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| 2. |
- Brenning, Nils, et al.
(author)
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Interpretation of the Electric Fields Measured in an Ionospheric Critical Ionization Velocity Experiment
- 1991
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96, s. 9719-9733
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Journal article (peer-reviewed)abstract
- This paper deals with the quasi-dc electric fields measured in the CRIT I ionospheric release experiment, which was launched from Wallops Island on May 13, 1986. The purpose of the experiment was to study the critical ionization velocity (CIV) mechanism in the ionosphere. Two identical barium shaped charges were fired from distances of 1.99 km and 4.34 km towards a main payload, which made full three-dimensional measurements of the electric field inside the streams. There was also a subpayload separated from the main payload by a couple of kilometers along the magnetic field. The relevance of earlier proposed mechanisms for electron heating in CIV is investigated in the light of the CRIT I results. It is concluded that both the “homogeneous” and the “ionizing front” models probably apply, but in different parts of the stream. It is also possible that electrons are directly accelerated by a magnetic-field-aligned component of the electric field; the quasi-dc electric field observed within the streams had a large magnetic-field-aligned component, persisting on the time scale of the passage of the streams. The coupling between the ambient ionosphere and the ionized barium stream in CRIT I was more complicated than is usually assumed in CIV theories, with strong magnetic-field-aligned electric fields and probably current limitation as important processes. One interpretation of the quasi-dc electric field data is that the internal electric fields of the streams were not greatly modified by magnetic-field-aligned currents, i.e., a state was established where the transverse currents were to a first approximation divergence-free. It is argued that this interpretation can explain both a reversal of the strong explosion-directed electric field in burst 1 and the absence of such a reversal in burst 2.
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| 4. |
- Rothwell, P.L., et al.
(author)
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Acceleration and Stochastic Heating of Ions Drifting Through an Auroral Arc
- 1992
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 97, s. 19333-19339
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Journal article (peer-reviewed)abstract
- We find that ions E x B drifting through an auroral arc can undergo transverse acceleration and stochastic heating. This result is very analogous to recent work regarding similar phenomena in the magnetotail (Buchner and Zelenyi, 1990; Chen and Palmadesso, 1986; Brittnacher and Whipple, 199 1). An analytic expression for the maximum arc width for which chaotic behavior is present is derived and numerically verified. We find, for example, that a 1.5-km-thick arc at LAMBDA = 65-degrees requires a minimum potential drop of 3 kV for transverse ion acceleration and heating to occur. Thicker arcs require higher potential drops for stochasticity to occur. This mechanism could be a source for conic ions.
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| 5. |
- Rothwell, P.L., et al.
(author)
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O+ phase bunching and auroral arc structure
- 1994
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 99, s. 2461-2470
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Journal article (peer-reviewed)abstract
- The equations of motion are solved for ions moving in a model electric field that corresponds to the nightside equatorial region df the magnetosphere. The model represents the poleward region of the Harang discontinuity mapped to the magnetosphere. Within this region the model electric field has a constant earthward gradient superimposed on a constant dawn-to-dusk electric field. In combination with the earthward drift motion due to the dawn-to-dusk field, the electric field gradient introduces an earthward inertia drift, which is proportional to the ion mass and therefore faster for O+ ions than for H+ ions or electrons. It is also found that the entry of the ions into the gradient region causes phase bunching and as a result ion density striations form. The striations are enhanced for more abrupt changes in the electric field gradient, a weaker magnetic field, a stronger cross-tail electric field and colder O+ ions. The first two conditions apply during the growth phase of a substorm. Using the Tsyganenko (1987) model a minimum electric field gradient value of 1 x 1O(-9) V/m(2) ((1 mV/m)/1000 km) at L = 6-7 is found. Charge neutrality requires coupling with the ionosphere through electrons moving along magnetic held lines, and such electrons may be the cause of multiple auroral arcs.
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| 6. |
- Rothwell, P.L, et al.
(author)
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Particle dynamics in a spatially varying electric field
- 1995
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 100, s. 14875-14885
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Journal article (peer-reviewed)abstract
- For an MHD description of a plasma a distinct separation between the macroscopic and microscopic spatial and temporal scales is assumed. In this paper we solve the particle dynamics with finite first and second spatial derivatives in the electric field. We find that (1) MHD (ideal and nonideal) becomes invalid for a sufficiently strong constant electric field gradient perpendicular to the magnetic field; (2) a sufficiently large second derivative in the electric field can cause heavy ions to become chaotically untrapped; (3) for an electric field with a constant gradient the ion drift velocity is equal to (ExB)/\textbackslashB\textbackslash(2) as long as the orbit-averaged value off is used. There are no finite currents associated with the ion drift for such an electric field; (4) perturbation technique gives a poor approximation to the ion drift velocity even for values of the second derivative that may well occur in the magnetosphere. Results 1 and 2 provide necessary criteria for the applicability of magnetospheric MHD models of spatially varying electric fields. They also predict an asymmetry in the heavy ion fluxes, a feature that could be useful in inferring magnetospheric electric field structure. We illustrate these results by application to the Harang discontinuity. It is found that if the interplanetary magnetic field swings northward under substorm growth conditions the orbits of the equatorial O+ may dramatically change due to result 2. This effect may contribute to the substorm onset process.
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| 8. |
- Rothwell, P.L., et al.
(author)
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PREBREAKUP ARCS - A COMPARISON BETWEEN THEORY AND EXPERIMENT
- 1991
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 96, s. 13967-13975
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Journal article (peer-reviewed)abstract
- We have developed a model describing the structure of a prebreakup arc based on an ionospheric Cowling channel and its extension into the magnetosphere. A coupled two-circuit representation of the substorm current wedge is used which is locally superimposed on both westward and eastward electrojets. We find that brighter, more unstable prebreakup arcs are formed in the premidnight (southwest of the Harang Discontinuity) than in the postmidnight (northeast of the Harang Discontinuity) sector. This contributes to the observed prevalence of auroral activity in the premidnight sector. Also, our model predicts that the north-south dimensions of the current wedge in the ionosphere should vary from a few kilometers at an invariant latitude (LAMBDA) of 62-degrees to hundreds of kilometers above LAMBDA = 68-degrees. Comparison of the model results with the extensive observations of Marklund et al. (1983) for a specific arc observed just after onset shows good agreement, particularly for the magnitude of the polarization electric field and the arc size. We conclude that this agreement is further evidence that the substorm breakup arises from magnetosphere-ionosphere coupling in the near magnetosphere and that the steady state model developed here is descriptive of the breakup arc before inductive effects become dominant.
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| 9. |
- Silevitch, M B, et al.
(author)
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O+ phase bunching as a source for stable auroral arcs
- 2000
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In: Journal of Geophysical Research. - 0148-0227 .- 2156-2202. ; 105, s. 10739-10749
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Journal article (peer-reviewed)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.
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