Disturbance Storm Time Index Dst Research Articles

Modulation of Solar Wind Impact on the Earth’s Magnetosphere during the Solar Cycle

The understanding of extreme geomagnetic storms is one of the key issues in space weather. Such phenomena have been receiving increasing attention, especially with the aim of forecasting strong geomagnetic storms generated by high-energy solar events since they can severely perturb the near-Earth space environment. Here, the disturbance storm time index Dst, a crucial geomagnetic activity proxy for Sun–Earth interactions, is analyzed as a function of the energy carried by different solar wind streams. To determine the solar cycle activity influence on Dst, a 12-year dataset was split into sub-periods of maximum and minimum solar activity. Solar wind energy and geomagnetic activity were closely correlated for both periods of activity. Slow wind streams had negligible effects on Earth regardless of their energy, while high-speed streams may induce severe geomagnetic storming depending on the energy (kinetic or magnetic) carried by the flow. The difference between the two periods may be related to the higher rate of geo-effective events during the maximum activity, where coronal mass ejections represent the most energetic and geo-effective driver. During the minimum period, despite a lower rate of high energetic events, a moderate disturbance in the Dst index can be induced.

The effect of different phases of severe geomagnetic storms on the low latitude ionospheric critical frequencies

In this paper, the effect of different phases of severe geomagnetic storms on low latitude ionospheric critical frequencies (foF2) is investigated. For this purpose, hourly ionospheric critical frequency (foF2) data measured at the low latitude ionosonde station Manila during 1981 and 1991 is examined. The investigation is carried out using superposed epoch analysis method considering the disturbance storm time index Dst ≤ −100 nT hours as event times. To examine depending on local time the effect of the phases of geomagnetic storms on foF2, this analysis was conducted for separately for the day hours, night hours, and all hours during both the main and the recovery phases of the severe geomagnetic storms and the results were compared with each other. It is observed that for both 1981 and 1991, the highest change (increase or decrease) in foF2 values occurs at the event times for all hours of day during both the main and the recovery phase of severe geomagnetic storms. Also, during the main phases of severe geomagnetic storms, the foF2 values increase at day hours, while the foF2 values decrease at night hours. However, during the recovery phases of severe geomagnetic storms, the foF2 values decrease at day hours, while the foF2 values increase at night hours. For both the day and night hours, the changes in the foF2 values during the recovery phase of severe geomagnetic storms are greater than the changes in the foF2 values during the main phase of severe geomagnetic storms.

Highly Geo-effective Solar Transients and Their Associated Geomagnetic Activities

The solar Cycle 23 has shown some peculiar features, i.e. slow and prolonged decline phase. When it is combined with the ascending phase of Cycle 24, it provides a long phase during which the overall magnetic activity was very low. During this interval the average sunspot number appeared on the solar disk was very low and signified the weak polar magnetic fields, and solar wind streams mainly originating from coronal holes. The study investigated the relationship between magnetic structure of coronal holes and/or coronal mass ejection (CME) source region and their influence on Earth's geomagnetic field, i.e. storms and sub storms. The study is performed mainly considering very intense geomagnetic storms that occurred during Solar Cycle 23. The disturbance storm time index Dst is taken as an indicator of geomagnetic activity by setting a value of Dstmin ≤ −200 nT as threshold. By examining halo CMEs that erupted between 2000 and 2008, we selected 07 events associated with M-class and X-class solar flares. Furthermore, as the geomagnetic field (BGeomag) puts a lower cutoff rigidity (Rc) to the entry of cosmic particles in to the earth's atmosphere. It depends upon the nature of geomagnetic activity and also upon the reconnection of field with the plasma regions formed due to the solar ejections. Sometimes when this entry of charged particles exhibits very sudden sharp and short lived increases in cosmic ray intensities, registered by the neutron monitor, it is termed as Ground-level enhancement (GLEs). These enhancements are known to take place during the result of powerful solar eruption. In present investigation GLE event associated with solar flare and coronal mass ejection (CME) occurred during the study period are also studied. The spacecraft data acquired by STEREO mission and those provided by SOHO, ACE and geomagnetic data from the geomagnetic stations like WDC-Kyoto are utilized in the study. It is found that the GLE's are well associated to X-class solar flares. The average speed of GLE associated CME was much faster than the average speed of non GLE associated CMEs.

Low-Amplitude Anisotropic Wave Train Events during Passage of Interplanetary Magnetic Clouds / Mazas Amplitūdas Anizotropiju Viļņu Parādību Secība Starpplanētu Magnētisko Mākoņu Rašanās Laikā

The work presents a continuation in the series related to the long-term space observations made by ground-based neutron monitoring stations. The cosmic ray intensity variation is considered as affected by interplanetary magnetic clouds during low-amplitude anisotropic wave train (LAAWT) events. It was observed that the solar wind velocity is higher than normal (> 300 km/s) while the interplanetary magnetic field (IMF) strength is lower than normal on the arrival of magnetic cloud during LAAWT events. The proton density is found to remain significantly low at high solar-wind velocity, which was expected. The north/south component of interplanetary magnetic field turns southward one day before the arrival of cloud and remains in this direction after that. The cosmic ray intensity is found to increase with the solar wind velocity. It is noteworthy that the cosmic ray intensity significantly increases before and 90 h after the arrival of such a cloud, and decreases gradually after its passage. The north/south component of IMF (Bz) is found to significantly correlate with latitude angle (Ө) and disturbance storm time index Dst, whereas the geomagnetic activity index (Ap) significantly anti-correlates with these parameters, decreasing with (Ө) and Dst increasing on the arrival of interplanetary magnetic cloud during LAAWT events.

Interplanetary turbulences causing unusual anisotropic wave trains in cosmic rays

The various observed harmonics of the cosmic ray variation may be understood on a unified basis if the free space cosmic ray anisotropy is non-sinusoidal in form. The major objective of this paper is to study the first three harmonics of high amplitude wave trains of cosmic ray intensity over the period 1991–1994 for Deep River Neutron Monitoring Station. The main characteristic of these events is that the high amplitude wave trains shows a maximum intensity of diurnal component in a direction earlier than 1800 Hr/co-rotational direction. It is noticed that these events are not caused either by the high-speed solar wind streams or by the sources on the Sun responsible for producing these streams such as polar coronal holes. The direction of semi-diurnal anisotropy shows negative correlation with Bz. The occurrence of high amplitude events is dominant for the positive polarity of Bz component of IMF. The diurnal amplitude of these events shows a negative and the time of maximum shows a weak correlation with disturbance storm time index Dst. The direction of tri-diurnal anisotropy of these events is found to significantly correlate with geomagnetic activity index Ap.