Dependence of Quiet Time Geomagnetic Activity Seasonal Variation on the Solar Magnetic Polarity

Oh, Suyeon 한국우주과학회 2013 Journal of Astronomy and Space Sciences Vol.30 No.1

The geomagnetic activity shows the semiannual variation stronger in vernal and autumnal equinoxes than in summer and winter solstices. The semiannual variation has been explained by three main hypotheses such as Axial hypothesis, Equinoctial hypothesis, and Russell-McPherron Effect. Many studies using the various geomagnetic indices have done to support three main hypotheses. In recent, Oh & Yi (2011) examined the solar magnetic polarity dependency of the geomagnetic storm occurrence defined by Dst index. They reported that there is no dependency of the semiannual variation on the sign of the solar polar fields. This study examines the solar magnetic polarity dependency of quiet time geomagnetic activity. Using Dxt index (Karinen & Mursula 2005) and Dcx index (Mursula & Karinen 2005) which are recently suggested, in addition to Dst index, we analyze the data of three-year at each solar minimum for eight solar cycles since 1932. As a result, the geomagnetic activity is stronger in the period that the solar magnetic polarity is anti-parallel with the Earth’s magnetic polarity. There exists the difference between vernal and autumnal equinoxes regarding the solar magnetic polarity dependency. However, the difference is not statistically significant. Thus, we conclude that there is no solar magnetic polarity dependency of the semiannual variation for quiet time geomagnetic activity.

Frequency of Solar Spotless Days and Flare Index as Indices of Solar Cycle Activity

Oh, Suyeon The Korean Space Science Society 2014 Journal of Astronomy and Space Sciences Vol.31 No.2

There was a research on the prolongation of solar cycle 23 by the solar cyclic variation of solar, interplanetary geomagnetic parameters by Oh & Kim (2013). They also suggested that the sunspot number cannot typically explain the variation of total solar irradiance any more. Instead of the sunspot number, a new index is introduced to explain the degree of solar activity. We have analyzed the frequency of sunspot appearance, the length of solar cycle, and the rise time to a solar maximum as the characteristics of solar cycle. Then, we have examined the predictability of solar activity by the characteristics of preceding solar cycle. We have also investigated the hemispheric variation of flare index for the periods that the leading sunspot has the same magnetic polarity. As a result, it was found that there was a good correlation between the length of preceding solar cycle and spotless days. When the length of preceding solar cycle gets longer, the spotless days increase. It is also shown that the shorter rise time to a solar maximum is highly correlated with the increase of sunspots at a solar maximum. Therefore, the appearance frequency of spotless days and the length of solar cycle are more significant than the general sunspot number as an index of declining solar activity. Additionally, the activity of flares leads in the northern hemisphere and is stronger in the hemisphere with leading sunspots in positive polarity than in the hemisphere with leading sunspots in negative polarity. This result suggests that it is necessary to analyze the magnetic polarity's effect on the flares and to interpret the period from the solar maximum to solar maximum as the definition of solar cycle.

Simultaneous Forbush Decrease caused by a CME shot by the STEREO

Suyeon Oh,Yu Yi 한국천문학회 2011 天文學會報 Vol.36 No.2

Influence of solar wind structure on cosmic ray modulation during epochs of alternate solar magnetic polarity

Oh, Suyeon,Yi, Yu,Evenson, Paul American Institute of Physics 2018 Physics of plasmas Vol.25 No.10

Observation of Periodic and Transient Cosmic Ray Flux Variations by the Daejeon Neutron Monitor and the Seoul muon Detector

Oh, Suyeon,Kang, Jeongsoo 한국우주과학회 2013 Journal of Astronomy and Space Sciences Vol.30 No.3

Recently, two instruments of cosmic ray are operating in South Korea. One is Seoul muon detector after October 1999 and the other is Daejeon neutron monitor (Kang et al. 2012) after October 2011. The former consists of four small plastic scintillators and the latter is the standard 18 NM 64 type. In this report, we introduce the characteristics of both instruments. We also analyze the flux variations of cosmic ray such as diurnal variation and Forbush decrease. As the result, the muon flux shows the typical seasonal and diurnal variations. The neutron flux also shows the diurnal variation. The phase which shows the maximum flux in the diurnal variation is around 13-14 local time. We found a Forbush decrease on 7 March 2012 by both instruments. It is also identified by Nagoya multi-direction muon telescope and Oulu neutron monitor. The observation of cosmic ray at Jangbogo station as well as in Korean peninsula can support the important information on space weather in local area. It can also enhance the status of Korea in the international community of cosmic ray experiments.

Observation and Research Activities of Neutron Monitors in Korea

Suyeon Oh 한국천문학회 2023 天文學會報 Vol.48 No.2

Record neutron monitor counting rates from galactic cosmic rays : RECORD NM COUNTING RATES FROM GCRS

Oh, Suyeon,Bieber, John W.,Evenson, Paul,Clem, John,Yi, Yu,Kim, Yongkyun American Geophysical Union 2013 Journal of geophysical research. Space physics Vol.118 No.9

Signals of solar cosmic ray flux variations inferred from the noise in raw CCD solar images taken by SOHO/EIT : SCR from solar images taken by SOHO/EIT

Oh, Suyeon,Park, Hyungmin,Yi, Yu,Chae, Jongchul American Geophysical Union 2014 JOURNAL OF GEOPHYSICAL RESEARCH. SPACE PHYSICS Vol.119 No.1

The noise embedded in the raw data in one scientific discipline has sometimes been proven to be a valuable signal for another discipline, and there are examples throughout science history. The solar images taken by the solid state detectors (CCDs) of the Sun monitoring satellites are usually cleaned by removing the traces of cosmic rays on the raw CCD data files. Thus, while applying the method of removing the cosmic ray traces, we may be able to estimate the cosmic ray flux by counting the number of traces. The net cosmic ray flux is the sum of galactic cosmic rays and solar-originating particles. The latter are seen as highly transient flux changes related to solar eruptions. We can identify this kind of cosmic ray event from the association with phenomena revealed in processed solar images, and we show this using the data of SOHO/extreme ultraviolet imaging telescope (EIT). On the other hand, the estimated cosmic ray flux in the steady state is anticorrelated with solar cycle sunspot number. The profiles of estimated solar cosmic ray flux showing significant increase are found to be strongly correlated with the ground neutron monitor ground level enhancements. Additionally, the profile of estimated cosmic ray flux is consistent with that of the GOES P6 channel. It indicates that the particles with energy higher than 80 MeV may mainly produce the tracks on CCD of EIT. In conclusion, the raw solar images are valuable data for estimating both long-term cosmic ray variations and transient solar particles events.Key Points<list list-type='bulleted' id='jgra50782-list-0001'> <list-item id='jgra50782-li-0001'>Traces of cosmic rays on the raw CCD solar images taken by SOHO/EIT <list-item id='jgra50782-li-0002'>To convert real signals of solar cosmic ray flux variations by counting traces <list-item id='jgra50782-li-0003'>The noises in raw solar images are as a kind of solar cosmic ray flux meter

Frequency of Solar Spotless Days and Flare Index as Indices of Solar Cycle Activity

Suyeon Oh 한국우주과학회 2014 Journal of Astronomy and Space Sciences Vol.31 No.2

There was a research on the prolongation of solar cycle 23 by the solar cyclic variation of solar, interplanetary geomagnetic parameters by Oh & Kim (2013). They also suggested that the sunspot number cannot typically explain the variation of total solar irradiance any more. Instead of the sunspot number, a new index is introduced to explain the degree of solar activity. We have analyzed the frequency of sunspot appearance, the length of solar cycle, and the rise time to a solar maximum as the characteristics of solar cycle. Then, we have examined the predictability of solar activity by the characteristics of preceding solar cycle. We have also investigated the hemispheric variation of flare index for the periods that the leading sunspot has the same magnetic polarity. As a result, it was found that there was a good correlation between the length of preceding solar cycle and spotless days. When the length of preceding solar cycle gets longer, the spotless days increase. It is also shown that the shorter rise time to a solar maximum is highly correlated with the increase of sunspots at a solar maximum. Therefore, the appearance frequency of spotless days and the length of solar cycle are more significant than the general sunspot number as an index of declining solar activity. Additionally, the activity of flares leads in the northern hemisphere and is stronger in the hemisphere with leading sunspots in positive polarity than in the hemisphere with leading sunspots in negative polarity. This result suggests that it is necessary to analyze the magnetic polarity’s effect on the flares and to interpret the period from the solar maximum to solar maximum as the definition of solar cycle.

Variation of Solar, Interplanetary and Geomagnetic Parameters during Solar Cycles 21-24

Oh, Suyeon,Kim, Bogyeong 한국우주과학회 2013 Journal of Astronomy and Space Sciences Vol.30 No.2

The length of solar cycle 23 has been prolonged up to about 13 years. Many studies have speculated that the solar cycle 23/24 minimum will indicate the onset of a grand minimum of solar activity, such as the Maunder Minimum. We check the trends of solar (sunspot number, solar magnetic fields, total solar irradiance, solar radio flux, and frequency of solar X-ray flare), interplanetary (interplanetary magnetic field, solar wind and galactic cosmic ray intensity), and geomagnetic (Ap index) parameters (SIG parameters) during solar cycles 21-24. Most SIG parameters during the period of the solar cycle 23/24 minimum have remarkably low values. Since the 1970s, the space environment has been monitored by ground observatories and satellites. Such prevalently low values of SIG parameters have never been seen. We suggest that these unprecedented conditions of SIG parameters originate from the weakened solar magnetic fields. Meanwhile, the deep 23/24 solar cycle minimum might be the portent of a grand minimum in which the global mean temperature of the lower atmosphere is as low as in the period of Dalton or Maunder minimum.