arXiv subjects

K. Sasikumar Raja

Publications and source records attributed to K. Sasikumar Raja.

30 records · Page 2Linked to original sources

arXiv2020

A Statistical Study of Solar Radio Type III Bursts and Space Weather Implication

Solar radio bursts (SRBs) are the signatures of various phenomena that happen in the solar corona and interplanetary medium (IPM). In this article, we have studied the occurrence of Type III bursts and their association with the Sunspot number. This study confirms that the occurrence of Type III bursts correlates well with the Sunspot number. Further, using the data obtained using the e-CALLISTO network, we have investigated the drift rates of isolated Type III bursts and the duration of the group of Type III bursts. Since Type II, Type III, and Type IV bursts are signatures of solar flares and/or CMEs, we can use the radio observations to predict space weather hazards. In this article, we have discussed two events that have caused near-Earth radio blackouts. Since e-CALLISTO comprises more than 152 stations at different longitudes, we can use it to monitor the radio emissions from the solar corona 24 hours a day. Such observations play a crucial role in monitoring and predicting space weather hazards within a few minutes to hours of time.

astro-ph.SR↗

arXiv2020

A Statistical Study of Low-frequency Solar Radio Type-III Bursts

We have studied low-frequency (45 - 410 MHz) type III solar radio bursts observed using the e-CALLISTO spectrometer located at Gauribidanur radio observatory, India during 2013 - 2017. After inspecting the 1531 type III bursts we found that 426 bursts were associated with flares, while the other bursts might have triggered by small scale features / weak energy events present in the solar corona. In this study, we have carried out a statistical analysis of various observational parameters like start time, lower and upper-frequency cut-offs of type III bursts and their association with flares, variation of such parameters with flare parameters such as location, class, onset and peak timings. From this study, we found that most of the high-frequency bursts (whose upper-frequency cut-off $> 350$ MHz) are originated from the western longitudes. We interpret that it could be due to the fact that Parker spirals from these longitudes are directed towards the earth and high-frequency bursts are more directive. Further, we report that number of bursts that reach earth from western longitudes are higher than eastern longitudes.

astro-ph.SR↗

arXiv2020

pyCallisto: A Python Library To Process The CALLISTO Spectrometer Data

CALLISTO is a radio spectrometer designed to monitor the transient radio emissions/bursts originated from the solar corona in the frequency range $45-870$ MHz. At present, there are $\gtrsim 150$ stations (together forms an e-CALLISTO network) around the globe continuously monitoring the Sun 24 hours a day. We have developed a pyCallisto, a python library to process the CALLISTO data observed by all stations of the e-CALLISTO network. In this article, we demonstrate various useful functions that are routinely used to process the CALLISTO data with suitable examples. This library is not only efficient in processing the data but plays a significant role in developing automatic classification algorithms of different types of solar radio bursts.

astro-ph.IM↗

arXiv2019

Automated Detection of Solar Radio Bursts using a Statistical Method

Radio bursts from the solar corona can provide clues to forecast space weather hazards. After recent technology advancements, regular monitoring of radio bursts has increased and large observational data sets are produced. Hence, manual identification and classification of them is a challenging task. In this paper, we describe an algorithm to automatically identify radio bursts from dynamic solar radio spectrograms using a novel statistical method. We used e-CALLISTO radio spectrometer data observed at Gauribidanur observatory near Bangalore in India during 2013 - 2014. We have studied the classifier performance using the receiver operating characteristics. Further, we studied type III bursts observed in the year 2014 and found that $75\%$ of the observed bursts were below 200 MHz. Our analysis shows that the positions of the flare sites which are associated with the type III bursts with upper-frequency cut-off $\gtrsim 200$ MHz originate close to the solar disk center

astro-ph.SR↗

arXiv2019

Dissipation scale lengths of density turbulence in the inner solar wind

Knowing the lengthscales at which turbulent fluctuations dissipate is key to understanding the nature of weakly compressible magnetohydrodynamic turbulence. We use radio wavelength interferometric imaging observations which measure the extent to which distant cosmic sources observed against the inner solar wind are scatter-broadened. We interpret these observations to determine that the dissipation scales of solar wind density turbulence at heliocentric distances of 2.5 -- 20.27 $R_{\odot}$ range from $\approx$ 13500 to 520 m. Our estimates from $\approx$ 10--20 $R_{\odot}$ suggest that the dissipation scale corresponds to the proton gyroradius. They are relevant to in-situ observations to be made by the Parker Solar Probe, and are expected to enhance our understanding of solar wind acceleration.

astro-ph.SR↗

arXiv2018

Clearing certain misconception in the common explanations of the aerodynamic lift

Air travel has become one of the most common means of transportation. The most common question which is generally asked is: How does an airplane gain lift? And the most common answer is via the Bernoulli principle. It turns out that it is wrongly applied in common explanations, and there are certain misconceptions. In an alternative explanation the push of air from below the wing is argued to be the lift generating force via Newton's law. There are problems with this explanation too. In this paper we try to clear these misconceptions, and the correct explanation, using the Lancaster-Prandtl circulation theory, is discussed. We argue that even the Lancaster-Prandtl theory at the zero angle of attack needs further insights. To this end, we put forward a theory which is applicable at zero angle of attack. A new length scale perpendicular to the lower surface of the wing is introduced and it turns out that the ratio of this length scale to the cord length of a wing is roughly $0.4930\pm0.09498$ for typical NACA airfoils that we analyzed. This invariance points to something fundamental. The idea of our theory is simple. The "squeezing" effect of the flow above the wing due to camber leads to an effective Venturi tube formation and leads to higher velocity over the upper surface of the wing and thereby reducing pressure according the Bernoulli theorem and generating lift. Thus at zero angle of attack there is no need to invoke vortex and anti-vortex pair generation. In fact vortex and anti-vortex pair generation cannot be justified. We come up with the equation for the lift coefficient at zero angle of attack: \[C_l = \frac{1}{c}\int_0^c dx \left(\frac{h_c^2}{(h_c - f(x))^2}-1\right).\] Here $C_l$ is the lift coefficient, $h_c$ is our new length scale perpendicular to the lower surface of the wing and $f(x)$ is the functional profile of the upper surface of the wing.

physics.pop-ph↗

arXiv2018

CALLISTO Spectrometer at IISER-Pune

A CALLISTO spectrometer to monitor solar radio transient emissions from $\approx 0.8-1.6~R_{\odot}$ (above photosphere) is installed at IISER, Pune, India (longitude $73^{\circ} 55'$ E and latitude $18^{\circ}31'$ N). In this paper, we illustrate the instrumental details (log-periodic dipole antenna and the receiver system) along with the recorded solar radio bursts and radio frequency interferences produced by the thunderstorms in the frequency range 45-870 MHz. We also developed the image processing pipelines using `sunpy' and in-house developed python library called `pycallisto'.

astro-ph.IM↗

arXiv2017

Turbulent density fluctuations and proton heating rate in the solar wind from 9-20~R_{\odot}

We obtain scatter broadened images of the Crab Nebula at 80 MHz as it transits through the inner solar wind in June 2016 and 2017. These images are anisotropic, with the major axis oriented perpendicular to the radially outward coronal magnetic field. Using these data, we deduce that the density modulation index (δN_e/N_e) caused by turbulent density fluctuations in the solar wind ranges from 1.9 \times 10^{-3} to 7.7 \times 10^{-3} between 9-20 R_{\odot}. We also find that the heating rate of solar wind protons at these distances ranges from 2.2 \times 10^{-13} to 1.0 \times 10^{-11} ~erg~cm^{-3}~s^{-1}. On two occasions, the line of sight intercepted a coronal streamer. We find that the presence of the streamer approximately doubles the thickness of the scattering screen.

astro-ph.SR↗

arXiv2016

Low frequency observations of transient quasi-periodic radio emission from the solar atmosphere

We report low frequency observations of the quasi-periodic, circularly polarized, harmonic type III radio bursts whose associated sunspot active regions were located close to the solar limb. The measured periodicity of the bursts at 80 MHz was $\approx$ 5.2 s and their average degree of circular polarization ($dcp$) was $\approx 0.12$. We calculated the associated magnetic field $B$ : (1) using the empirical relationship between the $dcp$ and $B$ for the harmonic type III emission, and (2) from the observed quasi-periodicity of the bursts. Both the methods result in $B \approx$ 4.2 G at the location of the 80 MHz plasma level (radial distance $r \approx 1.3~\rm R_{\odot}$) in the active region corona.

astro-ph.SR↗

arXiv2016

An estimate of the magnetic field strength associated with a solar coronal mass ejection from low frequency radio observations

We report ground based, low frequency heliograph (80 MHz), spectral (85-35 MHz) and polarimeter (80 and 40 MHz) observations of drifting, non-thermal radio continuum associated with the `halo' coronal mass ejection (CME) that occurred in the solar atmosphere on 2013 March 15. The magnetic field strengths ($B$) near the radio source were estimated to be $B \approx 2.2 \pm 0.4$ G at 80 MHz and $B \approx 1.4 \pm 0.2$ G at 40 MHz. The corresponding radial distances ($r$) are $r \approx 1.9~R_{\odot}$ (80 MHz) and $r \approx 2.2~R_{\odot}$ (40 MHz).

astro-ph.SR↗

arXiv2016

Design and performance of a low frequency cross-polarized log-periodic dipole antenna

We report the design and performance of a cross-polarized log-periodic dipole (CLPD) antenna for observations of polarized radio emission from the solar corona at low frequencies. The measured isolation between the two mutually orthogonal log periodic dipole antennas was as low as $\approx$ -43 dBm in the 65-95 MHz range. We carried out observations of the solar corona at 80 MHz with the above CLPD and successfully recorded circularly polarized emission.

astro-ph.IM↗

arXiv2016

Amplitude of solar wind density turbulence from 10--45 $R_{\odot}$

We report on the amplitude of the density turbulence spectrum ($C_{N}^{2}$) and the density modulation index ($δN/N$) in the solar wind between $10$ and $45 R_{\odot}$. We derive these quantities using a structure function that is observationally constrained by occultation observations of the Crab nebula made in 2011 and 2013 and similar observations published earlier. We use the most general form of the structure function, together with currently used prescriptions for the inner/dissipation scale of the turbulence spectrum. Our work yields a comprehensive picture of a) the manner in which $C_{N}^{2}$ and $δN/N$ vary with heliocentric distance in the solar wind and b) of the solar cycle dependence of these quantities.

astro-ph.SR↗