Pi2 pulsations observed around the dawn terminator

Imajo, Shun; Yoshikawa, Akimasa; Уозуми, Т.; Ohtani, S.; Nakamizo, Aoi; Marshall, R. A.; Шевцов, Б. М.; Akulichev, V. A.; Sukhbaatar, Usnikh; Liedloff, Adam C.; Yumoto, K.

doi:10.1002/2013ja019691

Cited by 11 publications

(28 citation statements)

References 35 publications

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“…Models based on the fast mode wave [plasmaspheric resonance (e.g., Yeoman and Orr 1989) or the direct driven fast mode wave (e.g., Kepko and Kivelson 1999)] cannot therefore explain the D phase reversal near the terminator. Recently Imajo et al (2015) suggested that the terminator effect could be explained by a change in the contributing currents from the FACs on the nightside to the meridional ionospheric currents on the sunlit side of the terminator, and vice versa. The meridional ionospheric currents are part of the dayside ionospheric current closure, including the equatorial Cowling current (see figure 8 in the 2015 paper).…”

Section: Discussionmentioning

confidence: 99%

“…If only a sharp gradient of conductivities is considered, the terminator effects are almost the same at dawn and dusk. However, case studies by Imajo et al (2015) showed that the distributions of wave properties with respect to the location of the terminators are not the same, although similar D component phase reversals were also found on the dusk side. A more detailed understanding of the differences between the effects of dawn and dusk terminators may provide clues to the determination of geometrical differences in currents.…”

Section: Introductionmentioning

confidence: 94%

“…They found that an increase in the D-to-H (east-west to north-south) amplitude ratio occurred simultaneously with E layer ionization at sunrise. Imajo et al (2015) presented case studies of Pi2 pulsations simultaneously observed at multiple low-latitude stations (L = 1.15-2.33) around the dawn terminator. They found that the D component oscillations in the dark and sunlit hemispheres were in antiphase, whereas the H component oscillated in phase.…”

Section: Introductionmentioning

confidence: 99%

“…To clarify the significance and effect of the terminator, statistically robust comparisons should be made between the longitudinal structures based on the location of the terminator and local time (LT) dependences. Imajo et al (2015) analyzed only the D component phase structure at the ASB station (GMLat = 36.43°) on the dawn side using such a statistical analysis.…”

Section: Introductionmentioning

confidence: 99%

“…The sources of the ionospheric electric fields associated with Pi2, which are thought to be field-aligned currents (FACs), are typically located at high latitudes (Shinohara et al 1998). Yoshikawa et al (2012) suggested that the meridional ionospheric current near the terminator connected the high-latitude FACs to the equatorial Cowling current, and Imajo et al (2015) applied such current closure to Pi2 pulsations. The morphology of the meridional ionospheric current can be understood by investigating the latitudinal variation in the terminator effects.…”

Section: Introductionmentioning

confidence: 99%

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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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Section: Discussionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 94%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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

et al. 2016

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Occurrence and characteristics of nighttime ULF waves at low latitude: The results of a comprehensive analysis

Villante

Tiberi²

2016

JGR Space Physics

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The occurrence and characteristics of ULF events (f ≈ 10–100 mHz) detected during the night at low latitude (L'Aquila, Italy, λ ≈ 36.3°), during quiet and moderately perturbed magnetospheric conditions, have been examined by means of a long‐term analysis between 1996 and 2012. Clearly defined events (≈8000 on each component) are typically more energetic in H than in D and basically consist of penetrating upstream waves, resonances of local field lines, and Pi2 waves. The global event occurrence shows a strong asymmetry about midnight, with a much higher wave activity before dawn than after dusk: it mostly comes from the intense penetration of upstream waves through the dawn flank of the magnetopause. D events are more frequent in summer and H events more frequent in winter, suggesting a different influence of the ionospheric modification of the downgoing signals. Between f ≈ 30 and 45 mHz, the reversal of the dominant polarization across midnight reveals tailward propagation of penetrating waves. Below f ≈ 25 mHz, intermingled with continuous Pc3 and Pc4 waves, a large fraction of events exhibit Pi2 characteristics: the dominant left‐handed polarization and the switch of the tilt angle of the major axis of the polarization ellipses are consistent with the pattern expected for waves related to the substorm current wedge. A relevant percentage of the power spectra shows a second enhancement above f ≈ 55 mHz, revealing resonance of local field lines also during the night.

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Application of a global magnetospheric‐ionospheric current model for dayside and terminator Pi2 pulsations

et al. 2017

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Pi2 magnetic oscillations on the dayside are considered to be produced by the ionospheric current that is driven by Pi2‐associated electric fields from the high‐latitude region, but this idea has not been quantitatively tested. The present study numerically tested the magnetospheric‐ionospheric current system for Pi2 consisting of field‐aligned currents (FACs) localized in the nightside auroral region, the perpendicular magnetospheric current flowing in the azimuthal direction, and horizontal ionospheric currents driven by the FACs. We calculated the spatial distribution of the ground magnetic field produced by these currents using the Biot‐Savart law in a stationary state. The calculated magnetic field reproduced the observational features reported by previous studies: (1) the sense of the H component does not change a wide range of local time sectors at low latitudes, (2) the amplitude of the H component on the dayside is enhanced at the equator, (3) the D component reverses its phase near the dawn and dusk terminators, (4) the meridian of the D component phase reversal near the dusk terminator is shifted more sunward than that near the dawn terminator, and (5) the amplitude of the D component in the morning is larger than that in the early evening. We also derived the global distributions of observed equivalent currents for two Pi2 events. The spatial patterns of dayside equivalent currents were similar to the spatial pattern of numerically derived equivalent currents. The results indicate that the oscillation of the magnetospheric‐ionospheric current system is a plausible explanation of Pi2s on the dayside and near the terminator.

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Pi2 pulsations observed around the dawn terminator

Cited by 11 publications

References 35 publications

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

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

Occurrence and characteristics of nighttime ULF waves at low latitude: The results of a comprehensive analysis

Application of a global magnetospheric‐ionospheric current model for dayside and terminator Pi2 pulsations

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