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Multispacecraft observations of fundamental poloidal waves without ground magnetic signatures

机译:多空间飞行观测没有地磁记录的基本极向波

摘要

Poloidal standing Alfvén waves observed by spacecraft usually have a second harmonic standing wave structure. On very rare occasions, fundamental poloidal waves have been observed in association with giant pulsations observed on the ground. In this paper, we report multisatellite observations of fundamental poloidal waves that did not produce any clearly associated magnetic pulsations on the ground. The waves were observed on 10 November 2008, at ~1830 Universal Time (UT) at THEMIS‐A and THEMIS‐D and at ~2010 UT at THEMIS‐E as these spacecraft passed L  ~ 11 and magnetic local time (MLT) ~ 0900. The GOES‐11 geostationary satellite ( L  ~ 7) also observed poloidal waves at ~1730 UT when it was at ~0900 MLT. The poloidal waves at THEMIS were characterized by narrow‐band oscillations (frequency ~4 mHz) of the ion bulk velocity and magnetic field in the radial direction. We identify the waves at THEMIS to be the fundamental mode on the basis of the wave properties observed slightly south of the magnetic equator: large velocity amplitude, small magnetic field amplitude, and ~90° phase delay of the magnetic field relative to the velocity. The azimuthal wave number is found to be ~70 (if we assume westward propagation) or ~200 (if we assume eastward propagation) from the phase delay between THEMIS‐A and THEMIS‐D. This wave number explains why there were no corresponding magnetic field oscillations on the ground. These observations imply that only a subset of fundamental poloidal waves excited in the magnetosphere is observed on the ground as giant pulsations. Key Points Poloidal waves were observed by THEMIS and GOES. The waves were high‐m fundamental standing Alfven waves. The waves did not propagate to the ground.
机译:航天器观测到的极向性Alfvén驻波通常具有二次谐波驻波结构。在极少数情况下,观察到基本的极向波与在地面上观察到的巨大脉动有关。在本文中,我们报告了基本多倍体波的多卫星观测,这些观测在地面上未产生任何明显相关的磁脉冲。随着这些航天器经过L〜11和磁性当地时间(MLT)〜0900,在2008年11月10日,THEMIS‐A和THEMIS‐D的〜1830世界时间(UT)和THEMIS‐E的〜2010 UT观察到了这些波。 。GOES-11对地静止卫星(L〜7)在约0900 MLT时也观测到约1730 UT的极向波。 THEMIS的极向波的特征是离子体积速度和径向磁场的窄带振荡(频率约4 mHz)。根据在电磁赤道以南略微观察到的波属性,我们将THEMIS处的波识别为基本模式:大的速度振幅,小的磁场振幅以及相对于速度的〜90°相位延迟。从THEMIS‐A和THEMIS‐D之间的相位延迟发现,方位波数约为〜70(如果假设为向西传播)或〜200(如果假设为向东传播)。该波数解释了为什么地面上没有相应的磁场振荡。这些观察结果暗示,在地面上仅观察到磁层中激发的基本极性波的一个子集,即巨脉冲。关键点THEMIS和GOES观测到极向波。这些波浪是高m基本常驻Alfven波浪。波浪没有传播到地面。

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