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Saturn's ULF wave foreshock boundary: Cassini observations

机译：土星的ULF波前震边界：卡西尼号观测

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摘要

Even though the solar wind is highly supersonic, intense ultra-low frequency (ULF) wave activity has been detected in regions just upstream of the bow shocks of magnetized planets. This feature was first observed ahead of the Earth's bow shock, and the corresponding region was called the ULF wave foreshock, which is embedded within the planet's foreshock. The properties as well as the spatial distribution of ULF waves within the Earth's foreshock have been extensively studied over the last three decades and have been explained as a result of plasma instabilities triggered by solar wind ions backstreaming from the bow shock. Since July 2004, the Cassini spacecraft has characterized the Saturnian plasma environment including its upstream region. Since Cassini's Saturn orbit insertion (SOI) in June 2004 through August 2005, we conducted a detailed survey and analysis of observations made by the Vector Helium Magnetometer (VHM). The purpose of the present study is to characterize the properties of waves observed in Saturn's ULF wave foreshock and identify its boundary using single spacecraft techniques. The amplitude of these waves is usually comparable to the mean magnetic field intensity, while their frequencies in the spacecraft frame yields two clearly different types of waves: one with frequencies below the local proton cyclotron frequency (Q_H+) and another with frequencies above Q_H+- All the wave crossings described here, clearly show that these waves are associated to Saturn's foreshock. In particular, the presence of waves is associated with the change in θ_(Bn) to quasi-parallel geometries. Our results show the existence of a clear boundary for Saturn's ULF wave foreshock, compatible with θ_(Bn) ~ 45° surfaces.
机译：即使太阳风是高度超音速的，在磁化行星的弓激波上游的区域中也检测到了强烈的超低频（ULF）波活动。该特征首先在地球的弓形冲击之前被观察到，相应的区域称为ULF波前震，它嵌入在行星的前震中。在过去的三十年中，已经对地球前震中ULF波的特性和空间分布进行了广泛研究，并已解释为由于弓形激波逆流而来的太阳风离子引发的等离子体不稳定性。自2004年7月以来，卡西尼号飞船已对包括其上游区域在内的土星等离子体环境进行了表征。自2004年6月至2005年8月卡西尼号（Cassini）的土星轨道（SOI）插入以来，我们对矢量氦磁力计（VHM）进行的观测进行了详细的调查和分析。本研究的目的是表征在土星的ULF波前震中观测到的波的特性，并使用单个航天器技术确定其边界。这些波的振幅通常可与平均磁场强度相媲美，而它们在航天器框架中的频率会产生两种明显不同的波：一种具有低于局部质子回旋加速器频率（Q_H +）的频率，另一种具有高于Q_H +-的频率。此处描述的波浪穿越清楚地表明，这些波浪与土星的前震有关。特别地，波的存在与θ_（Bn）到准平行几何形状的变化相关。我们的结果表明，土星的ULF波前震存在清晰的边界，与θ_（Bn）〜45°表面兼容。

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  • 来源
    《Planetary and space science》 |2013年第5期|64-75|共12页
  • 作者

    N. Andres; D.O. Gomez; C. Bertucci; C. Mazelle; M.K. Dougherty;

  • 作者单位

    Instituto de Astronomia y Fisica del Espacio, Av. Cantilo 2620 Pabelldn IAFE (cp 1428), CABA, Buenos Aires, Argentina;

    Instituto de Astronomia y Fisica del Espacio, Av. Cantilo 2620 Pabelldn IAFE (cp 1428), CABA, Buenos Aires, Argentina,Departamento de Fisica, FCEN, UBA, Buenos Aires, Argentina;

    Instituto de Astronomia y Fisica del Espacio, Av. Cantilo 2620 Pabelldn IAFE (cp 1428), CABA, Buenos Aires, Argentina;

    Institut de Recherche en Astrophysique et Planetologie, Toulouse, France;

    Space and Atmospheric Physics, The Blackett Laboratory, Imperial College, London, United Kingdom;

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  • 正文语种 eng
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  • 关键词

    foreshock; ULF waves; saturn; cassini;

    机译：前兆超低频波;土星卡西尼号;

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