Table Of ContentHemispheric Asymmetries of Solar Photospheric Magnetism: Radiative,
Particulate, and Heliospheric Impacts
ScottW.McIntosh1,RobertJ.Leamon2,JosephB.Gurman3,Jean-PhilippeOlive4,JonathanW.
Cirtain5,DavidH.Hathaway5,JoanBurkepile1,MarkMiesch1,RobertS.Markel1,Leonard
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1HighAltitudeObservatory,NationalCenterforAtmosphericResearch,P.O.Box3000,Boul-
derCO80307
2DepartmentofPhysics,MontanaStateUniversity,Bozeman,MT59717
3SolarPhysicsLaboratory,NASAGoddardSpaceFlightCenter,GreenbeltMD20771
4AstriumSAS,6rueLaurentPichat,75016Paris,France
5MarshallSpaceFlightCenter,Huntsville,AL35812
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ABSTRACT
Amongmanyothermeasurablequantitiesthesummerof2009sawaconsiderable
low in the radiative output of the Sun that was temporally coincident with the largest
cosmic ray flux ever measured at 1AU. Combining measurements and observations
made by the Solar and Heliospheric Observatory (SOHO) and Solar Dynamics Ob-
servatory (SDO) spacecraft we begin to explore the complexities of the descending
phase of solar cycle 23, through the 2009 minimum into the ascending phase of solar
cycle 24. A hemispheric asymmetry in magnetic activity is clearly observed and its
evolutionmonitoredandthe resulting(prolonged)magneticimbalancemusthavehad
a considerable impact on the structure and energetics of the heliosphere. While we
cannot uniquely tie the variance and scale of the surface magnetism to the dwindling
radiative and particulate output of the star, or the increased cosmic ray flux through
the 2009 minimum, the timing of the decline and rapid recovery in early 2010 would
appeartoinextricablylinkthem. TheseobservationssupportapicturewheretheSuns
hemispheresaresignificantlyoutofphasewitheachother. Studyinghistoricalsunspot
recordswiththispictureinmindshowsthatthenorthernhemispherehasbeenleading
since the middle of the last century and that the hemispheric dominance has changed
twice in the past 130 years. The observations presented give clear cause for concern,
especially with respect to our present understanding of the processes that produce the
surfacemagnetisminthe(hidden)solarinterior-hemisphericasymmetryisthenormal
state - the strong symmetry shown in 1996 was abnormal. Further, these observations
show that the mechanism(s) which create and transport the magnetic flux magnetic
flux are slowly changing with time and, it appears, with only loose coupling across
the equator such that those asymmetries can persist for a considerable time. As the
currentasymmetrypersistsandthebasalenergeticsofthesystemcontinuetodwindle
we anticipate new radiative and particulate lows coupled with increased cosmic ray
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fluxesheadingintothenextsolarminimum.
Subjectheadings: Sun: solarwind–Sun:magneticfields–Sun:corona
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1. Introduction
The most recent solar minimum has drawn a great deal of interest (e.g., McComas et
al. 2008; Gibson et al. 2009; Kirk et al. 2009; Mewaldt et al. 2010; Penn & Livingston 2010;
Galvin et al. 2010; Solomon et al. 2010; Altrock 2011; Wang & Robbrecht 2011; McIntosh et al.
2011a,b),butlargelybecauseithasnotbehavedinthisfashionduringtheepochofroutinedetailed
observation ofsurface andsub-surface phenomena. Further, there wasa beliefthat we(somehow)
understood the basal modulation of the Suns radiative and particulate output and that it was
invariant; that is not the caseno two solar minima are the same, we have known this, but the Sun
ably demonstrated it for us over the past few years. In our pursuit of a physical understanding of
theSunsimpactonEarth(andtheSolarSystem)wehave(correctly)beendrawntoprocessesthat
relentlessly form magnetic flux in the interior (e.g., Dikpati & Charbonneau 1999; Charbonneau
2005, 2007; Callebaut et al. 2007; Brandenburg 2009; Hathaway 2010; Nandy et al. 2011) and
those that govern the eventual eruption of that magnetic flux through the photosphere into the
portions of the atmosphere which we can directly observe. The tail of those processes in the
outer solar atmosphere, from the perspective of energy density, dictate the output of our star, but
our historical reference point has been limited to observing gross (readily observable) proxies
of magnetic evolution. It is likely that these proxies carry differently weighed information
about our stars evolution and it is our task to know the meaningful information content of each,
beyond zeroth order application, if we are to move towards comprehensive modeling of the solar
atmosphereasasystem.
In this Paper we consider several records of quiescent activity over the past solar cycle using
measurements from the SOHO, SDO, ACE, and Ulysses spacecraft. We pay particular attention
to the line-of-sight (LOS) magnetogram data from the SOHO Michelson Doppler Imager (MDI;
Scherrer et al. 1995) to investigate trends in the evolution of surface magnetism that may point to
complexitiesinlongertermprocessesandhowweconsider/modelthem.
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In §2 we demonstrate the minimum-minimum modulation of several Solar outputs and their
connection to the relentlessly evolving surface magnetism that motivated this study. Indeed, we
infer that these subtle changes may have a broader impact — on the structure of the heliosphere
itself (§2.1). Studying the MDI record of magnetograms in §3 we look for gross trends of
surface magnetism that may underly these varying quantities. The behavior that we identify
in the variation of the magnetogram record is subtle, but clear, and spawns the investigation
into other measures of outer atmospheric structure and small-scale activity in §6 in search
consistentbehaviorinthehypothesisdrawnthatthemagneticactivityofthenorthernandsouthern
hemispheres are not in phase with each other. Extending our analysis to a longer sunspot record
(differentiated by hemisphere; §4) we see that this behavior is not isolated to the recent solar
minimumbuthasbeenvisibleforaconsiderabletime. Wediscuss,intheclosingsections,thatthe
observed variance in surface magnetism must place stronger physical constraints on the modeling
of interior magnetic flux generation and how that evolving flux governs the energetic output of
the star as well as the mould of the heliosphere itself. There are strong indications that the two
hemispheres are nearly two years out of phase at the time of writing, and it is unclear what the
next descending phase and solar minimum will offer, far less those going further into the future
— what evolutionary path the Sun will take next? It is likely that only a significant increase in
complexity of modeling can help us establish the real knock-on effects of what can happen to
reversethehemisphericasymmetryandifthatisconnectedtotheslowlydecreasinglevelsofsolar
activity.
2. 2009SolarMinimumMeasurements
Figure 1 shows the variation of several measures of different origin over the past solar cycle.
From top to bottom the figure shows the total solar irradiance (TSI) measured by SOHO/VIRGO,
the mean Fe charge state ((cid:104)Q (cid:105)) measured by ACE/SWICS (Gloeckler et al. 1992; Stone et al.
Fe
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A) SOHO/VIRGO Total Solar Irradiance
1367.0
−2TSI [W m]111333666566...505 2007/01/012007/01/01 2009/01/012009/01/01
1365.0
B) ACE/SWICS Mean Iron Charge State
SWICS <Q>Fe111012 Fe9+ [= Fe X] 2007/01/01 2009/01/012009/01/01 111...246mation Temp [MK]
ACE/ 89 1.0Ion For
C) WIND/SWE 250−Day Average
6
260−322 322−343 343−362 362−381 381−400 400−423 423−451 451−492 492−560
5 2007/01/01 2009/01/01
AHe4
E
W
S3
D/
N
WI
2
1
0
work Scale [Mm]233802 D) SOHO/EIT 304Å 28−Day Average2007/01/012007/01/01 2009/01/012009/01/01
R Net26
Mean T24
E) SOHO/MDI MRoI Power Law Index
0.5
δMRoI −−010...005 2007/01/012007/01/01 2009/01/012009/01/01
−1.5
−2.0
1996 1998 2000 2002 2004 2006 2008 2010
Time [Years]
Fig. 1.— From top to bottom the total solar irradiance measured by SOHO/VIRGO, the mean Fe
chargestate((cid:104)Q (cid:105))measuredbyACE/SWICS,theabundanceofHelium(A )measuredbyACE,
Fe He
theapparentscalelengthoftheSOHO/EITtransitionregionnetworkemission,andthepower-law
index of the SOHO/MDI “Magnetic Range of Influence” (MRoI over solar cycle 23 and into the
early part of cycle 24. For reference we show 2 dates: January 1, 2007 (dashed vertical line) and
January1,2009(dottedverticalline)andhorizontaldashedlinestoindicatethe1996levelofeach
measurement. Thesolarwindmeasuresaredifferentiatedbyspeedasindicated—notethatthereis
nomeasurementof(cid:104)Q (cid:105)before1998.
Fe
–7–
1998), the abundance of Helium (A ) measured by WIND (e.g., Aellig et al. 2001; Kasper et al.
He
2007),theapparentscalelengthofthetransitionregionnetworkemissionobservedbySOHO/EIT,
and the power-law index of the MRoI inferred (see below) from SOHO/MDI magnetograms
over solar cycle 23 and into the early part of cycle 24 (McIntosh et al. 2011a). The bottom
pair of panels illustrate the variation in the separation distance of magnetic field elements in the
photosphere and the prevalent “network” emission length scale in the quiescent solar atmosphere
(McIntosh et al. 2011a)—their variation is remarkably consistent and demonstrates the “imprint”
of the underlying magnetism on the radiative and particulate output of our star (Schwadron et al.
2011; McIntosh et al. 2011b; Kasper et al. 2012). For reference we show 2 dates: January 1, 2007
(dashed vertical line) and January 1, 2009 (dotted vertical line) and horizontal dashed lines to
indicatethe1996levelofeachmeasurement.
2.1. The2009HighinCosmicRayFluxMeasuredAtEarth: CoincidenceorConnection?
At almost exactly the same time that the energetic and magnetic proxies shown in Fig. 1
reached new lows the cosmic ray flux (CRF) as measured at Earth (and in interplanetary space)
reached an all time high (in the context of the space age Mewaldt et al. 2010). We wonder if the
apparent temporal correlation of the increased CRF and decrease in proxies of solar magnetism
arecausallyrelated;orifthereisanappreciablecouplingbetweenthesmall-scaleandglobal-scale
magnetismthataffecttheenergeticsandstructureoftheheliosphere?
The CRF is measured on Earth by a network of neutron monitors that detect the decay
products arising from the collision of incoming cosmic rays with nitrogen and oxygen
molecules in the upper atmosphere. In this study, we have used neutron monitor data (red
points) from the Sodankyla Geophysical Observatory of the University of Oulu, Finland
(http://cosmicrays.oulu.fi/). We use data corrected for variations in local
atmospheric pressure since count rates of neutrons at the detector are proportional to the number
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of collision targets in the upper atmosphere. Further, to demonstrate that the anomalous CRF
variation observed during this most recent solar minimum is not an artifact produced by the
Earth’s atmosphere or magnetosphere, we must consider a non-Earthbound monitor of the space
particle environment, such as that studied by Mewaldt et al. (2010). Such a data set is provided
(serendipitously)bythe“SingleEventFlag”,orbad-bitcountrateofthesolidstaterecorder(SSR)
on the Solar and Heliospheric Observatory (SOHO; Domingo et al. 1995; Harboe-Sorensen et al.
2002). TheSOHOSSRdataisdiscussedinmoredetailinAppendixA.
The spacecraft’s management software keeps track of the number of bad data bits that are
correctedon theSSR,the rateofthese corrections isshownin Fig.2(blue dots). Theaveragerate
fluctuatesaroundonebad-bitperminute,rangingfrom0.3to1.4,plottedonthesamegraphisthe
12-hour average of the Oulu station CRF (red). There are numerous sharp peaks in the timeseries
that are driven by solar energetic particle storms, but the general modulation of the bad-bit count
rateis drivenbychangesin basalsolaractivitycycle thatareapparentin thedeclineinthe bad-bit
rate as solar maximum is approached. This spacecraft history is correlated well with ground-level
neutron monitor data as we observe a remarkable correspondence in the (scaled) variation in
range of both measurements over a broad range of timescales, from short flare-related particles,
to the longer solar-cycle related variation. In particular, the bad-bit count rate shows a similarly
anomalous increase to the CRF, starting in the middle of 2008, and recovering synchronously in
late 2009. The approximately 1.5 million km distance of SOHO from the Earth reinforces the
ACE measurements (Mewaldt et al. 2010) that the source of the anomaly observed in the Neutron
MonitorStationsdoesnotoriginateat,orneartheEarth,butinsteadresultsfromcuriousbehavior
oftheSunoverthisperiod.
We draw the reader’s attention to the apparent 28-day periodicity visible in the CRF during
the solar minima, around late 1996–mid 1997 and again in late 2007 through early 2008. This
very low-amplitude, highly modulated variation in the CRF at solar minima is not an isolated
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Neutron Monitor Count Rate From Oulu Station in the SOHO Era
e n]
at 1.4Mi
nt R7000 1.2unt/
u o
o C
onitor C6500 01..80nt Flag [
M6000 ve
ulu Neutron 5500 2007/01/01 2009/01/01 000...246OHO Single E
O5000 0.0S
1998 2000 2002 2004 2006 2008 2010
Time [years]
Fig. 2.— Comparing the daily average of the Oulu Station Cosmic Ray Flux (CRF; red) with the
SOHO Single Event Flag Count (blue dots) over the period from March 1996 to January 2011
which covers solar cycle 23 (cf. Fig 1). The SOHO SSR measurements track the variations in
the CRF measured at Earth with almost no delay. Like that shown in Fig. 1 the dashed vertical
black line marks June 1 2008 and indicates the start of the anomalous rise in the CRF. Notice also
the apparent low-amplitude 28-day periodicity in the CRF at solar minimum (e.g., Kudela 1999;
Bazilevskaya2000),adetailexploredAppendixB.1.
phenomenon (e.g., Kudela 1999; Bazilevskaya 2000). The apparent cause of this low-amplitude
modulation—a repeating pattern of different of solar magnetic activity on one side of the Sun
compared to the other. However, understanding this modulatory phenomenon bears some
relevance to the conclusion we will reach below. The response of the CRF to solar surface
magnetism fluctuations can be as rapid as only a couple of days, or the time that it takes the fast
solar wind traveling at 700 km/s to reach the Earth from the Sun, or the co-rotating interaction
region delineating fast and slow wind streams. These are not disturbances in the heliosphere’s
outer structure, they are directly forced by conditions on the Sun days, and not months, earlier.
However, since this topic is not directly relevant to the bulk of the manuscript we defer further
discussiontoanAppendix(B.1).
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3. Cycle23/24MagnetogramTrends
SOHO uses the Michelson Doppler Imager (MDI; Scherrer et al. 1995) to probe the interior
of the Sun by measuring the photospheric manifestation of solar oscillations and magnetism.
The instrument images the Sun on a 10242 CCD camera through a series of five narrow spectral
filters (FWHM bandwidth of 94 mA˚) that span the Ni I 6768A˚ spectral line. When polarizers are
included in the light path MDI records the line-of-sight (LOS) magnetic field with a resolution of
4(cid:48)(cid:48) over the whole solar disk using a simple linear combination of the signal in the five sampled
linepositions(asdiscussedinScherreretal.1995).
[Gauss] [Mm]
−25 −15 −5 5 15 25 0 25 50 75 100 125 150
A) SOHO/MDI Line−Of−Sight Magnetogram B) SOHO/MDI "Magnetic Range of Influence" − MRoI
1000
500
0.6 0.6
s)
csec 0 0.6 0.6
ar
Y (
−500
0.6 0.6
−1000
−1000 −500 0 500 1000 −1000 −500 0 500 1000
X (arcsecs) X (arcsecs)
Fig. 3.— The SOHO/MDI line-of-sight magnetogram (A) from 00:00UT on April 10 2008 and
“Magnetic Range of Influence” (B; MRoI) map during the extremely quiet phase near the cycle
23/24 solar minimum. Concentrations of negative magnetic field are black in color while the
negative magnetic field are white. The dashed line encircling disk center at a value of 0.6R and
(cid:12)
outlinestheregionforwhichwehaveperformedthemagnetogramanalysisshowninFig.4.
