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اخترفیزیک و علوم فضایی 2013
جزئیات رویداد
همایش ، رویداد ، ژورنال
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  • effective recombination coefficient and solar zenith angle effects on low-latitude d-region ionosphere evaluated from vlf signal amplitude and its time delay during x-ray solar flares

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    • تاریخ ارائه: 1392/07/24
    • تاریخ انتشار در تی پی بین: 1392/07/24
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     excess solar x-ray radiation during solar flares causes an enhancement of ionization in the ionospheric d-region and hence affects sub-ionospherically propagating vlf signal amplitude and phase. vlf signal amplitude perturbation (δa) and amplitude time delay (δt) (vis-á-vis corresponding x-ray light curve as measured by goes-15) of nwc/19.8 khz signal have been computed for solar flares which is detected by us during jan–sep 2011. the signal is recorded by softpal facility of ierc/icsp, sitapur (22∘ 27′n, 87∘ 45′e), west bengal, india. in first part of the work, using the well known lwpc technique, we simulated the flare induced excess lower ionospheric electron density by amplitude perturbation method. unperturbed d-region electron density is also obtained from simulation and compared with iri-model results. using these simulation results and time delay as key parameters, we calculate the effective electron recombination coefficient (α eff ) at solar flare peak region. our results match with the same obtained by other established models. in the second part, we dealt with the solar zenith angle effect on d-region during flares. we relate this vlf data with the solar x-ray data. we find that the peak of the vlf amplitude occurs later than the time of the x-ray peak for each flare. we investigate this so-called time delay (δt). for the c-class flares we find that there is a direct correspondence between δt of a solar flare and the average solar zenith angle z over the signal propagation path at flare occurrence time. now for deeper analysis, we compute the δt for different local diurnal time slots dt. we find that while the time delay is anti-correlated with the flare peak energy flux ϕ max independent of these time slots, the goodness of fit, as measured by reduced-χ 2, actually worsens as the day progresses. the variation of the z dependence of reduced-χ 2 seems to follow the variation of standard deviation of z along the t x -r x propagation path. in other words, for the flares having almost constant z over the path a tighter anti-correlation between δt and ϕ max was observed.

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