• Nandita Srivastava

      Articles written in Journal of Astrophysics and Astronomy

    • Chromospheric evolution and the flare activity of super-active region NOAA 6555

      C. Debi Prasad Ashok Ambastha Nandita Srivastava Sushanta C. Tripathy Mona J. Hagyard

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      Super-active region NOAA 6555 was highly flare productive during the period March 21st–27th, 1991 of its disk passage. We have st udied its chromospheric activity using high spatial resolution Hα filtergrams taken at Udaipur along with MSFC vector magnetograms. A possible relationship of flare productivity and the variation in shear has been explored. Flares were generally seen in those subareas of the active region which possessed closed magnetic field configuration, whereas only minor flares and/or surges occurred in subareas showing open magnetic field configuration. Physical mechanisms responsible for the observed surges are also discussed.

    • The challenge of predicting the occurrence of intense storms

      Nandita Srivastava

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      Geomagnetic super-storms of October and November 2003 are compared in order to identify solar and interplanetary variables that influence the magnitude of geomagnetic storms. Although these superstorms (DST < -300 nT) are associated with high speed CMEs, their DST indices show large variation. The most intense storm of November 20, 2003 (DSt∼ - 472 nT) had its source in a comparatively small active region and was associated with a relatively weaker, M-class flare, while the others had their origins in large active regions and were associated with strong X-class flares. An attempt has been made to implement a logistic regression model for the prediction of the occurrence of intense/superintense geomagnetic storms. The model parameters (regression coefficients) were estimated from a training data-set extracted from a data-set of 64 geo-effective CMEs observed during 1996–2002. The results indicate that logistic regression models can be effectively used for predicting the occurrence of major geomagnetic storms from a set of solar and interplanetary factors. The model validation shows that 100% of the intense storms (-200 nT < DSt < -100 nT) and only 50% of the super-intense (DST < -200 nT) storms could be correctly predicted.

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