NmF2 and hmF2 measurements at 95° E and 127° E around the EIA northern crest during 2010–2014

Bitap Raj Kalita,Akimasa Yoshikawa,Pradip Kumar Bhuyan

doi:10.1186/s40623-015-0355-3

Bitap Raj Kalita, Akimasa Yoshikawa + Show 1 more

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https://doi.org/10.1186/s40623-015-0355-3

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Abstract

The characteristics of the F2 layer parameters NmF2 and hmF2 over Dibrugarh (27.5° N, 95° E, 17° N geomagnetic, 43° dip) measured by a Canadian Advanced Digital Ionosonde (CADI) for the period of August 2010 to July 2014 are reported for the first time from this low mid-latitude station lying within the daytime peak of the longitudinal wave number 4 structure of equatorial anomaly (EIA) around the northern edge of anomaly crest. Equinoctial asymmetry is clearly observed at all solar activity levels whereas the midday winter anomaly is observed only during high solar activity years and disappears during the temporary dip in solar activity in 2013 but forenoon winter anomaly can be observed even at moderate solar activity. The NmF2/hmF2 variations over Dibrugarh are compared with that of Okinawa (26.5° N, 127° E, 17° N geomagnetic), and the eastward propagation speed of the wave number 4 longitudinal structure from 95° E to 127° E is estimated. The speed is found to be close to the theoretical speed of the wave number 4 (WN4) structure. The correlation of daily NmF2 over Dibrugarh and Okinawa with solar activity exhibits diurnal and seasonal variations. The highest correlation in daytime is observed during the forenoon hours in equinox. The correlation of daily NmF2 (linear or non-linear) with solar activity exhibits diurnal variation. A tendency for amplification with solar activity is observed in the forenoon and late evening period of March equinox and the postsunset period of December solstice. NmF2 saturation effect is observed only in the midday period of equinox. Non-linear variation of neutral composition at higher altitudes and variation of recombination rates with solar activity via temperature dependence may be related to the non-linear trend. The noon time maximum NmF2 over Dibrugarh exhibits better correlation with equatorial electrojet (EEJ) than with solar activity and, therefore, new low-latitude NmF2 index is proposed taking both solar activity and EEJ strength into account.

Highlights

The earth’s ionosphere is formed due to photoionization of neutral atmosphere by solar radiation and exhibits latitudinal, longitudinal, altitudinal as well as diurnal and seasonal variations
Diurnal and seasonal variations of NmF2 The seasonal variation of NmF2 is illustrated as a function of local time in Fig. 2 for low solar activity (August 2010–August 2011) and moderately high solar activity (September 2011–July 2014), respectively
In both low and high solar activity conditions, equinoctial asymmetry is observed, i.e., NmF2 is higher in March equinox than in September equinox

Summary

Introduction

The earth’s ionosphere is formed due to photoionization of neutral atmosphere by solar radiation and exhibits latitudinal, longitudinal, altitudinal as well as diurnal and seasonal variations. The diurnal variation of solar activity effect on daily NmF2 and hmF2 measured over Dibrugarh and Okinawa from August 2010 till July 2014 and its seasonal variation is studied using solar activity proxies.

Results

Conclusion