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 (ΔNmF2) 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). Key Points Pre‐Magnetic Storm Signatures (PMS) on NmF2 were observed to be maximum at the equatorial ionosphere Ionospheric Responses during PMS were observed to be similar to the effect of geomagnetic storms There is a need to consider a certain threshold of the aurora indices (AE, AL, and AU) in the definition of a geomagnetic quiet day