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We illustrate the radio intensity map at a frequency of 125 MHz for CR 2156, corresponding to 2014 October-November. Our results show a good first-order agreement with the observations by MWA in Mohan & Oberoi (2017).
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Radio observations grant access to a wide range of physical processes through different emission mechanisms. These processes range from thermal and quiescent to eruptive phenomena, such as shock waves and particle beams. We present a new synthetic radio imaging tool that calculates and visualizes the bremsstrahlung radio emission. This tool works c...
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... Figure 3, we show the radio synthetic intensity map as calculated by our simulation in the low-frequency regime and in particular for a frequency of 125 MHz. Recently, Mohan & Oberoi (2017) used the Murchison Widefield Array 8 (MWA) to obtain low-frequency radio observations of the Sun. ...
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... is playing a major role in the low-frequency regime. As shown in Figure 3, the solar disk appears more extended in low frequencies due to the critical surface being at a higher altitude. Please note that only our synthetic results are illustrated in Figure 3. ...
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... shown in Figure 3, the solar disk appears more extended in low frequencies due to the critical surface being at a higher altitude. Please note that only our synthetic results are illustrated in Figure 3. Our results show a good first-order agreement with the observations illustrated in Figure 6 of Mohan & Oberoi (2017). ...
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... are using synoptic magnetograms, which are time averaged over the time span of an entire CR. The simulation in Figure 3 therefore captures a monthly average distribution of the radio features on the solar disk, while the MWA observations of Mohan & Oberoi (2017) also capture their dynamic behavior. ...
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... Figure 5, we show four different synthetic images at frequencies of 125, 250, 300, and 500 MHz, after 10 (best CME volume illustration) and 38 minutes (best illustration of point-source peaks) from the CME initiation time. These bremsstrahlung CME images can be compared with the radio CME observed by Bastian et al. (2001; see their Figure 3), even though those authors attribute that radio emission to nonthermal synchrotron radiation. More specifically, the top part of the CME is less radio intense, while the footpoints appear to be more radio-loud. ...
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Citations
... We use the utility presented in Moschou et al. (2018) for generating synthetic radio images of the stellar corona from our MHD wind solutions. This algorithm captures the process of radio emission from the stellar corona due to bremsstrahlung emission and the way it propagates through the circumstellar medium of the nonuniform density (Benkevitch et al. 2010(Benkevitch et al. , 2012Moschou et al. 2018). ...
... We use the utility presented in Moschou et al. (2018) for generating synthetic radio images of the stellar corona from our MHD wind solutions. This algorithm captures the process of radio emission from the stellar corona due to bremsstrahlung emission and the way it propagates through the circumstellar medium of the nonuniform density (Benkevitch et al. 2010(Benkevitch et al. , 2012Moschou et al. 2018). During the propagation, radio waves suffer refraction. ...
... The higher the frequency is, the lower the drop we see. This high-frequency blocking trend follows the logic that the high-frequency emissions come from the hot, dense regions at the low corona where the magnetic field is stronger (e.g., Cohen et al. 2018;Moschou et al. 2018), which are simply shaded by the planet. We notice a slight increase in the intensity when the planet starts to move out of or in transit. ...
We perform a series of time-dependent magnetohydrodynamic simulations of the HD 189733 star–planet system in order to predict radio transit modulations due to the interaction between the stellar wind and planetary magnetic field. The simulation combines a model for the stellar corona and wind with an exoplanet that is orbiting the star in a fully dynamic, time-dependent manner. Our simulations generate synthetic radio images that enable us to obtain synthetic radio light curves in different frequencies. We find a clear evidence for the planetary motion in the radio light curves. Moreover, we find specific repeated features in the light curves that are attributed to the passage of the planetary magnetosphere in front of the star during transit. More importantly, we find a clear dependence in magnitude and phase of these light-curve features on the strength of the planetary magnetic field. Our work demonstrates that if radio transits could be observed, they could indeed provide information about the magnetic field strength of the transiting exoplanet. Future work to parameterize these light-curve features and their dependence on the planetary field strength would provide tools to search for these features in radio observation data sets. As we only consider the thermal radio emission from the host star for our study, very sensitive radio interferometers are necessary to detect these kinds of planetary transits in radio.
... We use the utility presented in Moschou et al. (2018) for generating synthetic radio images of the stellar corona from our MHD wind solutions. This algorithm captures the process of radio emission from the stellar corona due to Bremstrahulong emission and the way it propagates through the circumstellar medium of the non-uniform density (Benkevitch et al. 2010(Benkevitch et al. , 2012Moschou et al. 2018). ...
... We use the utility presented in Moschou et al. (2018) for generating synthetic radio images of the stellar corona from our MHD wind solutions. This algorithm captures the process of radio emission from the stellar corona due to Bremstrahulong emission and the way it propagates through the circumstellar medium of the non-uniform density (Benkevitch et al. 2010(Benkevitch et al. , 2012Moschou et al. 2018). During the propagation, radio waves suffer refraction. ...
... The higher the frequency is, the lower the drop we see. This high frequency blocking trend follows the logic that the high frequency emissions come from the hot, dense regions at the low corona where the magnetic field is stronger (e.g., Cohen et al. 2018;Moschou et al. 2018), which are simply shaded by the planet. We notice a slight increase in the intensity when planet starts to move out or in of transit. ...
We perform a series of time dependent Magnetohydrodynamic simulations of the HD 189733 star-planet system in order to predict radio transit modulations due to the interaction between the stellar wind and planetary magnetic field. The simulation combines a model for the stellar corona and wind with an exoplanet that is orbiting the star in a fully dynamic, time-dependent manner. Our simulations generate synthetic radio images that enable us to obtain synthetic radio lightcurves in different frequencies. We find a clear evidence for the planetary motion in the radio light curves. Moreover, we find specific repeated features in the light curves that are attributed to the passage of the planetary magnetosphere in front of the star during transit. More importantly, we find a clear dependence in the magnitude and phase of these lightcurve features on the strength of the planetary magnetic field. Our work demonstrates that if radio transits could be observed, they could indeed provide information about the magnetic field strength of the transiting exoplanet. Future work to parameterize these lightcurve features and their dependence on the planetary field strength would provide tools to search for these features in radio observations datasets. As we only consider the thermal radio emission from the host star for our study, very sensitive radio interferometers are necessary to detect these kinds of planetary transit in radio.
... [27]. Synthetic radio observations of stellar coronal radio emissions [28] and propagation through space weather in stellar environments will also be influenced by permittivity gradient based depolarization [29]. Astronomical signals such as observed with low frequency radio arrays for extra-galactic sources may depolarize along their entire path and in the last moments of their journey to the detector. ...
Recent interest in 3D vectorial sensors requires development of vectorial propagation methods rather than scalar wave equation approaches. We derive a vector wave equation from Maxwells equations for a medium which has an inhomogeneous dielectric permittivity dominated by variation along one dimension. It is well known that the electric field components decouple for homogeneous media. However, one-dimensional permittivity variations yield an upper triangular system of scalar wave equations with the wave polarization component parallel to the inhomogeneous direction/axis acting as a forcing term for the orthogonal components. The main implication is that waves with polarization oriented parallel to the permittivity gradient will act as a forcing term and excite other polarization components and thus induce depolarization. Contemporary studies treat the permittivity as a constant when deriving a wave equation or paraxial approximation, and then re-introduce via inhomogeneous wave speed, variable permittivity, thus missing important terms and physical mechanisms in their resulting equations. Contemporary studies neglect the term in the Maxwell vector wave equation responsible for this effect. Application of the EM propagation depolarization effect is demonstrated numerically for an air-sea interface evaporation duct with a 500 MHz source.
... Having confirmed recent results of Soja et al. (2014Soja et al. ( , 2015Soja et al. ( , 2018, we also note that the AUA020 and AOV022 sessions show superiority of the AWSoM model over the power law, and suggest that the power law is not an accurate representation of the solar corona electron density and that further analysis can significantly benefit from more sophisticated modern 3D solar corona models. The AWSoM model results were successfully compared to different kinds of observations in multiple works (see e.g., Oran et al. 2013;Sokolov et al. 2013;Moschou et al. 2018); VLBI observations of quasars provide an additional source of validation. A more careful analysis is needed in the presence of strong CMEs, where AWSoM data suggests extremely high electron density in certain regions. ...
The Sun’s corona has interested researchers for multiple reasons, including the search for a solution to the famous coronal heating problem and a purely practical consideration of predicting geomagnetic storms on Earth. There exist numerous different theories regarding the solar corona; therefore, it is important to be able to perform comparative analysis and validation of those theories. One way that could help us move toward the answers to those problems is the search for observational methods that could obtain information about the physical properties of the solar corona and provide means for comparing different solar corona models. In this work we present evidence that very long baseline interferometry (VLBI) observations are, in certain conditions, sensitive to the electron density of the solar corona and are able to distinguish between different electron density models, which makes the technique of VLBI valuable for solar corona investigations. Recent works on the subject used a symmetric power-law model of the electron density in solar plasma; in this work, an improvement is proposed based on a three-dimensional numerical model.
... Also available are results of validation of the AWSoM model against observational data. For instance, van der Holst et al. (2014) have compared simulated multi-wavelength extreme ultraviolet (EUV) images with observations from Solar Terrestrial Relations Observatory (STEREO) and Solar Dynamics Observatory (SDO); Oran et al. (2013) have compared AWSoM model results with remote observations in the extreme-ultraviolet and X-ray ranges from STEREO, Solar and Heliospheric Observatory, and Hinode spacecraft and with in situ measurements by Ulysses; Moschou et al. (2018) have compared predictions of the bremsstrahlung radio emission from the AWSoM model with observations of the Sun at MHz and GHz frequencies. All the studies have found the AWSoM model predictions to be in agreement with observational data. ...
... The AWSoM model results were successfully compared to different kinds of observations in multiple works (see e.g. Sokolov et al. 2013;Oran et al. 2013;Moschou et al. 2018); VLBI observations of quasars provide an additional source of validation. A more careful analysis is needed in presence of strong coronal mass ejections (CMEs), where AWSoM data suggests extremely high electron density in certain regions. ...
The Sun's corona has interested researchers for multiple reasons, including the search for solution for the famous coronal heating problem and a purely practical consideration of predicting geomagnetic storms on Earth. There exist numerous different theories regarding the solar corona; therefore, it is important to be able to perform comparative analysis and validation of those theories. One way that could help us move towards the answers to those problems is the search for observational methods that could obtain information about the physical properties of the solar corona and provide means for comparing different solar corona models. In this work we present evidence that VLBI observations are, in certain conditions, sensitive to the electron density of the solar corona and are able to distinguish between different electron density models, which makes the technique of VLBI valuable for solar corona investigations. Recent works on the subject used a symmetric power-law model of the electron density in solar plasma; in this work, an improvement is proposed based on a 3D numerical model.
... In general, in order to estimate synthetic radio fluxes that can be directly compared to observations, a number of additional factors are required, including source geometry considerations, signal propagation effects, and the specific details of various non-thermal emission mechanisms, as discussed by Moschou et al. (2018) and references therein. ...
Observations from the Kepler mission have revealed frequent superflares on young and active solar-like stars. Superflares result from the large-scale restructuring of stellar magnetic fields, and are associated with the eruption of coronal material (a coronal mass ejection, or CME) and energy release that can be orders of magnitude greater than those observed in the largest solar flares. These catastrophic events, if frequent, can significantly impact the potential habitability of terrestrial exoplanets through atmospheric erosion or intense radiation exposure at the surface. We present results from numerical modeling designed to understand how an eruptive superflare from a young solar-type star, κ1Cet, could occur and would impact its astrospheric environment. Our data-inspired, three-dimensional magnetohydrodynamic modeling shows that global-scale shear concentrated near the radial-field polarity inversion line can energize the closed-field stellar corona sufficiently to power a global, eruptive superflare that releases approximately the same energy as the extreme 1859 Carrington event from the Sun. We examine proxy measures of synthetic emission during the flare and estimate the observational signatures of our CME-driven shock, both of which could have extreme space-weather impacts on the habitability of any Earth-like exoplanets. We also speculate that the observed 1986 Robinson-Bopp superflare from κ1Cet was perhaps as extreme for that star as the Carrington flare was for the Sun.
... Most of these models assume a uniform source. More recently, Moschou et al. (2018) used global simulations of three-dimensional (3D) MHD via the block-adaptive-treesolarwind-roe-upwind-scheme (BATS-R-US) code (Powell et al. 1999) and performed ray tracing in 3D space. Otherwise, the inhomogeneous magnetic structure in the source has posed a problem in spectral inversions. ...
... Specifically, we expressed the source area, A, on the projected sky-plane and its thickness, L, along the line of sight in the form of scaling laws of magnetic field, B, as: 2 relevant parameters including magnetic field and adjust them by mi calculated and the observed two-dimensional (2D) maps of microwa assume a uniform source. More recently, Moschou et al. (2018) used gl (3D) MHD via the block-adaptive-tree-solarwind-roe-upwind-scheme 1999) and performed ray tracing in 3D space. Otherwise, the inhom source has posed a problem in spectral inversions. ...
Solar microwave bursts carry information about the magnetic field in the emitting region as well as about electrons accelerated during solar flares. While this sensitivity to the coronal magnetic field must be a unique advantage of solar microwave burst observations, it also adds a complexity to spectral analysis targeted to electron diagnostics. This paper introduces a new spectral analysis procedure in which the cross-section and thickness of a microwave source are expressed as power-law functions of the magnetic field so that the degree of magnetic inhomogeneity can systematically be derived. We applied this spectral analysis tool to two contrasting events observed by the Owens Valley Solar Array: the SOL2003-04-04T20:55 flare with a steep microwave spectrum and the SOL2003-10-19T16:50 flare with a broader spectrum. Our analysis shows that the strong flare with the broader microwave spectrum occurred in a region of highly inhomogeneous magnetic field and vice versa. We further demonstrate that such source properties are consistent with the magnetic field observations from the Michelson Doppler Imager instrument onboard the Solar and Heliospheric Observatory (SOHO) spacecraft and the extreme ultraviolet imaging observations from the SOHO extreme ultraviolet imaging telescope. This spectral inversion tool is particularly useful for analyzing microwave flux spectra of strong flares from magnetically complex systems.
Newly available high-resolution imaging of solar radio emission at many closely spaced frequencies and times provides new physical insight into the processes, structure, and dynamics of the solar atmosphere. The observational advances have spurred renewed interest in topics dating from the early days of solar radio astronomy and have led to considerable advances in our knowledge. Highlights of recent advances include the following: ▪ Quantitatively measuring the dynamic magnetic field strength, particle acceleration, and hot thermal plasma at the heart of solar flares and hinting at the processes that relate them. ▪ Resolving in space and time the energization and transport of electrons in a wide range of contexts. ▪ Mapping the magnetized thermal plasma structure of the solar chromosphere and corona over a substantial range of heights in active and quiet regions of the Sun.
This review explains why solar radio imaging spectroscopy is so powerful, describes the body of recent results, and outlines the future work needed to fully realize its potential. The application of radio imaging spectroscopy to stars and planets is also briefly reviewed.
Expected final online publication date for the Annual Review of Astronomy and Astrophysics, Volume 61 is August 2023. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.
We present an analytical model for the free-free emission at radio wavelengths produced by the interaction of a Coronal Mass Ejection with the ambient solar wind. Using our previous models, we show that a dense shell bounded by two shock fronts is formed from this interaction, whose dynamical evolution can be calculated based on considerations of the mass and momentum conservation for the shell. This structure undergoes two stages in its dynamical evolution: (1) A first one of constant velocity, when the shell is bounded by two shock fronts, and (2) a second one, in which a one-shock structure (the leading shock) is decelerated. Here, we compute the emission produced by these shocks, and present a comparison with synthetic observations of the 7 March 2011 Coronal Mass Ejection. Our simplified model gives a physical insight into the free-free emission produced by shocks of Coronal Mass Ejections.
We investigate the wind of λ And, a solar-mass star that has evolved off the main sequence becoming a subgiant. We present spectropolarimetric observations and use them to reconstruct the surface magnetic field of λ And. Although much older than our Sun, this star exhibits a stronger (reaching up to 83 G) large-scale magnetic field, which is dominated by the poloidal component. To investigate the wind of λ And, we use the derived magnetic map to simulate two stellar wind scenarios, namely a ‘polytropic wind’ (thermally driven) and an ‘Alfven-wave-driven wind’ with turbulent dissipation. From our 3D magnetohydrodynamics simulations, we calculate the wind thermal emission and compare it to previously published radio observations and more recent Very Large Array observations, which we present here. These observations show a basal sub-mJy quiescent flux level at ∼5 GHz and, at epochs, a much larger flux density (>37 mJy), likely due to radio flares. By comparing our model results with the radio observations of λ And, we can constrain its mass-loss rate $\dot{M}$. There are two possible conclusions. (1) Assuming the quiescent radio emission originates from the stellar wind, we conclude that λ And has $\dot{M} \simeq 3 \times 10^{-9}$ M⊙ yr −1, which agrees with the evolving mass-loss rate trend for evolved solar-mass stars. (2) Alternatively, if the quiescent emission does not originate from the wind, our models can only place an upper limit on mass-loss rates, indicating that $\dot{M} \lesssim 3 \times 10^{-9}$ M⊙ yr −1.
