Reconnection in the Magnetotail

6 Reconnection in the Magnetotail

Space weather events are largely driven by dynamic processes that occur within the magnetotail plasma sheet separating the low-density tail lobes. As was shown before, the large-scale structure of the magnetosphere causes the incoming Poynting flux to focus toward the plasma sheet. This leads to structural changes in the current sheet separating the antiparallel magnetic fields in the lobes, and may lead to bursts of reconnection associated with fast flows both Earthward and tailward of the reconnection region.

The magnetotail plasma sheet is a highly dynamic and structured region. Plasma flows in this region are not laminar, and during most times the net Earthward plasma flow imposed by the large-scale convection pattern is composed of short-lived (1 – 10 min) bursts of fast flow while the ambient plasma velocity distribution is very nearly isotropic. These flows are most likely created by localized reconnection events initiated either by internal tail processes or by external driving conditions (Baumjohann, 1993 ; Nagai et al., 2005 ). While such bursts of fast flow can be observed during all magnetospheric activity conditions, they become more numerous, more intense, and have larger scale sizes during magnetically active conditions. As this is the region which feeds the inner magnetosphere with both plasma, energetic particles, and magnetic flux, the plasma sheet dynamics is crucially important for space weather applications.

As the magnetospheric activity conditions are largely controlled by the stability properties of the cross-tail current sheet, and as the magnetotail current sheet shares many of the properties of dynamically important current sheets found e.g. in the solar plasmas, the dynamics of the cross-tail current has been intensively studied in recent years. In the quiet state, the cross-tail current sheet is rather thick, and the plasma and current sheet tailward of the quasi-dipolar region can be described by a simple one-dimensional Harris current sheet, where the magnetic field is given by B_x = B₀ tanh(Z∕λ), where B₀ is the lobe magnetic field, Z is the coordinate across the current sheet, and λ defines the scale thickness of the current sheet. Closer to the Earth, inside of about 15 R_E distance, the current sheet starts to deviate from the one-dimensional structure as the dipole introduces a component perpendicular to the current sheet. In the inner magnetosphere, the multiple plasma populations introduce their own complexity to the system.

As the IMF turns southward, dayside reconnection changes the conditions at the magnetospheric boundaries and begins to increase the open flux content in the magnetotail. In order to maintain pressure balance between the plasma sheet plasma pressure and the lobe magnetic pressure, the cross-tail current intensifies and the plasma sheet is compressed. However, in the region tailward of geostationary orbit out to about 20 – 30 R_E the changes in the current density are not uniform: the total current is distributed between the pre-existing thick plasma/current sheet and a newly formed thin current sheet embedded within the plasma sheet. The thin current sheet is often in the ion gyroradius scale, and can host very high current densities at the field reversal region (Runov et al., 2006 ; Sitnov et al., 2006 ). Furthermore, complex, bifurcated current sheets and large-scale wavy structures have been identified from multi-spacecraft analyses.

Figure 15:

Substorm on December 10, 1996. Left, from top to bottom: North-south component of the IMF. Magnetotail observations of the thinning and intensification of the tail current sheet. The period of energy storage is bracketed with the vertical lines. Increasing B_x in the magnetotail lobe recorded by Interball is a measure of increasing magnetic flux content in the tail. Decreasing B_z in the plasma sheet at geostationary orbit (recorded by GOES-9) and at about 25 R_E distance (recorded by Geotail) reflects the increasing tail current intensity.

Figure 16:

mpg-Movie (3926 KB) Substorm on December 10, 1996. Four snapshots of the LFM global MHD simulation. The color coding shows the plasma density in the noon-midnight meridian plane. The gray shading outlines the last closed flux surface indicating the large-scale magnetic topology of the system. The topological changes are consistent with those presented in the schematic picture in Section 3. The direction of the Sun is to the left.

During magnetospheric substorms, a thin and intense current sheet forms in the inner part of the magnetotail as a consequence of the intensified driving. This current intensification is seen as an increase of the lobe field B_x and a decrease in the normal component B_z at the inner magnetotail current sheet. Figures 15

and 16

show observations during a substorm event on December 10, 1996. The top panel of Figure 15

shows the IMF B_z which turned southward shortly after 06:00 UT. The next panels show the intensification of the lobe field recorded by the Interball satellite and thinning of the current sheet measured by the geostationary orbit satellite GOES-9 and the Geotail spacecraft in the tail plasma sheet. Figure 16

shows frames of the Lyon–Fedder–Mobarry (LFM) global MHD simulation code that was run for the substorm. The gray shading outlines the closed flux within the plasma sheet. The changes from the first to second frame illustrate the strong compression and thinning of the plasma sheet. The third panel shows the pinching off of the plasmoid and reduction of the volume of the closed-flux region. The last panel illustrates return to quiet conditions. These changes are consistent with those suggested by the schematic in Figure 5

. Figure 17

shows a frame from the same simulation, now plotted in an equatorial plane projection and showing the electric field with the color coding. The region of the thin current sheet is shown with the red and yellow colors indicating highest electric field intensity. The white circle shows the thin current sheet location as derived from an empirical model (Pulkkinen and Wiltberger, 2000 ), in remarkable agreement with the simulation result. The white arrows representing the flow velocity show a flow channel that is intruding into the inner magnetosphere and interacting with the thin current sheet region. The simulation shows a series of such localized flow bursts that penetrate to the inner magnetosphere, finally disrupting the thin current sheet when the substorm expansion phase begins.

Figure 17:

mpg-Movie (16561 KB) Substorm on December 10, 1996. LFM global MHD simulation results of the magnetotail flows in an equatorial plane projection. The direction of the Sun is to the left. The color coding shows the cross-tail electric field, with brighter colors showing higher electric field values indicative of a thin and intense current sheet at that location (highlighted by the white circle). The white arrows depict the flow velocity. Note a flow channel initiating from about 40 R_E distance and focusing toward the thin current sheet region (outlined with the white dotted line). Such flow channels become larger and more frequent during substorm activity and finally lead to disruption of the intense inner-tail current sheet during the substorm expansion phase.

As the flows created by magnetic reconnection in the tail enter the inner magnetosphere, the large-scale magnetic field configuration changes rapidly from highly taillike to a much more quasidipolar state. Furthermore, the reconnection process is associated with rapid and significant energy conversion from magnetic energy in the magnetotail lobes to particle kinetic energy and heat in the plasma sheet. The tail field reconfiguration is also associated with strong field-aligned currents to and from the ionosphere, which in part contribute to the energy dissipation in the ionosphere. Thus, the large-scale current disruption, configuration change, and field reconfiguration all are associated with a major energy dissipation process in the magnetosphere.

The structure and dynamics of the current sheet in the magnetotail control the energy storage and release processes initiated with the enhanced dayside reconnection at the magnetopause. The high-speed plasma flows, strong particle energization processes, and rapid reconfiguration all are major parts in creating the space weather effects in the inner magnetosphere.