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IAGA GAII-03 (Space Weather) Paper# IAGA2005-A-00844

This paper discusses the significant impact of the giant gamma-ray flare from Magnetar SGR 1806-20 on the Earth's ionosphere, particularly the daytime lower ionosphere. The study examines the VLF detection of disturbances caused by the flare and explores the mysterious low-frequency emissions observed. It also delves into the characteristics of gamma flares emitted by magnetars, highlighting specific events like those from SGR 0526-66 and SGR 1900+14. Furthermore, the paper presents detailed analysis of the light curve for the December 27, 2004 flare from SGR 1806-20, discussing the peculiarities observed and their implications. The study also addresses the absence of evidence for periodicity in disturbances caused by SGR 1900+14 and provides insights into ionization production by incident gamma rays. By incorporating modeling of VLF propagation in the Earth-Ionosphere waveguide and discussing electron detachment rates, the paper contributes to a deeper understanding of the effects of gamma-ray flares on Earth's ionosphere.

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IAGA GAII-03 (Space Weather) Paper# IAGA2005-A-00844

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  1. IAGA GAII-03 (Space Weather) Paper# IAGA2005-A-00844 Massive Disturbance of the Daytime Lower Ionosphere by the Giant g-ray Flare from Magnetar SGR 1806-20 U. S. Inan1, N. Lehtinen1, R. Moore1, K. Hurley2, S. Boggs2 , D. Smith3 , G. Fishman4 1Space, Telecommunications and Radioscience Laboratory Stanford University, Stanford, California 94305 http://www-star.stanford.edu/~vlf/ 2Space Sciences Laboratory, University of California at Berkeley 3Santa Cruz Institute for Particle Physics, University of Santa Cruz 4NASA, Marshall Space Flight Center

  2. Disturbance of the Nighttime Ionosphere by SGR 1900+14

  3. Disturbance of the Nighttime Ionosphere by SGR 1900+14

  4. Repeated Disturbance of the Ionosphere by SGR 1900+14

  5. VLF Detection of the Disturbance of the Daytime Lower Ionosphere • Very large amplitude/phase perturbations on VLF signals, the great-circle propagation paths of which are illuminated by the Flare • Flare center very near the sun; ionosphere was day-lit

  6. Event Setting and VLF Detection Event onset clearly evident in broadband spectra, as amplitude reduction on all signals, including VLF transmitters and radio atmospherics RHESSI Detectors saturate and stop counting

  7. Mysterious Low Frequency Emission? • An low frequency emission (or an enhanced background) briefly appears at ~2 to 6 kHz • Not known whether a new signal generated by the impact of g-ray flare or propagation effect

  8. Three Giant Gamma Flares[from Halzen et al, 2005] • Emitted by soft gamma repeaters (magnetars) • ~1044 ergs vs ~1054 ergs for extragalactic GRBs; however, these objects are in our Galaxy • May 5, 1979, 15:51 UT • SGR 0526-66, 50 kpc distance • Aug 27, 1998, 10:22:15.7 UT • SGR 1900+14, 14.5 kpc distance • 5.5x10-3 ergs/cm2 fluence in 0.35 s • Perturbed nighttime ionosphere [Inan et al., 1999] • Dec 27, 2004, 21:30:26.65 UT • SGR 1806-20, 15.1 kpc distance • 2 ergs/cm2 fluence in 0.2 s • ~8x1045 ergs emitted in g-rays

  9. g-ray Light Curve for 27 Dec 04 Flare (RHESSI+INTEGRAL) • SGR 1806-20, 21:30:26.5 UT, Dec 27, 2004 • Precursor (~ 1 s) not shown

  10. Event Onset (Initial Peak)[GEOTAIL, Terasawa et al, 2005] • Flux = 20 ergs/s/cm2 • Fluence = 2 ergs/cm2 • T~175 keV • Saturated most detectors • Even the reflection from the Moon was detected! [Mazets et al., 2005]

  11. The oscillations[RHESSI, Hurley et al., 2005] • The oscillations are due to rotation of the magnetar • Duration ~ 380 s • Fluence = 4.6x10-3 erg/cm2 (~0.3% of the total) • T ~ 30 keV • Period = 7.56 s

  12. The afterglow [INTEGRAL, Mereghetti et al., 2005] • Flux ~ t-0.85 • Not detected by other detectors • Spectrum not well known, probably T>~30 keV • Total fluence is roughly same as the oscillating tail • Flux too low to produce sufficient ionization

  13. g-ray Light Curve for 27 Dec 04 Flare (RHESSI+INTEGRAL) • SGR 1806-20, 21:30:26.5 UT, Dec 27, 2004 • Precursor (~ 1 s) not shown

  14. No Evidence for Periodicity

  15. Disturbance of the Nighttime Ionosphere by SGR 1900+14

  16. Incidence Geometry and VLF Detection RHESSI Detectors saturate and stop counting

  17. Narrowband VLF Perturbation Timescales

  18. Ionization Production by Incident g-ray Flare [Inan et al., 1999] • Monte Carlo model (~107 photons) used for g-ray propagation/interaction with atmosphere: • Use published data on photon cross sections, attenuation, and energy absorption coefficients • Compton scattering of photons by electrons • Absorption of photon upon removal of electron from its shell (photoeffect), including relativistic effects • Compton electrons deposit energy within 1 km • Neglected Rayleigh scattering and pair production • Secondary electron production rate is determined using ~35 eV energy loss per electron-ion pair

  19. Four species: Ne : electrons N- : negative ions, N+ : positive ions, Nx+ : positive cluster ions Processes: Ionization by the g-ray flare e- detachment e- attachment Conversion of N+ into Nx+ Recombination of e- and Nx+ Dissociative recombination of e- and N+ Mutual neutralization of N- with N+ or Nx+ Simple D-region Chemistry Model [Pasko and Inan, 1994] [Developed to interpret recovery signatures of subionospheric VLF perturbations due to lightning-induced electron precipitation]

  20. Simple D-region Chemistry Model [Pasko and Inan, 1994]

  21. Zenith Angle (y) Dependence of Ionization by g-rays

  22. Time Evolution of Electron Density (cm-3) background y=0o y=89o

  23. Narrowband VLF Perturbation Timescales

  24. Modeling VLF Propagation in the Earth-Ionosphere Waveguide • Multiple-mode solution accounts for: • Realistic Ground Conductivities • Path broken into 100-km slabs with varying Ne as dictated by the zenith angle dependence of ionization • The Earth’s Magnetic Field • Altitude-dependent ionospheric conductivity profile • Modal coupling at the slab boundaries

  25. Electron Detachment Rate g • Daytime g is determined by photodetachment • Nighttime g ~e-6000/T is sensitive to temperature • Shown with electron attachment rate b

  26. Onset for Different Values of Electron Detachment Rate g

  27. Recovery for Different Values of Electron Detachment Rate g Data Data

  28. Long Event Recovery for Electron Detachment g=3x10-18N Data Data

  29. VLF Signatures of Event Onset • High absorption of all signals, due to increased ionization • A low frequency emission between 3 and 6 kHz ; generated by the flare impact or a propagation effect?

  30. Summary • Giant g-ray flare from SGR 1806-20 massively disturbed the daytime lower ionosphere (<85 km) • Electron density increased to ~104 cm-3 at ~60 km and to ~10 cm-3 at ~25 km • Although the afterglow of the g-ray flare lasted for more than an hour, the g-ray flux was too low to produce sufficient ionization beyond the peak • At <60 km, enhanced electron densities due to the flare onset endured for more than an hour, due to high values of the electron-detachment rate • The nature of the low frequency emission which appears in the 3 to 6 kHz range is not yet clear

  31. g-ray Light Curve for 27 Dec 04 Flare (RHESSI+INTEGRAL) • SGR 1806-20, 21:30:26.5 UT, Dec 27, 2004 • Precursor (~ 1 s) not shown

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