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Why Temperature and Velocity have Different Relationships in the Solar Wind and in Interplanetary Coronal Mass Ejections?  

Pascal Demoulin   Submitted: 2009-04-11 11:18

In-situ observations of the solar wind (SW) show temperature increasing with the wind speed, while such dependence is not observed in interplanetary coronal mass ejections (ICMEs). The aim of this paper is to understand the main origin of this correlation in the SW and its absence in ICMEs. For that purpose both the internal-energy and momentum equations are solved analytically with various approximations. The internal-energy equation does not provide a strong link between temperature and velocity, but the momentum equation does. Indeed, the observed correlation in the open magnetic-field configuration of the SW is the result of its acceleration and heating close to the Sun. In contrast, the magnetic configuration of ICMEs is closed, and moreover the momentum equation is dominated by magnetic forces. It implies no significant correlation between temperature and velocity, as observed.

Authors: Demoulin P.
Projects: None

Publication Status: in press, Solar Physics
Last Modified: 2009-04-11 15:14
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Causes and consequences of magnetic cloud expansion  

Pascal Demoulin   Submitted: 2009-03-05 07:40

A magnetic cloud (MC) is a magnetic flux rope in the solar wind (SW), which, at 1 AU, is observed ~ 2-5 days after its expulsion from the Sun. The associated solar eruption is observed as a coronal mass ejection (CME). Both the in situ observations of plasma velocity distribution and the increase in their size with solar distance demonstrate that MCs are strongly expanding structures. The aim of this work is to find the main causes of this expansion and to derive a model to explain the plasma velocity profiles typically observed inside MCs. We model the flux rope evolution as a series of force-free field states with two extreme limits: (a) ideal magneto-hydrodynamics (MHD) and (b) minimization of the magnetic energy with conserved magnetic helicity. We consider cylindrical flux ropes to reduce the problem to the integration of ordinary differential equations. This allows us to explore a wide variety of magnetic fields at a broad range of distances to the Sun. We demonstrate that the rapid decrease in the total SW pressure with solar distance is the main driver of the flux-rope radial expansion. Other effects, such as the internal over-pressure, the radial distribution, and the amount of twist within the flux rope have a much weaker influence on the expansion. We demonstrate that any force-free flux rope will have a self-similar expansion if its total boundary pressure evolves as the inverse of its length to the fourth power. With the total pressure gradient observed in the SW, the radial expansion of flux ropes is close to self-similar with a nearly linear radial velocity profile across the flux rope, as observed. Moreover, we show that the expansion rate is proportional to the radius and to the global velocity away from the Sun. The simple and universal law found for the radial expansion of flux ropes in the SW predicts the typical size, magnetic structure, and radial velocity of MCs at various solar distances.

Authors: Demoulin P. & Dasso S.
Projects: None

Publication Status: A&A in press
Last Modified: 2009-03-05 07:43
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Modeling and Observations of Photospheric Magnetic Helicity  

Pascal Demoulin   Submitted: 2008-09-03 08:00

Mounting observational evidence of the emergence of twisted magnetic flux tubes through the photosphere have now been published. Such flux tubes, formed by the solar dynamo and transported through the convection zone, eventually reach the solar atmosphere. Their accumulation in the solar corona leads to flares and coronal mass ejections. Since reconnections occur during the evolution of the flux tubes, the concepts of twist and magnetic stress become inappropriate. Magnetic helicity, as a well preserved quantity, even during reconnection, is a more suitable physical quantity to use. Only recently, it has been realized that the flux of magnetic helicity can be derived from magnetogram time series. This paper reviews the advances made in measuring the helicity injection rate at the photospheric level, mostly in active regions. It relates the observations to our present theoretical understanding of the emergence process. Most of the helicity injection is found during magnetic flux emergence, whereas the effect of differential rotation is small, and the long-term evolution of active regions is still puzzling. The photospheric maps for the injection of magnetic helicity provide a new spatial information about the basic properties of the link between the solar activity and its sub-photospheric roots. Finally, the newest technique to measure photospheric flows are reviewed.

Authors: Demoulin, P. & Pariat, E.
Projects: None

Publication Status: Advances in Space Research, submitted
Last Modified: 2008-09-03 13:35
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Subject will be restored when possible  

Pascal Demoulin   Submitted: 2008-06-02 11:08

In situ data provide only a one dimensional sample of the plasma velocity along the spacecraft trajectory crossing an interplanetary coronal mass ejection (ICME). Then, to understand the dynamics of ICMEs it is necessary to consider some model to describe it. We derive a series of equations in a hierarchical order, from more general to more specific cases, to provide a general theoretical basis for the interpretation of in situ observations, extending and generalizing previous studies. The main hypothesis is a self-similar expansion, but with the freedom of possible different expansion rates in three orthogonal directions. The most detailed application of the equations is though for a subset of ICMEs, magnetic clouds (MCs), where a magnetic flux rope can be identified. The main conclusions are the following ones. First, we obtain theoretical expressions showing that the observed velocity gradient within an ICME is not a direct characteristic of its expansion, but that it depends also on other physical quantities such as its global velocity and acceleration. The derived equations quantify these dependencies for the three components of the velocity. Second, using three different types of data we show that the global acceleration of ICMEs has, at most, a small contribution to the in situ measurements of the velocity. This eliminates practically one contribution to the observed velocity gradient within ICMEs. Third, we provide a method to quantify the expansion rate from velocity data. We apply it to a set of 26~MCs observed by Wind or ACE spacecrafts. They are typical MCs, and their main physical parameters cover the typical range observed in MCs in previous statistical studies. Though the velocity difference between their front and back includes a broad range of values, we find a narrow range for the determined dimensionless expansion rate. This implies that MCs are expanding at a comparable rate, independently of their size or field strength, despite very different magnitudes in their velocity profiles. Furthermore, the equations derived provide a base to further analyze the dynamics of MCs/ICMEs.

Authors: Demoulin, P., Nakwacki, M.S., Dasso, S., Mandrini, C.H.
Projects: None

Publication Status: in press, Solar Physics
Last Modified: 2008-06-03 09:13
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Expected in Situ Velocities from a Heirarchical Model for Exapnding Interplanetary Coronal Mass Ejections  

Pascal Demoulin   Submitted: 2008-06-02 11:08

In situ data provide only a one dimensional sample of the plasma velocity along the spacecraft trajectory crossing an interplanetary coronal mass ejection (ICME). Then, to understand the dynamics of ICMEs it is necessary to consider some model to describe it. We derive a series of equations in a hierarchical order, from more general to more specific cases, to provide a general theoretical basis for the interpretation of in situ observations, extending and generalizing previous studies. The main hypothesis is a self-similar expansion, but with the freedom of possible different expansion rates in three orthogonal directions. The most detailed application of the equations is though for a subset of ICMEs, magnetic clouds (MCs), where a magnetic flux rope can be identified. The main conclusions are the following ones. First, we obtain theoretical expressions showing that the observed velocity gradient within an ICME is not a direct characteristic of its expansion, but that it depends also on other physical quantities such as its global velocity and acceleration. The derived equations quantify these dependencies for the three components of the velocity. Second, using three different types of data we show that the global acceleration of ICMEs has, at most, a small contribution to the in situ measurements of the velocity. This eliminates practically one contribution to the observed velocity gradient within ICMEs. Third, we provide a method to quantify the expansion rate from velocity data. We apply it to a set of 26~MCs observed by Wind or ACE spacecrafts. They are typical MCs, and their main physical parameters cover the typical range observed in MCs in previous statistical studies. Though the velocity difference between their front and back includes a broad range of values, we find a narrow range for the determined dimensionless expansion rate. This implies that MCs are expanding at a comparable rate, independently of their size or field strength, despite very different magnitudes in their velocity profiles. Furthermore, the equations derived provide a base to further analyze the dynamics of MCs/ICMEs.

Authors: Demoulin, P., Nakwacki, M.S., Dasso, S., Mandrini, C.H.
Projects:

Publication Status: Solar Physics, 2008, 250, 347
Last Modified: 2008-09-03 08:04
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Subject will be restored when possible  

Pascal Demoulin   Submitted: 2007-09-21 09:55

Magnetic clouds (MCs), and more generally interplanetary coronal mass ejections (ICMEs), are believed to be the interplanetary counterparts of CMEs. The link has usually been shown by taking into account the CME launch position on the Sun, the expected time delay and by comparing the orientation of the coronal and interplanetary magnetic field. Making such a link more quantitative is challenging since it requires the relation of very different kinds of magnetic field measurements: (i) photospheric magnetic maps, which are observed from a distant vantage point (remote sensing) and (ii) in situ measurements of MCs, which provide precise, directly measured, magnetic field data merely from one-dimensional linear samples. The association between events in these different domains can be made using adequate coronal and MC models. Then, global quantities like magnetic fluxes and helicity can be derived and compared. All the associations criteria are reviewed, with a description of the general trends found. A special focus is given on the cases which do not follow the earlier derived mean laws since interesting physics is usually associated to them.

Authors: P. Demoulin
Projects: None

Publication Status: submitted to Annales Geophysicae (review)
Last Modified: 2007-09-24 04:02
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Where will efficient energy release occur in 3D magnetic configurations?  

Pascal Demoulin   Submitted: 2007-02-17 07:13

The energy needed to power flares is thought to be stored in the coronal magnetic field. However, the energy release is efficient only at very small scales. Magnetic configurations with a complex topology, i.e. with separatrices, are the most obvious configurations where current sheets can form, and then, reconnection can efficiently occur. This has been confirmed for several flares computing the coronal field and comparing the locations of the flare loops and ribbons to the deduced 3D magnetic topology. However, this view is too restrictive taking into account the variety of observed solar flaring configurations. Indeed, ``Quasi-Separatrix Layers'' (QSLs), which are regions where there is a drastic change in field-line linkage, generalize the definition of separatrices. They let us understand where reconnection occurs in a broader variety of flares than separatrices do. The strongest electric field and current are generated at, or close to where the QSLs are thinnest. This defines the region where particle acceleration can efficiently occur. A new feature of 3-D reconnection is the natural presence of fast field line slippage along the QSLs, a process called ``slip-running reconnection''. This is a plausible origin for the motions of the X-ray sources along flare ribbons.

Authors: Demoulin, P.
Projects: None

Publication Status: Advances in Space Research, in press
Last Modified: 2007-02-17 10:09
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Recent theoretical and observational developments in magnetic helicity studies  

Pascal Demoulin   Submitted: 2006-12-15 09:29

Magnetic helicity quantifies how the magnetic field is sheared and twisted compared to its lowest energy state (potential field). Such stressed magnetic fields are usually observed in association with flares, eruptive filaments, and coronal mass ejections (CMEs). Magnetic helicity plays a key role in magnetohydrodynamics because it is almost preserved on a timescale less than the global diffusion time scale. Its conservation defines a constraint to the magnetic field evolution. Only relatively recently, scientists have realized that magnetic helicity can be computed from observations, and methods have been derived to bridge the gap between theory and observations. At the photospheric level, the rate (or flux) of magnetic helicity can be computed from the evolution of longitudinal magnetograms. The coronal helicity is estimated from magnetic extrapolation, while the helicity ejected in magnetic clouds (interplanetary counter-part of CMEs) is derived through modelling of in-situ magnetic field measurements. Using its conserved property, a quantitative link between phenomena observed in the corona and then in the interplanetary medium has been achieved.

Authors: Démoulin, Pascal
Projects: SoHO-MDI

Publication Status: Advances in Space Research, in press
Last Modified: 2006-12-15 10:37
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A Multiple flare scenario where the classic long duration flare was not the source of a CME  

Pascal Demoulin   Submitted: 2006-11-22 04:18

A series of flares (GOES class M, M and C) and a CME were observed in close succession on 20th January 2004 in NOAA 10540. Radio observations, which took the form of types II, III and N bursts, were associated with these events. We use the combined observations from TRACE, EIT, Hα images from Kawsan, MDI magnetograms, and GOES to understand the complex development of this event. Contrary to a standard interpretation, we conclude that the first two impulsive flares are part of the CME launch process while the following LDE flare represents simply the recovery phase. Observations show that the flare ribbons not only separate but also shift along the magnetic inversion line so that magnetic reconnection progresses stepwise to neighbouring flux tubes. We conclude that ''tether cutting'' reconnection in the sheared arcade progressively transforms it to a twisted flux tube which becomes unstable, leading to a CME. We interpret the third flare, a long-duration event, as a combination of the classical two-ribbon flare with the relaxation process following forced reconnection between the expanding CME structure and neighbouring magnetic fields.

Authors: Goff, C.P., van Driel-Gesztelyi, L., Démoulin, P., Culhane, J.L., Matthews, S.A., Harra, L.K., Mandrini, C.H., Klein, K.L., Kurokawa, H.
Projects: SoHO-EIT,SoHO-MDI

Publication Status: submitted, Solar Physics
Last Modified: 2006-11-22 09:53
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Decametric N burst: a consequence of the interaction of two coronal mass ejections  

Pascal Demoulin   Submitted: 2006-11-08 04:15

Radio emissions of electron beams in the solar corona and interplanetary space are tracers of the underlying magnetic configuration and of its evolution. We analyse radio observations from the Culgoora and Wind/WAVES spectrographs, in combination with SOHO/LASCO and SOHO/MDI data, to understand the origin of a type N burst originating from NOAA AR 10540 on January 20, 2004, and its relationship with type II and type III emissions. All bursts are related to the flares and the CME analysed in a previous paper (Goff et al., 2006). A very unusual feature of this event was a decametric type N burst, where a type III-like burst, drifting toward low frequencies (negative drift), changes drift first to positive, then again to negative. At metre wavelengths, i.e. heliocentric distances < 1.5 Rs, these bursts are ascribed to electron beams bouncing in a closed loop. Neither U nor N bursts are expected at decametric wavelengths because closed quasi-static loops are not thought to extend to distances >> 1.5 Rs. We take the opportunity of the good multi-instrument coverage of this event to analyse the origin of type N bursts in the high corona. Reconnection of the expanding ejecta with the magnetic structure of a previous CME, launched about 8 hours earlier, injects electrons in the same manner as with type III bursts but into open field lines having a local dip and apex. The latter shape was created by magnetic reconnection between the expanding CME and neighbouring (open) streamer field lines. This particular flux tube shape in the high corona, between 5-10 Rs, explains the observed type N burst. Since the required magnetic configuration is only a transient phenomenon formed by reconnection, severe timing and topological constraints are present to form the observed decametric N-burst. They are therefore expected to be rare features.

Authors: Demoulin, P., Klein, K.L., Goff, C.P., van Driel-Gesztelyi, L., Culhane, J.L., Mandrini, C.H., Matthews, S.A., Harra, L.K.
Projects:

Publication Status: In press, Solar Physics
Last Modified: 2006-11-23 08:56
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Magnetic topologies: where will reconnection occur ?  

Pascal Demoulin   Submitted: 2005-10-12 06:05

The energy needed to power flares is thought to come from the coronal magnetic field. However, such energy release is efficient only at very small scales. Magnetic configurations with a complex topology, i.e. with separatrices, are the most obvious configurations where current layers can spontaneously form. 3-D magnetic configurations have a variety of magnetic topologies not suspected before. If the photospheric field is described by an ensemble of magnetic charges, separated by flux-free regions, a complete topological description of the associated potential field is provided by the skeleton formed by the null points, spines, fans and separators. In order to better match the observed photospheric magnetograms, the magnetic charges can be set below the photosphere; then an extra topological element can appear, so-called bald patches with associated separatrices. In several flaring configurations the computed separatrices allows to understand the localization of the flare ribbons in the framework of magnetic reconnection. However, this view is too restrictive taking into account the variety of observed solar flaring configurations. Indeed ``quasi-separatrix layers'' (QSLs), which are regions where there is a drastic change in field-line linkage, generalize the definition of separatrices for magnetic fields extending in the full volume (photosphere and corona). The concept of ``hyperbolic flux tube'' (HFT) also generalizes the concept of separator. These studies indeed teach us that coronal magnetic reconnection occurs in a broader variety of magnetic configurations than traditionally thought, and this variety is reviewed.

Authors: Demoulin, P.
Projects: None

Publication Status: ESA publication, in press
Last Modified: 2005-10-12 06:05
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Extending the Concept of Separatrices to QSLs for Magnetic Reconnection  

Pascal Demoulin   Submitted: 2005-05-20 06:39

Magnetic reconnection is usually thought to be linked to the presence of magnetic null points and to be accompanied by the transport of magnetic field lines across separatrices, the set of field lines where the field-line linkage is discontinuous. However, this view is too restrictive taking into account the variety of observed solar flaring configurations. Indeed ``quasi-separatrix layers'' (QSLs), which are regions where there is a drastic change in field-line linkage, generalize the definition of separatrices. Magnetic reconnection is expected to occur preferentially at QSLs in three-dimensional magnetic configurations. This paper surveys the evolution of the QSL concept from the beginning to its recent status. The theory was successfully tested with multi-wavelength observations of solar flares. This validates the reconnection scenario as the main physical process at the origin of flares. The confrontation of observations with the state-of-the-art theory gives us also hints how to further develop our understanding of 3-D magnetic reconnection.

Authors: P. Demoulin
Projects: Soho-MDI

Publication Status: COSPAR, in press
Last Modified: 2005-05-20 06:39
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Radio and X-ray signatures of magnetic reconnection behind an ejected flux rope  

Pascal Demoulin   Submitted: 2005-02-10 07:09

We present a detailed study of a complex solar event observed on June 02 2002. Joint imaging EUV, X-ray and multi-wavelength radio observations allow us to trace the development of the magnetic structure involved in this solar event up to a radial distance of the order of two solar radii . The event involves type II, III and IV bursts. The type~IV burst is formed by two sources: a fast moving one (M) and a ``quasi-stationary'' one (S). The time coincidence in the flux peaks of these radio sources and the underlying hard X-ray sources implies a causal link. In a first part we provide a summary of the observations without reference to any CME model. The experimental results impose strong constraints on the physical processes. In a second part, we find that a model with an erupting twisted flux rope, with the formation of a current sheet behind, best relates the different observations in a coherent physical evolution (even if there is no direct evidence of the twisted flux rope). Our results show that multi-wavelength radio imaging represent a powerful tool to trace the dynamical evolution of the reconnecting current sheet behind ejected flux-ropes (in between sources M and S) and over an altitude range not accessible by X-ray observations.

Authors: Pick, M., Démoulin, P., Krucker, S., Malandraki, O., Maia, D.
Projects: RHESSI,Soho-EIT

Publication Status: ApJ, in press (625, June 2 issue)
Last Modified: 2005-02-10 07:09
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Amplitude and orientation of prominence magnetic fields from constant α magnetohydrostatic models  

Pascal Demoulin   Submitted: 2003-04-25 08:30

We analyze outputs from three-dimensional models for three observed filaments, which belong to the quiescent, intermediate and plage class respectively. Each model was calculated from a constant α magnetohydrostatic extrapolation, assuming that the prominence material is located in magnetic dips, so that the field is nearly horizontal throughout the prominence body and feet. We calculate the spatial distribution of the magnetic field amplitude B and orientation theta with respect to the filament axis, neither of which were imposed a priori in the models. In accordance with past magnetic field measurements within prominence bodies, we also obtain nearly homogeneous magnetic fields, respectively of about B ~ 3, 14 and 40 G for the quiescent, intermediate and plage prominence, with a systematic weak vertical field gradient of dB / dz ~ 0.1-1.5 X 10-4 G.km-1. We also find that the inverse polarity configuration is dominant with theta ~ -20 to 0 degrees, which is slightly smaller than in some observations. We also report some other properties, which have either rarely or never been observed. We find at prominence tops some localized normal polarity regions with theta < +10 degrees. At prominence bottoms below 20 Mm in altitude, we find stronger field gradients dB / dz ~ 1-10 X 10-4 G.km-1 and a wider range of field directions theta ~ -90 to 0 degrees. These properties can be interpreted by the perturbation of the prominence flux tube by strong photospheric polarities located in the neighborhood of the prominence. We also report some full portions of prominences that have the normal polarity. The latter are simply due to the local curvature of the filaments with respect to their average axis, which was used to define theta. These results could either be used as predictions for further testing of this class of models with new observations, or as quantitative tools for the interpretation of observations which show complex patterns.

Authors: Aulanier, G., Demoulin, P.
Projects: None

Publication Status: 2003, A&A, 402, 769
Last Modified: 2003-04-25 08:30
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Obervations of magnetic helicity  

Pascal Demoulin   Submitted: 2003-04-25 08:19

The first observational signature of magnetic helicity in the solar atmosphere (sunspot whirls) was discovered 77 years ago. Since then, the existence of a cycle-invariant hemispheric helicity pattern has been firmly established through current helicity and morphological studies. During the last years, attempts were made to estimate/measure magnetic helicity from solar and interplanetary observations. Magnetic helicity (unlike current helicity) is one of the few global quantities that is conserved even in resistive magnetohydrodynamics (MHD) on a timescale less than the global diffusion timescale, thus magnetic helicity studies make it possible to trace helicity as it emerges from the sub-photospheric layers to the corona and then is ejected via coronal mass ejections (CMEs) into the interplanetary space reaching the Earth in a magnetic cloud. We give an overview of observational studies on the relative importance of different sources of magnetic helicity, i.e. whether photospheric plasma motions (photospheric differential rotation and localized shearing motions) or the twist of the emerging flux tubes created under the photosphere (presumably by the radial shear in the differential rotation in the tachocline) is the dominant helicity source. We examine the sources of errors present in these early results and try to judge how realistic they are.

Authors: L. van Driel-Gesztelyi, P. Demoulin, C.H. Mandrini
Projects: Soho-MDI

Publication Status: COSPAR, Adv. Space Research, in press
Last Modified: 2003-04-25 08:19
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Magnetic energy and helicity fluxes at the photospheric level  

Pascal Demoulin   Submitted: 2003-04-24 09:42

The source of coronal magnetic energy and helicity lies below the surface of the sun, probably in the convective zone dynamo. Measurements of magnetic and velocity fields can capture the fluxes of both magnetic energy and helicity crossing the photosphere. We point out the ambiguities which can occur when observations are used to compute these fluxes. In particular, we show that these fluxes should be computed only from the horizontal motions deduced by tracking the photospheric cut of magnetic flux tubes. These horizontal motions include the effect of both the emergence and the shearing motions what ever the magnetic configuration complexity is. We finally analyze the observational difficulties involved in deriving such fluxes, in particular the limitations of the correlation tracking methods.

Authors: Demoulin, P., Berger, M.A.
Projects: Soho-MDI

Publication Status: Solar Physics, in press
Last Modified: 2003-04-24 09:45
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The scalings of the coronal plasma parameters with the mean photospheric magnetic field: The long-term evolution of AR 7978  

Pascal Demoulin   Submitted: 2002-12-05 08:20

We analyze the evolution of the fluxes observed in X-rays and correlate them with the magnetic flux density in active region NOAA 7978 from its birth throughout its decay, for five solar rotations. We use SoHO/MDI data to derive magnetic observables, together with Yohkoh/SXT and Yohkoh/BCS data to determine the global evolution of the temperature and the emission measure of the coronal plasma at times when no significant brightenings were observed. We show that the mean X-ray flux and derived parameters, temperature and emission measure (together with other quantities deduced from them, such as the density and the pressure), of the plasma in the AR follow power-law relationships with the mean magnetic flux density (ar{B}). The exponents (b) of these power-law functions (a ar{B}b) are derived using two different statistical methods, a classical least-squares method in log-log plots and a non-parametric method, which takes into account the fact that errors in the data may not be normally distributed. Both methods give similar exponents, within error bars, for the mean temperature and for both instruments (SXT and BCS); in particular, b stays in the range [0.27,0.31] and [0.24,0.55] for full resolution SXT images and BCS data, respectively. For the emission measure the exponent b lies in the range [0.85,1.35] and [0.45,1.96] for SXT and BCS, respectively. The determination of such power-law relations, when combined with the results from coronal heating models, can provide us with powerful tools for determining the mechanism responsible for the existence of the high temperature corona.

Authors: van Driel-Gesztelyi L., Demoulin, P., Mandrini C.H., Harra, L., Klimchuk, J.A.
Projects:

Publication Status: ApJ, preprint, in press
Last Modified: 2002-12-05 08:20
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Testing coronal heating models: The long-term evolution of AR 7978  

Pascal Demoulin   Submitted: 2002-12-05 08:15

We derive the dependence of the mean coronal heating rate on the magnetic flux density. Our results are based on a previous study of the plasma parameters and the magnetic flux density (ar{B}) in the active region NOAA 7978 from its birth to its decay, throughout five solar rotations using SoHO/MDI, Yohkoh/SXT and Yohkoh/BCS. We use the scaling laws of coronal loops in thermal equilibrium to derive four observational estimates of the scaling of the coronal heating with ar{B} (two from SXT and two from BCS observations). These results are used to test the validity of coronal heating models. We find that models based on the dissipation of stressed, current-carrying magnetic fields are in better agreement with the observations than models that attribute coronal heating to the dissipation of MHD waves injected at the base of the corona. This confirms, with smaller error bars, previous results obtained for individual coronal loops, as well as for the global coronal emission of the Sun and cool stars. Taking into account that the photospheric field is concentrated in thin magnetic flux tubes, both SXT and BCS data are in best agreement with models invoking a stochastic buildup of energy, current layers and MHD turbulence.

Authors: D'emoulin, P., van Driel-Gesztelyi, L., Mandrini C.H., Klimchuk, J.A., Harra, L.
Projects:

Publication Status: ApJ, preprint, in press
Last Modified: 2002-12-05 08:15
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What is the source of the magnetic helicity shed by CMEs. The long-term helicity budget of AR 7978  

Pascal Demoulin   Submitted: 2001-12-19 08:57

An isolated active region (AR) was observed on the Sun during seven rotations, starting from its birth in July 1996 to its full dispersion in December 1996. We analyse the long-term budget of the AR relative magnetic helicity. Firstly, we calculate the helicity injected by differential rotation at the photospheric level using MDI/SoHO magnetograms. Secondly, we compute the coronal magnetic field and its helicity selecting the model which best fits the soft X-ray loops observed with SXT/Yohkoh. Finally, we identify all the coronal mass ejections (CMEs) that originated from the AR during its lifetime using LASCO and EIT/SoHO. Assuming a one to one correspondence between CMEs and magnetic clouds, we estimate the magnetic helicity which could be shed via CMEs. We find that differential rotation can neither provide the required magnetic helicity to the coronal field (at least a factor 2.5 to 4 larger), nor to the field ejected to the interplanetary space (a factor 4 to 20 larger), even in the case of this AR for which the total helicity injected by differential rotation is close to the maximum possible value. However, the total helicity ejected is equivalent to that of a twisted flux tube having the same magnetic flux as the studied AR and a number of turns in the interval [0.5,2.0]. We suggest that the main source of helicity is the inherent twist of the magnetic flux tube forming the active region. This magnetic helicity is transferred to the corona either by the continuous emergence of the flux tube for several solar rotations (i.e. on a time scale much longer than the classical emergence phase), or by torsional Alfvén waves.

Authors: Démoulin P., van Driel-Gesztelyi L., Mandrini C.H., Thompson B., Plunkett S., Kovari Zs., Aulanier, G. & Young A.
Projects:

Publication Status: A&A (in press)
Last Modified: 2001-12-19 09:01
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The magnetic helicity injected by shearing motions  

Pascal Demoulin   Submitted: 2001-12-19 08:43

Photospheric shearing motions are one of the possible ways to inject magnetic helicity into the corona. We explore their efficiency as a function of their particular properties and those of the magnetic field configuration. Based on the work of M.A. Berger, we separate the helicity injection into two terms: twist and writhe. For shearing motions concentrated between the centres of two magnetic polarities the helicity injected by twist and writhe add up, while for spatially more extended shearing motions, such as differential rotation, twist and writhe helicity have opposite signs and partially cancel. This implies that the amount of injected helicity can change in sign with time even if the shear velocity is time independent. We confirm the amount of helicity injected by differential rotation in a bipole in the two particular cases studied by DeVore (2000), and further explore the parameter space on which this injection depends. For a given latitude, tilt and magnetic flux, the generation of helicity is slightly more efficient in young active regions than in decayed ones (up to a factor 2). The helicity injection is mostly affected by the tilt of the AR with respect to the solar equator. The total helicity injected by shearing motions, with both spatial and temporal coherence, is at most equivalent to that of a twisted flux tube having the same magnetic flux and a number of turns of 0.3. In the solar case, where the motions have not such global coherence, the injection of helicity is expected to be much smaller, while for differential rotation this maximum value reduces to 0.2 turns. We conclude that shearing motions are a relatively inefficient way to bring magnetic helicity into the corona (compared to the helicity carried by a significantly twisted flux tube).

Authors: Démoulin P., van Driel-Gesztelyi L., Mandrini C.H., Lòpez Fuentes M. & Aulanier, G.
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Publication Status: Solar Physics, in press
Last Modified: 2001-12-19 08:43
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