Termination Shock Asymmetries as Seen by the Voyager Spacecraft: The Role of the Interstellar Magnetic Field and Neutral Hydrogen

We show that asymmetries of the termination shock due to the influence of the interstellar magnetic field (ISMF) are considerably smaller in the presence of neutral hydrogen atoms, which tend to symmetrize the heliopause, the termination shock, and the bow shock due to charge exchange with charged particles. This leads to a much stronger restriction on the ISMF direction and its strength. We demonstrate that in the presence of the interplanetary magnetic field the plane defined by the local interstellar medium (LISM) velocity and magnetic field vectors does not exactly coincide with the plane defined by the interstellar neutral helium and hydrogen velocity vectors in the supersonic solar wind region, which limits the accuracy of the inferred direction of the ISMF. We take into account the tilt of the LISM velocity vector with respect to the ecliptic plane and show that magnetic fields as strong as 3 μG or greater may be necessary to account for the observed asymmetry. Estimates are made of the longitudinal streaming anisotropy of energetic charged particles at the termination shock caused by the nonalignment of the interplanetary magnetic field with its surface. By investigating the behavior of interplanetary magnetic field lines that cross the Voyager 1 trajectory in the inner heliosheath, we estimate the length of the trajectory segment that is directly connected by these lines to the termination shock. A possible effect of the ISMF draping over the heliopause is discussed in connection with radio emission generated in the outer heliosheath

Dispersive Fast Magnetosonic Waves and Shock‐Driven Compressible Turbulence in the Inner Heliosheath

The solar wind in the inner heliosheath beyond the termination shock (TS) is a nonequilibrium collisionless plasma consisting of thermal solar wind ions, suprathermal pickup ions, and electrons. In such multi‐ion plasma, two fast magnetosonic wave modes exist, the low‐frequency fast mode and the high‐frequency fast mode. Both fast modes are dispersive on fluid and ion scales, which results in nonlinear dispersive shock waves. We present high‐resolution three‐fluid simulations of the TS and the inner heliosheath up to a few astronomical units (AU) downstream of the TS. We show that downstream propagating nonlinear fast magnetosonic waves grow until they steepen into shocklets, overturn, and start to propagate backward in the frame of the downstream propagating wave. The counterpropagating nonlinear waves result in 2‐D fast magnetosonic turbulence, which is driven by the ion‐ion hybrid resonance instability. Energy is transferred from small scales to large scales in the inverse cascade range, and enstrophy is transferred from large scales to small scales in the direct cascade range. We validate our three‐fluid simulations with in situ high‐resolution Voyager 2 magnetic field observations in the inner heliosheath. Our simulations reproduce the observed magnetic turbulence spectrum with a spectral slope of −5/3 in frequency domain. However, the fluid‐scale turbulence spectrum is not a Kolmogorov spectrum in wave number domain because Taylor’s hypothesis breaks down in the inner heliosheath. The magnetic structure functions of the simulated and observed turbulence follow the Kolmogorov‐Kraichnan scaling, which implies self‐similarity.Key PointsNonlinear dispersive fast magnetosonic waves produce 2‐D compressible turbulence downstream of the termination shockTaylor’s hypothesis breaks down in the subfast magnetosonic solar wind in the inner heliosheathThe magnetic turbulence spectrum observed by Voyager 2 in the inner heliosheath is reproduced by self‐consistent three‐fluid MHD simulationPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/163373/2/jgra56004_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/163373/1/jgra56004.pd

Galactic Cosmic Rays Modulation in the Vicinity of Corotating Interaction Regions: Observations During the Last Two Solar Minima

Corotating interaction regions (CIRs) are responsible for short-term recurrent cosmic-ray modulation, prominent near solar minima. Using the OMNI data sets for two periods of low solar activity near the beginning and end of solar cycle 24, superposed epoch analysis was performed on the solar wind plasma features for 53 and 43 events during periods 2007–2008 and 2017–2018, respectively. Turbulent properties of the solar wind were studied using the variance method for each CIR. Power spectra have been constructed for overlapped subintervals in the vicinity of stream interfaces (SIs). Using measured correlation lengths and turbulent energies, parallel and perpendicular diffusion mean free paths for cosmic-ray ions have been inferred based on two distinct theoretical formulations. For the two periods with opposite solar polarities, our results show that unlike solar wind speed, magnetic field strength, flow pressure, and proton density are relatively higher during the latest period. Increased turbulent energy and reduced parallel transport coefficients of energetic particles at the SIs are observed. The diffusion coefficients follow the same trends during both periods. The perpendicular diffusion starts increasing nearly a day before SIs and is higher in the fast wind. Superposed epoch analysis is performed on the >120 MeV proton count rate obtained from the CRIS instrument on board the ACE spacecraft for the same events. The recorded proton rates have peaks half a day before a SI and reach their minimum more than a day after a SI and have a high anticorrelation with the perpendicular diffusion coefficient

Galactic Cosmic Rays Modulation in the Vicinity of Corotating Interaction Regions: Observations During the Last Two Solar Minima

Corotating interaction regions (CIRs) are responsible for short-term recurrent cosmic-ray modulation, prominent near solar minima. Using the OMNI data sets for two periods of low solar activity near the beginning and end of solar cycle 24, superposed epoch analysis was performed on the solar wind plasma features for 53 and 43 events during periods 2007–2008 and 2017–2018, respectively. Turbulent properties of the solar wind were studied using the variance method for each CIR. Power spectra have been constructed for overlapped subintervals in the vicinity of stream interfaces (SIs). Using measured correlation lengths and turbulent energies, parallel and perpendicular diffusion mean free paths for cosmic-ray ions have been inferred based on two distinct theoretical formulations. For the two periods with opposite solar polarities, our results show that unlike solar wind speed, magnetic field strength, flow pressure, and proton density are relatively higher during the latest period. Increased turbulent energy and reduced parallel transport coefficients of energetic particles at the SIs are observed. The diffusion coefficients follow the same trends during both periods. The perpendicular diffusion starts increasing nearly a day before SIs and is higher in the fast wind. Superposed epoch analysis is performed on the >120 MeV proton count rate obtained from the CRIS instrument on board the ACE spacecraft for the same events. The recorded proton rates have peaks half a day before a SI and reach their minimum more than a day after a SI and have a high anticorrelation with the perpendicular diffusion coefficient

Analysis of Voyager Observed High-Energy Electron Fluxes in the Heliosheath Using MHD Simulations

The Voyager spacecraft (V1 and V2) observed electrons of 6-14 MeV in the heliosheath which showed several incidences of flux variation relative to a background of gradually increasing flux with distance from the Sun. The increasing flux of background electrons is thought to result from inward radial diffusion. We compare the temporal electron flux variation with dynamical phenomena in the heliosheath that are obtained from our MHD simulations. Because our simulation is based on V2 observed plasma data before V2 crossed the termination shock, this analysis is effective up to late 2008, i.e., about a year after the V2-crossing, during which disturbances, driven prior to the crossing time, survived in the heliosheath. Several electron flux variations correspond to times directly associated with interplanetary shock events. One noteworthy example corresponds to various times associated with the March 2006 interplanetary shock, these being the collision with the termination shock, the passage past the V1 spacecraft, and the collision with the region near the heliopause, as identified by W.R. Webber et al. for proton/helium of 7-200 MeV. Our simulations indicate that all other electron flux variations, except one, correspond well to the times when a shock-driven magneto-sonic pulse and its reflection in the heliosheath either passed across V1/V2, or collided with the termination shock or with the plasma sheet near the heliopause. This result suggests that variation in the electron flux should be due to either direct or indirect effects of magnetosonic pulses in the heliosheath driven by interplanetary shock

On the Energy Dependence of Galactic Cosmic Ray Anisotropies in the Very Local Interstellar Medium

We report on the energy dependence of galactic cosmic rays (GCRs) in the very local interstellar medium (VLISM) as measured by the Low Energy Charged Particle (LECP) instrument on the Voyager 1 (V1) spacecraft. The LECP instrument includes a dual-ended solid state detector particle telescope mechanically scanning through 360 deg across eight equally-spaced angular sectors. As reported previously, LECP measurements showed a dramatic increase in GCR intensities for all sectors of the >=211 MeV count rate (CH31) at the V1 heliopause (HP) crossing in 2012, however, since then the count rate data have demonstrated systematic episodes of intensity decrease for particles around 90{\deg} pitch angle. To shed light on the energy dependence of these GCR anisotropies over a wide range of energies, we use V1 LECP count rate and pulse height analyzer (PHA) data from >=211 MeV channel together with lower energy LECP channels. Our analysis shows that while GCR anisotropies are present over a wide range of energies, there is a decreasing trend in the amplitude of second-order anisotropy with increasing energy during anisotropy episodes. A stronger pitch-angle scattering at the higher velocities is argued as a potential cause for this energy dependence. A possible cause for this velocity dependence arising from weak rigidity dependence of the scattering mean free path and resulting velocity-dominated scattering rate is discussed. This interpretation is consistent with a recently reported lack of corresponding GCR electron anisotropies

Heliospheric Response to Different Possible Interstellar Environments

At present, the heliosphere is embedded in a warm, low-density interstellar cloud that belongs to a cloud system flowing through the local standard of rest with a velocity near ~18 km s-1. The velocity structure of the nearest interstellar material (ISM), combined with theoretical models of the local interstellar cloud (LIC), suggest that the Sun passes through cloudlets on timescales of ≤103-104 yr, so the heliosphere has been, and will be, exposed to different interstellar environments over time. By means of a multifluid model that treats plasma and neutral hydrogen self-consistently, the interaction of the solar wind with a variety of partially ionized ISM is investigated, with the focus on low-density cloudlets such as are currently near the Sun. Under the assumption that the basic solar wind parameters remain/were as they are today, a range of ISM parameters (from cold neutral to hot ionized, with various densities and velocities) is considered. In response to different interstellar boundary conditions, the heliospheric size and structure change, as does the abundance of interstellar and secondary neutrals in the inner heliosphere, and the cosmic-ray level in the vicinity of Earth. Some empirical relations between interstellar parameters and heliospheric boundary locations, as well as neutral densities, are extracted from the models

A two-dimensional, self-consistent model of galactic and anomalous cosmic rays in the solar wind

We have developed a two-dimensional heliospheric model that includes galactic and anomalous cosmic rays as well as pickup ions. Cosmic rays are described via their number density in phase space, rather than pressure, as every preceding 2-D model has done. Cosmic-ray pressure is included in the total energy budget, allowing us to compute dynamical effects of the energetic particles on the solar wind. We include the magnetic field as well in order to consistently compute cosmic-ray diffusion coefficients. To accommodate' lower-energy cosmic rays with their short diffusion length, we implemented an adaptive mesh refinement code featuring improved spatial resolution near the termination shock. Our simulations show that galactic cosmic rays could substantially change the solar wind flow in the outer heliosphere. In particular, the solar wind is deflected towards the ecliptic plane during the positive solar cycle, resulting in faster wind near the current sheet. This is a result of large latitudinal gradients in the cosmic-ray pressure, caused by the difference in cosmic-ray drift patterns over latitude. We also found that anomalous cosmic rays have a minor effect on the solar wind. Their pressure is not sufficient to modify the termination shock significantly, a conclusion based on comparing model cosmic-ray spectra with observations. However, anomalous cosmic-ray acceleration occurs somewhat differently than thought before, and shock drift effects are not prominent. The spectra of these particles have an enhancement near the cutoff, that is not caused by shock drifts

Simulation of Solar Wind Turbulence near Corotating Interaction Regions: Superposed Epoch Analysis of Simulations and Observations

The effect of the turbulence that is associated with solar wind corotating interaction regions (CIRs) on transport of galactic cosmic rays remains an outstanding problem in space science. Observations show that the intensities of the plasma and magnetic fluctuations are enhanced within a CIR. The velocity shear layer between the slow and fast wind embedded in a CIR is thought to be responsible for this enhancement in turbulent energy. We perform physics-based magnetohydrodynamic simulations of the plasma background and turbulent fluctuations in the solar wind dominated by CIRs for radial distances between 0.3 and 5 au. A simple but effective approach is used to incorporate the inner boundary conditions for the solar wind and magnetic field for the periods 2007–2008 and 2017–2018. Legendre coefficients at the source surface obtained from the Wilcox Solar Observatory library are utilized for dynamic reconstructions of the current sheet and the fast and slow streams at the inner boundary. The dynamic inner boundary enables our simulations to generate CIRs that are reasonably comparable with observations near Earth. While the magnetic field structure is reasonably well reproduced, the enhancements in the turbulent energy at the stream interfaces are smaller than observed. A superposed epoch analysis is performed over several CIRs from the simulation and compared to the superposed epoch analysis of the observed CIRs. The results for the turbulent energy and correlation length are used to estimate the diffusion tensor of galactic cosmic rays. The derived diffusion coefficients could be used for more realistic modeling of cosmic rays in a dynamically evolving inner heliosphere