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Papers by nandita srivastava

Monthly Notices of the Royal Astronomical Society: Letters, Apr 27, 2022

Despite helium abundance (𝐴 𝐻 𝑒 = 𝑛 𝐻 /𝑛 𝐻 𝑒 ) is ∼ 8% at the solar photospheric/chromospheric he... more Despite helium abundance (𝐴 𝐻 𝑒 = 𝑛 𝐻 /𝑛 𝐻 𝑒 ) is ∼ 8% at the solar photospheric/chromospheric heights, 𝐴 𝐻 𝑒 can be found to exceed 8% in interplanetary coronal mass ejections (ICMEs) on many occasions. Although various factors like interplanetary shocks, chromospheric evaporation and "sludge removal" have been separately invoked in the past to address the 𝐴 𝐻 𝑒 enhancements in ICMEs, none of these processes could explain the variability of 𝐴 𝐻 𝑒 in ICMEs comprehensively. Based on extensive analysis of 275 ICME events, we show that there is a solar activity variation of ICME averaged 𝐴 𝐻 𝑒 values. The investigation also reveals that the first ionization potential effect as well as coronal temperature are not the major contributing factors for AHe enhancements in ICMEs. Investigation on concurrent solar flares and ICME events for 63 cases reveals that chromospheric evaporation in tandem with gravitational settling determine the 𝐴 𝐻 𝑒 enhancements and variabilities beyond 8% in ICMEs. While chromospheric evaporation releases the helium from chromosphere into the corona, the gravitationally settled heliums are thrown out during the ICME eruptions. We show that the intensity and timing of the preceding flares from the same active region from where the CME erupts are important factors to understand the 𝐴 𝐻 𝑒 enhancements in ICMEs.

Solar Physics, 2018

We report on the kinematics of two interacting CMEs observed on 13 and 14 June 2012. Both CMEs or... more 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 R from the Sun. The interaction led to a moderate geomagnetic storm at the Earth with D st 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) (≈ 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.

The Astrophysical Journal, Oct 28, 2016

We attempt to understand the collision characteristics of two coronal mass ejections (CMEs) launc... more We attempt 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 theSECCHI/Coronagraphic (COR) and Heliospheric Imagers, reveal their collision around 37 R ☉ from the Sun. In the analysis, we take into account the propagation and expansion speeds, impact direction, and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. We showthat the large expansion speed of the following CME compared with 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 asuper-elastic collision. The analysis under reasonable errorsin theobserved parameters of the CMEreveals a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.

Proceedings of the International Astronomical Union, Jun 1, 2013

We report observations of a long filament that underwent recurrent partial eruptions on August 4,... more We report observations of a long filament that underwent recurrent partial eruptions on August 4, 6, and 8, 2012. The filament reappeared in the subsequent rotation of the Sun, and disappeared completely on August 31, 2012. We implemented an automated filament detection algorithm developed by us for estimating different attributes of these filaments few hours prior to its disappearance in Hα and studied their evolution. Based on these attributes, we determine the onset time of the disappearance of Hα filaments. We then compared these onset times with that of the associated CMEs observed by LASCO/SOHO coronagraphs. This is also useful to understand temporal relationship of EUV and X-ray flux variation associated with filament disappearances in Hα. Our results show the importance of such studies in understanding the mechanism of CME initiation, particularly the role of eruptive filaments, in this process.

LASCO FeXIV and FeX observations of the solar coronal rotation during the recent solar activity minimum

AIP Conference Proceedings, 1999

ABSTRACT

Coronal Holes and Solar Wind Acceleration, 1999

We analyze data observed by the LASCO C1 coronagraph on board the SOHO spacecraft during the sola... more We analyze data observed by the LASCO C1 coronagraph on board the SOHO spacecraft during the solar minimum activity from April 1996 to March 1997. Using the phase dispersion technique, we investigate the periodicity and recurrence of Fe XIV emission structures with heliospheric latitude and distance above the Sun's surface with high spatial resolution. We find no significant deviation from a rigidly rotating Fe XIV corona with latitude or with distance from the Sun even on these small scales. In agreement with earlier work, the coronal rotation period at solar minimum is about 27.5 1 days.

Monthly Notices of the Royal Astronomical Society: Letters, Feb 13, 2021

The relative abundance of alpha particles with respect to proton, usually expressed as 𝐴 𝐻 𝑒 = (𝑛... more The relative abundance of alpha particles with respect to proton, usually expressed as 𝐴 𝐻 𝑒 = (𝑛 𝛼 /𝑛 𝑝 )*100, is known to respond to solar activity although changes in its behaviour in the last four solar cycles are not known. In this letter, by systematically analysing inter-calibrated 𝐴 𝐻 𝑒 data obtained from the first Lagrangian point of the Sun-Earth system, we show that 𝐴 𝐻 𝑒 variations are distinctively different in solar cycle 24 as compared to the last three cycles. The frequency of 𝐴 𝐻 𝑒 = 2-3% events is significantly higher in slow/intermediate solar winds in cycle 24 as opposed to the dominance of the typical 𝐴 𝐻 𝑒 = 4-5% events in the previous three cycles. Further, the occurrence of 𝐴 𝐻 𝑒 ≥ 10% events is significantly reduced in cycle 24. Not only that, the changes in delay of 𝐴 𝐻 𝑒 with respect to peak sunspot numbers are less sensitive to changes in solar wind velocity in cycle 24. The investigation suggests that the coronal magnetic field configuration started undergoing systematic changes starting from cycle 23 and this altered magnetic field configuration affected the way helium got processed and depleted in the solar atmosphere.

arXiv (Cornell University), Dec 25, 2021

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 4 He, 3 He, 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 particle flux data obtained from Ultra-Low Energy Isotope Spectrometer on board Advanced Composition Explorer satellite during the solar cycle 23 and 24. The analysis reveals that helium (4 He) 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 zero or positive lag in cycle 23 similar to 4 He, it shows only zero lag in cycle 24 and no negative lag is seen. Further, significant differences in the spectral indices are seen between 4 He and Fe in cycle 24 compared to the cycle 23. These results suggest that generation mechanisms responsible for suprathermal 4 He and Fe underwent changes in cycle 24 and these mechanisms are probably dependent on the first ionization potential and mass to charge ratio. This proposition gets credence from the fact that changes in the lag and spectral slopes for C and O are not significantly different in cycle 23 and 24.

Solar Hysteresis Pattern and Spectral Components in TEC Time Series (GPS and TIE‐GCM) of the Quadrilaterally Coupled Geomagnetic Conjugate Low‐Latitude Stations

Journal of Geophysical Research: Space Physics

Stealth CMEs: A Challenge for Solar Physics and Space Weather

AGUFM, Dec 1, 2013

Magnetic field experiment at L1 point onboard Aditya-L1 mission

On the Rotation Rate of the Emission Solar Corona

Conditions Leading to Eruptions of CMEs Associated with Eruptive Filaments

We report on a few recent and interesting observations of coronal mass ejections associated with ... more We report on a few recent and interesting observations of coronal mass ejections associated with eruptive filaments recorded in multiwavelengths using various data sets obtained from ground- and space-based observatories. These include chromospheric observations in H-alpha and the inner coronal data recorded from Mauna Loa Solar observatory (MLSO). The CME recorded in white light observations from the LASCO/SoHO is included

Advances in Space Research, 1997

Radio observations of the eclipse on November 3, 1994, were carried out at Chapecci, Brazil by us... more Radio observations of the eclipse on November 3, 1994, were carried out at Chapecci, Brazil by using a decimetric spectrograph having high spectral and time resolution. The light curve shows that: (1) Time variation of the radio flux before the totality was more compared to that after. (2) During the totality radio emission at 1.5 GHz was observed. Advantage of high spatial resolution (-3.2 arc set) possible during solar eclipse enabled us to determine the height of radio emission at 1.5 GHz. (3) Microwave bursts were observed associated with metric Type III-RS bursts. The source size of one of the microwave bursts was N 7 arc set and its physical parameters have been estimated. (4) The time difference between radio and optical contacts suggested for the first time asymmetrical limb brightening at 1.5 GHz.

Frontiers in Astronomy and Space Sciences

Interacting coronal mass ejections (CMEs) have been commonly reported during the STEREO era. With... more Interacting coronal mass ejections (CMEs) have been commonly reported during the STEREO era. With the interaction of CMEs in the heliosphere, it is expected that the participating CMEs will either merge to form a single interplanetary CME (ICME) or will arrive as distinct entities or ICMEs at 1 AU. Previous studies have focused on in situ observations of solar wind, i.e., plasma and magnetic field properties to understand the nature of the CME–CME interaction and its impact. In this study, we examine the observations of composition parameters of those ICMEs that resulted due to the interaction of two CMEs during their propagation between the Sun and the Earth. We report two events of the CME–CME interaction observed in 2012, of which one led to a merged structure after the interaction, as observed at 1 AU. The second interaction event was reported to arrive at L1 as two distinct structures. Our analysis reveals distinct composition signatures in the form of ion charge state enhancem...

Frontiers in Astronomy and Space Sciences, Dec 17, 2021

A Parametric Study of Performance of Two Solar Wind Velocity Forecasting Models During 2006–2011

Space Weather-the International Journal of Research and Applications, Sep 1, 2022

There is an increasing need for the development of a robust space weather forecasting framework. ... more There is an increasing need for the development of a robust space weather forecasting framework. State‐of‐the‐art MHD space weather forecasting frameworks are based upon the Potential Field Source Surface (PFSS) and Schatten Current Sheet (SCS) extrapolation models for the magnetic field using synoptic magnetograms. These models create a solar wind (SW) background for the simulations using empirical relations of Wang, Sheeley and Arge (WSA), at the inner boundary of heliosphere and have been used to simulate coronal mass ejections for specific cases in previous studies. Besides these MHD frameworks, the Heliospheric Upwind eXtrapolation (HUX) technique can extrapolate SW from inner heliospheric boundaries to L1 and can give a reliable estimate of the SW velocity at L1 comparable to MHD models but in a short computational time. We carried out an extensive parametric study of the performance of the Model1 (PFSS+WSA+HUX) and Model2 (PFSS+SCS+WSA+HUX) for SW velocity prediction at L1. We implemented this framework on 60 Carrington Rotations from CR2047 to CR2107 during 2006–2011, covering the descending and deep minimum phase of solar cycle (SC) 23, and the ascending phase of SC 24. Our results show an unexpected decrease in the performance of the framework during the deep minimum phase of cycle 23, which is attributed to the decrease in the observed coronal hole area. As SC 24 began, this decreasing trend vanished due to an increase in the coronal hole (CH) area at the low and mid‐latitudes, suggesting a good correlation between the performance of the framework and the variation in the CH area.

Proceedings of the International Astronomical Union, Feb 1, 2018

Generally Coronal Mass Ejections (CMEs) are large eruptions of plasma and magnetic field from the... more Generally Coronal Mass Ejections (CMEs) are large eruptions of plasma and magnetic field from the Sun into interplanetary space. CMEs are most frequently associated with a variety of phenomena occurring in the lower corona before, during and after onset of eruption and generally are visible in coronagraph observation. Stealth CMEs do not obviously exhibit any of the low-coronal signatures (LCS) like solar flares, flows, jets, coronal dimmings or brightenings, filament eruptions or the formation of flare loop arcades. In this study, five stealth CMEs are selected using LASCO/SOHO CME catalogue and associated ICMEs (Interplanetaty CMEs) are identified using data from STEREO, ACE and WIND.

Multipoint remote and in situ observations of interplanetary coronal mass ejection structures during 2011 and associated geomagnetic storms

Monthly Notices of the Royal Astronomical Society, Jul 13, 2021

ABSTRACT We present multipoint remote and in situ observations of interplanetary coronal mass eje... more ABSTRACT We present multipoint remote and in situ observations of interplanetary coronal mass ejection (ICME) structures during the year 2011. The selected ICMEs arrived at Earth on 2011 March 11 and 2011 August 6, and led to geomagnetic storms. Around the launch of these CMEs from the Sun, the coronagraphs onboard STEREO-Aand-B and SOHO enabled the CMEs to be imaged from three longitudinally separated viewpoints. We attempt to identify the in situ plasma and magnetic parameters of the ICME structures at multiple locations, for example at both STEREO spacecraft and also at the ACE/Wind spacecraft near the first Sun–Earth Lagrangian point (L1), to investigate the global configuration, interplanetary propagation, arrival times and geomagnetic response of the ICMEs. The near-Earth identified ICMEs of March 11 and August 6 formed as a result of the interaction of two successive CMEs observed in the inner corona on March 7 (for the March 11 ICME) and on August 3–4 (for the August 6 ICME). Our study suggests that the structures associated with interacting CMEs, possibly as a result of deflection or large sizes, may reach to even larger longitudinally separated locations in the heliosphere. Our multipoint in situ analysis shows that the characteristics of the same shock, propagating in a pre-conditioned medium, may be different at different longitudinal locations in the heliosphere. Similarly, multiple cuts through the same ejecta/complex ejecta, formed as a result of CME–CME interaction, are found to have inhomogeneous properties. The study highlights the difficulties in connecting the local observations of an ICME from a single in situ spacecraft to its global structures.