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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
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.


Yamazaki Y.,Kyushu University | Yumoto K.,Kyushu University | Cardinal M.G.,Kyushu University | Fraser B.J.,University of Newcastle | And 14 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.

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