Plasma Astrophysics, Part II: Reconnection and Flares

513 Pages·2013·5.306 MB·English

by  Boris V. Somov (auth.)

Plasma Astrophysics, Part II
Astrophysics and Space Science Library
EDITORIALBOARD
Chairman
W.B.BURTON,NationalRadioAstronomyObservatory,Charlottesville,Virginia,U.S.A.
([email protected]);UniversityofLeiden,TheNetherlands
([email protected])
F.BERTOLA,UniversityofPadua,Italy
J.P.CASSINELLI,UniversityofWisconsin,Madison,U.S.A.
C.J.CESARSKY,CommissionforAtomicEnergy,Saclay,France
P.EHRENFREUND,LeidenUniversity,TheNetherlands
O.ENGVOLD,UniversityofOslo,Norway
A.HECK,StrasbourgAstronomicalObservatory,France
E.P.J.VANDENHEUVEL,UniversityofAmsterdam,TheNetherlands
V.M.KASPI,McGillUniversity,Montreal,Canada
J.M.E.KUIJPERS,UniversityofNijmegen,TheNetherlands
H.VANDERLAAN,UniversityofUtrecht,TheNetherlands
P.G.MURDIN,InstituteofAstronomy,Cambridge,UK
F.PACINI,IstitutoAstronomiaArcetri,Firenze,Italy
V.RADHAKRISHNAN,RamanResearchInstitute,Bangalore,India
B.V.SOMOV,AstronomicalInstitute,MoscowStateUniversity,Russia
R.A.SUNYAEV,SpaceResearchInstitute,Moscow,Russia
Forfurthervolumes:
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Boris V. Somov
Plasma Astrophysics, Part II
Reconnection and Flares
Second Edition
123
BorisV.Somov
AstronomicalInstituteandFaculty
ofPhysics
M.V.LomonosovMoscowStateUniversity
UniversitetskijProspekt13
119991Moskva,Russia
ISSN0067-0057
ISBN978-1-4614-4294-3 ISBN978-1-4614-4295-0(eBook)
DOI10.1007/978-1-4614-4295-0
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Coverillustration:Totalsolareclipseon2006March29inTurkey.Theshapeofthecoronarevealsthe
global structure ofthedipole-like magnetic fieldwith openfieldlines atthepoles. Outofthe poles,
the so-called coronal streamers are the spectacular manifestations of the solar wind interaction with
magneticfield.HeremagneticreconnectiondetachesthesolarmagneticfieldfromtheSun.
Photograph reproduced with kind permission by Hanna Druckmu¨llerova´ and Miloslave Druckmu¨ller
(http://www.zam.fme.vutbr.cz/(cid:2)druck/Eclipse).
Printedonacid-freepaper
SpringerispartofSpringerScience+BusinessMedia(www.springer.com)
PhotofromSyrovarskii’sfamilyarchive
Atthattime
themagneticreconnection
wasanewidea...

Reconnection and Flares
Introduction
Magnetic fields are easily generated in astrophysical plasma owing to its high
conductivity. Magnetic fields, having strengths of order few 10(cid:2)6 G, correlated
on several kiloparsec scales are seen in spiral galaxies. Their origin could be
due to amplification of a small seed field by a turbulent galactic dynamo. In
severalgalaxies,likethefamousM51,magneticfieldsarewellcorrelated(oranti-
correlated) with the optical spiral arms. These are the weakest large-scale fields
observedin cosmic space. The strongestmagnetsin space are presumablythe so-
called magnetars, the highly magnetized (with the strength of the field of about
1015G)youngneutronstars(DuncanandThompson1992;Becker2009)formedin
thesupernovaexplosions.
The energy of magnetic fields is accumulated in astrophysicalplasma, and the
suddenreleaseofthisenergy–anoriginalelectrodynamical‘burst’or‘explosion’
– takes place under definite but quite general conditions (Peratt 1992; Sturrock
1994; Kivelson and Russell 1995; Rose 1998; Priest and Forbes 2000; Somov
2000; Kundt 2001; Hurley et al. 2005). Such a ‘flare’ in astrophysical plasma is
accompanied by fast directed ejections (jets) of plasma, powerful flows of heat
andhardelectromagneticradiationaswellasbyimpulsiveaccelerationofcharged
particlestohighenergies.
This phenomenon is quite a widespread one. It can be observed in flares on
the Sun and other stars (Haisch et al. 1991), in the Earth’s magnetosphere as
magnetic storms and substorms (Nishida and Nagayama 1973; Tsurutani et al.
1997;KokubunandKamide1998;Nagaietal.1998;Nishidaetal.1998),incoronae
ofaccretiondisksofcosmicX-raysources(Galeevetal.1979;Somovetal.2003a),
innucleiofactivegalaxiesandquasars(OzernoyandSomov1971;Begelmanetal.
1984). However this process, while being typical of astrophysical plasma, can be
directlyandfullystudiedontheSun.
The Sun is the only star that can be imaged with spatial resolution high
enough to reveal its key (fine as well as large-scale) structures and their dynamic
behaviors. This simple fact makes the Sun one of the most important objectives
in astronomy. The solar atmosphere can be regarded as a natural ‘laboratory’ of
vii
viii ReconnectionandFlares
astrophysical plasmas in which we can study the physical processes involved in
cosmicelectrodynamicalexplosions.
Weobservehowmagneticfieldsaregenerated(strictlyspeaking,howtheycome
to the surface of the Sun, called the photosphere). We observe the development
of solar flares (e.g., Strong et al. 1999) and other non-stationary large-scale
phenomena,such as a gigantic arcade formation,coronaltransients, coronalmass
ejections into the interplanetary medium (see Crooker et al. 1997), by means
of ground observatories (in radio and optical wavelength ranges) and spaceships
(practicallyinthewholeelectromagneticspectrum).
As a very good example, on board the Yohkoh satellite (Ogawara et al. 1991;
Acton et al. 1992), two telescopes worked in soft and hard X-ray bands (Tsuneta
etal.1991;Kosugietal.1991)during10yearsandallowedustostudythecreation
and developmentof non-steadyprocessesin the solar atmosphere(Ichimotoet al.
1992; Tsuneta et al. 1992; Tsuneta 1993), acceleration of electrons in flares. Of
particular interest to the acceleration process in solar flares were the hard X-
ray emissions from the so-called chromospheric ribbons produced by accelerated
electronswithenergy(cid:3)>10keV(Masudaetal.2001).
The LASCO experiment on board the Solar and Heliospheric Observatory,
SOHO(Domingoetal.1995)makesobservationsofsucheventsinthesolarcorona
outto30solarradii.MoreoverSOHOisequippedwithaninstrument,thefulldisk
magnetographMDI(Scherreretal.1995),forobservingthesurfacemagneticfields
oftheSun.FollowingSOHO,thesatelliteTransitionRegionandCoronalExplorer
(TRACE)was launchedto obtain highspatial resolution X-ray images(see Golub
etal.1999).Withthesolarmaximumof2000,wehadanunprecedentedopportunity
tousethethreesatellitesforcoordinatedobservationsandstudyofsolarflares.
The Reuven Ramaty High-Energy Solar Spectroscopic Imager (RHESSI) was
launched in 2002 and observes solar hard X-rays and gamma-rays from 3keV
to 17MeV with spatial resolution as high as 2.3arcsec (Lin et al. 2002, 2003a).
Imagingofgamma-raylines,producedbynuclearcollisionsofenergeticionswith
thesolaratmosphere,provideddirectinformationofthespatialpropertiesoftheion
accelerationin solar flares(Hurfordetal. 2003).RHESSI observationsallowedus
toinvestigatephysicalpropertiesofsolarflaresin manydetails(e.g., Fletcherand
Hudson2002;Kruckeretal.2003).
ThreeexperimentsareflownontheHinodemission(Kosugietal.2007)launched
in2006.TheprimaryobjectiveofHinodeistostudytheoriginofthecoronaandthe
couplingbetween the fine magnetic structure in the photosphereand the dynamic
processes occurring in the corona. Hinode has three high-resolution telescopes in
visible light,softX-ray,and extremeultra-violet(EUV)wavelengths:(a)a 50-cm
opticaltelescope,theSolarOpticalTelescope(SOT),(b)anX-raytelescope(XRT)
forimagingthehigh-temperaturecoronalplasmawithangularresolution(cid:2)1arcsec,
(c)anEUVimagingspectrometer(EIS)fordiagnosingeventsobserved.
The telescope SOT (Tsuneta et al. 2008) gives measurements of the magnetic
fields in features as small as 100km in size thereby providing ten times better
resolutionthanotherspace-andground-basedmagneticfieldmeasurements.Sothe
SOTinstrumentgivesopportunitytoobservetheSuncontinuouslywiththelevelof
Introduction ix
resolutionthatground-basedobservationscanmatchonlyunderexceptionallygood
conditions.SOTaimsatmeasuringthemagneticfieldandtheDopplervelocityfield
inthephotosphere.
Newspace-borneobservationsoftheSunfromHinode,RHESSI,SolarDynam-
icsObservatory(SDO;TarbellandAIATeam2011),andSolarTerrestrialRelations
Observatory(STEREO)haveproducedstunningresults,invigoratedsolarresearch
andchallengedexistingtheoreticalmodels.
Thelinkbetweenthesolarflaresobservedandtopologyofthemagneticfieldin
active regions,in whichthese flaresoccurred,was investigatedby Gorbachevand
Somov(1989,1990).Theydevelopedthefirsttopologicalmodelofanactualflare,
theflareon1980,November5,andhaveshownthat
all large-scale characteristic features of a flare can be explained by the
presenceofacurrentlayerformedontheso-calledseparatorwhichisthe
intersectionoftheseparatrixsurfaces.
In particular, the flare ribbons in the chromosphere as well as the ‘intersecting’
soft X-ray loops in the corona are the consequences of a topological structure of
amagneticfieldneartheseparator.
An increasing number of investigations clearly relates the location of a ‘chro-
mospheric flare’ – the flare’s manifestation in the solar chromosphere – with
the topological magnetic features of active regions (Mandrini et al. 1991, 1993;
De´moulin et al. 1993;Bagala´ et al. 1995; Longcope and Silva 1998). In all these
works it was confirmed that the solar flares can be considered as a result of the
interaction of large-scale magnetic structures; the authors derived the location of
theseparatrices–surfacesthatseparatecellsofdifferentfieldlineconnectivities–
andoftheseparator.
Thesestudiesstronglysupportedtheconceptofmagneticreconnectioninsolar
flares (Giovanelli 1946; Dungey 1958; Sweet 1958). Solar observations with the
Hard X-ray Telescope (HXT) and the Soft X-ray Telescope (SXT) on board the
Yohkohsatelliteclearlyshowedthat
themagneticreconnectioneffectiscommontoimpulsive(compact)and
gradual(largescale)solarflares
(Masuda et al. 1994, 1995). However, in the interpretation of the Yohkoh data,
the basic physics of magnetic reconnection in the solar atmosphere remained
uncertain(seeKosugiandSomov1998).Significantpartsofthebookinyourhands
are devoted to the physics of the reconnection process, a fundamental feature of
astrophysicalandlaboratoryplasmas.
Solarflaresandcoronalmassejections(CMEs)stronglyinfluencetheinterplan-
etaryandterrestrialspacebyvirtueofshockwaves,hardelectromagneticradiation
andacceleratedparticles(KivelsonandRussell1995;Miroshnichenko2001).That
iswhytheproblemof‘weatherandclimate’predictioninthenearspacebecomes
moreandmoreimportant.The term‘nearspace’refersto thespace thatiswithin
thereachoforbitingstations,bothmannedandautomated.Thenumberofsatellites
x ReconnectionandFlares
(meteorological,geophysical,navigationalones) with electronic systems sensitive
totheionizingradiationofsolarflaresissteadilygrowing.
It has been established that adverse conditions in the space environment can
cause disruption of satellite operations, communications, and electric power dis-
tribution grids, thereby leading to socioeconomic losses (Wright 1997). Space
weather (Hanslmeier 2007; Lilensten 2007) is of growing importance to the
scientific community and refers to conditions at a particular place and time on
the Sun and in the solar wind, magnetosphere,ionosphere,and thermospherethat
can influence the performance and reliability of spaceborne and ground-based
technologicalsystemsandcanaffecthumanlifeorhealth.
It is no mere chancethat solar flares and coronalmass ejectionsare of interest
to physicians, biologists and climatologists. Flares influence not only geospace –
theterrestrialmagnetosphere,ionosphereandupperatmosphere(Hargreaves1992;
Horwitzetal.1998;deJager2005)butalsothebiosphereandtheatmosphereofthe
Earth.Theyarethereforenotonlyofpurescientificimportance;theyalsohavean
appliedorpracticalrelevance.Forthisreason,thecomingyearspromisetobethe
golderaofsolarandheliosphericphysics.
Thelatteraspectis,however,certainlybeyondthescopeofthistext,thesecond
volume of the book “Plasma Astrophysics”, lectures given the students of the
Astronomical Division of the Faculty of Physics at the Moscow State University
in spring semesters over the years after 2000. The subject of the present book
“Plasma Astrophysics. Part II. Reconnection and Flares” is the basic physics of
the magnetic reconnection phenomenon and the reconnection related flares in
astrophysicalplasmas.Thefirstvolumeofthebook,“PlasmaAstrophysics.PartI.
FundamentalsandPractice”(Somov2012a,referredinthetextasPartI),isunique
in coveringthe main principlesand practicaltoolsrequiredforunderstandingand
workinmodernplasmaastrophysics.
Magneticreconnectionisafundamentalprocessinplasmasbywhichmagnetic
fieldtopologychangesandconnectionsofplasmaparticleswiththemagneticfield
are re-arranged.Reconnectionplaysa keyrole in explosiveenergyrelease events,
flares in astrophysical plasma. However a direct observations of 3D geometry of
magnetic reconnection in space was never able to make until launching of ESA
Cluster constellation. The Cluster mission providesthe first opportunityto detect
the 3D magnetic structurethrough4-pointmeasurementas the spacecrafttraverse
theheartofareconnectionregion.
Inaddition,theCluster observationscoordinatedwithothersatellites enableus
toseetheevolutionofstructuresatsmallscaleswithintheClustertetrahedron,and
also at large scales with other satellites. These measurements make it possible to
observetheglobalpatternofreconnectionatthemagnetopauseandinthemagneto-
tailoftheEarth(e.g.,Xiaoetal.2007).Insituevidenceofthefull3Dreconnection
geometryandassociateddynamicsprovidesanimportantsteptowardsestablishing
anobservationalframeworkof3Dreconnection.
In the past half a century, great progresses in understanding of the magnetic
reconnection effect has been gained through theoretical analysis, numerical sim-
ulations, and experimental and satellite observations. We would like to see these
progresses,statedmostsimply.