A physical mechanism producing suprathermal populations and initiating substorms in the Earth's magnetotail

摘要

We suggest a candidate physical mechanism, combining there dimensionalstructure and temporal development, which is potentially able to producesuprathermal populations and cross-tail current disruptions in the Earth'splasma sheet. At the core of the proposed process is the "akis" structure; in a thin current sheet (TCS) the stretched (tail-like)magnetic field lines locally terminate into a sharp tip around the tailmidplane. At this sharp tip of the TCS, ions become non-adiabatic, while apercentage of electrons are accumulated and trapped: The strong andtransient electrostatic electric fields established along themagnetic field lines produce suprathermal populations. Inparallel, the tip structure is associated with field aligned and mutuallyattracted parallel filamentary currents which progressively become moreintense and inevitably the structure collapses, and so does the local TCS.The mechanism is observationally based on elementary, almost autonomous andspatiotemporal entities that correspond each to a localthinning/dipolarization pair having duration of ~1 min.Energetic proton and electron populations do not occursimultaneously, and we infer that they are separately accelerated at localthinnings and dipolarizations, respectively. In one example energeticparticles are accelerated without any dB/dt variation and before thesubstorm expansion phase onset. A particular effort is undertakendemonstrating that the proposed acceleration mechanism may explainthe plasma sheet ratio T_i/T_e≈7. Allour inferences are checked by the highest resolution datasets obtained bythe Geotail Energetic Particles and Ion Composition (EPIC)instrument. The energetic particles are used as the best diagnostics forthe accelerating source. Near Earth (X≈10 R_E) selectedevents support our basic concept. The proposed mechanism seems toreveal a fundamental building block of the substorm phenomenon and may bethe basic process/structure, which is now missing, that might help explainthe persistent, outstanding deficiencies in our physical description ofmagnetospheric substorms. The mechanism is tested, checked, and foundconsistent with substorm associated observations performed ~30 and 60 R_E away from Earth.