Energy Conversion And Particle Acceleration In The Solar Corona

307 Pages·2003·8.37 MB·English

by  Klein (ed)

Introduction
Karl-Ludwig Klein
Solar flares and coronal mass ejections (CMEs) are prominent signatures of the
explosive release of energy stored in the coronal magnetic field. At some point
during the evolution of a magnetic field configuration the stored energy is con-
verted into heat, kinetic energy of bulk flows (ejecta) and energetic particle
populations. Observations with increasing sensitivity and temporal resolution
have shown that the corona varies on a broad range of time and length scales.
It is now thought that the apparently steadily heated corona itself may owe its
existence to similar explosive phenomena of interactions between plasmas and
magnetic fields. The manner in which magnetically stored energy is converted
to other forms, and prominently to energetic particle populations, is clearly of
central importance for coronal physics.
TheobservationalcoverageoftheSunimprovedtremendouslyduringthelast
decade. The SoHO, TRACE and Yohkoh satellites have been providing imaging
andspectroscopicdatawhichfurtheredourcurrentunderstandingthatmagnetic
reconnectionisakeyprocessofenergyreleaseinthesolaratmosphere.Energetic
particle diagnostics from gamma-rays to radio waves have shown the ubiquity
of non thermal particle populations in the corona, while in situ measurements
ofsolarenergeticparticlesnotablywithUlysses,WINDandACEdemonstrated
thevarietyofthesepopulationsandprovidednewelementsfortheirunderstand-
ing. The role of radio observations in this field is twofold: radio emissions are
an extremely sensitive tracer of non thermal electrons and therefore of energy
release processes, and the broad range of frequencies covered by contemporary
instruments probes these electrons from the low corona to interplanetary space.
Radio observations are therefore crucial to make the connection between the
dynamical plasma structures in the corona and in situ measurements of solar
energetic particles.
Given the wealth of new observations, and the significant progress in the
understanding of magnetic reconnection in collisionless plasmas from numeri-
cal studies and from measurements in the Earth’s magnetosphere, it appeared
timely, at the beginning of the RHESSI mission (Ramaty High Energy Spec-
troscopic Imager), to critically assess the state of the art, including input from
neighbouring fields of solar physics, especially the Earth’s magnetosphere. This
book is divided into three parts: energy conversion with emphasis on magnetic
reconnection,particleacceleration,andadiscussionofcurrentresearchandnew
diagnostic tools. Extended introductions to these subjects are given in [1, 2, 3].
K.-L.Klein(Ed.):LNP612,pp.1–6,2003.
(cid:1)c Springer-VerlagBerlinHeidelberg2003
2 Karl-Ludwig Klein
1 Energy Release and Magnetic Reconnection
In studies of the Earth’s magnetosphere and of solar flares it has been realised
that the interaction of magnetic flux from different sources creates regions of
strong magnetic field gradients (hence intense electric currents – current sheets)
wherethemagneticfieldmaydiffuseandchangeconnectivity.Thisisthecaseat
the interface between the solar wind and the magnetosphere as well as between
magnetic field structures in the solar corona and new flux that emerges from
the Sun’s interior. One consequence of magnetic reconnection is the detachment
andejectionofplasma-magneticfieldconfigurations(plasmoids).Suchplasmoid
ejections are inferred to exist in the solar corona from imaging observations in
visiblelight,atradioandX-raywavelengths,andarewellstudiedbyinsitumea-
surements in the Earth’s magnetotail. Another result of magnetic reconnection
is the conversion of energy stored in current systems into heat and bulk motion.
The original idea of magnetic reconnection first proposed by Sweet, followed
by Parker, in the 1950s is still valid. But their models of relatively long cur-
rent sheets could not explain the rapidity with which the process is observed to
proceed. Petschek’s conjecture that reconnection actually involves only a small
region where the magnetic field diffuses, whereas the bulk of the plasma flow is
diverted outside this region, at two pairs of slow-mode shocks, gave a physical
justification of small reconnection regions with a correspondingly high recon-
nection rate, but did not prove that current sheets actually evolve in this way.
The microphysical mechanism of reconnection is different for plasmas that are
dominated by collisions and those which are collisionless. Collisionless effects
are expected to play a key role in both the solar corona and the Earth’s mag-
netosphere. The reader is referred to [3] for an introductory text on magnetic
reconnection.
M.Scholer’sintroductorylecturegivesanoverviewofphenomenawheremag-
netic reconnection seems to play a role in the solar atmosphere and the Earth’s
magnetosphere.Imagingandspectroscopicobservationsofthesolaratmosphere,
especially at EUV and X-ray wavelengths, show a growing wealth of structures
and flows which are reminiscent of scenarios of magnetic reconnection. In situ
measurements at the magnetopause and in the magnetic tail of the Earth allow
for much more detailed quantitative analyses. They reveal that not all high-
speed flows show the relations expected for magnetic reconnection. Slow-mode
shocks predicted by Petschek-type reconnection models have been found in the
magnetotail, but only in a minority of cases where reconnection seems to occur.
The topologies of magnetic fields and scenarios of magnetic reconnection
in the solar atmosphere, as inferred from imaging observations, are discussed
by B. Vrˇsnak. He makes use of cartoon scenarios to show how the interaction of
magneticstructuresmayleadtothereconfigurationofthecoronalfield,plasmoid
formation and energy release in 2D and 3D configurations. These scenarios are
compared with the morphologies observed during flares and filament eruptions.
While the magnetic field in the corona is the crucial ingredient for the stor-
age and release of energy, direct measurements are difficult because of Doppler
broadening of visible emission lines. Infrared measurements recently provided
Introduction 3
first results, while cyclotron radiation at radio wavelengths is a well-established
tool to measure the field strength above sunspots. Other radio techniques hold
somepromiseforthefuturethroughmultifrequencyimaging.Forthetimebeing,
however, quantitative information on the coronal magnetic fields comes mostly
from the extrapolation of measurements in the photosphere. Y. Yan’s paper il-
lustrates how the strength and topology of the coronal magnetic field can be
inferred through a non-linear force-free field extrapolation. He discusses in par-
ticular the 14 July 2000 flare, including evidence for a magnetic flux rope and
an estimation of its energy content.
A crucial check of the calculated magnetic field topology in the corona are
imagingobservationsofplasmastructuresthataresupposedtobeshapedbythe
magnetic field. The high-cadence observations at arc-second scale provided by
TRACEhavesetnewstandardsincoronalimaging.L.FletcherandH.P.Warren
illustratethevarietyofcomplexmagneticgeometriesinvolvedinflaresandtheir
build-up phase. Detailed comparison with hard X-ray observations of acceler-
ated electrons underlines the role that these particles play in energy transport
from coronal sites of energy release to the chromosphere. The smallness of the
flux tubes guiding the particles is a challenge for our understanding of beam
propagation in plasmas.
Coherent plasma emission at decimetric and longer wavelengths from the
corona is a tracer of electron beam propagation and other processes, which the
observationssuggesttoberelatedtoenergyreleaseduringflaresofvarioussizes.
A. Benz reviews the basics of these processes and discusses the relevance of the
observations for understanding energy release and particle acceleration. Com-
bined spectrography and imaging at radio wavelengths, together with X-ray,
EUV and visible-light imaging of the corona, show geometries consistent with
particle acceleration and energy conversion in common scenarios of magnetic
reconnection. However, most of these observations pertain to higher regions in
the solar corona than those where radio spectrography suggests the bulk of the
energeticparticlestobeproducedduringsolarflares.Theopeningofthewindow
betweenabout500MHzandseveralGHztoimagingobservationsisanecessary
step to furthering our understanding of flare energy release.
Radio emission at centimetre and shorter wavelengths is mostly thermal
bremsstrahlung (also called free-free emission) from the quiescent atmosphere
andactiveregions,withsomecontributionofcyclotron(orgyroresonance)emis-
sionatlowharmonicsoftheelectroncyclotronfrequencyabovesunspots.During
flares, gyrosynchrotron radiation from non thermal electrons, sometimes up to
relativistic energies (where it is commonly called synchrotron radiation), domi-
natesmostoften.K.Shibasakireviewstheprocessesbywhichelectronsgenerate
these radiations and how they can be used to infer information on the radiating
particles, and presents flare observations with the Nobeyama Radioheliograph.
Hisinterpretationofaflareasadisruptionofcoronalmagneticstructuresunder
the effect of plasma pressure challenges the reconnection scenarios in a magnet-
icallydominatedatmosphere,andhighlightstheambiguitiesinherentinpresent
observations of solar flares.
4 Karl-Ludwig Klein
Progressinthetheoreticalunderstandingofmagneticreconnectioninnatural
plasmasisdiscussedbyD.Biskamp.Along-standingproblemistheslowrateof
reconnectioninaSweet-Parker-typecurrentsheet.Usinga2Dmodelofresistive
reconnection between two magnetic flux bundles with an arbitrarily imposed
localisation of high resistivity, Biskamp shows that reconnection becomes fast,
with a ratesimilarto thatpredicted by thePetschek scenario.Whilethereisno
reasontohaveanincreasedcollisionalresistivityinthereconnectionregion,non
collisional effects related to the dispersion of hydromagnetic waves are shown to
providetherequiredscales.Thepresentstatusofmodellingsuggeststhatenergy
ismostlyconvertedtoionbulkflowsinthereconnectingcurrentsheet,andonly
aminorfractionintoenergeticelectrons.Thismayimplythatprocesseswhichdo
notoccurinthediffusionregion,butaretheconsequenceoftheionoutflow,like
turbulence or shock waves, play the key role in particle acceleration, especially
of ions (see, however, Litvinenko’s paper).
2 Energetic Particles at and from the Sun
A fundamental consequence of energy release in a plasma is the generation of
energetic particle populations. Signatures of energetic particles at and from the
Sun range from frequently observed transient enhancements of electron fluxes
at typical energies up to a few keV that last between a second and several
days to the occasional events where gamma-ray observations and in situ mea-
surements in interplanetary space reveal relativistic electrons and ions. Remote
sensing observations and in situ measurements with high time resolution and
high sensitivity show that particle acceleration on all scales results from explo-
sive energy release. Flares are still the dominant source of information through
remotesensingdiagnostics,becausethewholerangeoftheelectromagneticspec-
trum,includingnucleargamma-rays,canbestudied.Coronalandinterplanetary
shocks are another source of energetic particle populations. The respective role
oftheseprocessesisasubjectofongoingdebatewhichisreflectedinthisvolume.
That a major fraction of energy released during flares is actually transferred
to accelerated electrons and nucleons is a consistent result of many years of
hard X-ray and gamma-ray analyses of solar flares reviewed by N. Vilmer and
A. MacKinnon. The authors describe the basic features of these emissions, as
well as radio and visible light diagnostics of energetic particles. They discuss in
detail the time scales of the radiation and how they relate to the acceleration
and transport of the particles in the coronal magnetic field. A fundamental
requirementfromtheobservationsisthattheaccelerationprocessesactontime
scales of seconds or less as shown by the variations of spectra and abundances.
The observations are discussed in the light of currently favoured acceleration
processes, involving direct electric fields, shock waves and turbulence.
The key role of energetic particles in transporting energy released in the
corona to the chromosphere, where the hard X-ray, gamma-ray and optical
emissions arise, is demonstrated by the advent of Hα observations with time
resolution≤1s.P.HeinzelandM.Karlicky´ reviewjointobservationsinHα and
Introduction 5
hard X-rays which show that energy may be transported much faster than by
thermal conduction, and discuss how these observations can be used for quan-
titative analyses. To this end they study the line formation under the effect of
pulsed particle beams, using a numerical model which describes both the fluid
behaviour of the system and the radiative transfer.
Apart from remote sensing of their characteristic radiations, particles ac-
celerated at the Sun can also be studied through in situ measurements in inter-
planetaryspace.Onemightexpectthatatcoronalaccelerationsiteswherethose
particles are energised which interact with the solar atmosphere, thereby gener-
ating the radiative signatures, particles are also injected into space. This would
infer a close correlation between e.g. hard X-ray and radio emission of electrons
in the solar atmosphere and electrons detected at 1 AU. S. Krucker’s overview
of electron and proton measurements with the WIND satellite shows that this
simple expectation is sometimes met, but that in a significant number of events
the injection of electrons into space is delayed with respect to the coronal ra-
dio emission. Suprathermal, but subrelativistic protons often behave in a still
different way which may reveal an energy-dependent release at different coronal
heights. Krucker argues that besides flares, shock waves driven by large-scale
coronal disturbances are significant accelerators of escaping particles at heights
>1R ,withoutinjectingelectronsinthelowcoronawheretheywouldgenerate
(cid:1)
radio emission.
Theinteractionofenergeticparticlepopulationswiththeturbulentinterplan-
etarymagneticfield,especiallythroughpitchanglescattering,blursthetracesof
theoriginalaccelerationprocesses.W.Dr¨ogereviewsquasi-linearmodelsofpar-
ticle transport and how they enable one to derive energy or momentum spectra
of electrons and protons from in situ measurements. He argues that the spectra
can be understood in terms of stochastic acceleration. Again, electrons and pro-
tons may have different behaviour. Where the acceleration occurs, and how it is
related to small-scale restructuring of the corona as in flares and to large-scale
reconfigurations like in coronal mass ejections, is not settled. The question can
certainly not be answered by in situ measurements alone, but measurements
in the inner heliosphere, where transport effects are minimised because of the
shorter travel paths, appear to be an essential tool for further progress.
Theories of particle acceleration processes relevant to natural plasmas are
discussed in the articles by Litvinenko and Schlickeiser. R. Schlickeiser presents
generalaspectsshowingthenecessityoftime-varyingelectricfields.Hedescribes
insomedetailtheaccelerationofparticlesthroughtheirinteractionwithvarious
typesofplasmawaveswhichareexpectedtobeabundantlygeneratedinthesolar
corona, e.g. in the interaction between a reconnection outflow and the ambient
plasma or in the vicinity of shock waves in the corona and the interplanetary
medium. An interesting feature of stochastic acceleration is that it predicts the
selective energisation of particles depending on their charge-to-mass ratio, as is
observed in many particle events in interplanetary space, in gamma-ray flares,
andmorerecentlyintheheavyiontemperaturesmeasuredinthequietcoronaby
SoHO.Wave-particleinteractionscanalsoleadtoaccelerationwhenarapidmass
6 Karl-Ludwig Klein
flow interacts with an ambient plasma. Schlickeiser shows this for a relativistic
flow which may have some relevance in active galaxies. An interesting question
in the solar context is if a rapid, though largely non-relativistic, bulk flow such
asacoronalmassejectioncanaccelerateparticlesthroughasimilarmechanism.
Y.Litvinenkoanalysesparticletrajectoriesinreconnectingcurrentsheetsand
evaluates the energy gain and acceleration time scales. The three-dimensional
magnetic field structure in the current sheet determines the energy spectra of
the accelerated particles. Deka-keV electron beams are expected in large-scale
current sheets, and electrons can be accelerated up to MeV energies if a tear-
ing instability creates multiple singular lines. Protons respond differently to the
non uniform magnetic field, but acceleration up to GeV energies seems to be
suggested by preliminary 2D analyses for relatively strong magnetic fields.
3 Aspects of Current Research
Currentresearchinthefieldofenergeticparticlesandenergyreleaseinthesolar
atmosphere was discussed by the participants of the CESRA workshop during
three half-day working group sessions. The summary by B. Kliem, A. MacKin-
non, G. Trottet and T. Bastian gives a flavour of the discussions and puts them
intothebroadercontextofresearch,startingwithabriefcomparativedescription
of scenarios of magnetic field reconfiguration during flares and mass ejections.
Very recent work on the signatures of magnetic reconnection and MHD turbu-
lence,especiallyatradiowavelengths,ofparticleaccelerationandtransport,and
of large-scale eruptive phenomena and energetic particles in space is discussed.
A major new diagnostic of the flaring solar atmosphere is observations at
wavelengths ≤1 mm. Pioneering work using the University of Cologne telescope
at Gornergrat (Switzerland) and the newly developed Brazilian Solar Submil-
limetre Telescope (SST) in the Argentinan Andes was reported at the workshop
bythegroupsfromBerneandSa˜oPaulo.P.Kaufmannreviewsthehistoryofso-
lar observations at these wavelengths, presents the setup of SST, and illustrates
its diagnostic potential for measuring the synchrotron radiation from relativis-
tic electrons. Sub-millimetre and far infrared observations of solar flares hold
promise for providing new constraints on the acceleration processes and also as
a diagnostic of the energy transport in the flaring solar atmosphere.
References
1. A.O. Benz: Plasma Astrophysics - Kinetic Processes in Solar and Stellar Coronae
(Kluwer Academic Publishers, Dordrecht, 1993)
2. T.E.Cravens:PhysicsofSolarSystemPlasmas (CambridgeUniversityPress,Cam-
bridge, 1997)
3. R.A. Treumann, W. Baumjohann: Advanced Space Plasma Physics (Imperial Col-
lege Press, London, 1997)
Magnetic Reconnection on the Sun
and in the Earth’s Magnetosphere
Manfred Scholer
Centre for Interdisciplinary Plasma Science,
Max-Planck-Institut fu¨r extraterrestrische Physik, 85740 Garching, Germany
Abstract. ObservationalevidenceformagneticreconnectionintheSun’satmosphere
and in the Earth’s magnetosphere is reviewed. On the Sun reconnection is inferred
from observations of high speed flows derived from motion of X-ray or EUV features,
fromtheidentificationofstructuresinthesoftX-rayimages,andfromtheobservation
of energetic particle events in hard X-ray images. In particular the soft X-ray images
during flares are supposed to exhibit the large scale magnetic field structure. In the
Earth’smagnetosphereinsituobservationsbysatellitesataparticularlocationreveal
magneticfieldandflowvariations.Totheseonecanapplyconservationlawsandverify
whether boundaries, like the magnetopause, have a normal magnetic field component,
i.e., are rotational discontinuities. In the magnetotail the magnetic field structure can
be deduced from the temporal variation of the magnetic field during high speed flow
events.
1 Evidence for Reconnection on the Sun
1.1 Long Duration Event Flares
Indicationforreconnectionbeingresponsibleforlongdurationeventflarescomes
from the observations of cusp-shaped soft X-ray flare loops, from observations
of chromospheric evaporation by using extreme-ultraviolet (EUV) spectrohelio-
grams, and from observation of inflow into the neutral line by using the motion
of structures in EUV images.
YohkohsoftX-rayobservationsofcusp-shapedstructuresintheseeventshave
firstsupportedtheviewthatanexpansionandrestructuringofthemagneticfield
occurs prior to the flare and that reconnection in a newly created neutral sheet
playsakeyroleintheflareenergyrelease(Tsunetaetal.,1992;Tsuneta,1996).
Thetipofthecuspisinterpretedtobetheremnantofthekinkofthereconnected
field lines. This tip usually increases in height, at the same time the distance
between the footpoints increases. A reconstruction of the temperature structure
duringthedecayphaseofaflarebyTsuneta(1996)hasshownthattheoutershell
ofthearcheshasthehighesttemperature(1.1×107 K),whereasthetemperature
decreases toward the inner arches (7×106 K). This is interpreted in terms of
energy supply by reconnection near the top of the loop and subsequent cooling
of more inner loops by heat conduction and filling with evaporated plasma.
Figure 1a shows an X-ray map (negative image) from an east limb flare. North
is up and east to the left. Figure 1b shows the plasma temperature; while the
K.-L.Klein(Ed.):LNP612,pp.9–27,2003.
(cid:1)c Springer-VerlagBerlinHeidelberg2003
10 Manfred Scholer
Fig. 1. (a) X-ray and (b) temperature maps of the 1992 February 21 flare. Both the
X-raybrightnessandtemperatureareshownbynegativeimage(afterTsuneta,1996).
X-raybrightnesshadapeakintheinnermostpartoftheflareregion,theplasma
temperature was hotter in the outer loops (where darker color corresponds to
hotter region). Note that outside the loop the X-ray intensity rapidly drops
so that here the temperature map becomes unreliable. Color-coded maps with
contours of the X-ray intensity level superimposed can be found in Tsuneta
(1996). Although direct evidence for coronal slow MHD shocks has not yet been
obtained, the overall morphology and temperature distribution in this type of
flares are consistent with the prediction of Petschek-type reconnection with two
MHD slow shocks bounding the reconnection outflow region.
Two-dimensional MHD simulations of reconnection including heat conduc-
tion predict chromospheric evaporation (i.e. upflows) in the outermost recon-
nected loops (Yokoyama and Shibata, 1997; 1998). As the loops become discon-
nected from the reconnection site, they cool and condensation sets in, leading
to downflows in the older loop legs. Czaykowska et al. (1999) used data from
theCoronalDiagnosticSpectrometer(CDS)onSOHOtodemonstratethatina
two-ribbon flare there was indeed gentle upflow near the outer edges of the Hα
ribbons.TheupflowwasobservedasblueshiftsintheEUVOV,FeXVI,andFe
XIXlinesinlowemissionregions,and,astimeproceeds,theblueshiftedregions
movedwiththeHαribbonstogreaterdistancesawayfromthemagneticneutral
line. In the region between the Hα ribbons and the neutral line bright down-
flowingplasmaisobserved.Inalaterpaper,Czaykowskaetal.(2001)compared
the upflow velocities with those expected from different chromospheric heating
models.Sinceanonthermalbeamofenergeticelectrons>15keVwouldproduce
significant hard X-ray emission, which is not observed, they concluded that the
most likely energy transport mechnism is thermal conduction, as in the model
of Yokoyama and Shibata (1997; 1998).
The final missing piece of evidence for reconnection, that is the inflow into
the reconnection region, has recently been reported by Yokoyama et al. (2001).
Theinflowvelocitywasderivedbytracingthemovementsofthreadlikepatterns
in images obtained by the EUV Imaging Telescope (EIT) on SOHO. Yohkoh
soft X-ray images during this flare showed a plasmoid ejection before the inflow
Reconnection on the Sun and in the Magnetosphere 11
sets in and a cusp-shaped loop during the inflow as a result of piling up of
reconnected field lines. Yokoyama et al. (2001) derived from the upper limit of
the inflow velocity of 5 km/s a reconnection rate of M = 0.001−0.03, where
A
M is the inflow Alfv´en Mach number.
A
1.2 Impulsive Flares
For a long time short-lived impulsive flares were believed to be “loop flares” in
which the energy release occurs within the loop and not due to reconnection
at an X point/line above the loop. This was based on the lack of cusp-shaped
structuresofimpulsiveflaresinsoftX-rayimages.However,Masudaetal.(1994)
discovered that in some of the impulsive compact loop flares occuring near the
solarlimbaloop-tophardX-ray(HXR)sourceappearedwellaboveasoftX-ray
bright source during the impulsive phase. Figure 2 shows hard X-ray contour
maps obtained during the rising phase of the 13 January 1992 flare near the
westsolarlimb.ThesoftX-rayloopcoincidesapproximatelywiththe14-23keV
contour maps (the backbone of the soft X-ray loop is shown by solid lines). The
higher energy maps reveal double foot point sources and an additional source
well above the apex of the soft X-ray loop. The hard X-ray source above the
soft X-ray loop indicates a source of energetic electrons outside of the closed
loop. Masuda et al. (1994) proposed that the loop-top hard X-rays are due to
nonthermal electrons produced by a fast mode shock when the reconnection
flow impinges on the loop. The nonthermal electrons stream along field lines
and produce the bright footpoints. This scenario is sketched in Fig. 3.
As pointed out by Shibata et al. (1995) (and indicated in Fig. 3) two ribbon
flaresshouldbeaccompaniedbyaplasmoidejectionhighabovethereconnected
loops seen as soft X-ray loops. Shibata et al. (1995) actually found an erupting
feature in soft X-rays during the 13 January 1992 flare, although the velocity
was considerably smaller than the theoretically expected velocity (= Alfv´en ve-
locity). Ohyama and Shibata (1998) have analyzed in detail the ejecta during
an impulsive flare (Fig. 4). The speed of the ejecta as obtained from the time
Fig. 2. Hard X-ray images of an impulsive west limb flare in three energy bands
obtained from the hard X-ray telescope on Yohkoh (after Masuda et al., 1994).
12 Manfred Scholer
Fig. 3. The reconnection - plasmoid ejection model for compact loop flares (from
Shibata et al., 1995).
Fig. 4. Left:timesequenceofsoftX-rayimagesduringthe5October1992flare.The
ejected plasma is indicated by an arrow. Right: apparent heights of the plasmoid and
flare loop, and hard X-ray intensity versus time (from Ohyama and Shibata, 1998).
sequence analysis shows an outward acceleration (Fig. 4, right hand side) The
ejectaispenetratedbyalarge-scaleexpandingloopasindicatedintheschematic
in Fig. 3. The kinetic energy of the ejecta was by an order of magnitude smaller