65 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 âŒ1 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 Ύ ⌠3 in a
CME magnetic field of 4.4 G at a height of 1.3 Rïżœ, 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
Evolution of the Alfvén Mach number associated with a coronal mass ejection shock
The Sun regularly produces large-scale eruptive events, such as coronal mass ejections (CMEs) that can drive shock waves through the solar corona. Such shocks can result in electron acceleration and subsequent radio emission in the form of a type II radio burst. However, the early-phase evolution of shock properties and its relationship to type II burst evolution is still subject to investigation. Here we study the evolution of a CME-driven shock by comparing three commonly used methods of calculating the Alfvén Mach number (MA), namely: shock geometry, a comparison of CME speed to a model of the coronal Alfvén speed, and the type II bandsplitting method. We applied the three methods to the 2017 September 2 event, focusing on the shock wave observed in extreme
ultraviolet by the Solar Ultraviolet Imager on board GOES-16, in white-light by the Large Angle and Spectrometric Coronagraph on board SOHO, and the type II radio burst observed by the Irish Low Frequency Array. We show that the three different methods of estimating shock MA yield consistent results and provide a means of relating shock property evolution to the type II emission duration. The type II radio emission emerged from near the nose of the CME when MA was in the range 1.4â2.4 at a heliocentric distance of âŒ1.6 Râ. The emission ceased when the CME nose reached âŒ2.4 Râ, despite an increasing AlfvĂ©n Mach number (up to 4). We suggest the radio emission cessation is due to the lack of quasi-perpendicular geometry at this altitude, which inhibits efficient electron acceleration and subsequent radio emission
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
Imaging a large coronal loop using type U solar radio burst interferometry
Solar radio U-bursts are generated by electron beams traveling along closed
magnetic loops in the solar corona. Low-frequency ( 100 MHz) U-bursts serve
as powerful diagnostic tools for studying large-sized coronal loops that extend
into the middle corona. However, the positive frequency drift component
(descending leg) of U-bursts has received less attention in previous studies,
as the descending radio flux is weak. In this study, we utilized LOFAR
interferometric solar imaging data from a U-burst that has a significant
descending leg component, observed between 10 to 90 MHz on June 5th, 2020. By
analyzing the radio source centroid positions, we determined the beam
velocities and physical parameters of a large coronal magnetic loop that
reached just about 1.3 in altitude. At this altitude, we found
the plasma temperature to be around 1.1 MK, the plasma pressure around 0.20
, and the minimum magnetic field strength around 0.07 G. The
similarity in physical properties determined from the image suggests a
symmetric loop. The average electron beam velocity on the ascending leg was
found to be 0.21 c, while it was 0.14 c on the descending leg. This apparent
deceleration is attributed to a decrease in the range of electron energies that
resonate with Langmuir waves, likely due to the positive background plasma
density gradient along the downward loop leg
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
A WORKFLOW-ORIENTED APPROACH TO PROPAGATION MODELS IN HELIOPHYSICS
The Sun is responsible for the eruption of billions of tons of plasma andthe generation of near light-speed particles that propagate throughout the solarsystem and beyond. If directed towards Earth, these events can be damaging toour tecnological infrastructure. Hence there is an effort to understand the causeof the eruptive events and how they propagate from Sun to Earth. However, thephysics governing their propagation is not well understood, so there is a need todevelop a theoretical description of their propagation, known as a PropagationModel, in order to predict when they may impact Earth. It is often difficultto define a single propagation model that correctly describes the physics ofsolar eruptive events, and even more difficult to implement models capable ofcatering for all these complexities and to validate them using real observational data.In this paper, we envisage that workflows offer both a theoretical andpractical framerwork for a novel approach to propagation models. We definea mathematical framework that aims at encompassing the different modalitieswith which workflows can be used, and provide a set of generic building blockswritten in the TAVERNA workflow language that users can use to build theirown propagation models. Finally we test both the theoretical model and thecomposite building blocks of the workflow with a real Science Use Case that wasdiscussed during the 4th CDAW (Coordinated Data Analysis Workshop) eventheld by the HELIO project. We show that generic workflow building blocks canbe used to construct a propagation model that succesfully describes the transitof solar eruptive events toward Earth and predict a correct Earth-impact tim
Technology Challenges of SURROUND: A Constellation of Small Satellites Around the Sun for Tracking Solar Radio Bursts
The SURROUND mission proposes the operational monitoring and forecasting of space weather events using a constellation of five small satellites in orbit around the Sun. This unique mission concept would enable the localisation and tracking of solar events with unprecedented accuracy. The small payload combined with high launch requirements makes this an ideal candidate mission for a distributed constellation of small spacecraft and provides an opportunity for technical development in the areas of deep space communication, propulsion, and survivability. The baseline configuration for SURROUND proposes the deployment of spacecraft to Earth-Sun Lagrange points L1, L4, and L5, and two additional spacecraft in Earth leading (\u3c 1AU) and trailing (\u3e 1AU) orbits. However, the development and realisation of such a constellation in deep space presents a number of challenges, particularly when the use of small spacecraft is considered. This paper presents the conceptual design for the proposed SURROUND constellation, principally focusing on the key technical challenges of deploying the spacecraft into their desired locations around the Sun and subsequently communicating the collected data back to Earth. In addition to the key propulsion system and communications architecture trades, additional technological challenges of the mission are also considered, including attitude control, radiation hardening, and electromagnetic compatibility
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