Ionospheric Systems Research

Noosaville, Australia

Ionospheric Systems Research

Noosaville, Australia
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Yamazaki Y.,Kyushu University | Yumoto K.,Kyushu University | Cardinal M.G.,Kyushu University | Fraser B.J.,University of Newcastle | And 15 more authors.
Journal of Geophysical Research: Space Physics | Year: 2011

An empirical model of the quiet daily geomagnetic field variation has been constructed based on geomagnetic data obtained from 21 stations along the 210 Magnetic Meridian of the Circum-pan Pacific Magnetometer Network (CPMN) from 1996 to 2007. Using the least squares fitting method for geomagnetically quiet days (Kp ≤ 2+), the quiet daily geomagnetic field variation at each station was described as a function of solar activity SA, day of year DOY, lunar age LA, and local time LT. After interpolation in latitude, the model can describe solar-activity dependence and seasonal dependence of solar quiet daily variations (S) and lunar quiet daily variations (L). We performed a spherical harmonic analysis (SHA) on these S and L variations to examine average characteristics of the equivalent external current systems. We found three particularly noteworthy results. First, the total current intensity of the S current system is largely controlled by solar activity while its focus position is not significantly affected by solar activity. Second, we found that seasonal variations of the S current intensity exhibit north-south asymmetry; the current intensity of the northern vortex shows a prominent annual variation while the southern vortex shows a clear semi-annual variation as well as annual variation. Thirdly, we found that the total intensity of the L current system changes depending on solar activity and season; seasonal variations of the L current intensity show an enhancement during the December solstice, independent of the level of solar activity. Copyright 2011 by the American Geophysical Union.


Lynn K.J.W.,Ionospheric Systems Research | Gardiner-Garden R.S.,Defence Science and Technology Organisation, Australia | Heitmann A.,Defence Science and Technology Organisation, Australia
Journal of Geophysical Research A: Space Physics | Year: 2014

Morning and afternoon peaks in daytime critical frequency foF2 defining a midday bite out were found to occur regularly across northern Australia during April 2008. This behavior was sufficiently repetitive to appear in the monthly median values of foF2. The twin peak and bite out phenomenon was observable across the entire longitudinal range of ionosondes which were accessible for this study, from Niue in mid-Pacific to Learmonth on the western side of Australia. The high geographic density of ionosondes operating in Australia enabled the limited latitudinal range of occurrence to be established, the phenomenon ceasing to be present in the southern half of Australia or at equatorial latitudes. While strongest in 2008, the foF2 bite out and associated variation in hmF2, the peak height of the ionosphere, continued to be seen in monthly April medians from sunspot minimum (2007-2008) to the current low sunspot maximum (2011-2013) while diminishing in magnitude. This phenomenon was also present around the September equinox, though not of such magnitude or consistency as during the April equinox. The morning and afternoon peaks in foF2 occurred during periods of falling virtual and true height and were associated with the maximum compression of the ionosphere at this time as measured by the subpeak equivalent parabolic layer thickness ym. Meridional winds in the F2 layer are suggested as a driver of the twin peak enhancements in foF2, which may possibly be related to a simultaneous occurrence of strong tidal influences as seen to be present in descending sporadic E. © 2014. American Geophysical Union. All Rights Reserved.


Balan N.,University of Sheffield | Balan N.,Kyoto University | Yamamoto M.,Kyoto University | Sreeja V.,Vikram Sarabhai Space Center | And 8 more authors.
Journal of Geophysical Research: Space Physics | Year: 2011

The response of the dayside equatorial F2 layer to the main phases of the 22 intense geomagnetic storms (Dst < -150 nT) in 1998-2008 is investigated using the digital ionosonde data from the equatorial stations in Brazilian, Indian, and Australian longitudes together with equatorial electrojet strength and IMF Bz; the storms include 15 superstorms (Dst < -200 nT). The observations show that there is a period during all MPs when the F2 layer peak rises (and falls) rapidly with large peak electron density (Nmax) reduction, the rise velocity strongly correlates with the intensity (Dst) of the storms, and the duration of the Nmax reduction corresponds to that of strong eastward electrojet when IMF Bz remains highly negative. The observations indicate the occurrence of strong eastward prompt penetration electric fields (PPEF) during the rapid F2 layer response. The PPEF drives the F2 layer peak rapidly upward, which reduces Nmax due to vertical expansion and diffusion. The results therefore suggest that the rapid F2 layer response (rapid rise (and fall) of peak height (hmax) with large Nmax reduction) observed by ionosondes can be used to detect the occurrence of the daytime eastward PPEF during intense geomagnetic storms irrespective of season and level of solar activity. The data also show two rare events of strong daytime westward electric fields due to disturbance dynamo and/or prompt penetration. The results are important when radars are not available to monitor the occurrence of the PPEF. Copyright 2011 by the American Geophysical Union.


Lynn K.J.W.,Ionospheric Systems Research
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2013

Three consecutive magnetic storms during the month of September 1982 were found to be associated with solar proton events (SPE) observed over a number of high latitude VLF propagation paths. The penetration of solar protons into the auroral zone produced a marked reduction in reflection height at night for high latitude VLF paths resulting in a reduced diurnal phase shift. This effect has been known for some 50 years. However in this paper, a previously unidentified response is described consisting of an increase in the night 90. km reflection height over middle latitude and transequatorial VLF paths. Solar protons do not penetrate to these latitudes and this slight increase in VLF reflection height was associated with typical negative ionospheric storm effects in the F2 region. Dynamics at the 90. km base of the night ionosphere are little known and difficult to investigate except at VLF. These results are the first to suggest a response of the night ionospheric base to events leading to the well known negative ionospheric storm seen at greater heights. Such negative storms seen in the F2 region have been associated with an equatorward wind surge and change in neutral atmospheric chemistry driven by joule heating in the auroral zone produced by solar proton precipitation. © 2013 Elsevier Ltd.


Lynn K.J.W.,Ionospheric Systems Research | Otsuka Y.,Nagoya University | Shiokawa K.,Nagoya University
Journal of Atmospheric and Solar-Terrestrial Physics | Year: 2013

A form of range-time plots derived from ionograms taken from a standard digital ionosondes, situated at the low latitude sites of Vanimo, Port Moresby and Darwin, exhibit bursts of spread F at the center of descending and ascending off-angle reflectors. This particular type of event has since been identified with the passage of optically imaged ionospheric plasma depletions (bubbles) over a Darwin ionosonde. This paper describes the process for producing this form of range-time display and its relationship to ionospheric height, satellite traces and range spread F as seen on individual ionograms. First hop satellite traces are proposed to be via direct reflection from the steep electron density gradients at the base of bubbles while second hop satellite traces then involve a single additional ground reflection. Measurements of night equatorial drift velocity were made from the range-time displays and found to be in the range 20-220. m/s peaking at approximately 90-100. m/s in good agreement with values derived from drift measurements made by a variety of other types of equipment. © 2013 Elsevier Ltd.


Lynn K.J.W.,Ionospheric Systems Research
AIP Conference Proceedings | Year: 2010

In recent times, research has moved towards using VLF radio transmissions propagating in the earth-ionosphere waveguide as a detector of a variety of transient geophysical phenomena. A correct interpretation of such results depends critically on understanding the propagation characteristics of the path being monitored. The observed effects will vary depending on time of day, path length, path orientation, magnetic latitude and VLF frequency. This paper provides a brief tutorial of the relevant propagation dependencies for medium to long VLF paths best understood in terms of waveguide mode theory together with results either not previously published, not published in the open scientific literature or whose significance has been little recognised. © 2010 American Institute of Physics.


Lynn K.J.W.,Ionospheric Systems Research | Otsuka Y.,Nagoya University | Shiokawa K.,Nagoya University
Geophysical Research Letters | Year: 2011

Airglow observations of ionospheric electron density depletions made at Darwin, Australia have demonstrated that the tree-like structure of bubbles developed at the magnetic equator are mapped along magnetic field lines with considerable accuracy to the base of the ionosphere at higher latitudes. Ionosonde range-time displays made at Darwin and other equatorial sites in the Australian region show characteristic approaching and receding echoes which converge on a typical spread-F event. These off-angle echoes have often been referred to in the literature as satellite traces and associated with spread F with little recognition of their true significance. All four optical depletions previously reported in the literature as being seen at Darwin are found in this paper to be accompanied by such typical off-angle/spread F events. The zonal drift velocity of the moving reflectors can be measured from the speed at which such echoes approach and recede. Since digital ionosondes in equatorial sites have existed for many years, existing ionogram data, when suitably processed into range-time displays, may allow the occurrence of such events over several sunspot cycles to be found. A question remains as to whether all or only some of such equatorial range-time events correspond to electron density depletions. Copyright 2011 by the American Geophysical Union.


Shiokawa K.,Nagoya University | Otsuka Y.,Nagoya University | Lynn K.J.W.,Ionospheric Systems Research | Wilkinson P.,Bureau of Meteorology | Tsugawa T.,Japan National Institute of Information and Communications Technology
Earth, Planets and Space | Year: 2015

Abstract We report the first observation of the disappearance of a plasma bubble over geomagnetically conjugate points. It was observed by airglow imagers at Darwin, Australia (magnetic latitude: -22°N) and Sata, Japan (21°N) on 8 August 2002. The plasma bubble was observed in 630-nm airglow images from 1530 (0030 LT) to 1800 UT (0300 LT) and disappeared equatorward at 1800 to 1900 UT (0300 to 0400 LT) in the field of view. The ionograms at Darwin and Yamagawa (20 km north of Sata) show strong spread-F signatures at approximately 16 to 21 UT. At Darwin, the F-layer virtual height suddenly increased from approximately 200 to approximately 260 km at the time of bubble disappearance. However, a similar F-layer height increase was not observed over the conjugate point at Yamagawa, indicating that this F-layer rise was caused not by an eastward electric field but by enhancement of the equatorward thermospheric wind over Darwin. We think that this enhancement of the equatorward neutral wind was caused by an equatorward-propagating large-scale traveling ionospheric disturbance, which was identified in the north-south keogram of 630-nm airglow images. We speculate that polarization electric field associated with this equatorward neutral wind drive plasma drift across the magnetic field line to cause the observed bubble disappearance. © 2015 Shiokawa et al.; licensee Springer.


Lynn K.J.W.,Ionospheric Systems Research | Gardiner-Garden R.S.,Defence Science and Technology Group | Heitmann A.,Defence Science and Technology Group
Radio Science | Year: 2016

This paper examines a number of sources of ionospheric variability and demonstrates that they have relationships in common which are currently not recognized. The paper initially deals with medium to large-scale traveling ionospheric disturbances. Following sections deal with nontraveling ionospheric disturbance (TID) ionospheric variations which are often repetitious from day to day. The latter includes the temporary rise in F2 height associated with sunset in equatorial latitudes resulting from strong upward drift in ionization driven by an E × B force. The following fall in height is often referred to as the premidnight collapse and is accompanied by a temporary increase in foF2 as a result of ionospheric compression. An entirely different repetitious phenomenon reported recently from middle latitudes in the Southern Hemisphere consists of strong morning and afternoon peaks in foF2 which define a midday bite-out and occur at the equinoxes. This behavior has been speculated to be tidal in origin. All the sources of ionospheric variability listed above exhibit similar relationships associated with a temporary expansion and upward lift of the ionospheric profile and a fall involving a compression of the ionospheric profile producing a peak in foF2 at the time of maximum compression. Such ionospheric compression/decompression is followed by a period in which the ionospheric profile recovers. Such relationships in traveling ionospheric disturbances (TIDs) have been noted previously. The present paper establishes for the first time that relationships hitherto seen as occurring only with TIDs are also present in association with other drivers of ionospheric variability. ©2016. American Geophysical Union. All Rights Reserved.

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