Interstellar Neutral He Parameters from Crossing Parameter Tubes with the Interstellar Mapping and Acceleration Probe Informed by 10 yr of Interstellar Boundary Explorer Observations

The Sun's motion through the interstellar medium leads to an interstellar neutral (ISN) wind through the heliosphere. Several ISN species, including He, moderately depleted by ionization are observed with pickup ions and directly imaged. Since 2009, analyzed Interstellar Boundary Explorer (IBEX) observations returned a precise 4D parameter tube associated with the bulk velocity vector and the temperature of ISN flow distribution. This 4D parameter tube is typically expressed in terms of the ISN speed, the inflow latitudinal direction, and the temperature as a function of the inflow longitudinal direction and the local flow Mach number. We have used IBEX observations and those from other spacecraft to reduce statistical parameter uncertainties: V ISN ∞ = 25.99 ± 0.18 km s−1, λ ISN ∞ = 75 .° 28 ± 0 .° 13 , β ISN ∞ = −5 .° 200 ± 0 .° 075 , and T ISN ∞ = 7496 ± 172 K. IBEX ISN viewing is restricted almost perpendicular to the Earth–Sun line, which limits observations in ecliptic longitude to ∼130° ± 30° and results in relatively small uncertainties across the IBEX parameter tube but large uncertainties along it. Operations over the last three years enabled the IBEX spin axis to drift to the maximum operational offset (7°) west of the Sun, helping to break the ISN parameter degeneracy by weakly crossing the IBEX parameter tubes: the range of possible inflow longitudes extends over the range λ ISN ∞ = 75 .° 28 − 2.21 + 2.27 and the corresponding range of other ISN parameters is V ISN ∞ = 25.99 − 1.76 + 1.86 km s−1, β ISN ∞ = −5 .° 200 − 0.085 + 0.093 , and T ISN ∞ = 7496 − 1528 + 1274 K. This enhances the full χ 2 analysis of ISN parameters through comparison with detailed models. The next-generation IBEX-Lo sensor on IMAP will be mounted on a pivot platform, enabling IMAP-Lo to follow the ISN flow over almost the entire spacecraft orbit around the Sun. A near-continuous set of 4D parameter tube orientations on IMAP will be observed for He and for O, Ne, and H that cross at varying angles to substantially reduce the ISN flow parameter uncertainties and mitigate systematic uncertainties (e.g., from ionization effects and the presence of secondary components) to derive the precise parameters of the primary and secondary local interstellar plasma flows.

The Spectrum Power of Interstellar Plasma Inhomogeneities in the Direction of Eleven Radio Pulsars

We have analyzed two-dimensional correlation functions from the dynamic spectra of 11 pulsars using the archival data of the “Radioastron” project. The time-sections of these functions were approximated by exponential functions with a power $$\alpha $$. It is shown that this approximation describes the shape of the correlation function much better than the Gaussian. The temporal structure function $$D(\Delta t)$$ for small values of the delay $$\Delta t$$is a power law with an index $$\alpha $$. The spectrum power of spatial inhomogeneities of the interstellar plasma is related to the power of the structure function as $$n = \alpha + 2$$. We have determined the characteristic scintillation time and the power $$n$$ in the direction of 11 pulsars. In the direction of three pulsars (B0329+54, B0823+26, and B1929+10), the spectrum power of spatial inhomogeneities of the interstellar plasma turned out to be very close to the value for the Kolmogorov spectrum ($$n = 3.67$$). For other pulsars, it ranges from 3.18 to 3.86. It is shown that the measured scintillation parameters are significantly influenced by the duration of the observation session, expressed by its ratio to the characteristic scintillation time. If this parameter is less than 10, the parameter estimates may be biased: the values of $$\alpha $$ and the characteristic scintillation time $${{t}_{{{\text{scint}}}}}$$ may decrease.

Non-equilibrium ionisation plasmas in the interstellar medium

Obtaining astrophysical information from diffuse cool, warm and hot plasmas in interstellar and intergalactic media by electromagnetic radiation is based on highly non-linear heating and cooling processes, which are largely determined by atomic physical time scales and reaction rates. To calculate spectra is further complicated by gas dynamical interactions and processes, such as e.g. shock waves, fast adiabatic expansion and catastrophic cooling. In essence this leads to a non-linear coupling between atomic physics and hydro- or magnetohydrodynamics, which renders radiative cooling to become time- and space-dependent, contrary to the often conveniently used assumption of collisional ionisation equilibrium for optically thin plasmas. Computing power and new algorithms for high performance computing have made it possible to trace the dynamical and thermal evolution of a sufficiently large section of interstellar space over an appreciable time scale to derive characteristic quantities like temperature and density distribution as well as spectra, which can be compared to X-ray, UV and optical observations. In this review we describe diffuse interstellar plasma simulations, the physical processes which drive the temporal and spatial evolution, and present high resolution numerical simulations, including time-dependent cooling, which further our understanding of the state and evolution of interstellar (magnetised) plasmas. We also discuss briefly the rôle of cosmic rays and their interaction with the plasma.

Extreme intra-hour variability of the radio source J1402+5347 discovered with Apertif

The propagation of radio waves from distant compact radio sources through turbulent interstellar plasma in our Galaxy causes these sources to twinkle, a phenomenon called interstellar scintillation. Such scintillations are a unique probe of the micro-arcsecond structure of radio sources as well as of the sub-AU-scale structure of the Galactic interstellar medium. Weak scintillations (i.e. an intensity modulation of a few percent) on timescales of a few days or longer are commonly seen at centimetre wavelengths and are thought to result from the line-of-sight integrated turbulence in the interstellar plasma of the Milky Way. So far, only three sources were known that show more extreme variations, with modulations at the level of some dozen percent on timescales shorter than an hour. This requires propagation through nearby (d ≲ 10 pc) anomalously dense (ne ∼ 102 cm−3) plasma clouds. Here we report the discovery with Apertif of a source (J1402+5347) showing extreme (∼50%) and rapid variations on a timescale of just 6.5 min in the decimetre band (1.4 GHz). The spatial scintillation pattern is highly anisotropic, with a semi-minor axis of about 20 000 km. The canonical theory of refractive scintillation constrains the scattering plasma to be within the Oort cloud. The sightline to J1402+5347, however, passes unusually close to the B3 star Alkaid (η UMa) at a distance of 32 pc. If the scintillations are associated with Alkaid, then the angular size of J1402+5347 along the minor axis of the scintels must be smaller than ≈10 μas, yielding an apparent brightness temperature for an isotropic source of ≳1014 K.

Does Nature use neutral beams for interstellar plasma heating around compact objects?

ABSTRACT A neutral beam injection technique is employed in all major TOKAMAK facilities for heating of magnetically confined plasma. The question then arises, whether a similar mechanism might work in astrophysical objects? For instance, a hyper-Eddington Galactic binary SS433 possesses baryonic jets, moving at a quarter of the speed of light, and observations revealed signs of gas cooling and recombination on sub-pc scales and equally strong signs of powerful energy deposition on much larger scales ∼100 pc. Here, we consider a model where neutral atoms transport this energy. A sub-relativistic beam of neutral atoms penetrates the interstellar medium; these atoms gradually get ionized and deposit their energy over a region, whose longitudinal dimension is set by the ‘ionization length’. The channel, where the energy is deposited, expands sideways and drives a shock in the lateral direction. Once the density in the channel drops, the heating rate by the beam drops accordingly, and the region of the energy release moves along the direction of the beam. We discuss distinct features associated with this scenario and speculate that such configuration might also boost shock acceleration of the ‘pick-up’ protons that arise due to ionization of neutral atoms both upstream and downstream of the shock.

Disentangling interstellar plasma screens with pulsar VLBI: combining auto- and cross-correlations

Current methods of measuring distances to pulsar scattering screens rely on a single screen dominating the scintillation pattern. We present a novel technique to reconstruct the scattered flux of a pulsar and solve for the scattering geometry in cases where the scattering environment along the line of sight to the pulsar is complex and may be composed of multiple scattering screens. This technique combines interferometric visibilities with cross-correlations of single-station intensities. It takes advantage of the fact that if one considers the interference of radiation from two points in the scattered image in delay–delay rate space, the visibilities are sensitive to the mean angular position of the points, while the cross-correlated intensities are sensitive to their angular separation. By combining the visibilities and the cross-correlated intensities, it is possible to measure the angular locations of both points in the pair. We show that this technique is able to reconstruct the published scattering geometry of PSR B0834+06. We then apply this technique to one-dimensional simulations of more complicated scattering systems, where we find that it can distinguish features from different scattering screens. This technique holds promise for studies of the interstellar medium and pulsars themselves: It will allow the application of scintillometry techniques, such as resolving pulsar emission regions using interstellar scattering, to sources for which a current lack of understanding of the scattering environment has precluded their use.

Intermittency and Anisotropy in the Ionized Interstellar Medium

The discovery of pulsars was closely followed by the discovery of dispersion and scattering in the interstellar plasma (ionized interstellar medium - IISM). The rich phenomena of scattering and scintillation have since been successfully modelled as propagation through a statistically uniform plasma turbulence with an isotropic Kolmogorov spectrum of density. However, this enticingly simple model fails to explain the many recent observations, that show anisotropic scattering from highly localized regions of the IISM often referred to as phase screens. I summarize the recent evidence from pulsars and also from very compact AGN sources, which can exhibit rapid scintillation and occasionally ESEs. The unknown astrophysical origin of these phenomena includes thin current sheets, the diffuse remnants of old supernova shells, and plasma filaments surrounding ubiquitous molecular clumps near young hot stars.