A minimum in the latitude variation of spread-F at March equinox

N Balan,V L Narayanan,A K Patra,T Maruyama

doi:10.1186/s40645-018-0180-y

Abstract

This short paper presents the statistics of spread-F occurrence at four low-mid latitude locations (17.0° N, 21.2° N, 26.5° N and 36.5° N mag. lat.) in Japan longitude sector (~ 135° E) in March to April 2002, 2003 and 2006 (high, medium and low solar activity). The location of a spread-F minimum predicted by theoretical models is identified for the first time. The spread-F minimum occurs at the poleward side of the equatorial ionisation anomaly crest and shifts equatorward from ~ 25° N mag. lat. at high solar activity to below 17° N at low solar activity. The corresponding spread-F maximum occurring on the poleward side of the minimum also shifts equatorward from ~ 35° N (or beyond) at medium solar activity to 20°–25° N at low solar activity. The spread-F occurrence increases with decreasing solar activity at all locations especially at higher latitudes (> ~ 25° N) where there is almost no occurrence at high solar activity.

Highlights

Earth’s ionosphere has a smooth density distribution during daytime when there is production of ionisation
The spread-F occurrence (Fig. 6a) at all locations increases with decreasing solar activity at higher latitudes (26.5°–36.5° N) where the occurrence increases from a low of ~ 2% at high solar activity to a high of over 65% at low solar activity
The analysis has identified a spread-F maximum on the poleward side of the minimum expected from the Perkins mechanism (e.g. Tsunoda 2006); the maximum shifts equatorward with decreasing solar activity

Summary

Introduction

Earth’s ionosphere has a smooth density distribution during daytime when there is production of ionisation. In other words, during daytime when the E-region conductivity is strong, the ionospheric drivers such as electric fields and neutral winds and their fluctuations cannot destabilise the ionosphere. After sunset when the Eregion conductivity becomes weak, the drivers can destabilise the ionosphere and generate plasma irregularities of various scale sizes that manifest as spread-F (Booker and Wells 1938) and plasma bubbles (Woodman and La Hoz 1976) causing scintillations in communication signals (Aarons 1993). The main driver that destabilises the ionosphere at post-sunset hours is the pre-reversal strengthening of the eastward electric field (e.g. Heelis et al 1974; Eccles et al 2015), which causes the pre-reversal enhancement of the vertical upward E × B plasma drift velocity Vz referred as PRE The irregularities have a linear growth rate (Ossakow 1981):

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Discussion

Conclusion