Abstract Details
| Name: Susanta Kumar Bisoi Affiliation: Physical Research Laboratory, Ahmedabad Conference ID: ASI2015_550 Title : Solar and Solar Wind Studies Using Ground and Space Based Observations Authors and Co-Authors : P.Janardhan Physical Research Laboratory, Ahmedabad Abstract Type : Oral Abstract Category : Thesis Abstract : Using synoptic magnetograms from the ground-based NSO/KP and space-based SoHO/MDI database, we examined solar photospheric magnetic fields for Cycles 21, 22, and 23. Specifically, we examined polar magnetic fields at high latitudes (78ο-90ο) in both the hemispheres and found a steady decline in the unsigned polar fields from the late declining phase of Cycle 22 to the maximum of Cycle 23. We found a good correlation, in Cycle 23, between the long-term changes in the unsigned polar fields and changes in meridional flow speeds during Cycle 23. In addition, our observations of continuously weaker polar fields, in Cycle 23, led us to believe that these weaker fields were the cause of the extremely prolonged minimum, at the end of Cycle 23 and would lead to a weaker Cycle 24. We undertook investigation of quasi-periodic variations in solar photospheric fields in the build-up to one of the deepest solar minima experienced in recent times. We found a hemispheric asymmetry in quasi-periodicity of the photospheric fields, confined to the latitude range 45ο to 60ο. This observed asymmetry, when coupled with the fact that both solar fields above 45ο and micro-turbulence levels in the inner-heliosphere have been decreasing steadily since the early to mid-1990s suggested that active changes occurred in the solar dynamo around this time. These changes, in turn, probably initiated the build-up to the deep solar minimum around mid-1990s. Any long term changes in solar photospheric fields during solar cycles must leave its signatures in the solar wind and can be effectively detected through interplanetary scintillation (IPS) measurements. We used IPS observations at 327 MHz, obtained between 1983 and 2009, to investigate the long-term temporal variations in the scintillation levels and found a steady and significant decline in the micro-turbulence levels in the entire inner heliosphere, which was started since ~1995. This large-scale heliospheric IPS signature, coupled with the similar steady decline of solar polar magnetic fields since ~1995, provide a consistent result that showed the buildup to the deepest solar minima, experienced in the past 100 years, actually began in a decade earlier, that is, in the early- to mid-1990s. We also studied a pair of sudden impulses (SI) in the Earth's magnetic fields, first identified at the Indian magnetic observatories, on 23 – 24 April 1998. We discussed the close correlations between the SI pair and the corresponding variations in solar wind density, while the solar wind velocity and the southward component of the interplanetary magnetic field (IMF-Bz) did not show any correspondence. Further, we also showed that it is possible for a rear-side solar flare to propagate a shock towards the earth. |