Response of the low‐latitude D region ionosphere to extreme space weather event of 14–16 December 2006

Abstract

The response of the D region low-latitude ionosphere has been examined for extreme space weather event of 14-16 December 2006 associated with a X1.5 solar flare and an intense geomagnetic storm (Dst = À146 nT) using VLF signals from Northwest Cape, Australia (NWC) (19.8 kHz) and Lualualei, Hawaii (callsign NPM) (21.4 kHz) transmitters monitored at Suva (Geographic Coordinates, 18.10°S, 178.40°E), Fiji. Modeling of flare associated amplitude and phase enhancements of NWC (3.6 dB, 223°) and NPM (5 dB, 153°) using Long-Wave Propagation Capability code shows reduction in the D region reflection height (H′) by 11.1 km and 9.4 km, and enhancement in ionization gradients described by increases in the exponential sharpness factor ( β) by 0.122 and 0.126 km À1, for the NWC and NPM paths, respectively. During the storm the daytime signal strengths of the NWC and NPM signals were reduced by 3.2 dB on 15 and 16 December (for about 46 h) and recovered by 17 December. Modeling for the NWC path shows that storm time values of H′ and β were reduced by 1.2 km and 0.06 km À1, respectively. Morlet wavelet analysis of signal amplitudes shows no clearly strong signatures of gravity wave propagation to low latitudes during the main and recovery phases. The reduction in VLF signal strength is due to increased signal attenuation and absorption by the Earth-ionosphere waveguide due to storm-induced D region ionization changes and hence changes in D region parameters. The long duration of the storm effect results from the slow diffusion of changed composition/ionization at D region altitudes compared with higher altitudes in the ionosphere.

The 22 July 2009 Total Solar Eclipse: Modeling D Region Ionosphere Using Narrowband VLF Observations

“…The result is understandable since the NWC‐SUVA TRGCP did not see any considerable effect of the eclipse. It must be noted that N e profile shown in Figure for control days and reduction in N e on 22 July 2009 TSE day at four sites, ALD, VNS, NAT, and BUSAN, is not a reduction of N e in solar eclipse affected segment of TRGCP; rather, it is an average decrease in N e all along the whole TRGCP (Kumar et al, ) that would cause the same changes in the H ′ and β as estimated in this study. Considering 75‐km altitude as the daytime D region reference height, we next compare N e profiles presented in Figure on control and TSE days.…”

“…LWPC model obtained H ′ and β presented in Table show increase in the H ′ on TSE day. Using LWPC obtained H ′ and β several workers have used conventional Wait profile (Wait & Spies, ) to create D region electron density profiles, N e ( h ) in per cubic centimeter up to altitude of 100 km under normal and perturbed geophysical conditions (e.g., Kumar et al, ; McRae & Thomson, ; Thomson et al, ). Figures a–e present the N e ( h ) at all the five stations: ALD, VNS, NAT, BUSAN, and SUVA obtained using equation , wherein values of input parameters H ′ and β were taken from Table on respective control days and on 22 July TSE day.…”

Changes in the D region associated with three recent solar eclipses in the South Pacific region

“…There was an intense geomagnetic storm ( Dst = −108 nT) with main phase during 13/14 November 2012, but we do not see any WLS corresponding to storm activity during the period of our analysis shown in Figure which is consistent with results of Kumar et al . []. However, based on this analysis comparatively for shorter period of about 6 h (Figure ), we cannot completely rule out the occurrence of WLS associated with storms as already reported by Vlasov et al .…”

“…Several researchers have used this conventional Wait profile (equation (1)) to determine the D region electron density under the normal conditions and disturbed conditions due to solar and geomagnetic activities [e.g., McRae and Thomson, 2004;Thomson et al, 2005;Kumar et al, 2015]. This electron density profile (equation (1)) gives a simple effective average change during the SEs along the TRGCPs; however, a more realistic D region profile using the technique given by Todoroki et al [2007] could also be determined.…”

Long-duration positive ionospheric storm during the December 2006 geomagnetic storm: Ionizing effect of forbidden electrons

“…Extreme solar and space weather events of 13-16 December 2006 have comprehensively studied earlier (Dmitriev et al, , 2010Lei et al, 2008aLei et al, , 2008bEbihara et al, 2009;Myagkova et al, 2009;Pedatella et al, 2009;de Jesus et al, 2010;Kikuchi et al, 2010;Wang et al, 2010;Wei et al, 2011;Dahlgren et al, 2012;Klimenko and Klimenko, 2012) and still attract much attention, particularly in studies of the upper atmosphere and ionosphere (Koshiishi, 2014;Zigman et al, 2014;Kumar et al, 2015). Also, the positive ionospheric storm and FEE enhancement during the main phase and maximum of the 15 December geomagnetic storm were analyzed in our previous works (Suvorova et al, 2012(Suvorova et al, , 2013.…”