Hoppock Ian W, Chandran Benjamin D G, Klein Kristopher G, Mallet Alfred, Verscharen Daniel
Space Science Center, University of New Hampshire, Durham, NH, 03824, USA.
Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ, 85719, USA.
J Plasma Phys. 2018 Dec;84(6). doi: 10.1017/S0022377818001277. Epub 2018 Dec 19.
Stochastic heating refers to an increase in the average magnetic moment of charged particles interacting with electromagnetic fluctuations whose frequencies are much smaller than the particles' cyclotron frequencies. This type of heating arises when the amplitude of the gyroscale fluctuations exceeds a certain threshold, causing particle orbits in the plane perpendicular to the magnetic field to become stochastic rather than nearly periodic. We consider the stochastic heating of protons by Alfvén-wave (AW) and kinetic-Alfvén-wave (KAW) turbulence, which may make an important contribution to the heating of the solar wind. Using phenomenological arguments, we derive the stochastic-proton-heating rate in plasmas in which ∼ 1 - 30, where is the ratio of the proton pressure to the magnetic pressure. (We do not consider the ≳ 30 regime, in which KAWs at the proton gyroscale become non-propagating.) We test our formula for the stochastic-heating rate by numerically tracking test-particle protons interacting with a spectrum of randomly phased AWs and KAWs. Previous studies have demonstrated that at ≲1, particles are energized primarily by time variations in the electrostatic potential and thermal-proton gyro-orbits are stochasticized primarily by gyroscale fluctuations in the electrostatic potential. In contrast, at ≳ 1, particles are energized primarily by the solenoidal component of the electric field and thermal-proton gyro-orbits are stochasticized primarily by gyroscale fluctuations in the magnetic field.
随机加热是指与频率远低于粒子回旋频率的电磁涨落相互作用的带电粒子平均磁矩增加。当回旋尺度涨落的幅度超过某个阈值时，就会出现这种加热类型，导致垂直于磁场平面内的粒子轨道变得随机而非近似周期性。我们考虑了阿尔文波（AW）和动理学阿尔文波（KAW）湍流对质子的随机加热，这可能对太阳风的加热有重要贡献。利用唯象论证，我们推导出了质子压力与磁压力之比(\beta\sim1 - 30)的等离子体中的随机质子加热率。（我们不考虑(\beta\gtrsim30)的情况，在这种情况下，质子回旋尺度的KAWs变得非传播。）我们通过数值跟踪与一系列随机相位的AWs和KAWs相互作用的测试粒子质子，来检验我们的随机加热率公式。先前的研究表明，在(\beta\lesssim1)时，粒子主要通过静电势的时间变化获得能量，热质子的回旋轨道主要通过静电势的回旋尺度涨落而随机化。相比之下，在(\beta\gtrsim1)时，粒子主要通过电场的螺线管分量获得能量，热质子的回旋轨道主要通过磁场的回旋尺度涨落而随机化。
