Substorm Evolution Research Articles

A quantitative assessment of energy storage and release in the Earth's magnetotail

Magnetospheric substorms represent a global interaction between the solar wind, the magnetosphere, and the ionosphere. Energy extracted from the solar wind is episodically stored in the magnetosphere, with a large fraction of this energy being in the form of excess magnetic flux in the magnetotail lobes. The stored energy is periodically dissipated in an explosive instability that occurs in the near‐Earth plasma sheet at substorm expansive phase onset. Methods are discussed to estimate the amount of energy transferred from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the plasma sheet and magnetotail lobes. Possible energy sources to drive substorm evolution are evaluated in one particularly well‐observed case (May 3, 1986). By examining the major avenues of energy dissipation, the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, etc. is assessed during the substorm. It is found that stored tail‐lobe energy is sufficient (by a factor of 3 or more) to drive observed substorm dissipation processes. On the other hand, energy in the closed field lines of the plasma sheet is insufficient (by a substantial margin) to supply the substorm energy.

Outflowing ionospheric oxygen-ion motion in a reconfigurating magnetosphere

Abstract. During substorms, large-scale changes of the topology of the Earth's magnetosphere following the variation of the characteristics of the interplanetary medium are accompanied by the induction of the electric field. In this study a model of a time-dependent magnetosphere is constructed and the large-scale features of the induced electric field are described. Local-time sectors with upward or downward field-aligned component and with intense perpendicular component of the electric field are distinguished. The electric-field structure implies the existence of outflow regions particularly effective in ion energization. With the vector potential adopted in the study, the region from which the most energized ions originate is defined by the local-time sector near 2100 MLT and latitude zone near 71° MLAT. The motion of ionospheric oxygen ions of energy 0.3–3 keV is investigated during a 5-min reconfiguration event when the tail-like magnetospheric field relaxes to the dipole-like field. As the characteristics of plasma in the regions near the equatorial plane affect the substorm evolution, the energy, pitch angle, and the magnetic moment of ions in these regions are analyzed. These quantities depend on the initial energy and pitch angle of the ion and on the magnetic and electric field it encounters on its way. With the vector potential adopted, the energy attained in the equatorial regions can reach hundreds of keV. Three regimes of magnetic-moment changes are identified: adiabatic, oscillating, and monotonous, depending on the ion initial energy and pitch angle and on the magnetic- and electric-field spatial and temporal scales. The implications for the global substorm dynamics are discussed

Energy requirement of magnetic reconnection during magnetospheric substorms

Methods are discussed to estimate energy transfer from the solar wind to the magnetosphere during substorm growth phases. Observational and modeling constraints are then used to assess quantitatively the total amount of energy stored in the magnetotail. The major avenues of energy dissipation are examined and the energy that is released in the form of plasma sheet heating, ionospheric Joule heating, plasmoids, and energetic particle production during substorms is assessed. Energy sources are evaluated to drive substorm evolution in a particularly well-observed case: stored tail-lobe energy is sufficient to drive observed substorm dissipation processes (by large margins). On the other hand, energy in the closed field lines of the plasma sheet is insufficient to supply the substorm energy. Hence, magnetic reconnection is required on energy grounds during well-observed substorm cases.

MHD simulations of the transition of magnetic reconnection from closed to open field lines

Two‐ and three‐dimensional resistive MHD simulations are used to study the behavior of a dynamic magnetotail model when reconnection proceeds from closed to open field lines. For this purpose, the initial, quasi‐static magnetotail configuration is set up for various lobe density values. Initiated by the increase or occurrence of anomalous resistivity, all simulations then exhibit the growth of an unstable tail mode, which leads to neutral line and plasmoid formation. The reconnection process starts in a plasma sheet region of high mass density and progresses to open field lines with much lower plasma density and much lower plasma β. At the time of transition, drastic changes of typical dynamical parameters occur, involving strongly enhanced electric fields and reconnection rates and the generation of field‐aligned currents. Further, typical substorm signatures such as the dipolarization of the magnetic field in the inner regions of the magnetotail appear on very short timescales, comparable to those found in spacecraft observations. It is argued that this behavior should indeed become important during substorm evolution, in support of substorm models based on this transition.

Hybrid modeling of collisionless reconnection in two‐dimensional current sheets: Simulations

The stability of thin current sheets, such as commonly occur in space plasmas, is investigated in the presence of a magnetic normal component. The investigation is based on hybrid simulations using a suitably adapted electron pressure model. In order to properly represent the effects of the electron cyclotron motion in the magnetic normal component, we include the cyclotron part of the electron pressure evolution in an implicit manner. The results of a number of these simulations are presented with emphasis on a comparison with the linear analytical theory (Kuznetsova et al., this issue), and on the nonlinear evolution of the tearing instability. We find, in agreement with linear theory, a transition to stability for electron mass below a certain threshold, which depends on the initial configuration. The results suggest that the onset of fast substorm evolution in the magnetotail may be a two‐step process, where another presently not included mechanism leads to the destabilization of the tearing instability followed by the nonlinear growth of a large scale instability.

Formation of thin current sheets in a quasistatic magnetotail model

Observations suggest that thin current sheets forming in the near‐Earth magnetotail late in substorm growth phases may be a crucial part of substorm evolution. In a simple theoretical model the current density was shown to become singular for suitable external perturbations. Here, we address the same problem in a more realistic model based on the adiabatic MHD ‐ theory developed by Schindler and Birn [1982]. We show that under suitable conditions the formation of a thin current sheet in the near‐Earth tail is an intrinsic aspect of flux transfer to the magnetotail. The mechanism is based on the strong variation of flux tube volume with the magnetic flux function.

Convecting towards the catastrophe of substorm onset

A nonlinear model of substorm evolution is constructed which self-consistently unifies the directly driven and the loading-unloading systems. Using only the principle that the magnetosphere minimizes its potential energy within the constraints imposed by external conditions, substorms are described with reference to trajectories on a cusp catastrophe surface. The model predicts the conditions required for substorm onset and the amount of energy explosively released at the beginning of the expansive phase.

A proposed production model of rapid subauroral ion drifts and their relationship to substorm evolution

Multisatellite data are used to examine the temporal relationship between Subauroral Ion Drifts (SAID) and the phases of an auroral substorm. Utilizing images of auroral luminosities taken by the Dynamics Explorer 1 (DE 1) spacecraft and observations of particle injection at geosynchronous orbit, we identify the time of expansive phase onset and estimate the time at which recovery begins. Noting the times at which SAID are observed simultaneously by the DE 2 spacecraft, we find that SAID typically occur well after substorm onset (more than 30 min), during the substorm recovery phase. Substantial westward ion drifts and field‐aligned currents are observed well equatorward of the auroral oval during the expansion phase of a substorm, but the drifts lack the narrow spike signature associated with SAID. Prior to substorm onset and after substorm recovery, field‐aligned currents are absent equatorward of the auroral oval and the ionosphere is very nearly corotating. A phenomenological model of SAID production is proposed that qualitatively agrees with the observed ionospheric signatures and substorm temporal relationship. In this model, substorm‐generated, subauroral field‐aligned currents close via Pedersen currents with the outward flowing, region 1 currents at higher latitudes. These Pedersen currents flow in the region of low conductivity equatorward of the auroral oval and are associated with relatively large, poleward directed electric fields. The frictional heating of the ions caused by collisions with the corotating neutral atmosphere substantially increases the rate of ion‐atom interchange between O+ and N2. Subsequent fast recombination of NO+ with electrons further reduces the subauroral F region conductivities with a corresponding increase in the electric field and the frictional heating. This heating leads to thermal expansion, substantial field‐aligned plasma flow, and very large depletions in the F peak concentration, thus additionally reducing the height‐integrated Pedersen conductivity.

Magnetosphere‐ionosphere coupling during plasmoid evolution: First results

The influence of magnetosphere‐ionosphere coupling on the dynamic evolution of the magnetotail is investigated by a three‐dimensional resistive MHD code that includes the effects of the closure of field‐aligned currents in a simple resistive model ionosphere. Particular emphasis is on the role of this coupling during substorm evolution and the modification of the latter by the convection driven by the ionospheric electric fields. For comparison, we present results from a simulation which uses an infinitely conducting ionosphere but is otherwise identical. Comparison of the two simulations shows that the major impact of magnetosphere‐ionosphere communication is an acceleration of magnetotail evolution. Otherwise, phenomena in the two models are qualitatively similar. We conclude that ionospheric effects do not significantly affect substorm associated magnetotail dynamics.

Detailed observations of the plasma sheet during a substorm on April 24, 1979

This paper reports a detailed study of plasma, magnetic field, and energetic particle data acquired by the ISEE 1 and 2 satellites, about 20 RE from earth in the magnetotail plasma sheet during a well‐defined substorm on April 24, 1979. The paper addresses, particularly, the plasma flows and energetic particle anisotropies that were encountered during the various stages of the substorm's evolution and compares these to predictions of the neutral line model of substorms. We find that the observations support that model in considerable detail. The interval from substorm onset at earth to plasma dropout was only 4½ min, and we stress the importance of high time resolution in all measurements if a full understanding of substorm initiation is to be developed. We found that the energetic ions (up to 125 keV) and plasma ions, although they sometimes showed somewhat different angular distributions in the plasma rest frame, typically shared a common bulk speed, i.e., the bulk flow speed of the plasma, parallel and perpendicular to the magnetic field. Thus the anisotropies of the energetic ion fluxes that have sometimes led to their characterization as earthward or tailward “beams” were found to be largely a consequence of their bulk motion with the plasma. We identify the region at the lobe‐plasma sheet interface, which has been called the plasma sheet boundary layer, with the separatrix layer in the neutral line model. This is the region of field lines including, and just equatorward of, the lobe‐plasma sheet separatrix, which should carry the most recent dynamic signatures of reconnection. Our results seem to support that expectation. Several features of our observations were inconsistent with the substorm picture, called the boundary layer dynamics model, in which a satellite's observation of fast plasma and particle flow is due primarily to its envelopment by a preexisting boundary layer of flowing plasma.

Correlated observations of two auroral substorms from an aircraft and from a Vela satellite

A jet aircraft, flying from Goose Bay, Labrador, to Fairbanks, Alaska, made auroral observations at nearly constant magnetic local time (approx.2100 MLT) in the auroral zone while a Vela satellite passed through the plasma sheet at rapprox. =18R/subE/ at nearly the same magnetic local time. Comparison of data from the two locations provide further confirmation of the 'poleward leap' of the auroral electrojet which occurs in a late phase of an auroral substorm and is associated with a rapid tailward motion of an X-type neutral line in the magnetotail. The poleward leap is a a distinctive feature of the substorm evolution and is not simply the superposition of a new substorm on the recovery phase of a preceding substorm. It probably marks the sudden transition of the magnetotail from one quasi-stable configuration to another more stable one. Onset of a substorm expansive phase brings about a change of tail magnetic field from a configuration that is extremely tailike, with field lines from lambda/subm/approximately-less-than66degree stretching to the Vela orbit, to one that is much less taillike, with field lines from lambdam/sub approximately-greater-than/70degree not stretching as far as the Vela orbit. (AIP)