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.