Vorticity in the magnetosphere is of importance because it has been associated with field-aligned current (FAC) systems flowing during substorms. One such current system is the substorm current wedge (SCW). An expression for the current density due to the change of vorticity and the magnetic field is:
where Ω is the vorticity, B is the magnetic field (subscript "i" refers to the ionosphere), ρ is the mass density, and the integration is along the magnetic field line from the equator to the ionosphere.
Several substorm scenarios, such as plasma flow breaking/diversion and magnetic pileup, cross-field current instability, and ballooning instabilities of various types, allow for the formation of flow vorticity and associated FAC generation.
In this study, we present a comparison of conjugate magnetospheric and ionospheric vortices, and we demonstrate that the ionospheric vortices were driven by the space vortices (Figure 1). Additionally, evidence is given that the space vortices contributed to the field-aligned current of the SCW by applying the above equation and by comparison with model results of a reconstructed SCW.
On 19 February 2008, an isolated substorm occurred. The THEMIS spacecraft were located in the nightside plasma sheet, close to the neutral sheet. While TH-C, D, and E recorded strong plasma flow in the positive X direction (GSM), TH-A recorded strong flow in the negative X direction. The flows show characteristics of counter-rotating vortices (Figure 1).
|Figure 1. Poleward expanding aurora and two plasma |
flow vortices in space causing the aurora.
|Click here to enlarge the image.|
Simultaneously, two equivalent ionospheric current (EIC) vortices with opposite rotation formed (Figure 2). The blue vectors indicate the current direction and intensity. Clockwise and counterclockwise rotations correspond to downward and upward FAC, respectively. Magnetic-field-line mapping shows that the space vortices were conjugate to the EIC vortices.
|Figure 2. Two counter-rotating |
(equivalent) ionospheric current
systems (see red circles) during the
development of a substorm. Blue
arrows show the current directions.
|Click here to enlarge the image.|
Using the above equation and data from the THEMIS spacecraft, we estimate the contribution to the field-aligned current by the counterclockwise-rotating space vortex. We obtain a current density of about 2.8 nA/m2 or a total current of 0.1 MA.
In comparison, the modeled current of the SCW (not shown here, see Keiling et al., 2009) shows a gradual increase of the current with a peak value of 0.7 MA about 10 min after onset. Closer to the time of the space vortex observations, the modeled current was between 0.1-0.2 MA. Therefore, this comparison suggests that the space vortices contributed a significant part or all of the FAC of the SCW at onset of the substorm expansion phase, which could later be replaced by other current generating mechanisms.
Here, the novel result is the calculation of the substorm current by direct application of the vortex equation which relates the field-aligned current generation to the forming space vortex. Furthermore, the combined ground and space observations, together with the model results, present a scenario in which the space vortices generated the field-aligned current of the SCW at the beginning of the substorm expansion phase and coupled to the ionosphere, causing the ionospheric vortices.
For more details, see A. Keiling, et al. (2009), Substorm current wedge driven by plasma flow vortices: THEMIS observations, J. Geophys. Res., 114, A00C22, doi:10.1029/2009JA014114. Additional, more popular descriptions of these results can be found at the following websites:
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Andreas Keiling is a research space physicist with the Space Sciences Laboratory at the University of California, Berkeley. His current research interests are magnetosphere-ionosphere coupling, ULF waves, and substorm/auroral dynamics.