Spatial structures of magnetic depression in the Earth's high-altitude cusp: Cluster multipoint observations

• Published in: Journal of Geophysical Research. B. Solid Earth 2009-10, Vol.114 (A10), p.n/a
• Main Authors: Shi, Q. Q., Pu, Z. Y., Soucek, J., Zong, Q.-G., Fu, S. Y., Xie, L., Chen, Y., Zhang, H., Li, L., Xia, L. D., Liu, Z. X., Lucek, E., Fazakerley, A. N., Reme, H.
• Format: Article
• Language: English
• Subjects: cusp; magnetic hole; spatial structure
• ISSN: 0148-0227; 2169-9380; 2156-2202; 2169-9402
• DOI: 10.1029/2009JA014283

Magnetic depression structures (magnetic holes) of short time duration from seconds to minutes have been studied using Cluster data in the high‐latitude cusp. Our multispacecraft analysis revealed that the magnetic depressions are spatial structures traveling across the spacecraft, and this result was further strengthened by the calculation of the boundary normal directions and velocities using various methods. In this article, we show that multiple properties of the magnetic depressions are consistent with those of mirror structures observed in the magnetosheath or solar wind. The plasma in the cusp is rarely unstable with respect to mirror instability. However, as has been shown by previous studies, once a large magnetic hole is created by mirror instability, it becomes relatively stable and can survive for extended periods of time even if surrounding plasma conditions drop well below the mirror threshold. Although local generation of these structures cannot be completely ruled out in some cases, we propose an interpretation of the magnetic depressions observed in the cusp as mirror structures generated upstream and convected to the cusp by plasma flow. Specifically, the magnetic holes could be generated in the magnetosheath and enter the cusp due to the open geometry of the cusp magnetic field.