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Effects of Severe Geomagnetic Storms on Low-Equatorial D-Region Ionosphere Using VLF Radio Waves

This study examines the effects of severe geomagnetic storms in March and June 2015 on the low-equatorial D-region ionosphere using very low-frequency (VLF) radio waves. VLF observations and LWPC modeling are used to analyze the storms' impact on VLF signal amplitude and explore the mechanisms behind these effects. The study highlights the decrease in VLF signal amplitude during the storms and suggests possible mechanisms such as changes in reflection height and electron density. Wavelet analysis results also indicate the presence of atmospheric gravity waves (AGWs) during the storms. Overall, this study provides valuable insights into the effects of geomagnetic storms on the low-equatorial D-region ionosphere using VLF radio waves.

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Effects of Severe Geomagnetic Storms on Low-Equatorial D-Region Ionosphere Using VLF Radio Waves

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  1. On the effects of severe geomagnetic storms of March and June 2015 at low-equatorial D-region ionosphere using very low-frequency radio waves Ajeet K Maurya, Department of Physics, Doon University, Dehradun, India International space weather initiative workshop 20-24 May 2019

  2. Plan of Talk • Introduction • Storms of March and June 2015 • VLF observations and LWPC Modeling – Low Latitude path • Wavelet analysis • Summary and Conclusion

  3. Geomagnetic storm and its characteristics • Geomagnetic storm is the temporary disturbance on Earth’s magnetosphere • Geomagnetic storm mechanism • Signature Storm produces magnetic field opposite to Earth’s Horizontal component, decreases Dst index Different effects could be seen at different regions of Ionosphere Dst (nT)

  4. What are VLF waves? • Very low frequency waves (3-30 kHz) • Natural broadband sources of VLF waves (covers entire frequency range) • Fixed frequency navigational transmitters (Narrowband sources) • At VLF frequencies the D-region and the Earth’s surface acts as conductor and form a waveguide, Allows VLF waves to travel around the globe

  5. Daily variation of VLF narrowband signal • No solar radiation • Minor sources are present • Causes variable signal TRGCP in day TRGCP in night TRGCP in day/night transition TRGCP in day/night transition Lyman-α

  6. VLF Receiver and Transmitter Narrowband VLF signal (Low-equatorial Latitude path) Low-Equatorial latitude path

  7. Geomagnetic storm of March 2015 (St. Patrick’s day Storm) Occurred on the Birth Anniversary of St. Patrick (17th March) The most severe (min Dst = -223 nT) storm of solar cycle 24, Caused by a double-halo CME The storm reached its peak (severe) intensity at ~00:00 UT on 18 March 2015 (Dst ~ -223 nT) Recovered fully on 25 March, 2015 Widely studied at E and F regions of Ionosphere (JGR-Space Physics special issue in 2015) (e.g. Ramsingh et al., 2015; Nava et al., 2016; Jacobsen et al., 2016 and many more)

  8. Effect of March storm on VLF signals Time LT (hours)

  9. March storm effect on VLF signals • VLF signal Amplitude decrease • Decrase on main phase day (17 March) ~3.5 dB (w. r. to minima2) • Maximum amplitude change ~3.6dB (w. r. to minima 2) on 19 March 2 days after main phase

  10. Geomagnetic storm of June 2015 The second most severe storm of solar cycle 24, caused by two CMEs on 22 June 2015 The storm reached its peak (severe) intensity at ~04:30 UT on 23 June 2015 (Dst ~ -204 nT) Recovered fully on 27 June, 2015 Less studied compare to March 2015 storm, and no work for D-region effect

  11. Effect of June storm on VLF signals • VLF singal Amplitude decrease • Between 17:00 – 19:00 LT during 25th - 26th June • No decrease on main phase day (23 June) • Maximum decrease ~3.4 dB on 25th June (2 days after main phase)

  12. LWPC Modeling of Storm perturbations • LWPC : Long Wavelength Propagation Capability Theoretical modeling code: for VLF signal propagation Input Change in VLF amplitude h/ = Reflection height (km) β = Sharpness factor (km-1) • h/controls altitude of electron density profile • βcontrols sharpness of electron density profile Input Electron density [Wait and Species, 1964]

  13. LWPC modeling of NWC Signal: March Storm h’ β Electron density NWC ~ 48%

  14. LWPC modeling of NWC Signal: June Storm h’ β Electron density NWC ~ 40%

  15. Probable Mechanism Mechanism Decrease in VLF signal amplitude Longer VLF wave path Increase in D-region reflection height (h’) R Wave energy loss Earth Decrease in D-region electron density EIWG Storm EIWG Normal Decrease in VLF amplitude Mechanism for electron density change during storm (1) Prompt penetration (PP) of the high-latitude electric fields to low latitudes Characterized by few minutes of main phase onset Sudden change in the signal strength Not found in present observations (2) The storm induced circulations in terms of TIDs/AGWs

  16. The storm induced circulations in terms of TIDs/AGWs • Wavelet analysis result: AGWs in NWC signal March Storm From 17th – 20th March; Period ~40-60 mins AGWs in NWC signal June Storm On 25th June; Period ~40-60 mins • AGWs increases temperature and Neutral density • Results in a decrease in ionization due to an increase in the recombination coefficient.

  17. Sources of TIDs/AGWs Direct observations of AGWs suggest important role of TID/AGWs mechanism. Question: How the AGWs have generated? • The Figures shows, the large variations in the AE index (marked by red circles). • Larger variation of AE indices could be the direct indication of Joule heating in the Auroral region and causing observed AGWs.

  18. Summary and Conclusion Summary and Conclusion • We performed first detailed observation and modeling of effect of two major storms events of solar cycle 24 on the low latitude D-region. • VLF signal amplitude showed decrease during evening terminator • For March storm decrease observed during 17-27 March, while for June storm it was only two days 25-26 June. • March storm show storm after effect • The modeling results show decrease in the D-region electron density during storm at low latitude. • The storm induced circulation from high latitude to low latitude in term ofTIDs/AGW are clearly seen

  19. References • Astafyeva, E., I. Zakharenkova, and M. Förster (2015), Ionospheric response to the 2015 St. Patrick’s Day storm: A global multi-instrumental overview, J. Geophys. Res. Space Physics, 120, 9023–9037, doi:10.1002/2015JA021629 • Bozoki T. et al. (2017), Signature of St. Patrick Geomagnetic Storm on Schumann Resonances, 19th EGU General Assembly (EGU2017) proceedings from the conference held 23-28 April, 2017, Vienna, Austria., p.1858 • Hargreaves, J. K. (1992), The Solar-Terrestrial Environment, Cambridge Univ. Press, New York. • Ramsingh, S. Sripathi, S. Sreekumar, S. Banola, K. Emperumal, P. Tiwari, and B. S. Kumar (2015), Low-latitude ionosphere response to super geomagnetic storm of 17/18 March 2015: Results from a chain of ground based observations over Indian sector, J. Geophys. Res. Space Physics, 120, 10,864–10,882, doi:10.1002/2015JA021509 • Maurya, A. K., Venkatesham, K., Kumar, S., Singh, R., Tiwari, P., & Singh, A. K. (2018). 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: Space Physics, 123. https://doi.org/10.1029/2018JA025536

  20. Thank you for your attention ! question/suggestions ? Contact information Dr. Ajeet K Maurya UGC-Assistant Professor Ramanujan Fellow Department of Physics, Doon University, Dehradun, India Email: ajeet.iig@gmail.com; Mob: 9454932852

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