On the Pre-Magnetic Storm Signatures in NmF2 in Some Equatorial, Low- and Mid-Latitude Stations

In this paper, the ionospheric quiet-time disturbances otherwise known as Pre-Magnetic Storm Signatures (PMS) have been studied using the F2-layer peak electron density (NmF2) data obtained from 12 Digisonde/ionosonde stations distributed across equatorial, low and mid-latitudes. The datasets used spans the years 2010-2012. Results from this study reveals strong PMS in NmF2 with percentage deviations (Delta NmF2) ranging from -91% to 500% at the equatorial, low and mid-latitudes, with maxima occurring at the equatorial region. Significant effects on the peak height of the F2-layer (hmF2) were also observed, and they are correlated with the variations in NmF2 particularly at the equatorial station during the PMS. The duration of a PMS is found to be 12-48 h. Although, it was difficult to state clearly the connection between the PMS and the geomagnetic storm that usually follows within 24-48 h; but the NmF2 and hmF2 responses during the PMS were quite similar to those observed during geomagnetic storms. A slight increase in the Solar-wind-plasma speed (>20 km/s) was also observed during PMS. The PMS occur under a southward IMF-Bz, moderate aurora activity (AE ranging from 114 to 560 nT) and quiet ring current (Dst >-10 nT). Therefore, it is pertinent to consider a certain threshold of the aurora indices (AE, AL, and AU) in addition to the Dst, ap, and Kp in the definition of a geomagnetically quiet day. This may eliminate the ambiguity in explaining the ionospheric variability that occurs few days before Sudden Storm Commencement (SSC).