1. Introduction

[2]  Traditionally, ionospheric F2 -layer disturbances are related to solar activity variations (geomagnetic activity being a part of it), but there exists a large class of disturbances which are not directly due to geomagnetic activity but have their origin in the atmosphere itself. Partly, such quiet time disturbances (Q disturbances) may be attributed to the impact from below. Presumably, the energy is transferred by internal gravity waves propagating from troposphere and stratosphere and producing perturbations in upper atmosphere not only at the heights of the lower ionosphere ( D region), where the meteorological control is well known [e.g., Danilov, 1986; Danilov et al., 1987], but also in the F region [Forbes et al., 2000; Kazimirovsky and Kokourov, 1991; Kazimirovsky et al., 2003; Khachikjan, 1987; Rishbeth and Mendillo, 2001]. One of the first attempts to analyze "pure" ionospheric disturbances in the F2 layer was undertaken by Zevakina and Hill [1978] using ionosonde observations at the low-latitude station, San Jose, for solar minimum (1964-1965) and solar maximum (1968) conditions. Quiet time N_mF2 deviations in the daytime F2 region observed around equinoxes were analyzed by Mikhailov and Schlegel [2001]. Another example of strong N_mF2 quiet time deviations presents the midlatitude nighttime F2 layer where strong up to a factor of 3-5 and even larger deviations in N_mF2 is a common feature of the night-to-night variability [Farelo et al., 2002; Mikhailov and Förster, 1999; Mikhailov et al., 2000a, 2000b]. Well-known quasi-2-day oscillation in the ionosphere [Altadill and Apostolov, 2001; Apostolov et al., 1995; Chen, 1992; Forbes and Zhang, 1997; Forbes et al., 2000; Rishbeth and Mendillo, 2001] also may be attributed to the meteorological effects in the F2 region as they are not related to geomagnetic activity. It should be noted that Q disturbances are not only rather frequent but their amplitude is comparable to the amplitude of moderate F2 -layer storm effects. It seems that F2 -layer Q disturbances have different origins depending on conditions, but no systematic analysis of this problem has been done yet. This paper is devoted to the morphological analysis of F2 -layer Q disturbances using the worldwide ground-based ionosonde observations from high to lower latitudes in the Northern Hemisphere. Equatorial F2 layer exhibits different morphology, and it will be considered in a separate paper. Physical interpretation of the revealed morphological features will be given later elsewhere.