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Papers by nandita srivastava
Frontiers in Astronomy and Space Sciences, 2021
We attempt to understand the influence of the heliospheric state on the expansion behavior of cor... more We attempt to understand the influence of the heliospheric state on the expansion behavior of coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs) in solar cycles 23 and 24. Our study focuses on the distributions of the radial sizes and duration of ICMEs, their sheaths, and magnetic clouds (MCs). We find that the average radial size of ICMEs (MCs) at 1 AU in cycle 24 is decreased by ∼33% (∼24%) of its value in cycle 23. This is unexpected as the reduced total pressure in cycle 24 should have allowed the ICMEs in cycle 24 to expand considerably to larger sizes at 1 AU. To understand this, we study the evolution of radial expansion speeds of CME-MC pairs between the Sun and Earth based on their remote and in situ observations. We find that radial expansion speeds of MCs at 1 AU in solar cycles 23 and 24 are only 9% and 6%, respectively, of their radial propagation speeds. Also, the fraction of radial propagation speeds as expansion speeds of CMEs close to the Su...
Monthly Notices of the Royal Astronomical Society, 2019
Similar to the Sun, other stars shed mass and magnetic flux via ubiquitous quasi-steady wind and ... more 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 cycles 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 ph...
Space Science Reviews, 2021
Geomagnetic storms are an important aspect of space weather and can result in significant impacts... more Geomagnetic storms are an important aspect of space weather and can result in significant impacts on space- and ground-based assets. The majority of strong storms are associated with the passage of interplanetary coronal mass ejections (ICMEs) in the near-Earth environment. In many cases, these ICMEs can be traced back unambiguously to a specific coronal mass ejection (CME) and solar activity on the frontside of the Sun. Hence, predicting the arrival of ICMEs at Earth from routine observations of CMEs and solar activity currently makes a major contribution to the forecasting of geomagnetic storms. However, it is clear that some ICMEs, which may also cause enhanced geomagnetic activity, cannot be traced back to an observed CME, or, if the CME is identified, its origin may be elusive or ambiguous in coronal images. Such CMEs have been termed “stealth CMEs”. In this review, we focus on these “problem” geomagnetic storms in the sense that the solar/CME precursors are enigmatic and steal...
The Astrophysical Journal
Investigations on the solar cycle variation of the properties of suprathermal populations (H and ... more Investigations on the solar cycle variation of the properties of suprathermal populations (H and other heavy ions like 4He, 3He, C, O, and Fe) in the solar wind are sparse and hence poorly understood. In the present investigation, solar cycle variations of “quiet” time suprathermal elements are investigated using <∼ 1 MeV n−1 particle flux data obtained from the Ultra-Low Energy Isotope Spectrometer on board the Advanced Composition Explorer satellite during solar cycles 23 and 24. The analysis reveals that helium (4He) shows zero or positive lags with respect to sunspot numbers in solar cycle 23 while it shows zero or negative lag in solar cycle 24. On the contrary, although iron (Fe) shows a zero or positive lag in cycle 23 similar to 4He, it shows only a zero lag in cycle 24 and no negative lag is seen. Further, significant differences in the spectral indices are seen between 4He and Fe in cycle 24 compared to cycle 23. These results suggest that generation mechanisms responsi...
The Astrophysical Journal
Solar flares create adverse space weather impacting space- and Earth-based technologies. However,... more Solar flares create adverse space weather impacting space- and Earth-based technologies. However, the difficulty of forecasting flares, and by extension severe space weather, is accentuated by the lack of any unique flare trigger or a single physical pathway. Studies indicate that multiple physical properties contribute to active region flare potential, compounding the challenge. Recent developments in machine learning (ML) have enabled analysis of higher-dimensional data leading to increasingly better flare forecasting techniques. However, consensus on high-performing flare predictors remains elusive. In the most comprehensive study to date, we conduct a comparative analysis of four popular ML techniques (k nearest neighbors, logistic regression, random forest classifier, and support vector machine) by training these on magnetic parameters obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory for the entirety of solar cycle 24. We demonstrate th...
Astronomical Society of India Conference Series, 2013
Bulletin of the Astronomical Society of India
Bulletin of the Astronomical Society of India
Coronal Mass Ejections (CMEs) associated with solar filament eruptions expel huge amount of mass,... more Coronal Mass Ejections (CMEs) associated with solar filament eruptions expel huge amount of mass, energy and magnetic helicity in interplanetary medium as part of Interplanetary Coronal Mass Ejections (ICMEs). Despite of the fact that around 70% of CMEs are associated with filament eruptions, observations of filament (cold) material are rare when sampled by the in situ spacecraft (ACE/Wind) at L1 point and were reported quantitatively in only few earlier studies. We examine cases where cold material is identified as a direct consequence of eruptive filaments during last solar cycle as density enhancements, temperature depressions and low ionic charge states with depressed deviations in RMS parameters (magnetic and plasma). We further attempt to determine the threshold magnitudes of relative changes in magnetic, plasma and compositional parameters associated with in- situ observations which could serve as indicators for filament plasma.
AIP Conference Proceedings, 2012
We propose a space based solar coronagraph for the vector magnetic field measurements. A space ba... more We propose a space based solar coronagraph for the vector magnetic field measurements. A space based coronagraph is highly desirable in order to avoid the earth’s atmospheric scattered light related problems in coronal magnetic field measurements. There are several interesting problems which could be addressed by knowing the vector magnetic field in the corona. Some of the most interesting problems are, the magnetic field configuration for the origin of coronal mass ejections, role of magnetic field in the coronal heating, and solar g-mode detection. There are a handful of lines in the infrared wavelength which could be used for the coronal magnetic measurements, among those Fe XIII line is the most suitable line for this purpose. For the magnetic field measurements using this line, we describe the expected polarization signal level and the measurement technique employed to retrieve the polarization signals. Towards the end, we discuss the optical layout of a space based coronagraph...
Frontiers in Astronomy and Space Sciences, 2021
Eruptions of coronal mass ejections (CMEs) from the Sun are usually associated with a number of s... more Eruptions of coronal mass ejections (CMEs) from the Sun are usually associated with a number of signatures that can be identified in solar disc imagery. However, there are cases in which a CME that is well observed in coronagraph data is missing a clear low-coronal counterpart. These events have received attention during recent years, mainly as a result of the increased availability of multi-point observations, and are now known as “stealth CMEs.” In this work, we analyze examples of stealth CMEs featuring various levels of ambiguity. All the selected case studies produced a large-scale CME detected by coronagraphs and were observed from at least one secondary viewpoint, enabling a priori knowledge of their approximate source region. To each event, we apply several image processing and geometric techniques with the aim to evaluate whether such methods can provide additional information compared to the study of “normal” intensity images. We are able to identify at least weak eruptive...
Advances in Space Research, 2018
Monthly Notices of the Royal Astronomical Society, 2018
AIP Conference Proceedings, 1999
Frontiers in Astronomy and Space Sciences, 2021
We attempt to understand the influence of the heliospheric state on the expansion behavior of cor... more We attempt to understand the influence of the heliospheric state on the expansion behavior of coronal mass ejections (CMEs) and their interplanetary counterparts (ICMEs) in solar cycles 23 and 24. Our study focuses on the distributions of the radial sizes and duration of ICMEs, their sheaths, and magnetic clouds (MCs). We find that the average radial size of ICMEs (MCs) at 1 AU in cycle 24 is decreased by ∼33% (∼24%) of its value in cycle 23. This is unexpected as the reduced total pressure in cycle 24 should have allowed the ICMEs in cycle 24 to expand considerably to larger sizes at 1 AU. To understand this, we study the evolution of radial expansion speeds of CME-MC pairs between the Sun and Earth based on their remote and in situ observations. We find that radial expansion speeds of MCs at 1 AU in solar cycles 23 and 24 are only 9% and 6%, respectively, of their radial propagation speeds. Also, the fraction of radial propagation speeds as expansion speeds of CMEs close to the Su...
Monthly Notices of the Royal Astronomical Society, 2019
Similar to the Sun, other stars shed mass and magnetic flux via ubiquitous quasi-steady wind and ... more 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 cycles 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 ph...
Space Science Reviews, 2021
Geomagnetic storms are an important aspect of space weather and can result in significant impacts... more Geomagnetic storms are an important aspect of space weather and can result in significant impacts on space- and ground-based assets. The majority of strong storms are associated with the passage of interplanetary coronal mass ejections (ICMEs) in the near-Earth environment. In many cases, these ICMEs can be traced back unambiguously to a specific coronal mass ejection (CME) and solar activity on the frontside of the Sun. Hence, predicting the arrival of ICMEs at Earth from routine observations of CMEs and solar activity currently makes a major contribution to the forecasting of geomagnetic storms. However, it is clear that some ICMEs, which may also cause enhanced geomagnetic activity, cannot be traced back to an observed CME, or, if the CME is identified, its origin may be elusive or ambiguous in coronal images. Such CMEs have been termed “stealth CMEs”. In this review, we focus on these “problem” geomagnetic storms in the sense that the solar/CME precursors are enigmatic and steal...
The Astrophysical Journal
Investigations on the solar cycle variation of the properties of suprathermal populations (H and ... more Investigations on the solar cycle variation of the properties of suprathermal populations (H and other heavy ions like 4He, 3He, C, O, and Fe) in the solar wind are sparse and hence poorly understood. In the present investigation, solar cycle variations of “quiet” time suprathermal elements are investigated using <∼ 1 MeV n−1 particle flux data obtained from the Ultra-Low Energy Isotope Spectrometer on board the Advanced Composition Explorer satellite during solar cycles 23 and 24. The analysis reveals that helium (4He) shows zero or positive lags with respect to sunspot numbers in solar cycle 23 while it shows zero or negative lag in solar cycle 24. On the contrary, although iron (Fe) shows a zero or positive lag in cycle 23 similar to 4He, it shows only a zero lag in cycle 24 and no negative lag is seen. Further, significant differences in the spectral indices are seen between 4He and Fe in cycle 24 compared to cycle 23. These results suggest that generation mechanisms responsi...
The Astrophysical Journal
Solar flares create adverse space weather impacting space- and Earth-based technologies. However,... more Solar flares create adverse space weather impacting space- and Earth-based technologies. However, the difficulty of forecasting flares, and by extension severe space weather, is accentuated by the lack of any unique flare trigger or a single physical pathway. Studies indicate that multiple physical properties contribute to active region flare potential, compounding the challenge. Recent developments in machine learning (ML) have enabled analysis of higher-dimensional data leading to increasingly better flare forecasting techniques. However, consensus on high-performing flare predictors remains elusive. In the most comprehensive study to date, we conduct a comparative analysis of four popular ML techniques (k nearest neighbors, logistic regression, random forest classifier, and support vector machine) by training these on magnetic parameters obtained from the Helioseismic and Magnetic Imager on board the Solar Dynamics Observatory for the entirety of solar cycle 24. We demonstrate th...
Astronomical Society of India Conference Series, 2013
Bulletin of the Astronomical Society of India
Bulletin of the Astronomical Society of India
Coronal Mass Ejections (CMEs) associated with solar filament eruptions expel huge amount of mass,... more Coronal Mass Ejections (CMEs) associated with solar filament eruptions expel huge amount of mass, energy and magnetic helicity in interplanetary medium as part of Interplanetary Coronal Mass Ejections (ICMEs). Despite of the fact that around 70% of CMEs are associated with filament eruptions, observations of filament (cold) material are rare when sampled by the in situ spacecraft (ACE/Wind) at L1 point and were reported quantitatively in only few earlier studies. We examine cases where cold material is identified as a direct consequence of eruptive filaments during last solar cycle as density enhancements, temperature depressions and low ionic charge states with depressed deviations in RMS parameters (magnetic and plasma). We further attempt to determine the threshold magnitudes of relative changes in magnetic, plasma and compositional parameters associated with in- situ observations which could serve as indicators for filament plasma.
AIP Conference Proceedings, 2012
We propose a space based solar coronagraph for the vector magnetic field measurements. A space ba... more We propose a space based solar coronagraph for the vector magnetic field measurements. A space based coronagraph is highly desirable in order to avoid the earth’s atmospheric scattered light related problems in coronal magnetic field measurements. There are several interesting problems which could be addressed by knowing the vector magnetic field in the corona. Some of the most interesting problems are, the magnetic field configuration for the origin of coronal mass ejections, role of magnetic field in the coronal heating, and solar g-mode detection. There are a handful of lines in the infrared wavelength which could be used for the coronal magnetic measurements, among those Fe XIII line is the most suitable line for this purpose. For the magnetic field measurements using this line, we describe the expected polarization signal level and the measurement technique employed to retrieve the polarization signals. Towards the end, we discuss the optical layout of a space based coronagraph...
Frontiers in Astronomy and Space Sciences, 2021
Eruptions of coronal mass ejections (CMEs) from the Sun are usually associated with a number of s... more Eruptions of coronal mass ejections (CMEs) from the Sun are usually associated with a number of signatures that can be identified in solar disc imagery. However, there are cases in which a CME that is well observed in coronagraph data is missing a clear low-coronal counterpart. These events have received attention during recent years, mainly as a result of the increased availability of multi-point observations, and are now known as “stealth CMEs.” In this work, we analyze examples of stealth CMEs featuring various levels of ambiguity. All the selected case studies produced a large-scale CME detected by coronagraphs and were observed from at least one secondary viewpoint, enabling a priori knowledge of their approximate source region. To each event, we apply several image processing and geometric techniques with the aim to evaluate whether such methods can provide additional information compared to the study of “normal” intensity images. We are able to identify at least weak eruptive...
Advances in Space Research, 2018
Monthly Notices of the Royal Astronomical Society, 2018
AIP Conference Proceedings, 1999