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Merav Opher - One of the best experts on this subject based on the ideXlab platform.
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Magnetic field spectral analysis in the Heliosheath from Voyagers data
2016Co-Authors: Federico Fraternale, Merav Opher, John D. Richardson, Luca Gallana, Michele Iovieno, Daniela TordellaAbstract:In 2004 and 2007 the two Voyagers crossed the termination shock entering the Heliosheath, where the solar wind is slowed down and interacts with the interstellar medium. Many observations are not fully understood, one of these is the difference in the profiles of energetic ions and electrons fluxes recorded by V1 and V2. The high fluctuations of particles fluxes at V2 led researchers suppose the existence of two regions with distinct magnetic field features inside the HS: the sectored Heliosheath, at low latitudes, and the unipolar Heliosheath. These regions should have different transport properties, and the presence of turbulence may play a key role. In this work we analyze the spectral behaviour of magnetic field from V1 and V2 data in both the SHS and the UHS after 2009. The power spectra at V1 show a higher anisotropy level and slope than V2 in the intermediate range. A spectral bumb at f = 5e4 Hz is observed before 2010.5 and awaits a consistent physical interpretation
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Turbulence in the Heliosheath: spectral analysis from Voyager 1 and 2 data
Bulletin of the American Physical Society, 2015Co-Authors: Federico Fraternale, Merav Opher, John D. Richardson, Luca Gallana, Michele Iovieno, Sophie M. Fosson, Enrico Magli, Rachel Morgan, Daniela TordellaAbstract:The Voyager 2 spacecraft is traveling through the Heliosheath, the outermost layer in heliosphere where the solar wind is slowed by the interstellar gas, while Voyager 1 has entered the local interstellar medium. The they are providing the fist in-situ measurement of plasma and magnetic fields in that regions. We focus on the differences between the energetic particle intensity variations seen by the Voyager 1 and 2 crafts that are crossing the sectored and the unipolar as well as the sectored Heliosheath regions, respectively. We try to provide a spectral analysis of the full Heliosheath, characterizing the plasma and magnetic field turbulence through the estimate of the spectral properties in the different frequency ranges. Signal reconstruction techniques are mandatory to reconstruct spectra due to extreme data sparsity (up to 97\%\ missings in high resolution data beyond 80 AU). We use three different methods: correlation computation coupled with the maximum likelihood reconstruction, compress sensing and a genetic algorithm to estimate the gap influence on reconstructed spectra. These methods have been previously validated on 1979 data and synthetic hydrodynamics fluid turbulent fields.
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DEPENDENCE OF ENERGETIC ION AND ELECTRON INTENSITIES ON PROXIMITY TO THE MAGNETICALLY SECTORED Heliosheath: VOYAGER 1 AND 2 OBSERVATIONS
The Astrophysical Journal, 2014Co-Authors: Matthew E. Hill, S M Krimigis, Robert B. Decker, L. E. Brown, James Drake, D. C. Hamilton, Merav OpherAbstract:Taken together, the Voyager 1 and 2 (V1 and V2) spacecraft have collected over 11 yr of data in the Heliosheath. Despite extensive study, energetic particles and magnetic fields measured in the Heliosheath have not been reconciled by existing models. In particular, the differences between the energetic particle intensity variations at V1 and V2 are unexplained. While energetic particle intensities at V1 change gradually over 7 yr in the Heliosheath, those at V2 vary by a factor ~10 in 1 yr. Energetic particle intensities at V2 show temporally coherent variations over a broad range of species and energies: from suprathermal ions (10s of keV) to galactic cosmic rays (>1 GeV), as well as electrons from 10s of keV to >100 MeV, corresponding to a range ~104 in particle gyroradii. Here we suggest that many of the intensity variations of energetic particle populations in the Heliosheath are organized by their proximity to two fundamentally different regions—the unipolar Heliosheath (UHS) and the sectored Heliosheath (SHS). The SHS is a region of enhanced particle intensities, wherein particle transport, acceleration, and magnetic connectivity differ from those in the UHS. The SHS may serve as either a reservoir of energetic particles or as a region of enhanced transport, depending on the particle species and energy. Comparatively, particle intensities in the UHS are greatly reduced. We propose that the boundary between the SHS and UHS plays as important a role in the physics of Heliosheath particles and fields as do the termination shock and heliopause.
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probing the nature of the Heliosheath with the neutral atom spectra measured by ibex in the voyager 1 direction
The Astrophysical Journal, 2013Co-Authors: Merav Opher, C L Prested, D J Mccomas, N A Schwadron, J F DrakeAbstract:We are able to show by comparing modeled energetic neutral atoms (ENAs) spectra to those measured by Interstellar Boundary Explorer (IBEX) that the models along the Voyager 1 (V1) trajectory that best agree with the low energy IBEX data include extra heating due to ram and magnetic energy in the quasi-stagnation region or a kappa ion distribution (with κ = 2.0) in the outer Heliosheath. The model explored is the multi-ion, multi-fluid (MI-MF) which treats the pick-up ions and the thermal ion fluids with separate Maxwellian distributions. These effects are included ad hoc in the modeled ENA since they are not present in the model. These results indicate that the low energy spectra of ENAs as measured by IBEX is sensitive to the physical nature of the Heliosheath and to effects not traditionally present in current global models. Therefore, by comparing the low energy ENA spectra to models, we can potentially probe the Heliosheath in locations beyond those probed by V1 and Voyager 2 (V2).
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Propagation into the Heliosheath of a large-scale solar wind disturbance bounded by a pair of shocks
Astronomy & Astrophysics, 2013Co-Authors: E. Provornikova, Vladislav V. Izmodenov, Merav Opher, Gabor TothAbstract:Context. After the termination shock (TS) crossing, the Voyager 2 spacecraft has been observing strong variations of the magnetic field and solar wind parameters in the Heliosheath. Anomalous cosmic rays, electrons, and galactic cosmic rays present strong intensity fluctuations. Several works suggested that the fluctuations might be attributed to spatial variations within the Heliosheath. Additionally, the variability of the solar wind in this region is caused by different temporal events that occur near the Sun and propagate to the outer heliosphere. Aims. To understand the spatial and temporal effects in the Heliosheath, it is important to study these effects separately. In this work we explore the role of shocks as one type of temporal effects in the dynamics of the Heliosheath. Although currently plasma in the Heliosheath is dominated by solar minima conditions, with increasing solar cycle shocks associated with transients will play an important role. Methods. We used a 3D MHD multi-fluid model of the interaction between the solar wind and the local interstellar medium to study the propagation of a pair of forward-reverse shocks in the supersonic solar wind, interaction with the TS, and propagation to the Heliosheath. Results. We found that in the supersonic solar wind the interaction region between the shocks expands, the shocks weaken and decelerate. The fluctuation amplitudes of the plasma parameters vary with heliocentric distance. The interaction of the pair of shocks with the TS creates a variety of new waves and discontinuities in the Heliosheath, which produce a highly variable solar wind flow. The collision of the forward shock with the heliopause causes a reflection of fast magnetosonic waves inside the Heliosheath.
L. F. Burlaga - One of the best experts on this subject based on the ideXlab platform.
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Magnetic field observations as Voyager 1 entered the Heliosheath depletion region
Science (New York N.Y.), 2013Co-Authors: L. F. Burlaga, N. F. Ness, E C StoneAbstract: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.
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Heliosheath magnetic fields between 104 and 113 au in a region of declining speeds and a stagnation region
The Astrophysical Journal, 2012Co-Authors: L. F. Burlaga, N. F. NessAbstract:We examine the relationships between the magnetic field and the radial velocity component VR observed in the Heliosheath by instruments on Voyager 1 (V1). No increase in the magnetic field strength B was observed in a region where VR decreased linearly from 70 km s–1 to 0 km s–1 as plasma moved outward past V1. An unusually broad transition from positive to negative polarity was observed during a ≈26 day interval when the heliospheric current sheet (HCS) moved below the latitude of V1 and the speed of V1 was comparable to the radial speed of the Heliosheath flow. When V1 moved through a region where VR ≈ 0 (the "stagnation region"), B increased linearly with time by a factor of two, and the average of B was 0.14 nT. Nothing comparable to this was observed previously. The magnetic polarity was negative throughout the stagnation region for ≈580 days until 2011 DOY 235, indicating that the HCS was below the latitude of V1. The average passage times of the magnetic holes and proton boundary layers were the same during 2009 and 2011, because the plasma moved past V1 during 2009 at the same speed that V1 moved through the stagnation region during 2011. The microscale fluctuations of B in the stagnation region during 2011 are qualitatively the same as those observed in the Heliosheath during 2009. These results suggest that the stagnation region is a part of the Heliosheath, rather than a "transition region" associated with the heliopause.
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The Solar Wind in the Outer Heliosphere and Heliosheath
Space Science Reviews, 2011Co-Authors: J. D. Richardson, L. F. BurlagaAbstract:The solar wind environment has a large influence on the transport of cosmic rays. This chapter discusses the observations of the solar wind plasma and magnetic field in the outer heliosphere and the Heliosheath. In the supersonic solar wind, interaction regions with large magnetic fields form barriers to cosmic ray transport. This effect, the “CR-B” relationship, has been quantified and is shown to be valid everywhere inside the termination shock (TS). In the Heliosheath, this relationship breaks down, perhaps because of a change in the nature of the turbulence. Turbulence is compressive in the Heliosheath, whereas it was noncompressive in the solar wind. The plasma pressure in the outer heliosphere is dominated by the pickup ions which gain most of the flow energy at the TS. The Heliosheath plasma and magnetic field are highly variable on scales as small as ten minutes. The plasma flow turns away from the nose roughly as predicted, but the radial speeds at Voyager 1 are much less than those at Voyager 2, which is not understood. Despite predictions to the contrary, magnetic reconnection is not an important process in the inner Heliosheath with only one observed occurrence to date.United States. National Aeronautics and Space Administration (NASA contract 959203
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Sectors and Large-Scale Magnetic Field Strength Fluctuations in the Heliosheath Near 110 AU: Voyager 1,2009
The Astrophysical Journal, 2010Co-Authors: L. F. Burlaga, N. F. NessAbstract:This paper describes observations of daily averages of the magnetic field strength B and the magnetic polarity measured by Voyager 1 (V1) in the Heliosheath during 2009 between 108.5 and 112.1 AU and at heliographic latitude 344. A negative magnetic polarity sector was observed during 2009 DOY 43-255. A positive polarity sector was observed during 2009 DOY 256-365. We offer the hypothesis that the existence of the two sectors is the result of the displacement of the wavy heliospheric current sheet to the position of V1 as a result of northward flow in the Heliosheath. The large size of the sectors is caused by the slow radial motion of the flow observed by V1 in the Heliosheath. The distribution of B during 2009 was lognormal, in contrast to the Gaussian distributions observed by V1 in the Heliosheath prior to 2009. The large-scale fluctuations of B, described by the distribution of increments of daily averages of B, have a Tsallis distribution with q = 1.6. The large-scale fluctuations of B observed by V1 during 2009 have a multifractal spectrum with the same parameters that V1 observed during 2005 close to the termination shock at 94 AU. These results suggest that the large-scale magnetic fluctuations of B are in a metastable equilibrium state in the Heliosheath between 94 AU and 112.1 AU.
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observations of the Heliosheath and solar wind near the termination shock by voyager 2
The Astrophysical Journal, 2009Co-Authors: L. F. Burlaga, N. F. Ness, M H Acuna, E C Stone, J. D. Richardson, F B McdonaldAbstract:This paper describes the principal features of 24 hr averages of the magnetic field strength variations B(t) and their relationships to the plasma and energetic particles observed prior to and after the crossing of the termination shock (TS) by Voyager 2 (V2). The solar wind (pre-TS crossing) and Heliosheath (post-TS crossing) data extend from day of year (DOY) 1 through 241, 2007 and from 2007 DOY 245 through 2008 DOY 80, respectively. In the solar wind, two merged interaction regions (MIRs) were observed in which the ratio of plasma pressure to magnetic pressure in the solar wind was relatively low. Strong magnetic fields and low values of beta were also observed just prior to its crossing of the TS. The predicted correlation between peaks in the intensity of energetic particles in the solar wind when V2 crossed the heliospheric current sheet from positive to negative magnetic polarity in the solar wind was not observed. In the Heliosheath, V2 observed a feature characterized by large enhancements of the density N and the proton temperature T, a small increase in speed V, and a depression in B. The distributions of 24 hr averages of B and beta were approximately log-normal in both the solar wind and the Heliosheath. A unipolar region was observed for 73 days in the Heliosheath, as the heliospheric current sheet moved toward the equatorial plane to latitudes lower than V2.
John D. Richardson - One of the best experts on this subject based on the ideXlab platform.
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Heliosheath Properties Measured from a Voyager 2 to Voyager 1 Transient
The Astrophysical Journal, 2019Co-Authors: J. S. Rankin, John D. Richardson, David J. Mccomas, N A SchwadronAbstract:In mid-2012, a GMIR observed by Voyager 2 crossed through the Heliosheath and collided with the heliopause, generating a pressure pulse that propagated into the very local interstellar medium. The effects of the transmitted wave were seen by Voyager 1 just 93 days after its own heliopause crossing. The passage of the transient was accompanied by long-lasting decreases in galactic cosmic ray intensities that occurred from ~2012.55 to ~2013.35 and ~2012.91 to ~2013.70 at Voyager 2 and Voyager 1, respectively. Omnidirectional (>20 MeV) proton-dominated measurements from each spacecraft's Cosmic Ray Subsystem reveal a remarkable similarity between these causally-related events, with a correlation coefficient of 91.2% and a time-lag of 130 days. Knowing the locations of the two spacecraft, we use the observed time-delay to calculate the GMIR's average speed through the Heliosheath (inside the heliopause) as a function of temperature in the very local interstellar medium. This, combined with particle, field, and plasma observations enables us to infer previously unmeasured properties of the Heliosheath, including a range of sound speeds and total effective pressures. For a nominal temperature of ~20,000 K just outside the heliopause, we find a sound speed of 314 (+/-) 32 km/s and total effective pressure of 267 (+/-) 55 fPa inside the heliopause. We compare these results with the Interstellar Boundary Explorer's data-driven models of Heliosheath pressures derived from energetic neutral atom fluxes (the globally distributed flux) and present them as additional evidence that the Heliosheath's dynamics are driven by suprathermal energetic processes.
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Uncertainties in the Heliosheath ion temperatures
Annales Geophysicae, 2018Co-Authors: Klaus Scherer, John D. Richardson, Horst Fichtner, Hans J. Fahr, Adama Sylla, Marian LazarAbstract:Abstract. The Voyager plasma observations show that the physics of the Heliosheath is rather complex and that the temperature derived from observation particularly differs from expectations. To explain this fact, the temperature in the Heliosheath should be based on κ distributions instead of Maxwellians because the former allows for much higher temperature. Here we show an easy way to calculate the κ temperatures when those estimated from the data are given as Maxwellian temperatures. We use the moments of the Maxwellian and κ distributions to estimate the κ temperature. Moreover, we show that the pressure (temperature) given by a truncated κ distribution is similar to that given by a Maxwellian and only starts to increase for higher truncation velocities. We deduce a simple formula to convert the Maxwellian to κ pressure or temperature. We apply this result to the Voyager 2 observations in the Heliosheath. Keywords. Space plasma physics (kinetic and MHD theory)
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Magnetic field spectral analysis in the Heliosheath from Voyagers data
2016Co-Authors: Federico Fraternale, Merav Opher, John D. Richardson, Luca Gallana, Michele Iovieno, Daniela TordellaAbstract:In 2004 and 2007 the two Voyagers crossed the termination shock entering the Heliosheath, where the solar wind is slowed down and interacts with the interstellar medium. Many observations are not fully understood, one of these is the difference in the profiles of energetic ions and electrons fluxes recorded by V1 and V2. The high fluctuations of particles fluxes at V2 led researchers suppose the existence of two regions with distinct magnetic field features inside the HS: the sectored Heliosheath, at low latitudes, and the unipolar Heliosheath. These regions should have different transport properties, and the presence of turbulence may play a key role. In this work we analyze the spectral behaviour of magnetic field from V1 and V2 data in both the SHS and the UHS after 2009. The power spectra at V1 show a higher anisotropy level and slope than V2 in the intermediate range. A spectral bumb at f = 5e4 Hz is observed before 2010.5 and awaits a consistent physical interpretation
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Turbulence in the Heliosheath: spectral analysis from Voyager 1 and 2 data
Bulletin of the American Physical Society, 2015Co-Authors: Federico Fraternale, Merav Opher, John D. Richardson, Luca Gallana, Michele Iovieno, Sophie M. Fosson, Enrico Magli, Rachel Morgan, Daniela TordellaAbstract:The Voyager 2 spacecraft is traveling through the Heliosheath, the outermost layer in heliosphere where the solar wind is slowed by the interstellar gas, while Voyager 1 has entered the local interstellar medium. The they are providing the fist in-situ measurement of plasma and magnetic fields in that regions. We focus on the differences between the energetic particle intensity variations seen by the Voyager 1 and 2 crafts that are crossing the sectored and the unipolar as well as the sectored Heliosheath regions, respectively. We try to provide a spectral analysis of the full Heliosheath, characterizing the plasma and magnetic field turbulence through the estimate of the spectral properties in the different frequency ranges. Signal reconstruction techniques are mandatory to reconstruct spectra due to extreme data sparsity (up to 97\%\ missings in high resolution data beyond 80 AU). We use three different methods: correlation computation coupled with the maximum likelihood reconstruction, compress sensing and a genetic algorithm to estimate the gap influence on reconstructed spectra. These methods have been previously validated on 1979 data and synthetic hydrodynamics fluid turbulent fields.
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Plasma and Variability in the Heliosheath
Journal of Physics: Conference Series, 2015Co-Authors: John D. RichardsonAbstract:Voyager 2 (V2) is making the first direct plasma measurements of the Heliosheath and is moving outward about 3 AU/yr. This paper has two foci. It provides an update on plasma conditions in the Heliosheath out to 108 AU and discusses the variability of plasma parameters in the Heliosheath. The plasma speed continues to be on average constant across the Heliosheath and the flow continues to turn tailward, with a flow angle greater than 70° in 2014. The average density and temperature have remained roughly constant since an increase in 2011. The Heliosheath continues to be highly variable; this paper shows examples of small- scale variability and shows the changes of the plasma parameters on various scales. Future observations of plasma in the local interstellar medium (LISM) are discussed.
M. S. Potgieter - One of the best experts on this subject based on the ideXlab platform.
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Modelling of low-energy galactic electrons in the Heliosheath
Advances in Space Research, 2011Co-Authors: Godfrey Sibusiso Nkosi, M. S. Potgieter, William R. WebberAbstract:The modulation of cosmic ray electrons in the heliosphere plays an important role in improving our understanding and assessment of the processes applicable to low-energy galactic electrons. A full three-dimensional numerical model based on Parker’s transport equation is used to study the modulation of 10 MeV galactic electrons, in particular inside the Heliosheath. The emphasis is placed on the role that perpendicular diffusion plays in causing the extraordinary large increase in the observed intensities of these electrons in the Heliosheath. The modelling is compared with observations of 6–14 MeV electrons from the Voyager 1 mission. Results are shown for the radial intensity profiles of these electrons, as well as the modulation effects of varying the extent of the Heliosheath by changing the location of the termination shock and the heliopause and the value of the local interstellar spectrum. We confirm that the Heliosheath acts as a modulation ‘barrier’ for low-energy galactic electrons. The significance of this result depends on how wide the inner Heliosheath is; on how high the very local interstellar spectrum is at these low energies (E
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Stochastic acceleration and adiabatic heating of anomalous cosmic rays in the inner Heliosheath
AIP Conference Proceedings, 2008Co-Authors: Stefan Ferreira, M. S. Potgieter, Klaus SchererAbstract:A brief discussion on the transport and acceleration of anomalous cosmic rays in the inner Heliosheath is given with emphasis on stochastic acceleration and adiabatic heating of particles. Results from a numerical model is shown and compared to observations. We show that these acceleration processes play a major role in explaining Voyager 1 anomalous cosmic ray observations at the termination shock and in the inner Heliosheath.
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Transport and acceleration of anomalous cosmic rays in the inner Heliosheath
Journal of Geophysical Research: Space Physics, 2007Co-Authors: Stefan Ferreira, M. S. Potgieter, Klaus SchererAbstract:[1] The transport and acceleration of anomalous cosmic rays in the inner Heliosheath is studied. A unique two-dimensional hydrodynamic numerical model is used to calculate the interaction of the solar wind and the local interstellar medium, neutral hydrogen, and pickup ions. The divergence of the flow, heliospheric magnetic field, and Alfven speed are calculated and then inserted into model which calculates cosmic ray transport and acceleration in this heliosphere by solving the Parker particle transport equation. We show that adiabatic heating and stochastic acceleration plays a major role in explaining Voyager 1 observations both at the termination shock and in the inner Heliosheath. While the inclusion of adiabatic heating in a numerical modulation model results in the correct spectral shape of accelerated anomalous particles for energies ≤∼10 MeV at the termination shock, stochastic acceleration effectively accelerates these particles further to anomalous energies out in the inner Heliosheath. These accelerated particles are then modulated back inward, resulting in realistic radial gradients as well as an upturn in the anomalous cosmic ray spectra. A parameter study is also done to illustrate the sensitivity of these acceleration processes to different diffusion coefficients.
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Effects of the solar wind termination shock and Heliosheath on theheliospheric modulation of galactic and anomalous Helium
Annales Geophysicae, 2004Co-Authors: U. W. Langner, M. S. PotgieterAbstract:The interest in the role of the solar wind termination shock and Heliosheath in cosmic ray modulation studies has increased significantly as the Voyager 1 and 2 spacecraft approach the estimated position of the solar wind termination shock. The effect of the solar wind termination shock on charge-sign dependent modulation, as is experienced by galactic cosmic ray Helium (He++) and anomalous Helium (He+), is the main topic of this work, and is complementary to the previous work on protons, anti-protons, electrons, and positrons. The modulation of galactic and anomalous Helium is studied with a numerical model including a more fundamental and comprehensive set of diffusion coefficients, a solar wind termination shock with diffusive shock acceleration, a Heliosheath and particle drifts. The model allows a comparison of modulation with and without a solar wind termination shock and is applicable to a number of cosmic ray species during both magnetic polarity cycles of the Sun. The modulation of Helium, including an anomalous component, is also done to establish charge-sign dependence at low energies. We found that the Heliosheath is important for cosmic ray modulation and that its effect on modulation is very similar for protons and Helium. The local Helium interstellar spectrum may not be known at energies
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Heliospheric Modulation of Cosmic-Ray Positrons and Electrons: Effects of the Heliosheath and the Solar Wind Termination Shock
The Astrophysical Journal, 2004Co-Authors: M. S. Potgieter, U. W. LangnerAbstract:The importance of the solar wind termination shock (TS) and the modulation that may occur in the Heliosheath have become most relevant, as emphasized by cosmic-ray observations of the Voyager spacecraft in the distant heliosphere close to the TS. The role of the TS and that of the Heliosheath on cosmic-ray electrons and positrons are studied with a numerical model, including global drifts and a Jovian electron source. The consequent charge-sign dependence is computed using improved local interstellar spectra and new fundamentally derived diffusion coefficients. The importance of modulation in the Heliosheath is studied for the two species, as well as the differences between a model with and one without a TS. We found that the modulation in the Heliosheath depends on the particle species, is strongly dependent on the energy of the cosmic rays and the magnetic polarity cycle, and is enhanced by the inclusion of the TS. From the computations it is possible to estimate the ratio of modulation occurring in the Heliosheath to the total modulation between the heliopause and Earth. For electrons the factor modulation in the Heliosheath becomes comparable to the factor between the TS and Earth at low energies for both polarity epochs. During A > 0 cycles the Heliosheath cannot really be considered a modulation "barrier" above ~150 MeV for electrons, while for positrons this occurs at somewhat higher energies during the A 0 polarity cycles but for positron modulation during A < 0 cycles. For positrons, with a completely differently shaped local interstellar spectrum, the modulated spectra have a very mild energy dependence below ~300 MeV, even at Earth, in contrast to electrons, protons, and antiprotons. These characteristic spectral features may be helpful for distinguishing between electron and positron spectra when measured near and at Earth. These simulations can be of use for future missions to the outer heliosphere and beyond.
Gabor Toth - One of the best experts on this subject based on the ideXlab platform.
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Propagation into the Heliosheath of a large-scale solar wind disturbance bounded by a pair of shocks
Astronomy & Astrophysics, 2013Co-Authors: E. Provornikova, Vladislav V. Izmodenov, Merav Opher, Gabor TothAbstract:Context. After the termination shock (TS) crossing, the Voyager 2 spacecraft has been observing strong variations of the magnetic field and solar wind parameters in the Heliosheath. Anomalous cosmic rays, electrons, and galactic cosmic rays present strong intensity fluctuations. Several works suggested that the fluctuations might be attributed to spatial variations within the Heliosheath. Additionally, the variability of the solar wind in this region is caused by different temporal events that occur near the Sun and propagate to the outer heliosphere. Aims. To understand the spatial and temporal effects in the Heliosheath, it is important to study these effects separately. In this work we explore the role of shocks as one type of temporal effects in the dynamics of the Heliosheath. Although currently plasma in the Heliosheath is dominated by solar minima conditions, with increasing solar cycle shocks associated with transients will play an important role. Methods. We used a 3D MHD multi-fluid model of the interaction between the solar wind and the local interstellar medium to study the propagation of a pair of forward-reverse shocks in the supersonic solar wind, interaction with the TS, and propagation to the Heliosheath. Results. We found that in the supersonic solar wind the interaction region between the shocks expands, the shocks weaken and decelerate. The fluctuation amplitudes of the plasma parameters vary with heliocentric distance. The interaction of the pair of shocks with the TS creates a variety of new waves and discontinuities in the Heliosheath, which produce a highly variable solar wind flow. The collision of the forward shock with the heliopause causes a reflection of fast magnetosonic waves inside the Heliosheath.
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DO COROTATING INTERACTION REGION ASSOCIATED SHOCKS SURVIVE WHEN THEY PROPAGATE INTO THE Heliosheath
The Astrophysical Journal, 2012Co-Authors: E. Provornikova, Vladislav V. Izmodenov, Merav Opher, Gabor TothAbstract:During the solar minimum at the distance of 42-52 AU from the Sun, Voyager 2 observed recurrent sharp, shock-like increases in the solar wind speed that look very much like forward shocks (Lazarus et al.). The shocks were produced by corotating interaction regions (CIRs) that originated near the Sun. After the termination shock (TS) crossing in 2007, Voyager 2 entered the Heliosheath and has been observing the plasma emanated during the recent solar minima. Measurements show high variable flow, but there were no shocks detected in the Heliosheath. When CIR-driven shocks propagate to the outer heliosphere, their structure changes due to collision and merging processes of CIRs. In this Letter, we explore an effect of the merging of CIRs on the structure of CIR-associated shocks. We use a three-dimensional MHD model to study the outward propagation of the shocks with characteristics similar to those observed by Voyager 2 at ~45 AU (Lazarus et al. 1999). We show that due to merging of CIRs (1) reverse shocks disappear, (2) forward shocks become weaker due to interaction with rarefaction regions from preceding CIRs, and (3) forward shocks significantly weaken in the Heliosheath. Merged CIRs produce compression regions in the Heliosheath with small fluctuations of plasma parameters. Amplitudes of the fluctuations diminish as they propagate deeper in the sheath. We conclude that interaction of shocks and rarefaction regions could be one of the explanations, why shocks produced by CIRs are not observed in the Heliosheath by Voyager 2 while they were frequently observed upstream the TS.
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Is the magnetic field in the Heliosheath laminar or a turbulent bath of bubbles
The Astrophysical Journal, 2011Co-Authors: Merav Opher, John D. Richardson, R B Decker, James Drake, M Swisdak, K M Schoeffler, Gabor TothAbstract:All the current global models of the heliosphere are based on the assumption that the magnetic field in the Heliosheath, in the region close to the heliopause is laminar. We argue that in that region the heliospheric magnetic field is not laminar but instead consists of magnetic bubbles. Recently, we proposed that the annihilation of the "sectored" magnetic field within the Heliosheath as it is compressed on its approach to the heliopause produces the anomalous cosmic rays and also energetic electrons. As a product of the annihilation of the sectored magnetic field, densely-packed magnetic islands/bubbles are produced. These magnetic islands/bubbles will be convected with the ambient flows as the sector region is carried to higher latitudes filling the Heliosheath. We further argue that the magnetic islands/bubbles will develop upstream within the Heliosheath. As a result, the magnetic field in the Heliosheath sector region will be disordered well upstream of the heliopause. We present a 3D MHD simulation with very high numerical resolution that captures the north-south boundaries of the sector region. We show that due to the high pressure of the interstellar magnetic field a north-south asymmetry develops such that the disordered sectored region fills a large portion of the northern part of the heliosphere with a smaller extension in the southern hemisphere. We suggest that this scenario is supported by the following changes that occur around 2008 and from 2009.16 onward: a) the sudden decrease in the intensity of low energy electrons detected by Voyager 2; b) a sharp reduction in the intensity of fluctuations of the radial flow; and c) the dramatic differences in intensity trends between GCRs at V1 and 2. We argue that these observations are a consequence of V2 leaving the sector region of disordered field during these periods and crossing into a region of unipolar laminar field.
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Kinetic versus Multi-fluid Approach for Interstellar Neutrals in the Heliosphere: Exploration of the Interstellar Magnetic Field Effects
The Astrophysical Journal, 2011Co-Authors: Fathallah Alouani-bibi, Vladislav V. Izmodenov, Merav Opher, Dimitry Alexashov, Gabor TothAbstract:We present a new three-dimensional (3D) self-consistent two-component (plasma and neutral hydrogen) model of the solar wind interaction with the local interstellar medium (LISM). This model (K-MHD) combines the magnetohydrodynamic treatment of the solar wind and the ionized LISM component with a kinetic model of neutral interstellar hydrogen (LISH). The local interstellar magnetic field (B LISM) intensity and orientation are chosen based on an early analysis of the Heliosheath flows. The properties of the plasma and neutrals obtained using the K-MHD model are compared to previous multi-fluid and kinetic models. The new treatment of LISH revealed important changes in the heliospheric properties not captured by the multi-fluid model. These include a decrease in the heliocentric distance to the termination shock (TS), a thinner Heliosheath, and a reduced deflection angle (θ) of the Heliosheath flows. The asymmetry of the TS, however, seems to be unchanged by the kinetic aspect of the LISH.
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Magnetic Effects Change Our View of the Heliosheath
arXiv: Astrophysics, 2004Co-Authors: Merav Opher, Paulett C. Liewer, Marco Velli, Ward B. Manchester, Darren L. Dezeeuw, Gabor Toth, Tamas I. Gombosi, Igor M. SokolovAbstract:There is currently a controversy as to whether Voyager 1 has already crossed the termination Shock, the first boundary of the heliosphere. The region between the termination shock and the heliopause, the Heliosheath, is one of the most unknown regions theoretically. In the Heliosheath magnetic effects are crucial, as the solar magnetic field is compressed at the termination shock by the slowing flow. Recently, our simulations showed that the Heliosheath presents remarkable dynamics, with turbulent flows and the presence of a jet flow at the current sheet that is unstable due to magnetohydrodynamic instabilities. In this paper we review these recent results, and present an additional simulation with constant neutral atom background. In this case the jet is still present but with reduced intensity. Further study, e.g., including neutrals and the tilt of the solar rotation from the magnetic axis, is required before we can definitively address how the Heliosheath behaves. Already we can say that this region p...