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Anisotropic Radio-wave Scattering and the Interpretation of Solar Radio Emission Observations
Eduard P. Kontar1, Xingyao Chen1,2, Nicolina Chrysaphi1, Natasha L. S. Jeffrey1,3, A. Gordon Emslie4, Vratislav Krupar5,6, Milan Maksimovic7, Mykola Gordovskyy8, and Philippa K. Browning9

Published 2019 October 17 • © 2019. The American Astronomical Society. All rights reserved.
The Astrophysical Journal, Volume 884, Number 2
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Abstract
The observed properties (i.e., source size, source position, time duration, and decay time) of solar radio emission produced through plasma processes near the local plasma frequency, and hence the interpretation of solar radio bursts, are strongly influenced by propagation effects in the inhomogeneous turbulent solar corona. In this work, a 3D stochastic description of the propagation process is presented, based on the Fokker–Planck and Langevin equations of radio-wave transport in a medium containing anisotropic electron density fluctuations. Using a numerical treatment based on this model, we investigate the characteristic source sizes and burst decay times for Type III solar radio bursts. Comparison of the simulations with the observations of solar radio bursts shows that predominantly perpendicular density fluctuations in the solar corona are required, with an anisotropy factor of ~0.3 for sources observed at around 30 MHz. The simulations also demonstrate that the photons are isotropized near the region of primary emission, but the waves are then focused by large-scale refraction, leading to plasma radio emission directivity that is characterized by a half width at half maximum of about 40° near 30 MHz. The results are applicable to various solar radio bursts produced via plasma emission.

/pdf/anisotropic-radio-wave-scattering-and-the-interpretation-of-13q7hm0wes.pdf

Anisotropic Radio-Wave Scattering and the Interpretation of Solar Radio Emission Observations

Coronal mass ejections (CMEs) are believed to be effective in producing shocks in the solar corona and interplanetary space. One of the important signatures of shocks and shock acceleration are Type II solar radio bursts that drift with the shock speed and produce bands of fundamental and higher harmonic plasma radio emission. An intriguing aspect of Type II radio bursts is the occasional split of a harmonic band into thinner lanes, known as band splitting. Here we report a detailed imaging and spectroscopic observation of a CME-driven shock producing band splitting in a Type II burst. Using LOFAR, we examine the spatial and temporal relation of the Type II burst to the associated CME event, use source imaging to calculate the apparent coronal density, and demonstrate how source imaging can be used to estimate projection effects. We consider two widely accepted band-splitting models that make opposing predictions regarding the locations of the true emission sources with respect to the shock front. Our observations suggest that the locations of the upper and lower subband sources are spatially separated by ~0.2 ± 0.05 R ⊙. However, we quantitatively show, for the first time, that such separation is consistent with radio-wave scattering of plasma radio emission from a single region, implying that the split-band Type II sources could originate from nearly cospatial locations. Considering the effects of scattering, the observations provide supporting evidence for the model that interprets the band splitting as emission originating in the upstream and downstream regions of the shock front, two virtually cospatial areas.

/pdf/cme-driven-shock-and-type-ii-solar-radio-burst-band-b9amyg8jul.pdf

CME-driven Shock and Type II Solar Radio Burst Band Splitting

The Radio and Plasma Waves (RPW) instrument on the ESA Solar Orbiter mission is described in this paper. This instrument is designed to measure in-situ magnetic and electric fields and waves from the continuous to a few hundreds of kHz. RPW will also observe solar radio emissions up to 16 MHz. The RPW instrument is of primary importance to the Solar Orbiter mission and science requirements since it is essential to answer three of the four mission overarching science objectives. In addition RPW will exchange on-board data with the other in-situ instruments in order to process algorithms for interplanetary shocks and type III langmuir waves detections.

/pdf/the-solar-orbiter-radio-and-plasma-waves-rpw-instrument-1nhv9v4vui.pdf

The Solar Orbiter Radio and Plasma Waves (RPW) instrument

We analyze the high-energy particle emission from the Sun in two extreme solar particle events, in which protons are accelerated to relativistic energies and can cause a significant signal even in the ground-based particle detectors. Analysis of a relativistic proton event is based on modeling of the particle transport and interaction, from a near-Sun source through the solar wind and the Earth’s magnetosphere and atmosphere to a detector on the ground. This allows us to deduce the time profile of the proton source at the Sun and compare it with observed electromagnetic emissions. The 2 May 1998 event is associated with flare and coronal mass ejection (CME) well observed by the Nan¸cay Radioheliograph, so that the images of radio sources are available. For the 2 November 2003 event, there are available the low-corona images of the CME liftoff obtained at the Mauna Loa Solar Observatory. Those complementary data sets are analyzed jointly with the broadband dynamic radio spectra, EUV images and other data available for both events. We find a common scenario for both eruptions, including the flare’s dual impulsive phase, the CME-launch-associated decimetric-continuum burst, and the late, low-frequency type III radio bursts at the time of the relativistic proton injection into the interplanetary medium. The analysis supports the idea that the two considered events start with emission of relativistic protons previously accelerated during the flare and CME launch, then trapped in large-scale magnetic loops and later released by the expanding CME.

/pdf/investigating-the-origins-of-two-extreme-solar-particle-vyv8k3yr9d.pdf

Investigating the Origins of Two Extreme Solar Particle Events: Proton Source Profile and Associated Electromagnetic Emissions

Radio waves are strongly scattered in the solar wind, so that their apparent sources seem to be considerably larger and shifted than the actual ones. Since the scattering depends on the spectrum of density turbulence, better understanding of the radio wave propagation provides indirect information on the relative density fluctuations $\epsilon=\langle\delta n\rangle/\langle n\rangle$ at the effective turbulence scale length. Here, we have analyzed 30 type III bursts detected by Parker Solar Probe (PSP). For the first time, we have retrieved type III burst decay times $\tau_{\rm{d}}$ between 1 MHz and 10 MHz thanks to an unparalleled temporal resolution of PSP. We observed a significant deviation in a power-law slope for frequencies above 1 MHz when compared to previous measurements below 1 MHz by the twin-spacecraft Solar TErrestrial RElations Observatory (STEREO) mission. We note that altitudes of radio bursts generated at 1 MHz roughly coincide with an expected location of the Alfven point, where the solar wind becomes super-Alfvenic. By comparing PSP observations and Monte Carlo simulations, we predict relative density fluctuations $\epsilon$ at the effective turbulence scale length at radial distances between 2.5$R_\odot$ and 14$R_\odot$ to range from $0.22$ and $0.09$. Finally, we calculated relative density fluctuations $\epsilon$ measured in situ by PSP at a radial distance from the Sun of $35.7$~$R_\odot$ during the perihelion \#1, and the perihelion \#2 to be $0.07$ and $0.06$, respectively. It is in a very good agreement with previous STEREO predictions ($\epsilon=0.06-0.07$) obtained by remote measurements of radio sources generated at this radial distance.

https://iopscience.iop.org/article/10.3847/1538-4365/ab65bd/pdf

Density Fluctuations in the Solar Wind Based on Type III Radio Bursts Observed by Parker Solar Probe

Type III bursts are generated by fast electron beams originated from magnetic reconnection sites of solar flares. As propagation of radio waves in the interplanetary medium is strongly affected by random electron density fluctuations, type III bursts provide us with a unique diagnostic tool for solar wind remote plasma measurements. Here, we performed a statistical survey of 152 simple and isolated type III bursts observed by the twin-spacecraft Solar TErrestrial RElations Observatory mission. We investigated their time–frequency profiles in order to retrieve decay times as a function of frequency. Next, we performed Monte Carlo simulations to study the role of scattering due to random electron density fluctuations on time–frequency profiles of radio emissions generated in the interplanetary medium. For simplification, we assumed the presence of isotropic electron density fluctuations described by a power law with the Kolmogorov spectral index. Decay times obtained from observations and simulations were compared. We found that the characteristic exponential decay profile of type III bursts can be explained by the scattering of the fundamental component between the source and the observer despite restrictive assumptions included in the Monte Carlo simulation algorithm. Our results suggest that relative electron density fluctuations /ne in the solar wind are 0.06–0.07 over wide range of heliospheric distances.

Interplanetary Type III Bursts and Electron Density Fluctuations in the Solar Wind

Coronal mass ejections (CMEs) are large-scale eruptions of magnetized plasma that may cause severe geomagnetic storms if Earth-directed. Here we report a rare instance with comprehensive in situ and remote sensing observa- tions of a CME combining white-light, radio, and plasma measurements from four different vantage points. For the first time, we have successfully applied a radio direction-finding technique to an interplanetary type II burst detected by two identical widely separated radio receivers. The derived locations of the type II and type III bursts are in general agreement with the white light CME recon- struction. We find that the radio emission arises from the flanks of the CME, and are most likely associated with the CME-driven shock. Our work demon- strates the complementarity between radio triangulation and 3D reconstruction techniques for space weather applications.

https://iopscience.iop.org/article/10.3847/2041-8205/823/1/L5/pdf

An analysis of interplanetary solar radio emissions associated with a coronal mass ejection

Very low frequency wave intensity measurements provided by the French low‐altitude DEMETER spacecraft are studied using the principal component analysis (PCA). We focus on both the physical interpretation of the first two principal components and their application to real physical problems. Variations of the first principal component (PC1) coefficients due to the geomagnetic activity and seasonal/longitudinal changes are studied. It is shown that their distribution corresponds to the wave intensity dependences obtained in previous studies. Moreover, the variations of PC1 coefficients around interplanetary shock arrivals are analyzed. The study is performed for fast forward (FF), fast reverse, slow forward, and slow reverse shocks separately. It shows that the most significant effect on the wave intensity is displayed in the FF case. Furthermore, it turns out that the wave intensity variations depend on the wave intensity detected before the shock arrival. Finally, the shock strength and interplanetary magnetic field orientation are also important. The performed analysis shows that PCA can be successfully applied to characterize large data sets of spacecraft measurements by limited sets of numbers—principal component coefficients (typically first one or two are enough), which still maintain a sufficient amount of information.

/pdf/using-principal-component-analysis-to-characterize-the-334w44qizn.pdf

Using Principal Component Analysis to Characterize the Variability of VLF Wave Intensities Measured by a Low‐Altitude Spacecraft and Caused by Interplanetary Shocks

O. Kruparova

Papers

Interplanetary Type III Bursts and Electron Density Fluctuations in the Solar Wind

An analysis of interplanetary solar radio emissions associated with a coronal mass ejection

Using Principal Component Analysis to Characterize the Variability of VLF Wave Intensities Measured by a Low‐Altitude Spacecraft and Caused by Interplanetary Shocks