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Kim J.-H.,Chungnam National University | Kim Y.H.,Chungnam National University | Oh S.-J.,Space Environment Laboratory Inc.
Journal of the Korean Physical Society | Year: 2016

As a base model for space weather application, we develop an ionospheric model (SAMI2-CNU) by revising the open source SAMI2 model. The revision includes the photoionization of X-rays in the range below 50 Å and photoelectron impact ionization. As a solar flux input to the model, we utilize the real-time solar extreme ultraviolet (EUV) and X-ray (1 ~ 105.0 nm) fluxes of the FISM (flare irradiance spectral model). In order to verify the SAMI2-CNU model, we compare the peak densities of the E and the F2 layers with those from the GIRO (Global Ionospheric Radio Observatory) network for 110 days with the F10.7 range of 70 - 280 in the last solar cycle. The model provides larger values of NmF2 by 30 - 40%, but the NmE values are reasonably close to the ionosonde values. By carrying out a linear correlation analysis between the model and the ionosonde values, we derived scale factors of 0.67 and 0.41 for the photoionization rates in the F2 and the E regions, respectively. With these scale factors, the SAMI2-CNU model allows us to match the ionosonde NmF2 and NmE values within 7% on the average. We also simulated ionospheric changes during eight solar flare events and found averaged 20% and 46% enhancements of NmF2 and NmE, respectively, over the pre-flare values. Although the SAMI2-CNU model has the same problem with matching the NmF2 enhancement as the original SAMI2, it provides reasonable estimates of the NmE increase, as well as reasonable matches of the NmF2 and the NmE values with the ionosonde values. Thus, the SAMI2-CNU model can be used as a physics model in the data-assimilated model that is being developed for the regional ionosphere around the Korea peninsula. © 2016, The Korean Physical Society.

Kil H.,Johns Hopkins University | Oh S.-J.,Space Environment Laboratory Inc.
Journal of Geophysical Research: Space Physics | Year: 2011

The variability of the evening prereversal enhancement (PRE) of the vertical plasma drift has been investigated by using the average velocity at a fixed local time range, but this method does not provide a clear distinction between the PRE occurrence frequency and magnitude. In this study, we examine the dependence of the occurrence frequency, occurrence local time, and magnitude of the PRE on geophysical parameters by using the retrievals of the individual PRE from the first Republic of China satellite data. Our result shows that the seasonal-longitudinal variation of the average vertical velocity in the evening that has been identified in previous studies is associated with both the PRE occurrence frequency and PRE magnitude. The magnitude of the PRE shows an increasing trend with an increase of the solar flux and with a decrease of the conjugate E region sunset time lag. The seasonal-longitudinal variation of the occurrence local time of the peak PRE shows a dependence on the E region sunset time at the magnetic equator rather than on the magnetic declination or the sunset time lag in the conjugate E regions. This observation indicates that the optimum time for the creation of the peak PRE is when the sunset terminator at the E region height and the magnetic meridian intersect near the magnetic equator. Copyright 2011 by the American Geophysical Union.

Kil H.,Johns Hopkins University | Kil H.,Korea Astronomy and Space Science Institute | Kwak Y.-S.,Korea Astronomy and Space Science Institute | Kwak Y.-S.,Korea University | And 4 more authors.
Journal of Geophysical Research A: Space Physics | Year: 2015

Plasma density depletions (bubbles) and enhancements (blobs) with respect to the background ionosphere occur at night in the low-latitude F region. Those phenomena are understood to be either causally linked or independent. The idea of the causal relationship between bubbles and blobs is on the basis of the observations of them in the same longitude. However, the occurrence of bubbles and blobs in the same longitude can also be just a coincidence. We investigate causal linkage of bubbles and blobs using the measurements of the ion density on 5-days in June 2008 and April 2009 by the Communication/Navigation Outage Forecasting System and CHAllenging Minisatellite Payload satellites. The observations during the solar minimum show that blobs occur in broader longitudes than do bubbles and occur in any longitudes regardless of the existence of bubbles. These observations indicate that a significant portion of blobs are not associated with bubbles. Even if some blobs are associated with bubbles, those blobs are indistinguishable from those produced by other sources. Therefore, the observations of bubbles and blobs at the same longitudes do not warrant their causal relationship. The independent behavior of bubbles and blobs rather indicates that their occurrences in the same longitudes are mostly coincidences. Considering the frequent occurrence of blobs near midnight, June solstice, and the solar minimum, medium-scale traveling ionospheric disturbances are likely the major source of blobs. This idea is supported by the observations of blobs with the ionospheric disturbances in broad longitudes and latitudes. Key Points The occurrences of plasma blobs and bubbles are not correlated Blobs accompany ionospheric disturbances in broad longitudes and latitudes MSTIDs are the plausible source of blobs during a solar minimum ©2015. American Geophysical Union. All Rights Reserved.

Kwak Y.-S.,Korea Astronomy and Space Science Institute | Kil H.,Johns Hopkins University | Lee W.K.,Korea Astronomy and Space Science Institute | Oh S.-J.,Space Environment Laboratory Inc. | Ren Z.,CAS Institute of Geology and Geophysics
Journal of Geophysical Research: Space Physics | Year: 2012

The wave number 4 (wave 4) and wave number 3 (wave 3) longitudinal structures in the thermospheric neutral mass density are understood as tidal structures driven by diurnal eastward propagating zonal wave number 3 (DE3) and wave number 2 (DE2) tides, respectively. However, those structures have been identified using data from limited time periods, and the consistency and recurrence of those structures have not yet been examined using long-term observation data. We examine the persistence of those structures by analyzing the neutral mass density data for the years 2001-2008 taken by the Challenging Minisatellite Payload (CHAMP) satellite. During years of low solar activity the amplitude of the wave 4 structure is pronounced during August and September, and the wave 4 phase shows a consistent eastward phase progression of 90° within 24h local time in different months and years. During years of high solar activity the wave 4 amplitude is small and does not show a distinctive annual pattern, but the tendency of the eastward phase shift at a rate of 90°/24h exists. Thus the DE3 signature in the wave 4 structure is considered as a persistent feature. The wave 3 structure is a weak feature in most months and years. The amplitude and phase of the wave 3 structure do not show a notable solar cycle dependence. Among the contributing tidal modes to the wave 3 structure, the DE2 amplitude is most pronounced. This result may suggest that the DE2 signature, although it is a weak signature, is a perceivable persistent feature in the thermosphere. Copyright 2012 by the American Geophysical Union.

Kil H.,Johns Hopkins University | Paxton L.J.,Johns Hopkins University | Kim K.-H.,Kyung Hee University | Park S.,Kyung Hee University | And 2 more authors.
Journal of Geophysical Research: Space Physics | Year: 2011

The variations of ionospheric and thermospheric disturbances in satellite data during geomagnetic storms are often understood as storm-phase-dependent temporal variations. However, the nonuniform spatial distribution of disturbances and their corotation also produce an apparent temporal variation. In this case study of the ionospheric disturbances during the 20 November 2003 storm, we examine the contribution of the spatial component to the storm-time behavior of ionospheric disturbances. The equatorward boundary of the negative ionospheric storm observed at noon by the Challenging Minisatellite Payload (CHAMP) shows apparent equatorward and poleward movements as the storm progresses. The global total electron content (TEC) maps show that the apparent behavior of the negative ionospheric storm is related to the corotation of the spatially nonuniform ionospheric disturbances. The behavior of the negative ionospheric storm is closely associated with the evolution of the thermospheric neutral composition disturbance. The middle-latitude steep plasma density gradient appears in the dusk and premidnight sectors during the storm period. The temporal evolution of the global TEC maps shows that the steep density gradient is not created by an instantaneous response of the ionosphere to the storm. A severe positive ionospheric storm is created in the dayside during the main phase of the storm, and the corotation of the daytime positive ionospheric storm is primarily responsible for the steep density gradients in the dusk and premidnight sectors. Copyright 2011 by the American Geophysical Union.

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