spectral analysis over a frequency range of five decades, from 5e-8 Hz to 1e-2 Hz [Gallana et al., JGR 121 (2016)]. Special spectral estimation techniques are used to deal with the large amount of missing data (70%). We provide the first clear evidence of an inertial-cascade range of turbulence (spectral index is between -2 and -1.5). A spectral break at about 1e-5 Hz is found to separate the inertial range from the enegy-injection range (1/f energy decay). Instrumental noise bounds our investigation to frequencies lower than 5e-4 Hz. By considering several consecutive periods after 2009 at both V1 and V2, we show that the extension and the spectral energy decay of these two regimes may be indicators of IHS regions governed by different physical processes. We describe fluctuations’ regimes in terms of spectral energy density, anisotropy, compressibility, and statistical analysis of intermittency.   In the LISM, it was theorized that pristine interstellar turbulence may coexist with waves from the IHS, however this is still a debated topic. We observe that the fluctuating magnetic energy cascades as a power law with spectral index in the range [-1.35, -1.65] in the whole range of frequencies unaffected by noise. No spectral break is observed, nor decaying turbulence