Lightning Evolution and VLF Perturbations Associated With Category 5 TC Yasa in the South Pacific Region

Abstract In this paper, we present the D-region ionospheric response during the lifespan (10–19 December 2020) of a severe category 5 tropical cyclone (TC) Yasa in the South Pacific by using the very low frequency (VLF, 3-30 kHz) signals from NPM, NLK, and JJI transmitters recorded at Suva, Fiji. Results indicate enhanced lightning and convective activity in all three regions (eyewall, inner rainbands, and outer rainbands) during the TC Yasa that are also linked to the wave sensitive zones of these transmitter-receiver great circle paths. Of the three regions, the outer rainbands showed the maximum lightning occurrence; hence convective activity. Prominent eyewall lightning was observed just before the TC started to weaken following its peak intensity. Analysis of VLF signal amplitudes showed both negative and positive perturbations (amplitudes exceeding ±3σ mark) lasting for more than 2 hours with maximum change in the daytime and nighttime signal amplitudes of -4.9 dB (NPM) and -19.8 dB (NLK), respectively. The signal perturbations were wave-like, exhibiting periods of oscillations between ~2.2-5.5 hours as revealed by the Morlet wavelet analysis. Additionally, the LWPC modeling of the signal perturbations indicated a 10 km increase in daytime D-region reference height, H¢, and a 12 km decrease in nighttime D-region H¢ during TC Yasa. The D-region density gradients (sharpness), b, showed small perturbations of 0.01–0.14 km-1 from its normal values. We suggest that the observed changes to the D-region parameters are due to the enhanced convection during TC Yasa which excites atmospheric gravity waves producing travelling ionospheric disturbances to the D-region.

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Circulation Regimes and Low-Frequency Oscillations in the South Pacific Sector

Monthly Weather Review ◽

10.1175//2548.1 ◽

2003 ◽

Vol 131 (8) ◽

pp. 1566-1576 ◽

Cited By ~ 31

Author(s):

Andrew W. Robertson ◽

Carlos R. Mechoso

Keyword(s):

Singular Spectrum Analysis ◽

Low Frequency ◽

South Pacific ◽

Stationary Wave ◽

Gaussian Mixture ◽

Reanalysis Data ◽

Oscillatory Behavior ◽

South American ◽

The South ◽

Austral Spring

Abstract The characteristics of subseasonal circulation variability over the South Pacific are examined using 10-day lowpass-filtered 700-hPa geopotential height NCEP–NCAR reanalysis data. The extent to which the variability in each season is characterized by recurrent geographically fixed circulation regimes and/or oscillatory behavior is determined. Two methods of analysis (a K-means cluster analysis and a cross-validated Gaussian mixture model) both indicate three to four geographically fixed circulation regimes in austral fall, winter, and (to some extent) spring. The spatial regime structures are found to be quite similar in each season; they resemble the so-called Pacific–South American (PSA) patterns discussed in previous studies and often referred to as PSA 1 and PSA 2. Oscillatory behavior is investigated using singular spectrum analysis. This identifies a predominantly stationary wave with a period of about 40 days and a spatial structure similar to PSA 1; it is most pronounced in winter and spring and exhibits a noticeable eastward drift as it decays. The power spectrum of variability is otherwise well approximated by a red spectrum, together with enhanced broader-band 15–30-day variability. The results presented herein indicate that low-frequency variability over the South Pacific is not dominated by a propagating wave whose quadrature phases are PSA 1 and PSA 2, as hitherto described. Rather, it is found that the variability is well described by the occurrence of three to four geographically fixed circulation regimes, with a (near) 40-day oscillation that is predominantly stationary in space. The potential subseasonal predictability implied by this duality is discussed. Only during austral spring is a strong correlation found between El Niño and the frequency of occurrence of the circulation regimes.

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Low‐Frequency Acoustic Attenuation in the South Pacific Ocean

The Journal of the Acoustical Society of America ◽

10.1121/1.1912421 ◽

1971 ◽

Vol 49 (3B) ◽

pp. 810-815 ◽

Cited By ~ 7

Author(s):

A. C. Kibblewhite ◽

R. N. Denham

Keyword(s):

Pacific Ocean ◽

Low Frequency ◽

South Pacific ◽

The South ◽

Acoustic Attenuation ◽

South Pacific Ocean

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Nighttime D-region equivalent electron density determined from tweek sferics observed in the South Pacific Region

Earth Planets and Space ◽

10.1186/bf03353201 ◽

2009 ◽

Vol 61 (7) ◽

pp. 905-911 ◽

Cited By ~ 21

Author(s):

Sushil Kumar ◽

Anil Deo ◽

V. Ramachandran

Keyword(s):

Electron Density ◽

South Pacific ◽

Pacific Region ◽

The South ◽

D Region ◽

Equivalent Electron

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Low-Frequency Variability of the South Pacific Subtropical Gyre as Seen from Satellite Altimetry and Argo

Journal of Physical Oceanography ◽

10.1175/jpo-d-15-0026.1 ◽

2015 ◽

Vol 45 (12) ◽

pp. 3083-3098 ◽

Cited By ~ 9

Author(s):

Linlin Zhang ◽

Tangdong Qu

Keyword(s):

Thermal Expansion ◽

Low Frequency ◽

South Pacific ◽

Subtropical Gyre ◽

Sea Surface ◽

The South ◽

Argo Data ◽

Steric Height ◽

Low Frequency Variability ◽

Height Increase

AbstractLow-frequency variability of the South Pacific Subtropical Gyre is investigated using satellite altimeter and Argo data. In most of the region studied, both sea surface height and steric height exhibit a linearly increasing trend, with its largest amplitude in the western part of the basin. Analysis of the Argo data reveals that the steric height increase north of 30°S is primarily caused by variations in the upper 500 m, while the steric height increase south of 30°S is determined by variations in the whole depths from the sea surface to 1800 m, with contributions from below 1000 m accounting for about 50% of the total variance. Most of the steric height increase is due to thermal expansion, except below 1000 m where haline contraction is of comparable magnitude with thermal expansion. Correspondingly, the South Pacific Subtropical Gyre has strengthened in the past decade. Within the latitude range between 10° and 35°S, transport of the gyre circulation increased by 20%–30% in the upper 1000 m and by 10%–30% in the deeper layers from 2004 to 2013. Further analysis shows that these variations are closely related to the southern annular mode in the South Pacific.

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Low Ionosphere under Influence of Strong Solar Radiation: Diagnostics and Modeling

Applied Sciences ◽

10.3390/app11167194 ◽

2021 ◽

Vol 11 (16) ◽

pp. 7194

Author(s):

Vladimir A. Srećković ◽

Desanka M. Šulić ◽

Ljubinko Ignjatović ◽

Veljko Vujčić

Keyword(s):

Solar Radiation ◽

Solar Flares ◽

Low Frequency ◽

Ionospheric Disturbances ◽

D Region ◽

Radio Signals ◽

The Impact ◽

First Time ◽

Region Plasma ◽

Intense Radiation

Solar flares (SFs) and intense radiation can generate additional ionization in the Earth’s atmosphere and affect its structure. These types of solar radiation and activity create sudden ionospheric disturbances (SIDs), affect electronic equipment on the ground along with signals from space, and potentially induce various natural disasters. Focus of this work is on the study of SIDs induced by X-ray SFs using very low frequency (VLF) radio signals in order to predict the impact of SFs on Earth and analyze ionosphere plasmas and its parameters. All data are recorded by VLF BEL stations and the model computation is used to obtain the daytime atmosphere parameters induced by this extreme radiation. The obtained ionospheric parameters are compared with results of other authors. For the first time we analyzed physics of the D-region—during consecutive huge SFs which continuously perturbed this layer for a few hours—in detail. We have developed an empirical model of the D-region plasma density and gave a simple approximative formula for electron density.

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