Short-Period(5-33 min) Variations in Vertical Drift of F-Region Plasma near the Magnetic Equator.

Abstract

Measurements ofFregion vertical drift, V~ made at Kodaikanal (dip 4°N) on 133 days during March 1991-February 1992 under non-equatorial spread-F(ESF) conditions, are analysed forthe characteristics of short-period (5-33 min) variations in Vr in the evening hours (1630-0130 IST). The spectral content of the fluctuations in V. is found to depend on local time. The amplitude of the fluctuations, quantified in terms of the variance (a2), increases during the period of postsunset enhancement of upward vertical drift and decreases thereafter reaching a minimum during 2200-0000 IST. In consonance with this, while fnctuations in Vi in the entire band 5-33 min manifest in the postsunset hours, the longer-period (13-33 min) components dominate thereafter (2200-0000 IST). The amplitude of the fluctuations and its dayto-day variability are higher in local summer than in local winter. On a day-to-day basis, the amplitude of Vi functuations increases with peak height in the interval 1756-2004 IST corresponding to the epoch ofthe evening enhancement in Vr. The results support the view that the short-period variations in Fregion vertical drift near the dip equator during the evening hours could be due to electric fields associated with the commonly occurring atmospheric gravity waves.

Letter to the editor: Electric field fluctuations (25-35 min) in the midnight dip equatorial ionosphere

“…This is the reason for the absence of an apparent downward drift over Kodaikanal prior and during the QP¯uctuations. Corrections for chemical loss e ects are not made for V z data because they are basically a d.c e ect and¯uctuations with periods <40 min of speci®c interest here are unlikely to be in¯uenced by the altitude-dependent changes in the chemical loss or in the ionization scale height (Sastri, 1995). The sense of the perturbation peaks in V z is to be positive (upward drift) in view of their close temporal association with IMF B z and magnetic variations elsewhere (see Fig.…”

The diurnal, annual and sunspot-cycle variations of ESFm at Huancayo compared with similar variations of spread-F occurrence at other longitudes and latitudes

“…An increased wave amplitude with height for an equatorial location has been recorded by Sastri (1995) when AGWs with shallow wave amplitudes were being investigated with no spread-F recorded. Sastri (1995) states, p. 1215, ''The amplitude of the fluctuations y. increases during the period of post-sunset enhancement of upward vertical drift y.'' For increased wave amplitudes with height for equatorial post-sunset periods, see also Nair et al (1992).…”

Ionogram‐recorded equatorial spread‐F and height changes at Huancayo during sunspot‐maximum years

“…Shallow‐wave amplitudes, which do not lead to spread‐F traces on ionograms, have been shown to occur regularly in equatorial latitudes. Sastri [1995] for the postsunset period and in the absence of spread‐F has recorded AGWs with periodicities in the range 5–33 min. The wave amplitudes of these AGWs increased with height [see also Subbarao and Krishna Murthy , 1994].…”

“…Whether or not spread‐F is recorded seems to depend on wave amplitudes. The spatial scale of ESF m will depend on medium‐scale structures with periodicities in the range 5–33 min [ Sastri , 1995] and AGWs with speeds averaging 97 m s −1 [ Huang , 1990].…”