Temperature Anisotropy Relaxation in Plasmas

• Temperature anisotropy relaxation is the process by which plasma with unequal parallel and perpendicular temperatures returns to isotropy via collisions, instabilities, and turbulent interactions.
• The mechanisms vary across regimes, with kinetic microinstabilities dominating in collisionless plasmas and Coulomb collisions in strongly coupled or unmagnetized conditions.
• Simulations and theory reveal that suprathermal effects and strong magnetization can alter relaxation times by orders of magnitude, influencing energy partition and stability in both astrophysical and laboratory settings.

Temperature anisotropy relaxation is the process by which a plasma with unequal parallel ($T_\parallel$) and perpendicular ($T_\perp$) kinetic temperatures with respect to the magnetic field returns toward isotropy on microphysical and macroscopic timescales. The relaxation can occur via Coulomb collisions, quasilinear wave–particle interactions, microinstabilities, or anomalous scattering, with the governing mechanisms and timescales strongly dependent on the plasma regime (collisionality, magnetization, particle distributions, and turbulence). Temperature anisotropy relaxation is fundamental in space, laboratory, and astrophysical plasmas, determining kinetic stability, transport, energy partition, and the persistence or dissipation of non-equilibrium states.

1. Fundamental Mechanisms and Regimes

In weakly coupled or collisionless plasmas, anisotropy is primarily relaxed by kinetic microinstabilities—most notably the electromagnetic ion-cyclotron (EMIC), proton firehose (PFH), and mirror instabilities—and by quasilinear interactions with electromagnetic turbulence (Shaaban et al., 13 Sep 2024, Moya et al., 2020, Shaaban et al., 2016, Santos-Lima et al., 2016). In the collisional, unmagnetized limit (one-component plasma, OCP), relaxation is governed by Coulomb scattering and tractable by kinetic theory or molecular dynamics (Baalrud et al., 2017). In strongly magnetized or partially magnetized regimes relevant to laboratory and non-neutral plasmas, the distinction between parallel and perpendicular temperature relaxation rates becomes critical; gyromotion introduces different collisional and wave–particle pathways for each component, often with orders-of-magnitude separation in relaxation timescales (Jose et al., 10 Nov 2024, Welch et al., 12 Nov 2025).

2. Kinetic Descriptions and Instability Thresholds

In space and astrophysical plasmas, temperature anisotropy is typically quantified by $A = T_\perp / T_\parallel$. Plasma microinstabilities can be triggered for sufficient excess or deficit of $A$ away from unity at given plasma beta ($\beta_\parallel = 8\pi n k_B T_\parallel / B^2$). For $A>1$, the parallel EMIC and mirror instabilities grow; for $A<1$, the PFH and aperiodic firehose modes dominate (Shaaban et al., 2016, Shaaban et al., 13 Sep 2024, Santos-Lima et al., 2016):

$$\begin{align*} &\text{Ion-cyclotron (parallel):}\qquad&&A > 1 + a\,\beta_\parallel^{-b}\ &\text{Mirror (oblique):}\qquad&&A > 1 + a'\,\beta_\perp^{-b'}\ &\text{Firehose (parallel):}\qquad&&A < 1 - a''\,\beta_\parallel^{-b''} \end{align*}$$

with empirical coefficients $a,b$ dependent on the mode, plasma composition, and suprathermal content (e.g., $a\approx 0.43$, $b\approx 0.42$ for EMIC (Moya et al., 2020); $a''\approx 0.61$, $b''\approx 0.63$ for PFH (Santos-Lima et al., 2016)).

The stability thresholds can be shifted by non-Maxwellian (kappa) velocity distributions and by electron–proton anisotropy correlations (Shaaban et al., 13 Sep 2024, Shaaban et al., 2016).

3. Relaxation Dynamics: Linear Theory, Quasilinear Evolution, and Simulation

Collisionless and Weakly Collisional Regimes

Linear kinetic theory predicts that growth rates of relevant instabilities ($\gamma$) reach a significant fraction of the proton gyrofrequency ($\gamma_{\mathrm{max}} \sim 0.05-0.2\,\Omega_p$), corresponding to e-folding relaxation times of $\tau_{\rm relax}\sim(5-20)\,\Omega_p^{-1}$—vastly faster than collisional or expansion timescales in the solar wind or ICM (Shaaban et al., 2016, Santos-Lima et al., 2016). Quasilinear theory and simulations show that once unstable, these microinstabilities generate growing electromagnetic fluctuations ($\delta B$) which scatter particle pitch angles, rapidly driving $A$ back toward marginal stability.

Hybrid simulations and quasilinear calculations demonstrate accelerated and deeper anisotropy relaxation in the presence of suprathermal (bi-Kappa) populations, driven by higher $\gamma$ and amplified fluctuation power. For example, in an EMIC regime with $A_p(0)=6$, $\beta_{p\parallel}=0.1$, bi-Kappa distributions ($\kappa=3$) yield a relaxation time $\tau_{\rm relax}\approx80\,\Omega_p^{-1}$, a $\sim30\%$ reduction compared to $\tau_{\rm relax}\approx120\,\Omega_p^{-1}$ for a bi-Maxwellian, reflecting enhancement factors of 1.5–2 in growth rates and final fluctuation energy (Shaaban et al., 13 Sep 2024).

Collisional and Strongly Coupled OCP

In the OCP, molecular dynamics show exponential relaxation of anisotropy with a rate $\nu$, well described by the Landau formula at weak coupling ($\Gamma\lesssim0.1$), and increasingly by effective-potential theory at moderate coupling ($0.1\lesssim\Gamma\lesssim10$). Strongly coupled plasmas exhibit oscillatory structure and memory effects. Markovian kinetic theories fail to capture these features at high $\Gamma$ (Baalrud et al., 2017).

4. Turbulence-Driven Anisotropy and Self-Regulation

Background electromagnetic turbulence can generate temperature anisotropy even in nominally stable, isotropic plasmas. Quasilinear kinetic theory for a turbulent spectrum shows that damping of high-$k$ fluctuations (e.g., at $k\rho_i\sim2-5$) channels energy into $T_\perp$, promoting $T_\perp>T_\parallel$ and reducing $\beta_\parallel$. Sufficiently energetic tails ($W_B/B_0^2\gtrsim10^{-3}$) can drive $A$ above instability thresholds, at which point instabilities re-grow and regulate the distribution near marginality. The final state is a self-regulated equilibrium with $A\gtrsim1$ and reduced $\beta_\parallel$, matching in situ observations in the solar wind (Moya et al., 2020).

In the turbulent intracluster medium (ICM), the instability-driven (quasilinear) relaxation rate dramatically outpaces the anisotropy driving rate from large-scale AMHD turbulence by $12-14$ orders of magnitude, confining the plasma to values of $A$ arbitrarily close to the kinetic instability thresholds (Santos-Lima et al., 2016).

5. Suprathermal Effects and Species Interplay

Suprathermal tails, described by finite-$\kappa$ bi-Kappa distributions, lower the threshold for instability, enhance growth rates, and broaden unstable wavenumber bands for both EMIC and PFH modes. Hybrid simulations confirm that suprathermals accelerate and deepen anisotropy relaxation—reducing $\tau_{\rm relax}$ by up to 30\%, raising $\delta B^2$ by factors of 2–10, and driving $A$ closer to unity or even overshooting marginal stability (Shaaban et al., 13 Sep 2024).

Electron–proton anisotropy correlations further modify thresholds and relaxation. Direct or anti-correlation (parameterized by $\delta$ in $A_e=A_p^\delta$) can either inhibit or enhance instability development, shifting observed bounds in the $(A_p,\beta_{p\parallel})$ plane (Shaaban et al., 2016).

6. Strong Magnetization: Anisotropic Temperature Relaxation Rates

In strongly magnetized systems ($\beta\equiv\omega_c/\omega_p\gg1$), Coulomb collisions and wave–particle interactions become highly anisotropic. The parallel ($\nu_\parallel$) and perpendicular ($\nu_\perp$) temperature relaxation rates for a given species ($s$) can differ by orders of magnitude (Jose et al., 10 Nov 2024, Welch et al., 12 Nov 2025). Characteristic behavior is as follows:

• For electrons with $\beta_e\gg1$, ions weakly magnetized ($\beta_i\ll1$):
  • $\nu_{e\perp i}$ is suppressed by $2$–$3$ orders of magnitude ($\alpha_\perp^{ei}\ll1$), $\nu_{e\parallel i}$ modestly enhanced ($\alpha_\parallel^{ei}\sim1.1$–$1.2$).
  • Ion relaxation rates both modestly enhanced ($\sim1.2$–$1.3$).
• When both electrons and ions are strongly magnetized, ion–electron perpendicular rates collapse ($\alpha_\perp^{ie}\to0$ for $\beta_i\gg1$), parallel rates remain of order the unmagnetized value.

As a result, T$_{\parallel}$ equilibrates rapidly (on timescales $\sim1/\nu_\parallel$), while T$_{\perp}$ persists for much longer ($\sim1/\nu_\perp$), leading to long-lived temperature anisotropies (Jose et al., 10 Nov 2024, Welch et al., 12 Nov 2025).

Relaxation Dynamics and Scaling Example

$\beta_e$	$\nu_{e\parallel i}/\mu_0^{ei}$	$\nu_{e\perp i}/\mu_0^{ei}$
0.5	1.00	1.00
8.7	1.15	0.05
34	1.17	0.013

(Jose et al., 10 Nov 2024)

This anisotropy in temperature relaxation can impose stringent bottlenecks on plasma cooling strategies in traps and astrophysical settings, requiring explicit tracking of T$_\parallel$ and T$_\perp$ instead of assuming isotropy.

7. Implications, Applications, and Self-Regulation in Astrophysical and Laboratory Plasmas

The fast relaxation of temperature anisotropy in collisionless and weakly collisional plasmas via kinetic instabilities ensures that observed distributions in the solar wind and ICM remain tightly bounded near instability thresholds, explaining the apparent proximity to marginal stability observed in situ (Santos-Lima et al., 2016, Shaaban et al., 2016, Moya et al., 2020, Shaaban et al., 13 Sep 2024).

In laboratory contexts with strong magnetization (Penning–Malmberg traps, antihydrogen experiments), the vastly different T$_\parallel$ and T$_\perp$ relaxation timescales necessitate multi-temperature models. The slow relaxation of T$_\perp$ at high $\beta$ can limit the efficiency of sympathetic cooling and the achievement of ultracold conditions (Welch et al., 12 Nov 2025, Jose et al., 10 Nov 2024).

Temperature anisotropy relaxation, whether via Coulomb collisions, microinstabilities, or turbulence-mediated pathways, constitutes a core transport process and a key constraint on plasma non-equilibrium structure in both natural and experimental plasmas.