Analysis of diurnal double maxima observed above Italy during 1975–1991 Z.T. Katamzi a,n , N.D. Smith b , C.N. Mitchell b , P. Spalla c a South African National Space Agency (SANSA) Space Science, P O Box 32, Hermanus 7200, South Africa b Department of Electronic and Electrical Engineering, University of Bath, Bath BA2 7AY, United Kingdom c Istituto Di Fisica Applicata Nello Carrara, Consiglio Nazionale delle Ricerche, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy article info Article history: Received 3 February 2011 Received in revised form 1 August 2012 Accepted 2 August 2012 Available online 10 August 2012 Keywords: Double diurnal maxima Double peak Total electron content Storms and substorms abstract One of the major features of the mid-latitude ionosphere is the anomalous enhancements that often either occur before or after the more usual post mid-day peak. The structure in the total electron content comprising a pre- or post-midday peak and another enhancement is referred to by Pi et al. (1993) as a diurnal double maxima (DDM). DDM structures were sometimes observed during daytime in a long time-series of mid-latitude slant total electron content (sTEC) measurements collected in Italy between 1975 and 1991. From a total of 1366 daily TEC observations, DDM structures were observed on 302 days. The first peak usually occurred between the hours of 10–11 UT, and the second peak usually occurred either between the hours of 13–14 UT or 18–19 UT, where UT  LT1 hour. The times of occurrence of each peak demonstrated a seasonal variation, with the second peak appearing later in the day during summer. The rate of occurrence of the DDM structure was higher in winter and summer than in autumn and spring. Independent observations were sought by using foF2 and hmF2 measurements from an ionosonde in Rome. Some evidence shows that vertical plasma motions in the F-region can cause DDM, however the cause of many other DDM events is still not clear. Furthermore, a comparison of DDM structures across different geomagnetic and auroral activities indicated that the structures were not necessarily related to geomagnetic storms or substorm events. & 2012 Elsevier Ltd. All rights reserved. 1. Introduction The general diurnal variation of the ionosphere in response to solar radiation as the primary source of ionisation is well under- stood; an example of the general trend of the ionospheric parameter, slant total electron content (sTEC), is shown in Fig. 1. Of course there are deviations from this diurnal variation caused by events like travelling ionospheric disturbances which produce wave-like structures in the electron density or TEC data, for example see Soicher (1988). There are also night-time enhancements which are small peaks on the electron density or TEC data during night time, for example see Farelo et al. (2002). Another example is the general Weddell Sea Anomaly where the diurnal peak of ionospheric electron density or total electron content appears in the evening or at night during summer, for example see He et al. (2009). Also, the ionosphere behaves differently under severe storm conditions, as documented in a review by Mendillo (2006). An ionospheric phenomenon in which the temporal variation of the ionospheric parameter, e.g. max- imum frequency reflected from the F2 region (foF2) or TEC, displays two peaks during day time has been known as either midday bite-out or diurnal double maxima (DDM). This has been studied in detail by Pi et al. (1993, 1995). The double peak structures have also been observed in earlier work by Kohl et al. (1968) and Saryo et al. (1989). The DDM structures have been observed at low and mid latitudes. It has been suggested that the bite-out at low latitude regions is due to the Appleton anomaly caused by the eastward electric field, (Saryo et al., 1989), but this was before bite-out structures were observed at higher latitudes. Later studies have suggested two different mechanisms for the occurrence of the double peaks, namely the neutral winds and E  B drifts: (1) depletion in the DDM structure may be due to the meridional winds blowing polewards causing ions to move along the geo- magnetic field lines to lower altitudes, where the loss rate is higher, while the enhancements may be due to equatorward winds causing the ions to move to higher altitudes where the rate loss is lower, (2) the E  B drift causes vertical motion of the ionospheric plasma that results in either a depletion or enhance- ment. There are arguments for and against these mechanisms (see Saryo et al., 1989; Pi et al., 1993, 1995), and this suggests that the cause of the midday bite-out or enhancements is not fully understood. This paper presents a statistical analysis on the occurrence of the DDM structure in slant TEC measurements; its duration and relative magnitudes of the peaks for different Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jastp Journal of Atmospheric and Solar-Terrestrial Physics 1364-6826/$ - see front matter & 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jastp.2012.08.001 n Corresponding author. Tel.: þ27283121196. E-mail addresses: z.t.katamzi@bath.ac.uk (Z.T. Katamzi), n.smith@bath.ac.uk (N.D. Smith), c.n.mitchell@bath.ac.uk (C.N. Mitchell), p.spalla@ifac.cnr.it (P. Spalla). Journal of Atmospheric and Solar-Terrestrial Physics 89 (2012) 67–75