Seasonal and solar cycle variations of equatorial spread-F in the American zone

Abstract

The range type of spread-F near the magnetic equator seems to be associated with the post sunset uplifting of the F-layer which itself is caused by the strong eastward horizontal electric field in the evening before its reversal to the normal nighttime westward direction. The frequency type of spread-F seems to be the latter development of the F-region irregularities. During any solar epoch, the occurrence of range spread is maximum diuranally at 2100 LT and seasonally around November and February months, a secondary peak develops around 0500 during June solstices. The occurrence of frequency spread is maximum around 0000 LT diurnally and seasonally around June-July. The solar cycle effect of range spread is quite significant raising the rate and peak value of post sunset increase of its occurrence; however, the occurrence decreases very rapidly with time after 2100 LT during high sunspot years. Further, the range spread occurs fairly frequently during the post midnight hours of the low sunspot years especially for June solstices. Thus the relation between sunspots and range spread is positive for pre-midnight hours and strongly negative for post midnight hours giving an inverse relation between sunspots and whole night average range spread.