棕矮星相干回旋辐射中的闪光电离特征
Flash ionisation signature in coherent cyclotron emission from Brown Dwarfs
摘要：
棕矮星在其大气层中形成矿物质云，带电粒子可以产生闪电形式的大规模放电，导致局部电离的大幅突然增加。观测到棕矮星会发出回旋射电辐射。我们表明，强瞬态大气电离事件（闪光电离）的签名可以印在预先存在的辐射。对这种闪光电离事件的探测将开启对电离状态和大气动力学的研究。这种电离事件也可能是爆炸冲击波、爆发或喷发造成的。我们提出了一个分析模型，它描述了调制的一个预先存在的电磁辐射的时间依赖性（闪光）的电导率，是闪光电离事件，如闪电的特性。我们的电导率模型再现了来自地面伽玛射线闪光观测的电导率函数，并适用于在本地电离强的时间变化的天体，如在行星大气和原行星盘。我们表明，该领域响应与特征的闪光形状的脉冲的导电性闪光的中间强度。更强大的电离事件导致初始辐射的较小变化，或其衰减。我们表明，高功率初始辐射的响应字段的特征阻尼进行有关的电离闪光的幅度和持续时间的信息。对于较大的电导率变化，脉冲放大或阻尼的持续时间始终较短，并且可以用于评估闪光电离的强度。我们的工作表明回旋辐射可以作为探测BD大气带电过程的探测信号。
英文摘要：
Brown dwarfs form mineral clouds in their atmospheres, where charged particles can produce large-scale discharges in form of lightning resulting in a substantial sudden increase of local ionisation. Brown dwarfs are observed to emit cyclotron radio emission. We show that signatures of strong transient atmospheric ionisation events (flash ionisation) can be imprinted on a pre-existing radiation. Detection of such flash ionisation events will open investigations into the ionisation state and atmospheric dynamics. Such ionisation events can also result from explosion shock waves, bursts or eruptions. We present an analytical model that describes the modulation of a pre-existing electromagnetic radiation by a time-dependent (flash) conductivity that is characteristic for flash ionisation events like lightning. Our conductivity model reproduces the conductivity function derived from observations of Terrestrial Gamma Ray Flashes, and is applicable to astrophysical objects with strong temporal variations in the local ionization, as in planetary atmospheres and protoplanetary disks. We show that the field responds with a characteristic flash-shaped pulse to a conductivity flash of intermediate intensity. More powerful ionisation events result in smaller variations of the initial radiation, or in its damping. We show that the characteristic damping of the response field for high-power initial radiation carries information about the ionisation flash magnitude and duration. The duration of the pulse amplification or the damping is consistently shorter for larger conductivity variations and can be used to evaluate the intensity of the flash ionisation. Our work suggests that cyclotron emission could be probe signals for electrification processes inside BD atmosphere.
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