60 research outputs found
Quiet Sun coronal heating: analyzing large scale magnetic structures driven by different small-scale uniform sources
Recent measurements of quiet Sun heating events by Krucker and Benz (1998)
give strong support to Parker's (1988) hypothesis that small scale dissipative
events make the main contribution to the quiet heating. Moreover, combining
their observations with the analysis by Priest et al. (2000), it can be
concluded that the sources driving these dissipative events are also small
scale sources, typically of the order of (or smaller than) 2000 km and the
resolution of modern instruments. Thus arises the question of how these small
scale events participate into the larger scale observable phenomena, and how
the information about small scales can be extracted from observations. This
problem is treated in the framework of a simple phenomenological model
introduced in Krasnoselskikh et al. (2001), which allows to switch between
various small scale sources and dissipative processes. The large scale
structure of the magnetic field is studied by means of Singular Value
Decomposition (SVD) and a derived entropy, techniques which are readily
applicable to experimental data.Comment: 9 pages, 9 figure
Deceleration and Dispersion of Large-scale Coronal Bright Fronts
One of the most dramatic manifestations of solar activity are large-scale
coronal bright fronts (CBFs) observed in extreme ultraviolet (EUV) images of
the solar atmosphere. To date, the energetics and kinematics of CBFs remain
poorly understood, due to the low image cadence and sensitivity of previous EUV
imagers and the limited methods used to extract the features. In this paper,
the trajectory and morphology of CBFs was determined in order to investigate
the varying properties of a sample of CBFs, including their kinematics and
pulse shape, dispersion, and dissipation. We have developed a semi-automatic
intensity profiling technique to extract the morphology and accurate positions
of CBFs in 2.5-10 min cadence images from STEREO/EUVI. The technique was
applied to sequences of 171A and 195A images from STEREO/EUVI in order to
measure the wave properties of four separate CBF events. Following launch at
velocities of ~240-450kms^{-1} each of the four events studied showed
significant negative acceleration ranging from ~ -290 to -60ms^{-2}. The CBF
spatial and temporal widths were found to increase from ~50 Mm to ~200 Mm and
~100 s to ~1500 s respectively, suggesting that they are dispersive in nature.
The variation in position-angle averaged pulse-integrated intensity with
propagation shows no clear trend across the four events studied. These results
are most consistent with CBFs being dispersive magnetoacoustic waves.Comment: 15 pages, 18 figure
Multi-point study of the energy release and impulsive CME dynamics in an eruptive C7 flare
We combine observations from different vantage points to perform a detailed
study of a long duration eruptive C7 class flare that occurred on 17 April 2021
and was partially occulted from Earth view. The dynamics and thermal properties
of the flare-related plasma flows, the flaring arcade, and the energy releases
and particle acceleration are studied together with the kinematic evolution of
the associated CME in order to place this long duration event in context of
previous eruptive flare studies. The flare showed hard X-ray (HXR) bursts over
the duration of an hour in two phases lasting from 16:04 UT to 17:05 UT. During
the first phase, a strong increase in emission from hot plasma and impulsive
acceleration of the CME was observed. The CME acceleration profile shows a
three-part evolution of slow rise, acceleration, and propagation in line with
the first STIX HXR burst phase, which is triggered by a rising hot (14 MK)
plasmoid. During the CME acceleration phase, we find signatures of ongoing
magnetic reconnection behind the erupting structure, in agreement with the
standard eruptive flare scenario. The subsequent HXR bursts that occur about 30
minutes after the primary CME acceleration show a spectral hardening (from
7 to 4) but do not correspond to further
CME acceleration and chromospheric evaporation. Therefore, the CME-flare
feedback relationship may only be of significance within the first 25 minutes
of the event under study, as thereafter the flare and the CME eruption evolve
independently of each other
Genesis and Impulsive Evolution of the 2017 September 10 Coronal Mass Ejection
The X8.2 event of 2017 September 10 provides unique observations to study the genesis, magnetic morphology,
and impulsive dynamics of a very fast coronal mass ejection (CME). Combining GOES-16/SUVI and SDO/AIA
EUV imagery, we identify a hot (T ≈ 10–15 MK) bright rim around a quickly expanding cavity, embedded inside
a much larger CME shell (T ≈ 1–2 MK). The CME shell develops from a dense set of large AR loops (0.5Rs)
and seamlessly evolves into the CME front observed in LASCO C2. The strong lateral overexpansion of the CME
shell acts as a piston initiating the fast EUV wave. The hot cavity rim is demonstrated to be a manifestation of the
dominantly poloidal flux and frozen-in plasma added to the rising flux rope by magnetic reconnection in the current
sheet beneath. The same structure is later observed as the core of the white-light CME, challenging the traditional
interpretation of the CME three-part morphology. The large amount of added magnetic flux suggested by these
observations explains the extreme accelerations of the radial and lateral expansion of the CME shell and cavity, all
reaching values of 5–10 km s
−2
. The acceleration peaks occur simultaneously with the first RHESSI 100–300 keV
hard X-ray burst of the associated flare, further underlining the importance of the reconnection process for the
impulsive CME evolution. Finally, the much higher radial propagation speed of the flux rope in relation to the
CME shell causes a distinct deformation of the white-light CME front and shock
What is the Nature of EUV Waves? First STEREO 3D Observations and Comparison with Theoretical Models
One of the major discoveries of the Extreme ultraviolet Imaging Telescope
(EIT) on SOHO were intensity enhancements propagating over a large fraction of
the solar surface. The physical origin(s) of the so-called `EIT' waves is still
strongly debated. They are considered to be either wave (primarily fast-mode
MHD waves) or non-wave (pseudo-wave) interpretations. The difficulty in
understanding the nature of EUV waves lies with the limitations of the EIT
observations which have been used almost exclusively for their study. Their
limitations are largely overcome by the SECCHI/EUVI observations on-board the
STEREO mission. The EUVI telescopes provide high cadence, simultaneous
multi-temperature coverage, and two well-separated viewpoints. We present here
the first detailed analysis of an EUV wave observed by the EUVI disk imagers on
December 07, 2007 when the STEREO spacecraft separation was .
Both a small flare and a CME were associated with the wave cadence, and single
temperature and viewpoint coverage. These limitations are largely overcome by
the SECCHI/EUVI observations on-board the STEREO mission. The EUVI telescopes
provide high cadence, simultaneous multi-temperature coverage, and two
well-separated viewpoints. Our findings give significant support for a
fast-mode interpretation of EUV waves and indicate that they are probably
triggered by the rapid expansion of the loops associated with the CME.Comment: Solar Physics, 2009, Special STEREO Issue, in pres
Large-scale Bright Fronts in the Solar Corona: A Review of "EIT waves"
``EIT waves" are large-scale coronal bright fronts (CBFs) that were first
observed in 195 \AA\ images obtained using the Extreme-ultraviolet Imaging
Telescope (EIT) onboard the \emph{Solar and Heliospheric Observatory (SOHO)}.
Commonly called ``EIT waves", CBFs typically appear as diffuse fronts that
propagate pseudo-radially across the solar disk at velocities of 100--700 km
s with front widths of 50-100 Mm. As their speed is greater than the
quiet coronal sound speed (200 km s) and comparable to the
local Alfv\'{e}n speed (1000 km s), they were initially
interpreted as fast-mode magnetoacoustic waves ().
Their propagation is now known to be modified by regions where the magnetosonic
sound speed varies, such as active regions and coronal holes, but there is also
evidence for stationary CBFs at coronal hole boundaries. The latter has led to
the suggestion that they may be a manifestation of a processes such as Joule
heating or magnetic reconnection, rather than a wave-related phenomena. While
the general morphological and kinematic properties of CBFs and their
association with coronal mass ejections have now been well described, there are
many questions regarding their excitation and propagation. In particular, the
theoretical interpretation of these enigmatic events as magnetohydrodynamic
waves or due to changes in magnetic topology remains the topic of much debate.Comment: 34 pages, 19 figure
On the Nature and Genesis of EUV Waves: A Synthesis of Observations from SOHO, STEREO, SDO, and Hinode
A major, albeit serendipitous, discovery of the SOlar and Heliospheric
Observatory mission was the observation by the Extreme Ultraviolet Telescope
(EIT) of large-scale Extreme Ultraviolet (EUV) intensity fronts propagating
over a significant fraction of the Sun's surface. These so-called EIT or EUV
waves are associated with eruptive phenomena and have been studied intensely.
However, their wave nature has been challenged by non-wave (or pseudo-wave)
interpretations and the subject remains under debate. A string of recent solar
missions has provided a wealth of detailed EUV observations of these waves
bringing us closer to resolving their nature. With this review, we gather the
current state-of-art knowledge in the field and synthesize it into a picture of
an EUV wave driven by the lateral expansion of the CME. This picture can
account for both wave and pseudo-wave interpretations of the observations, thus
resolving the controversy over the nature of EUV waves to a large degree but
not completely. We close with a discussion of several remaining open questions
in the field of EUV waves research.Comment: Solar Physics, Special Issue "The Sun in 360",2012, accepted for
publicatio
Numerical simulation of the 12 May 1997 CME Event: The role of magnetic reconnection
We perform a numerical study of the evolution of a Coronal Mass Ejection (CME) and its interaction with the coronal magnetic field based on the 12 May 1997, CME event using a global MagnetoHydroDynamic (MHD) model for the solar corona. The ambient solar wind steady-state solution is driven by photospheric magnetic field data, while the solar eruption is obtained by superimposing an unstable flux rope onto the steady-state solution. During the initial stage of CME expansion, the core flux rope reconnects with the neighboring field, which facilitates lateral expansion of the CME footprint in the low corona. The flux rope field also reconnects with the oppositely orientated overlying magnetic field in the manner of the breakout model. During this stage of the eruption, the simulated CME rotates counter-clockwise to achieve an orientation that is in agreement with the interplanetary flux rope observed at 1 AU. A significant component of the CME that expands into interplanetary space comprises one of the side lobes created mainly as a result of reconnection with the overlying field. Within 3 hours, reconnection effectively modifies the CME connectivity from the initial condition where both footpoints are rooted in the active region to a situation where one footpoint is displaced into the quiet Sun, at a significant distance (≈1R ) from the original source region. The expansion and rotation due to interaction with the overlying magnetic field stops when the CME reaches the outer edge of the helmet streamer belt, where the field is organized on a global scale. The simulation thus offers a new view of the role reconnection plays in rotating a CME flux rope and transporting its footpoints while preserving its core structure
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The European Solar Telescope
The European Solar Telescope (EST) is a project aimed at studying the magnetic connectivity of the solar atmosphere, from the deep photosphere to the upper chromosphere. Its design combines the knowledge and expertise gathered by the European solar physics community during the construction and operation of state-of-the-art solar telescopes operating in visible and near-infrared wavelengths: the Swedish 1m Solar Telescope, the German Vacuum Tower Telescope and GREGOR, the French Télescope Héliographique pour l'Étude du Magnétisme et des Instabilités Solaires, and the Dutch Open Telescope. With its 4.2 m primary mirror and an open configuration, EST will become the most powerful European ground-based facility to study the Sun in the coming decades in the visible and near-infrared bands. EST uses the most innovative technological advances: the first adaptive secondary mirror ever used in a solar telescope, a complex multi-conjugate adaptive optics with deformable mirrors that form part of the optical design in a natural way, a polarimetrically compensated telescope design that eliminates the complex temporal variation and wavelength dependence of the telescope Mueller matrix, and an instrument suite containing several (etalon-based) tunable imaging spectropolarimeters and several integral field unit spectropolarimeters. This publication summarises some fundamental science questions that can be addressed with the telescope, together with a complete description of its major subsystems
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