Significance of ionospheric VLF measurements for the Earth's (solar cycle 24) atmosphere
In this presentation we show results from very low frequency (VLF, 3-30 kHz) amplitude and phase measurements related to ionospheric D-region disturbances due to solar flares for the period 2009 to 2017. The record is constantly updated, at the moment ~400 C/M/X-class solar flare events are in the database and compared with NOAA GOES x-ray flux measurements. In general ionospheric variations attract high interest, e.g. due to environmental studies, satellite navigation systems and remote sensing applications.
The VLF receiver measures amplitude and phase from sub-ionospheric paths of 12 navigational transmitters to the Graz station, which is part of a larger network. In the VLF frequency range the signals are propagating in the so-called Earth-ionosphere waveguide and are subject to various modulations, e.g. (i) lower ionospheric variations connected with flares, energetic particle precipitation, cosmic rays, and/or (ii) influences from the surface and lithosphere, among them natural phenomena like earthquakes and volcanic eruptions.
The VLF receiving station is primarily dedicated to investigate earthquake related phenomena and associated lithospheric atmospheric ionospheric (LAI) coupling mechanisms, complementary measurements are from satellites. A mere coincidence is the planned launch of the China Seismo-Electromagnetic Satellite (CSES) with IWF participation one day after this talk on Feb. 2nd, 2018. The scientific goals from the CSES mission and the VLF measurements are very similar, the oral presentation summarize planned common investigations in the near future.