Generation of electric fields and currents by neutral flows in weakly ionized plasmas through collisional dynamos

Abstract

Intense electric currents called electrojets occur in weakly ionized magnetized plasmas. An example occurs in the Earth's ionosphere near the magnetic equator where neutral winds drive the plasma across the geomagnetic field. Similar processes take place in the Solar chromosphere and MHD generators. This letter argues that not all convective neutral flows generate electrojets and it introduces the corresponding universal criterion for electrojet formation, ∇ × (U × B) = ∂B/∂t, where U is the neutral flow velocity, B is the magnetic field, and t is time. This criterion does not depend on the conductivity tensor,σ. For many systems, the displacement current, ∂B/∂t, is negligible, making the criterion even simpler. This theory also shows that the neutral-dynamo driver that generates electrojets plays the same role as the DC electric current plays for the generation of the magnetic field in the Biot-Savart law.PACS numbers: 51.50.+v,52.30.Cv,52.25.Ya, A weakly ionized plasma in a strong magnetic field, colliding with a neutral gas, can generate electric currents called electrojets. One such neutral-driven electrojet, named the equatorial electrojet, forms in the Earth's Eregion ionosphere around the magnetic equator 1-4 . This electrojet results from a large E-field that ultimately derives its energy from neutral winds, abundant in the bottom of the thermosphere (90 -130 km altitude). The resulting E × B drifting electrons cause the primary electrojet current. Similar electrojets exist along the magnetic equators of other magnetized planets 5 . Strong convective neutral flows across B in the highly collisional solar chromosphere will also generate electrojets 6-8 . Under special conditions, similar processes can take place in magnetohydrodynamic (MHD) generators 9,10 . This paper presents a novel approach in analyzing the strength of electrojets and develops a universal criterion for the existence of these currents. It also shows that the simple 1D model often used by textbooks and papers to illustrate the origin of electrojets will not, in fact, create an electrojet.Electrojets develop a complex array of behaviors beyond just generating currents strong enough to cause large magnetic field perturbations. These currents also frequently drive various plasma instabilities that result in plasma density irregularities and fluctuating electric fields 11 . These irregularities and fields have been observed for a long time by radars and rockets 4,12 . These instabilities can cause intense electron heating and anomalous conductivities [13][14][15][16] . Fontenla 6 has speculated that such instabilities may play an important role in chromospheric heating.Theoretical studies of the Earth's equatorial electrojet have a long history 17-20 , providing detailed quantitative a) dimant@bu.edu descriptions of the electrodynamic effects due to different components of the neutral wind. However, they have never addressed the following simple questions: 1) Do the neutral convection flows always drive electrojets? and 2) What component of ...

Multifluid Simulation of Solar Chromospheric Turbulence and Heating Due to Thermal Farley–Buneman Instability

“…However, we also note that removing the imposed current from our simulation causes the necessary velocity differences to vanish significantly faster than the instability growth rate. This behavior is consistent with Dimant et al (2016), which detailed the criterion for generating currents from neutral flows in a weakly ionized collisional plasma.…”

Effects of Ion Magnetization on the Farley–Buneman Instability in the Solar Chromosphere

“…in order to compare to the neutral acoustic speed and the neutral flows that may be responsible for the ambient electric field (Dimant et. al.…”

“…In the equatorial region on Earth, the enhancement is due not only to the neutral wind induced drift, but also changing conductivity layers as a function of height. This is the criteria for electrojet formation laid out in Dimant et. al.…”