Mechanism by which merging at X lines causes discrete auroral arcs

Atkinson, G.

doi:10.1029/91ja02443

Cited by 69 publications

(84 citation statements)

References 19 publications

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“…The magnetic reconfiguration in the merging process must be communicated away from the X-line along the separatrices via Alfve´n waves carrying field aligned currents (see Atkinson, 1992;Ma and Bhattachargee, 2001). These waves will have a parallel Poynting flux.…”

Section: Merging Primermentioning

confidence: 99%

Coupling the Solar-Wind/IMF to the Ionosphere Through the High Latitude CUSPS

Maynard¹

2005

Surv Geophys

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Magnetic merging is a primary means for coupling energy from the solar wind into the magnetosphere-ionosphere system. The location and nature of the process remain as open questions. By correlating measurements from diverse locations and using large-scale MHD models to put the measurements in context, it is possible to constrain our interpretations of the global and meso-scale dynamics of magnetic merging. Recent evidence demonstrates that merging often occurs at high latitudes in the vicinity of the cusps. The location is in part controlled by the clock angle in the interplanetary magnetic field (IMF) Y-Z plane. In fact, B Y bifurcates the cusp relative to source regions. The newly opened field lines may couple to the ionosphere at MLT locations of as much as 3 hr away from local noon. On the other side of noon the cusp may be connected to merging sites in the opposite hemisphere. In fact, the small convection cell is generally driven by opposite hemisphere merging. B X controls the timing of the interaction and merging sites in each hemisphere, which may respond to planar features in the IMF at different times. Correlation times are variable and are controlled by the dynamics of the tilt of the interplanetary electric field phase plane. The orientation of the phase plane may change significantly on time scales of tens of minutes. Merging is temporally variable and may be occurring at multiple sites simultaneously. Accelerated electrons from the merging process excite optical signatures at the foot of the newly opened field lines. All-sky photometer observations of 557.7 nm emissions in the cusp region provide a ''television picture'' of the merging process and may be used to infer the temporal and spatial variability of merging, tied to variations in the IMF.

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Section: Merging Primermentioning

confidence: 99%

Coupling the Solar-Wind/IMF to the Ionosphere Through the High Latitude CUSPS

Maynard¹

2005

Surv Geophys

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“…This information must be conveyed away from the separator along the merged field lines. Atkinson (1992), following earlier calculations by Sonnerup and Wang (1987), showed that Alfvén waves propagate away from the diffusion region, carrying the field-aligned currents (FAC) needed to discharge Hall electric fields that develop in the collisionless merging process. These FACs are carried by electrons (since the ions cannot respond in these thin layers) with relatively large drift speeds parallel to B, having parallel electron Alfvén Mach numbers as high as 4, as recently been reported by Scudder et al (2002a).…”

Section: Wave Poynting Flux As a Merging Discriminatormentioning

confidence: 99%

“…When merging occurs, Alfvén waves are launched to communicate information to magnetically connected regions (Atkinson, 1992). The associated wave Poynting vector is directed away from the source since it represents escaping electromagnetic energy.…”

Section: Introductionmentioning

confidence: 99%

Polar, Cluster and SuperDARN evidence for high-latitude merging during southward IMF: temporal/spatial evolution

Maynard¹,

Ober²,

Burke

et al. 2003

Ann. Geophys.

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Abstract. Magnetic merging on the dayside magnetopause often occurs at high latitudes. Polar measured fluxes of accelerated ions and wave Poynting vectors while skimming the subsolar magnetopause. The measurements indicate that their source was located to the north of the spacecraft, well removed from expected component merging sites. This represents the first use of wave Poynting flux as a merging discriminator at the magnetopause. We argue that wave Poynting vectors, like accelerated particle fluxes and the Walén tests, are necessary, but not sufficient, conditions for identifying merging events. The Polar data are complemented with nearly simultaneous measurements from Cluster in the northern cusp, with correlated observations from the Super-DARN radar, to show that the locations and rates of merging vary. Magnetohydrodynamic (MHD) simulations are used to place the measurements into a global context. The MHD simulations confirm the existence of a high-latitude merging site and suggest that Polar and SuperDARN observed effects are attributable to both exhaust regions of a temporally varying X-line. A survey of 13 merging events places the location at high latitudes whenever the interplanetary magnetic field (IMF) clock angle is less than ∼150 • . While inferred highlatitude merging sites favor the antiparallel merging hypothesis, our data alone cannot exclude the possible existence of a guide field. Merging can even move away from equatorial latitudes when the IMF has a strong southward component. MHD simulations suggest that this happens when the dipole tilt angle increases or when IMF B X increases the effective dipole tilt.

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“…The generator is the source of free energy (usually in the magnetosphere) that produces the field-aligned currents, and the magnetic and electric fields associated with auroral arcs. Some possible generators include shear flow in magnetospheric convection, pressure gradients at small angles to gradients in flux tube volume (Lyons and Samson, 1992), reconnection and X-line formation (Atkinson, 1992) and compressional MHD energy that excites FLRs . Borovsky (1993) discusses a variety of the mechanisms, excluding the FLR.…”

Section: Introductionmentioning

confidence: 99%

Optical signatures of auroral arcs produced by field line resonances: comparison with satellite observations and modeling

2003

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Abstract. We show two examples from the CANOPUS array of the optical signatures of auroral arcs produced by field line resonances on the night of 31 January 1997. The first example occurs during local evening at about 18:00 MLT (Magnetic Local Time), where CANOPUS meridian scanning photometer data show all the classic features of field line resonances. There are two, near-monochromatic resonances (at approximately 2.0 and 2.5 mHz) and both show latitudinal peaks in amplitude with an approximately 180 degree latitudinal phase shift across the maximum. The second field line resonance event occurs closer to local midnight, between approximately 22:00 and 22:40 MLT. Magnetometer and optical data show that the field line resonance has a very low frequency, near 1.3 mHz. All-sky imager data from CANOPUS show that in this event the field line resonances produce auroral arcs with westward propagation, with arc widths of about 10 km. Electron energies are on the order of 1 keV. This event was also seen in data from the FAST satellite (Lotko et al., 1998), and we compare our observations with those of Lotko et al. (1998). A remarkable feature of this field line resonance is that the latitudinal phase shift was substantially greater than 180 degrees. In our discussion, we present a model of field line resonances which accounts for the dominant physical effects and which is in good agreement with the observations. We emphasize three points. First, the low frequency of the field line resonance in the second event is likely due to the stretched topology of the magnetotail field lines, with the field line resonance on field lines threading the earthward edge of the plasma sheet. Second, the latitudinal phase structure may indicate dispersive effects due to electron trapping or finite ion gyroradius. Third, we show that a nonlocal conductivity model can easily explain the parallel electric fields and the precipitating electron energies seen in the field line resonance.

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Mechanism by which merging at X lines causes discrete auroral arcs

Cited by 69 publications

References 19 publications

Coupling the Solar-Wind/IMF to the Ionosphere Through the High Latitude CUSPS

Coupling the Solar-Wind/IMF to the Ionosphere Through the High Latitude CUSPS

Polar, Cluster and SuperDARN evidence for high-latitude merging during southward IMF: temporal/spatial evolution

Optical signatures of auroral arcs produced by field line resonances: comparison with satellite observations and modeling

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