Ankush Bhaskar

Quantitative understanding of Forbush decrease drivers based on shock-only and CME-only models using global signature of February 14, 1978 event

We have studied the Forbush decrease (FD) event that occurred on February 14, 1978 using 43 neutron monitor observatories to understand the global signature of FD. We have studied rigidity dependence of shock amplitude and total FD amplitude. We have found almost the same power law index for both shock phase amplitude and total FD amplitude. Local time variation of shock phase amplitude and maximum depression time of FD have been investigated which indicate possible effect of shock/CME orientation. We have analyzed rigidity dependence of time constants of two phase recovery. Time constants of slow component of recovery phase show rigidity dependence and imply possible effect of diffusion. Solar wind speed was observed to be well correlated with slow component of FD recovery phase. This indicates solar wind speed as possible driver of recovery phase. To investigate the contribution of interplanetary drivers, shock and CME in FD, we have used shock-only and CME-only models. We have applied these models separately to shock phase and main phase amplitudes respectively. This confirms presently accepted physical scenario that the first step of FD is due to propagating shock barrier and second step is due to flux rope of CME/magnetic cloud.

Ionospheric current contribution to the main impulse of a negative sudden impulse

The geomagnetic field response to a moderate-amplitude negative sudden impulse (SI−) that occurred on 14 May 2009 at 10:30 UT was examined at 97 geomagnetic observatories situated all over the globe. The response signature contains a contribution from magnetospheric as well as ionospheric currents. The main impulse (MI) is defined as the maximum depression in the observed geomagnetic field. It is observed that for low-to-high latitudes, the amplitude of the MI is larger in the afternoon to post-dusk sector than in the dawn-noon sector, indicating asymmetry in the MI amplitude. We estimated the contribution at various observatories due to the Chapman-Ferraro magnetopause currents using the Tsyganenko model (T01) and subtracted this from the observed MI amplitude to obtain the contribution due to ionospheric currents. It is found that the ionospheric currents contribute significantly to the MI amplitude of moderate SI− even at low-to-mid latitudes and that the contribution is in the same direction as that from the magnetopause currents near dusk and in the opposite direction near dawn. The equivalent current vectors reveal a clockwise (anticlockwise) ionospheric current loop in the afternoon (morning) sector during the MI of the negative pressure impulse. This evidences an ionospheric twin-cell-vortex current system (DP2) due to field-aligned currents (FACs) associated with the dusk-to-dawn convection electric field during the MI of an SI−. We also estimated the magnetic field variation due to prompt penetration electric fields, which is found to be very small at low latitudes in the present case. The studied SI− is not associated with shock, and hence no preliminary reverse impulse was evident. In addition, the summer hemisphere reveals larger MI amplitudes than the winter hemisphere, indicating once again the role of ionospheric currents.

Forbush Decrease: A New Perspective with Classification

The sudden short duration decrease in cosmic ray flux is known as Forbush decrease which is mainly caused by interplanetary disturbances. A generally accepted view is that the first step of Forbush decrease is due to shock sheath and second step is due to magnetic cloud (MC) of interplanetary coronal mass ejection (ICME). This simplistic picture does not consider several physical aspects, such as, whether the complete shock-sheath or MC (or only part of these) are contributing to the decrease, what effect does the internal structure within the shock-sheath region / MC have on the decrease, etc. We present a summary of the analysis of a total of 18 large (≥ 8%) Forbush decrease events and the associated ICMEs, a majority of which show multiple steps in the Forbush decrease profile. We propose a re-classification of Forbush decrease events depending upon the number of steps observed in their respective profile, and the physical origin of these steps. Our analysis clearly indicates that not only broad regions (shock-sheath and MC), but also localized structures within the shock-sheath and MC, have a very significant role in influencing the Forbush decrease profile. The detailed analysis in the present work is expected to contribute toward understanding the relationship between FD and ICME parameters in better way. key words: Shock-sheath, magnetic cloud (MC), ICME, cosmic ray, Forbush decrease, local magnetic structures. Corresponding author.

Quantitative assessment of drivers of recent global temperature variability: an information theoretic approach

Identification and quantification of possible drivers of recent global temperature variability remains a challenging task. This important issue is addressed adopting a non-parametric information theory technique, the Transfer Entropy and its normalized variant. It distinctly quantifies actual information exchanged along with the directional flow of information between any two variables with no bearing on their common history or inputs, unlike correlation, mutual information etc. Measurements of greenhouse gases:

The Presence of Turbulent and Ordered Local Structure within the ICME Shock-sheath and Its Contribution to Forbush Decrease

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A study of secondary cosmic ray flux variation during the annular eclipse of 15 January 2010 at Rameswaram, India

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Confirmation of secondary cosmic ray flux enhancement during the total lunar eclipse of 10 December 2011

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