Anomalous variations of <I>Nm</I>F2 over the Argentine Islands: a statistical study

Abstract

Abstract. We present a statistical study of variations in the F2-layer peak electron density, NmF2, and altitude, hmF2, over the Argentine Islands ionosonde. The critical frequencies, foF2, and, foE, of the F2 and E-layers, and the propagation factor, M(3000)F2, measured by the ionosonde during the 1957-1959 and 1962-1995 time periods were used in the statistical analysis to determine the values of NmF2 and hmF2. The probabilities to observe maximum and minimum values of NmF2 and hmF2 in a diurnal variation of the electron density are calculated. Our study shows that the main part of the maximum diurnal values of NmF2 is observed in a time sector close to midnight in November, December, January, and February exhibiting the anomalous diurnal variations of NmF2. Another anomalous feature of the diurnal variations of NmF2 exhibited during November, December, and January when the minimum diurnal value of NmF2 is mainly located close to the noon sector. These anomalous diurnal variations of NmF2 are found to be during both geomagnetically quiet and disturbed conditions. Anomalous features are not found in the diurnal variations of hmF2. The statistical study of the NmF2 winter anomaly phenomena over the Argentine Islands ionosonde was carried out. The variations in a maximum daytime value, R, of a ratio of a geomagnetically quiet daytime winter NmF2 to a geomagnetically quiet daytime summer NmF2 taken at a given UT and for approximately the same level of solar activity were studied. The conditional probability of the occurrence of R in an interval of R, the most frequent value of R, the mean expected value of R, and the conditional probability to observe the F2-region winter anomaly during a daytime period were calculated for low, moderate, and high solar activity. The calCorrespondence to: A. V. Pavlov (pavlov@izmiran.ru) culations show that the mean expected value of R and the occurrence frequency of the F2-region winter anomaly increase with increasing solar activity.

On the solar cycle variation of the winter anomaly

“…Lee et al [2011] recently described these ably in their introduction, so only a few papers will be mentioned here. A number of studies extended Berkner et al's [1936] original work [e.g., Yonezawa, 1971;Torr and Torr, 1973;Rishbeth and MullerWodarg, 2006;Pavlov and Pavlova, 2009]. The Torr and Torr [1973] study described the difference in the winter anomaly at solar maximum and minimum.…”

“…Their work, however, used a definition of the winter anomaly that effectively included the annual anomaly as well and did not attempt to describe how the winter anomaly developed over a solar cycle. Pavlov and Pavlova [2009] suggested that the Torr and Torr [1973] study might not make an accurate calculation of the intensity of the winter anomaly because of contamination by magnetically disturbed conditions. However, as geomagnetic storms are relatively rare events (as defined by Kp > 5), their contribution to the averages used by Torr and Torr was probably relatively small.…”

Multipoint observations of Ionospheric Alfvén Resonance

“…There is an anomaly in f 0 F 2 behaviour at UAS in summer when the critical frequency in nighttime is higher than in daytime [13]. This explains why the probability of IAR registration at UAS in summer is much lower than at the other stations.…”

Variations of f o F 2 and GPS total electron content over the Antarctic sector

“…Close to the midnight sector, the F 2 -layer is raised and maintained by plasma drift. As the ionosphere is sunlit during the long summer day in January N m F 2 increases producing the observed higher N m F 2 values close to midnight (Pavlov et al, 2008;Pavlov and Pavlova, 2009). …”

“…This behaviour indicates that diurnal variations of f o F 2 have a weak solar zenith dependence. These diurnal variations of f o F 2 are explained by Pavlov and Pavlova (2009) by changes in the neutral wind-induced drift along the magnetic field lines and by neutral density variations. A poleward neutral wind causes a lowering of the F 2 -region height and a reduction of the density peak N m F 2 due to an increase in the loss rate of O + ions with N 2 and O 2 .…”