Keywords

Plasma kinetic waves and alpha–proton differential flow are two important subjects on the topic of solar wind evolution. Based on the Wind data during 2005–2015, this paper reports that the occurrence of electromagnetic cyclotron waves (ECWs) near the proton cyclotron frequency significantly depends on the direction of alpha–proton differential flow ${{\boldsymbol{V}}}_{d}$. As ${{\boldsymbol{V}}}_{d}$ rotates from the anti-Sunward direction to the Sunward direction, the occurrence rate of ECWs as well as the percentage of left-handed (LH) polarized ECWs decreases considerably. In particular, it is shown that the dominant polarization changes from LH polarization to right-handed polarization during the rotation. The investigation on proton and alpha particle parameters ordered by the direction of ${{\boldsymbol{V}}}_{d}$ further illustrates that large kinetic energies of alpha–proton differential flow correspond to high occurrence rates of ECWs. These results are consistent with theoretical predictions for effects of alpha–proton differential flow on proton temperature anisotropy instabilities.

In collision-poor plasmas from space, e.g., solar wind or stellar outflows, the heat flux carried by the strahl or beaming electrons is expected to be regulated by the self-generated instabilities. Recently, simultaneous field and particle observations have indeed revealed enhanced whistler-like fluctuations in the presence of counter-beaming populations of electrons, connecting these fluctuations to the whistler heat-flux instability (WHFI). This instability is predicted only for limited conditions of electron beam-plasmas, and has not yet been captured in numerical simulations. In this Letter we report the first simulations of WHFI in particle-in-cell setups, realistic for the solar wind conditions, and without temperature gradients or anisotropies to trigger the instability in the initiation phase. The velocity distributions have a complex reaction to the enhanced whistler fluctuations conditioning the instability saturation by a decrease of the relative drifts combined with induced (effective) temperature anisotropies (heating the core electrons and pitch-angle and energy scattering the strahl). These results are in good agreement with a recent quasilinear approach, and support therefore a largely accepted belief that WHFI saturates at moderate amplitudes. In the anti-sunward direction the strahl becomes skewed with a pitch-angle distribution decreasing in width as electron energy increases, which seems to be characteristic of self-generated whistlers and not to small-scale turbulence.

Hydration is a major mineral alteration process in primitive asteroids and it might occur in comet nuclei; however, it is poorly understood at low temperatures, especially below the freezing point of water. Long-duration experiments were performed with exposures of amorphous silicate nanoparticles and organic compounds (glycine and ribose) to D₂O and D₂O + NH₃ ices and vapors at temperatures of −17°C and −27°C for 10–120 days; and with exposure of amorphous silicates to H₂O vapor/liquid at >25°C for 10 days. The amorphous silicates were analyzed by X-ray diffraction and Fourier-transform infrared spectroscopy, and recovery of organic molecules was determined by liquid chromatography–mass spectrometry. No hydration of amorphous silicates or organic compounds was observed after exposure at temperatures below −17°C for 120 days to ices with or without NH₃, whereas hydration of the amorphous silicates was observed in experiments above room temperature. The estimated thermal history of the nucleus of the short-period comet 67P/Churyumov–Gerasimenko indicates that the surface temperature does not exceed −45°C, even in a region exposed to strong solar illumination during the perihelion passage. Assuming hydration is controlled by the collision frequency between H₂O molecules and dust particles, the present results indicate that cometary dust does not hydrate for more than 25–510 periods of comet 67P. This is consistent with the absence of phyllosilicates on 67P and suggests that amino acids and sugars have not been altered.

We study the relationship between the solar wind helium-to-hydrogen abundance ratio (A_He), solar wind speed (v_sw), and sunspot number (SSN) over solar cycles 23 and 24. This is the first full 22 year Hale cycle measured with the Wind spacecraft covering a full cycle of the solar dynamo with two polarity reversals. While previous studies have established a strong correlation between A_He and SSN, we show that the phase delay between A_He and SSN is a monotonic increasing function of v_sw. Correcting for this lag, A_He returns to the same value at a given SSN over all rising and falling phases and across solar wind speeds. We infer that this speed-dependent lag is a consequence of the mechanism that depletes slow wind A_He from its fast wind value during solar wind formation.

We calculate the interplanetary magnetic field path lengths traveled by electrons in solar electron events detected by the WIND 3DP instrument from 1994 to 2016. The velocity dispersion analysis method is applied for electrons at energies of ∼27–310 keV. Previous velocity dispersion analyses employ the onset times, which are often affected by instrumental effects and the pre-existing background flux, leading to large uncertainties. We propose a new method here. Instead of using the peak or onset time, we apply the velocity dispersion analysis to the times that correspond to the rising phase of the fluxes that are a fraction, η, of the peak flux. We perform statistical analysis on selected events whose calculated path lengths have uncertainties smaller than 0.1 au. The mean and standard deviation, (μ, σ), of the calculated path lengths corresponding to η = 3/4, 1/2, and 1/3 of the peak flux is (1.17 au, 0.17 au), (1.11 au, 0.14 au), and (1.06 au, 0.15 au). The distribution of the calculated path lengths is also well fitted by a Gaussian distribution for the η = 3/4 and 1/3 cases. These results suggest that in these electron events the interplanetary magnetic field topology is close to the nominal Parker spiral, with little field line meandering. Our results have important implications for particles' perpendicular diffusion.

We investigate the evolution of the suprathermal (ST) proton population as interplanetary shocks cross 1 au. The variability of the ST proton intensities and energy spectra upstream of the shocks is analyzed in terms of the shock parameters, upstream magnetic field configurations, and preexisting upstream populations. Propitious conditions for the observation of ST particles at distances far upstream from the shock occur in parallel shock configurations when particles can easily escape from the shock vicinity. In this situation, ST intensity enhancements show onsets characterized by velocity dispersion effects and energy spectra that develop into a "hump" profile peaking around ∼10 keV just before the arrival of the shock. The observation of field-aligned proton beams at low energies (5–10 keV) is possible under conditions that facilitate the scatter-free propagation of the particles streaming out of the shock. Upstream of perpendicular shocks, ST intensity enhancements are only observed in close proximity to the shock. Power-law proton spectra develop downstream of the shocks. The functional form for the downstream phase-space density proportional to v⁻⁵ is observed only over a limited range of ST energies. The absence of ST populations observed far upstream of interplanetary shocks raises questions about whether ST protons contribute as a seed particle population in the processes of particle acceleration at shocks.

We present the multifractal study of the intermittency of the magnetic field turbulence in the fast and slow solar wind beyond the ecliptic plane during two solar minima (1997–1998, 2007–2008) and solar maximum (1999–2001). More precisely, we consider 126 time intervals of Ulysses magnetic field measurements, obtain the multifractal spectra, and examine the degree of multifractality as the measure of intermittency in the MHD range of scales, for a wide range of heliocentric distances and heliolatitudes. The results show a slow decrease of intermittency with the radial distance, which is more significant for the fast than for the slow solar wind. Analysis of Alfvénic and compressive fluctuations confirms the decrease of intermittency with distance and latitude. This radial dependence of multifractality/intermittency may be explained by a slower evolution of turbulence beyond the ecliptic plane and by the reduced efficiency of intermittency drivers with the distance from the Sun. Additionally, our analysis shows that the greatest differences between magnetic field components are revealed close to the Sun, where intermittency is the strongest. Moreover, we observe that the slow solar wind from the maximum of the solar cycle 23 exhibits in general, a lower level of multifractality (intermittency) than fast solar wind, which can be related to the idea of a new type of Alfvénic slow solar wind.

Suprathermal ions form from interstellar gas that is first ionized into pickup ions and then accelerated to tens and hundreds of keV in energy. The resulting suprathermal ion spectra with hundreds of keV have been previously observed throughout the heliosphere; however, measurements at lower energies, around the pickup ion cutoff energy where they are accelerated from, were limited to <10 au. Here we present a statistical study of suprathermal ions in the keV to hundred keV energy range. We use the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) instrument on the New Horizons spacecraft, which recorded observations at a wide range of heliocentric distances, and compare these measurements to charge energy mass spectrometer (CHEMS) observations on Cassini, which cruised to and remained at Saturn. We find that the power-law exponents of suprathermal ion intensity over energy are between −1 and −2, change abruptly close to discontinuities that are likely corotating merged interaction regions, correlate with the solar wind bulk speed, and show a long-term evolution on the timescale of the solar cycle. The independent measurements from New Horizons and Cassini are consistent, confirming the first fully calibrated measurements from the New Horizons/PEPSSI instrument.

In the onset phase of large gradual solar energetic particle (SEP) events, the first particles of a given rigidity to arrive at Earth are accelerated in the low corona, focused into a narrow cone of pitch angles by the diverging magnetic field, and transported from near the Sun to 1 au with minimal scattering. The effects of focused transport on the evolution of the beam-like SEPs are investigated analytically. The model assumes for simplicity a constant focusing length and a constant pitch-angle diffusion coefficient for SEPs at small pitch angles. Cross-field transport is ignored. This analytical approximation provides a reasonable representation of the spatial and pitch-angle distribution of the beam-like SEPs. Assuming an instantaneous injection of SEPs near the Sun, the model naturally reproduces several features of the SEP onset profiles observed at 1 au, including a spike-like time–intensity profile with rapid rising and declining edges that resemble a Reid–Axford profile. By assuming an extended injection profile with the shape of an isosceles triangle, we fit the onset phase data of the 2005 January 20 GLE event to our model. The derived mean free path (∼4 au) for relativistic protons is much larger than the theoretical prediction based on the standard quasilinear theory but consistent with our assumption of nearly scatter-free transport and can be explained by a reduced scattering rate, due to particles interacting with ambient turbulence with a Goldreich–Sridhar spectrum. Assuming that the SEPs that are scattered out of the beam are governed by spatial diffusive transport in interplanetary space, we perform an illustrative calculation to account for the nearly isotropic phase following the anisotropic onset as a natural result of the interplanetary transport of SEPs.

Non-recurrent short-term variations of the galactic cosmic-ray (GCR) flux above 70 MeV n⁻¹ were observed between 2016 February 18 and 2017 July 3 on board the European Space Agency LISA Pathfinder (LPF) mission orbiting around the Lagrange point L1 at 1.5 × 10⁶ km from Earth. The energy dependence of three Forbush decreases is studied and reported here. A comparison of these observations with others carried out in space down to the energy of a few tens of MeV n⁻¹ shows that the same GCR flux parameterization applies to events of different intensity during the main phase. FD observations in L1 with LPF and geomagnetic storm occurrence are also presented. Finally, the characteristics of GCR flux non-recurrent variations (peaks and depressions) of duration <2 days and their association with interplanetary structures are investigated. It is found that, most likely, plasma compression regions between subsequent corotating high-speed streams cause peaks, while heliospheric current sheet crossing causes the majority of the depressions.