Postmidnight chorus: A substorm phenomenon
Postmidnight chorus: A substorm phenomenon
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DOI：
10.1029/ja079i001p00118
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发表时间：
1974
影响因子：
--
通讯作者：
B. Tsurutani;E. Smith
中科院分区：
文献类型：
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作者：
B. Tsurutani;E. Smith
利用Ogo - 5型搜索线圈磁强计数据研究了外磁层午夜扇区极低频（10-1500 Hz）电磁发射。在L = 5至L = 9的整个区域内，都可以与磁层亚暴一起检测到合唱，但仅在午夜后的时间内。在午夜前的3小时内，即使是在亚暴的过程中，也没有看到合唱。午夜后的合唱只发生在地磁赤道±15°以内，而且最经常发生在地磁赤道。时间平均强度从10−8到10−6 γ²/Hz不等，这比之前报道的白天合唱的最大值低一个数量级。合唱发生在带宽为50-200赫兹的窄频带内。合唱频率变化从不到赤道电子回旋频率的四分之一到高达四分之三，由机载磁力计确定。在这个范围内的所有频率都被检测到，除了一个接近电子回旋频率一半的窄带，在那里合唱似乎被强烈衰减。副歌经常被观察到在陀螺频率的一半以上和以下有两个不同的波段。两种最常见的合唱类型是没有结构的窄乐队合唱和降调合唱。上升的音调和钩子也被观察到。通常观察到合唱脉动的准周期为5-15秒。在环境磁场的微脉动和合唱脉动之间寻找相关性，但没有发现。午夜后合唱的许多特征可以用波和次风暴电子之间的回旋加速器共振相互作用来解释。副歌的分布作为当地时间和L的函数，与能量为bbb40 keV的增强、捕获和沉淀的亚风暴电子的分布惊人地相似。午夜后发生的合唱是由于注入电子的向东曲率和梯度漂移所致。正如观测到的那样，回旋加速器共振相互作用在赤道附近应该是最强的。由于低能量（1-10 keV）的极光电子的朗道阻尼，限制在赤道15°以内。电子回旋频率的一半处的衰减带可能是由电子的朗道阻尼造成的，这些电子具有与回旋加速器共振相对应的能量，但与波的方向相同。午夜后的合唱和白天合唱之间的联系是密切的。在1000 LT左右的日侧合唱强度最大值与高能电子降水的日侧最大值相关，这可能代表了在午夜附近注入的亚暴电子的进一步降水。
The ELF (10–1500 Hz) electromagnetic emissions in the midnight sector of the outer magnetosphere have been studied using Ogo 5 search coil magnetometer data. Chorus was detected in conjunction with magnetospheric substorms throughout the region from L = 5 to L = 9 but only during postmidnight hours. No chorus was seen in the 3 hr preceding midnight even when substorms were in progress. The postmidnight chorus occurred only within ±15° of, and most frequently at, the geomagnetic equator. Time-averaged intensities varied from 10−8 to 10−6 γ²/Hz, which is more than an order below the maximum values reported previously for dayside chorus. The chorus occurred in narrow frequency bands having a bandwidth of 50–200 Hz. Chorus frequencies varied from less than one-fourth to as high as three-fourths of the equatorial electron gyrofrequency as determined by the on-board magnetometer. All frequencies in this range were detected except for a narrow band near one-half the electron gyrofrequency where the chorus appeared to be strongly attenuated. Chorus was often observed as two distinct bands above and below one-half the gyrofrequency. The two most common types of chorus were narrow band chorus without structure and falling tones. Rising tones and hooks were also observed. Chorus pulsations were observed often with quasi-periods of 5–15 s. A correlation was sought, but none was found, between the micropulsations in the ambient magnetic field and the chorus pulsations. Many features of postmidnight chorus can be explained by a cyclotron resonant interaction between the waves and the substorm electrons. The distribution of the chorus as a function of local time and L is strikingly similar to the distribution of enhanced, trapped, and precipitated substorm electrons with energies >40 keV. The postmidnight occurrence of chorus is attributable to the eastward curvature and gradient drift of the injected electrons. Cyclotron resonant interactions should be strongest near the equator, as was observed. The confinement to within 15° of the equator is attributed to Landau damping by low-energy (1–10 keV) auroral electrons. The attenuation band at one-half the electron gyrofrequency may result from Landau damping by electrons that have energy corresponding to cyclotron resonance but are traveling in the same direction as the waves. A close correspondence is expected between the occurrence of postmidnight and dayside chorus. The maximum in dayside chorus intensity at approximately 1000 LT, which is correlated with the dayside maximum of energetic electron precipitation, may represent further precipitation of the substorm electrons injected near midnight.