Plasma Sheet Reconnection Research Articles

Development of Magnetic Topology During the Growth Phase of the Substorm Inducing the Onset of the Near‐Earth Neutral Line

AbstractThe transition of magnetic topology is analyzed during the substorm. During the growth phase in the polar ionosphere, magnetic fields consist of three categories: closed magnetic field, open magnetic field leading to the northward interplanetary magnetic field (IMF), and open field leading to the southward IMF. While the open magnetic field region leading to the southward IMF expands consistently during the growth phase, the ionospheric onset starts inside the closed field line region. There should be triple points where three kinds of magnetic field coexist in the ionosphere. Connected to the triple points, complex magnetic topologies appear in the magnetosphere. There occur definitive changes in the global magnetic topology such as retreat of cusp nulls generated under the northward IMF, generation of plasma sheet magnetic fields with reverse curvature, generation of bifurcation areas on the magnetopause, formation of new dayside nulls, development of null lines on the flank, and generation of bifurcation areas (group of nulls) on the magnetopause. During the late growth phase, divergence Vx flow is excited in the central plasma sheet associated with the appearance of reverse curvature. The plasma sheet reconnection starts 5 min before the ionospheric onset from a localized area at X = −17.5 Re due to the reduction of the normal component Bz caused by divergence Vx flow. It occurs in the residual magnetic structure formed under the northward IMF as a three‐dimensional reconnection with a guide field By. The resulting two‐turn coil of magnetic field forms a plasmoid, which is ejected downtail.

Auroral streamers implication for the substorm progression on September 14, 2004

Magnetospheric substorms manifest a fundamental disturbance in geospace. Understanding of substorm progression is essential for solar terrestrial physics and space weather research. The auroral streamer and the bursty bulk flows (BBFs) were simultaneously observed by the IMAGE/WIC and the Cluster in the midtail (X∼−16RE), whose footprint was near the auroral streamer, respectively, during a substorm on September 14, 2004. The auroral poleward boundaries (APBs) are 2∼3° MLAT lower than the open–closed field line boundaries (OCBs) during the growth phase. After the expansion onset, streamer elongated poleward, and the location of APBs became coincident with OCBs. These observations are consistent with the scenario that substorm activations start from plasma sheet reconnection of closed field lines during growth phase; in the expansion phase the open field line in the lobes are involved in reconnection in tail.

Active phase of a substorm as a chain of two types of reconnection: In the closed plasma sheet and in the open tail lobes

The maps of the field-aligned current (FAC) density distribution in the ionosphere obtained by the TIM-2 magnetogram inversion technique are used to investigate the August 27, 2001 substorm. The open magnetic flux Ψ and intensity J of the substorm current wedge (SCW) have been determined with a step of 1–5 min. The substorm onsets are divided into two types, PSR (plasma sheet reconnection) and TLR (tail lobe reconnection). The fast expansion tailward of the PSR region is described as the transition from PSR to PSR+TLR. Assuming that the SCW FACs flow down into the ionosphere from the edges of the disruption region of the cross-tail dawn-dusk current, several parameters of the disruption region have been estimated. The disrupted magnetic field has been found to be ∼5% of the undisrupted one for PSR and ∼95% for PSR+TLR. The disturbance power Q for PSR is an order of magnitude lower than that for PSR+TLR. The abrupt growth of Q during the transition from PSR to PSR+TLR is observed over the entire SCW area from its near-Earth part to the midtail and distant tail.

Multispacecraft and ground‐based observations of substorm timing and activations: Two case studies

Two case studies are performed to investigate substorm timing and activations based on Double Star TC1, Cluster, Polar, IMAGE, LANL geostationary satellites and ground‐based geomagnetic field measurements. In both events, an earthward flow associated with plasma sheet thinning is measured by Cluster 8–10 min ahead of the auroral breakup. A couple of minutes after the breakup, either TC1 at ∼X‐10 RE first detects plasma sheet expansion and then the LANL satellites near the midnight measure energetic electron injections at geostationary orbit or the LANL satellites first measure the electron injections and then TC1 detects the plasma sheet expansion. More than about 20 min later, Cluster at X∼16 RE and Polar (at higher latitude) successively observe plasma sheet expansion. The open magnetic flux of the polar cap, Ψ, is found to continually increase during the early substorm phase and then to rapidly fall when the IMF turns northward. When Ψ reaches its minimum value, bright and broad auroral activities start to decrease. Tailward progression of the magnetic dipolarization and a poleward expansion of auroral bulges are shown to closely map to one another. These results suggest that substorm activations start in the midtail before ground onset and then move earthward, which leads to an expansion onset in the near‐Earth tail around X∼ ‐(8–9) RE. After onset, the activations progress both earthward and tailward. Substorm onset is possibly related to plasma sheet reconnection of close field lines, while tail lobe reconnection of open field lines release more energy to support the full expansion of the substorm. In a fully developed expansion phase, an initial dipolarization in the near‐Earth may eventually evolve to enable disruption of the cross‐tail current over a wide region of the magnetotail.

Comment on “Jupiter: A fundamentally different magnetospheric interaction with the solar wind” by D. J. McComas and F. Bagenal

Comment on “Jupiter: A fundamentally different magnetospheric interaction with the solar wind” by D. J. McComas and F. Bagenal

Ion heat flux and energy transport near the magnetotail neutral sheet

Ten‐year averages of energy transport rates near the neutral sheet showed that the enthalpy flux density or thermal energy term QT = (5/2)PV was the largest, where P is the isotropic pressure and V is the bulk flow velocity. The ion heat flux, qi, was the next largest term. Sorting data using a magnetic flux transport parameter showed that qi could become dominant during periods of slow flow. Both qi and the ion bulk velocity Vi were duskward on the dusk side of the neutral sheet. This relationship is characteristic of cross‐tail drift and a heat flux that can be attributed to the energy dependent gradient and curvature drifts. The qi and Vi vectors often pointed in different directions on the dawn side. The x component of qi on the dawn side pointed tailward, suggesting entry through the magnetopause of a suprathermal ion component. On the dusk side the qix plots that were sorted using a magnetic flux transport parameter showed evidence of plasma sheet reconnection. The long‐term averaged x component of QT pointed earthward almost everywhere in the neutral sheet, and was attributed to periods of very fast plasma flow. The cross‐tail component of QT was separated into two contributions. One part of QTy involved a common drift away from midnight during both earthward and tailward fast flows. This feature suggests that thermal energy and plasma flow from the outer plasma sheet toward the neutral sheet near midnight, and then toward the flanks. The other part of QTy involved a differential duskward drift during fast earthward flows and a dawnward drift during fast tailward flows. The incremental E fields that would produce such convection point tailward during the fastest earthward flows and earthward during the fastest tailward flows. The dependencies of Vi, qi and QT on the interplanetary magnetic field (IMF) clock angle also were studied. Both Vi and QT were reduced when the IMF was northward and the neutral sheet plasma became cold and dense. However, no dependence of qi on the IMF direction was seen. This shows that the generation of a cold dense plasma sheet does not substantially change the distribution of the suprathermal ions that are most important in the production of qi.

Geotail observations of north‐south plasma velocity enhancements in the lobe near substorm expansion phase onset

This paper reports on the enhancement of the lobe plasma velocity towards the neutral sheet. Analyzing the Geotail plasma and magnetic field data in the lobes earthward of X = −45 RE, we identified 30 events of several‐minute magnetic field increase‐and‐decrease during which the field tilts only southward; that is, the BZ variation is unipolar. For most of these events, we found that the lobe plasma velocity towards the neutral sheet is enhanced and peaked around the most southward tilting of the field. The average of the peak magnitude is about 130 km/s in a plane perpendicular to the magnetic field. For each case, this magnitude is roughly 1–10 % of the lobe Alfven speed. We interpret the velocity enhancement having these characteristics as signatures of the transition from plasma sheet reconnection to lobe reconnection at the near‐Earth neutral line.

Substorm expansion phase caused by an intense localized convection imposed on the ionosphere

It is shown that the substorm expansion phase requires an intense localized convection in the midnight sector. The earlier magnetosphere‐ionosphere coupling (MIC) model showed that the enhanced global convection imposed on the ionosphere can only produce the growth phase. By imposing the intense localized convection on the ionosphere at the end of the growth phase, the new MIC model presented in the present paper can quantitatively produce the signatures of all substorm phases. The pattern of the localized convection is consistent with the wedge‐like field‐aligned current obtained from the solution to the Alfvén wave “field‐current” equation. During the expansion phase in the new MIC model, the ionospheric convection field increases to ∼40 mV/m, the Hall conductance is enhanced to ∼33 mho, the upward field‐aligned current increases to ∼1.8 μA/m2, and the auroral electrojet current increases to ∼0.8 × 106 A. The results of the MIC model are in good quantitative agreement with substorm observations. We suggest that the intense localized nightside convection is driven by localized reconnection of closed field lines in the plasma sheet. A remaining great challenge in substorm research is to uncover the cause of the localized plasma sheet reconnection.

A global simulation of the magnetosphere with a long tail: Southward and northward interplanetary magnetic field

Through a global three‐dimensional MHD simulation, which makes it possible to reveal the physical processes and causalities of the global interaction between the solar wind and the magnetosphere (Watanabe and Sato, 1990; Kageyama et al., 1992), the effect of the interplanetary magnetic field (IMF), both northward and southward, on Earth's magnetosphere has been investigated. A southward IMF, upon reconnecting at the dayside magnetopause, sweeps the Earth's magnetic field toward the nightside and drapes the magnetotail. This gives rise to a plasma sheet cross‐tail current increase and explosive magnetic reconnection at around 15 RE from the Earth. This reconnection results in the formation of a large plasmoid which grows much faster than for the simulation with no IMF, thus supporting previous notions that a southward IMF is a driving mechanism of plasma sheet reconnection. A northward IMF is observed to reconnect with the magnetosphere along the cusp shoulder, stripping magnetic field lines away and weakening compression of the plasma sheet, thus inhibiting plasma sheet reconnection. In order to accommodate a balance of magnetic to dynamic pressure along the magnetopause, the magnetosphere changes from its usual cometlike shape to that resembling a tadpole as it attempts to return to a dipolar structure. Plasma sheet reconnection is thereby inhibited. A case when the direction of the IMF turns from south to north is also studied.

Magnetic storms, substorms and microsubstorms

The present conception of substorms is now under development within the scope of two basic models. In one of them (model 1) an active substorm phase is due to instability (plasma sheet reconnection) resulting from the accumulation of solar wind energy, transported to the magnetosphere, in the form of magnetotail energy. In model 2 the active phase requires no energy accumulation. In this paper a semi-empirical model is proposed where the active substorm phase comprises two periods. The first period (convection-type disturbances) corresponds to model 2, the second one (anticonvection-type disturbances) does not contradict model 1.