Energy Flow Exciting Field‐Aligned Current at Substorm Expansion Onset

Abstract

At substorm expansion onset, upward field‐aligned currents (FACs) increase abruptly, and a large amount of electromagnetic energy starts to consume in the polar ionosphere. A question arises as to where the energy comes from. Based on the results obtained by the global magnetohydrodynamics simulation, we present energy flow and energy conversion associated with the upward FACs that manifest the onset. Our simulations show that the cusp/mantle region transmits electromagnetic energy to almost the entire region of the magnetosphere when the interplanetary magnetic field is southward. Integral curve of the Poynting flux shows a spiral moving toward the ionosphere, probably suggesting the pathway of electromagnetic energy from the cusp/mantle dynamo to the ionosphere. The near‐Earth reconnection initiates three‐dimensional redistribution of the magnetosphere. Flow shear in the near‐Earth region results in the generation of the near‐Earth dynamo and the onset FACs. The onset FACs are responsible to transport the electromagnetic energy toward the Earth. In the near‐Earth region, the electromagnetic energy coming from the cusp/mantle dynamo is converted to the kinetic energy (known as bursty bulk flow) and the thermal energy (associated with high‐pressure region in the inner magnetosphere). Then, they are converted to the electromagnetic energy associated with the onset FACs. A part of electromagnetic energy is stored in the lobe region during the growth phase. The release of the stored energy, together with the continuously supplied energy from the cusp/mantle dynamo, contributes to the energy supply to the ionosphere during the expansion phase.

New Diagnosis for Energy Flow From Solar Wind to Ionosphere During Substorm: Global MHD Simulation

“…For some runs, the maximum Joule heating rate is smaller than the maximum power released from the lobe (Runs 3, 4, 5, 6, and 8). This is not a surprising result because the contribution from the magnetic energy directly supplied from the solar wind is significant and is comparable to that released from the lobe (Ebihara & Tanaka, ).…”

“…The continuity equation for the internal energy is given by where u and γ is the thermal energy density of plasma (=3 p /2) and the ratio of specific heat (=5/3), respectively. Readers may refer to Birn and Hesse () and Ebihara and Tanaka () for the derivation of equations and . The energy conversion takes place through the right‐hand sides of equations –.…”

“…First, the magnetic energy stored in the lobe is not only the energy source that is consumed in the ionosphere. A large amount magnetic energy originating from the solar wind is continuously and directly supplied toward the plasma sheet during the expansion phase (Ebihara & Tanaka, ). Second, the rates of the magnetic energy stored in the lobe and released from the lobe depend on the solar wind parameters, V sw , N sw , and SBZ.…”

“…The global the global magnetohydrodynamics (MHD) simulation provides a direct way to investigate the flow and conversion of the energy. One such way is to draw an integral curve of the Poynting flux (hereinafter, referred to as an S curve) (Ebihara & Tanaka, ; Papadopoulos et al, ). The S curve originating from the solar wind penetrates the lobe region and traverses the magnetic separatrix.…”

Magnetosphere‐Ionosphere Convection Under the Due Northward IMF

“…Associated with dynamo formation, conversion of thermal energy to electromagnetic energy, divergence of pointing flux, and the generation of the FAC occur as mutually coupling processes (Ebihara & Tanaka, 2017a). The circulation of magnetic field must be constructed as one body with convection.…”