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We carried out a MHD simulation of deep interplanetary space from 2001 to 2009 with a realistic inner boundary condition obtained from a time-dependent 3-D ideal MHD model incorporating interplanetary scintillation and synoptic solar magnetic field data. As a result, we found that wavy solar wind structure with amplitudes of 10–50 km/s and a monochromatic wavelength of ~2 AU filled deep interplanetary space beyond ~50 AU from the Sun, which corresponds to a velocity oscillation detected by Voyager 2. The phase of the velocity oscillation is anticorrelated with density fluctuations in our simulation, which suggests that the velocity oscillation should propagate inward toward the Sun in the frame moving with the solar wind but is advected outward by the solar wind in the rest frame of the Sun.

Recent spacecraft observations reveal that energetic tails in ion spectra are present continually in the slow low-latitude solar wind, even in the absence of nearby shocks. To explain this phenomenon we take into account growing evidence that MHD turbulence in the solar wind has a strong two-dimensional (2D) component. 2D MHD turbulence simulations exhibit large-scale field-aligned electric field fluctuations which are also observable in the solar wind. We explore pickup ion propagation and acceleration between the Sun and Earth with a newly developed kinetic numerical model which includes a random distribution of large-scale field-aligned electric field fluctuations consistent with observations in the low-latitude solar wind. The model is found to qualitatively reproduce the accelerated interstellar pickup He+ spectra observed at Earth. Like the observations, the simulated accelerated “tail” is approximately a power law f(υ) ∝ υ−4.5 until it reaches particle speeds about ten times the solar wind speed where a rollover occurs.

Evidence exists suggesting that not only do heliospheric shocks accelerate particles up to energies as high as a few GeV/nuc. but that the source of these energetic particles are pickup ions or low energy particles present in the suprathermal tail of the solar wind distribution. To be diffusively accelerated at a shock wave a particle needs an energy significantly greater than that of the expected source particles, the so-called “injection problem”. We present here a model of particle acceleration at shocks of arbitrary obliquity that produces particle energies sufficiently high for diffusive shock acceleration to occur.

A unified description of pickup and anomalous cosmic ray proton transport in the near ecliptic regions of the upwind heliosphere is presented. Multiply reflected ion acceleration theory is used for the injection of pickup protons into the diffusive shock acceleration mechanism, so yielding the anomalous cosmic ray component at the quasi-perpendicular termination shock. The results show that both the multiply reflected and simultaneously accelerated pickup protons and the anomalous protons significantly modify the termination shock. Although the shock mediation lowers the cut-off speed of the accelerated pickup protons, a sufficient large population of anomalous protons is accelerated at the termination shock to give a favorable comparison of the modulated anomalous proton spectra up-stream with observations at large heliocentric distances.

Voyager 2 data from 47 AU shows a delay of about 6 days between the arrival of interplanetary shock fronts and the peaks in the 0.52–1.45 MeV energetic proton flux. Similar data recorded when Voyager 2 was at 5 AU shows that the shock fronts and the peaks in the energetic proton flux are virtually coincident. If injection and acceleration is taking place at the shock fronts one would expect the peaks in the energetic proton flux and the shocks to coincide. We show that the delay observed at 47 AU can be understood if the injection and acceleration of energetic protons becomes negligibly small some time before the shocks are observed.

[1] Voyager 2 crossed the solar wind termination shock several times during August 30 – September 1 of 2007. During the last forty days before the crossing intensities of energetic ions measured by the LECP instrument were increasing exponentially, and their spectrum showed clear evidence of unfolding at low energies. Plasma data featured a broad velocity precursor, where solar wind speed decreased from about 380 km/s far upstream to ∼300 km/s at the shock. By using plasma and energetic particle conservation laws we demonstrate that the speed decrease during Voyager 2's last ∼40 days in the solar wind could be plausibly produced by the back-pressure of energetic ions with energies of a few MeV on the upstream plasma flow. These particles propagate diffusively upstream from the termination shock, producing a shock precursor that has a characteristic lengthscale of 0.35 AU, assuming the shock has not moved substantially during that time.

Recent observations at comets Giacobini-Zinner and Halley suggest that simple nonreacting gas dynamics or MHD is an inappropriate description for the bow shock. The thickness of the observed (sub)shock implies that mass-loading is an important dynamical process within the shock itself, thereby requiring that the Rankine-Hugoniot conditions possess source terms. This leads to shocks with properties similar to those of combustion shocks. The paper considers parallel magnetized shocks subjected to mass-loading, describes some properties which distinguish them from classical MHD parallel shocks, and establishes the existence of a new kind of MHD compound shock. These results will be of importance both to observations and numerical simulations of the comet-solar wind interaction.

Recent theoretical investigations of low Mach number flows, that describe two distinct approaches by fluids to the incompressible regime are summarized. The first includes the effects of relatively strong density and temperature fluctuations (Type I), while the second places fluctuations in mechanical pressure, density, and temperature on an equal footing (Type II). In the latter case, the relations between density and pressure are recovered, whereas the former case yields departures from incompressible behavior in that density and temperature fluctuations are predicted to be anti-correlated. It is suggested that nearly incompressible fluids can be classified as either Type I or II, and it is shown that the well-known pressure-balanced structures represent a subclass of static solutions within this classification. Two examples from Voyager data illustrate the potential for observing these distinct nearly incompressible dynamical ordering in the solar wind.

[1] We propose a solitary wave model for small scale magnetic structures observed in the solar wind and more recently in the Voyager 1 observations of the heliosheath. The model is based on the recent, fully nonlinear theory of solitary waves by McKenzie et al. (2001, 2004). Our solutions i.e., magnetic holes, humps, trains of holes and humps, are strongly nonlinear (70 to 80% change in the magnetic field at the centre), propagate at large angles (>60°) to the mean magnetic field and are well approximated by Gaussians. The structures are almost pressure balanced with an anti-correlation between the magnetic field and the plasma density, and no change in the magnetic vector across the structure. These features are consistent with observations of magnetic structures in the heliosheath.

[1] We investigate the effects of a large-scale interplanetary disturbance, such as a merged interaction region (MIR), on the properties of the heliospheric termination shock and particle population accelerated at the shock. The interaction process between transient structures and the shock is reflected in energetic particle spectra through variability in shock location, its compression ratio, and particle mean free paths that control their acceleration rate. It is shown that particle distributions observed at a specific moment of time exhibit features, such as spectral breaks, from earlier acceleration stages with lowest energy particles produced most recently and the more energetic anomalous cosmic rays (ACRs) accelerated at earlier times. We also show that a MIR collision can produce an extended period of anomalously low radial velocity in the heliosheath if density and velocity variations are correlated within a MIR and a following rarefaction.

[1] High power radio emissions near 2–3 kHz originating in the outer heliosphere have been observed by the Voyager spacecraft. Recent theories for these emissions combine acceleration of nonthermal electrons at global merged interaction region shocks with formation of a superthermal electron tail by lower-hybrid drive associated with pick-up ions, beyond the heliopause. This paper examines the efficiency of this process during the solar cycle, based on variations of the availability of pick-up ions in the source region associated with periodic solar cycle variations in the solar wind ram pressure. It is predicted that high levels of pick-up ions are present in the source region approximately 2–3 years after solar maximum, leading to strong emissions, in close agreement with the observed timing of events: the 1983–84, 1992–93 and 2003 events were preceded by solar maxima in 1980, 1990 and 2000, respectively. These results provide further evidence for the proposed model.

[1] A recent study of Cane et al. [2003] showed that in some intense SEP events, the time-intensity profiles exhibit two peaks, with an earlier one having a high Fe/O and a later one with a low Fe/O ratio. They suggested that these two-component events are due to CMEs and their accompanying flares occurring together, with the first peak being flare-related and the second peak being CME-driven shock related. In this paper, we develop a model which examines particle acceleration and transport when both flares and CME-driven shocks are present. We study time-intensity profiles for three different scenarios: a pure shock case, a pure flare case and a shock-flare-mixed case. Using reasonable estimates of the relative timing between CMEs and associated flares, we find that a large portion of the flare accelerated material is subject to absorption and re-acceleration by the CME-driven shock. Consequently, the time intensity profile for the shock-flare-mixed case shows an initial rapid increase, owing to particles accelerated at the flare and followed by a plateau similar to that of a pure shock case.

[1] The structure of the heliosphere is investigated for a period when the Sun enters a cold dense unmagnetized interstellar cloud. Under these conditions the heliospheric interface resembles the two-shock standard model with the termination shock located close to 23 AU in the upwind direction. Anomalous and galactic cosmic ray fluxes are calculated and compared with their present day intensities in the inner heliosphere. A tenfold increase in anomalous cosmic ray and a twofold increase in galactic cosmic ray intensities are possible if heliosheath modulation is reduced, in this high-density case. On the basis of our heliospheric modeling, we show that a possible explanation of the enhancement of cosmogenic elements in ice and sediment samples 35,000 and 60,000 years BP might be an encounter of the Sun with a dense interstellar cloud.

[1] The Voyager spacecraft observe radio emissions near 2-3 kHz that appear to be generated when an interplanetary shock nears the heliopause. This paper presents a detailed, qualitative theory for where and why the radiation turns on. The radiation turns on just beyond the heliopause due to the shock moving into a region primed with a strong superthermal electron tail. Lower-hybrid drive produces the tail: ambient electrons are resonantly accelerated by lower hybrid waves driven by a ring-beam instability of pickup protons. The pickup protons originate as neutrals in the inner heliosheath. Strong arguments suggest that the tail exists only beyond the heliopause (within ≃ 20-50 AU of the nose) and that shock acceleration of the tail dramatically enhances the foreshock electron beams, Langmuir waves and radiation, so that the shock triggers observable radiation only when beyond the heliopause.