Spectropolarimetric Observations of Solar Noise Storms at Low Frequencies

Mugundhan, V.; Ramesh, R.; Kathiravan, C.; Gireesh, G. V. S.; Hegde, Aathira

doi:10.1007/s11207-018-1260-2

Cited by 21 publications

(20 citation statements)

References 39 publications

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“…In addition to the above, we verified the presence/absence of radio burst activities using data obtained with GLOSS, GRASP, and e‐CALLISTO also. Observations with the GRIP were particularly useful to verify the presence/absence of discrete sources of weak nonthermal emission like noise storm continuum on any given day via their circularly polarized radio emission and accordingly select the GRAPH data (see, e.g., McCauley et al, 2019; Mugundhan et al, 2018; Ramesh et al, 2011). Note that observations with a correlation radio interferometer with mutually perpendicular linearly polarized antennas as in GRIP are sensitive for circular polarization measurements (see, e.g., Thompson et al, 2001).…”

Section: Observationsmentioning

confidence: 99%

Low‐Frequency Radio Observations of the “Quiet” Corona During the Descending Phase of Sunspot Cycle 24

Ramesh

Kumari

Kathiravan

et al. 2020

Geophysical Research Letters

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We carried out a statistical study of the "quiet" solar corona during the descending phase of the Sunspot Cycle 24 (i.e., January 2015 to May 2019) using data obtained with the Gauribidanur RAdioheliograPH (GRAPH) at 53 and 80 MHz simultaneously. Our results show that the equatorial (east-west) diameters of the solar corona at the above two frequencies shrunk steadily. The decrease was found to be due to a gradual reduction in the coronal electron density (N e). Independent estimates of N e in the equatorial region of the "background" corona using white-light coronagraph observations indicate a decline consistent with our findings.

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Section: Observationsmentioning

confidence: 99%

Low‐Frequency Radio Observations of the “Quiet” Corona During the Descending Phase of Sunspot Cycle 24

Ramesh

Kumari

Kathiravan

et al. 2020

Geophysical Research Letters

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“…Not all active regions that are productive at other wavelengths produce noise storms, and the non-radio signatures are often scant (Willson, 2005;Iwai et al, 2012;Li et al, 2017), unlike Type II and III bursts, which have obvious associations with CMEs and flares (Cairns et al, 2003;Reid and Ratcliffe, 2014). There is general agreement that both the burst and continuum components of noise storms are produced by plasma emission, largely due to their often high circular polarizations (Aschwanden, 1986;Mugundhan et al, 2018), but what accelerates the electrons is still debated. Small-scale reconnection events (Benz and Wentzel, 1981) or weak shocks associated with upwardpropagating waves (Spicer, Benz, and Huba, 1982) are the two leading ideas, and recent work has favored persistent interchange reconnection between open and closed fields at the boundaries of active regions (Del Zanna et al, 2011;Mandrini et al, 2015) or reconnection driven by moving magnetic features (Bentley et al, 2000;Li et al, 2017).…”

Section: Active Region Noise Storm Sourcesmentioning

confidence: 99%

The Low-Frequency Solar Corona in Circular Polarization

et al. 2019

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We present spectropolarimetric imaging observations of the solar corona at low frequencies (80 -240 MHz) using the Murchison Widefield Array (MWA). These images are the first of their kind, and we introduce an algorithm to mitigate an instrumental artefact by which the total intensity signal contaminates the polarimetric images due to calibration errors. We then survey the range of circular polarization (Stokes V ) features detected in over 100 observing runs near solar maximum during quiescent periods. First, we detect around 700 compact polarized sources across our dataset with polarization fractions ranging from less than 0.5 % to nearly 100 %. These sources exhibit a positive correlation between polarization fraction and total intensity, and we interpret them as a continuum of plasma emission noise storm (Type I burst) continua sources associated with active regions. Second, we report a characteristic "bullseye" structure observed for many low-latitude coronal holes in which a central polarized component is surrounded by a ring of the opposite sense. The central component does not match the sign expected from thermal bremsstrahlung emission, and we speculate that propagation effects or an alternative emission mechanism may be responsible. Third, we show that the large-scale polarimetric structure at our lowest frequencies is reasonably well-correlated with the line-of-sight (LOS) magnetic field component inferred from a global potential field source surface (PFSS) model. The boundaries between opposite circular polarization signs are generally aligned with polarity inversion lines in the model at a height roughly corresponding to that of the radio limb. This is not true at our highest frequencies, however, where the LOS magnetic field direction and polarization sign are often not straightforwardly correlated.

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“…The patches of bright emission during the period ≈05:09 -05:11 UT are typical of type I or noise storm bursts from the solar corona (see for e.g. Iwai et al (2013); Mugundhan et al (2018b)). It is widely believed that the bursts are due to plasma radiation at the fundamental plasma frequency (Melrose 1980;Kai et al 1985).…”

Section: Observationsmentioning

confidence: 99%

Radio, X-ray and extreme-ultraviolet observations of weak energy releases in the `quiet' Sun

Ramesh,

Kathiravan,

Mithun

et al. 2021

Preprint

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We analyzed ground-based low frequency (<100 MHz) radio spectral and imaging data of the solar corona obtained with the facilities in the Gauribidanur observatory during the same time as the very weak soft X-ray flares (sub A-class, flux <10 −7 Wm −2 in the 1 -8 Å wavelength range) from the 'quiet' Sun observed with the X-ray Solar Monitor (XSM) onboard Chandrayaan-2 during the recent solar minimum. Non-thermal type I radio burst activity were noticed in close temporal association with the X-ray events. The estimated brightness temperature (T b ) of the bursts at a typical frequency like 80 MHz is ≈3×10 5 K. Extreme-ultraviolet (EUV) observations at 94 Å with the Atmospheric Imaging Assembly (AIA) onboard the Solar Dynamics Observatory (SDO) revealed a brightening close to the same location and time as the type I radio bursts. As far as we know reports of simultaneous observations of X-ray and/or EUV counterpart to weak transient radio emission at low frequencies from the 'quiet' Sun in particular are rare. Considering this and the fact that low frequency radio observations are sensitive to weak energy releases in the solar atmosphere, the results indicate that coordinated observations of similar events would be useful to understand transient activities in the 'quiet' Sun.

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Spectropolarimetric Observations of Solar Noise Storms at Low Frequencies

Cited by 21 publications

References 39 publications

Low‐Frequency Radio Observations of the “Quiet” Corona During the Descending Phase of Sunspot Cycle 24

Low‐Frequency Radio Observations of the “Quiet” Corona During the Descending Phase of Sunspot Cycle 24

The Low-Frequency Solar Corona in Circular Polarization

Radio, X-ray and extreme-ultraviolet observations of weak energy releases in the `quiet' Sun

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