Evidence of Magnetic Nulls in Electron Diffusion Region

Abstract

Theoretically, magnetic reconnection—the process responsible for solar flares and magnetospheric substorms—occurs at the X‐line or radial null in the electron diffusion region (EDR). However, whether this theory is correct is unknown, because the radial null (X‐line) has never been observed inside the EDR due to the lack of efficient techniques and the scarcity of EDR measurements. Here we report such evidence, using data from the recent MMS mission and the newly developed First‐Order Taylor Expansion (FOTE) Expansion technique. We investigate 12 EDR candidates at the Earth's magnetopause and find radial nulls (X‐lines) in all of them. In some events, spacecraft are only 3 km (one electron inertial length) away from the null. We reconstruct the magnetic topology of these nulls and find it agrees well with theoretical models. These nulls, as reconstructed for the first time inside the EDR by the FOTE technique, indicate that the EDR is active and the reconnection process is ongoing.

Electron‐Driven Dissipation in a Tailward Flow Burst

“…Specifically, we trace and inverse trace a few points around the spacecraft tetrahedron to obtain the magnetic topology, as done in previous studies (e.g., X. H. Chen et al, ; Z. Z. Chen et al, ; Fu et al, ; Fu, Cao, et al, ; Liu, Fu, Cao, et al, ; Z. Wang et al, ). We use the local magnetic field strength as the trace step length.…”

Electron Distribution Functions Around a Reconnection X‐Line Resolved by the FOTE Method

“…Zhong et al (2018), etc. However, to confirm the encounter of an EDR, an X-line pattern (2-D description) or radial null (3-D description) must be detected (Chen et al, 2018;Fu, Cao, et al, 2019). However, to well describe the reconnection current sheet, we transfer data to local magnetic normal coordinates, which are determined using the minimum variance analysis of the magnetic field between 7:44:08 and 7:44:54 UT.…”

Evidence of Magnetic Nulls in the Reconnection at Bow Shock

“…The method we use is the FOTE method, which was initially developed by Fu et al (), later validated by Fu et al (), and hitherto has been extensively applied to the analyses of reconnection events in the Earth's magnetosphere (e.g., Chen et al, ; Chen, Fu, Liu, et al, ; Chen, Fu, Wang, et al, ; Fu et al, ; Fu, Cao, et al, ; Huang et al, ; Liu, Fu, Cao, et al, ; Liu et al, ; Man et al, ; Wang et al, ; Wang, Fu, et al, ). This method, based on four‐spacecraft measurements, can position a magnetic null both inside and outside the spacecraft tetrahedron and can also reconstruct the magnetic topology of the null.…”

“…Instead, there will be magnetic nulls in the reconnection plane, which are termed “in‐plane” nulls (Burch, Torbert, et al, ; Fu et al, ). For such case, we artificially subtract the guide field to find an in‐plane null, as that done in Fu, Cao, et al (). It should be noticed that such removal of guide field does not affect the identification of magnetic‐null type, because the Jacobian matrix ∇ B is same when subtracting a guide field uniformly from four‐spacecraft measurements (see the discussion by Fu et al, ).…”

“…Magnetic reconnection, a fundamental plasma process converting magnetic energy into particles' kinetic and thermal energy (Priest & Forbes, ; Yamada et al, ; Wang et al, ; Fu, Khotyaintsev, et al, , Fu, Xu, et al, ), is believed to be responsible for many explosive phenomena in astrophysical plasmas such as solar flares (Shibata et al, ) and terrestrial substorms (Angelopoulos et al, ; Cao et al, , , ; Fu, Cao, et al, ; Xu et al, ), and in the disruptions of fusion experiments (Ji et al, ). Such process theoretically occurs at the magnetic null where the magnetic strength becomes zero and plasmas get unmagnetized (Greene, ; Lau & Finn, ; Pontin et al, ; Priest & Titov, ; Wyper & Pontin, ) or occurs at the “in‐plane” magnetic null where magnetic field vanishes in the reconnection plane (Fu et al, ; Fu, Cao, et al, ) but still exists in the direction perpendicular to the reconnection plane—known as “guide field” (Peng et al, ; Wang et al, ). Therefore, the magnetic null or “in‐plane” magnetic null plays very important roles in the energy dissipation/conversion during reconnection processes (Fu et al, ; Olshevsky et al, ).…”