On the occurrence and characteristics of intense low-altitude electric fields observed by Freja
On the occurrence and characteristics of intense low-altitude electric fields observed by Freja
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Freja观测到的低空强电场的发生及特征
DOI：
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发表时间：
1995
期刊：
影响因子：
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通讯作者：
L. Eliasson
中科院分区：
文献类型：
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作者：
G. Marklund;L. Blomberg;C. Fälthammar;P. Lindqvist;L. Eliasson
介绍了Freja卫星上双探头电场仪的高分辨率测量结果。观测结果表明，在Freja探索的高度范围内（高达1700公里），极强（高达1 V m−1）和精细结构（<1公里）的电场存在于极光纬度。强磁场事件通常发生在地磁活动期间的极光椭圆（01-07 MLT）的清晨部分。与极光加速区域内的观测结果相反，Freja观测到的强电场往往是发散的，位于电子极光外的向下场向电流区域内，其特征是与电子沉淀、向上离子束和向上场向电流相关的强会聚电场。此外，观察到的强场结合沉淀和横向能量为0.5-1 keV的离子，并可能在离子加热中发挥重要作用。观测结果表明，强电场事件与小规模的低电导率电离层区域有关，这些区域没有极光发射，例如东西对齐的暗丝或位于晨间弥漫极光区域内极光弧之间或邻近极光弧的黑色极光卷曲的涡旋街道。我们认为，这些强烈的领域也存在于电离层高度，虽然还没有这样的观测。这是可能的，因为与暗丝的高度集成的电导率可能低至0.1 S或更少。此外，Freja电场数据收集的极光区外进行了讨论，特别强调亚极光电场观察到的19-01 MLT部门之间的赤道边缘的极光椭圆形和环电流的内边缘。
High-resolution measurements by the double probe electric field instrument on the Freja satellite are presented. The observations show that extremely intense (up to 1 V m−1) and fine-structured (<1 km) electric fields exist at auroral latitudes within the altitude regime explored by Freja (up to 1700 km). The intense field events typically occur within the early morning sector of the auroral oval (01-07 MLT) during times of geomagnetic activity. In contrast to the observations within the auroral acceleration region characterized by intense converging electric fields associated with electron precipitation, upward ion beams and upward field-aligned currents, the intense electric fields observed by Freja are often found to be diverging and located within regions of downward field-aligned currents outside the electron aurora. Moreover, the intense fields are observed in conjunction with precipitating and transversely energized ions of energies 0.5-1 keV and may play an important role in the ion heating. The observations suggest that the intense electric field events are associated with small-scale low-conductivity ionospheric regions void of auroral emissions such as east-west aligned dark filaments or vortex streets of black auroral curls located between or adjacent to auroral arcs within the morningside diffuse auroral region. We suggest that these intense fields also exist at ionospheric altitudes although no such observations have yet been made. This is possible since the height-integrated conductivity associated with the dark filaments may be as low as 0.1 S or less. In addition, Freja electric field data collected outside the auroral region are discussed with particular emphasis on subauroral electric fields which are observed within the 19–01 MLT sector between the equatorward edge of the auroral oval and the inner edge of the ring current.