Observations of mode coupling in the solar corona and bipolar noise storms

White, S. M.; Thejappa, G.; Kundu, M. R.

doi:10.1007/bf00146202

Cited by 38 publications

(18 citation statements)

References 40 publications

(19 reference statements)

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“…The brightness temperatures in the two modes are summed and differenced to report Stokes I and V, or one can display V/I, the degree of circular polarization, as in Figures 7, 8. Mode coupling between the x and o modes, which can result in reversal in the sense of circular polarization at points where the magnetic field direction along the line of sight reverses (e.g., White et al, 1992), is not yet included in the FORWARD calculation but will be in future releases. Figure 8 shows an example of a FORWARD radio emission calculation using a threedimensional hydrodynamic active region model (density, vector magnetic field and temperature) with thermal conduction and radiative cooling (Lionello et al, 2013).…”

Section: Radio Emission: Thermal Bremstrahllung and Gyroresonancementioning

confidence: 99%

FORWARD: A Toolset for Multiwavelength Coronal Magnetometry

Gibson

Kucera

White

et al. 2016

Front. Astron. Space Sci.

101

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Determining the 3D coronal magnetic field is a critical, but extremely difficult problem to solve. Since different types of multiwavelength coronal data probe different aspects of the coronal magnetic field, ideally these data should be used together to validate and constrain specifications of that field. Such a task requires the ability to create observable quantities at a range of wavelengths from a distribution of magnetic field and associated plasma-i.e., to perform forward calculations. In this paper we describe the capabilities of the FORWARD SolarSoft IDL package, a uniquely comprehensive toolset for coronal magnetometry. FORWARD is a community resource that may be used both to synthesize a broad range of coronal observables, and to access and compare synthetic observables to existing data. It enables forward fitting of specific observations, and helps to build intuition into how the physical properties of coronal magnetic structures translate to observable properties. FORWARD can also be used to generate synthetic test beds from MHD simulations in order to facilitate the development of coronal magnetometric inversion methods, and to prepare for the analysis of future large solar telescope data.

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Section: Radio Emission: Thermal Bremstrahllung and Gyroresonancementioning

confidence: 99%

FORWARD: A Toolset for Multiwavelength Coronal Magnetometry

Gibson

Kucera

White

et al. 2016

Front. Astron. Space Sci.

101

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“…We therefore expect sources 1, 2, and 3 to be left circularly polarized, but not sources 4 and 5. Because the emission from these sources must pass through the neutral line above the active region on its path to us, we assume that their polarization is reversed there due to mode coupling (White, Thejappa, & Kundu 1992).…”

Section: Radio Polarizationmentioning

confidence: 99%

Quasi‐periodic Pulsations in a Solar Microwave Burst

Grechnev

White

Kundu

2003

ApJ

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Quasi-periodic pulsations in solar flares can provide important information on physical conditions in flaring regions. In this paper, we study a microwave burst that showed deep quasi-periodic pulsations. The most dramatic feature of this event has been discussed by Asai and coworkers. In the second of four bursts during the flare, strongly modulated pulsations appear in radio images from the eastern end of a long loop and in hard X-rays from the western end of the loop. We show, in addition, that (1) at least five distinct radio sources with very different time profiles can be identified, including emission from the long loop connecting the modulated radio and X-ray sources; (2) substructure is also present in the radio emission from the eastern end of the long loop during the first burst of the flare, but with timescales shorter than in the second burst; (3) radio modulations are seen at the western end of the loop during the second burst but at a level some 20 times weaker than at the eastern end; (4) these radio modulations at the western end of the loop, like the hard X-ray modulations at the same location, appear to lead the modulations at the eastern end by about 0.5 s, but all have the same period. The period of the modulation can be explained by MHD oscillations of the loop %120 00 long connecting the sources: both oscillations that change the magnetic field strength in the loop, such as propagating fast-mode waves, and torsional oscillations that change the direction of the magnetic field in the loop can explain the observed properties of the modulation of the radio emission. An impulsive reconnection episode is a plausible source of oscillating fast-mode waves and is consistent with some other aspects of the event. However, it is difficult to reconcile the strength of the radio modulations at the eastern end of the loop with their delay relative to the emissions at the western end, where the modulation is observed to be much weaker. If the electrons originate at the western end where the main energy release seems to occur, and the modulation is imposed on them there before they propagate 10 10 cm to the other end of the loop, any effects due to a spread in electron energies or pitch angles would lead to a spread in propagation times that should smooth out the modulation of the radio emission from the remote source, as should any trapping of electrons in the loop: yet the radio emission from the eastern end of the loop shows much stronger modulation than the radio and hard X-ray emission from the western end of the loop.

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“…The dashed line is meant to be schematic only but agrees quanti-tatively with other estimates. For example, the quiet Sun spectrum is observed to have a brightness temperature of-5 x 10 s K at 327 MHz [e.g., White et al, 1992] and should continue to decline at lower frequencies [e.g., Wang et al, 1987] due to the density dependence of the two relevant characteristic frequencies, •pe and %= • [see Gary and Hurford, 1989]. For comparison, we plot the range of brightness temperature measurements at a few relevant frequencies from Lantos [1980] and Wang et al [1987].…”

Section: Free-free Emissionmentioning

confidence: 99%

On the feasibility of imaging coronal mass ejections at radio wavelengths

Bastian¹,

Gary²

1997

J. Geophys. Res.

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Abstract. Coronal mass ejections (CMEs) can have a profound impact on the interplanetary medium and the near-Earth environment. We discuss the feasibility of detecting coronal mass ejections at radio wavelengths with a ground-based instrument. In particular, we explore the possibility that a radio telescope employing Fourier synthesis imaging techniques can detect thermal bremsstrahlung emission from CMEs. Using a simulated database from such a telescope, we explore three detection schemes: direct detection, an approximate differential detection scheme, and an "exact" differential detection scheme. We conclude that thermal bremsstrahlung emission from CMEs can be detected by such a telescope provided differential techniques are employed. While the approximate differential detection scheme may be sufficient for CMEs viewed near the solar limb, detection of CMEs against the solar disk may require the more sensitive exact differential scheme. The detection and imaging of nonthermal radio emissions from CMEs is also discussed. IntroductionA coronal mass ejection (CME) is an eruption of coronal magnetic field and plasma from the Sun into interplanetary space. CMEs can have a profound influence on the interplanetary medium and the near-Earth environment. The relationship between CMEs, flares, prominence eruptions, coronal and interplanetary shocks, and various kinds of radio bursts is complex and is an extremely active area of research. In white light (as observed by a space-based coronagraph, for example), CMEs typically show a three-part structure composed of a relatively fast, bright, leading arc of emission from material at coronal temperatures, followed by a dark cavity, and slower moving, dense, cool (prominence) material. In soft X rays (SXRs), eruptive morphologies have been identified with overlying CMEs, but no direct counterpart to the white light CME has been conclusively identified [Hudson et al., 1996].In this paper we investigate the detectability of the radio counterpart to white light CMEs via the thermal bremsstrahlung (flee-free) radiation they emit. In particular, we consider detection schemes best suited to an imaging instrument operating at radio wavelengths. While Rodriguez [1996] has considered the idea of detecting CMEs using radar techniques in the 10-80 MHz range, such techniques probe the upper corona and rely upon the detection of the Doppler-shifted signal reflected from the CME. In contrast, we consider direct detection of the radiation emitted by the CME and by associated phenomena at significantly higher radio frequencies.Observations of CMEs at radio wavelengths offer several advantages over those in other wavelength regimes. Because there is no occulting disk, CMEs can, in principle, be imaged against the disk of the Sun in a manner similar to SXR detec- free-free emission, the temperature and emission measure can be derived and compared with complementary soft X ray and white light observations. If the emission is due to gyrosynchrotron emission from nonthermal electrons entrained ...

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Observations of mode coupling in the solar corona and bipolar noise storms

Cited by 38 publications

References 40 publications

FORWARD: A Toolset for Multiwavelength Coronal Magnetometry

FORWARD: A Toolset for Multiwavelength Coronal Magnetometry

Quasi‐periodic Pulsations in a Solar Microwave Burst

On the feasibility of imaging coronal mass ejections at radio wavelengths

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