The Morphology of Equatorial Plasma Bubbles – a review

길효섭 한국우주과학회 2015 Journal of Astronomy and Space Sciences Vol.32 No.1

Plasma bubbles that occur in the equatorial F-region make up one of the most distinguishing phenomena in the ionosphere. Bubbles represent plasma depletions with respect to the background ionosphere, and are the major source ofelectron density irregularities in the equatorial F-region. Such bubbles are seen as plasma depletion holes (in situ satelliteobservations), vertical plumes (radar observations), and emission-depletion bands elongated in the north-south direction(optical observations). However, no technique can observe the whole three-dimensional structure of a bubble. Variousaspects of bubbles identified using different techniques indicate that a bubble has a “shell” structure. This paper reviewsthe development of the concepts of “bubble” and “shell” in this context.

The Occurrence Climatology of Equatorial Plasma Bubbles: A Review

길효섭 한국우주과학회 2022 Journal of Astronomy and Space Sciences Vol.39 No.2

Electron density irregularities in the equatorial ionosphere at night are understood in terms of plasma bubbles, which are produced by the transport of low-density plasma from the bottomside of the F region to the topside. Equatorial plasma bubbles (EPBs) have been detected by various techniques on the ground and from space. One of the distinguishing characteristics of EPBs identified from long-term observations is the systematic seasonal and longitudinal variation of the EPB activity. Several hypotheses have been developed to explain the systematic EPB behavior, and now we have good knowledge about the key factors that determine the behavior. However, gaps in our understanding of the EPB climatology still remain primarily because we do not yet have the capability to observe seed perturbations and their growth simultaneously and globally. This paper reviews the occurrence climatology of EPBs identified from observations and the current understanding of its driving mechanisms.

Statistical studies of trough at middle latitudes observed by DMSP F15

박사라,길효섭,김관혁,Park, Sarah,Kil, Hyosub,Kim, Khan-Hyuk 한국천문학회 2012 天文學會報 Vol.37 No.2

The middle-latitude ionization trough, the plasma density depletion in the subauroral region, has been extensively studied since its discovery in the 1960s. Our knowledge of the variability of the trough is mostly based on the observations in the northern hemisphere. Recently, the COSMIC observations enabled the investigation of the trough in both hemispheres at all local time. However, the investigation was limited to the period of the low solar activity. In this study, we investigate the variability of the trough location and morphology with local time, solar cycle, magnetic activity, and interplanetary magnetic field. For this purpose, we analyze the DMSP F15 data acquired during 2000-2010.

Nonmigrating tidal characteristics in the thermospheric neutral mass density

곽영실,길효섭,이우경,오성준,양태용,Kwak, Young-Sil,Kil, Hyosub,Lee, Woo-Kyoung,Oh, Seung-Jun,Yang, Tae-Yong 한국천문학회 2012 天文學會報 Vol.37 No.2

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 $90^{\circ}$ within 24 h 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 $90^{\circ}$/24 h 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.

Variation of the Hemispheric Asymmetry of the Equatorial Ionization Anomaly with Solar Cycle

곽영실,길효섭,이우경,양태용 한국우주과학회 2019 Journal of Astronomy and Space Sciences Vol.36 No.3

In solstices during the solar minimum, the hemispheric difference of the equatorial ionization anomaly (EIA) intensity (hereafter hemispheric asymmetry) is understood as being opposite in the morning and afternoon. This phenomenon is explained by the temporal variation of the combined effects of the fountain process and interhemispheric wind. However, the mechanism applied to the observations during the solar minimum has not yet been validated with observations made during other periods of the solar cycle. We investigate the variability of the hemispheric asymmetry with local time (LT), altitude, season, and solar cycle using the electron density taken by the CHAllenging Minisatellite Payload satellite and the global total electron content (TEC) maps acquired during 2001–2008. The electron density profiles provided by the Constellation Observing System for Meteorology, Ionosphere, and Climate satellites during 2007–2008 are also used to investigate the variation of the hemispheric asymmetry with altitude during the solar minimum. During the solar minimum, the location of a stronger EIA moves from the winter hemisphere to the summer hemisphere around 1200–1400 LT. The reversal of the hemispheric asymmetry is more clearly visible in the F-peak density than in TEC or in topside plasma density. During the solar maximum, the EIA in the winter hemisphere is stronger than that in the summer hemisphere in both the morning and afternoon. When the location of a stronger EIA in the afternoon is viewed as a function of the year, the transition from the winter hemisphere to the summer hemisphere occurs near 2004 (yearly average F10.7 index = 106). We discuss the mechanisms that cause the variation of the hemispheric asymmetry with LT and solar cycle.

Characteristics of mid-latitude field-aligned irregularities observed with VHF coherent scatter ionospheric radar over Korea

양태용,곽영실,길효섭,박영득,Yang, Tae-Yong,Kwak, Young-Sil,Kil, Hyosub,Park, Young-Deuk 한국천문학회 2012 天文學會報 Vol.37 No.2

The 40.8-MHz VHF coherent scatter ionospheric radar, located in South Korea (Gyeryong, $36.18^{\circ}N$, $127.14^{\circ}E$), has been operating since December 2009 to investigate ionosphere E- and F-region field-aligned irregularities (FAIs) of mid-latitude. During the observation, we found E- and F-region FAIs appeared frequently: continuous echoes during the post-sunrise period and Quasi-Periodic (QP) echoes at nighttime for E region ; strong post-sunset and pre-sunrise FAIs for F region. The characteristics of E- and F-region FAIs are presented in terms of seasonal and local time variations of occurrence during December 2009 to August 2012. In addition, to investigate the correlation with geomagnetic activity to FAIs occurrence, we compared K-index variations to local time occurrence. It is worth to note our occurrence result since long term observation over several years in the mid-latitude has not yet been carried out.

Characteristics of the E - and F -region field-aligned irregularities in middle latitudes: Initial results obtained from the Daejeon 40.8 MHz VHF radar in South Korea

곽영실,양태용,길효섭,Devulapalli Venkata Phanikumar,허복행,이재진,황정아,최승환,박영덕,최호성 한국우주과학회 2014 Journal of Astronomy and Space Sciences Vol.31 No.1

We present preliminary observations of the field-aligned-irregularities (FAIs) in the E and F regions during the solarminimum (2009 - 2010) using the 40.8 MHz coherent backscatter radar at Daejeon (36.18°N, 127.14°E, 26.7°N dip latitude)in South Korea. The radar, which consists of 24 Yagi antennas, observes the FAIs using a single beam with a peak power of24 kW. The radar has been continuously operated since December 2009. Depending on the manner of occurrence of thebackscatter echoes, the E-region echoes are largely divided into two types: quasi-periodic (QP) and continuous echoes. Our observations show that the QP echoes occur frequently above an altitude of 105 km in the post-sunset period andcontinuous echoes occur preferentially around an altitude of 105 km in the post-sunrise period. QP echoes appear asstriated discrete echoes for a period of about 10 - 20 min. The QP-type echoes occur more frequently than the continuoustypeechoes do and the echo intensity of the QP type is stronger than that of the continuous type. In the F region, the FAIsoccur at night at an altitude interval of 250 - 450 km. As time proceeds, the occurrence height of the FAIs gradually increasesuntil early in the morning and then decreases. The duration of the F-region FAIs is typically a few hours at night, although,in rare cases, FAIs persist throughout the night or appear even after sunrise. We discuss the similarities and differences ofthe FAIs observed by the Daejeon radar in comparison with other radar observations.