1 Introduction
The majority of astrophysical plasmas are likely to be permeated with cosmic rays (CRs); these include protoplanetary disks, the interstellar and circumgalactic and intracluster media. In the Milky Way, the CR energy density (dominated by a CR population at around GeV energies) is in equipartition with the average thermal and magnetic energy densities (Boulares & Cox Reference Boulares and Cox1990). Hence, these CRs constitute an essential non-thermal component that provides dynamical feedback to the interstellar medium (Girichidis et al. Reference Girichidis, Naab, Walch, Hanasz, Mac Low, Ostriker, Gatto, Peters, Wünsch, Glover, Klessen, Clark and Baczynski2016, Reference Girichidis, Naab, Hanasz and Walch2018; Simpson et al. Reference Simpson, Pakmor, Marinacci, Pfrommer, Springel, Glover, Clark and Smith2016, Reference Simpson, Pakmor, Pfrommer, Glover and Smith2023; Farber et al. Reference Farber, Ruszkowski, Yang and Zweibel2018) and can launch galaxy-scale outflows, as demonstrated in one-dimensional models (Ipavich Reference Ipavich1975; Breitschwerdt, McKenzie & Voelk Reference Breitschwerdt, McKenzie and Voelk1991; Recchia, Blasi & Morlino Reference Recchia, Blasi and Morlino2016; Quataert, Jiang & Thompson Reference Quataert, Jiang and Thompson2022) as well as in three-dimensional simulations of galaxies forming both in isolation (Uhlig et al. Reference Uhlig, Pfrommer, Sharma, Nath, Enßlin and Springel2012; Salem & Bryan Reference Salem and Bryan2014; Pakmor et al. Reference Pakmor, Pfrommer, Simpson and Springel2016; Ruszkowski, Yang & Zweibel Reference Ruszkowski, Yang and Zweibel2017; Thomas, Pfrommer & Pakmor Reference Thomas, Pfrommer and Pakmor2023) and in cosmological environments (Salem, Bryan & Hummels Reference Salem, Bryan and Hummels2014; Buck et al. Reference Buck, Pfrommer, Pakmor, Grand and Springel2020; Hopkins et al. Reference Hopkins, Chan, Garrison-Kimmel, Ji, Su, Hummels, Kereš, Quataert and Faucher-Giguère2020). The propagation of CRs with energies below 100 GeV is believed to be predominantly governed by self-generated magnetic perturbations (Blasi, Amato & Serpico Reference Blasi, Amato and Serpico2012; Evoli et al. Reference Evoli, Blasi, Morlino and Aloisio2018). These perturbations efficiently scatter the CRs, resulting in a significant decrease in their mean transport speed. The interplay of CR-driven growth of plasma waves and collisionless wave damping processes determines the effective transport speed of CRs, and hence the coupling strengths to the ambient plasma: strong scattering causes CR isotropisation in the Alfvén wave frame and forces CRs to stream at mean speeds close to the Alfvén speed while faster CR diffusion prevails in the case of weak scattering if the waves are strongly damped (Zweibel Reference Zweibel2017).
Most importantly, the strength of CR feedback critically depends on these microscopic transport properties, including mass and energy loading factors of galactic winds, the wind speed and the emerging CR pressure support in the circumgalactic medium (Buck et al. Reference Buck, Pfrommer, Pakmor, Grand and Springel2020; Ji et al. Reference Ji, Chan, Hummels, Hopkins, Stern, Kereš, Quataert, Faucher-Giguère and Murray2020). Likewise, the evolution of gas and chemical compositions in diffusive regions and dense interstellar clouds are affected by CR ionisation to the extent that CRs are the fundamental source of residual ionisation inside shielded molecular clouds (Phan, Morlino & Gabici Reference Phan, Morlino and Gabici2018). Because of the sensitive dependence of the emerging galaxies and the phase structure of the interstellar medium on CR feedback strength, the investigation of CR-driven instabilities becomes of paramount importance in order to attain predictive capabilities in simulation campaigns. Thus, understanding and accurately modelling these instabilities is crucial for effectively regulating the transport of CRs in interstellar, circumgalactic and intracluster plasmas.
Our focus here is on instabilities of parallel wave modes along the background magnetic field. This is for two reasons, which follow for astrophysical plasmas, where the CR density is much lower compared with the background plasma. First, when considering the full spectra of CR driven waves, the growth rates due to resonance with hydrodynamical waves are highest for parallel wave mode, as demonstrated in (4) of Kulsrud & Pearce (Reference Kulsrud and Pearce1969). Second, even if oblique waves, which propagate at an angle to the magnetic field, were to grow, these waves would be strongly Landau damped by thermal background ions, especially in high plasma beta conditions, i.e., where the thermal energy is greater than the magnetic energy. Foote & Kulsrud (Reference Foote and Kulsrud1979) estimated the damping rate and showed that it is significantly faster than the typical growth rates of obliquely propagating wave modes (see (68) of Zweibel (Reference Zweibel2017) for a concise expression of the damping rate).
In CR-driven instabilities, CR ions represent the primary source of free energy. This energy is channelled through these instabilities into unstable electromagnetic wave modes of the background plasma. This is in contrast to beam-plasma instabilities (Breǐzman, Ryutov & Chebotaev Reference Breǐzman, Ryutov and Chebotaev1972; Bret, Gremillet & Dieckmann Reference Bret, Gremillet and Dieckmann2010b; Chang et al. Reference Chang, Broderick, Pfrommer, Puchwein, Lamberts and Shalaby2014; Shalaby et al. Reference Shalaby, Broderick, Chang, Pfrommer, Lamberts and Puchwein2017b), where the source of free energy is an electron–positron beam that drives wave modes on the electron skin depth scale or shorter unstable. By contrast, CR-driven instabilities excite unstable electromagnetic wave modes on scales much larger than the electron skin depth. These instabilities can be classified generally as follows. First, there are non-resonant instabilities, such as the Bell instability (Bell Reference Bell2004), which occur when the CR current is very high. These instabilities are highly relevant for studying the escape of CR ions after acceleration at supernova remnant shocks. Second, there are resonant instabilities, which occur when the CR current is low but the CR mean drift is faster compared with the local ion Alfvén speed. Within galactic and stellar environments, CR transport (with energies below 100 GeV) is believed to be primarily regulated by two dominant resonant instabilities: the gyro-resonant instability (Kulsrud & Pearce Reference Kulsrud and Pearce1969; Lebiga, Santos-Lima & Yan Reference Lebiga, Santos-Lima and Yan2018; Bai et al. Reference Bai, Ostriker, Plotnikov and Stone2019; Holcomb & Spitkovsky Reference Holcomb and Spitkovsky2019; Bai Reference Bai2021; Bambic, Bai & Ostriker Reference Bambic, Bai and Ostriker2021; Plotnikov, Ostriker & Bai Reference Plotnikov, Ostriker and Bai2021) and the recently discussed intermediate-scale instability (Shalaby, Thomas & Pfrommer Reference Shalaby, Thomas and Pfrommer2021; Lemmerz et al. Reference Lemmerz, Shalaby, Thomas and Pfrommer2023). Provided CRs propagate with a finite pitch angle relative to the large-scale magnetic field, they induce an instability in electromagnetic waves (propagating along the background magnetic field) on scales intermediate between the gyroradii of ions and electrons. This instability occurs as long as CRs drift at velocities less than half of the Alfvén speed of the electrons. The emerging unstable modes are identified as background ion-cyclotron modes in the reference frame co-moving with the CRs. Interestingly, this newly found instability typically exhibits significantly faster growth, exceeding the growth rate of the commonly discussed resonant instability at the ion gyroscale by more than an order of magnitude (Shalaby et al. Reference Shalaby, Thomas and Pfrommer2021).
We focus, moreover, on instabilities in the cold limit for background plasma species. While it remains to be shown analytically and numerically that this assumption does not impact the nature of the instabilities we study in this paper, the unstable wave modes are typically present at scales much larger than the electron skin depth (which is typically larger than both ion and electron Debye lengths) and of an electromagnetic nature. Thus, background temperatures are typically argued to have no impact on such long-wavelength unstable wave modes (Zweibel Reference Zweibel2003; Bell Reference Bell2004; Shalaby et al. Reference Shalaby, Thomas and Pfrommer2021). This is in contrast to beam-plasma instabilities that can be greatly impacted by thermal effects (Bret, Gremillet & Bénisti Reference Bret, Gremillet and Bénisti2010a; Chang et al. Reference Chang, Broderick, Pfrommer, Puchwein, Lamberts, Shalaby and Vasil2016; Shalaby Reference Shalaby2017) or structures (Krafft, Volokitin & Krasnoselskikh Reference Krafft, Volokitin and Krasnoselskikh2013; Shalaby et al. Reference Shalaby, Broderick, Chang, Pfrommer, Lamberts and Puchwein2018, Reference Shalaby, Broderick, Chang, Pfrommer, Puchwein and Lamberts2020) in the background plasma on scales close to the electron skin depth. The intermediate-scale instability can also play a significant role in electron acceleration within non-relativistic shocks (Shalaby et al. Reference Shalaby, Lemmerz, Thomas and Pfrommer2022). This study focuses on unravelling the physical origins of these resonant instabilities, particularly in the context of CRs with a gyrotropic momentum distribution. Through a transparent visualisation of the fundamental mechanisms, we gain a deeper understanding of the behaviour and characteristics of these instabilities and their influence on CR transport.
The structure of the paper is as follows. First, in § 2, we analyse the normal modes supported by a magnetised electron–ion plasma system and investigate how these wave modes are affected when an additional population of electron–ion CRs, characterised by a relative drift speed, is included. This inclusion leads to the emergence of additional Doppler-shifted CR wave modes, and the interaction between the background and CR ion wave modes gives rise to resonant instabilities, with the maximum growth rate occurring at the wavelengths where these modes resonantly interact. Next, in § 3, we demonstrate that approximating the background plasma using either magnetohydrodynamics (MHD) or Hall-MHD erroneously overlooks the fastest-growing modes in the linear regime. This highlights the importance of considering the full dynamics of the system down to the electron scale. Furthermore, in § 4, we explore the impact of the driven wave modes on particle–wave scattering and how the presence of these wave modes influences the particle trajectories. Finally, in § 5 we summarise our findings and present an outlook for potential implications in § 6. Throughout this work, we use the SI system of units.
2 Electromagnetic linear dispersion relation
In a magnetised plasma with various species $s$, the linear dispersion relation for electromagnetic wave modes of (complex) frequency $\omega$ and wave mode $k$ that propagate parallel to a constant background magnetic field (here taken to be ${{\boldsymbol B}}_0 = B_0 \hat {\boldsymbol {x}}$) is given by (Schlickeiser Reference Schlickeiser2002)
Here, $\chi _s^{\pm }$ is the linear response for species $s$, which is characterised by charge $q_s$, mass $m_s$ and the equilibrium gyrotropic phase-space distribution, $f_{s,0}$. The light speed in vacuum is denoted by $c$ and the spatial part of the 4-velocity is ${{\boldsymbol u}} =\gamma \boldsymbol{\upsilon}$, where $u_{\parallel }$ and $u_{\perp }$ are the parallel and perpendicular velocities, respectively, which are defined with respect to the direction of the uniform background magnetic field ${{\boldsymbol B}}_0$. The magnitude of the velocity is defined such that $u^2 = u_{\parallel }^2 + u_{\perp }^2$ and $\gamma =\sqrt {1+u^2/c^2}$. The non-relativistic cyclotron frequency of species $s$ is $\varOmega _{s,0} = q_s B_0/ m_s$, and the relativistic one is $\varOmega _s = \varOmega _{s,0} / \gamma$.
For simplicity, we take $f_{s,0}$ to be a gyrotropic ring distribution with fixed parallel and perpendicular velocities for all particles of species $s$, i.e. $f_{s,0}(u_{\parallel },u_{\perp }) = n_s \delta (u_{\parallel } - \gamma \upsilon_{\rm dr}) \delta (u_{\perp } - \gamma \upsilon_{\perp })/ (2 \pi u_{\perp })$, where $n_s$ is a uniform density of the plasma species $s$, and all of its particles are drifting along ${{\boldsymbol B}}_0$ with constant speed $\upsilon_{{\rm dr},s}$ and have the same perpendicular velocity $\upsilon_{\perp,s}$. In this case, the linear response reduces to $\zeta _s^{\pm }$ given by (Holcomb & Spitkovsky Reference Holcomb and Spitkovsky2019; Weidl, Winske & Niemann Reference Weidl, Winske and Niemann2019; Shalaby et al. Reference Shalaby, Thomas and Pfrommer2021)
Here, $\omega _{s} = \sqrt { n_{ s} q^2_{s}/(m_{ s} \epsilon _0) }$ is the plasma frequency of species $s$ and $\epsilon _0$ is the permittivity of free space. The solutions of $D^{\pm }=0$ correspond to right/left polarisation states, and the corresponding first-order perturbed electric and magnetic fields obey $E_{y,1}(k,\omega ) \pm i E_{z,1}(k,\omega ) =0$ and $B_{y,1}(k,\omega ) \pm i B_{z,1}(k,\omega ) =0$, respectively. In the analysis below, the velocity distributions for both the background plasma and CR species are graphically illustrated in figure 1; a cold distribution for background electrons and ions and a gyrotropic distribution for CRs.
2.1 Normal modes of electron–ion plasma
The normal modes of an electron–ion plasma are well known and extensively studied in the literature (see, e.g. Boyd & Sanderson Reference Boyd and Sanderson2003). In the cold limit (also called the cold-hydrodynamic (HD) limit), one can obtain the plasma rest-frame wave modes by solving the following dispersion relation:
where $\zeta _{\rm i(e)}$ denotes the contribution of the cold stationary ion (electron) species with a fixed uniform number density $n_0$. Therefore, for $k>0$, the real part of the solutions, $\omega _{\rm r}={\rm Re}(\omega )>0 ~ (<0)$, indicate that the sense of rotation of the magnetic eigenmodes is the same as that of electrons (ions) around the constant background magnetic field ${{\boldsymbol B}}_0$. The normal modes for such a case include light characteristics ($\omega _{\rm r} = k c$) in the short-wavelength regime, which turn into $\omega _{\rm r} \sim \omega _{\rm p}$ for small values of $k$, where $\omega _{\rm p} = \sqrt {\omega _{\rm i}^2+\omega _{\rm e}^2}$ is the total plasma frequency.
The other set of wave modes is the electron- and ion-cyclotron waves in the short-wavelength regime, $k d_{\rm i} \ll 1$, where $d_{\rm i}=c/\omega _{\rm i}$ is the ion skin depth. More precisely, the electron-cyclotron branch includes the forward-propagating Alfvén waves ($\omega _{\rm r} = k \upsilon_\mathrm {A}$) for $k d_{\rm i} \ll 1$, where $\upsilon_\mathrm {A} = B_0/\sqrt { \mu _0 n_{\rm i} m_{\rm i}} = \varOmega _{\rm i,0} c/\omega _{\rm i}$ is the Alfvén speed for ions. This branch turns into whistler waves for shorter wavelengths at around $k d_{\rm i} > 1$, which become electron-cyclotron waves with $\omega _{\rm r} = |\varOmega _{\rm e}|$ for $k d_{\rm e} = k d_{\rm i}/\sqrt {m_{\rm r}} >1$, where $d_{\rm e} = c /\omega _{\rm e}$ is the electron skin depth. The ion-cyclotron branch is conceptually simpler: it includes the backward-propagating Alfvén waves for $k d_{\rm i} \ll 1$ and the ion-cyclotron wave modes with $\omega _{\rm r} = - \varOmega _{\rm i}$ for $k d_{\rm i} \geq 1$. An example for the electron- and ion-cyclotron branches is depicted by the black solid lines in the top panel of figure 2.
2.2 Resonances in the presence of drifting low-density CRs
Whether a drifting electron–ion plasma population (CR population) can excite resonant plasma instabilities is most easily seen by identifying the intersection points of background and CR wave modes in the $\omega$-$k$ plane: this yields solutions of wave modes for which the rotation rate of both background and CRs are in resonance. Thus, it is for these wave modes that we expect the largest energy exchange between CRs and the background plasma populations. In the following, we take a closer look at these resonant interactions in the case of CRs with a low number density, $n_{\rm cr}$, in comparison with the density of the background plasma $n_0$. We characterise this by the CR-to-background number density ratioFootnote 1 $\alpha \equiv n_{\rm cr}/n_0 \ll 1$ and adopt a mean relative drift speed $v_{\rm dr}$ of the CR population along the large-scale background magnetic field. In the background frame, one can derive the expected CR modes by solving the dispersion relation
As anticipated, one solution of (2.5) is the CR ion-cyclotron branch, which is comparable to that of the background, albeit with a Doppler-shifted rotation frequency, that is $\omega \rightarrow \omega + k \upsilon_{\rm dr}$. Additionally, in the ion-cyclotron branch, the backward-propagating Alfvén waves start to rotate at the Doppler-shifted ion-cyclotron frequency at $k d_{\rm cr,i} = k d_{\rm i}/ \sqrt {\alpha }\sim 1$. In other words, within the ion-cyclotron branch, wave modes rotate with the Doppler-shifted ion-cyclotron frequency at $k d_{\rm i} \gtrsim \sqrt {\alpha }$, which is much less than unity. This means that, as observed from the background, this branch mainly produces waves rotating with Doppler-shifted ion-cyclotron frequency $\omega _{\rm r} = - \varOmega _{\rm i} + k \upsilon_{\rm dr}$. We refer to this mode as the CR ion-cyclotron wave mode.
To verify that the individual wave modes of CRs and those of the background plasma are indeed the sum of the background and CR wave modes as seen in the background frame, we solve the full dispersion relation
Note that, in our set-up, the zeroth-order current due to CR ions is compensated by CR electrons drifting at the same speed. Here, we have added the possibility for CR populations to have a non-zero perpendicular velocity. That is, CRs are distributed uniformly on a ring in the perpendicular velocity space, characterised by a radius of $\upsilon_{\perp }$. Solutions for the full dispersion relation are shown in figure 2, where we use a reduced mass ratio $m_{\rm r} = m_{\rm i}/m_{\rm e} = 36$ for visual purposes to reduce the separation between electron- and ion-cyclotron frequencies; $\sqrt {m_{\rm r}}/2 = 3$. We also adopt an ion Alfvén speed $\upsilon_\mathrm {A}$ at $10^{-4}c$, the density ratio $\alpha = n_{\rm cr}/n_0 = 10^{-6}$, $\upsilon_{\perp, {\rm e}}=0$ and $\upsilon_{\perp,{\rm i}} = \upsilon_\mathrm {A}$. We use different values of $\upsilon_{\rm dr}/\upsilon_\mathrm {A} \approx \{3.7, 3, 2.7\}$, such that various interesting types of resonances are expected in the solutions of the dispersion relation. We note that this choice of parameters enables us to show the physics of resonance on a linear scale in the background plasma frame. In § 2.3, we adopt more physically motivated parameters and show the resulting solutions of the dispersion relation in the CR drift frame.
For all values of $\upsilon_{\rm dr}$, we obtain the same exact background wave modes as described in the previous section, which are shown as black lines in the top panel of figure 2. Differing values of $\upsilon_{\rm dr}$ result in various CR ion-cyclotron wave modes, which are shown as different colours and which exactly agree with $\omega _{\rm r} = - \varOmega _{\rm i} + k \upsilon_{\rm dr}$ expected for the various cases. In the bottom panel of figure 2, we show the fastest growth rate obtained by solving the full dispersion relation in (2.6). The real part of the most unstable modes are the CR ion-cyclotron waves that we show with different colours and line styles in the top panel of figure 2. That is, in the unstable regions, only the rotating and propagating electromagnetic waves that are supported by the gyrating and drifting CRs are exponentially growing. We note that in regions where the fastest growth rate is zero, any branch can be chosen, and we select the CR ion-cyclotron wave mode in this case.
To find the locations where the CR ion-cyclotron wave modes are in resonance with the background wave modes, we solve
for wave mode $k$. That is, the solutions are those wave modes for which the CR ion-cyclotron branch intersects the background wave modes in the cold-HD limit. This gives a fourth-order polynomial which, in general, has four solutions for $k$ for any value $\upsilon_{\rm dr}/\upsilon_\mathrm {A}$, albeit these solutions may be degenerate.
For the case of $\upsilon_{\rm dr}/\upsilon_\mathrm {A} = 2.7 < \sqrt {m_{\rm r}}/2$ (solid-red curves in figure 2), there exist four locations where the CR ion-cyclotron wave modes are in resonance with the background wave modes, i.e. (2.7) has four distinct roots. These expected locations for resonances are indicated with red-dashed vertical lines in the top and bottom panels of figure 2: there are two resonances for $k d_{\rm i} <1$ and two resonances for $k d_{\rm i} >1$. The two solutions for $k d_{\rm i} <1$ represent the intersections of the CR ion-cyclotron wave mode with both forward- and backward-propagating Alfvén waves, and thus resonance occurs approximately at $k d_{\rm i} = \upsilon_\mathrm {A}/(\upsilon_{\rm dr} \pm \upsilon_\mathrm {A})$. This corresponds to the two resonant peaks in the growth rate due to the gyro-resonant (streaming) instability (Kulsrud & Pearce Reference Kulsrud and Pearce1969). The other two resonances at smaller scales at $k d_{\rm i} >1$ correspond to the two peaks in the growth rate due to the intermediate-scale instability (Shalaby et al. Reference Shalaby, Thomas and Pfrommer2021). In the next section, we show the growth rates when using realistic values of the mass ratio, for which the fastest growth rate due to the intermediate-scale instability is more than an order of magnitude greater than that at the gyroscales.
In figure 2, the purple dash-dotted curves represent the case of $\upsilon_{\rm dr}/\upsilon_\mathrm {A} \sim \sqrt {m_{\rm r}}/2$. We see that the two resonant peaks of the intermediate-scale instability at $k d_{\rm i} >1$ merge into a single resonance. As in the previous case, the two peaks of the gyroscale instability for $k d_{\rm i} <1$ remain, but move closer together. As $\upsilon_{\rm dr}/\upsilon_\mathrm {A}$ grows larger than $\sqrt {m_{\rm r}}/2$, the resonances at $k d_{\rm i} >1$ disappear, and thus, the intermediate-scale instability is no longer able to drive wave modes with $k d_{\rm i} > 1$ unstable (blue dashed curves in figure 2). That is, $\upsilon_{\rm dr}/\upsilon_\mathrm {A} \leq \sqrt {m_{\rm r}}/2$ is a condition for the possibility of resonance between CR ion-cyclotron wave modes and background wave modes at $k d_{\rm i} >1$, which is also the condition for the intermediate-scale instability.Footnote 2 The numerical identification of this instability condition was previously conducted by Shalaby et al. (Reference Shalaby, Thomas and Pfrommer2021).
It is important to acknowledge that, in a realistic environment, background plasmas possess a finite temperature. Consequently, when incorporating this into the dispersion relation, it can influence wave modes occurring on scales smaller than the ion Debye length, which is much smaller in comparison with the ion skin depth for non-relativistic plasmas (Reville et al. Reference Reville, Kirk, Duffy and O'Sullivan2008; Zweibel & Everett Reference Zweibel and Everett2010). Thus, it does not have any impact on the instabilities discussed in this paper. Furthermore, discussions and analysis of simulations in Shalaby et al. (Reference Shalaby, Thomas and Pfrommer2021) and Lemmerz et al. (Reference Lemmerz, Shalaby, Thomas and Pfrommer2023) reveal that ion-cyclotron thermal damping has negligible effects on the driven wave modes.
2.3 Instabilities in the rest frame of the CRs
In this section, we demonstrate that the intermediate-scale instability indeed drives CR comoving ion-cyclotron wave modes as found in Shalaby et al. (Reference Shalaby, Thomas and Pfrommer2021). To this end, we present the solution of the dispersion relation in the CR rest frame, where the background plasma is drifting with velocity $-\upsilon_{\rm dr}$ and hence drifting anti-parallel with respect to the background magnetic field ${{\boldsymbol B}}_0$. The dispersion relation that we solve is given by
Figure 3 shows some solutions of (2.8) for $\upsilon_\mathrm {A} = 10^{-4} c$, $\alpha = n_{\rm cr}/n_0 = 10^{-6}$,Footnote 3 $m_{\rm r} = m_{\rm i}/m_{\rm e} = 1836$, $v_{\perp, {\rm e}}=0$ and $\upsilon_{\perp, {\rm i}} = \upsilon_\mathrm {A}$, along with various background plasma drift speeds such that $\upsilon_{\rm dr}/\upsilon_\mathrm {A} \sim \{22.28, 21.45 , 19.281 \} \sim \{1.05, 1, 0.9 \} \sqrt {m_{\rm r}}/2$. In the top panel of that figure, the CR ion-cyclotron wave modes are shown as a black line for different values of $\upsilon_{\rm dr}$. Meanwhile, the background cyclotron wave modes are rotated differently depending on the drift speed, leading to intersections at both long ($kd_{\rm i} <1$) and short ($kd_{\rm i} >1$) wavelengths for $\upsilon_{\rm dr}/\upsilon_\mathrm {A} \leq \sqrt {m_{\rm r}}/2$. These intersections lead to instabilities at these wave modes. The bottom panel of figure 3 shows the growth rates of the ion-cyclotron wave modes, which are also the fastest-growing modes. This demonstrates that the driven modes are ion-cyclotron waves at all wavelengths in the rest frame of CRs. If the condition for the intermediate-scale instability is not fulfilled ($\upsilon_{\rm dr}/\upsilon_\mathrm {A} > \sqrt {m_{\rm r}}/2$), the instability no longer operates and short-wavelengths modes are stable.
The solutions in figure 3 show that, for realistic values of $m_{\rm r}$, the intermediate-scale instability growth rates significantly dominate over gyroscale growth rates. This dominance is even more pronounced at larger pitch angles because the growth rate at the peaks of the intermediate-scale instability is proportional to $(\upsilon_{\perp,{\rm i}}/\upsilon_\mathrm {A})^{2/3}$ (Shalaby et al. Reference Shalaby, Thomas and Pfrommer2021). Additionally, it is important to note that the growth rate is higher for the resonance with forward-propagating Alfvén waves at the gyroscale ($kd_{\rm i}<1$) compared with that with the backward-propagating waves.
3 Instabilities with approximate background plasma descriptions
In this section, we revisit the dispersion relation in the background plasma rest frame to investigate the effect of approximating the background plasma description on the nature of the emerging instabilities. One commonly used approximation for the background plasma dispersion assumes the MHD dispersion relation, where it is assumed that $\omega \ll \varOmega _{\rm i} \ll |\varOmega _{\rm e}|$. In this limit, the third and fourth terms in (2.6) are reduced to $\omega ^2 c^2/\upsilon_\mathrm {A}^2$ and the first term ($\omega ^2$) is neglected, due to the fact that $\upsilon_\mathrm {A}^2 \ll c^2 \Rightarrow \omega ^2 \ll \omega ^2 c^2/\upsilon_\mathrm {A}^2$ . The dispersion relation is for
This approximation is typical in works aiming at computing various types of CR-driven instabilities (see, e.g. Zweibel Reference Zweibel2003; Bell Reference Bell2004; Amato & Blasi Reference Amato and Blasi2009; Bai et al. Reference Bai, Ostriker, Plotnikov and Stone2019). Another possible approximation involves considering the impact of finite but small values of $k d_{\rm i}$, which are wave modes comparable to the ion skin depth, as is typically done in the Hall-MHD approximation. In this case, the dispersion relation approximates the behaviour of Alfvén and whistler waves found in the cold HD dispersion relation. Within the Hall-MHD approximation, the background plasma contributions (see, e.g. § 14.4.4 of Goedbloed, Keppens & Poedts Reference Goedbloed, Keppens and Poedts2010) are reduced to $\omega ^2 c^2/[\upsilon_\mathrm {A}^2 (1+ k d_{\rm i})^2]$, leading to the following modified dispersion relation:
In figure 4, we show the solutions of the dispersion relations with various background plasma assumptions, i.e. the solutions for ((2.6), (3.1) and (3.2)) using the following parameters: $\upsilon_\mathrm {A} = 10^{-4} c$, $\alpha = n_{\rm cr}/n_0 = 10^{-6}$, $m_{\rm i}/m_{\rm e} = 1836$, $v_{\perp, {\rm e}}=0$, $\upsilon_{\perp, {\rm i}} = \upsilon_\mathrm {A}$ and $\upsilon_{\rm dr}/\upsilon_\mathrm {A} = 0.98 \sqrt {m_{\rm r}}/2 \approx 20.99$. In all cases, we fix the CR drift speed, resulting in the same CR ion-cyclotron wave mode (shown as a black line in the top panel) that follows the expected dispersion relation $\omega _{\rm r} = k \upsilon_\mathrm {A} - \varOmega _{\rm i}$. When using the MHD background approximation, the background wave modes follow the expected dispersion relation $\omega _{\rm r} = \pm k \upsilon_\mathrm {A}$ (shown as orange dash-dotted curves in the top panel). In the case of Hall-MHD, the dispersion of background wave modes, as expected, follows $\omega _{\rm r} = \pm k \upsilon_\mathrm {A} (1+k d_{\rm i})$; shown as green dashed curves in the top panel.
In the bottom panel of figure 4, we show the growth rates of the CR ion-cyclotron waves (which are the fastest growth rates) for different approximations of the background plasma. At large (gyro) scales, all approximations produce similar growth rates peaking at the same wave modes, indicating that using any of these approximations for the background plasma leads to correctly capturing the gyroscale instability. However, at smaller scales, i.e. scales where the intermediate-scale instability operates ($kd_{\rm i}>1$), the MHD approximation of the background plasma dispersion wrongly predicts complete stability at these scales. In the case of Hall-MHD, the growth of the first peak of the intermediate-scale instability is approximately reproduced but at a longer wavelength. For shorter wavelengths, the use of the Hall-MHD approximation predicts completely wrong growth rates compared with those found when using the cold-HD dispersion for the background plasma. An additional worrisome implication arising from the use of the Hall-MHD approximation is its incorrect prediction that shorter-wavelength modes are universally unstable, even in cases where the condition for driving the intermediate-scale instability is not satisfied.
4 Nature of CR scattering at different resonant scales
So far, we have focused on the influence of gyrotropic CR populations in driving perpendicular electromagnetic perturbations. In the absence of such perturbations, particles follow trajectories characterised by a constant drift speed along the direction of the magnetic field vector, while simultaneously gyrating at their gyrofrequency. In this section, our goal is to evaluate the effects of these perturbations on the particle trajectories, specifically how particles are scattered by the induced parallel electromagnetic perturbations. To accomplish this, we examine the Lorentz force acting on a particle with velocity $\boldsymbol{\upsilon} =\{ \upsilon_{x} , \upsilon_{y}, \upsilon_{z} \}$, charge $q_\mathrm {s}$ and mass $m_\mathrm {s}$ caused by these perturbations. In Fourier space, along the particle trajectory, the momentum equation is
Here, we use the fact that ${{\boldsymbol k}} \parallel {{\boldsymbol B}}_0$ and both vectors are aligned with $\hat {\boldsymbol {x}}$. Furthermore, we utilise the relation ${{\boldsymbol k}} \boldsymbol {\times } \delta {{\boldsymbol E}}^k = \omega \delta {{\boldsymbol B}}^k$ to find $\delta {{\boldsymbol E}}^k = - \boldsymbol{\upsilon} _{\rm ph} \boldsymbol {\times } \delta {{\boldsymbol B}}^k$. This indicates that particle scattering in the parallel direction is significantly influenced by the phase difference between the particle's perpendicular velocities and the magnetic field perturbations. On the other hand, scattering of particles in the perpendicular direction crucially depends on the difference between the relative drift speed of the particles and the waves.
In previous sections, we found that the propagation of CRs destabilise waves at different wavelengths. The real frequency of the unstable wave modes, in the rest frame of the background, is always given by $\omega _{\rm r} = k \upsilon_{\rm dr} - \varOmega _{\rm i}$. Consequently, the phase velocity of the unstable waves can be expressed as $\upsilon_{\rm ph} = \upsilon_{\rm dr} - \upsilon_\mathrm {A}/(k d_{\rm i})$. At the gyroscale ($k d_{\rm i} < 1$), wave growth peaks at wave modes where $k d_{\rm i} \approx \upsilon_\mathrm {A}/(\upsilon_{\rm dr} \mp \upsilon_\mathrm {A})$. This leads to a phase velocity of approximately $\upsilon_{\rm ph} \approx \pm \upsilon_\mathrm {A}$ for forward ($+$) and backward ($-$) propagating Alfvén waves. On the other hand, the fastest growth due to the intermediate-scale instability occurs for $k d_{\rm i} > 1$. Since typically $\upsilon_{\rm dr} \gg \upsilon_\mathrm {A}$, the phase velocity of the driven unstable modes is approximately $\upsilon_{\rm ph} \approx \upsilon_{\rm dr}$. To summarise, the phase speed of the growing wave modes can be expressed as follows:
Therefore, although both the gyro and intermediate-scale instabilities are resonant instabilities, they lead to perturbations with distinct phase speeds. Due to the significant disparity in phase velocity between these two scales, the scattering of CR ions by these electromagnetic perturbations exhibits notable differences, as can be seen from (4.1). The wave modes driven by the intermediate-scale instability scatter particles that drive them only in the direction parallel to ${{\boldsymbol B}}_0$, since $\upsilon_{x} = \upsilon_{\rm dr} \sim \upsilon_{\rm ph}$. However, particles with different parallel drift speeds can be scattered both in parallel and perpendicular directions due to wave modes driven by the intermediate-scale instability. On the other hand, scattering occurs in both parallel and perpendicular directions at the gyroscale. It can be demonstrated from (4.1) that the gyroscale waves result in energy-conserving scattering in the frame of the driven Alfvén waves (Shalaby et al. Reference Shalaby, Thomas and Pfrommer2021).
5 Summary
This paper examines the physics of resonant instabilities driven by CR ions with a gyrotropic momentum distribution. These instabilities occur most rapidly when a resonance between the Doppler-shifted background and CR wave modes occurs. The Doppler shift arises from the relative drift between the background and CR plasma. The relative drift sets the location of resonances and hence the most unstable wavelengths of various instabilities. It leads to two important resonant instabilities: the gyroscale instability at large scales (Kulsrud & Pearce Reference Kulsrud and Pearce1969) and the recently found intermediate-scale instability (Shalaby et al. Reference Shalaby, Thomas and Pfrommer2021).
The gyroscale instability has peak growth rates when the CR ion-cyclotron wave mode resonates with both, forward- and backward-propagating Alfvén waves of the background plasma. In the background frame, the resonances occur when $- \varOmega _{\rm i} + k \upsilon_{\rm dr} \rightarrow \pm k \upsilon_\mathrm {A}$, which results in the most unstable wave modes at $k_{\rm g,i}^{\pm }d_{\rm i} = \upsilon_\mathrm {A} /(\upsilon_{\rm dr} \mp \upsilon_\mathrm {A})$. From figure 3, it is evident that resonance with the forward-propagating Alfvén wave leads to faster growth at larger wave modes $k_{\rm g,i}^+$, thus it is called the ion gyroscale. Since $\upsilon_{\rm dr} \gg \upsilon_\mathrm {A}$, both growth peaks at the gyroscales occur for wavelengths larger than the ion skin depth, i.e. $k_{\rm g,i}^{\pm }d_{\rm i} <1$. Therefore, in the linear regime, the gyroscale instability can be accurately described when approximating the electron–ion background plasma using either MHD or Hall-MHD approximations (see figure 4).
For an electron–ion background plasma, additional resonances occur at shorter wavelengths, where $k d_{\rm i} > 1$. These resonances give rise to the intermediate-scale instability that destabilise wave modes between the ion gyroscale, $k_{\rm g,i}^+$ and the electron gyroscale, $k_{\rm g,e} = m_{\rm r} k_{\rm g,i}^+$, where $m_{\rm r}$ represents the ion-to-electron mass ratio. At these resonant scales, the peak growth rates of the intermediate-scale modes are significantly larger compared with those at the gyroscales. This establishes the intermediate-scale instability as the fastest instability in the linear regime of the resonant CR-driven instabilities. Moreover, the dominance of the peak growth of the intermediate-scale instability is further amplified when CR ions possess larger pitch angles, corresponding to a larger perpendicular velocity ($\upsilon_{\perp }$) (see table 1 of Shalaby et al. Reference Shalaby, Thomas and Pfrommer2021).
As depicted in figure 4, the utilisation of the MHD approximation for describing the linear response of the background plasma hinders the occurrence of resonances at short wavelengths. Consequently, such an approximation suppresses the dominant, intermediate-scale instability. Conversely, employing the Hall-MHD approximation for the background plasma captures a resonance at $k d_{\rm i} > 1$, and, while the fastest growth rate associated with the intermediate-scale instability is approximately captured, it is associated with a wrong wavenumber. Additionally, this approximation erroneously predicts the presence of intermediate-scale instability even when the conditions for the instability are not met.
We argue in § 1 that background temperatures are unlikely to impact the growth of the instabilities addressed in this paper. Supporting evidence for this assertion can be found in the simulations conducted by Shalaby et al. (Reference Shalaby, Thomas and Pfrommer2021), where the background plasma was characterised by high temperatures, yet exhibited excellent agreement with the growth rates predicted from the dispersion relation assuming a cold background plasma. On the other hand, it is natural to contemplate whether these instabilities, particularly the newly discovered intermediate-scale instability, persist under different and potentially more realistic velocity distributions for CR ions. The fundamental explanation for the origin of these instabilities lies in the resonance between the ion-cyclotron wave modes of CR ions and the background wave modes. Consequently, any velocity distribution that supports CR ion-cyclotron modes while adhering to the instability conditions will excite this instability. This argument is supported by circumstantial evidence from particle-in-cell simulations of shocks conducted by Shalaby et al. (Reference Shalaby, Lemmerz, Thomas and Pfrommer2022), where the instability is clearly driven by CRs with a thermal velocity distribution.
Thus, while our choice of the CR ion distribution ensures the ease of repeatability in our analytical calculations, it does not imply that the instability is exclusively associated with such a choice of CR ion velocity distribution. The choice of a particular CR ion velocity distribution could impact the growth rates of different instabilities. However, qualitative considerations show that it is highly likely that the intermediate-scale instabilities will remain dominant even in these cases. An analytical demonstration of this is, however, deferred to future studies.
6 Outlook
The presence of the intermediate-scale instability is vital for the efficiency of electron acceleration in parallel electron–ion non-relativistic shocks as seen in fully kinetic particle-in-cell simulations (Shalaby et al. Reference Shalaby, Broderick, Chang, Pfrommer, Lamberts and Puchwein2017a, Reference Shalaby, Thomas and Pfrommer2021). Moreover, when the condition for the instability is not met in simulations, a notable decrease in the efficiency of electron acceleration is observed (Shalaby et al. Reference Shalaby, Lemmerz, Thomas and Pfrommer2022). That is, the significance of this instability extends beyond its potential to regulate the transport of CRs in various astrophysical scenarios. It could substantially influence particle injection and acceleration processes occurring at shocks, the escape of CRs from their sources into the interstellar medium and CR-driven galactic winds. This novel understanding of the fundamental physics underlying CR-driven resonant plasma instabilities will contribute to elucidate the critical role played by CRs in many astrophysical environments.
Acknowledgements
Editor Luís O. Silva thanks the referees for their advice in evaluating this article.
Declaration of interest
The authors report no conflict of interest.
Funding
The authors acknowledge support by the European Research Council under ERC-AdG grant PICOGAL-101019746.