scholarly journals Tracking of an electron beam through the solar corona with LOFAR

2018 ◽  
Vol 611 ◽  
pp. A57 ◽  
Author(s):  
G. Mann ◽  
F. Breitling ◽  
C. Vocks ◽  
H. Aurass ◽  
M. Steinmetz ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Electron Beam ◽  
Low Frequency ◽  
Coronal Magnetic Field ◽  
Propagation Path ◽  
Radio Bursts ◽  
Low Frequencies ◽  
Energetic Electrons ◽  
Type Iii ◽  
Field Lines

The Sun’s activity leads to bursts of radio emission, among other phenomena. An example is type-III radio bursts. They occur frequently and appear as short-lived structures rapidly drifting from high to low frequencies in dynamic radio spectra. They are usually interpreted as signatures of beams of energetic electrons propagating along coronal magnetic field lines. Here we present novel interferometric LOFAR (LOw Frequency ARray) observations of three solar type-III radio bursts and their reverse bursts with high spectral, spatial, and temporal resolution. They are consistent with a propagation of the radio sources along the coronal magnetic field lines with nonuniform speed. Hence, the type-III radio bursts cannot be generated by a monoenergetic electron beam, but by an ensemble of energetic electrons with a spread distribution in velocity and energy. Additionally, the density profile along the propagation path is derived in the corona. It agrees well with three-fold coronal density model by (1961, ApJ, 133, 983).

Exploring the circular polarisation of low-frequency solar radio bursts with LOFAR and estimating the coronal magnetic field

2021 ◽  
Author(s):  
Diana Morosan ◽  
Anshu Kumari ◽  
Juska Räsänen ◽  
Emilia Kilpua ◽  
Pietro Zucca ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Radio ◽  
Low Frequency ◽  
Coronal Magnetic Field ◽  
Plasma Emission ◽  
The Sun ◽  
Radio Bursts ◽  
Solar Radio Bursts ◽  
Circular Polarisation ◽  
Type Iii

<p>The Sun is an active star that often produces numerous bursts of electromagnetic radiation at radio wavelengths. In particular, low frequency (< 150 MHz)  radio bursts have recently been brought back to light with the advancement of novel radio interferometric arrays. However, the polarisation properties of solar radio bursts have not yet been explored in detail, especially with the Low Frequency Array (LOFAR). Here, we explore the circular polarisation of type III radio bursts and a type I noise storm and present the first Stokes V low frequency radio images of the Sun with LOFAR in tied array mode observations. We find that the degree of circular polarisation for each of the selected bursts increases with frequency for fundamental plasma emission, while this trend is either not clear or absent for harmonic plasma emission. In the case of type III bursts, we also find that the sense of circular polarisation varies with each burst, most likely due to their different propagation directions, despite all of these bursts being part of a long-lasting type III storm. Furthermore, we use the degree of circular polarisation of the harmonic emission of type III bursts to estimate the coronal magnetic field at distances of 1.4 to 4 solar radii from the centre of the Sun. We found that the magnetic field has a power law variation with a power index in the range 2.4-3.6, depending on the individual type III burst observed.</p>

scholarly journals Properties and magnetic origins of solar S-bursts

2019 ◽  
Vol 622 ◽  
pp. A204 ◽  
Author(s):  
Brendan P. Clarke ◽  
Diana E. Morosan ◽  
Peter T. Gallagher ◽  
Vladimir V. Dorovskyy ◽  
Alexander A. Konovalenko ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Solar Activity ◽  
Solar Radio ◽  
Low Frequency ◽  
High Sensitivity ◽  
Coronal Magnetic Field ◽  
Solar Radio Emission ◽  
Radio Bursts ◽  
Burst Source ◽  
Low Frequencies

Context. Solar activity is often accompanied by solar radio emission, consisting of numerous types of solar radio bursts. At low frequencies (<100 MHz) radio bursts with short durations of milliseconds, such as solar S-bursts, have been identified. To date, their origin and many of their characteristics remain unclear. Aims. We report observations from the Ukrainian T-shaped Radio telescope, (UTR-2), and the LOw Frequency ARray (LOFAR) which give us new insight into their nature. Methods. Over 3000 S-bursts were observed on 9 July 2013 at frequencies of 17.4–83.1 MHz during a period of low solar activity. Leading models of S-burst generation were tested by analysing the spectral properties of S-bursts and estimating coronal magnetic field strengths. Results. S-bursts were found to have short durations of 0.5–0.9 s. Multiple instruments were used to measure the dependence of drift rate on frequency which is represented by a power law with an index of 1.57. For the first time, we show a linear relation between instantaneous bandwidth and frequency over a wide frequency band. The flux calibration and high sensitivity of UTR-2 enabled measurements of their fluxes, which yielded 11 ± 3 solar flux units (1 SFU ≡ 104 Jy). The source particle velocities of S-bursts were found to be ∼0.07 c. S-burst source heights were found to range from 1.3 R⊙ to 2 R⊙. Furthermore, a contemporary theoretical model of S-burst generation was used to conduct remote sensing of the coronal magnetic field at these heights which yielded values of 0.9–5.8 G. Within error, these values are comparable to those predicted by various relations between magnetic field strength and height in the corona.

scholarly journals Transition Between Type I and Type III Bursts in Closed or Open Magnetic Field Lines

1980 ◽  
Vol 86 ◽  
pp. 363-368
Author(s):  
Monique G. Aubier
Keyword(s):  
Magnetic Field ◽  
Velocity Component ◽  
Low Frequency ◽  
Critical Value ◽  
Type I ◽  
Accelerated Electrons ◽  
Type Iii ◽  
Field Lines ◽  
Parallel Momentum ◽  
Open Magnetic Field

When studying the propagation of accelerated electrons outwards in the corona, we have shown that the perpendicular momentum of the electrons remaining after the type I process is transformed into parallel momentum during the propagation along the decreasing magnetic field, and that type III emission can occur when the parallel velocity component reaches a critical value. With this model we explain in particular the low frequency cut-off of type I emission, the characteristics of the type III bursts near their starting frequency and the transition between type III- and type I-like decameter emission observed in few cases.

Nonrelativistic electron beam expansion in the solar corona/wind  and their type III radio bursts observed with LOFAR

2021 ◽  
Author(s):  
Hamish Reid ◽  
Eduard Kontar
Keyword(s):  
Electron Beam ◽  
Energy Density ◽  
Solar Corona ◽  
Electron Beams ◽  
Low Frequency ◽  
High Energy ◽  
High Energy Density ◽  
Moderate Intensity ◽  
Radio Bursts ◽  
Type Iii

&lt;div&gt; &lt;div&gt;&lt;span&gt;Solar type III radio bursts contain a wealth of information about the dynamics of&amp;#160;near-relativistic&amp;#160;electron beams in the solar corona and the inner heliosphere; this information is currently unobtainable through other means. &amp;#160;Whilst electron beams expand along their trajectory, the motion of different regions of an electron beam (front, middle, and back) had never been systematically analysed before.&amp;#160;&amp;#160;Using LOw Frequency ARray (LOFAR) observations between 30-70 MHz of type III radio bursts, and kinetic simulations of electron beams producing derived type III radio brightness temperatures, we explored the expansion as electrons propagate away from the Sun.&amp;#160;&amp;#160;From relatively moderate intensity type III bursts, we found mean electron beam speeds for the front, middle and back of 0.2, 0.17 and 0.15 c, respectively.&amp;#160;&amp;#160;Simulations demonstrated that the electron beam energy density, controlled by the initial beam density and energy distribution have a significant effect on the beam speeds, with high energy density beams reaching front and back velocities of 0.7 and 0.35 c, respectively.&amp;#160;&amp;#160;Both observations and simulations found that higher inferred velocities correlated with shorter FWHM durations of radio emission at individual frequencies.&amp;#160;&amp;#160;Our radial predictions of electron beam speed and expansion can be tested by the upcoming in situ electron beam measurements made by Solar Orbiter and Parker Solar Probe.&lt;/span&gt;&lt;/div&gt; &lt;/div&gt;

Type III Radio Bursts and Langmuir Wave Excitation

2020 ◽  
Author(s):  
Gottfried Mann ◽  
Christian Vocks ◽  
Mario Bisi ◽  
Eoin Carley ◽  
Bartosz Dabrowski ◽  
...  
Keyword(s):  
Distribution Function ◽  
Interplanetary Space ◽  
Langmuir Wave ◽  
Velocity Distribution Function ◽  
Radio Bursts ◽  
Radio Waves ◽  
Langmuir Waves ◽  
Low Frequencies ◽  
Energetic Electrons ◽  
Type Iii

&lt;p&gt;Type III radio bursts are a common phenomenon the Sun&amp;#8217;s nonthermal radio radiation. They appear as stripes of enhanced radio emission with a rapid drift from high to low frequencies in dynamic radio spectra. They are considered as the radio signatures of beams of energetic electrons travelling along magnetic field lines from the solar corona into the interplanetary space. With the ground based radio interferometer LOFAR and the instrument FIELDS onboard NASA&amp;#8217;s &amp;#8220;Parker Solar Probe&amp;#8221; (PSP) , type III radio bursts can be observed simultaneously from high (10-240 MHz) to low frequencies (0.01-20 MHz) with LOFAR and PSP&amp;#8217;s FIELDs, respectively. That allows to track these electron beams from the corona up to the interplanetary space. Assuming that a population of energetic electrons is initially injected, the velocity distribution function of these electrons evolves into a beam like one. Such distribution function leads to the excitation of Langmuir waves which convert into radio waves finally observed as type II radio bursts. Numerical calculations of the electron-beam-plasma interaction reveal that the Langmuir waves are excited by different parts of the energetic electrons at different distances in the corona and interplanetary space. This result is compared with special type III radio bursts observed with LOFAR and PSP&amp;#8217;s FIELDS.&lt;/p&gt;

Solar Radio Pulsations

1974 ◽  
Vol 57 ◽  
pp. 471-471
Author(s):  
B. L. Gotwols
Keyword(s):  
Magnetic Field ◽  
Solar Radio ◽  
Low Frequency ◽  
Source Area ◽  
Synchrotron Emission ◽  
Radio Bursts ◽  
Low Frequencies ◽  
Synchrotron Source ◽  
Type Iv ◽  
The Magnetic Field

(Solar Phys.). Several models for pulsating type IV radio bursts are presented based on the assumption that the pulsations are the result of fluctuations in the synchrotron emission due to small variations in the magnetic field of the source. It is shown that a source that is optically thick at low frequencies due to synchrotron self absorption exhibits pulsations that occur in two bands situated on either side of the spectral peak. The pulsations in the two bands are 180° out of phase and the band of pulsations at the higher frequencies is the more intense. In contrast, a synchrotron source that is optically thin at all frequencies and whose low frequency emission is suppressed due to the Razin effect develops only a single band of pulsations around the frequency of maximum emission. However, the flux density associated with the later model would be too small to explain the more intense pulsations that have been observed unless the source area is considerably larger than presently seems reasonable.

On the Fundamental and Harmonic Components of Low-Frequency Type III Solar Radio Bursts

1982 ◽  
Vol 4 (4) ◽  
pp. 382-386 ◽  
Author(s):  
S. Suzuki ◽  
K.V. Sheridan
Keyword(s):  
Solar Radio ◽  
Plasma Frequency ◽  
Low Frequency ◽  
Radio Bursts ◽  
Solar Radio Bursts ◽  
Type Iii ◽  
Field Lines ◽  
Harmonic Components ◽  
Electro Magnetic ◽  
Open Magnetic Field

Ground-based observations of Type III bursts made with spectrographs and spectro-polarimeters, at frequencies above the ionospheric cut-off, reveal that most bursts (excluding storm Type IIIs) have fundamental (F) and harmonic (H) structure (Wild et al. 1959; Dulk and Suzuki 1980). An example of F-H bursts is given by Sheridan (1978). Such bursts are produced by streams of electrons travelling along open magnetic field lines and exciting plasma oscillations which are converted to electro-magnetic waves at both the F and H frequencies of the local plasma frequency in the corona.

scholarly journals Solar Radio Bursts at Low Frequencies

1974 ◽  
Vol 57 ◽  
pp. 183-200 ◽  
Author(s):  
J. Fainberg
Keyword(s):  
Solar Radio ◽  
Low Frequency ◽  
Leading Edge ◽  
Emission Region ◽  
The Sun ◽  
Radio Bursts ◽  
Solar Radio Bursts ◽  
Low Frequencies ◽  
Type Iii ◽  
Radio Measurements

Properties of solar radio bursts observed by spacecraft at frequencies below several MHz are reviewed. In this frequency range most of the observed bursts are type III events (associated with particles) but several cases of type II emission (associated with shocks) have been reported. The analyses which lead to emission levels of type III solar bursts out to beyond 1 AU from the Sun also indicate that the low frequency radiation is observed at the harmonic of the emission region plasma frequency. Simultaneous particle and radio measurements imply that the bursts are generated near the leading edge of impulsive streams of solar electrons with energies extending from several hundred keV to several keV. Recent experiments measuring the direction of arrival of the radio emission allow the exciter particles to be tracked along the interplanetary magnetic field from regions near the Sun out to 1 AU.

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