Anshu Kumari, PhD

Anshu Kumari, PhD

NASA Postdoctoral Program (NPP) Fellow

Sitemap

A list of all the posts and pages found on the site. For you robots out there is an XML version available for digesting as well.

Pages

Posts

awards

publications

On the usefulness of existing solar wind models for pulsar timing corrections

Published in Monthly Notices of the Royal Astronomical Society, Volume 487, Issue 1, p.394-408, 2019

Authors: Tiburz, C, Verbiest, J. P. W., Shaifullah, G. M., Janssen, G. H., Anderson, J. M., Horneffer, A., K ̈unsem ̈oller, J., Osłowski, S., Donner, J. Y., Kramer, M., Kumari, A., Porayko, N. K., Zucca, P., Ciardi, B., Dettmar, R-J., Grießmeier, J-M., Hoeft, M. & Serylak, M.

research

talks

New Evidence for a Coronal Mass Ejection-driven High Frequency Type II Burst near the Sun

Published:

Corona, the outermost region of the Sun, is believed to be present from a heliocentric distance of ~1.01 R ⊙ (where R is the radius of the solar photosphere) to more than 1 AU. Being highly tenuous plasma medium, it harbours large scale structures, as multi-frequency observations reveal. One such powerful solar phenomenon is Coronal Mass Ejections (CMEs), which has radio signatures as Type II bursts. Type II solar radio bursts are considered to originate from plasma waves excited by magnetohydrodynamic (MHD) shocks and converted into radio waves at the local plasma frequency and/or its harmonics. They are the direct diagnostic of MHD shocks in the solar atmosphere.

LOFAR Interferometric Observation of a group of Type III Solar Radio Bursts

Published:

Corona, the outermost layer of the Sun, is believed to permeate from a heliocentric distance of ~1.01 Rʘ to more than 1 AU. Being highly tenuous plasma medium, it harbours large scale structures, as multi-frequency observations reveal. One of the most common signatures of any flicker on the Sun is known as solar type III radio bursts. These bursts are an important diagnostic tool to understand the acceleration of non-thermal electron beams along the coronal magnetic field lines. Using the interferometric and beam formed capabilities of LOw Frequency ARray (LOFAR), we analysed a group of type III radio bursts observed between 80-20 MHz, on 30 March 2018. Taking advantage of the high spectral, temporal and spatial resolution of LOFAR, we were able to distinguish five different trajectories of propagation of the electron beams in the type III group. Using full Stokes observations (frequency and time resolution of 10 ms and 12 kHz, respectively) by the simultaneous beam formed LOFAR observations, we estimated the coronal magnetic field along these five electron beam trajectories. This was done by calculating the degree of circular polarisation of the harmonic plasma emission from the type III bursts. The methods and results will be discussed in this talk.

Estimation of Strength of the Solar Coronal Magnetic Field using Contemporaneous Radio and Whitelight Observations

Published:

Estimation of the magnetic field strength (B) in the solar corona is one of the widely pursued areas of research in observational solar physics. We estimated the coronal magnetic field strength during the 23 July 2016 coronal mass ejection (CME) event using i) the flux rope structure of the CME in the whitelight coronagraph images and ii) the band splitting in the associated type II burst. No models were assumed for the coronal electron density (N (r)) used in the estimation. The results obtained using the above two independent methods correspond to different heliocentric distances (r) in the range ≈ 2.5 – 4.5Ro, but they show excellent consistency and could be fitted with a single power-law distribution of the type B(r) = 5.7r^{−2.6} G, which is applicable in the aforementioned distance range. The power law index (i.e. −2.6) is in good agreement with the results obtained in previous studies by different methods, for example, using Faraday rotation observations of the linearly polarized carrier signals of the HELIOS spacecraft and using observations of similar signals from extragalactic radio sources occulted by the solar corona.

On the occurrence of type IV solar radio bursts in the solar cycle 24 and their association with coronal mass ejections

Published:

Solar activity, in particular coronal mass ejections, are often accompanied by bursts of radiation at meter wavelengths, some of which are long duration and broadband in nature, such as type IV radio bursts. However, the association of type IV bursts with coronal mass ejections is still not well understood. We performed the first statistical study of type IV solar radio bursts in the solar cycle 24. Our studies include a total of 445 type IV radio bursts that occurred during this solar cycle. Our results show that ~56 % of type IV bursts were accompanied by CMEs, based on a temporal association with white-light CME observations. Our results indicated that ~65 % bursts had source region on the disk and ~35 % bursts had source region on the limb. Our studies also suggests that type IV bursts can occur with both ‘Fast’ (≥ 500 km/s) and ‘Slow’ (< 500 km/s) CMEs, however most of the type IV bursts (~ 66 %) were associated with ‘Wide’ (≥ 60 º) CMEs. Only ~43 % of the type IV bursts in this solar cycle were associated with ‘Fast and Wide’ CMEs. These results are unlike the majority of type II radio bursts that are mostly associated with ‘Fast and Wide’ CMEs.

Radio Polarimetric Studies of the Solar Corona at Low Frequencies

Published:

Measurements of the coronal magnetic field strength particularly in the radial distance range ~ 1.1 – 2.0 R, (where R is the radius of the solar photosphere) is presently difficult because of practical reasons. Polarization observations, by measuring the Stokes-V parameter of the received radio signal, are generally used as a tool to measure the magnetic field strength associated with the radio emission; the latter is one of the widely pursued areas of research in the solar coronal physics, in addition to the currently available but limited methods of estimating the magnetic field strength using simultaneous radio imaging and spectral observations. In this talk, I will be talking about: i) design of a new wideband, low frequency antennas, ii) design, development and characterization of a high temporal and spectral resolution multi frequency polarimeteric receiver system for solar observations at low radio frequencies, iii) studies of the solar radio bursts observed with these instruments and their counterparts as observed in multi frequencies with other space and ground-based instruments.

Radio Polarimetric Imaging of the Solar Corona at Low Frequencies

Published:

Measurements of the coronal magnetic field strength particularly in the radial distance range ~ 1.1 - 2.0 R, (where R is the radius of the solar photosphere) is presently difficult because of practical reasons. Polarization observations, by measuring the Stokes-V parameter of the received radio signal, are generally used as a tool to measure the magnetic field strength associated with the radio emission; the latter is one of the widely pursued areas of research in the solar coronal physics, in addition to the currently available but limited methods of estimating the magnetic field strength using simultaneous radio imaging and spectral observations. This work includes: i) design of a new wideband, low frequency antennas, ii) design, development and characterization of a high temporal and spectral resolution multi frequency polarimeteric receiver system for solar observations at low radio frequencies, iii) studies of the solar radio bursts observed with these instruments and their counterparts as observed in multi frequencies with other space and ground-based instruments.

Association of type IV radio bursts with CMEs in solar cycle 24

Published:

Measurements of the coronal magnetic field strength particularly in the radial distance range ~ 1.1 - 2.0 R, (where R is the radius of the solar photosphere) is presently difficult because of practical reasons. Polarization observations, by measuring the Stokes-V parameter of the received radio signal, are generally used as a tool to measure the magnetic field strength associated with the radio emission; the latter is one of the widely pursued areas of research in the solar coronal physics, in addition to the currently available but limited methods of estimating the magnetic field strength using simultaneous radio imaging and spectral observations. This work includes: i) design of a new wideband, low frequency antennas, ii) design, development and characterization of a high temporal and spectral resolution multi frequency polarimeteric receiver system for solar observations at low radio frequencies, iii) studies of the solar radio bursts observed with these instruments and their counterparts as observed in multi frequencies with other space and ground-based instruments.

On the occurrence of type IV solar radio bursts in the solar cycle 24 and their association with coronal mass ejections

Published:

Solar activities, in particular coronal mass ejections (CMEs), are often accompanied by bursts of radiation at metre wavelengths, some of which are long duration and broadband in nature, such as type IV radio bursts. However, the association of type IV bursts with coronal mass ejections is still not well understood. We perform the first statistical study of type IV solar radio bursts in the solar cycle 24. Our study includes a total of 446 type IV radio bursts that occurred during this cycle. Our results show that a clear majority, ∼81%of type IV bursts, were accompanied by CMEs, based on a temporal association with white-light CME observations. However, we found that only ∼2.2%of the CMEs are accompanied by type IV radio bursts. We categorised the type IV bursts as moving or stationary based on their spectral characteristics and found that only ∼18% of the total type IV bursts in this study were moving type IV bursts. Our study suggests that type IV bursts can occur with both ‘Fast’ (≥500km/s) and ‘Slow’ (<500km/s), and also both ‘Wide’ (≥60◦) and ‘Narrow’ (<60◦) CMEs. However, the moving type IV bursts in our study were mostly associated with ‘Fast’ and ‘Wide’ CMEs (∼52%), similar to type II radio bursts. Contrary to type II bursts, stationary type IV bursts have a more uniform association with all CME types

Effects of optimisation parameters on data-driven magnetofrictional modelling

Published:

The solar coronal magnetic field plays an important role in the formation, evolution, and dynamics of small and large-scale structures in the corona. Estimation of the coronal magnetic field, the ultimate driver of space weather, particularly in the ‘low’ and ‘middle’ corona, is presently limited due to practical difficulties. Data-driven time-dependent magnetofrictional modelling (TMFM) of active region magnetic fields has been proven as a tool to observe and study the corona. In this work, we present a detailed study of data-driven TMFM of active region 12473 to trace the early evolution of the flux rope related to the coronal mass ejection that occurred on 28 December 2015. Non-inductive electric field component in the photosphere is critical for energizing and introducing twist to the coronal magnetic field, thereby allowing unstable configurations to be formed. We estimate this component using an approach based on optimizing the injection of magnetic energy. We study the effects of these optimisation parameters on the data driven coronal simulations. By varying the free optimisation parameters, we explore the changes in flux rope formation and their early evolution, as well other parameters, e.g. axial flux, magnetic field magnitude.

Solar Radio Spectro-Polarimetry (50-500 Mhz) : Design And Development Of Cross polarized Log-Periodic Dipole Antenna

Published:

A radio spectro-polarimeter was developed at the Gauribidanur radio observatory (Longitude : 77◦27′07′′; Latitude : 13◦36′12′′) to study the characteristics of the polarized radio waves that are emitted by the impetuous solar corona in the 50 - 500 MHz frequency range. The instrument has three major components : a Cross-polarized Log-Periodic Dipole Antenna (CLPDA) [1], an analog receiver, and a digital receiver (spectrum analyzer). This article elaborates the design and developmental aspects of the CLPDA, its characteristics and briefs about the configurations of the analog and digital receivers, setting up of the spectro-polarimeter, stage- wise tests performed to characterize it, etc. To demonstrate the instrumental capability, the estimation of the solar coronal magnetic field strength (B vs heliocentric height), using the spectral data obtained with it, is exemplified.

Effects of optimisation parameters on data-driven magnetofrictional modelling

Published:

The solar coronal magnetic field plays an important role in the formation, evolution, and dynamics of small and large-scale structures in the corona. Estimation of the coronal magnetic field, the ultimate driver of space weather, particularly in the “low” and “middle” corona, is presently limited due to practical difficulties. Data-driven time-dependent magnetofrictional modelling (TMFM) of active region magnetic fields has been proven as a tool to observe and study the corona. In this work, we present a detailed study of data-driven TMFM of active region 12473 to trace the early evolution of the flux rope related to the coronal mass ejection that occurred on 28 December 2015. Non-inductive electric field component in the photosphere is critical for energizing and introducing twist to the coronal magnetic field, thereby allowing unstable configurations to be formed. We estimate this component using an approach based on optimizing the injection of magnetic energy. We study the effects of these optimisation parameters on the data driven coronal simulations. By varying the free optimisation parameters, we explore the changes in flux rope formation and their early evolution, as well other parameters, e.g. axial flux, magnetic field magnitude.

Effects of optimisation parameters on data-driven magnetofrictional modelling od AR12473

Published:

The solar magnetic field plays an essential role in the formation, evolution, and dynamics of large-scale eruptive structures in the corona. Estimation of the coronal magnetic field, the ultimate driver of space weather, particularly in the ‘low’ and ‘middle’ corona, is presently limited due to practical difficulties. Data-driven time-dependent magneticfrictional modelling (TMFM) of active region magnetic fields has been proven to be a useful tool to study the corona. The input to the model is the photospheric electric field that is inverted from a time-series of the photospheric magnetic field. Constraining the complete electric field, i.e., including the non-inductive component, is critical for capturing the eruption dynamics. We present a detailed study of the effects of optimisation of the non-inductive electric field on TMFM of AR12473. We study the effects of these optimisation parameters on the data-driven coronal simulations. By varying the free optimisation parameters, we explore the changes in flux rope formation and their early evolution and other parameters, e.g., axial flux and magnetic field magnitude. We used the high temporal and spatial resolution cadence vector magnetograms from the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamics Observatory (SDO). The non-inductive electric field component in the photosphere is critical for energising and introducing twist to the coronal magnetic field, thereby allowing unstable configurations to be formed. We estimate this component using an approach based on optimising the injection of magnetic energy. Our simulations show that flux ropes were formed in all of the simulations except the lower values of these parameters. However, the flux rope formation, evolution and eruption time varies depending upon the values of the optimisation parameters. This study shows that irrespective of ad hoc free parameters values, flux ropes are formed and erupted, which indicates that data-driven TMFM can be used to estimate flux rope properties early in their evolution without needing to employ a lengthy optimisation process.

On the source sizes of fine structures of type II radio bursts using LOFAR

Published:

The magnetic field dominates the structure and dynamics of the solar corona and it is the primary driver of Space weather. Radio observations are one of the most common approaches to diagnosing the magnetic field in the solar atmosphere. One of the direct signatures of explosive solar phenomena, such as coronal mass ejections (CMEs) in radio wavelengths, is called metric type II radio bursts. Type II bursts originate from plasma waves converted into radio waves at the local plasma frequency and its harmonics. These radio bursts can be considered a direct diagnosis of MHD shocks in the solar atmosphere. These bursts can be used to study the kinematics, energetics, and dynamics of the associated eruptive events. With state-of-the-art radio instruments such as LOw Frequency ARray (LOFAR), it has now been possible to study these bursts and the structures within them in great spectral, temporal and spatial resolutions. We studied the source sizes and shapes of the fine structures of type II radio bursts observed with LOFAR and their variation with frequency in metric wavelengths.

Coronal Mass Ejections: From Observations to Simulations

Published:

The coronal magnetic field, the ultimate driver of space weather, plays an essential role in the formation, evolution, and dynamics of the small and large-scale structures in the solar corona. These structures may lead to gigantic explosions in the solar atmosphere in the form of large-scale eruptions, such as coronal mass ejections (CMEs), which may severely impact near-Earth space. CMEs can reach Earth within several hours to days, and depending on the orientation of its internal magnetic field; they can interact with the Earth’s magnetosphere causing severe geomagnetic storms. Moreover, the shocks generated by CMEs can accelerate the energetic particles leading to highly energetic solar radiation storms. These extreme space weather conditions may damage satellite operations and Earth’s communication and navigation system. Therefore, studying such violent solar eruptions is crucial to understand their consequences on space weather. CMEs are often accompanied by radio emissions, which provide access to observations of the related solar, heliospheric, and ionospheric space weather phenomena. Radio techniques can provide early signatures of particle acceleration associated with solar flares and CMEs, which give insights into CME initialisation and eruption processes (1). These observational techniques also serve as a powerful tool to constrain the coronal and heliospheric models. With state-of-the-art radio instruments such as LOw-Frequency ARray (LOFAR) and legacy instruments such as Nançay Radio Heliograph (NRH), it has now been possible to study these bursts and their structures in great spectral, temporal and spatial resolutions (2). Using these observations and time-dependent data-driven numerical modeling of active region magnetic fields (3), we study the formation and eruption of the coronal flux-ropes leading to CME eruptions. We estimate various properties of the CME flux-rope and compare them with the associated multi-wavelength ground- and space-based observations. In this talk, I will highlight the radio techniques to constrain the initial CME properties close to the Sun and the numerical modeling approach to understanding the initiation and evolution of large-scale solar eruptions.

Coronal Mass Ejections: From Observations to Simulations

Published:

The coronal magnetic field, the ultimate driver of space weather, plays an essential role in the formation, evolution, and dynamics of the small and large-scale structures in the solar corona. These structures may lead to gigantic explosions in the solar atmosphere in the form of large-scale eruptions, such as coronal mass ejections (CMEs), which may severely impact near-Earth space. CMEs are often accompanied by radio emissions, which provide access to observations of the related solar, heliospheric, and ionospheric space weather phenomena. Radio techniques can provide early signatures of particle acceleration associated with solar flares and CMEs. Using radio observations and time-dependent data-driven numerical modeling of active region, we study the formation and eruption of the coronal flux ropes. I will highlight the radio techniques to constrain the initial CME properties close to the Sun and the numerical modeling approach to understanding the initiation and evolution of large-scale solar eruptions.

teaching

Solar Physics (PAP321)

Advanced course (Bachelors, Masters and PhD), University of Helsinki, 2021

I worked as teaching assistance on Solar Physics (advanced cource) in Spring semester 2021.