Coronal Turbulence Research Articles

An Unsupervised Machine Learning-based Algorithm for Detecting Weak Impulsive Narrowband Quiet Sun Emissions and Characterizing Their Morphology

The solar corona is extremely dynamic. Every leap in observational capabilities has been accompanied by unexpected revelations of complex dynamic processes. The ever more sensitive instruments now allow us to probe events with increasingly weaker energetics. A recent leap in the low-frequency radio solar imaging ability has led to the discovery of a new class of emissions, namely weak impulsive narrowband quiet Sun emissions (WINQSEs). They are hypothesized to be the radio signatures of coronal nanoflares and could potentially have a bearing on the long standing coronal heating problem. In view of the significance of this discovery, this work has been followed up by multiple independent studies. These include detecting WINQSEs in multiple data sets, using independent detection techniques and software pipelines, and looking for their counterparts at other wavelengths. This work focuses on investigating morphological properties of WINQSEs and also improves upon the methodology used for detecting WINQSEs in earlier works. We present a machine learning-based algorithm to detect WINQSEs, classify them based on their morphology, and model the isolated ones using 2D Gaussians. We subject multiple data sets to this algorithm to test its veracity. Interestingly, despite the expectations of their arising from intrinsically compact sources, WINQSEs tend to be resolved in our observations. We propose that this angular broadening arises due to coronal scattering. Hence, WINQSEs can provide ubiquitous and ever-present diagnostic of coronal scattering (and, in turn, coronal turbulence) in the quiet Sun regions, which has not been possible until date.

Imaging-spectroscopy of a band-split type II solar radio burst with the Murchison Widefield Array

Type II solar radio bursts are caused by magnetohydrodynamic (MHD) shocks driven by solar eruptive events such as coronal mass ejections (CMEs). Often, both fundamental and harmonic bands of type II bursts are split into sub-bands, which are generally believed to be coming from upstream and downstream regions of the shock; however, this explanation remains unconfirmed. Here, we present combined results from imaging analyses of type II radio burst band splitting and other fine structures observed by the Murchison Widefield Array (MWA) and extreme ultraviolet observations from Solar Dynamics Observatory (SDO)/Atmospheric Imaging Assembly (AIA) on 28 September 2014. The MWA provides imaging-spectroscopy in the range 80−300 MHz with a time resolution of 0.5 s and frequency resolution of 40 kHz. Our analysis shows that the burst was caused by a piston-driven shock with a driver speed of ∼112 km s−1 and shock speed of ∼580 km s−1. We provide rare evidence that band splitting is caused by emission from multiple parts of the shock (as opposed to the upstream–downstream hypothesis). We also examine the small-scale motion of type II fine structure radio sources in MWA images, and suggest that this motion may arise because of radio propagation effects from coronal turbulence, and is not due to the physical motion of the shock location. We present a novel technique that uses imaging spectroscopy to directly determine the effective length scale of turbulent density perturbations, which is found to be 1−2 Mm. The study of the systematic and small-scale motion of fine structures may therefore provide a measure of turbulence in different regions of the shock and corona.

Solar Coronal Density Turbulence and Magnetic Field Strength at the Source Regions of Two Successive Metric Type II Radio Bursts

We report spectral and polarimeter observations of two weak, low-frequency (≈85–60 MHz) solar coronal type II radio bursts that occurred on 2020 May 29 within a time interval ≈2 minutes. The bursts had fine structures, and were due to harmonic plasma emission. Our analysis indicates that the magnetohydrodynamic shocks responsible for the first and second type II bursts were generated by the leading edge (LE) of an extreme-ultraviolet flux rope/coronal mass ejection (CME) and interaction of its flank with a neighboring coronal structure, respectively. The CME deflected from the radial direction by ≈25° during propagation in the near-Sun corona. The estimated power spectral density and magnetic field strength (B) near the location of the first burst at heliocentric distance r ≈ 1.35 R ⊙ are ≈2 × 10−3 W2m and ≈1.8 G, respectively. The corresponding values for the second burst at the same r are ≈10−3 W2m and ≈0.9 G. The significant spatial scales of the coronal turbulence at the location of the two type II bursts are ≈62–1 Mm. Our conclusions from the present work are that the turbulence and magnetic field strength in the coronal region near the CME LE are higher compared to the corresponding values close to its flank. The derived estimates of the two parameters correspond to the same r for both the CME LE and its flank, with a delay of ≈2 minutes for the latter.

A study on the solar coronal dynamics during the post-maxima phase of the solar cycle 24 using S-band radio signals from the Indian Mars Orbiter Mission

ABSTRACT Radio signals from India’s Mars Orbiter Mission (MOM) have been used to study turbulence in the solar plasma during the post-maximum phase of solar cycle 24. S-band (2.29 GHz) radio carrier downlink signals from MOM were received at the Indian Deep Space Network, Bangalore, and frequency residuals were spectrally analysed to obtain coronal turbulence spectra at heliocentric distances ranging between 4 and 20 R⊙, corresponding to coronal regions where the solar wind is primarily accelerated. The frequency fluctuation spectrum relates to the turbulence regime in the near-Sun region. The turbulence power spectrum (the temporal spectrum of frequency fluctuations) at smaller heliocentric distances (<10 R⊙) reveals flattening in lower-frequency regions, with a spectral index αf ∼ 0.3−0.5, which corresponds to the solar wind acceleration region. For larger heliocentric distances (>10 R⊙), the curve steepens with a spectral index αf ∼ 0.7−0.8, a value close to 2/3 and indicative of a developed Kolmogorov-type turbulence spectrum. The findings are consistent with earlier results. Plausible explanations to support the theory of coronal heating by magnetohydrodynamic waves and the acceleration of the solar wind are presented. An insight into the feeble maximum of solar cycle 24 is discussed.

Hybrid Dual-Hop RF/FSO Terrestrial-Deep Space Communication System under Solar Scintillation during Superior Solar Conjunction

Free-space optical communication (FSO) technology has wide prospects in deep space exploration, but it will encounter coronal turbulence during superior solar conjunction, and solar scintillation will seriously affect the communication quality. In this paper, we propose a terrestrial–deep space hybrid radio frequency (RF)/FSO system with the hybrid L-pulse position modulation-binary phase shift keying-subcarrier intensity modulation (L-PPM–BPSK–SIM) scheme, where the RF channel of the satellite-terrestrial relay follows the Rayleigh distribution, and the FSO channel of the relay satellite to the deep space probe adopts Gamma–Gamma distribution. Considering the pointing error, the expression of the bit error rate (BER), the outage probability, and the average channel capacity of the hybrid system are derived. In addition, we evaluated the influence of coronal turbulence parameters on the system through amplitude fluctuations. The simulation results demonstrate that the hybrid RF/FSO system improves the BER performance by 10 to 30 times in a deep space environment, and the use of a hybrid modulation can further reduce the BER. The non-Kolmogorov spectral index, outer scale, solar wind density fluctuation factor, and optical wavelength comprehensively affect the BER through amplitude fluctuations. Our research results have potential application value for evaluating the link performance of future deep space communications.

Spectral Diagnostics of Langmuir Coronal Plasma Turbulence Based on the Radio Emission at the Double Plasma Frequency

Spectral Diagnostics of Langmuir Coronal Plasma Turbulence Based on the Radio Emission at the Double Plasma Frequency

Characterising coronal turbulence using snapshot imaging of radio bursts in 80–200 MHz

Context. Metrewave solar type-III radio bursts offer a unique means to study the properties of turbulence across coronal heights. Theoretical models have shown that the apparent intensity and size of the burst sources evolve at sub-second scales under the influence of local turbulence. The properties of the evolution vary with observation frequency. However, observational studies remained difficult due to the lack of high fidelity imaging capabilities at these fine temporal scales simultaneously across wide spectral bands. Aims. I present a spectroscopic snapshot imaging (0.5 s, 160 kHz resolution) study of a type-III burst event across the 80–200 MHz band. By modelling the temporal variability of the source sizes and intensity at every observation frequency, the characteristics of coronal turbulence are studied across a heliocentric height range of ≈1.54–1.75 R⊙. Methods. To understand the morphological evolution of the type-III source, a 2D Gaussian fitting procedure is used. The observed trends in the source area and integrated flux density are analysed in the framework of theoretical and data-driven models. Results. The strength of density fluctuations (δN/N) in the corona is derived as a function of height (R). Combined with the archival low frequency data, δN/N values across ≈1.5–2.2 R⊙ agree within a few factors. The burst decay time (τdecay) and the full width at half maximum of the source showed a power-law dependence with frequency, roughly consistent with the results from data-driven models. However, the values of τdecay across frequencies turned out higher than the expected trend. The intrinsic sizes of the burst source were derived, correcting for scatter broadening. This roughly matched the expected size of flux tubes at the coronal heights explored. I also report the observation of an intrinsic anti-phased pulsation in the area and flux density of the source.

Mixed RF/FSO Deep Space Communication System Under Solar Scintillation Effect

The emerging technology of mixed radio frequency (RF)/free space optical (FSO) communications offers a great deal of merits in seamless channels and has potential for use in deep space exploration. In this article, a new paradigm with the dual-hop mixed RF/FSO system for the deep space communications link during a superior solar conjunction is proposed. The performance of this system, which is composed of an RF link between the Earth and a geosynchronous orbit relay satellite and an FSO link between the satellite and a deep space probe is evaluated for the first time to the best of our knowledge. An asymmetric environment is considered, where the RF link is subjected to Nakagami-m fading and the FSO link is assumed to follow an exponentiated Weibull distribution, which is induced by coronal turbulence. Under the assumption of amplify-and-forward relaying, we first derive the cumulative density function and the probability density function of this deep space mixed RF/FSO system. Thereafter, the closed-form expressions for the system performance—such as the end-to-end outage probability, the average bit error rate for the $M$-ary phase shift keying modulation scheme, and the channel ergodic capacity—are determined in terms of Meijer's G function. Numerical results are provided to validate our theoretical expressions. The results suggest that the performance of our proposed deep space mixed RF/FSO system is vulnerable to some key parameters, but it still outperforms its pure RF and FSO counterparts. Consequently, this study paves the way for constructing a future deep space communication network.

Turbulence transport in the solar corona: Theory, modeling, and Parker Solar Probe

A primary goal of the Parker Solar Probe (PSP) Mission is to answer the outstanding question of how the solar corona plasma is heated to the high temperatures needed for the acceleration of the solar wind. Various heating mechanisms have been suggested, but one that is gaining increasing credence is associated with the dissipation of low frequency magnetohyrodynamic (MHD) turbulence. However, the MHD turbulence models come in several flavors: one in which outwardly propagating Alfvén waves experience reflection from the large-scale flow and density gradients associated with the solar corona, and the resulting counterpropagating Alfvén waves couple nonlinearly to produce quasi-2D turbulence that dissipates and heats the corona, thereby driving the solar wind. The second approach eschews a dominant outward flux of Alfvén waves but argues instead that quasi-2D turbulence dominates the lower coronal plasma and is generated in the constantly upwelling magnetic carpet, experiencing dissipation as it is advected through the corona and into the solar wind, yielding temperatures in the corona that exceed a million degrees. We review the two turbulence models, describe the modeling that has been done, and relate PSP observations to the basic predictions of both models. Although PSP measurements are made in the super-Alfvénic solar wind, the observations are close to the coronal region, thus providing a glimpse into the likely properties of coronal turbulence. Observations of low-frequency MHD turbulence by PSP in the super-Alfvénic solar wind allow us to place constraints on models of the turbulently heated solar corona that drive the supersonic solar wind.

Dual-hop deep space-terrestrial FSO/RF communication under solar scintillation: Performance analysis and challenges

Deep space communication has been a fundamental guarantee for space exploration but encounters solar scintillation during superior solar conjunction. In this paper, a paradigm for deep space-terrestrial free space optical (FSO)/radio frequency (RF) communication is proposed, and the outage probability performance of the dual-hop amplify-and-for-ward (AF) based FSO/RF communication system is investigated. Specifically, the asymmetric dual-hop channel models are considered, where the Gamma-Gamma distribution characterizes the FSO link between the deep space probe and a relay satellite that undergoes coronal turbulence, while the RF link between the satellite and the terrestrial receiver follows the shadowed-Rician fading model due to the shadowing effect. Moreover, the closed-form expressions of the outage probability for both channel state information (CSI)-assisted and fixed gain relay systems are presented with the consideration of multiple antennas equipped on the destination, which provide practical insights to the link performance. Simulation results present the validation of our theoretical expressions and highlight a strategy for the improvement of the outage probability performance for deep space-terrestrial communication. Finally, some open issues and research directions that should be further investigated for the actual deep space communication system during superior solar conjunction are identified.

BER and channel capacity of a deep space FSO communication system using L-PPM-MSK-SIM scheme during superior solar conjunction.

Although free space optical (FSO) communication is a promising technique for deep space communication and it can help in the rapid development of space exploration missions, it encounters coronal turbulence during superior solar conjunction. To improve the bit error rate (BER) performance of FSO communication system under the influence of coronal turbulence, a hybrid modulation scheme, L-PPM-MSK-SIM-which is a combination of pulse position modulation (PPM), minimum shift keying (MSK), and sub-carrier intensity modulation (SIM) techniques-is proposed in this study. Considering various noise sources, both the BER and channel capacity of the communication system are evaluated under the lognormal (LN) turbulence channel. Our simulation results demonstrate that the BER performance with the L-PPM-MSK-SIM scheme is superior to that with L-PPM and BPSK-SIM schemes. In addition, the parameters of the coronal turbulence and FSO communication system have a tremendous influence on the link BER and channel capacity. Moreover, our results also revel that thermal noise is more predominant than the short noise and background noise for the BER performance of deep space FSO communication.

Solar Scintillation Effect for Optical Waves Propagating Through Gamma–Gamma Coronal Turbulence Channels

Optical communication is a promising strategy for deep space exploration but it is susceptible to coronal solar wind turbulence impairments during superior solar conjunction. The variance of amplitude fluctuations caused by coronal turbulence on optical waves propagation is proposed in this paper. Both the generalized non-Kolmogorov coronal turbulence spectrum and the aperture averaging effect are taken into account. The analytic expression of the bit error rate (BER) for free-space optical (FSO) link is then derived based on Gamma–Gamma distribution model under the weak-to-moderate coronal turbulence channels. Numerical evaluations results demonstrate that coronal turbulence with small non-Kolmogorov spectral index, large outer scale, and inner scale has more potential to deduce amplitude fluctuations. With the increment of antenna radius, the amplitude fluctuations decrease obviously. In addition, the variation tendencies of these parameters can further result in small BER. Therefore, the link performance will be improved by decreasing the optical wavelength and enlarging the antenna radius. Our proposed amplitude fluctuations model with aperture averaging effect has potential application for the future deep space FSO communication.

Error performance of deep space optical communication with M-ary pulse position modulation over coronal turbulence channels.

Free space optical (FSO) communication is a promising technology for future deep space exploration, but it encounters coronal turbulence during superior solar conjunction. In this paper, the bit-error rate (BER) performance of deep space FSO communication systems is evaluated for optical waves propagating in the non-Kolmogorov coronal turbulence. By virtue of its high energy efficiency, the pulse position modulation (PPM) technique is adopted to mitigate the influence of turbulence under lognormal distribution channels. The effects of the parameters of the coronal turbulence and of the FSO system, such as the turbulence outer scale, spectral index, symbol number, data bit rate, equivalent load resistor, and average gain, on the BER are investigated and discussed in this paper. In addition, the performance improvement from M-ary PPM and binary phase-shift keying subcarrier intensity modulation is studied. The results of a numerical evaluation illustrate that a deep space FSO communication system with PPM scheme can well be used to mitigate the impact of coronal turbulence.

Effects of Solar Scintillation on Deep Space Communications: Challenges and Prediction Techniques

A reliable deep space communications system plays a key role in extending the range of space communications and has become an indispensable part of space information networks. With the emergence of many deep space exploration projects, it can be envisioned that a new era of research on deep space communications has arrived. However, the performance of deep space communications is severely degraded by solar scintillation, which occurs frequently during superior solar conjunction. In this article, we first review the state of the art of solar scintillation research, including the definition of solar scintillation, influencing factors, assessment methods, and scintillation models. Then an effective solar scintillation prediction model is proposed to overcome the challenges related to solar scintillation for deep space communications. After that, a coronal turbulence channel model is investigated and discussed. Based on this coronal channel model and our proposed solar scintillation model, the link bit error rate performance is further analyzed. Finally, we identify some open problems and research directions that could be investigated for a deep space communications system during superior solar conjunctions, which will support space information networks.

Solar Scintillation Effects on the Deep Space Communication Performance for Radio Wave Propagation Through Non-Kolmogorov Turbulence

Solar scintillation refers to the rapid amplitude fluctuations of the radio signal passing through large-scale coronal irregularities. In this letter, the scintillation index model is first derived to characterize radio wave propagation in coronal turbulence during superior solar conjunction. The proposed model takes into account the non-Kolmogorov turbulence model with the Rytov approximation method. After that, the bit error rate (BER) performance is analyzed based on the proposed scintillation index model, which has instructional significance for deep space communication but still remains largely unexplored. Simulation results demonstrate that the coronal parameters have extensive effects on the scintillation index, and further affect the link BER performance. Our proposed scintillation index model also achieves better agreement with the observation data.

Amplitude fluctuations for optical waves propagation through non-Kolmogorov coronal solar wind turbulence channels.

Optical communication has a great potential for the future deep space communication, while the amplitude fluctuations caused by the coronal solar wind irregularities has been a challenging topic during superior solar conjunction. In this paper, a closed-form amplitude fluctuations expression for optical waves propagation through non-Kolmogorov solar wind turbulence is derived by establishing a generalized coronal turbulence spectrum model. The profound impact of the coronal parameters on the bit error rate (BER) performance of the free space optical system is also investigated based on the derived amplitude fluctuations model. The derived expression allows easy analysis of the evolution of the amplitude fluctuations and, in particular, an understanding of the imposed effects caused by the parameters during the waves propagation. The combined effect of the optical wavelength, non-Kolmogorov spectral index, turbulence outer scale, relative solar wind density fluctuation factor, and link distance on amplitude fluctuations are evaluated. Numerical calculations show that these parameters produce obvious effects on the amplitude fluctuations and the BER. The large optical wavelength can mitigate the influence of the coronal turbulence. Our results have potential applications for evaluating the link performance of the future deep space communication.

Fine Structures of Solar Radio Type III Bursts and Their Possible Relationship with Coronal Density Turbulence

Abstract Solar radio type III bursts are believed to be the most sensitive signatures of near-relativistic electron beam propagation in the corona. A solar radio type IIIb-III pair burst with fine frequency structures, observed by the Low Frequency Array (LOFAR) with high temporal (∼10 ms) and spectral (12.5 kHz) resolutions at 30–80 MHz, is presented. The observations show that the type III burst consists of many striae, which have a frequency scale of about 0.1 MHz in both the fundamental (plasma) and the harmonic (double plasma) emission. We investigate the effects of background density fluctuations based on the observation of striae structure to estimate the density perturbation in the solar corona. It is found that the spectral index of the density fluctuation spectrum is about −1.7, and the characteristic spatial scale of the density perturbation is around 700 km. This spectral index is very close to a Kolmogorov turbulence spectral index of −5/3, consistent with a turbulent cascade. This fact indicates that the coronal turbulence may play the important role of modulating the time structures of solar radio type III bursts, and the fine structure of radio type III bursts could provide a useful and unique tool to diagnose the turbulence in the solar corona.

Theoretical analysis of the angle-of-arrival fluctuations for optical wavext propagation through solar corona turbulence

Optical communication is promising in deep space exploration, but there is the problem of angle-of-arrival fluctuations due to solar scintillation during superior solar conjunction. In this paper, an analytic expression of the angle-of-arrival fluctuations was derived for optical waves propagating through coronal environment by establishing a solar corona non-Kolmogorov turbulence spectrum model. The combined effects of the non-Kolmogorov spectral index, turbulent outer scale, wavelength, and antenna radius are evaluated. Results demonstrate that the spectral index and the turbulent outer scale obviously affect the angle-of-arrival fluctuations. Moreover, minimizing the wavelength and enlarging the receiver aperture can effectively mitigate the angle-of-arrival fluctuations caused by coronal turbulence. It is expected that this work will be beneficial for the investigation of coronal turbulence and deep space optical communication.

A new model of amplitude fluctuations for radio propagation in solar corona during superior solar conjunction

AbstractRadio communication links through the solar corona are disturbed by electron density fluctuations caused by coronal turbulence. This has become a problem for deep space exploration when the spacecraft is near superior conjunction with the Sun. With a forecast of the signal fluctuations the link could be adapted to compensate for these impairments in real time. Motivated by this need, we present a theoretical study of the signal fluctuations including an analytical expression for the amplitude fluctuations. The proposed model includes an anisotropic spectrum of density fluctuations, the solar wind “outer scale,” and the spectral exponent. The performance of this analytical solution is demonstrated by comparison with published data from spacecraft and with other existing analytical methods.

Coronal turbulence and the angular broadening of radio sources – the role of the structure function

The amplitude of density turbulence in the extended solar corona, especially near the dissipation scale, impinges on several problems of current interest. Radio sources observed through the turbulent solar wind are broadened due to refraction by and scattering off density inhomogeneities, and observations of scatter broadening are often employed to constrain the turbulence amplitude. The extent of such scatter broadening is usually computed using the structure function, which gives a measure of the spatial correlation measured by an interferometer. Most such treatments have employed analytical approximations to the structure function that are valid in the asymptotic limits $s \gg l_{i}$ or $s \ll l_{i}$, where $s$ is the interferometer spacing and $l_{i}$ is the inner scale of the density turbulence spectrum. We instead use a general structure function (GSF) that straddles these regimes, and quantify the errors introduced by the use of these approximations. We have included the effects of anisotropic scattering for distant cosmic sources viewed through the solar wind at small elongations. We show that the regimes where the GSF predictions are more accurate than those of the asymptotic expressions are not only of practical relevance, but are where inner scale effects influence estimates of scatter broadening. Taken together, we argue that the GSF should henceforth be used for scatter broadening calculations and estimates of turbulence amplitudes in the solar corona and solar wind.