The phase structure of very low latitude ULF waves across dawn

Waters, C. L.; Sciffer, M. D.; Fraser, B. J.; Brand, K.; Foulkes, K.; Menk, F. W.; Saka, O.; Yumoto, K.

doi:10.1029/2000ja000339

Cited by 12 publications

(13 citation statements)

References 22 publications

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“…Item 3 above, newly found in this paper, can be rephrased as follows: The H ‐component Pc3 pulsations near the ground magnetic equator have a very small azimuthal wave number ( m ≈ 0.3). The 180° phase shift across 0730 LT (item 4) is very similar to the observational result shown in a case study by Waters et al [2001]; they observed ∼150° phase difference around dawn between POH (Φ = 0.09°, Λ = 229.19°) and YAP (Φ = 1.02°, Λ = 209.42°), which are longitudinally distributed along the dip equator. We have clarified that this sharp phase shift always occurs around 0730 LT slightly off the dip equator (Φ = 3.0–6.2°).…”

Section: Summary and Discussionsupporting

confidence: 84%

Longitudinal structure of Pc3 pulsations on the ground near the magnetic equator

et al. 2004

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[1] It has been generally accepted that there are two models for the propagation mechanism of Pc3 pulsations observed on the ground near the magnetic equator, as follows: (1) Compressional waves, propagating along the equatorial plane of the magnetosphere across the ambient magnetic field lines, arrive at the equatorial ionosphere and couple with the magnetic perturbations on the ground through the ionosphere (compressional wave model); (2) Alfvén waves or compressional waves, propagating into the high-latitude ionosphere, generate large-scale ionospheric current oscillations there, and then they leak to the low latitude and cause Pc3 pulsations near the magnetic equator (ionospheric current model). In order to clarify which of these models is more suitable for Pc3 pulsations on the ground near the magnetic equator, the longitudinal structures of their coherence, amplitude, and phase were statistically studied by using three longitudinally separated subequatorial stations, CRI (F = 3.01°, L = 273.44°), GUA (F = 5.60°, L = 215.50°), and MUT (F = 6.24°, L = 192.17°). We found a nearly inphase structure in the 0730-1700 LT sector and a nearly 180°phase shift across 0730 LT for high-coherence Pc3 events in the H component. The longitudinal phase structure can be explained by the ionospheric current model; in this case, the 180°phase shift across 0730 LT means a meridional current along the dawn terminator supplied from the source at higher latitudes. The compressional wave model cannot explain the 180°phase shift across 0730 LT if the ionosphere is assumed as an infinitely thin sheet; however, the compressional wave model still remains as a candidate if we treat the ionosphere as that having a finite thickness. The data at hand do not let us conclude which of the two models is more appropriate; still, the longitudinal phase structure, shown in this paper, is a new finding that has never been reported and is important for understanding how Pc3 pulsations propagate from the magnetosphere to the equatorial ionosphere.

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Section: Summary and Discussionsupporting

confidence: 84%

Longitudinal structure of Pc3 pulsations on the ground near the magnetic equator

et al. 2004

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“…9). Waters et al (2001) showed that the vertical propagation effect in the ionosphere, which is controlled by the ionospheric conductivity, and horizontal wave number, can be a cause of the gradual phase shift with longitude of equatorial ULF waves. We suggest two other possibilities.…”

Section: D-to-h Amplitude Ratiomentioning

confidence: 99%

Solar terminator effects on middle- to low-latitude Pi2 pulsations

et al. 2016

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To clarify the effect of the dawn and dusk terminators on Pi2 pulsations, we statistically analyzed the longitudinal phase and amplitude structures of Pi2 pulsations at middle-to low-latitude stations (GMLat = 5.30°-46.18°) around both the dawn and dusk terminators. Although the H (north-south) component Pi2s were affected by neither the local time (LT) nor the terminator location (at 100 km altitude in the highly conducting E region), some features of the D (east-west) component Pi2s depended on the location of the terminator rather than the LT. The phase reversal of the D component occurred 0.5-1 h after sunrise and 1-2 h before sunset. These phase reversals can be attributed to a change in the contributing currents from field-aligned currents (FACs) on the nightside to the meridional ionospheric currents on the sunlit side of the terminator, and vice versa. The phase reversal of the dawn terminator was more frequent than that of the dusk terminator. The D-to-H amplitude ratio on the dawn side began to increase at sunrise, reaching a peak approximately 2 h after sunrise (the sunward side of the phase reversal region), whereas the ratio on the dusk side reached a peak at sunset (the antisunward side). The dawn-dusk asymmetric features suggest that the magnetic contribution of the nightside FAC relative to the meridional ionospheric current on the dusk side is stronger than that on the dawn side, indicating that the center of Pi2-associated FACs, which probably corresponds to the Pi2 energy source, tends to be shifted duskward on average. Different features and weak sunrise/sunset dependences at the middle-latitude station (Paratunka, GMLat = 46.18°) can be attributed to the larger annual variation in the sunrise/sunset time and a stronger magnetic effect because of closeness from FACs. The D-to-H amplitude ratio decreased with decreasing latitude, suggesting that the azimuthal magnetic field produced by the FACs in darkness and the meridional ionospheric current in sunlight also decreased with decreasing latitude.

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“…At lower latitudes, the fast mode may no longer be evanescent (e.g. Waters et al, 2001) and the full reflection coefficient and mode conversion matrix, including an oblique B 0 , needs to be considered (Sciffer and Waters, 2002).…”

Section: Introductionmentioning

confidence: 99%

Propagation of ULF waves through the ionosphere: Inductive effect for oblique magnetic fields

2004

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Abstract. Solutions for ultra-low frequency (ULF) wave fields in the frequency range 1-100 mHz that interact with the Earth's ionosphere in the presence of oblique background magnetic fields are described. Analytic expressions for the electric and magnetic wave fields in the magnetosphere, ionosphere and atmosphere are derived within the context of an inductive ionosphere. The inductive shielding effect (ISE) arises from the generation of an "inductive" rotational current by the induced part of the divergent electric field in the ionosphere which reduces the wave amplitude detected on the ground. The inductive response of the ionosphere is described by Faraday's law and the ISE depends on the horizontal scale size of the ULF disturbance, its frequency and the ionosphere conductivities. The ISE for ULF waves in a vertical background magnetic field is limited in application to high latitudes. In this paper we examine the ISE within the context of oblique background magnetic fields, extending studies of an inductive ionosphere and the associated shielding of ULF waves to lower latitudes. It is found that the dip angle of the background magnetic field has a significant effect on signals detected at the ground. For incident shear Alfvén mode waves and oblique background magnetic fields, the horizontal component of the field-aligned current contributes to the signal detected at the ground. At low latitudes, the ISE is larger at smaller conductivity values compared with high latitudes.

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The phase structure of very low latitude ULF waves across dawn

Cited by 12 publications

References 22 publications

Longitudinal structure of Pc3 pulsations on the ground near the magnetic equator

Longitudinal structure of Pc3 pulsations on the ground near the magnetic equator

Solar terminator effects on middle- to low-latitude Pi2 pulsations

Propagation of ULF waves through the ionosphere: Inductive effect for oblique magnetic fields

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