28 research outputs found
Low frequency radio observations of bi-directional electron beams in the solar corona
The radio signature of a shock travelling through the solar corona is known
as a type II solar radio burst. In rare cases these bursts can exhibit a fine
structure known as `herringbones', which are a direct indicator of particle
acceleration occurring at the shock front. However, few studies have been
performed on herringbones and the details of the underlying particle
acceleration processes are unknown. Here, we use an image processing technique
known as the Hough transform to statistically analyse the herringbone fine
structure in a radio burst at 20-90 MHz observed from the Rosse
Solar-Terrestrial Observatory on 2011 September 22. We identify 188 individual
bursts which are signatures of bi-directional electron beams continuously
accelerated to speeds of 0.16. This occurs at a shock
acceleration site initially at a constant altitude of 0.6 R in
the corona, followed by a shift to 0.5 R. The anti-sunward
beams travel a distance of 170 Mm (and possibly further) away
from the acceleration site, while those travelling toward the sun come to a
stop sooner, reaching a smaller distance of 112 Mm. We show that
the stopping distance for the sunward beams may depend on the total number
density and the velocity of the beam. Our study concludes that a detailed
statistical analysis of herringbone fine structure can provide information on
the physical properties of the corona which lead to these relatively rare radio
bursts
Estimation of a coronal mass ejection magnetic field strength using radio observations of gyrosynchrotron radiation
Coronal mass ejections (CMEs) are large eruptions of plasma and magnetic field from the low solar corona into interplanetary space.
These eruptions are often associated with the acceleration of energetic electrons which produce various sources of high intensity
plasma emission. In relatively rare cases, the energetic electrons may also produce gyrosynchrotron emission from within the CME
itself, allowing for a diagnostic of the CME magnetic field strength. Such a magnetic field diagnostic is important for evaluating the
total magnetic energy content of the CME, which is ultimately what drives the eruption. Here, we report on an unusually large source
of gyrosynchrotron radiation in the form of a type IV radio burst associated with a CME occurring on 2014-September-01, observed
using instrumentation from the Nançay Radio Astronomy Facility. A combination of spectral flux density measurements from the
Nançay instruments and the Radio Solar Telescope Network (RSTN) from 300 MHz to 5 GHz reveals a gyrosynchrotron spectrum
with a peak flux density at ∼1 GHz. Using this radio analysis, a model for gyrosynchrotron radiation, a non-thermal electron density
diagnostic using the Fermi Gamma Ray Burst Monitor (GBM) and images of the eruption from the GOES Soft X-ray Imager (SXI),
we were able to calculate both the magnetic field strength and the properties of the X-ray and radio emitting energetic electrons within
the CME. We find the radio emission is produced by non-thermal electrons of energies >1 MeV with a spectral index of δ ∼ 3 in a
CME magnetic field of 4.4 G at a height of 1.3 R�, while the X-ray emission is produced from a similar distribution of electrons but
with much lower energies on the order of 10 keV. We conclude by comparing the electron distribution characteristics derived from
both X-ray and radio and show how such an analysis can be used to define the plasma and bulk properties of a CME
A Statistical Analysis of the Solar Phenomena Associated with Global EUV Waves
Solar eruptions are the most spectacular events in our solar system and are
associated with many different signatures of energy release including solar
flares, coronal mass ejections, global waves, radio emission and accelerated
particles. Here, we apply the Coronal Pulse Identification and Tracking
Algorithm (CorPITA) to the high cadence synoptic data provided by the Solar
Dynamic Observatory (SDO) to identify and track global waves observed by SDO.
164 of the 362 solar flare events studied (45%) are found to have associated
global waves with no waves found for the remaining 198 (55%). A clear linear
relationship was found between the median initial velocity and the acceleration
of the waves, with faster waves exhibiting a stronger deceleration (consistent
with previous results). No clear relationship was found between global waves
and type II radio bursts, electrons or protons detected in-situ near Earth.
While no relationship was found between the wave properties and the associated
flare size (with waves produced by flares from B to X-class), more than a
quarter of the active regions studied were found to produce more than one wave
event. These results suggest that the presence of a global wave in a solar
eruption is most likely determined by the structure and connectivity of the
erupting active region and the surrounding quiet solar corona rather than by
the amount of free energy available within the active region.Comment: 33 pages, 6 figures, 1 table. Accepted for publication in Solar
Physic
LOFAR observations of radio burst source sizes and scattering in the solar corona
Low frequency radio wave scattering and refraction can have a dramatic effect
on the observed size and position of radio sources in the solar corona. The
scattering and refraction is thought to be due to fluctuations in electron
density caused by turbulence. Hence, determining the true radio source size can
provide information on the turbulence in coronal plasma. However, the lack of
high spatial resolution radio interferometric observations at low frequencies,
such as with the LOw Frequency ARray (LOFAR), has made it difficult to
determine the true radio source size and level of radio wave scattering. Here
we directly fit the visibilities of a LOFAR observation of a Type IIIb radio
burst with an elliptical Gaussian to determine its source size and position.
This circumvents the need to image the source and then de-convolve LOFAR's
point spread function, which can introduce spurious effects to the source size
and shape. For a burst at 34.76 MHz, we find full width at half maximum (FWHM)
heights along the major and minor axes to be and
, respectively, at a plane of sky heliocentric
distance of 1.75 R. Our results suggest that the level of density
fluctuations in the solar corona is the main cause of the scattering of radio
waves, resulting in large source sizes. However, the magnitude of
may be smaller than what has been previously derived in observations of radio
wave scattering in tied-array images.Comment: 6 pages, 3 figures, accepted for publication in Astronomy &
Astrophysic
Loss-cone instability modulation due to a magnetohydrodynamic sausage mode oscillation in the solar corona
Solar flares often involve the acceleration of particles to relativistic energies and the generation of high-intensity bursts of radio emission. In some cases, the radio bursts can show periodic or quasiperiodic intensity pulsations. However, precisely how these pulsations are generated is still subject to debate. Prominent theories employ mechanisms such as periodic magnetic reconnection, magnetohydrodynamic (MHD) oscillations, or some combination of both. Here we report on high-cadence (0.25 s) radio imaging of a 228 MHz radio source pulsating with a period of 2.3 s during a solar flare on 2014-April-18. The pulsating source is due to an MHD sausage mode oscillation periodically triggering electron acceleration in the corona. The periodic electron acceleration results in the modulation of a loss-cone instability, ultimately resulting in pulsating plasma emission. The results show that a complex combination of MHD oscillations and plasma instability modulation can lead to pulsating radio emission in astrophysical environments.Peer reviewe
Properties and magnetic origins of solar S-bursts
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 17.4-83.1MHz 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 equivalent to 10(4) Jy). The source particle velocities of S-bursts were found to be similar to 0.07 c. S-burst source heights were found to range from 1.3 R-circle dot to 2 R-circle dot. 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.Peer reviewe
Tracking a beam of electrons from the low solar corona into interplanetary space with the Low Frequency Array, Parker Solar Probe and 1 au spacecraft
Type III radio bursts are the result of plasma emission from mildly
relativistic electron beams propagating from the low solar corona into the
heliosphere where they can eventually be detected in situ if they align with
the location of a heliospheric spacecraft. Here we observe a type III radio
burst from 0.1-16 MHz using the Parker Solar Probe (PSP) FIELDS Radio Frequency
Spectrometer (RFS), and from 10-80 MHz using the Low Frequency Array (LOFAR).
This event was not associated with any detectable flare activity but was part
of an ongoing noise storm that occurred during PSP encounter 2. A deprojection
of the LOFAR radio sources into 3D space shows that the type III radio burst
sources were located on open magnetic field from 1.6-3 and originated
from a specific active region near the East limb. Combining PSP/RFS
observations with WIND/WAVES and Solar Terrestrial Relations Observatory
(STEREO)/WAVES, we reconstruct the type III radio source trajectory in the
heliosphere interior to PSP's position, assuming ecliptic confinement. An
energetic electron enhancement is subsequently detected in situ at the STEREO-A
spacecraft at compatible times although the onset and duration suggests the
individual burst contributes a subset of the enhancement. This work shows
relatively small-scale flux emergence in the corona can cause the injection of
electron beams from the low corona into the heliosphere, without needing a
strong solar flare. The complementary nature of combined ground and space-based
radio observations, especially in the era of PSP, is also clearly highlighted
by this study.Comment: 17 pages, 10 figures, Submitted to ApJ, April 15 202
Comprehensive Characterization of Solar Eruptions with Remote and In-Situ Observations, and Modeling : The Major Solar Events on 4 November 2015
Solar energetic particles (SEPs) are an important product of solar activity. They are connected to solar active regions and flares, coronal mass ejections (CMEs), EUV waves, shocks, Type II and III radio emissions, and X-ray bursts. These phenomena are major probes of the partition of energy in solar eruptions, as well as for the organization, dynamics, and relaxation of coronal and interplanetary magnetic fields. Many of these phenomena cause terrestrial space weather, posing multiple hazards for humans and their technology from space to the ground. Since particular flares, shocks, CMEs, and EUV waves produce SEP events but others do not, since propagation effects from the low corona to 1 AU appear important for some events but not others, and since Type II and III radio emissions and X-ray bursts are sometimes produced by energetic particles leaving these acceleration sites, it is necessary to study the whole system with a multi-frequency and multi-instrument perspective that combines both in-situ and remote observations with detailed modeling of phenomena. This article demonstrates this comprehensive approach and shows its necessity by analyzing a trio of unusual and striking solar eruptions, radio and X-ray bursts, and SEP events that occurred on 4 November 2015. These events show both strong similarities and differences from standard events and each other, despite having very similar interplanetary conditions and only two flare sites and CME genesis regions. They are therefore major targets for further in-depth observational studies, and for testing both existing and new theories and models. We present the complete suite of relevant observations, complement them with initial modeling results for the SEPs and interplanetary magnetic connectivity, and develop a plausible scenario for the eruptions. Perhaps controversially, the SEPs appear to be reasonably modelled and evidence points to significant non-Parker magnetic fields. Based on the very limited modeling available, we identify the aspects that are and are not understood, and we discuss ideas that may lead to improved understanding of the SEP, radio, and space-weather events.Peer reviewe
Interferometric imaging of the type IIIb and U radio bursts observed with LOFAR on 22 August 2017
Context. The Sun is the source of different types of radio bursts that are associated with solar flares, for example. Among the most frequently observed phenomena are type III solar bursts. Their radio images at low frequencies (below 100 MHz) are relatively poorly studied due to the limitations of legacy radio telescopes.Aims. We study the general characteristics of types IIIb and U with stria structure solar radio bursts in the frequency range of 20-80 MHz, in particular the source size and evolution in different altitudes, as well as the velocity and energy of electron beams responsible for their generation.Methods. In this work types IIIb and U with stria structure radio bursts are analyzed using data from the LOFAR telescope including dynamic spectra and imaging observations, as well as data taken in the X-ray range (GOES and RHESSI satellites) and in the extreme ultraviolet (SDO satellite).Results. In this study we determined the source size limited by the actual shape of the contour at particular frequencies of type IIIb and U solar bursts in a relatively wide frequency band from 20 to 80 MHz. Two of the bursts seem to appear at roughly the same place in the studied active region and their source sizes are similar. It is different in the case of another burst, which seems to be related to another part of the magnetic field structure in this active region. The velocities of the electron beams responsible for the generation of the three bursts studied here were also found to be different.Peer reviewe