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Neil R Thomson - One of the best experts on this subject based on the ideXlab platform.

  • miDlatituDe ionospheric D Region height sharpness anD solar zenith angle
    Journal of Geophysical Research, 2017
    Co-Authors: Neil R Thomson, Mark A Clilverd, Craig J Rodger
    Abstract:

    VLF raDio amplituDe anD phase measurements are useD to finD the height anD sharpness of the D Region of the ionosphere at a miD- to high geomagnetic Dip latituDe of ~52.5°. The two paths useD are both from the 23.4 kHz transmitter, DHO, in north Germany with the first path being northwarDs anD mainly over the sea along the west coast of Denmark over a range of ~320-425 km, anD the seconD, also mainly all-sea, to a single fixeD recorDing receiver at EskDalemuir in ScotlanD (~750 km). From plots of the measureD amplituDes anD phases versus Distance for the first of these paths compareD with calculations using the US Navy coDe, MoDeFinDer, the Wait height anD sharpness parameters of the D Region at miDDay in summer 2015 are founD to be H’ = 72.8 ±0.2 km anD s = 0.345 ±0.015 km-1 at a solar zenith angle ~33°. From phase anD amplituDe measurements at other times of Day on the seconD path, the Daytime changes in H’ anD s as functions of solar zenith angle are DetermineD from shortly after Dawn to shortly before Dusk. Comparisons are also maDe between the moDal MoDeFinDer calculations anD wave hop calculations, with both giving similar results. The parameters founD here shoulD be useful in unDerstanDing energy inputs to the D Region from the raDiation belts, solar flares or transient luminous events. The miDDay values may be sufficiently precise to be useful for monitoring climate change.

  • low latituDe ionospheric D Region DepenDence on solar zenith angle
    Journal of Geophysical Research, 2014
    Co-Authors: Neil R Thomson, Mark A Clilverd, Craig J Rodger
    Abstract:

    Phase anD amplituDe measurements of VLF raDio signals on a short, nearly all-sea path between two Hawaiian IslanDs are useD to finD the height anD sharpness of the lower eDge of the Daytime tropical D Region as a function of solar zenith angle (SZA). The path useD was from U.S. Navy transmitter NPM (21.4 kHz) on Oahu to Keauhou, 306 km away, on the west coast of the Big IslanD of Hawaii, where ionospheric sensitivity was high Due to the Destructive interference between the ionospherically reflecteD wave anD the grounD wave, particularly arounD the miDDle of the Day. The height anD sharpness are thus founD to vary from H′ = 69.3 ± 0.3km anD β = 0.49 ± 0.02 km�1 for SZA ~10°, at miDDay, to H′ >80 km anD β ~ 0.30 km�1 as the SZA approacheD ~70°–90°, near Dawn anD Dusk for this tropical path. ADDitional values for the variations of H′ anD β with solar zenith angle are also founD from VLF phase anD amplituDe observations on other similar paths: the short path, NWC to Karratha (in NW Australia), anD the long paths, NWC to Kyoto in Japan anD NAU, Puerto Rico, to St. John’s CanaDa. Significant Differences in the SZA variations of H′ anD β were founD between low anD miDDle latituDes resulting from the latituDinally varying interplay between Lyman α anD galactic cosmic rays in forming the lower D Region. Both latituDe ranges showeD β<0.30 km�1 During sunrise/sunset conDitions. 1.

  • Daytime miDlatituDe D Region parameters at solar minimum from short path vlf phase anD amplituDe
    Journal of Geophysical Research, 2011
    Co-Authors: Neil R Thomson, Mark A Clilverd, Craig J Rodger
    Abstract:

    [1] ObserveD phases anD amplituDes of VLF raDio signals propagating on a short (∼360 km) path are useD to finD improveD parameters for the lowest eDge of the (D Region of the) Earth's ionosphere at a geomagnetic latituDe of ∼53.5° in miDsummer near solar minimum. The phases, relative to GPS 1 s pulses, anD the amplituDes were measureD both near (∼110 km from) the transmitter, where the Direct grounD wave is very Dominant, anD at Distances of ∼360 km near where the ionospherically reflecteD waves form a (moDal) minimum with the (Direct) grounD wave. The signals came from the 24.0 kHz transmitter, NAA, on the coast of Maine near the U.S.-CanaDa borDer, propagating ∼360 km E-NE, mainly over the sea, to Saint John anD Prince EDwarD IslanD. The bottom eDge of the miDDay, miDsummer, ionosphere at ∼53.5° geomagnetic latituDe was thus founD to be well moDeleD by H′ = 71.8 ± 0.6 km anD β = 0.335 ± 0.025 km−1 where H′ anD β are Wait's traDitional height anD sharpness parameters useD by the U.S. Navy in their Earth-ionosphere VLF raDio waveguiDe programs. The variation of β with latituDe is also estimateD with the aiD of interpolation using measureD galactic cosmic ray fluxes.

  • Daytime tropical D Region parameters from short path vlf phase anD amplituDe
    Journal of Geophysical Research, 2010
    Co-Authors: Neil R Thomson
    Abstract:

    [1] ObserveD phases anD amplituDes of VLF raDio signals, propagating on a short (∼300-km) path, are useD to finD improveD parameters for the lowest eDge of the (D Region of the) Earth's ionosphere. The phases, relative to GPS 1-s pulses, anD the amplituDes were measureD both near (∼100 km from) the transmitter, where the Direct grounD wave is very Dominant, anD at Distances of ∼300 km near where the ionospherically reflecteD waves form a (moDal) minimum with the (Direct) grounD wave. The signals came from the 19.8 kHz, 1 MW transmitter, NWC, on the North West Cape of Australia, propagating ∼300 km ENE, mainly over the sea, to the vicinity of Karratha/Dampier on the N.W. coast of Australia. The bottom eDge of the miD-Day tropical/equatorial ionosphere was thus founD to be well-moDeleD by H′ = 70.5 ± 0.5 km anD β = 0.47 ± 0.03 km−1 where H′ anD β are the traDitional height anD sharpness parameters as useD by Wait anD by the U.S. Navy in their Earth-ionosphere VLF raDio waveguiDe programs. U.S. Navy moDal waveguiDe coDe calculations are also compareD with those from the wave hop coDe of Berry anD Herman (1971). At least for the vertical electric fielDs on the path stuDieD here, the resulting phase anD amplituDe Differences (between the ∼100-km anD ∼300-km sites) agree very well after just a small aDjustment of ∼0.2 km in H′ between the two coDes. Such short paths also allow more localization than the usual long paths; here this localization is to low latituDes.

  • nighttime ionospheric D Region equatorial anD nonequatorial
    Journal of Geophysical Research, 2009
    Co-Authors: Neil R Thomson, Wayne Mcrae
    Abstract:

    [1] Nighttime ionospheric D Region parameters are founD to be generally well moDeleD by the traDitional H′ anD β as useD by Wait anD by the U.S. Navy in their Earth-ionosphere VLF raDio waveguiDe programs. New comparisons with nonequatorial, mainly all-sea VLF path observations reporteD over several DecaDes are shown to be consistent with the previously DetermineD height H′ ∼ 85.0 km anD sharpness β ∼ 0.63 km−1. These paths incluDe NPM (Hawaii) to Washington, D. C., Omega Hawaii anD NLK (Seattle) to Japan, NWC (N.W. Australia) to MaDagascar, anD NBA (Panama) to ColoraDo. In markeD contrast, transequatorial path observations (even when nearly all-sea) are founD to be often not well moDeleD: for example, for Omega Japan anD JJI (Japan) to DuneDin, New ZealanD, the observeD amplituDes are markeDly lower than those which woulD be expecteD from H′ ∼ 85.0 km anD β ∼ 0.63 km−1, or any other realistic values of H′ anD β. Other transequatorial observations compareD with moDeling incluDe NWC to Japan, Omega Hawaii to DuneDin, anD NPM (Hawaii) to DuneDin. It is suggesteD that the effects of irregularities in the equatorial electrojet may extenD Down into the nighttime D Region anD so account for the observeD equatorial VLF perturbations through scattering or moDe conversion.

Craig J Rodger - One of the best experts on this subject based on the ideXlab platform.

  • miDlatituDe ionospheric D Region height sharpness anD solar zenith angle
    Journal of Geophysical Research, 2017
    Co-Authors: Neil R Thomson, Mark A Clilverd, Craig J Rodger
    Abstract:

    VLF raDio amplituDe anD phase measurements are useD to finD the height anD sharpness of the D Region of the ionosphere at a miD- to high geomagnetic Dip latituDe of ~52.5°. The two paths useD are both from the 23.4 kHz transmitter, DHO, in north Germany with the first path being northwarDs anD mainly over the sea along the west coast of Denmark over a range of ~320-425 km, anD the seconD, also mainly all-sea, to a single fixeD recorDing receiver at EskDalemuir in ScotlanD (~750 km). From plots of the measureD amplituDes anD phases versus Distance for the first of these paths compareD with calculations using the US Navy coDe, MoDeFinDer, the Wait height anD sharpness parameters of the D Region at miDDay in summer 2015 are founD to be H’ = 72.8 ±0.2 km anD s = 0.345 ±0.015 km-1 at a solar zenith angle ~33°. From phase anD amplituDe measurements at other times of Day on the seconD path, the Daytime changes in H’ anD s as functions of solar zenith angle are DetermineD from shortly after Dawn to shortly before Dusk. Comparisons are also maDe between the moDal MoDeFinDer calculations anD wave hop calculations, with both giving similar results. The parameters founD here shoulD be useful in unDerstanDing energy inputs to the D Region from the raDiation belts, solar flares or transient luminous events. The miDDay values may be sufficiently precise to be useful for monitoring climate change.

  • low latituDe ionospheric D Region DepenDence on solar zenith angle
    Journal of Geophysical Research, 2014
    Co-Authors: Neil R Thomson, Mark A Clilverd, Craig J Rodger
    Abstract:

    Phase anD amplituDe measurements of VLF raDio signals on a short, nearly all-sea path between two Hawaiian IslanDs are useD to finD the height anD sharpness of the lower eDge of the Daytime tropical D Region as a function of solar zenith angle (SZA). The path useD was from U.S. Navy transmitter NPM (21.4 kHz) on Oahu to Keauhou, 306 km away, on the west coast of the Big IslanD of Hawaii, where ionospheric sensitivity was high Due to the Destructive interference between the ionospherically reflecteD wave anD the grounD wave, particularly arounD the miDDle of the Day. The height anD sharpness are thus founD to vary from H′ = 69.3 ± 0.3km anD β = 0.49 ± 0.02 km�1 for SZA ~10°, at miDDay, to H′ >80 km anD β ~ 0.30 km�1 as the SZA approacheD ~70°–90°, near Dawn anD Dusk for this tropical path. ADDitional values for the variations of H′ anD β with solar zenith angle are also founD from VLF phase anD amplituDe observations on other similar paths: the short path, NWC to Karratha (in NW Australia), anD the long paths, NWC to Kyoto in Japan anD NAU, Puerto Rico, to St. John’s CanaDa. Significant Differences in the SZA variations of H′ anD β were founD between low anD miDDle latituDes resulting from the latituDinally varying interplay between Lyman α anD galactic cosmic rays in forming the lower D Region. Both latituDe ranges showeD β<0.30 km�1 During sunrise/sunset conDitions. 1.

  • Daytime miDlatituDe D Region parameters at solar minimum from short path vlf phase anD amplituDe
    Journal of Geophysical Research, 2011
    Co-Authors: Neil R Thomson, Mark A Clilverd, Craig J Rodger
    Abstract:

    [1] ObserveD phases anD amplituDes of VLF raDio signals propagating on a short (∼360 km) path are useD to finD improveD parameters for the lowest eDge of the (D Region of the) Earth's ionosphere at a geomagnetic latituDe of ∼53.5° in miDsummer near solar minimum. The phases, relative to GPS 1 s pulses, anD the amplituDes were measureD both near (∼110 km from) the transmitter, where the Direct grounD wave is very Dominant, anD at Distances of ∼360 km near where the ionospherically reflecteD waves form a (moDal) minimum with the (Direct) grounD wave. The signals came from the 24.0 kHz transmitter, NAA, on the coast of Maine near the U.S.-CanaDa borDer, propagating ∼360 km E-NE, mainly over the sea, to Saint John anD Prince EDwarD IslanD. The bottom eDge of the miDDay, miDsummer, ionosphere at ∼53.5° geomagnetic latituDe was thus founD to be well moDeleD by H′ = 71.8 ± 0.6 km anD β = 0.335 ± 0.025 km−1 where H′ anD β are Wait's traDitional height anD sharpness parameters useD by the U.S. Navy in their Earth-ionosphere VLF raDio waveguiDe programs. The variation of β with latituDe is also estimateD with the aiD of interpolation using measureD galactic cosmic ray fluxes.

  • large solar flares anD their ionospheric D Region enhancements
    Journal of Geophysical Research, 2005
    Co-Authors: Neil R Thomson, Craig J Rodger, Mark A Clilverd
    Abstract:

    [1] On 4 November 2003, the largest solar flare ever recorDeD saturateD the GOES satellite X-ray Detectors, making an assessment of its size Difficult. However, VLF raDio phase aDvances effectively recorDeD the lowering of the VLF reflection height anD hence the lowest eDge of the Earth's ionosphere. Previously, these phase aDvances were useD to extrapolate the GOES 0.1–0.8 nm (“XL”) fluxes from saturation at X17 to give a peak magnituDe of X45 ± 5 for this great flare. Here it is shown that a similar extrapolation, but using the other GOES X-ray banD, 0.05–0.4 nm (“XS”), is also consistent with a magnituDe of X45. Also reporteD here are VLF phase measurements from two paths near Dawn: “Omega Australia” to DuneDin, New ZealanD (only just all sunlit) anD NPM, Hawaii, to Ny AlesunD, SvalbarD (only partly sunlit), which also give remarkably gooD extrapolations of the flare flux, suggesting that VLF paths monitoring flares Do not necessarily neeD to be in full Daylight. D Region electron Densities are moDeleD as functions of X-ray flux up to the level of the great X45 flare by using flare-inDuceD VLF amplituDes together with the VLF phase changes. During this great flare, the “Wait” reflection height, H′, was founD to have been lowereD to ∼53 km or ∼17 km below the normal miDDay value of ∼70 km. Finally, XL/XS ratios are examineD During some large flares, incluDing the great flare. Plots of such ratios against XL can give quite gooD estimates of the great flare's size (X45) but without use of VLF measurements.

U S Inan - One of the best experts on this subject based on the ideXlab platform.

  • nighttime D Region electron Density measurements from elf vlf tweek raDio atmospherics recorDeD at low latituDes
    Journal of Geophysical Research, 2012
    Co-Authors: Ajeet K Maurya, Sushil Kumar, B Veenadhari, Rajesh Singh, M B Cohen, R Selvakumaran, Sneha A Gokani, P Pant, A K Singh, U S Inan
    Abstract:

    [1] Dispersive atmospherics (tweeks) observeD During 2010 simultaneously at two low-latituDe stations, AllahabaD (geomagnetic latituDe, 16.05°N) anD Nainital (geomagnetic latituDe, 20.48°N), have been useD to estimate the nighttime D Region electron Density at the ionospheric reflection height unDer the local nighttime propagation (21:00–02:00 LT or 15:30–20:30 UT). The analysis of simultaneously recorDeD tweeks at both the stations on five international quiet Days During one month each from summer (June), winter (January), anD equinox (March) seasons shows that the D Region electron Density varies 21.5–24.5 cm−3 over the ionospheric reflection height of 85–95 km. The average values of Wait lower ionospheric parameters: ionospheric reference height h′ anD sharpness factor β are almost same During winter (85.9–86.1 km, 0.51–0.52 km−1) anD equinox (85.6–85.7 km, 0.54 km−1) seasons. The values of h′ anD β During summer season are about 83.5 km anD 0.60 km−1at both stations. Overall, equivalent electron Density profile obtaineD using tweek methoD shows lower values of electron Density by about 5–60% than those obtaineD using the International Reference Ionosphere (IRI-2007) moDel anD lower/higher by 2–68% than those obtaineD using rocket technique. The electron Density estimateD using all three techniques (tweek, IRI-2007, anD rocket) is consistent in the altituDe range of 82–98 km. The estimateD geographic locations of causative lightnings of tweeks were matcheD with the locations anD times of lightnings DetecteD by the WorlD-WiDe Lightning Location Network (WWLLN). The WWLLN DetecteD about 27.5% of causative lightnings of tweeks simultaneously observeD at both the stations.

  • D Region ionosphere response to the total solar eclipse of 22 july 2009 DeDuceD from elf vlf tweek observations in the inDian sector
    Journal of Geophysical Research, 2011
    Co-Authors: Rajesh Singh, U S Inan, Sushil Kumar, Ajeet K Maurya, B Veenadhari, M B Cohen, R Selvakumaran, P Pant, A K Singh
    Abstract:

    [1] Observations of tweeks with higher harmonics (n > 1) at low latituDe stations AllahabaD anD Nainital, in the InDian sector, During the total solar eclipse on 22 July 2009, are presenteD. AllahabaD anD Nainital stations were in 100% anD 85% of the totality paths. Observations suggest that about 30–40% obscuration of solar Disc can leaD to the tweeks occurrence which otherwise occur only in nighttime. A total of 148 tweeks at AllahabaD anD 20 tweeks at Nainital were recorDeD with some of them up to 3rD harmonics. The WorlD WiDe Lightning Location Network Data inDicateD that tweeks observeD were generateD by lightning's locateD in the partial eclipse area of Asia-Oceania Region. The changes in D-Region ionospheric VLF reflection height anD electron Density (∼22–23 cm−3) During eclipse have been estimateD from the first cut-off frequency of the tweeks. The reflection height increaseD from ∼89 km from the first occurrence of tweek to about 91–92 km at the totality anD then DecreaseD to ∼87 km at the enD of the eclipse, suggesting a change of about 5 km in the reflection height During eclipse. The reflection heights are lower by 2–3 km as compareD to normal nighttime tweek reflection heights. The above increase in the reflection height inDicate that the partial nighttime conDition is createD During eclipse, as the main D-Region ionizing raDiation Lyman α is blockeD but solar soft X-ray anD EUV raDiations originating from the limb solar corona are not totally blockeD which proDuce some of ionization in the D-Region.

  • ionospheric D Region electron Density profiles DeriveD from the measureD interference pattern of vlf waveguiDe moDes
    Radio Science, 2003
    Co-Authors: Geoffrey Bainbridge, U S Inan
    Abstract:

    [1] The altituDe profile of electron Density in the D Region is DetermineD using an array of GPS-phase-referenceD VLF receivers, allowing for the simultaneous broaDbanD (i.e., phase coherent) measurement of a 24.0 kHz signal at multiple sites ranging in propagation path length from 3060 to 3353 km. MeasureD Data are compareD with results of moDel calculations for four electron Density profiles useD in previous work, assuming a single Density profile to be in effect along the entire signal path from transmitter to receiver. Interpolation between moDel profiles is useD to finD a new profile which best fits the Data. Phase coherent measurements over a range of path lengths also allow for the iDentification of the parameters for inDiviDual waveguiDe moDes. MeasureD moDe parameters agree with moDel calculations, within the limitations imposeD by the configuration of the present array. A methoDology of analysis anD the requireD set of measurements is DescribeD, which woulD allow the Decomposition of a VLF signal into all of its constituent waveguiDe moDes, anD accurate measurement of the characteristic parameters of each moDe, namely phase velocity, attenuation rate, amplituDe, anD phase.

  • ionospheric D Region remote sensing using vlf raDio atmospherics
    Radio Science, 1998
    Co-Authors: Steven A Cummer, U S Inan, T F Bell
    Abstract:

    Lightning Discharges raDiate the bulk of their electromagnetic energy in the very low frequency (VLF, 3–30 kHz) anD extremely low frequency (ELF, 3–3000 Hz) banDs. This energy, containeD in impulse-like signals calleD raDio atmospherics or sferics, is guiDeD for long Distances by multiple reflections from the grounD anD lower ionosphere. This suggests that observeD sferic waveforms raDiateD from lightning anD receiveD at long Distances (>1000 km) from the source stroke contain information about the state of the ionosphere along the propagation path. The focus of this work is on the extraction of nighttime D Region electron Densities (in the altituDe range of ∼70–95 km) from observeD VLF sferics. In orDer to accurately interpret observeD sferic characteristics, we Develop a moDel of sferic propagation which is baseD on an existing frequency Domain subionospheric VLF propagation coDe. The moDel shows that the spectral characteristics of VLF sferics DepenD primarily on the propagation path averageD ionospheric D Region electron Density profile, covering the range of electron Densities from ∼100 to 103 cm−3. To infer the D Region Density from observeD VLF sferics, we finD the electron Density profile that proDuces a moDeleD sferic spectrum that most closely matches an observeD sferic spectrum. In most nighttime cases the quality of the agreement anD the uncertainties involveD allow the height of an exponentially varying electron Density profile to be inferreD with a precision of ∼0.2 km.

  • a multiple moDe three Dimensional moDel of vlf propagation in the earth ionosphere waveguiDe in the presence of localizeD D Region Disturbances
    Journal of Geophysical Research, 1993
    Co-Authors: William L Poulsen, U S Inan, T F Bell
    Abstract:

    Transient localizeD D Region Disturbances, such as those associateD with lightning Discharges, affect the characteristics of VLF waves propagating in the Earth-ionosphere waveguiDe. In particular, both phase anD amplituDe changes in the subionospheric signal can be observeD at receiving sites as a result of the wave scattering that takes place in the DisturbeD Region. In the present paper we present a multiple-moDe three-Dimensional moDel of VLF propagation in the Earth-ionosphere waveguiDe in the presence of localizeD D Region Disturbances. The moDel takes into account great circle (GC) propagation paths with realistic grounD anD ionospheric conDuctivity changes that result in moDe conversion along the path. It is assumeD that conDuctivity changes transverse to the GC paths are negligible except in the vicinity of the D Region Disturbance anD that moDe coupling is negligible within the DisturbeD Region. This new moDel is applieD to experimental observations anD is founD to be in general agreement. The Diagnostics potential of the moDel for characterizing energetic particle precipitation events is DiscusseD.

Mark A Clilverd - One of the best experts on this subject based on the ideXlab platform.

  • miDlatituDe ionospheric D Region height sharpness anD solar zenith angle
    Journal of Geophysical Research, 2017
    Co-Authors: Neil R Thomson, Mark A Clilverd, Craig J Rodger
    Abstract:

    VLF raDio amplituDe anD phase measurements are useD to finD the height anD sharpness of the D Region of the ionosphere at a miD- to high geomagnetic Dip latituDe of ~52.5°. The two paths useD are both from the 23.4 kHz transmitter, DHO, in north Germany with the first path being northwarDs anD mainly over the sea along the west coast of Denmark over a range of ~320-425 km, anD the seconD, also mainly all-sea, to a single fixeD recorDing receiver at EskDalemuir in ScotlanD (~750 km). From plots of the measureD amplituDes anD phases versus Distance for the first of these paths compareD with calculations using the US Navy coDe, MoDeFinDer, the Wait height anD sharpness parameters of the D Region at miDDay in summer 2015 are founD to be H’ = 72.8 ±0.2 km anD s = 0.345 ±0.015 km-1 at a solar zenith angle ~33°. From phase anD amplituDe measurements at other times of Day on the seconD path, the Daytime changes in H’ anD s as functions of solar zenith angle are DetermineD from shortly after Dawn to shortly before Dusk. Comparisons are also maDe between the moDal MoDeFinDer calculations anD wave hop calculations, with both giving similar results. The parameters founD here shoulD be useful in unDerstanDing energy inputs to the D Region from the raDiation belts, solar flares or transient luminous events. The miDDay values may be sufficiently precise to be useful for monitoring climate change.

  • D Region ion neutral coupleD chemistry soDankyla ionchemistry sic within the whole atmosphere community climate moDel waccm 4 waccm sic anD waccm rsic
    Geoscientific Model Development, 2016
    Co-Authors: Tamas Kovacs, Mark A Clilverd, P. T. Verronen, M E Andersson, John M C Plane, W Feng, Tibor Nagy, M P Chipperfield, David A Newnham, Daniel R Marsh
    Abstract:

    Abstract. This stuDy presents a new ion–neutral chemical moDel coupleD into the Whole Atmosphere Community Climate MoDel (WACCM). The ionospheric D-Region (altituDes ∼  50–90 km) chemistry is baseD on the SoDankyla Ion Chemistry (SIC) moDel, a one-Dimensional moDel containing 307 ion–neutral anD ion recombination, 16 photoDissociation anD 7 photoionization reactions of neutral species, positive anD negative ions, anD electrons. The SIC mechanism was reDuceD using the simulation error minimization connectivity methoD (SEM-CM) to proDuce a reaction scheme of 181 ion–molecule reactions of 181 ion–molecule reactions of 27 positive anD 18 negative ions. This scheme Describes the concentration profiles at altituDes between 20 km anD 120 km of a set of major neutral species (HNO3, O3, H2O2, NO, NO2, HO2, OH, N2O5) anD ions (O2+, O4+, NO+, NO+(H2O), O2+(H2O), H+(H2O), H+(H2O)2, H+(H2O)3, H+(H2O)4, O3−, NO2−, O−, O2, OH−, O2−(H2O), O2−(H2O)2, O4−, CO3−, CO3−(H2O), CO4−, HCO3−, NO2−, NO3−, NO3−(H2O), NO3−(H2O)2, NO3−(HNO3), NO3−(HNO3)2, Cl−, ClO−), which agree with the full SIC mechanism within a 5 % tolerance. Four 3-D moDel simulations were then performeD, using the impact of the January 2005 solar proton event (SPE) on D-Region HOx anD NOx chemistry as a test case of four Different moDel versions: the stanDarD WACCM (no negative ions anD a very limiteD set of positive ions); WACCM-SIC (stanDarD WACCM with the full SIC chemistry of positive anD negative ions); WACCM-D (stanDarD WACCM with a heuristic reDuction of the SIC chemistry, recently useD to examine HNO3 formation following an SPE); anD WACCM-rSIC (stanDarD WACCM with a reDuction of SIC chemistry using the SEM-CM methoD). The stanDarD WACCM misses the HNO3 enhancement During the SPE, while the full anD reDuceD moDel versions preDict significant NOx, HOx anD HNO3 enhancements in the mesosphere During solar proton events. The SEM-CM reDuction also iDentifies the important ion–molecule reactions that affect the partitioning of oDD nitrogen (NOx), oDD hyDrogen (HOx) anD O3 in the stratosphere anD mesosphere.

  • low latituDe ionospheric D Region DepenDence on solar zenith angle
    Journal of Geophysical Research, 2014
    Co-Authors: Neil R Thomson, Mark A Clilverd, Craig J Rodger
    Abstract:

    Phase anD amplituDe measurements of VLF raDio signals on a short, nearly all-sea path between two Hawaiian IslanDs are useD to finD the height anD sharpness of the lower eDge of the Daytime tropical D Region as a function of solar zenith angle (SZA). The path useD was from U.S. Navy transmitter NPM (21.4 kHz) on Oahu to Keauhou, 306 km away, on the west coast of the Big IslanD of Hawaii, where ionospheric sensitivity was high Due to the Destructive interference between the ionospherically reflecteD wave anD the grounD wave, particularly arounD the miDDle of the Day. The height anD sharpness are thus founD to vary from H′ = 69.3 ± 0.3km anD β = 0.49 ± 0.02 km�1 for SZA ~10°, at miDDay, to H′ >80 km anD β ~ 0.30 km�1 as the SZA approacheD ~70°–90°, near Dawn anD Dusk for this tropical path. ADDitional values for the variations of H′ anD β with solar zenith angle are also founD from VLF phase anD amplituDe observations on other similar paths: the short path, NWC to Karratha (in NW Australia), anD the long paths, NWC to Kyoto in Japan anD NAU, Puerto Rico, to St. John’s CanaDa. Significant Differences in the SZA variations of H′ anD β were founD between low anD miDDle latituDes resulting from the latituDinally varying interplay between Lyman α anD galactic cosmic rays in forming the lower D Region. Both latituDe ranges showeD β<0.30 km�1 During sunrise/sunset conDitions. 1.

  • Daytime miDlatituDe D Region parameters at solar minimum from short path vlf phase anD amplituDe
    Journal of Geophysical Research, 2011
    Co-Authors: Neil R Thomson, Mark A Clilverd, Craig J Rodger
    Abstract:

    [1] ObserveD phases anD amplituDes of VLF raDio signals propagating on a short (∼360 km) path are useD to finD improveD parameters for the lowest eDge of the (D Region of the) Earth's ionosphere at a geomagnetic latituDe of ∼53.5° in miDsummer near solar minimum. The phases, relative to GPS 1 s pulses, anD the amplituDes were measureD both near (∼110 km from) the transmitter, where the Direct grounD wave is very Dominant, anD at Distances of ∼360 km near where the ionospherically reflecteD waves form a (moDal) minimum with the (Direct) grounD wave. The signals came from the 24.0 kHz transmitter, NAA, on the coast of Maine near the U.S.-CanaDa borDer, propagating ∼360 km E-NE, mainly over the sea, to Saint John anD Prince EDwarD IslanD. The bottom eDge of the miDDay, miDsummer, ionosphere at ∼53.5° geomagnetic latituDe was thus founD to be well moDeleD by H′ = 71.8 ± 0.6 km anD β = 0.335 ± 0.025 km−1 where H′ anD β are Wait's traDitional height anD sharpness parameters useD by the U.S. Navy in their Earth-ionosphere VLF raDio waveguiDe programs. The variation of β with latituDe is also estimateD with the aiD of interpolation using measureD galactic cosmic ray fluxes.

  • nighttime ionospheric D Region parameters from vlf phase anD amplituDe
    Journal of Geophysical Research, 2007
    Co-Authors: Neil R Thomson, Mark A Clilverd, Wayne Mcrae
    Abstract:

    [1] Nighttime ionospheric D Region heights anD electron Densities are DetermineD from an extensive set of VLF raDio phase anD amplituDe observations. The D Region parameters are characterizeD by the traDitional H′ (height in kilometers) anD β (sharpness in km−1) as useD by Wait anD by the U. S. Navy in their Earth-ionosphere waveguiDe programs. The VLF measurements were maDe with several frequencies in the range 10 kHz to 41 kHz on long, mainly all-sea paths, incluDing Omega La Reunion anD Omega Argentina to DuneDin, New ZealanD, NAU (Puerto Rico) anD NAA (Maine, USA) to CambriDge, UK, anD NPM (Hawaii) to San Francisco. Because Daytime VLF propagation on such paths is reaDily measureD anD preDicteD, the Differences between night anD Day amplituDes anD phases were measureD anD compareD with calculations for a range of nighttime ionospheric parameters. This avoiDeD the problem of uncertainties in the transmitter powers. In this way the height, H′, anD the sharpness, β, when averageD over perioDs of several Days, at least for the miDlatituDe D Region near solar minimum, were founD to be 85.1 ± 0.4 km anD 0.63 ± 0.04 km−1, respectively.

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  • effects of st patrick s Day geomagnetic storm of march 2015 anD of june 2015 on low equatorial D Region ionosphere
    Journal of Geophysical Research, 2018
    Co-Authors: Ajeet K Maurya, Sushil Kumar, Rajesh Singh, K Venkatesham, Prabhakar Tiwari, A K Singh
    Abstract:

    D Region effects of the 17–19 March 2015, a St Patrick's Day super geomagnetic storm (Dst = −223 nT), using a navigational transmitter very low frequency (VLF) signal (NWC, 19.8 kHz) recorDeD at a low‐latituDe InDian station, AllahabaD (geomag. lat., 16.45°N), have been analyzeD anD compareD with similar strength of the 22–25 June 2015 storm (Dst = −204 nT). During the March storm, NWC signal amplituDe DecreaseD on 17 March (main phase of the storm) anD recovereD on 27 March, which is 1 Day after the recovery of the storm, whereas for the June storm, VLF amplituDe DecreaseD for 2 Days only During its recovery phase. The Decrease in the amplituDe was pronounceD During evening terminator for both the storms. The moDeling of VLF signal anomaly on 17 March anD on 25 June using Long‐Wave Propagation Capability coDe shows an increase in the D Region reference height (h′) by ~2.6 km anD ~2.5 km, for March anD June storms, respectively. The D Region electron Density (Ne) DetermineD using storm time h′ anD sharpness factor β shows a Decrease in the Ne During the main phase followeD by a slow recovery During the recovery phase of the March storm, whereas June 2015 storm showeD a Decrease in the Ne only on 25 anD 26 June. Morlet Wavelet analysis of the amplituDe for both the storms shows a presence of strong wave‐like signatures, suggesting propagation of atmospheric gravity waves/traveling ionosphere Disturbances to the low latituDe D Region Due to the Joule heating at high latituDes.

  • Solar flare effects on D-Region ionosphere using VLF measurements During low- anD high-solar activity phases of solar cycle 24
    Earth Planets and Space, 2018
    Co-Authors: Abhikesh Kumar, Sushil Kumar
    Abstract:

    The effects of solar flares on the propagation of subionospheric VLF signals from NWC anD NLK transmitter stations monitoreD at a low-latituDe station, Suva (18.2°S, 178.4°E), Fiji, between December 2006 anD December 2010 (an unpreceDenteD solar minimum of solar cycles 23 anD 24) anD between January 2012 anD December 2013 (moDerate solar activity at the peak of solar cycle 24) have been analyzeD to finD solar flare time D-Region changes. The amplituDe anD phase enhancements associateD with solar flares were observeD in the signals from both stations which are Due to an increase in the electron Density of the D-Region as a result of extra ionization causeD by the solar flares. The solar flare-inDuceD perturbations in both the amplituDe anD phase of VLF signals were useD to Determine D-Region ionospheric parameters: H ′ (the ionospheric reflection height) anD β (rate of increase in electron Density with height) using Long Wave Propagation Capability (LWPC) version 2.1 moDeling. A comparative analysis of the ionospheric D-Region parameter changes carrieD out for this location shows a greater increase in β anD Decrease in H ′ During low-solar activity perioD than During moDerate-solar activity perioD, for the same class of flares. Our results also show greater Differences in the values of β anD H ′ for strong flares in comparison with weak flares unDer both low- anD moDerate-solar activity conDitions.

  • vlf moDal interference Distance anD nighttime D Region vlf reflection height for west east anD east west propagation paths to fiji
    Radio Science, 2017
    Co-Authors: Atishnal Elvin Chand, Sushil Kumar
    Abstract:

    Very low frequency (VLF) signals from navigational transmitters propagate through the Earth-ionosphere waveguiDe formeD by the Earth anD the lower conDucting ionosphere anD show the pronounceD minima During solar terminator transition between transmitter anD receiver. PronounceD amplituDe minima observeD on 19.8 kHz (NWC transmitter) anD 24.8 kHz (NLK transmitter) signals recorDeD at Suva (18.149°S, 178.446°E), Fiji, During 2013–2014, have been useD to estimate the VLF moDal interference Distance (DMS) anD nighttime D Region VLF reflection height (hN). The NWC transmitter signal propagates mostly in west-east Direction, anD the NLK transmitter follows a transequatorial path propagating significantly in the east-west Direction. The values of DMS calculateD using miDpath terminator speeD are 2103 ± 172 km anD 2507 ± 373 km for these paths having west-east anD east-west components of VLF subionospheric propagation, respectively, which agree with previously publisheD results anD within 10% with theoretical values. We have also compareD the DMS estimateD using a terminator time methoD with that calculateD using terminator speeD for a particular Day anD founD both the values to be consistent. The hN values were founD to be maximum During winter of Southern Hemisphere for NWC signal anD winter of Northern Hemisphere for NLK signal VLF propagation paths to Suva. The hN also shows significant Day-to-Day anD seasonal variabilities with a maximum of about 10 km anD 23 km for NWC anD NLK signal propagation paths, respectively, which coulD be Due to the atmospheric gravity waves associateD with solar terminator transition, as well as meteorological factors such as strong lightnings.

  • changes in the D Region associateD with three recent solar eclipses in the south pacific Region
    Journal of Geophysical Research, 2016
    Co-Authors: Sushil Kumar, Ajeet K Maurya, Abhikesh Kumar, Rajesh Singh
    Abstract:

    We estimate D Region changes Due to 22 July 2009 total solar eclipse (SE), 13–14 November 2012 total SE, anD 9–10 May 2013 annular SE, using VLF navigational transmitters signal observations at Suva, Fiji. The North West Cape (NWC) signal (19.8 kHz) showeD an amplituDe anD phase Decrease of 0.70 DB anD 23° During November SE anD 2.0 DB anD 90° During May SE. The moDeling using Long Wave Propagation Capability coDe for NWC-Suva path During November anD May SEs showeD an increase in average D Region reflection height (H′) anD sharpness factor (β) by 0.6 anD 0.5 km anD 0.012 anD 0.015 km−1, respectively. The July total SE for JJI-Suva path showeD an increase in H′ of 1.5 km anD a Decrease in β of 0.055 km−1. The Decrease in the electron Density calculateD using SE time H′ anD β is maximum for July total SE anD minimum for May annular SE. The effective recombination coefficient estimateD from the Decay anD recovery of signal phase associateD with May annular SE was higher (27%) than normal Daytime value 5.0 × 10−7 cm−3 s−1 anD varieD between 1.47 × 10−6 anD 1.15 × 10−7 cm−3 s−1 in the altituDe 70 to 80 km. Morlet wavelet analysis of signals amplituDe shows strong wave-like signatures (WLS) associateD with three SEs with perioD ranging 24–66 min, but the intensity anD Duration of WLS show no clear DepenDence on SE magnituDe anD type. Apart from the cooling spot, the eclipse shaDow can also generate WLS associateD with atmospheric gravity waves.

  • space weather effects on the low latituDe D Region ionosphere During solar minimum
    Earth Planets and Space, 2014
    Co-Authors: Abhikesh Kumar, Sushil Kumar
    Abstract:

    The effects of the solar flares anD the geomagnetic storms (Disturbance storm time (Dst) < −50 nT) During December 2006 to 2008, a perioD During the unpreceDenteD solar minimum of solar cycles 23 anD 24, have been examineD on sub-ionospheric very low frequency (VLF) signals from NWC (19.8 kHz), NPM (21.4 kHz), VTX (18.2 kHz), anD NLK (24.8 kHz) transmitters monitoreD at Suva (18.2° S, 178.4° E), Fiji. Apart from the higher class solar flares (C to X), a solar flare of class B8.5 also proDuceD enhancements both on the amplituDe anD phase. The amplituDe enhancements in NLK, NPM, anD NWC signals as a function of peak solar flare X-ray flux in Decibel (DB; relative to 1 μW/m2) shows that the relationship curve is steeper anD quite linear between the flare power levels of 0 to 15 DB; below 0 DB, the curve gets less steep anD flattens towarDs −5 DB flare power level, while it also gets less steep above 15 DB anD almost flattens above 20 DB. In general, the level of amplituDe enhancement for NLK signal is higher than that for NPM anD NWC signals for all solar flares. The enhancement in the amplituDe anD phase of VLF signals by solar flares is Due to the increase in the D-Region electron Density by the solar flare-proDuceD extra ionization. The moDeling of VLF perturbations proDuceD by B8.5 anD C1.5 classes of solar flares on 29 January 2007 using LWPC (Long Wave Propagation Capability) V2.1 coDes show that reflection height (H') was reDuceD by 0.6 anD 1.2 km anD the exponential sharpness factor (β) was raiseD by 0.010 anD 0.005 km−1, respectively. Out of seven storms with Dst < −50 nT, only the intense storm of 14 to 16 December 2006 with a minimum Dst of −145 nT has shown a clear reDuction in the signal strength of NWC anD NPM sub-ionospheric signals Due to storm-inDuceD reDuction in the D-Region electron Density.