| 1. |
- Rothwell, P L, et al.
(författare)
-
The role of O+ ions in channeling solar wind energy to the ionosphere
- 2000
-
Ingår i: IEEE Transactions on Plasma Science. - : Institute of Electrical and Electronics Engineers (IEEE). - 0093-3813 .- 1939-9375. ; 28, s. 1912-1919
-
Tidskriftsartikel (refereegranskat)abstract
- In space weather prediction, the transport of solar wind energy through the magnetosphere is a major aspect. For the transport of energy from the magnetosphere to the ionosphere, magnetic field-aligned (Birkeland) currents are a very important agent. In the present paper, we discuss the role of O+ ions for driving field-aligned currents of spatially alternating polarity that may explain multiple auroral arcs, It is known from earlier work that nonadiabatic motion of O+ ions in the magnetotail plasma can lead to the formation of density striations that are stationary in the GSM frame, As the magnetospheric plasma drifts through these density striations, magnetic field-aligned currents of alternating signs are forced to flow in and out of the oxygen-rich region to maintain quasineutrality. This generates Alfven waves that propagate in the drifting plasma but can form stationary structures in the GSM frame. As the currents close in the ionosphere, the equatorial plasma constitutes a generator from which spatially alternating magnetic field-aligned currents carry energy to the ionospheric load, The wavelength of the density striations, mapped to the ionosphere, is compatible with the spacing of stable auroral arcs, and the power supplied by the equatorial generator region is estimated to be compatible with what is needed to drive auroral arcs, Thus, the consequences of nonadiabatic motion of O+ ions may explain how part of the energy extracted from the solar wind is channeled into multiple auroral arcs.
|
|
| 2. |
- Silevitch, M B, et al.
(författare)
-
O+ phase bunching as a source for stable auroral arcs
- 2000
-
Ingår i: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 105, s. 10739-10749
-
Tidskriftsartikel (refereegranskat)abstract
- We propose a model to explain how ion dynamics create an Alfven wave generator in the equatorial region that can be applied to the stable are problem. For example, in the earthward drifting magnetotail plasma, phase bunching of O+ ions land to a much lesser extent of the H+ ions) can be caused by a weak (similar to 1x10(-9) Vm(-2)) electric field gradient [Rothwell et al., 1994]. This leads to density striations in the GSM frame. O+ density striations in the earthward drifting plasma frame are seen as a tailward propagating source of Alfven waves where the hydrogen ions provide the polarization current of the wave. A transformation to the CSM frame will yield a static, oblique wave structure similar to that previously treated. The waves propagate from the equatorial region to both ionospheres where they are reflected. The ionospheric boundary condition when combined with a magnetospheric boundary condition allows a solution of the wave amplitudes in terms of the striation structure. The frequency of the Alfven wave and the associated wavelengths are also determined by the striation driver. We find that the magnitude of the parallel current density at the ionosphere has a spatial resonance when the distance between the ionosphere and the equatorial plane is equal to a quarter wavelength along B-o. In that case, the magnitude of the parallel current density at the ionosphere is of the order of 10 mu A m(-2) and peaks for striation wavelengths las mapped to the ionosphere) of 10 -40 km, which is comparable to the transverse scale of auroral arcs. The associated Poynting flux incident on the ionosphere is found to be similar to 2 mWm(-2) and represents a net transfer of energy from the magnetosphere to the ionosphere as recently observed by experimenters studying substorm onsets. We find that in the steady state the power extracted from the bulk flow to power the are is balanced by energy provided by the solar wind through the cross-tail electric field.
|
|
