In this study we use a recently developed technique for measuring the 2-D magnetic flux in the ecliptic plane to examine (1) the long term variation of the magnetic flux in interplanetary space and (2) the apparent rate at which coronal mass ejections (CMEs) may be opening new flux from the Sun. Since there is a substantial variation ({approximately}50%) of the flux in the ecliptic plane over the solar cycle, we conclude that there must be some means whereby new flux can be opened from the Sun and previously open magnetic flux can be closed off. We briefly describe recently discovered coronal disconnections events which could serve to close off previously open magnetic flux. CMEs appear to retain at least partial magnetic connection to the Sun and hence open new flux, while disconnections appear to be likely signatures of the process that returns closed flux to the Sun; the combination of these processes could regulate the amount of open magnetic flux in interplanetary space. 6 refs., 3 figs

Gosling, J.T.

McComas, D.J.

Phillips, J.L.

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/ iLA-UR 9 1 " 3 _ _ (" LA-UR--91-3377DE92 002545Los Alamos National Laboratory iS operated by the University o! California for the United Stales Department of Energy under contract W-7405-ENG-36.TITLE: Regulation of the Interplanetary Magnetic FluxAUTHOR(S): D.J. McComas, J.T. Gosling, J.L. PhilltpsSUBMITTEDTO: Proceedings of the Solar Wind 7 Conference, Goslar, Germany16-20 September 1991DISCLAIMERThis report was prepared as an account of work sponsored by an agency of the United StatesGovernment. Neither the United States Government nor any agency thereof, nor any of theiremployees, makes any warranty, express or implied, or assumes any legal liability or responsi- ,"_..bility for the accuracy, completeness, or usefulness of any information, apparatus, product, or ; . _ '_:.process disclosed, or represents that its use would not infringe privately owned rights. Refer- : " _._,:ence herein to any specific commercial product, process,or scrvic¢ by trade name, trademark,manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recom-mendation, or favoring by the United States Government or any agency thereof. The views " (i)ill1901and opinions of authors expressed herein do not necessarily state or reflect those of theUnited States Government or any agency thereof.By acceptance ofth_sarhcle, the publisher recognizes that tl_eU.S Government retatnsa nonexcluswe, royalty-free license to publish or reproducethe published form of th=SCOntribution. or to allow others to do so. for U.S. Government purposes.The Los Alamos Nat,onal Laboratory requests that the puDIisher <lentily th_s article as wOrkperformed under the ausp¢cesof the U.S. Department of EnergyMASTERLosA,amos Nationa,Laboratorylamn.% ewMex; ,,FORM NO B36 R4 0'_""_"STNO2s2,5,,, OISTRIBUTIONOFTltl_ DOCUMENTtS UNLIMITEDREGULATION OF THE INTERPLANETARY MAGNETIC FLUXD.J. McComas, J.T. Gosling, and J.L. PhillipsSpace Plasma Physics Group, MS-D438, Los Alamos NationalLaboratory, Los Alamos, NM 87545 USAABSTRACTIn this study we use a recently developed technique for measuring the 2-D magnetic flux in theecliptic plane to examine 1) the long term variation of the magnetic flux in interplanetary space and2) the apparent rate at which coronal mass ejections (CMEs) may be opening new flux from theSun. Since there is a substantial variation (~50%) of the flux in the ecliptic plane over the solarcycle, we conclude that there must be some means whereby new flux can be opened from the Sunand previously open magnetic flux can be closed off. We briefly describe recently discoveredcoronal disconnection events which could serve to close off previously open magnetic flux. CMEsappear to retain at least partial magnetic connection to the Sun and hence open new flux, whiledisconnections appear to be likely signatures of the process that returns closed flux to the Sun; thecombination of these processes could regulate the amount of open magnetic flux in interplanetaryspace.INTRODUCTIONOne of the most stunning comparisons to be made with eclipse and coronagraph images of thesolar corona is that between solar minimum and maximum conditions. At solar minimum thecorona generally displays a relatively simple structure with a coronal streamer belt at low" heliolatitudes and large coronal holes at the solar poles. At solar maximum the corona has a muchmore complicated configuration with streamers and coronal holes found at ali heliolatitudes. Onshorter time scales of several days to several solar rotations, coronal holes and streamers typicallyappear and disappear in the solar corona. Since the "open" magnetic fields observed in the solarcorona connect out into interplanetary space to form the interplanetary magnetic field (IMF), bothlong and short term modifications of the coronal magnetic field map into the IMF configuration.Figure 1 schematically portrays various magnetic topologies possible in interplanetary space (abovethe dashed line). Typical interplanetary field lines (A) are referred to as "open" although theyclearly must close somewhere in the outer heliosphere in order for the field to be divergence free.Coronal mass ejections (CMEs) are observed in sequences of coronagraph pictures as bright (highdensity) structures rising through the solar corona and expanding into interplanetary space. ClassicCMEs appear as closed loops of newly expanding magnetic fields. The outward progression ofthese loops ultimately produces new open field lines. The magnetic bottle geometry (B) is thenatural extension of CMEs which maintain simple magnetic connection to the Sun.One long-standing question of heliospheric physics is why doesn't the IMF magnitude growwithout bound if CMEs open new magnetic flux into interplanetary space? Possible solutions tothis problem include 1) CMEs falling back into the Sun and 2) CMEs being released as detached"plasmoid" structures (C in Figure 1). One possible intermediate configuration for CMEs is that ofmagnetic flux ropes (D in Figure 1) which would form when reconnection occurs across somefraction of a CME's sheared magnetic loops/1,2/. With one unpublished exception that we areaware of, CME material never appears to fall back sunward. Similarly, few coronagraphobservations support the hypothesis of magnetic detachment of CMEs as plasmoids. Instead, mostCMEs appear to open previously magnetically closed regions of the Sun.An alternate solution to the build-up of interplanetary magnetic flux has been suggested recently byMcComas et al./3,4/. These authors observed coronal disconnection events, which appear as theseparation and release of U-shaped magnetic structures above helmet streamers in the corona.Since helmet streamers are the coronal footprints of the heliospheric current sheet, we believe thatcoronal disconnections arise from reconnection between oppositely directed fields across thecurrent sheet. This reconnection process converts open magnetic fields (A in Figure 1) to closedfield arches (E in figure 1), thus reducing the amount of open magnetic flux in interplanetary space.OBSERVATIONSThe magnetic flux, _, integrated over a surface completely bounding the Sun must be identicallyequal to zero since V • B = 0. However, the amount of magnetic flux in interplanetary space(combining both inward and outward) could vary greatly. This amount of "total flux", _*, wouldbe found, for example, by integrating the absolute value of the radial field over a spherical surfacecentered on the Sun. Unfortunately, such complete measurements are not available.By way of contrast, only single point spacecraft measurements have generally been available ininterplanetary space and the standard proxy for the total flux has simply been long-term averages ofthe IMF magnitude. Using this proxy, solar cycle 20 displayed little long term variation in totalflux while solar cycle 21 displayed a substantial and systematic variation/5/. There are a numberof problems with inferring the global variation in the amount of total magnetic flux from simpleaverages of the IMF magnitude measured at a single point in space. Obviously, single pointmeasurements do not provide information about the flux elsewhere. In addition averages of theIMF magnitude do not correct for the rate at which magnetic flux is convected past themeasurement point. This is because simple time averages neglect the fact that for any IMFmagnitude more flux passes the spacecraft at times of greater than at lesser solar wind speeds.Recently a somewhat more sophisticated method for measuring the amount of magnetic fluxpassing a single observing spacecraft was developed/2/. While this technique still suffers from theunavoidable undersampling of single point spacecraft measurements, at least the rate at which fluxis convected past the spacecraft is taken into account. Using the 2-dimensional divergence theoremit is easy to show/2/that the 2-D total flux in the ecliptic plane is_* = _ IBy Ivx dt (1)in SE coordinates under the simplifying assumption that the magnetic flux in the ecliptic plane isconfined to that X-Y plane. This 2-D assumption is not precisely correct, however, McComas etal./2/demonstrated that it is a reasonable approximation for these purposes.Figure 2 displays annual averages of _* calculated using the NSSDC's OMNI data file of one houraveraged solar wind and IMF parameters from 1973 - 1988. This interval spans solar cycle 21which extends from 1976 through 1987. Clearly there is a significant (-50%) variation in theamount of magnetic flux encountered in the ecliptic plane at 1 AU over solar cycle 21, as discussedpreviously by Slavin et al./5/. We have not tried to separate normal open magnetic flux from thatwhich is newly opening (bottle geometry) or simply convecting past (plasmoid geometry) over theentire interval shown in Figure 2. However, such a significant variation in 0* over the solar cyclesuggests there must be some means by which 1) new flux can be opened (such as by CMEs whichremain at least partially magnetically attached to the Sun) and 2) open magnetic flux can be closedoff (such as by reconnection above helmet streamers).While the method for calculating _* discussed above is an improvement for examining the longterm variations in the amount of magnetic flux in interplanetary space, its real advantage is that itcan be applied to short duration, transient structures in the solar wind. This is because thecombination of magnetic field and flow speed in equation (1) provides a measurement independentof the rate of the structure's motion past the spacecraft. Coronal mass ejections can be most readilyidentified in the solar wind by the streaming of suprathermal halo electrons in both directions alongthe local magnetic field/1/. We have used the counterstreaming signature to identify 58 CMEs overthe 18 month period from mid-August 1987 through mid-February 1980 and have calculated _* foreach event/2/.Figure 3 displays the monthly observed 2-D flux (solid curve with dots) and the cumulative flux(solid trace). The latter gives the rate at which 2-D magnetic flux would build up in interplanetaryspace if ali CMEs remained simply connected to the Sun in bottle-type geometries. The dottedhorizontal line indicates the average total flux integral, 0", in the ecliptic plane from Figure 2. Atthe rate of build-up shown in Figure 3, the amount of open magnetic flux in the ecliptic planewould be expected to doubl.e over only --9 months. This rate is slower than the for doubling timeof ~ 100 days estimated by MacQueen /6/ from coronagraph observations of CMEs prior to 1980.Of course if CMEs remain only partially magnetically attached to the Sun the rate at which fluxbuilds up in interplanetary space would be slower.SUMMARY1. Coronal mass ejections appear to remain at least partially magnetically attached to the Sun and,hence, probably open new magnetic flux into interplanetary space.2. A recently developed technique for measuring the "total flux" passing an observing spacecraftin interplanetary space confirms that there is a substantial variation in the magnetic flux in theecliptic plane over solar cycle 21. This variation suggests that there must also be some means forclosing off flux previously open to interplanetary space•3. Integration of the flux involved in CMEs over an 18 month interval near solar maximumindicates that in the absence of other processes the 2-D flux in the ecliptic plane would double overonly -9 months if ali CMEs represented simple "bottle" geometry magnetic fields.CONCLUSIONWe believe that the evidence strongly suggests that the amount of magnetic flux in interplanetaryspace must be regulated by processes which open and close flux from the Sun. CMEs appear to bethe obvious choice for opening new magnetic flux while reconnection across the heliosphericcurrent sheet above helmet streamers is the most likely method for closing off flux.We thank E.J. Smith and the JPL magnetometer group for ISEE-3 data and the NSSDC for OMNIdata. This work was conducted under the auspices of the United States Department of Energy withsupport from NASA under S-04039-D.REFERENCES1. J.T. Gosling, Coronal mass ejections and magnetic flux ropes in interplanetary space, Physicsof Magnetic Flux Ropes, ed. C.T. Russell, E.R. Priest, and L.C. Lee, Geophys. Mono. 58,AGU, 1990, p. 343.2. D.J. McComas, J.T. Gosling, and J.L. Phillips, Interplanetary magnetic flux: measurement andbalance, J. Geophys. Res., in press (1991)•w *3. D.J. McComas, J.L. Phillips, A.J. Hundhausen, and J.T. Burkepile, Observations ofkdisconnection of open coronal magnetic structures, Geophys. Res. Lett., 18, 73 (1991)4. D.J. McComas, J.L. Phillips, A.J. Hundhausen, and J.T. Burkepile, Disconnection of opencoronal magnetic structures, this issue.5. J.A. Slavin, G. Jungman, and E.J. Smith, The interplanetary magnetic field during solar cycle21" ISEE-3/ICE observations, Geophys. Res. Lett., 13, 513 (1986)6. R.M. MacQueen, Coronal transients: a summary, Phil. Trans. R. Soc. Lond., 297, 605 (1980)Figure CaptionsFig. I. Schematic diagram of various possible magnetic topologies in interplanetary space (abovedashed line): (A) normal "open" magnetic flux; (B-D) possible CME geometries; and (E) a coronaldisconnection event topology resulting from reconnection above a helmet streamer.Fig. 2. Annual averages of the magnetic flux in the ecliptic plane from 1973 - 1988.Fig. 3. Monthly (trace with dots) and integrated (solid trace) ecliptic plane magnetic fluxes inCMEs observed by ISEE-3 at 1 AU. The dashed line indicates the average flux from Figure 2.Outer Hel iosp hereA B C D Ei _ Plasmold _ !/Open _@ l__Co r8otr.le Flux Ropet onalJi__iJ DisconnectionI I I I I I I • I II I I I I I I I I I I I I I I ICoronaSolar Photosphere60005000E4000"_ 30002000 , . , . , • , • , • , . , • , • , • , • , • , • , • , • , . ,73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88' Year0A S OND J FMAM J J A SOND J FMonth [8/78-2/80]

Regulation of the interplanetary magnetic flux

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Regulation of the interplanetary magnetic flux

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