Observations and theory of the formation of stable auroral red arcs
Observations and theory of the formation of stable auroral red arcs
复制标题
稳定极光红弧形成的观测和理论
DOI：
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发表时间：
1975
期刊：
影响因子：
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通讯作者：
R. Roble
中科院分区：
文献类型：
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作者：
M. Rees;R. Roble
在几个地球半径处的磁层赤道区域附近发现了一群能量为几十keV的质子，称为环电流。在地磁暴主相期间，环电流向更低的L值移动，进入等离子体层顶区域，该区域的特点是等离子体密度存在陡峭梯度。这种相互作用以及各向异性的投掷角分布导致环电流不稳定以及离子回旋波湍流的增长。当波能通过朗道阻尼在周围电子气中耗散时，等离子体层顶电子温度升高到几电子伏特，并相对于电离层产生了显著的温度梯度。在低碰撞频率区域通过投掷角散射以及在碰撞主导区域通过热传导传递给电离层的能量将电离层电子温度提高到几千度。因此，在大约400公里的电离层F区，麦克斯韦分布的高能尾部存在相当数量的电子，即能量大于2电子伏特的电子，这里原子氧是主要的中性气体成分。2电子伏特是氧原子激发到亚稳态¹D能级的阈值，这些O(¹D)原子发射6300埃的辐射，这是稳定极光红（SAR）弧的特征。尽管产生足以导致平均SAR弧的电子温度所需的能量输入率小于0.1尔格/平方厘米·秒，但红线辐射的能量仅约为0.003尔格/平方厘米·秒。因此，SAR弧是地磁暴期间磁层能量缓慢释放的一种光学表现。从能量角度看，它与高纬度极光过程相比是较小的。
A population of protons with energy of some tens of keV, called the ring current, is found near the equatorial region of the magnetosphere at several earth radii. During the main phase of geomagnetic storms the ring current shifts toward lower L values into the region of the plasmapause, which is characterized by steep gradients in the plasma density. This interaction together with an anisotropic pitch angle distribution leads to ring current instability and the growth of ion cyclotron wave turbulence. As wave energy is dissipated in the ambient electron gas by Landau damping, the plasmapause electron temperature is raised to a few electron volts, and a substantial temperature gradient is created with respect to the ionosphere. The energy transferred to the ionosphere by pitch angle scattering in the low collision frequency region and by heat conduction in the collision-dominated regime raises the ionospheric electron temperature to several thousand degrees. Therefore an appreciable number of electrons in the high-energy tail of the Maxwellian distribution, i.e., electrons with energy greater than 2 eV, exist in the F region of the ionosphere at about 400 km, where atomic oxygen is the dominant neutral gas constituent. Two eV is the threshold for excitation of oxygen atoms to the metastable ¹D level, and these O(¹D) atoms emit 6300-A radiation, the signature of stable auroral red (SAR) arcs. Although the energy input rate required to produce electron temperatures sufficient to cause average SAR arcs is less than 0.1 erg cm−2 s−1, the energy radiated in the red line is only about 0.003 erg cm−2 s−1. Thus an SAR arc is an optical manifestation of a slow release of energy from the magnetosphere during a geomagnetic storm. Energetically it is small in comparison with high-latitude auroral processes.