Solar surface rotation: N-S asymmetry and recent speed-up

0.48 MB·English

by  L. Zhang

Astronomy&Astrophysicsmanuscriptno.zhangetal2014 (cid:13)c ESO2015
January13,2015
LettertotheEditor
Solar surface rotation: N-S asymmetry and recent speed-up
L.Zhang1,2 ,K.Mursula1,andI.Usoskin1,3
1 ReSoLVECentreofExcellence,DepartmentofPhysics,UniversityofOulu,Finland,e-mail:[email protected]
2 KeyLaboratoryofSolarActivity,NationalAstronomicalObservatories,ChineseAcademyofSciences,Beijing,China
3 Sodankyla¨GeophysicalObservatory,UniversityofOulu,Finland
Preprintonlineversion:January13,2015
5
ABSTRACT
1
0
Context.Therelationbetweensolarsurfacerotationandsunspotactivitystillremainsopen.Sunspotactivityhasdramaticallyreduced
2
insolarcycle24andseveralsolaractivityindicesandfluxmeasurementsexperiencedunprecedentedlylowlevelsduringthelastsolar

n minimum.
a Aims.Weaimtorevealthemomentaryvariationofsolarsurfacerotation,especiallyduringtherecentyearsofreducingsolaractivity.
J Methods.Weusedadynamic,differentiallyrotatingreferencesystemtodeterminethebest-fitannualvaluesofthedifferentialrotation
2 parametersofactivelongitudesofsolarX-rayflaresandsunspotsin1977-2012.
1 Results.The evolution of rotation of solar active longitudes obtained with X-ray flares and with sunspots is very similar. Both
hemispheres speed up since the late1990s, withthe southern hemisphere rotating slightlyfaster than the north. Earlier, in1980s,
] rotationinthenorthernhemispherewasconsiderablyfaster,butexperiencedamajordecreaseintheearly1990s.Ontheotherhand,
R littlechangewasfoundinthesouthernrotationduringthesedecades.Thisledtoapositiveasymmetryinnorth-southrotationratein
S theearlypartofthetimeintervalstudied.
Conclusions.Therotationofbothhemispheres hasbeenspeedingupatroughly thesameratesincelate1990s, withthesouthern
.
h hemisphere rotating slightly faster than the north. This period coincides with the start of dramatic weakening of solar activity, as
p observedinsunspotsandseveralothersolar,interplanetaryandgeomagneticparameters.
-
o Keywords.Sun:activelongitudes—flares—sunspots—rotation
r
t
s
a
1. Introduction convincingly that the ALs are persistent structures that sustain
[
forseveraltensofyears,andpossiblyevenlonger.Alargefrac-

1 A number of solar activity indices and flux measurements tion of various forms of solar activity are produced by regions
v reached abnormally low values during last solar minimum, whichare themselvesdifferentiallyrotating.Thepersistenceof
8 which was also exceptionally long. The activity level in the ALs is clearly demonstrated in this reference frame, and the
9 ongoing cycle 24 dramatically reduced compared to previous level of longitudinal asymmetry increases significantly higher
6 few solar cycles. The remarkably long and deep solar mini- than in any rigidly rotating system. Moreover, the stronger
2
mum and the weak cycle 24 have caused intensive attention formsofsolaractivityaremoreasymmetricallydistributedthan
0
. in the solar and space physics community (Jianetal. 2011; theweaker,morediffusiveforms(Zhangetal.2011a).
1 Clette&Lefe`vre 2012; Wangetal. 2009; Solomonetal. 2010,
Solar surface rotation has been studied for a long time us-
0 2011;Cliver&Ling2011).
ing various forms of solar activity (Balthasar&Wo¨hl 1980;
5
Here we study the changes in solar surface rotation during Pulkkinen&Tuominen 1998; Wangetal. 1988; Brajsˇaetal.
1
: thelastfewdecades,includingtheperiodofactivityweakening, 2000).Theseculardecelerationofsolarrotationwassuggested
v by analyzing the rotation of solar active longitudes. Sunspots by Brajsˇaetal. (2006) and Lietal. (2014), while a secular ac-
Xi andotherformsofsolarmagneticactivityarenotuniformlydis- celerationtrendwasfoundbyHeristchi&Mouradian(2009).A
tributedin solar longitude,butare centeredaroundcertain lon- north-south(N-S)asymmetryinsolarrotationhasbeenreported
r
a gitude bands, which are called active longitudes (ALs). Active bymanyauthors(Brajsˇaetal.2000;Javaraiah&Komm1999).
longitudes have been observed in several studies using differ- Veryrecently,bystudyingtherotationofALsofsunspotsinthe
ent data bases (Temmeretal. 2006; Chenetal. 2011; Li 2011; lasttwelvesolarcyclesitwasfoundthatthelong-termevolution
Murako¨zy&Ludma´ny2012). ofthesolarsurfacerotationhasaquasi-periodicityabout80-90
Typically the ALs can be seen clearly only during a few years (Zhangetal. 2013). Rotation was also found to be N-S
solar rotations. The longitudinal distribution of active regions asymmetric duringmost of this time period.The consistent N-
becomes rather symmetric after a moderately long time inter- SasymmetryofsolarsurfacerotationwasconfirmedbySuzuki
val, typically a few years. Accordingly, a simple way of cal- (2014).
culating the longitudinaldistribution of solar activity using the The level of non-axisymmetrywas found to increase when
Carringtonlongitudeoftheactiveregionsdoesnotyieldaper- using shorter fit lengths (Zhangetal. 2013). The average non-
sistentlyinhomogeneousdistribution. axisymmetry of sunspots increased from 0.280 for the 5-year
A few recent studies (Berdyugina&Usoskin 2003; runningmethodto0.464forthe1-yearmethod.Sincetherota-
Usoskinetal. 2005; Zhangetal. 2011a,b) using a dynamic, tionratesvaryatatimescaleofoneyear,therotationparameters
differentiallyrotatingcoordinatesystemhavebeenabletoshow obtainedfortheshorterintervalsrepresentthemomentaryrota-
1
L.Zhang,K.Mursula,andI.Usoskin:RotationofALsofsolarX-rayflares
tionrate valuesofALsmoreclosely.Accordingly,theALsare We studied both the weighting method and no-weight
moreaccuratelydeterminedfromshorterintervals. method.Theresultsobtainedwiththetwomethodsareverysim-
However,veryshortintervalfitsofafewsolarrotationsonly ilar.Thus,onlytheresultsofno-weightmethodarepresentedin
may lose the continuous evolution of the rotational phase and thispaper.
therebycauseincreaseduncertaintywhendeterminingtherota-
tionparameters.Thethree-yearfitintervallengthhasbeenfound
tobetheoptimumlengthwhenbothsatisfactoryrepresentation
andcontinuousevolutionaremet(Zhangetal.2013).
3. Results
Herewestudythevariationofsolarrotationinrecentyears
bystudyingtherotationparametersofALsofsolarX-rayflares 3.1.N-Sasymmetryandrecentspeed-upofsolarrotation
in 1977-2012. To compare the results of flares with those of
sunspots, we also study the rotation parameters determined by WehavecalculatedtheyearlyrotationratesΩ ofALsdefined
17
sunspotsforthecommonperiod.Wepresenttheyearlyrotation byformula(1)atthereferencelatitude17◦ forX-rayflaresand
rates at the average latitude of flares and of sunspots, respec- sunspots, separately. The values of Ω are presented in Fig.
17
tively. 1 (left-hand panels for X-ray flares and right-hand panels for
sunspots;toppanelsfornorthernhemisphereandbottompanels
forsouthernhemisphere).Opencirclesdepicttheyearlyvalues
2. Dataandanalysismethod of Ω for the central year of each 3-year fit interval. The 1σ
17
errorforeachΩ valueiswithin ±0.015deg/daywhich istoo
We study the solar X-ray flares of class-B and higher ob- 17
small to be shown in the figure. To demonstrate the long-term
served by the NOAA GOES satellites during the period of
variation pattern more clearly, the 11-point running mean (av-
1977-2012. Most X-ray flares were identified by simulta-
erage over one solar cycle) is shown in the figure as red filled
neous optical flare observations in the data prior to 2006.
circles.
However, a large fraction of the X-ray flares are without an
accompanying optical flare since 2007. No X-ray flare is The rotation rates of Ω17 in the two hemispheres obtained
observed together with an optical flare in 2011. Fortunately, with X-ray flares depict little correlation from 1977 until pre-
the flare location can be identified during the whole time 2000. In the 1980s the north rotates faster than the south but
periodusingtheNOAA/USAFsunspotregionnumber.Tokeep slows down in the 1990s even below the southernrate. During
the treatment consistent throughout the entire study period, thistimethesouthernrotationrateremainsratherconstant.The
we use the location of the sunspot region where the flare rotation rate has been speeding up in both hemispheres since
occurred as the location of the flare. GOES X-ray flare data pre-2000,with thesouthernhemisphererotatingfasterthanthe
(ftp://ftp.ngdc.noaa.gov/STP/space-weather/solar-data/solar-featurenso/srothla.rT-flhaerreost/axt-iroanyse/vgooleust/i)onobtainedforsunspotsdepictsano-
provides the NOAA/USAF sunspot group number where tablysimilarpatternasflares.However,therecentrotationofthe
the flare is observed, but no location information for northern hemisphere according to sunspots is somewhat faster
the sunspot groups. The location information of sunspot than thataccordingto flares. Therefore,the hemisphericasym-
groups can be retrieved from NOAA/USAF sunspot data metry in the rotation rate is smaller for sunspots. This is prob-
(http://solarscience.msfc.nasa.gov/greenwch.shtml). For ably caused by a few large and flare productive active regions
sunspots we study the NOAA/USAF sunspot groups for whichrotateratherslowlyinthenorthernhemisphere.Figure2
thecommonperiodofsolarflares. depictstheN-Sasymmetry((N-S)/(N+S))ofsolarrotationat
the latitude of 17◦ obtained for X-ray flares (open circles) and
The analysis method can be found in earlier studies
sunspots(filledcircles).Theevolutionofthehemisphericasym-
(Usoskinetal. 2005; Zhangetal. 2011a, 2013), and is briefly
describedhere.Differentialrotationof the solar surfaceis usu- metry of solar rotation is quite similarly depicted by flares and
sunspots:whiletheasymmetryhasbeenfairlyconstantandneg-
allydescribedas
ative since the late 1990s,it was stronglypositivein the 1980s
anddecreasedrapidlyfrommid-1980stomid-1990s.
Ω =Ω −Bsin2φ, (1)
φ 0
The fact that the southern hemisphere rotates faster than
whereΩ standsforthesidereal(allrotationratesaretakenhere the north can also be seen in the migration of ALs in the two
φ
to be sidereal) angular velocity at latitude φ, Ω (deg/day) de- hemispheres.Figure3demonstratesthemigrationofALsinthe
0
notestheequatorialangularvelocity,and B(deg/day)describes Carrington reference frame in the northern (top) and southern
the differential rotation rate. The active longitudes are also as- (bottom) hemispheres in 2004. Black dots denote the B-flares,
sumed to follow the same form of differential rotation at their trianglesC-flares,reddotsM-flaresandstarsX-flares.Thetwo
ownspecificvaluesfortheparametersΩ andB. solid lines in each panel depict the migration of the two ALs
0
from the beginningto the end of 2004.The dotted lines on ei-
Assuming that the two ALs are at Carrington longitudeΛ
and Λ ±180◦ and follow the differentialrotation of solar sur0- thersideofthetwosolidALlinesdenotethe90◦(±45◦)regions
0
aroundthetwoALs.
face, one can measure the distance between the longitude of
a flare or sunspot group and the nearest AL. The merit func- One can see that in the northern hemisphere the longitudi-
tioncanbedefinedasthemeansquareofthesedistanceseither nal location of one AL increases graduallyfrom 120◦ to 180◦-
withoutanyweightingonflaresorsunspots(Zhangetal.2011a) 190◦ intheCarringtonreferenceframein2004yieldingtheto-
or by weighting the flares with their normalized peak intensity tal increase of 60-70◦ in one year, while in the southern hemi-
andthesunspotswiththeirnormalizedarea.Themeritfunction spherethetotalincreaseinthisyearisabout160◦-170◦.Thisin-
ǫ(Λ ,Ω ,B)dependsonthethreeparametersΛ ,Ω ,and Bof dicatesthatbothhemispheresrotateconsistentlyfasterthanthe
01 0 0 0
theactivelongitudes.We usetheleast-squaremethodtosearch Carringtonreferenceframe(asshowninFig.1)andthesouthern
forthebest-fitparameters.The3-yearrunningfitintervalisused hemisphererotatessignificantlyfasterthanthenorth(asdepicted
here,yieldingtherotationparametersforthemiddleyear. bythenegativevaluesinFig.2).
2
L.Zhang,K.Mursula,andI.Usoskin:RotationofALsofsolarX-rayflares
Fig.1.Left-handpanels:YearlyvaluesofΩ inthenorth(top)andsouth(bottom)forX-rayflares.Opencirclesstandfortheyearly
17
best-fitvaluesand red filled circles demonstratethe 11-pointrunningmeanvalues. Solid horizontalline denotesthe averageΩ
17
overtheentirestudyperiod,anddashedlinethesiderealCarringtonrotationrate.Right-handpanels:thesameasintheleft-hand
butforsunspots.
axisymmetryofflaresinthesouthernhemisphereincreasessys-
tematically with flare class from 0.422 (71.1%) of B-flares to
0.679 (83.9%) of X-flares. In the northern hemisphere, the in-
creaseofnon-axisymmetrywithflareclassislessdramaticand
lesssystematic.Thenon-axisymmetry0.488(74.4%)ofX-flares
isslightlylowerthanthenon-axisymmetry0.506(75.3%)ofM-
flares. This indicates that the rotation rates of these active re-
gionsinthenorthernhemispherethatproduceX-classflaresand
those that produce other flares of other classes have somewhat
largerdifferencesthaninthesouthernhemisphere.Allthenon-
axisymmetryvaluesofflaresinbothhemispheresarelargerthan
the non-axisymmetryof sunspot.Thisis in agreementwith the
earlierfindingthatmorepowerfulsolaractivityattendstoappear
closertothetwoALs(Zhangetal.2011a,b).
Fig.2.YearlyvaluesofN-Sasymmetryof solar rotationat lat-
itude 17◦ obtained for X-ray flares (open circles) and sunspots
(filledcircles). 4. Discussionandconclusion
The relationship between solar rotation and sunspot activity
3.2.Non-axisymmetryofALs is a foundamental problem. It is known that sunspot activity
presents a secular increase during the 20th century leading to
Wedefinethemeasureofnon-axisymmetryΓasfollows
the so-called Modern Maximum (MM) (Solankietal. 2004).
N −N Brajsˇaetal. (2006) and Kitchatinovetal. (1999) found a sec-
Γ= 1 2, (2) ular decelerationof solar rotation,implyinga negativecorrela-
N +N
1 2 tion between solar rotation rate and sunspot activity. However,
whereN andN denotethenumberofsolarflaresthatappeared a secular acceleration in solar rotation has also been pro-
1 2
within(N )oroutside(N )thetwoALregions,whicharetaken posed(Heristchi&Mouradian2009;Lietal. 2014).Moreover,
1 2
hereasthetwo90◦-longitudebands,depictedinFig.3. Ribes&Nesme-Ribes(1993)claimedthatthe solarsurfacero-
The averaged yearly values of Γ for sunspots and for each tationat20◦ latitudewas6%slowerduringMaunderMinimum
flare class are listed in Table 1 (online). The asymmetries for thaninmoderntimes.Theselatterstudiessuggestthatthecorre-
the different flare classes were calculated using the same rota- lationbetweensolarrotationrateandsunspotactivityispositive.
tionparametersofthe(common)ALs.Thecorrespondingfrac- We find that the northern and southern hemispheres both
tions of flares in the ALs are also listed in Table 1. The non- have been speeding up since late 1990s - the ending phase of
3
L.Zhang,K.Mursula,andI.Usoskin:RotationofALsofsolarX-rayflares
The recent speed-up of solar rotation coincided with
the breakdown of the mutual relationships among sev-
eral solar activity and geo-activity indices. A large diver-
gence was observed at about 2001-2002 between the sunspot
numbers and several UV/EUV flux proxies, including the
F10.7 radio flux (Floydetal. 2005; Lefe`vre&Clette 2011;
Lukianova&Mursula 2011; Kane 2003; Leanetal. 2011;
Liuetal. 2011). These changes can be understood in terms of
the changesin sunspotdistribution and the recent vanishingof
smallsunspots(Clette&Lefe`vre2012).
Tosummarize,wefindthatbothsolarhemisphereshavein-
creasedtheirrotationratesincethelate1990suntilrecentyears.
Also,therateofincreaseisfairlysimilarinthetwohemispheres
so that the hemispheric asymmetry in rotation rates has been
roughly constant, with southern rotation being slightly faster.
Thisperiodof recentsolar speed-upcoincideswith the decline
oftheModernMaximum(periodofexceptionallyhighactivity
duringmostofthe20thcentury),whichisevidencedbytheover-
allreductionofsunspotactivity,vanishingofsmallsunspots,de-
creasingsolarwinddensityandmagneticfieldetc.Wealsonote
thatsimilar,althoughshorterandlesssignificantperiodsofboth
hemispheresspeedingupwereonlyfoundduringthetwolowest
solarcycles12and14atthe turnof the19thand20thcentury.
These results strongly suggest that there is, at least momentar-
ily,anegativecorrelationbetweensolarsurfacerotationrateand
sunspot activity. However, the causes to these results and their
possible implications to, e.g., solar dynamo, remain for subse-
quentstudiestobesolved.
Fig.3. Migration of ALs in northern (top) and southern hemi- Acknowledgements. Theresearchleadingtotheseresultshasreceivedfunding
spheres(bottom)in2004.BlackdotsstandforB-flares,triangles fromtheEuropeanCommission’sSeventhFrameworkProgramme(FP7/2007-
2013)underthegrantagreementeHeroes(projectn◦284461,www.eheroes.eu).
C-flares,reddotsM-flaresandstarsX-flares.Thetwosolidlines
Wealsoacknowledge thefinancial supportbytheAcademyofFinlandtothe
depict the migration of the two ALs from the beginningto the
ReSoLVECentreofExcellence(projectno.272157).
endof2004with45◦ extensionsoneachsidedenotedbydotted
lines.
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1 1
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2
north south
Γ Γ
sunspots 0.313(65.7%) 0.330(66.5%)
B-flares 0.422(71.1%) 0.422(71.1%)
C-flares 0.474(73.7%) 0.425(71.2%)
M-flares 0.506(75.3%) 0.572(78.6%)
X-flares 0.488(74.4%) 0.679(83.9%)
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