Ionospheric conductance distribution and MHD wave structure: observation and model

Budnik, F.; Stellmacher, Martin; Glaßmeier, Karl‐Heinz; Buchert, Stephan

doi:10.1007/s00585-998-0140-8

Cited by 22 publications

(19 citation statements)

References 23 publications

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“…However, Allan [1983] did not have these techniques available. Budnik et al [1998] used time series and power spectra to examine data from one GOES satellite. It is not as easy to identify field line resonances solely from power spectra, especially when there is significant damping in the night hemisphere.…”

Section: Discussionmentioning

confidence: 99%

“…We have presented observations and simulations demonstrating the excitation of quarter‐wave modes near 0700 LT in June at L = 2.6 in the American sector. According to Allan and Knox [1979a, 1979b] and Budnik et al [1998], Σ A − Σ P should have different signs at conjugate ends of the field line when the quarter‐wave mode is generated. Figure 6a shows that the Pedersen conductance was larger than the Alfvén conductance at both ends of the field line when we observed these events.…”

Section: Discussionmentioning

confidence: 99%

“…Figure 6a shows that the Pedersen conductance was larger than the Alfvén conductance at both ends of the field line when we observed these events. The model of Budnik et al [1998] included no contribution from ionospheric Hall currents, and did not account for the magnetic field dip angle. Thus they described the ionospheric reflection coefficient as the simplest approximation suggested by Scholer [1970].…”

Section: Discussionmentioning

confidence: 99%

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Quarter‐wave modes of standing Alfvén waves detected by cross‐phase analysis

et al. 2008

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[1] We have examined the diurnal variation of the local field line eigenfrequency at L $ 2.6 using cross-phase analysis of Sub-Auroral Magnetometer Network and Magnetometers Along the Eastern Atlantic Seaboard for Undergraduate Research and Education ground magnetometer array data. On several days the eigenfrequency was remarkably low near the dawn terminator, when one end of the field line was sunlit and the other end was in darkness. Later in the morning the eigenfrequency gradually increased to the normal daytime value. This type of diurnal eigenfrequency variation was found in both European and American meridians and in several seasons (March, June, and December). By modeling this situation we show that the extraordinarily low eigenfrequency events appeared when the ionospheric Pedersen conductance was strongly asymmetric between both ends of the field line, leading to the formation of quarter-wavelength-mode standing waves that revert to half-wavelength modes as the dawn terminator passes both conjugate points. Ground-based magnetometer measurements of local toroidal field line eigenfrequencies are often inverted to infer plasma mass density in the magnetosphere by assuming half-wavelength-mode standing field line oscillations. However, the mode structure and hence field line eigenfrequency also depend on the ionospheric conductance. In particular, we find that there is a threshold of interhemispheric conductance ratio for the quarter-wavelength mode to be established. Our results therefore show that cross-phase techniques can detect quarter-wavelength-mode waves, where the inferred mass density would be overestimated.

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Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

Section: Discussionmentioning

confidence: 99%

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Quarter‐wave modes of standing Alfvén waves detected by cross‐phase analysis

et al. 2008

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“… Budnik et al [1998] presented a satellite observation at L = 6.8 of a pulsation event with a sharp frequency change observed both in the radial and azimuthal magnetic perturbations which they proposed was due to the transformation of a half‐wavelength harmonic mode into a quarter‐wavelength mode as the Pedersen conductivities became increasingly asymmetric across the dawn‐dusk terminator. Although Budnik et al [1998] did not solve for the field‐aligned eigenmodes, they inferred that when the Pedersen conductivity at one end of the field line became less than a critical conductivity given by Σ P critical = 1/μ 0 A I where A I is the Alfvén speed at the ionosphere (Σ P critical = 0.35 S for their case) the mode would change from a half‐ to a quarter‐wavelength harmonic. They observed a sharp frequency change from 18–19 mHz to 5–6 mHz, which could be consistent with this mode change.…”

Section: Discussionmentioning

confidence: 99%

Modeling the properties of guided poloidal Alfvén waves with finite asymmetric ionospheric conductivities in a dipole field

Ozeke

Mann

2004

J. Geophys. Res.

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[1] Analytical and numerical solutions to the guided poloidal Alfvén wave equation in a dipole field are developed, including the effects of realistic and asymmetric finite ionospheric conductivities. We show that when the ionospheric Pedersen conductivity in one hemisphere is less than a critical value, then quarter-wavelength harmonic modes become possible. We present solutions using realistic plasma density variations along the field line and illustrate how the electric and magnetic fields of a field-aligned halfwavelength harmonic mode change as the ionospheric conductivities are made increasingly asymmetric and the wave develops into a quarter-wavelength mode. Further, we show how over a small critical range of ionospheric Pedersen conductivities, the wave damping rates and frequencies change rapidly as this transition from half-to quarterwavelength mode occurs. We also show that the phase difference between the electric and magnetic field components is strongly dependent upon the distance along the magnetic field line, upon the asymmetry in the ionospheric Pedersen conductivities, and significantly upon which hemisphere has its footprint in the ionosphere whose conductivity is close to critical. This has important implications for inferring the fieldaligned harmonic mode of standing ULF pulsations when using single satellite data.

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“…This event showed highly asymmetric amplitudes and a 90° phase difference between the D component magnetic fields at both ends of the field line. Investigating the influence of ionospheric conductance on wave reflection conditions, Budnik et al [] reported a pulsation event recorded by the GOES‐6 (geosynchronous) satellite, in which the wave frequency abruptly decreased from 18–19 to 5–6 mHz when the satellite was crossing the dusk terminator. They explained this jump in frequency as a change in the wave structure.…”

Section: Introductionmentioning

confidence: 99%

Resonance structure and mode transition of quarter‐wave ULF pulsations around the dawn terminator

et al. 2015

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Quarter‐wave modes are standing shear Alfvén waves supported along geomagnetic field lines in space. They are predicted to be generated when the ionosphere has very different conductance between the north compared with the south ionosphere. Our previous observation reported that the resonant frequency is sometimes very low around the dawn terminator and suggested these were due to quarter‐wave modes. In this paper, we examine the resonance structure that provides further evidence of the presence of quarter‐wave modes. Data from three magnetometers in New Zealand were analyzed. Four events are discussed which show extraordinarily low eigenfrequencies, wide resonance widths, and strong damping when the ionosphere above New Zealand was in darkness while the conjugate northern hemisphere ionosphere was sunlit. Later in the morning, the eigenfrequencies and resonance widths changed to normal daytime values. The wide resonance width and the strong damping of the quarter‐wave modes arise from strong energy dissipation in the dark side ionosphere. One event exhibited field line resonance structure continuously through a transition from very low frequency to the normal daytime values. The frequency change began when the dawn terminator passed over New Zealand and finished 1 h later when the ratio of the interhemispheric ionospheric conductances decreased and reached ~5. These observations are strong evidence of the presence of quarter‐wave modes and mode conversion from quarter‐ to half‐wave resonances. These experimental results were compared with the ULF wave fields obtained from a 2.5‐dimensional simulation model.

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Ionospheric conductance distribution and MHD wave structure: observation and model

Cited by 22 publications

References 23 publications

Quarter‐wave modes of standing Alfvén waves detected by cross‐phase analysis

Quarter‐wave modes of standing Alfvén waves detected by cross‐phase analysis

Modeling the properties of guided poloidal Alfvén waves with finite asymmetric ionospheric conductivities in a dipole field

Resonance structure and mode transition of quarter‐wave ULF pulsations around the dawn terminator

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