[P-EM11] Dynamics of Magnetosphere and Ionosphere

[PEM11-16] Reconstruction of the electron diffusion region of magnetotail reconnection seen by the Magnetospheric Multiscale spacecraft

*長谷川 洋¹、Richard Denton²、中村 るみ³、Kevin Genestreti⁴、中村 琢磨³、Joo Hwang⁵、Tai Phan⁶、Roy Torbert⁴、Jim Burch⁵、Barbara Giles⁷、Dan Gershman⁷、Chris Russell⁸、Bob Strangeway⁸、Per-Arne Lindqvist⁹、Yuri Khotyaintsev¹⁰、Robert Ergun¹¹、北村 成寿¹²、斎藤 義文¹ (1.宇宙航空研究開発機構宇宙科学研究所、2.ダートマス大学、3.オーストリア宇宙科学研究所、4.ニューハンプシャー大学、5.サウスウェスト研究所、6.カリフォルニア大学バークレー校、7.NASAゴダード宇宙飛行センター、8.カリフォルニア大学ロサンゼルス校、9.スウェーデン王立工科大学、10.スウェーデン宇宙物理学研究所、11.コロラド大学、12.東京大学地球惑星科学専攻)

キーワード：磁気リコネクション、磁気圏尾部、電子拡散領域

Magnetic reconnection is a fundamental plasma process that controls transfer of solar wind energy and mass to planetary magnetospheres and causes explosive energy release associated with solar flares and sudden auroral brightening. NASA's Magnetospheric Multiscale (MMS) mission, which consists of four identical spacecraft launched in March 2015, aims at elucidating how magnetic reconnection works with unprecedented high temporal and spatial resolution measurements of charged particles and electromagnetic fields in space. MMS has been observing Earth's magnetotail since May 2017, and encountered the central region of magnetic reconnection, called the electron diffusion region (EDR), on 11 July 2017 (Torbert et al., Science, 2018). We present results from the reconstruction of the electron diffusion region (EDR) observed in this event. The conditions were suited for the reconstruction technique, developed by Sonnerup et al. (JGR, 2016), that produces magnetic field and electron streamline maps based on a two-dimensional (2-D), time independent, inertia-less form of electron magnetohydrodynamic equation, assuming an approximately symmetric current sheet and negligible guide magnetic field. Our reconstruction results (Hasegawa et al., JGR, 2019) indicate that although the X point was not captured inside its tetrahedron, MMS approached the X point as close as one electron inertial length ~27 km. The opening angle of the recovered separatrix field line, combined with theory, suggests that the dimensionless reconnection rate was 0.17, which is consistent with the measured reconnection electric field 2–4 mV/m. The stagnation point of the reconstructed electron flow is shifted earthward of the X point by ~90 km, one possible interpretation of which is discussed. The energy conversion rate j*E' in the electron frame tends to be higher near the stagnation point, consistent with earlier observations and simulations, and is not correlated with the amplitude of broadband electrostatic waves observed in the upper-hybrid frequency range. The latter suggests that the waves did not contribute to energy dissipation in this particular EDR.

References:
Hasegawa, H., Denton, R. E., Nakamura, R., Genestreti, K. J., Nakamura, T. K. M., Hwang, K.-J., et al., Reconstruction of the electron diffusion region of magnetotail reconnection seen by the MMS spacecraft on 11 July 2017. Journal of Geophysical Research: Space Physics, 124. https://doi.org/10.1029/2018JA026051, 2019.
Sonnerup, B. U. O., H. Hasegawa, R. E. Denton, and T. K. M. Nakamura, Reconstruction of the electron diffusion region, J. Geophys. Res. Space Physics, 121, 4279-4290, doi:10.1002/2016JA022430, 2016.
Torbert, R. B., J. L. Burch, T. D. Phan, ... Y. Saito, Electron-scale dynamics of the diffusion region during symmetric magnetic reconnection in space, Science, doi:10.1126/science.aat2998, 2018.