975 research outputs found
Solar filament eruptions and their physical role in triggering Coronal Mass Ejections
Solar filament eruptions play a crucial role in triggering coronal mass
ejections (CMEs). More than 80 % of eruptions lead to a CME. This correlation
has been studied extensively during the past solar cycles and the last long
solar minimum. The statistics made on events occurring during the rising phase
of the new solar cycle 24 is in agreement with this finding. Both filaments and
CMEs have been related to twisted magnetic fields. Therefore, nearly all the
MHD CME models include a twisted flux tube, called a flux rope. Either the flux
rope is present long before the eruption, or it is built up by reconnection of
a sheared arcade from the beginning of the eruption. In order to initiate
eruptions, different mechanisms have been proposed: new emergence of flux,
and/or dispersion of the external magnetic field, and/or reconnection of field
lines below or above the flux rope. These mechanisms reduce the downward
magnetic tension and favor the rise of the flux rope. Another mechanism is the
kink instability when the configuration is twisted too much. In this paper we
open a forum of discussions revisiting observational and theoretical papers to
understand which mechanisms trigger the eruption. We conclude that all the
above quoted mechanisms could bring the flux rope to an unstable state.
However, the most efficient mechanism for CMEs is the loss-of-equilibrium or
torus instability, when the flux rope has reached an unstable threshold
determined by a decay index of the external magnetic field.Comment: 23 pages, 13 figures, revie
Does the spacecraft trajectory strongly affect the detection of magnetic clouds?
Magnetic clouds (MCs) are a subset of interplanetary coronal mass ejections
(ICMEs) where a magnetic flux rope is detected. Is the difference between MCs
and ICMEs without detected flux rope intrinsic or rather due to an
observational bias? As the spacecraft has no relationship with the MC
trajectory, the frequency distribution of MCs versus the spacecraft distance to
the MCs axis is expected to be approximately flat. However, Lepping and Wu
(2010) confirmed that it is a strongly decreasing function of the estimated
impact parameter. Is a flux rope more frequently undetected for larger impact
parameter? In order to answer the questions above, we explore the parameter
space of flux rope models, especially the aspect ratio, boundary shape, and
current distribution. The proposed models are analyzed as MCs by fitting a
circular linear force-free field to the magnetic field computed along simulated
crossings.
We find that the distribution of the twist within the flux rope, the
non-detection due to too low field rotation angle or magnitude are only weakly
affecting the expected frequency distribution of MCs versus impact parameter.
However, the estimated impact parameter is increasingly biased to lower values
as the flux-rope cross section is more elongated orthogonally to the crossing
trajectory. The observed distribution of MCs is a natural consequence of a
flux-rope cross section flattened in average by a factor 2 to 3 depending on
the magnetic twist profile. However, the faster MCs at 1 AU, with V>550 km/s,
present an almost uniform distribution of MCs vs. impact parameter, which is
consistent with round shaped flux ropes, in contrast with the slower ones. We
conclude that either most of the non-MC ICMEs are encountered outside their
flux rope or near the leg region, or they do not contain any
Accuracy of magnetic energy computations
For magnetically driven events, the magnetic energy of the system is the
prime energy reservoir that fuels the dynamical evolution. In the solar
context, the free energy is one of the main indicators used in space weather
forecasts to predict the eruptivity of active regions. A trustworthy estimation
of the magnetic energy is therefore needed in three-dimensional models of the
solar atmosphere, eg in coronal fields reconstructions or numerical
simulations. The expression of the energy of a system as the sum of its
potential energy and its free energy (Thomson's theorem) is strictly valid when
the magnetic field is exactly solenoidal. For numerical realizations on a
discrete grid, this property may be only approximately fulfilled. We show that
the imperfect solenoidality induces terms in the energy that can lead to
misinterpreting the amount of free energy present in a magnetic configuration.
We consider a decomposition of the energy in solenoidal and nonsolenoidal parts
which allows the unambiguous estimation of the nonsolenoidal contribution to
the energy. We apply this decomposition to six typical cases broadly used in
solar physics. We quantify to what extent the Thomson theorem is not satisfied
when approximately solenoidal fields are used. The quantified errors on energy
vary from negligible to significant errors, depending on the extent of the
nonsolenoidal component. We identify the main source of errors and analyze the
implications of adding a variable amount of divergence to various solenoidal
fields. Finally, we present pathological unphysical situations where the
estimated free energy would appear to be negative, as found in some previous
works, and we identify the source of this error to be the presence of a finite
divergence. We provide a method of quantifying the effect of a finite
divergence in numerical fields, together with detailed diagnostics of its
sources
Are constant loop widths an artifact of the background and the spatial resolution?
We study the effect of the coronal background in the determination of the
diameter of EUV loops, and we analyze the suitability of the procedure followed
in a previous paper (L\'opez Fuentes, Klimchuk & D\'emoulin 2006) for
characterizing their expansion properties. For the analysis we create different
synthetic loops and we place them on real backgrounds from data obtained with
the Transition Region and Coronal Explorer (\textit{TRACE}). We apply to these
loops the same procedure followed in our previous works, and we compare the
results with real loop observations. We demonstrate that the procedure allows
us to distinguish constant width loops from loops that expand appreciably with
height, as predicted by simple force-free field models. This holds even for
loops near the resolution limit. The procedure can easily determine when loops
are below resolution limit and therefore not reliably measured. We find that
small-scale variations in the measured loop width are likely due to
imperfections in the background subtraction. The greatest errors occur in
especially narrow loops and in places where the background is especially bright
relative to the loop. We stress, however, that these effects do not impact the
ability to measure large-scale variations. The result that observed loops do
not expand systematically with height is robust.Comment: Accepted for publication in Ap
Constraints on filament models deduced from dynamical analysis
The conclusions deduced from simultaneous observations with the Ultra-Violet Spectrometer and Polarimeter (UVSP) on the Solar Maximum Mission satellite, and the Multichannel Subtractive Double Pass (MSPD) spectrographs at Meudon and Pic du Midi observatories are presented. The observations were obtained in 1980 and 1984. All instruments have almost the same field of view and provide intensity and velocity maps at two temperatures. The resolution is approx. 0.5 to 1.5" for H alpha line and 3" for C IV. The high resolution and simultaneity of the two types of observations allows a more accurate description of the flows in prominences as functions of temperature and position. The results put some contraints on the models and show that dynamical aspects must be taken into account
PUPHS2D 2.0 User\u27s Manual
The Purdue University Program for Heterostructure Simulation in Two Dimensions (PUPHS2D) solves Poisson\u27s equation and the electron and hole continuity equations within a two-dimensional heterostructure device. The program will compute the electrostatic potential, electron and hole densities, recombination rate, and other quantities of interest as a function of applied bias. Like its predecessor, version 2.0 allows extensive analysis of solar cells, including computation of the current-voltage characteristics of two-terminal devices, solar cell parameters, quantum efficiency, and current versus solar intensity. Extensions to version 2.Q include transient analysis and bipolar transistor capability. The heterojunction bipolar transistor routines allow computation of dc currents as a function of applied bias, as well as quasi-static evaluation of the high-frequency behavior. A simplified energy balance equation has been added in the interest of more accurately computing high-field characteristics, and should be viewed as a preliminary step toward this goal. PUPHS2D stands as an accurate model for computing low-field device characteristics and recombinative losses. While PUPHS2D was written for the ternary AlxGai1-xAs, all material specific parameters are contained within a single subroutine (BANDX), except for absorption coefficient and carrier mobilities which are computed in subroutines ALGABS and SETMOB, respectively. Material-specific parameters used for the energy balance equation are found in subroutines INITMU and INITAU. The program may be readily modified to analyze other semiconductors. For a more thorough discussion of the theoretical basis and numerical implementation of PUPHS2D, the user is directed to the references. Materials parameters are described in reference [I]. Various phases of the development of PUPHS2D have been supported by the Semiconductor Research Corporation, Sandia National Laboratories/ the Eastman Kodak Company, and by Research Triangle Institute
Eruption of a Kink-Unstable Filament in Active Region NOAA 10696
We present rapid-cadence Transition Region And Coronal Explorer (TRACE)
observations which show evidence of a filament eruption from active region NOAA
10696, accompanied by an X2.5 flare, on 2004 November 10. The eruptive
filament, which manifests as a fast coronal mass ejection some minutes later,
rises as a kinking structure with an apparently exponential growth of height
within TRACE's field of view. We compare the characteristics of this filament
eruption with MHD numerical simulations of a kink-unstable magnetic flux rope,
finding excellent qualitative agreement. We suggest that, while tether
weakening by breakout-like quadrupolar reconnection may be the release
mechanism for the previously confined flux rope, the driver of the expansion is
most likely the MHD helical kink instability.Comment: Accepted by ApJ Letters. 4 figures (Fig. 3 in two parts). For MPEG
files associated with Figure 1, see:
http://www.mssl.ucl.ac.uk/~drw/papers/kink/ktrace.mpg
http://www.mssl.ucl.ac.uk/~drw/papers/kink/kmdi.mpg
http://www.mssl.ucl.ac.uk/~drw/papers/kink/ksimu.mp
Plasma composition in a sigmoidal anemone active region
Using spectra obtained by the EIS instrument onboard Hinode, we present a
detailed spatially resolved abundance map of an active region (AR)-coronal hole
(CH) complex that covers an area of 359 arcsec x 485 arcsec. The abundance map
provides first ionization potential (FIP) bias levels in various coronal
structures within the large EIS field of view. Overall, FIP bias in the small,
relatively young AR is 2-3. This modest FIP bias is a consequence of the AR
age, its weak heating, and its partial reconnection with the surrounding CH.
Plasma with a coronal composition is concentrated at AR loop footpoints, close
to where fractionation is believed to take place in the chromosphere. In the
AR, we found a moderate positive correlation of FIP bias with nonthermal
velocity and magnetic flux density, both of which are also strongest at the AR
loop footpoints. Pathways of slightly enhanced FIP bias are traced along some
of the loops connecting opposite polarities within the AR. We interpret the
traces of enhanced FIP bias along these loops to be the beginning of
fractionated plasma mixing in the loops. Low FIP bias in a sigmoidal channel
above the AR's main polarity inversion line where ongoing flux cancellation is
taking place, provides new evidence of a bald patch magnetic topology of a
sigmoid/flux rope configfiuration.Comment: For on-line animation, see
http://www.mssl.ucl.ac.uk/~db2/fip_intensity.gif. Accepted by Ap
Shallow water tomography with a sparse array during the INTIMATE'98 sea trial
Invert acoustic data using sparse arrays - at
the limit with a single hydrophone - is a challenging task.
The final goal is to obtain a rapid environmental assessment with systems both easier to deploy and less expensive than
full vertical arrays. In this paper, it is shown that using a known broadband source signal and an array with few hydrophones,
ocean acoustic tomography can be performed,
even in a complex internal waves induced highly variable ocean. The inversion approach presented herein is based on an arrival matching processor and a genetic algorithm search procedure. Due to the poor accuracy on the a priori knowledge of the source range, source depth and water depth, the inversion procedure was split in two stages: in the first stage the geometric parameters where estimated and in the second stage sound speed estimates where obtained.
This procedure was applied to field data, acquired during the INTIMATE'98 sea trial, in a shallow water area off the coast of France in the Gulf of Biscay. That area
is expected to have a relatively high internal wave activity, specially during the summer. A 4 sec long - 700 Hz bandwidth linear frequency modulated signal was transmitted from a ship suspended sound source and received on a 4 element vertical array at a range of approximately 10.5 km, over a relatively range-independent area. The results from the inversion of the acoustic data are in line with those obtained by concurrent non acoustic data like GPS source range, measured source depth, XBT casts and temperature sensors
Interplanetary Magnetic Field Guiding Relativistic Particles
The origin and the propagation of relativistic solar particles (0.5 to few Ge V) in the interplanetary medium remains a debated topic. These relativistic particles, detected at the Earth by neutron monitors have been previously accelerated close to the Sun and are guided by the interplanetary magnetic field (IMF) lines, connecting the acceleration site and the Earth. Usually, the nominal Parker spiral is considered for ensuring the magnetic connection to the Earth. However, in most GLEs the IMF is highly disturbed, and the active regions associated to the GLEs are not always located close to the solar footprint of the nominal Parker spiral. A possible explanation is that relativistic particles are propagating in transient magnetic structures, such as Interplanetary Coronal Mass Ejections (ICMEs). In order to check this interpretation, we studied in detail the interplanetary medium where the particles propagate for 10 GLEs of the last solar cycle. Using the magnetic field and the plasma parameter measurements (ACE/MAG and ACE/SWEPAM), we found widely different IMF configurations. In an independent approach we develop and apply an improved method of the velocity dispersion analysis to energetic protons measured by SoHO/ERNE. We determined the effective path length and the solar release time of protons from these data and also combined them with the neutron monitor data. We found that in most of the GLEs, protons propagate in transient magnetic structures. Moreover, the comparison between the interplanetary magnetic structure and the interplanetary length suggest that the timing of particle arrival at Earth is dominantly determined by the type of IMF in which high energetic particles are propagating. Finally we find that these energetic protons are not significantly scattered during their transport to Earth
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