Proton anisotropy in velocity space has been generally accepted as a major parameter for exciting electromagnetic ion cyclotron (EMIC) waves. In this study, we estimate the proton anisotropy parameter as defined by the linear resonance theory using data from the Van Allen Probes mission. Our investigation uses the measurements of the inner magnetosphere (L < 6) from January 2013 to February 2018. We find that the proton anisotropy is always clearly limited by an upper bound and it well follows an inverse relationship with the parallel proton β (the ratio of the plasma pressure to the magnetic pressure) within a certain range. This upper bound exists over wide spatial regions, AE conditions, and resonance energies regardless of the presence of EMIC waves. EMIC waves occur when the anisotropy lies below but close to this upper bound within a narrow plasma β range: The lower cutoff β is due to an excessively high anisotropy threshold and the upper cutoff β is possibly due to the predominant role of a faster‐growing mirror mode instability. We also find that the anisotropy during the observed EMIC waves is unstable, leading to the linear ion cyclotron instability. This result implies that the upper bound of the anisotropy is due to nonlinear processes.

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磁层内质子各向异性的上限及其与电磁回旋波的关系

速度空间中的质子各向异性已被普遍接受为激发电磁离子回旋加速器（EMIC）波的主要参数。在这项研究中，我们使用Van Allen Probes任务的数据估算由线性共振理论定义的质子各向异性参数。我们的研究使用了 2013年1月至2018年2月内部磁层（L <6）的测量值。我们发现，质子各向异性总是明显受上限限制，并且与平行质子β呈反比关系（等离子压力与电磁压力之比）在一定范围内。该上限存在于宽的空间区域，AE条件和共振能量中，而与EMIC波的存在无关。当各向异性在狭窄的血浆β范围内但低于该上限但又接近该上限时发生EMIC波：较低的β截止值是由于各向异性阈值过高而较高的β值可能是由于快速增长的主要作用镜像模式不稳定。我们还发现，在观测到的EMIC波中，各向异性是不稳定的，从而导致线性离子回旋加速器不稳定。该结果暗示各向异性的上限是由于非线性过程造成的。