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.
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.
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.
We present low-frequency (80 – 240 MHz) radio observations of coronal holes (CHs) made with the
Murchison Widefield Array
(MWA). CHs are expected to be dark structures relative to the background ...corona across the MWA bandwidth due to their low densities. However, we observe that multiple CHs near disk center transition from being dark structures at higher frequencies to bright structures at lower frequencies (
≲
145
MHz
). We compare our observations to synthetic images obtained using the software suite FORWARD, in combination with the magnetohydrodynamic algorithm outside a sphere (MAS) model of the global coronal magnetic field, density, and temperature structure. The synthetic images do not exhibit this transition, and we quantify the discrepancy as a function of frequency. We propose that the dark-to-bright transition results from refraction of radio waves into the low-density CH regions, and we develop a qualitative model based on this idea and the relative optical depths inside and outside a CH as a function of frequency. We show that opacity estimates based on the MAS model are qualitatively consistent with our interpretation, and we conclude that propagation and relative absorption effects are a viable explanation for the dark-to-bright transition of CHs from high to low frequencies.
We present an analysis of extreme-ultraviolet and soft X-ray emission detected toward Comet Lovejoy (C/2011 W3) during its post-perihelion traverse of the solar corona on 2011 December 16. ...Observations were recorded by the Atmospheric Imaging Assembly (AIA) aboard the Solar Dynamics Observatory and the X-Ray Telescope (XRT) aboard Hinode. A single set of contemporaneous images is explored in detail, along with prefatory consideration for time evolution using only the 171 Angstrom data. For each of the eight passbands, we characterize the emission and derive outgassing rates where applicable. As material sublimates from the nucleus and is immersed in coronal plasma, it rapidly ionizes through charge states seldom seen in this environment. The AIA data show four stages of oxygen ionization (O III-O VI) along with C IV, while XRT likely captured emission from O VII, a line typical of the corona. With a nucleus of at least several hundred meters upon approach to a perihelion that brought the comet to within 0.2 R sub(middot in circle) of the photosphere, Lovejoy was the most significant sungrazer in recent history. Correspondingly high outgassing rates on the order of 10 super(32.5) oxygen atoms per second are estimated. Assuming that the neutral oxygen comes from water, this translates to a mass-loss rate of ~9.5 x 10 super(9) g s super(-1), and based only on the 171 middot in circle observations, we find a total mass loss of ~10 super(13) g over the AIA egress. Additional and supporting analyses include a differential emission measure to characterize the coronal environment, consideration for the opening angle, and a comparison of the emission's leading edge with the expected position of the nucleus.
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