Scientific rationale
1) Propagation and Dissipation of Shocks in the VLISM:
Large-scale disturbances and shocks generated by the Sun’s dynamics first propagate through the heliosphere, influence the heliosphere’s outer boundaries, and then are transmitted into the LISM (Mostafavi et al., 2022; Burlaga et al., 2022), see Fig. 1. We aim to determine the primary dissipation mechanisms for VLISM shocks, which remain poorly understood. Voyager has occasionally detected electron plasma oscillation events and jumps in the magnetic field, some of which are believed to be shocks. Data suggest that the structure of these shocks is unexpectedly thick. It is unclear why the shock thickness of these quasi-perpendicular shocks estimated from Voyager observations is ∼0.05-0.25 AU, smaller than the expected Coulomb collision mean free path (m.f.p.) of ∼4 AU for proton temperatures ∼30,000 K observed by Voyagers and predicted by heliospheric models.
Mostafavi et al. (2018) attributed shock properties to proton-proton Coulomb collisions, arguing that they provide the dissipation required to convert SW kinetic energy into thermal pressure for OHS shocks. However, this model requires negligible increase in temperature of the thermal LISM plasma due to deceleration at the HP. If the temperature of the unperturbed LISM ∼7500 K is assumed to persist at the HP, the collisional m.f.p. reduces to 0.2 – 0.3 AU, close to the width of observed structures. However, observations and simulations both indicate that the plasma temperature is closer to 30,000 K because (i) the secondary ions born by charge exchange have a temperature ∼35,000 K and constitute the dominant proton population by energy and density; (ii) V2/PLS recorded temperatures of 30,000 K – 50,000 K (Richardson et al. (2019), which despite the large uncertainties is in excellent agreement with global models (Pogorelov et al. 2017b; Fraternale et al., 2021b); and (iii) numerical models show that by suppressing charge exchange, plasma adiabatic compression alone may result in a plasma temperature at the HP of 15,000 – 20,000 K, for TLISM=7,500 K. In this case the shock structure would be 10 to 100 times thinner than the m.f.p. but still 104 times larger than the convective gyroscale (∼800 km), which determines the structure of collisionless shocks in the SW.
It is thus theoretically challenging to identify the dissipation mechanism responsible for shock structures in the VLISM. Due to the broad range of scales involved, simulations (even collisional hybrid simulations) have not been able to reproduce existing Voyager observations. We will examine whether turbulence plays a fundamental role in the dissipation mechanisms at shocks in the VLISM.
Fig. 1. Shocks, plasma waves, and turbulence in the VLISM. (a) A global heliospheric model (MS-FLUKSS code) showing compressible perturbations propagating into the VLISM (Pogorelov et al. 2017b). (b) Plasma oscillations detected by V1/PWS (Gurnett et al., 2021). (c) Intermittent magnetic field fluctuations in front and behind of a shock in 2014 (Fraternale et al., 2020a)
2) Generation of Langmuir Waves, Radio Emission, and Quasi-thermal Fluctuations:
VLISM shocks have been associated with Langmuir waves and radio emission (Kurth et al. 1984; Gurnett et al. 1993) observed in situ by Voyager (Gurnett at al.,2021). Langmuir waves and radio emissions at the plasma frequency fp∼ 2-3 kHz can be driven by electron beams accelerated at propagating shocks. However, this requires a mechanism to prime (pre-accelerate) electrons.
A long-prevailing theory attributes electron-priming to the lower-hybrid mechanism according to which lower-hybrid waves (LHWs) are driven by the instability of ring-beam pickup ion (PUI) distributions that prime electrons (Cairns & Zank, 2002). However, this instability is effective in transferring energy to electrons only if Vr/VA < 5, where Vr and VA are the PUI ring-beam speed and the Alfvén speed, respectively (Omelchenko 1989).
Voyager measurements suggest the Alfvén speed in the OHS is 25-45 km/s at V1 and 60-80 km/s at V2, implying that these theoretical criteria for efficient energization of electrons may not be satisfied (Pogorelov et al. 2021).
Moreover, only two of the electron plasma oscillation (EPO) events detected by V1 (in 2012.8 and 2014.35) and one by V2 (in 2020.5) were directly identified as precursors to shocks detected in magnetic field data. Some observed magnetic field jumps, possibly shocks, were not accompanied by plasma oscillations. Conversely, several EPO events have been detected far from observed shocks. We aim to resolve these apparent discrepancies by building on the foreshock models of Cairns & Zank (2002) and Gurnett et al (2020).
Our proposed study addresses fundamental questions about the nature of shock-related plasma wave activity: Is the LHW priming really necessary? What is the electron distribution function? What mechanisms accelerate electrons at shocks? Are electrons hot enough for VLISM shocks to create electron beams? To what distances can these beams travel? What is the extent of the foreshock?
In addition, a weak, continuous emission line was recently discovered in PWS spectrograms (Ocker et al., 2021a). Its nature is still not entirely clear: Gurnett et al. (2021) suggested that the emission is driven by a substantial fraction of suprathermal electrons that significantly contribute to the total pressure and excite Langmuir waves, whereas Meyer-Vernet (2022) attribute the observed line to a small proportion of suprathermal electrons that contribute negligibly to the pressure and suggest that, near the heliopause, compressive fluctuations prevent the line to be detected, by moving its frequency too much during the time of integration. We will use the latest available Voyager data to constrain the suprathermal electron component. The analysis of this proposed ISSI team may lead to important estimates of the electron distribution function in the VLISM.
3) The Nature and Role of Turbulence in the VLISM:
Voyager has thus far provided the only constraints on the interstellar turbulence power spectrum at kinetic and dissipative scales, which cannot be accessed with other remote probes of the ISM. The magnetic turbulence observed by Voyager in the VLISM is low-intensity, compressible, and inhomogeneous (Burlaga et al., 2013). It varies with distance and is enhanced in compressed plasma regions behind shocks/pressure waves (Fraternale et al., 2022). VLISM turbulence appears to be forced by the SW-driven motion of the HP and superimposed on LISM turbulence (Zank et al, 2017, Fraternale et al. 2021a). This scenario is supported by polarization data (Frisch et al. 2015b) and electron density spectra derived from V1/PWS data, which imply the local VLISM spectrum is amplified by a factor ∼100 compared to the remote LISM (Lee & Lee, 2020, Ocker et al., 2021a). Solar cycle periodicity and the heliosphere’s size likely determine the turbulence outer scale.
The range of magnetic spectral indices at low frequencies (<≈10-6 Hz) hint at the presence of “N’’-wave profiles and Burgers-like turbulence instead of a classic Kolmogorov-like cascade (Fraternale et al. 2021a).
The existence of Burgers turbulence may have important implications for particle acceleration and the establishment of a f(p)∼ p-5 tail distribution. An energy spectrum E⊥ ∼ k⊥-2 is also characteristic of wave turbulence phenomena, which cannot be excluded in the VLISM. Wave turbulence approximation will be tested with MAG data by studying intermittency.
Data suggest that part of the inertial range of turbulence lies at scales smaller than the scales where collisionless effects may become dominant L⊥ ∼ (λpp rc,p)1/2∼10-3 AU, L||∼λpp∼ 0.5-5 AU.
If the HP motion creating VLISM shocks is fast enough, the initial shock thickness may lie in this moderately collisional regime. Another possibility is that shocks are born at some distance from the HP by wave steepening of broad compression waves. The problem of wave steepening and shock formation is therefore of great interest. In addition, kinetic turbulence associated with the instability of PUI distributions may exist on scales smaller than ∼10,000 km. The presence of turbulence and wave activity might make the structure of shocks intrinsically nonstationary, and the turbulent Mach number may be as large as 0.1. Intermittent structures are typically present in front and behind collisionless shocks (Alexandrova et al. 2004) and have been observed in the VLISM (Fig. 1c). We will investigate their presence and role in trapping and accelerating charged particles.
We will investigate whether turbulence and intermittent structures affect the generation of plasma waves and radio emissions by modifying (i) the large-scale and local shock properties, or (ii) the propagation of electron beams through scattering or by modification of the electron distribution function via turbulent acceleration and heating.
The goal of this project is to develop new theoretical explanations for Voyager MAG and PWS measurements of shocks, turbulence, Langmuir waves and radio emissions and verify them with extensive numerical modeling.
The objectives of the proposal are:
- To identify the physical mechanisms and quantitative criteria for electron pre-acceleration at turbulence-modulated shocks.
- To investigate the 3-D properties and nature of compressible VLISM turbulence and its role in mediating shocks and providing the observed in-shock dissipation.
- To examine the structure of foreshock regions, the generation and propagation of electron beams, and their relationship to Langmuir waves and radio emissions observed by Voyager.
- To study the generation of Langmuir waves and radio emissions observed by V1 and V2.
- To determine the distance at which solar-origin shocks completely dissipate in the VLISM.
- To analyze the role of suprathermal ions and turbulence in the continuous plasma wave emission line discovered in the LISM at the electron plasma frequency.
- To reconcile the properties of VLISM turbulence with remote observations of the larger LISM.