Perturbations of midlatitude subionospheric VLF signals associated with lower ionospheric disturbances during major geomagnetic storms

Abstract

We examine the effects on the midlatitude ionospheric D region of the 7 April 2000 storm and the “Halloween storm” of late October 2003 by means of the associated perturbations of several subionospheric VLF signals propagating in both the northern and southern hemispheres. We use VLF nighttime data from the Holographic Array for Ionospheric/Lightning Research (HAIL), located in the United States (L = 2–3), as well as data from Palmer Station, Antarctica (L = 2.4). On 7 April 2000, a ∼5 dB depression in VLF amplitudes is observed at multiple HAIL stations, with a depression onset that occurs later for VLF signal paths at lower latitudes. On both 7 April 2000 and 31 October 2003, fluctuations in the amplitude of the VLF signals are first observed in the premidnight sector and persist through the end of the data‐recording period (dawn). The frequency content of the fluctuations is predominantly in the 0.01 to 0.02 Hz range but extends up to ∼0.03 Hz. Increases in the energetic electron flux in the loss cone as measured by the NOAA‐POES satellites are observed on both 7 April 2000 and 31 October 2003. We suggest that both the signal depressions and subsequent fluctuations are associated with variations in the precipitation flux of energetic electrons onto the upper atmosphere. Auroral activity patterns based on data from the NOAA‐POES satellites show that the equatorward edge of the auroral oval expanded equatorward to lower L shells (L < 3) during both geomagnetic storms. Using the auoral activity patterns and multiple VLF/LF signal paths, we provide evidence that the fluctuations and the signal depression coincide with the equatorward edge of the auroral oval extending over the perturbed VLF/LF Great Circle Paths. Quantitative modeling of subionospheric VLF wave propagation incorporating energetic electron flux measurements (and the associated altitude profiles of secondary ionization) yields results consistent with the variations in the VLF signal amplitude observed.

Space weather effects on the low latitude D-region ionosphere during solar minimum

“…The diurnal plots for both NWC and NPM signals were closely inspected for the period 12 to 18 December 2006. No apparent increase in signal fluctuations were noticed during day or nighttime as reported by previous researchers such as Peter et al (2006) who reported amplitude fluctuations in the nighttime data and Belrose and Thomas (1968) who reported fluctuations in the phase during twilight and nighttime during the geomagnetic storms in the mid-latitude regions. The relevant phase data which were intercepted by short-term switching off of transmitters have not been used for analysis, since phase does not follow the same value as before the transmitter was switched off.…”

“…They obtained negative phase and amplitude variations of the VLF signal both during day and nighttimes during the main storm phase days, but the nighttime variations were more pronounced. There have not been other studies on magnetic storm associated subionospheric changes purely at low latitude paths; however, there have been several studies on VLF perturbations due to storm-induced energetic electron precipitation at mid and high latitudes (e.g., Peter et al 2006;Clilverd et al 2010).…”

Effects of a Solar Eclipse on the Propagation of VLF-LF Signals: Observations and Results

“…The paper of Peter et al (2006) demonstrates that subionospheric VLF signals can be used as a diagnostic of high-energy auroral precipitation at midlatitudes during disturbed geomagnetic conditions. The onset of fluctuations in the VLF amplitude data occurred immediately after the depression of the signal amplitude.…”

“…In fact, they reported that VLF events occurred in higher number during recovery phase of geomagnetic storms. Peter et al (2006) examined the effects of two geomagnetic storms on the midlatitude D-region using several VLF signal paths in both the northern and southern hemispheres. The amplitude and phase of the VLF signals exhibited perturbations with increased geomagnetic activity and with fluctuations in amplitude persisting several hours after the occurrence of the geomagnetic storm.…”

Sub-ionospheric very low frequency perturbations associated with the 12 May 2008 &lt;i&gt;M&lt;/i&gt; = 7.9 Wenchuan earthquake

“…The enhancement in the VLF amplitude was most likely due to additional ionization (basically increase in electron concentration) produced by this earthquake in the lower ionosphere. We have looked for four possible mechanisms for electron density enhancement in the D region and hence the enhancement in JJI VLF amplitude: solar flares (Zigman et al, 2007), geomagnetic storm (Peter et al, 2006), lower ionospheric heating due to lightning discharges causing early/fast VLF events (Inan et al, 1996), and seismic origin due to 12 May 2008 EQ. We have examined details of all possible events; solar flare events are of few minute duration whose effect can be easily identified in the VLF amplitude data.…”