arXiv subjects

Nandita Srivastava

Publications and source records attributed to Nandita Srivastava.

At least 37 records · Page 2Linked to original sources

arXiv2019

Lorentz Force Evolution Reveals the Energy Buildup Processes during Recurrent Eruptive Solar Flares

The energy release and build-up processes in the solar corona have significant implications in particular for the case of large recurrent flares, which pose challenging questions about the conditions that lead to the episodic energy release processes. It is not yet clear whether these events occur due to the continuous supply of free magnetic energy to the solar corona or because not all of the available free magnetic energy is released during a single major flaring event. In order to address this question, we report on the evolution of photospheric magnetic field and the associated net Lorentz force changes in ARs 11261 and 11283, each of which gave rise to recurrent eruptive M- and X-class flares. Our study reveals that after the abrupt downward changes during each flare, the net Lorentz force increases by (2-5)x10^22 dyne in between the successive flares. This distinct rebuild-up of net Lorentz forces is the first observational evidence found in the evolution of any non-potential parameter of solar active regions (ARs), which suggests that new energy was supplied to the ARs in order to produce the recurrent large flares. The rebuild-up of magnetic free energy of the ARs is further confirmed by the observations of continuous shearing motion of moving magnetic features of opposite polarities near the polarity inversion line. The evolutionary pattern of the net Lorentz force changes reported in this study has significant implications, in particular, for the forecasting of recurrent large eruptive flares from the same AR and hence the chances of interaction between the associated CMEs.

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arXiv2019

Mass Loss via Solar Wind and Coronal Mass Ejections During Solar Cycle 23 and 24

Similar to the Sun, other stars shed mass and magnetic flux via ubiquitous quasi-steady wind and episodic stellar coronal mass ejections (CMEs). We investigate the mass loss rate via solar wind and CMEs as a function of solar magnetic variability represented in terms of sunspot number and solar X-ray background luminosity. We estimate the contribution of CMEs to the total solar wind mass flux in the ecliptic and beyond, and its variation over different phases of the solar activity cycles. The study exploits the number of sunspots observed, coronagraphic observations of CMEs near the Sun by SOHO/LASCO, in situ observations of the solar wind at 1 AU by WIND, and GOES X-ray flux during solar cycle 23 and 24. We note that 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. Our study confirms a true physical increase in CME activity relative to the sunspot number in cycle 24. We show 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. These results have implications to the study of solar-type stars.

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arXiv2019

Evolution of Coronal Cavity from Quiescent to Eruptive Phase in Association with Coronal Mass Ejection

We present the evolution of a coronal cavity encompassing its quiescent and eruptive phases in the lower corona. Using the multi-vantage point observations from the SDO/AIA, STEREO SECCHI/EUVI and PROBA2/SWAP EUV imagers, we capture the sequence of quasi-static equilibria of the quiescent cavity which exhibited a slow rise and expansion phase during its passage on the solar disc from 2010 May 30 to 2010 June 13. By comparing the decay-index profiles of the cavity system during the different stages of its quiescent and pre-eruptive phases we find that the decay-index value at the cavity centroid height can be used as a good indicator to predict the cavity eruption in the context of torus instability. Combining the observations of SWAP and LASCO C2/C3 we show the evolution of the EUV cavity into the white-light cavity as a three-part structure of the associated CME observed to erupt on 2010 June 13. By applying successive geometrical fits to the cavity morphology we find that the cavity exhibited non self-similar expansion in the lower corona, below 2.2 +/- 0.2 Rs, which points to the spatial scale for the radius of source surface where the coronal magnetic field lines are believed to become radial. Furthermore, the kinematic study of the erupting cavity captures both the "impulsive" and "residual" phases of acceleration along with a strong deflection of the cavity at 1.3 Rs. We also discuss the role of driving forces behind the dynamics of the morphological and kinematic evolution of the cavity.

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arXiv2018

Dependence of coronal mass ejection properties on their solar source active region characteristics and associated flare reconnection flux

The near-Sun kinematics of coronal mass ejections (CMEs) determine the severity and arrival time of associated geomagnetic storms. We investigate the relationship between the deprojected speed and kinetic energy of CMEs and magnetic measures of their solar sources, reconnection flux of associated eruptive events and intrinsic flux rope characteristics. Our data covers the period 2010-2014 in solar cycle 24. Using vector magnetograms of source active regions we estimate the size and nonpotentiality. We compute the total magnetic reconnection flux at the source regions of CMEs using the post-eruption arcade method. By forward modeling the CMEs we find their deprojected geometric parameters and constrain their kinematics and magnetic properties. Based on an analysis of this database we report that the correlation between CME speed and their source active region size and global nonpotentiality is weak, but not negligible. We find the near-Sun velocity and kinetic energy of CMEs to be well correlated with the associated magnetic reconnection flux. We establish a statistically significant empirical relationship between the CME speed and reconnection flux that may be utilized for prediction purposes. Furthermore, we find CME kinematics to be related with the axial magnetic field intensity and relative magnetic helicity of their intrinsic flux ropes. The amount of coronal magnetic helicity shed by CMEs is found to be well correlated with their near-Sun speeds. The kinetic energy of CMEs is well correlated with their intrinsic magnetic energy density. Our results constrain processes related to the origin and propagation of CMEs and may lead to better empirical forecasting of their arrival and geoeffectiveness.

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arXiv2018

Study of Interplanetary and Geomagnetic Response of Filament Associated CMEs

It has been established that Coronal Mass Ejections (CMEs) may have significant impact on terrestrial magnetic field and lead to space weather events. In the present study, we selected several CMEs which are associated with filament eruptions on the Sun. We attempt to identify the presence of filament material within ICME at 1AU. We discuss how different ICMEs associated with filaments lead to moderate or major geomagnetic activity on their arrival at the Earth. Our study also highlights the difficulties in identifying the filament material at 1AU within isolated and in interacting CMEs.

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arXiv2018

Geometric and magnetic properties of coronal flux ropes associated with CMEs leading to geomagnetic storms

We have studied three Interplanetary Coronal Mass Ejections (ICMEs) having clear signatures of magnetic cloud (MC) arrival at 1 AU and their associated solar sources during 2011 to 2013. Comparing the axial magnetic field strength (B0) of the near-Sun coronal flux-ropes with that of the MC at 1 AU, we have found that the average inferred value of B0 at 1 AU assuming the self-similar expansion of the flux-rope is two times smaller than the value of B0 obtained from the results of MC fitting. Furthermore, by comparing the initial orientation of the flux-rope near the Sun and its final orientation at 1 AU we have found that the three CMEs exhibited more than 80-degree rotation during its propagation through the interplanetary medium. Our study suggests that although the near-Sun magnetic properties of coronal flux-ropes can be used to infer the field strength of the associated MC at 1 AU, it is difficult to estimate the final orientation of the MC axis in order to predict the geo-effectiveness of the ICMEs.

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arXiv2018

Solar cycle variation of coronal mass ejections contribution to solar wind mass flux

Coronal Mass Ejections (CMEs) contributes to the perturbation of solar wind in the heliosphere. Thus, depending on the different phases of the solar cycle and the rate of CME occurrence, contribution of CMEs to solar wind parameters near the Earth changes. In the present study, we examine the long term occurrence rate of CMEs, their speeds, angular widths and masses. We attempt to find correlation between near sun parameters, determined using white light images from coronagraphs, with solar wind measurements near the Earth from in-situ instruments. Importantly, we attempt to find what fraction of the averaged solar wind mass near the Earth is provided by the CMEs during different phases of the solar cycles.

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arXiv2018

A Comparative Study of the Eruptive and Non-Eruptive Flares Produced by the Largest Active Region of Solar Cycle 24

We investigate the morphological and magnetic characteristics of solar active region (AR) NOAA 12192. AR 12192 was the largest region of Solar Cycle 24; it underwent noticeable growth and produced 6 X-class flares, 22 M-class flares, and 53 C-class flares in the course of its disc passage. However, the most peculiar fact of this AR is that it was associated with only one CME in spite of producing several X-class flares. In this work, we carry out a comparative study between the eruptive and non-eruptive flares produced by AR 12192. We find that the magnitude of abrupt and permanent changes in the horizontal magnetic field and Lorentz force are significantly smaller in the case of the confined flares compared to the eruptive one. We present the areal evolution of AR 12192 during its disc passage. We find the flare-related morphological changes to be weaker during the confined flares, whereas the eruptive flare exhibits a rapid and permanent disappearance of penumbral area away from the magnetic neutral line after the flare. Furthermore, from the extrapolated nonlinear force-free magnetic field, we examine the overlying coronal magnetic environment over the eruptive and non-eruptive zones of the AR. We find that the critical decay index for the onset of torus instability was achieved at a lower height over the eruptive flaring region, than for the non-eruptive core area. These results suggest that the decay rate of the gradient of overlying magnetic field strength may play a decisive role to determine the CME productivity of the AR. In addition, the magnitude of changes in the flare-related magnetic characteristics are found to be well correlated with the nature of solar eruptions.

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arXiv2017

Interplanetary and Geomagnetic Consequences of Interacting CMEs of 13-14 June 2012

We report on the kinematics of two interacting CMEs observed on 13 and 14 June 2012. Both CMEs originated from the same active region NOAA 11504. After their launches which were separated by several hours, they were observed to interact at a distance of 100 Rs from the Sun. The interaction led to a moderate geomagnetic storm at the Earth with Dst index of approximately, -86 nT. The kinematics of the two CMEs is estimated using data from the Sun Earth Connection Coronal and Heliospheric Investigation (SECCHI) onboard the Solar Terrestrial Relations Observatory (STEREO). Assuming a head-on collision scenario, we find that the collision is inelastic in nature. Further, the signatures of their interaction are examined using the in situ observations obtained by Wind and the Advance Composition Explorer (ACE) spacecraft. It is also found that this interaction event led to the strongest sudden storm commencement (SSC) (approximately 150 nT) of the present Solar Cycle 24. The SSC was of long duration, approximately 20 hours. The role of interacting CMEs in enhancing the geoeffectiveness is examined.

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arXiv2017

Assessing the collision nature of coronal mass ejections in the inner heliosphere

There have been few attempts in the past to understand the collision of individual cases of interacting Coronal Mass Ejections (CMEs). We selected 8 cases of interacting CMEs and estimated their propagation and expansion speeds, direction of impact and masses exploiting coronagraphic and heliospheric imaging observations. Using these estimates with ignoring the errors therein, we find that the nature of collision is perfectly inelastic for 2 cases (e.g., 2012 March and November), inelastic for 2 cases (e.g., 2012 June and 2011 August), elastic for 1 case (e.g., 2013 October) and super-elastic for 3 cases (e.g., 2011 February, 2010 May and 2012 September). Admitting large uncertainties in the estimated directions, angular widths and pre-collision speeds; the probability of perfectly inelastic collision for 2012 March and November cases diverge from 98%-60% and 100%-40%, respectively, reserving some probability for other nature of collision. Similarly, the probability of inelastic collision diverge from 95%-50% for 2012 June case, 85%-50% for 2011 August case, and 75%-15% for 2013 October case. We note that probability of super-elastic collision for 2011 February, 2010 May and 2012 September CMEs diverge from 90%-75%, 60%-45% and 90%-50%, respectively. Although the sample size is small, we find a good dependence of nature of collision on CMEs parameters. The crucial pre-collision parameters of the CMEs responsible for increasing the probability of super-elastic collision, in descending order of priority, are their lower approaching speed, higher expansion speed of the following CME over the preceding one, and longer duration of collision phase.

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arXiv2016

On Understanding the Nature of Collision of Coronal Mass Ejections Observed by \textit{STEREO}

Our study attempts to understand the collision characteristics of two coronal mass ejections (CMEs) launched successively from the Sun on 2013 October 25. The estimated kinematics, from three-dimensional (3D) reconstruction techniques applied to observations of CMEs by SECCHI/Coronagraphic (COR) and Heliospheric Imagers (HIs), reveal their collision around 37 $R_\sun$ from the Sun. In the analysis, we take into account the propagation and expansion speeds, impact direction, angular size as well as the masses of the CMEs. These parameters are derived from imaging observations, but may suffer from large uncertainties. Therefore, by adopting head-on as well as oblique collision scenarios, we have quantified the range of uncertainties involved in the calculation of the coefficient of restitution for expanding magnetized plasmoids. Our study shows that the comparatively large expansion speed of the following CME than that of the preceding CME, results in a higher probability of super-elastic collision. We also infer that a relative approaching speed of the CMEs lower than the sum of their expansion speeds increases the chance of super-elastic collision. The analysis under a reasonable errors in observed parameters of the CME, reveals the larger probability of occurrence of an inelastic collision for the selected CMEs. We suggest that the collision nature of two CMEs should be discussed in 3D, and the calculated value of the coefficient of restitution may suffer from a large uncertainty.

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arXiv2015

Kinematics of Interacting CMEs of September 25 and 28, 2012

We have studied two Coronal Mass Ejections (CMEs) that occurred on September 25 and 28, 2012 and interacted near the Earth. By fitting the Graduated Cylindrical Shell (GCS) model on the SECCHI/COR2 images and applying the Stereoscopic Self-Similar Expansion (SSSE) method on the SECCHI/HI images, the initial direction of both the CMEs is estimated to be west of the Sun-Earth line. Further, the three-dimensional (3D) heliospheric kinematics of these CMEs have been estimated using Self-Similar Expansion (SSE) reconstruction method. We show that use of SSE method with different values of angular extent of the CMEs, leads to significantly different kinematics estimates for the CMEs propagating away from the observer. Using the estimated kinematics and true masses of the CMEs, we have derived the coefficient of restitution for the collision which is found to be close to elastic. The in situ measurements at 1 AU show two distinct structures of interplanetary CMEs, heating of the following CME, as well as ongoing interaction between the preceding and the following CME. We highlight the signatures of interaction in remote and in situ observations of these CMEs and the role of interaction in producing a major geomagnetic storm.

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arXiv2015

Heliospheric tracking of enhanced density structures of the 6 October 2010 CME

A Coronal Mass Ejection (CME) is an inhomogeneous structure consisting of different features which evolve differently with the propagation of the CME. Simultaneous heliospheric tracking of different observed features of a CME can improve our understanding about relative forces acting on them. It also helps to estimate accurately their arrival times at the Earth and identify them in in- situ data. This also enables to find association between remotely observed features and in-situ observations near the Earth. In this paper, we attempt to continuously track two density enhanced features, one at the front and another at the rear edge of the 6 October 2010 CME. This is achieved by using time-elongation maps constructed from STEREO/SECCHI observations. We derive the kinematics of the tracked features using various reconstruction methods. The estimated kinematics are used as inputs in the Drag Based Model (DBM) to estimate the arrival time of the tracked features of the CME at L1. On comparing the estimated kinematics as well as the arrival times of the remotely observed features with in-situ observations by ACE and Wind, we find that the tracked bright feature in the J-map at the rear edge of 6 October 2010 CME corresponds most probably to the enhanced density structure after the magnetic cloud detected by ACE and Wind. In-situ plasma and compositional parameters provide evidence that the rear edge density structure may correspond to a filament associated with the CME while the density enhancement at the front corresponds to the leading edge of the CME. Based on this single event study, we discuss the relevance and significance of heliospheric imager (HI) observations in identification of the three-part structure of the CME.

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arXiv2014

Morphological and Kinematic Evolution of Three Interacting Coronal Mass Ejections of 2011 February 13-15

During 2011 February 13 to 15, three Earth-directed CMEs launched in successively were recorded as limb CMEs by coronagraphs (COR) of STEREO. These CMEs provided an opportunity to study their geometrical and kinematic evolution from multiple vantage points. In this paper, we examine the differences in geometrical evolution of slow and fast speed CMEs during their propagation in the heliosphere. We also study their interaction and collision using STEREO/SECCHI COR and Heliospheric Imager (HI) observations. We have found evidence of interaction and collision between the CMEs of February 15 and 14 in COR2 and HI1 FOV, respectively, while the CME of February 14 caught the CME of February 13 in HI2 FOV. By estimating the true mass of these CMEs and using their pre and post-collision dynamics, the momentum and energy exchange between them during collision phase are studied. We classify the nature of observed collision between CME of February 14 and 15 as inelastic, reaching close to elastic regime. Relating imaging observations with the in situ measurements, we find that the CMEs move adjacent to each other after their collision in the heliosphere and are recognized as distinct structures in in situ observations by WIND spacecraft at L1. Our results highlight the significance of HI observations in studying CME-CME collision for the purpose of improved space weather forecasting.

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arXiv2014

Evolution and Consequences of Interacting CMEs of 2012 November 9-10 using STEREO/SECCHI and In Situ Observations

Understanding of the kinematic evolution of Coronal Mass Ejections (CMEs) in the heliosphere is important to estimate their arrival time at the Earth. It is found that kinematics of CMEs can change when they interact or collide with each other as they propagate in the heliosphere. In this paper, we analyze the collision and post-interaction characteristics of two Earth-directed CMEs, launched successively on 2012 November 9 and 10, using white light imaging observations from STEREO/SECCHI and in situ observations taken from WIND spacecraft. We tracked two density enhanced features associated with leading and trailing edge of November 9 CME and one density enhanced feature associated with leading edge of November 10 CME by constructing J-maps. We found that the leading edge of November 10 CME interacted with the trailing edge of November 9 CME. We also estimated the kinematics of these features of the CMEs and found a significant change in their dynamics after interaction. In in situ observations, we identified distinct structures associated with interacted CMEs and also noticed their heating and compression as signatures of CME-CME interaction. Our analysis shows an improvement in arrival time prediction of CMEs using their post-collision dynamics than using pre-collision dynamics. Estimating the true masses and speeds of these colliding CMEs, we investigated the nature of observed collision which is found to be close to perfectly inelastic. The investigation also places in perspective the geomagnetic consequences of the two CMEs and their interaction in terms of occurrence of geomagnetic storm and triggering of magnetospheric substorms.

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arXiv2014

A comparison of reconstruction methods for the estimation of CME kinematics based on SECCHI/HI observations

A study of the kinematics and arrival times of CMEs at Earth, derived from time-elongation maps (J-maps) constructed from STEREO/Heliospheric Imager (HI) observations, provides an opportunity to understand the heliospheric evolution of CMEs in general. We implement various reconstruction techniques, based on the use of time-elongation profiles of propagating CMEs viewed from single or multiple vantage points, to estimate the dynamics of three geo-effective CMEs. We use the kinematic properties, derived from analysis of the elongation profiles, as inputs to the Drag Based Model for the distance beyond which the CMEs cannot be tracked unambiguously in the J-maps. The ambient solar wind into which these CMEs, which travel with different speeds, are launched, is different. Therefore, these CMEs will evolve differently throughout their journey from the Sun to 1 AU. We associate the CMEs, identified and tracked in the J-maps, with signatures observed in situ near 1 AU by the WIND spacecraft. By deriving the kinematic properties of each CME, using a variety of existing methods, we assess the relative performance of each method for the purpose of space weather forecasting. We discuss the limitations of each method, and identify the major constraints in predicting the arrival time of CMEs near 1 AU using heliospheric imager observations.

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arXiv2013

Estimating arrival time of Earth-directed CMEs at \textit{in-situ} spacecraft using COR & HI observations from STEREO

The prediction of the arrival time and transit speed of CMEs near the Earth is one of the key problems in understanding the solar terrestrial relationship. Although, STEREO observations now provide a multiple view of CMEs in the heliosphere, the true speeds derived from stereoscopic reconstruction of SECCHI coronagraph data are not quite sufficient in accurate forecasting of arrival time of a majority of CMEs at the Earth. This is due to many factors which change the CME kinematics, like interaction of two or more CMEs or the interaction of CMEs with the pervading solar wind. In order to understand the propagation of CMEs, we have used the 3D triangulation method on SECCHI coronagraph (COR2) images, and geometric triangulation on the J-maps constructed from Heliospheric Imagers HI1 and HI2 images for eight Earth-directed CMEs observed during 2008-2010. Based on the reconstruction, and implementing the Drag Based Model for the distance where the CMEs could not be tracked unambiguously in the interplanetary medium, the arrival time of these CMEs have been estimated. These arrival times have also been compared with the actual arrival time as observed by \textit{in-situ} instruments. The analysis reveals the importance of heliospheric imaging for improved forecasting of the arrival time and direction of propagation of CMEs in the interplanetary medium.

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arXiv2012

The Build-up to Eruptive Solar Events Viewed as the Development of Chiral Systems

When we examine the chirality or observed handedness of the chromospheric and coronal structures involved in the long-term build-up to eruptive events, we find that they evolve in very specific ways to form two and only two sets of large-scale chiral systems. Each system contains spatially separated components with both signs of chirality, the upper portion having negative (positive) chirality and the lower part possessing positive (negative) chirality. The components within a system are a filament channel (represented partially by sets of chromospheric fibrils), a filament (if present), a filament cavity, sometimes a sigmoid, and always an overlying arcade of coronal loops. When we view these components as parts of large-scale chiral systems, we more clearly see that it is not the individual components of chiral systems that erupt but rather it is the approximate upper parts of an entire evolving chiral system that erupts. We illustrate the typical pattern of build-up to eruptive solar events first without and then including the chirality in each stage of the build-up. We argue that a complete chiral system has one sign of handedness above the filament spine and the opposite handedness in the barbs and filament channel below the filament spine. If the spine has handedness, the observations favor its having the handedness of the filament cavity and coronal loops above. As the separate components of a chiral system form, we show that the system appears to maintain a balance of right-handed and left-handed features, thus preserving an initial near-zero net helicity. Each individual chiral system may produce many successive eruptive events above a single filament channel.

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