Weak heating events are frequent and ubiquitous in the solar corona. They derive their energy from the local magnetic field and form a major source of local heating, signatures of which are seen in ...EUV and X-ray bands. Their radio emission arises from various plasma instabilities that lead to coherent radiation, making even a weak flare appear very bright. Hence, the radio observations probe nonequilibrium dynamics, providing complementary information on plasma evolution. However, a robust study of radio emission from a weak event among many simultaneous events requires high dynamic range imaging at subsecond and sub-MHz resolutions owing to its high spectrotemporal variability. Such observations were not possible until recently. This is among the first spatially resolved multiwaveband studies of active region loops hosting transient brightenings (ARTBs) and is dynamically linked to meter-wave type I noise storms. Observations at meter-wave, EUV, and X-ray bands are used, along with magnetogram data. We believe that this is the first spectroscopic radio imaging study of a type I source, the data for which were obtained using the Murchison Widefield Array. We report the discovery of 30 s quasi-periodic oscillations (QPOs) in the radio light curve riding on a coherent baseline flux. The strength of the QPOs and the baseline flux were enhanced during a microflare associated with the ARTB. Our observations suggest a scenario where magnetic stress builds up over an Alfvén timescale (30 s) across the typical magnetic field braiding scale and then dissipates via a cascade of weak reconnection events.
We present coronal density profiles derived from low-frequency (80 – 240 MHz) imaging of three Type III solar radio bursts observed at the limb by the
Murchison Widefield Array
(MWA). Each event is ...associated with a white-light streamer at larger heights and is plausibly associated with thin extreme-ultraviolet rays at lower heights. Assuming harmonic plasma emission, we find average electron densities of 1.8
×
10
8
cm
−3
down to 0.20
×
10
8
cm
−3
at heights of 1.3 to 1.9 R
⊙
. These values represent approximately 2.4 – 5.4× enhancements over canonical background levels and are comparable to the highest streamer densities obtained from data at other wavelengths. Assuming fundamental emission instead would increase the densities by a factor of four. High densities inferred from Type III source heights can be explained by assuming that the exciting electron beams travel along overdense fibers or by radio propagation effects that may cause a source to appear at a larger height than the true emission site. We review the arguments for both scenarios in light of recent results. We compare the extent of the quiescent corona to model predictions to estimate the impact of propagation effects, which we conclude can only partially explain the apparent density enhancements. Finally, we use the time- and frequency-varying source positions to estimate electron beam speeds of between 0.24 and 0.60 c.
Self-assembled wormlike micelles (WLMs) are widely studied in small-molecule surfactants due to their unique ability to break and recombine; however, less is known about the structure and dynamics of ...nonionic polymer WLMs. Here, solutions of seven triblock poloxamers, composed of poly(propylene oxide) (PPO) midblocks and poly(ethylene oxide) (PEO) end blocks, are comprehensively examined to determine the role of poloxamer composition, temperature, and inorganic salt type and concentration on rod formation and subsequent elongation into WLMs. Phase separation and sphere-to-rod transition temperatures were quantified via cloud point measurements and shear rheology, respectively, and corroborated with small-angle neutron scattering (SANS). The local microstructure of resulting rodlike micelles is remarkably similar across poloxamer type and sodium fluoride (NaF) or sodium chloride (NaCl) content. Salt addition reduces transition temperatures, with the most pronounced effects for poloxamers with high PEO molecular weights and PEO fractions. Between these two temperatures, several poloxamers elongate into WLMs, where shear rheology detects increases in viscosity up to 6 orders of magnitude. Despite similar local microstructures, poloxamer identity and salt content impact micelle growth substantially, where large poloxamers with lower PEO fractions exhibit the highest viscosities and longest relaxation times. While sodium fluoride has little impact on micelle growth, increasing NaCl concentration dramatically increases the WLM viscosity and relaxation time. This result is explained by different interactions of each salt with the micelle: whereas NaF interacts primarily with PEO chains, NaCl may also partition to the PPO/PEO interface in low levels, increasing micelle surface tension, scission energy, and contour length.
ABSTRACT We examine a small prominence eruption that occurred on 2014 May 1 at 01:35 UT and was observed by the Interface Region Imaging Spectrometer (IRIS) and the Atmospheric Imaging Assembly (AIA) ...on the Solar Dynamics Observatory. Pre- and post-eruption images were taken by the X-ray Telescope (XRT) on Hinode. Pre-eruption, a dome-like structure exists above the prominence, as demarcated by coronal rain. As the eruption progresses, we find evidence for reconnection between the prominence magnetic field and the overlying field. Fast flows are seen in AIA and IRIS, indicating reconnection outflows. Plane-of-sky flows of 300 km s−1 are observed in the AIA 171 A channel along a potentially reconnected field line. IRIS detects intermittent fast line of sight flows of 200 km s−1 coincident with the AIA flows. Differential emission measure calculations show heating at the origin of the fast flows. Post-eruption XRT images show hot loops probably due to reconfiguration of magnetic fields during the eruption and subsequent heating of plasma in these loops. Although there is evidence for reconnection above the prominence during the eruption, high spatial resolution images from IRIS reveal potential reconnection sites below the prominence. A height-time analysis of the erupting prominence shows a slow initial rise with a velocity of 0.4 km s−1 followed by a rapid acceleration with a final velocity of 250 km s−1. Brightenings in IRIS during the transition between these two phases indicate the eruption trigger for the fast part of the eruption is likely a tether-cutting mechanism rather than a break-out mechanism.
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 artifact 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.
ABSTRACT We present an investigation of the polar crown prominence that erupted on 2012 March 12. This prominence is observed at the southeast limb by the Solar Dynamics Observatory (SDO)/Atmospheric ...Imaging Assembly (AIA; end-on view) and displays a quasi-vertical thread structure. A bright U-shaped or horn-like structure is observed surrounding the upper portion of the prominence at 171 before the eruption and becomes more prominent during the eruption. The disk view of shows that this long prominence is composed of a series of vertical threads and displays a half-loop-like structure during the eruption. We focus on the magnetic support of the prominence vertical threads by studying the structure and dynamics of the prominence before and during the eruption using observations from SDO and STEREO_B. We also construct a series of magnetic field models (sheared arcade model, twisted flux rope model, and unstable model with hyperbolic flux tube). Various observational characteristics appear to be in favor of the twisted flux rope model. We find that the flux rope supporting the prominence enters the regime of torus instability at the onset of the fast-rise phase, and signatures of reconnection (posteruption arcade, new U-shaped structure, rising blobs) appear about one hour later. During the eruption, AIA observes dark ribbons seen in absorption at 171 corresponding to the bright ribbons shown at 304 , which might be caused by the erupting filament material falling back along the newly reconfigured magnetic fields. Brightenings at the inner edge of the erupting prominence arcade are also observed in all AIA EUV channels, which might be caused by the heating due to energy released from reconnection below the rising prominence.
Rapid and stepwise changes of the magnetic field are often observed during flares but cannot be explained by models yet. Using a 45 minute sequence of Solar Dynamics Observatory/Helioseismic and ...Magnetic Imager 135 s fast-cadence vector magnetograms of the X1 flare on 2014 March 29 we construct, at each timestep, nonlinear force-free models for the coronal magnetic field. Observed flare-related changes in the line-of-sight magnetic field BLOS at the photosphere and chromosphere are compared with changes in the magnetic fields in the models. We find a moderate agreement at the photospheric layer (the basis for the models), but no agreement at chromospheric layers. The observed changes at the photosphere and chromosphere are surprisingly different, and are unlikely to be reproduced by a force-free model. The observed changes are likely to require a change in the magnitude of the field, not just in its direction.
We present low-frequency (80-240 MHz) radio imaging of type III solar radio bursts observed by the Murchison Widefield Array on 2015 September 21. The source region for each burst splits from one ...dominant component at higher frequencies into two increasingly separated components at lower frequencies. For channels below ∼132 MHz, the two components repetitively diverge at high speeds (0.1c-0.4c) along directions tangent to the limb, with each episode lasting just ∼2 s. We argue that both effects result from the strong magnetic field connectivity gradient that the burst-driving electron beams move into. Persistence mapping of extreme-ultraviolet jets observed by the Solar Dynamics Observatory reveals quasi-separatrix layers (QSLs) associated with coronal null points, including separatrix dome, spine, and curtain structures. Electrons are accelerated at the flare site toward an open QSL, where the beams follow diverging field lines to produce the source splitting, with larger separations at larger heights (lower frequencies). The splitting motion within individual frequency bands is interpreted as a projected time-of-flight effect, whereby electrons traveling along the outer field lines take slightly longer to excite emission at adjacent positions. Given this interpretation, we estimate an average beam speed of 0.2c. We also qualitatively describe the quiescent corona, noting in particular that a disk-center coronal hole transitions from being dark at higher frequencies to bright at lower frequencies, turning over around 120 MHz. These observations are compared to synthetic images based on the MHD algorithm outside a sphere (MAS) model, which we use to flux-calibrate the burst data.
We present low-frequency (80 – 240 MHz) radio observations of circular polarization in 16 isolated type III solar radio bursts using the Murchison Widefield Array (MWA) between August 2014 and ...November 2015. For most of the bursts, near burst onset, we find on average
9
%
circular polarization at 80 MHz and
22
%
at 240 MHz whereas these percentages are
5
%
and
20
%
near burst maximum. The polarization fractions are neither constant in time nor uniform over the spatial extents of the bursts. We measure polarization fractions as a function of burst source’s position. On average, near both burst onset and maximum, we find higher polarization near the disk center and lower polarization when the burst source is near the limb. We study total intensity (Stokes
I
), circularly polarized intensity (Stokes
V
), and polarization fraction (
|
V
|
/
I
) profiles for type III bursts with and without source motion as a function of position at times when the intensity of bursts is maximum. For the burst event with no source motion, we find symmetric profiles for Stokes
I
,
V
, and
|
V
|
/
I
. We find symmetric
I
and
V
but asymmetric
|
V
|
/
I
profiles for burst events which have source motion. We argue that this is due to the fundamental emission at the front of a type III electron beam and motion of the burst source. We then perform spectropolarimetric imaging studies of moving burst sources and analyze their motion. At burst onset, we obtain relatively higher polarization fractions, which is considered to be due to a large contribution from fundamental plasma emission at the front of the beam. At burst maximum, the polarization fraction is lower due to the combination of fundamental and harmonic components. After peak intensity, the emission is dominated again by the fundamental component that decays until the end of a burst with lesser polarization fraction than earlier. We argue that the fundamental radiation that decays over time after peak burst intensity is strongly scattered. This pattern of fundamental, fundamental and harmonic, and then fundamental emission with time at each frequency is consistent with the interpretations of Dulk, Suzuki, and Sheridan (
Astron. Astrophys.
130
, 39,
1984
), Robinson, Cairns, and Willes (
Astrophys. J.
422
, 870,
1994
), and Robinson and Cairns (
Solar Phys.
181
, 363,
1998
). We propose that scattering effects can be a viable reason for low polarization fractions in type III events. Finally, we investigate the variations of the decay time (
t
d
) for three events with frequency (
f
), finding that
t
d
∝
f
−
2.0
±
0.1
and decreases more rapidly with increasing
f
compared with previous lower-frequency observations (
t
d
∝
f
−
1.1
±
0.1
). This is interpreted in terms of radial variations of the turbulence properties.
This review addresses our current understanding of comets that venture close to the Sun, and are hence exposed to much more extreme conditions than comets that are typically studied from Earth. The ...extreme solar heating and plasma environments that these objects encounter change many aspects of their behaviour, thus yielding valuable information on both the comets themselves that complements other data we have on primitive solar system bodies, as well as on the near-solar environment which they traverse. We propose clear definitions for these comets: We use the term
near-Sun comets
to encompass all objects that pass sunward of the perihelion distance of planet Mercury (0.307 AU).
Sunskirters
are defined as objects that pass within 33 solar radii of the Sun’s centre, equal to half of Mercury’s perihelion distance, and the commonly-used phrase
sungrazers
to be objects that reach perihelion within 3.45 solar radii, i.e. the fluid Roche limit. Finally, comets with orbits that intersect the solar photosphere are termed
sundivers
. We summarize past studies of these objects, as well as the instruments and facilities used to study them, including space-based platforms that have led to a recent revolution in the quantity and quality of relevant observations. Relevant comet populations are described, including the Kreutz, Marsden, Kracht, and Meyer groups, near-Sun asteroids, and a brief discussion of their origins. The importance of light curves and the clues they provide on cometary composition are emphasized, together with what information has been gleaned about nucleus parameters, including the sizes and masses of objects and their families, and their tensile strengths. The physical processes occurring at these objects are considered in some detail, including the disruption of nuclei, sublimation, and ionisation, and we consider the mass, momentum, and energy loss of comets in the corona and those that venture to lower altitudes. The different components of comae and tails are described, including dust, neutral and ionised gases, their chemical reactions, and their contributions to the near-Sun environment. Comet-solar wind interactions are discussed, including the use of comets as probes of solar wind and coronal conditions in their vicinities. We address the relevance of work on comets near the Sun to similar objects orbiting other stars, and conclude with a discussion of future directions for the field and the planned ground- and space-based facilities that will allow us to address those science topics.
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