Research Vision and Key Contributions
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Research Interests:
Kinematics and Thermodynamics of Solar CMEs: Kinematic and thermodynamic evolution of coronal mass ejections (CMEs) in the inner heliosphere, 3D reconstruction using COR and HIs, Stealth CMEs, and their origin, propagation, and detection, Problematic ICMEs and their unique signatures and impacts
Characterizing the Substructures of CMEs: CME substructures using multi-spacecraft remote, in situ, and radio observations, local inhomogeneity in ICMEs, inclination and orientation of CME flux rope, the evolution of CME substructures plasma parameters in the inner heliosphere
CME-CME Interactions: Conditions leading to successive CME eruptions, Interaction of CMEs with other CMEs (CME-CME interactions) and their in-situ observations at 1 AU, Momentum exchange between interacting CMEs and nature of their collisions, Shock-shock or shock-CME interactions, Role of CME Properties in Interaction Outcomes, CME-CME interactions causing compound geomagnetic storms, Modeling CME-CME interactions using MHD codes
Interplanetary MHD shocks: Shock wave formation due to CMEs and CIRs, Nonlinear evolution of shocks, In-situ observations of different types of interplanetary shocks, shock structures and their Mach numbers, Role of solar wind turbulence in shock dissipation
Geomagnetic and Space Weather Impacts: Geomagnetic storms and their triggering mechanisms, Forbush decreases and their association with CMEs and solar wind disturbances, Impact of interplanetary shock on the planetary magnetosphere, pace weather forecasting and the impact of solar transients on Earth’s environment, Coupling of solar activity with the magnetosphere, ionosphere, and thermosphere
Long-term Variations in the Heliospheric Dynamics and Solar Wind: Long-term variations in the heliospheric state due to solar wind and CMEs, Contribution of CIRs to long-term solar wind variability, Abundance and charge states of heavy ions in the solar wind, SIRs, and ICMEs, Mass loss rates from the Sun and other solar-type stars, Heating and acceleration of solar wind, Properties of slow and fast solar wind at different distances from the Sun
Particle Acceleration and Energetics: Shock-driven acceleration of solar energetic particles (SEPs), Role of quasi-parallel and quasi-perpendicular shocks in particle acceleration, SEP transport, Magnetic connectivity, Particle species, and their charge states, and energy ranges
Solar-Stellar Connection: Comparative analysis of CMEs in the Sun and solar-type stars, Stellar Magnetic Fields and Cycles, Space Weather in Exoplanetary Systems, Interaction of stellar winds with planetary magnetospheres in exoplanetary systems, Comparative studies of the heliosphere and astrospheres around other stars
Indian Space Missions: Science from different payloads onboard Aditya-L1 missions launched by ISRO, Joint observations campaign for Aditya-L1 with other missions, Involvement in future missions dedicated to solar and heliospheric physics
Research Highlights: Solar Astrophysics and Heliophysics
An approach to identify the compressed region of the magnetic cloud (MC) by utilizing MC axis
The figure depicts the three scenarios to identify the compressed region of the MC which is based on the arrival of axis and size center of the MC.
The left and right y-axis of the figure show the in situ measured speed and latitude (angle of the magnetic field vector from the ecliptic plane) at STEREO-A and Wind in the top and bottom panels, respectively. The axis center is behind the size center of both spacecraft, which means that the MC is compressed at the rear portion. However, the compression is more pronounced at STEREO-A.
Disparities in MC observations at mesoscales
The figure shows the in situ measured total magnetic field and its components at STEREO-A and Wind in blue and red, respectively.
The figure depicts the differences in the total magnetic field and its components measured at both spacecraft, shown in a black solid line. The black dashed line represents the zero reference to visualize the difference in the magnetic field. The bottom panel shows the cosine similarity (the angle between two vectors) between the magnetic field vector measured at both spacecraft. The rear portion of the MC shows disparity.
Non-conventional approach for deriving the expansion speeds of CMEs at different instances
We demonstrate a non-conventional approach to examine the evolution of radial size and instantaneous expansion speed at different instances during the passage of the MC over the in situ spacecraft.
This illustrates the evolution of an expanding CME during its passage over the in situ spacecraft. The circles represent the geometry of a CME in the plane of an in situ spacecraft, and vertical lines denote the LE, size center, and TE of the MC, respectively. The top-to-bottom panels represent the arrival of LE (L), center (C), and TE (T) at 1 AU in different instances. The location of in situ spacecraft at 1 AU is marked on the horizontal black line with two additional distances, one greater than 1 AU and one lesser than 1 AU. The left-right arrow represents the distance traveled by different features and the evolution of the CME radial dimension during any two instances.
The figure shows the in situ measured speed profile for 2010 April 3 CME. Different vertical lines mark the arrival of different features. A greater disparity between the size and time center of virtual MC suggests its more substantial expansion during MC’s total duration.
The figure shows the in situ measured speed on the y-axis (left), while the y-axis (right) shows the acceleration. An attempt is made to derive the acceleration of different features at 1 AU using their in situ measured speed over a certain thickness that can be used to derive their speeds at different times when in situ measurements are unavailable.
Our non-conventional approach utilizes the propagation speed of any two CME substructures at the same instance to determine the instantaneous expansion speed. The propagation speed of a particular substructure at instances when situ observations are lacking is calculated assuming its constant acceleration, combined with the first equation of motion. The constant acceleration of the substructure is obtained from the gradient in the propagation speed across the considered thickness of the substructure identified in the single-point in situ observations. Additionally, this approach uses the second equation of motion to compute the radial size and the distance travelled by the CME substructures at various instances. We examine the aspect ratio of the CME, which influences its expansion behavior and shows the discrepancy between its value in the corona and interplanetary medium. We show that the introduced non-conventional approach and evolving aspect ratio of CMEs if taken into account, can provide better accuracy in estimating radial sizes and instantaneous expansion speeds of CMEs in different instances.
Geomagnetic storms and their recovery phases over solar cycles 23 and 24
Top panel: An ideal single-peaked great geomagnetic storm occurred in November 2003 with its magnitude of −422 nT, having smooth main and recovery phases shown on the left. A multiple-peaked storm in September 2017, with the primary Dst peak reaching −122 nT, is shown on the right. The shaded areas with cyan and yellow show the duration of the main and recovery phases for both the single and multiple-peaked storms. Middle panel: This pie chart represents the distribution of single-peaked and multiple-peaked stronger-than-intense storms during solar cycle 23 in the left-hand and right-hand panels. Bottom panel: It is the same as the middle panel, but for solar cycle 24.
The figure shows the correlation between the recovery phase duration of single peak storms of solar cycle 23 and storm characteristics. From the top to the panel, the correlation of the recovery phase with the peak Dst index, main phase build-up rate, duration of the fast decay phase, and slow decay phase is shown, respectively.
The figure shows the correlation between the recovery phase duration of multiple peak storms of solar cycle 23 and storm characteristics. From the top to the panel, the correlation of the recovery phase with the peak Dst index, main phase build-up rate, duration of the fast decay phase, and slow decay phase is shown, respectively.
The figure shows the correlation between the recovery phase duration of single peak storms of solar cycle 24 and storm characteristics. From the top to the panel, the correlation of the recovery phase with the peak Dst index, main phase build-up rate, duration of the fast decay phase, and slow decay phase is shown, respectively.
The figure shows the correlation between the recovery phase duration of multiple peak storms of solar cycle 24 and storm characteristics. From the top to the panel, the correlation of the recovery phase with the peak Dst index, main phase build-up rate, duration of the fast decay phase, and slow decay phase is shown, respectively.
The study concludes that multiple-peak storms in both cycles have a recovery phase duration strongly influenced by slow and fast decay phases, with no correlation with the main phase build-up rate and Dst peak. However, the recovery phase in single-peak storms for both cycles depends to some extent on the main phase build-up rate and Dst peak, in addition to slow and fast decay phases.
Investigating the thermodynamics of CMEs using FRIS model
Schematic of a flux-rope CME in the cylindrical coordinate system as assumed in the Flux Rope Internal State (FRIS) model. It shows the propagation speed of the flux rope axis, expansion speed, and poloidal speed, with blue, green, and violet arrows, respectively.
GCS-model-fitted wireframe in green and pink overlay on the contemporaneous coronagraphic images of the fast CME of 2011 September 24 shown in the left (left: STEREO-A/COR2, center: LASCO-C3, right: STEREO-B/COR2) and for the slow CME of 2018 August 20 shown in the right (left: STEREO-A/COR2, right: LASCO-C2).
Kinematics of fast CME of 2011 September 24 (left) and CME of 2018 August 20 (right) using the GCS model on the coronagraphic observations. Top panel: the measurements of the heliocentric distance (h) of the leading edge of the flux rope (FR) and its radius (R). Middle panel: propagation speed (v) and expansion speed (ve ) derived by taking the three-point derivatives of h and R, respectively. Bottom panel: propagation acceleration (a) and expansion acceleration (ae ) derived by taking the derivative of v and ve , respectively. The red vertical lines at each data point show the error bars derived by considering an error of 10% in the measurements of the flux rope's leading-edge height (h).
For the CME of 2011 September 24: Variation of the polytropic index, average heating rate, average temperature, and average proton number density of the CME with the heliocentric distance of the CME's leading edge is shown in the top left, top right, bottom left, and bottom right, respectively. The red vertical lines at each data point show the error bars derived by considering an error of 10% in measurements of the flux rope's leading-edge height.
For the CME of 2018 August 20: variation of the Polytropic index, average heating rate, average temperature, and average proton number density of the CME with the heliocentric distance of the CME's leading edge is shown in the top left, top right, bottom left, and bottom right, respectively. The red vertical lines at each data point show the error bars derived by considering an error of 10% in measurements of the flux rope's leading-edge height.
FRIS-model-derived average internal forces, such as Lorentz force, Thermal pressure force, and Centrifugal force, that are responsible for the radial expansion of the flux rope of 2011 September 24 CME (left) and 2018 August 20 CME (right). The red vertical lines at each data point show the error bars derived by considering an error of 10% in measurements of the leading-edge height of the flux rope. The solid and dashed lines show the power-law fitted values for the model-derived internal forces.
Kinematic evolution of the selected CMEs: (a) the variation in the leading-edge speed and (b) expansion speed, (c) leading-edge acceleration, and (d) expansion acceleration with leading-edge height of CMEs.
Thermodynamic evolution of CMEs is shown: (a) Variation of the polytropic index, (b) heating rate per unit mass, (c) temperature, and (d) thermal pressure of the CME with the heliocentric distance of the CME’s leading edge. The dashed horizontal line in panel (a) shows the adiabatic index value, representing a state with no heat exchange. The vertical lines show the errors associated with the FRIS mode-derived parameters estimated using an uncertainty of 10% in the input leading edge heights and the subsequent propagated errors in the kinematics.
Our investigation reveals that the selected fast CMEs exhibit a heat-release phase at the beginning, followed by a heat-absorption phase with a near-isothermal state during their later propagation phase. The thermal state transition, from heat release to heat absorption, occurs at around 3-7 solar radii for different CMEs. We found that the CMEs with higher expansion speeds experience a less pronounced sharp temperature decrease before gaining a near-isothermal state.
Mass loss from the Sun via CMEs and solar wind over solar cycle 23 and 24
The variation of mass loss via CMEs with solar X-ray background luminosity is shown over almost two decades.
The variation of CMEs and solar wind proton flux observed at 1 AU is shown for almost two decades.
Our study found that the CME occurrence rate and associated mass loss rate can be better predicted by X-ray background luminosity than the sunspot number. The solar wind mass loss rate, which is an order of magnitude more than the CME mass loss rate, shows no obvious dependency on cyclic variation in sunspot number and solar X-ray background luminosity. The X-ray background luminosity, occurrence rate of CMEs and ICMEs, solar wind mass flux, and associated mass loss rates from the Sun do not decrease as strongly as the sunspot number from the maximum of solar cycle 23 to the next maximum. Such a study, establishing a relation between X-ray luminosity and mass loss rate, has implications for estimating the mass loss rate from solar-type stars due to stellar CMEs and wind.
Need for the continuous tracking of CMEs using coronagraphic and heliospheric imaging (HI) observations for estimating their accurate arrival time at Earth
The animation shows the evolution of CMEs using STEREO coronagraph (COR) and wide-angle heliospheric imagers (HIs) observations.
It shows the J-maps (time-elongation variations) constructed from COR and HI observations.
Our analysis shows the importance of using J-maps constructed from heliospheric imaging observations for improved forecasting of the arrival time and direction of propagation of CMEs in the IP medium. Such a study helps us understand the CME acceleration, deceleration, and non-radial deflection beyond the coronal heights. It allows us to explore the role of different forces acting on CMEs to shape their kinematics in the background solar wind.
CME-CME interaction and its impacts on plasma parameters of CMEs and their kinematics
The 3D reconstruction of CMEs using the GCS model on the contemporaneous images from SECCHI/COR2-B, SOHO/LASCO, and SECCHI/COR2-A is shown for the successive CMEs of 25 September 2012 (CME1: top panel) and 28 September 2012 (CME2: bottom panel).
The images show the tracking of CMEs in the HI-1 (top) and HI-2 (bottom) field of view. The contours of the elongation angle (green) and the position angle (blue) are overlaid. The horizontal red line is along the ecliptic at the position angle of the Earth.
The figure shows the derived 3D kinematics of the interacting CMEs of 25 and 28 September 2012 (CME1 and CME2) using the Self-similar expansion (SSE) reconstruction method. The vertical lines show the error bars. The horizontal lines and filled circles in the speed panels, respectively, represent the in situ measured speed and arrival times of the CMEs.
The figure shows the interacting CMEs parameters observed in in situ observations at 1 AU. S1 and S2 mark the arrival of shocks associated with the CME1 and CME2, respectively. We notice ongoing interaction between the preceding and the following CME and their two distinct structures. We also note the heating of the following CME. The interaction structure formed close to 1 AU is found to be responsible for enhanced geomagnetic activity.
Our study clearly notes that post-collision kinematics improves the CME arrival time estimation on the Earth. Using the estimated kinematics and true masses of the CMEs, the coefficient of restitution for the collision can be calculated. There could be a huge momentum exchange between CMEs participating in the interaction. Based on this study, we conclude that CMEs cannot be treated as completely isolated magnetized plasma blobs, especially when they are launched in quick succession. The preceding CME plays an important role in preconditioning the ambient medium in which a following CME has to travel. The study also supports the idea that CME interaction or collision can lead to the heating and compression of both preceding and following CMEs. The interaction region (IR), formed by the interacting CMEs, has intensified plasma and magnetic field parameters, which are responsible for major geomagnetic activity. Based on several case studies, we find that collisions between CMEs can happen at varying distances from the Sun, depending on the relative dynamics of successively launched CMEs and differences in their launch times. The study found that the crucial pre-collision parameters of the CMEs responsible for increasing the probability of a super-elastic collision are, in descending order of priority, their lower approaching speed, the expansion speed of the following CME higher than the preceding one, and a longer duration of the collision phase.
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