We model the magnetospheric magnetic field during two storms, moderate and intense, using the event-oriented modelling technique which includes the representations of the magnetic field arising from the various magnetospheric current systems. The model free parameters are specified for each time step separately using observations from GOES 8, 9, and 10, Polar, Interball and Geotail satellites and Dst measurements. It is shown that the ring current is most important during intense storms, whereas the near Earth tail currents contribute more to the Dst index than the ring current during moderate storms

Christopher T. Russell

Howard J. Singer

Marina V. Kubyshkina

Masaki Ejiri

Natalia Yu. Ganushkina

Tuija I. Pulkkinen

National Institute of Polar Research Repository

1.  IntroductionMany changes occur in the Earth’s magnetosphere during magnetic storms, includingchanges in different current systems, and, hence, in the magnetic field. During the lastdecades several magnetospheric magnetic field models have been developed (Alexeev et al.,2001; Tsyganenko, 2002a, b). Magnetospheric configurations corresponding to averageconditions are quite well represented by these models, whereas, fine structure in the mag-netic field during substorms or large magnetic field changes during storms cannot beaccounted for by these models (Ganushkina et al., 2002). Several types of studies require anaccurate representation of the magnetospheric configuration during a specific event. It is themagnetospheric configuration that determines how particles move in the magnetosphere,and changes in that configuration provide the particle acceleration. For such cases, event-oriented modeling may be of key importance (Ganushkina et al., 2004).In this paper we model two storm events, one moderate storm on June 25–26, 1998,when Dst reached －120 nT, and one intense storm on April 6–8, 2000 when Dst droppedResearch noteEvent-oriented modelling of magnetic fields and currents during stormsNatalia Yu. Ganushkina1, Tuija I. Pulkkinen1, Marina V. Kubyshkina2, Masaki Ejiri3, Howard J. Singer4 and Christopher T. Russell51 Finnish Meteorological Institute, Geophysical Research Division,P.O. Box 503, FIN-00101 Helsinki, Finland2 Institute of Physics, University of St.-Petersburg, St.-Petersburg, 198904, Russia3 National Institute of Polar Research, Kaga 1-chome, Itabashi-ku, Tokyo 173-85154 NOAA Space Environment Center, Boulder, CO 80303-3328, U.S.A.5 Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, U.S.A.(Received November 12, 2003; Accepted January 16, 2004)Abstract: We model the magnetospheric magnetic field during two storms, moderateand intense, using the event-oriented modelling technique which includes the representa-tions of the magnetic field arising from the various magnetospheric current systems. Themodel free parameters are specified for each time step separately using observations fromGOES 8, 9, and 10, Polar, Interball and Geotail satellites and Dst measurements. It isshown that the ring current is most important during intense storms, whereas the nearEarth tail currents contribute more to the Dst index than the ring current during moderatestorms.key words: magnetospheric current systems, magnetic field modelling, storms and sub-stormsAdv. Polar Upper Atmos. Res., 18, 105–110, 2004 2004 National Institute of Polar Research105to －300 nT. We examine the long-term evolution of different current systems during stormtimes and compute the relative contributions from the ring, magnetotail and magnetopausecurrents to the Dst index.2.  Description of eventsOn June 25, 1998 the IMF Bz behavior (Fig. 1a, upper panel) reflected the passage ofmagnetic cloud: sudden jump to more than +15 nT at 1550 UT and southward turn up to －13 nT around 2300 UT. Solar wind dynamic pressure had several peaks around 8–10 nPawith one peak of about 18 nPa at about 1000 UT on June 26 (second panel from the top).The AE index (next panel) showed first increase at about 2300 UT on June 25 and reached apeak value of 1400 nT around 0255 UT on June 26. The Dst index (bottom panel) started todecrease at the beginning of June 26 and reached －120 nT around 0500 UT.April 6–7, 2000 intense storm was caused by coronal mass ejection which took placeon the Sun on April 4. IMF Bz reached －30 nT around 1800 UT on April 6 (Fig. 1b, upperpanel). Magnetosphere was very much compressed, solar wind dynamic pressure showedseveral peaks reaching 25 nPa on storm maximum at about 0000 UT on April 7 (secondpanel from the top). The AE index (next panel) showed great increase at about 1800 UT onApril 6 with value of 2400 nT. The Dst (bottom panel) started to decrease at around 1800UT on April 6 and reached －300 nT around 0000 UT on April 7.3.  Modelling resultsWe model the magnetic field during June 25–26, 1998 moderate storm and April 6–7,2000 intense storm using Ganushkina et al. (2004) event-oriented storm-time magnetic fieldmodel. The model used the Tsyganenko T89 magnetic field model (Tsyganenko, 1989) as abaseline, and the ring, tail and magnetopause currents were modified to give a good fit with106 N. Ganushkina et al.Fig. 1.  Event overview for June 25–26, 1998 storm (a) and April 6–7, 2000 storm (b).in-situ observations. To represent the storm-time ring current the T89 ring current wasreplaced by a bean-shaped current system, which has a cross-section that is close to theobserved distribution of trapped particles in the inner magnetosphere. It is achieved by rep-resenting the current density at a point outside the equatorial plane by the functional depen-dence similar to that of omni-directional flux along the field line (Roederer, 1970). Ringcurrent has eastward flowing inner and westward flowing outer components. In addition tothe symmetric ring current, an asymmetric partial ring current was taken into account withclosing Region 2 sense field-aligned currents. For the tail current system both global andlocal changes were introduced. Global changes include intensification of the tail currentsheet as a whole using a tail current amplification factor, which indicates the change of thetail current from the original value, i.e. that given by T89 model. A new thin tail currentsheet is added as a combination of two vector potentials similar to T89 to account for thelocal changes during substorms. Magnetopause currents are varied in accordance with solarwind dynamic pressure variations. The free parameters in the model are the radii of thewestward and partial ring currents, the maximum current densities for westward and partialring currents, the amplification factor for the tail current, and the amplitude of additionalthin current sheet intensity. Other parameters are either fixed or calculated from solar windand Dst measurements (for details, see Ganushkina et al., 2004). We searched the values ofthe free parameters that give the best fit between the model and the in-situ field observationsby GOES 8, GOES 9, GOES 10, Polar, Geotail and Interball satellites, and the Dst mea-surements. The details of the fitting procedure can be found in Ganushkina et al. (2002).To get the model Dst index, the magnetic field from the extraterrestrial currents wascomputed at the locations of several stations such as Sun Juan, Tenerife, Tbilisi, Lunping,Kakioka, Honolulu and Del Rio. However, before the model values can be compared withthe observed ones, the quiet time level must be subtracted from the model. This is done bymodeling the entire duration of the quietest day of the month for each storm event. Thequiet level of the magnetic field given by the model is then evaluated at the locations of Dststations. In order to be able to examine the contributions of the different current systems tothe Dst index, the quiet time levels are also evaluated for the ring, tail and magnetopausecurrents separately. Currents in the magnetosphere induce currents in the electrically con-ducting Earth, which are estimated to be about 25% of the measured Dst (Häkkinen et al.,2002). In comparing our model Dst with the observed one, we remove this 25% from theobserved Dst.The model allows us to calculate the contributions of tail, ring and magnetopause cur-rents to the Dst index. Quiet time contributions for each current system were computed first.Figure 2 shows contributions from the ring current (thick solid curve), tail current (dash-dotted curve) and magnetopause currents (dotted curve) to the observed Dst index (thinsolid curve) during June 25–26, 1998 moderate storm (a) and April 6–7, 2000 intense storm(b). It is clear that the contributions from different current systems to the Dst index dependon the storm strength and change during the storm development. During the moderate stormon June 25–26, 1998, the tail current contributes more than the ring current to the Dst. Thesituation is quite different during the intense storm on April 6–7, 2000: the main contribu-tion to Dst comes from the ring current. The ring current contribution gradually increasesand reaches maximum at the intense storm maximum, whereas the tail current contributionstarts to decrease before the storm maximum. For both storms the tail current intensifies107Modelling fields and currents during stormsfirst at the storm beginning when the Dst drops while the ring current develops later andstays longer at an increased level.4.  Conclusions and discussionIn this paper we discuss model results of the evolution of current systems and theircontribution to Dst index during two storm events on June 25–26, 1998 and April 6–7, 2000given by event-oriented storm-time magnetic field model by Ganushkina et al. (2002,2004). The model describes the ring current, the tail current, and the magnetopause currentswith functions containing free parameters, whose values are defined for each time step sep-arately based on in situ measurements in the magnetosphere, on solar wind dynamic pres-sure, and on the ground based storm index Dst. During intense storms the main contributionto the Dst index comes from the ring current, but during moderate storms the tail currentcontribution can be dominant or comparable to the ring current. The tail current intensifiesfirst, and follows the drop in the Dst index. The ring current develops slower, and stays at anincreased level longer than the tail current. Three moderate storms and one intense stormwere modelled in our previous study by Ganushkina et al. (2004), and similar conclusionswere drawn. Our results are similar to earlier work (Dremukhina et al., 1999; Alexeev et al.,2001; Ohtani et al., 2001) with regard to moderate storms, but give a quite different pictureof the dynamics during intense storms. From the other hand, our results do not agree withthe study of Turner et al. (2000) study where using Tsyganenko T96 magnetospheric mag-netic field model to model one moderate January 1997 storm it was concluded that the tailcurrent contribution is about 25% of the measured Dst variation.When discussing the relative contributions from the ring and tail currents, the keyquestion is how to separate partial ring current and tail current at the inner edge of the plas-ma sheet. There are two possible ways to give definitions: Using magnetic field observa-tions, current flowing in the region of dipolar field lines is (partial) ring current while cur-rent in tail-like field lines is cross-tail current. The other way is to measure the characteris-tic energy of the current-carrying population and assume that it is ring current (or partialring current) if the energies are in the range of several tens of keV and tail current if the108 N. Ganushkina et al.Fig. 2. Contributions from the ring current (thick solid curve), tail current (dash-dotted curve) and magne-topause currents (dotted curve) to the observed Dst index (thin solid curve) during June 25–26, 1998moderate storm (a) and April 6–7, 2000 intense storm (b).energies are more typical plasma sheet energies of 10–20 keV. In reality, the ring and tailcurrents can not be unambiguously separated in this transition region between dipole andtail-like field. Our model uses the observed magnetic field, and it does not contain any pre-determined separation between the ring and tail currents in the transition region. If theobserved magnetic field is stretched, tail-like (which was actually observed at GOES), themodel tries to intensify the tail current. Calculations of model contributions to Dst from theparts of the tail current confined in different regions in the tail showed that 40% of ourmodel tail current contribution to the Dst index comes from the current that flows inside 8RE. It is therefore important to realize that results indicating strong asymmetric ring currentand strong inner-tail current are not contradictory but may be the same physical processdescribed with different terminology.It is interesting to note that our model current systems have different characteristicresponse times. While the ring current increases quite slowly (much slower than the Dstenhancement), the tail current responds very rapidly. Thus, most of the fast decrease of theDst index during the storm main phase in this model is created by the intensifying cross-tailcurrent. The ring current intensification contributes to the magnitude and timing of thestorm maximum, as the ring current maximizes at storm maximum. Further analysis on rel-ative contributions to Dst from the different current systems including the ring and tail cur-rent contributions during storm recovery phase and the contribution of the magnetopausecurrents is still needed.AcknowledgmentsWe would like to thank K. Ogilvie and R. Lepping for the use of WIND data in thispaper, Interball-tail MIF-M team for providing high resolution data (routinely produced byS. Romanov), World Data Center C2 for Geomagnetism, Kyoto, for the provisional AE, Kpand Dst indices data. The data were obtained from the Coordinated Data Analysis Web(CDAWeb). GEOTAIL magnetic field data were provided by T. Nagai through DARTS atthe Institute of Space and Astronautical Science (ISAS) in Japan. This work was supportedby the Academy of Finland.The editor thanks Dr. A. Brekke and another referee for their help in evaluating thispaper.ReferencesAlexeev, I.I., Kalegaev, V.V., Belenkaya, Bobrovnikov, S.Y., Feldstein, Y.I. and Gromova, L.I. (2001): Dynamicmodel of the magnetosphere: Case study for January 9–12, 1997. J. Geophys. Res., 106, 25683–25693.Dremukhina, L.A., Feldstein, Y.I., Alexeev, I.I., Kalegaev, V.V. and Greenspan, M.E. (1999): Structure of the mag-netospheric magnetic field during magnetic storms. J. Geophys. Res., 104, 28351–28360.Ganushkina, N.Yu., Pulkkinen, T.I., Kubyshkina, M.V., Singer, H.J. and Russell, C.T. (2002): Modeling the ringcurrent magnetic field during storms. J. Geophys. Res., 107, 10.1029/2001JA900101, 02 July 2002.Ganushkina, N.Yu., Pulkkinen, T.I., Kubyshkina, M.V., Singer, H.J. and Russell, C.T. (2004): Long-term evolutionof magnetospheric current systems during storms. Ann. Geophys., 22, 1317–1334.Häkkinen, L.V.T., Pulkkinen, T.I., Nevanlinna, H., Pirjola, R.J. and Tanskanen, E.I. (2002): Effects of induced cur-rents on Dst and on magnetic variations at midlatitude stations. J. Geophys. Res., 107, SMP1-7, CitedID1014, DOI 10.1029/2001JA900130.Ohtani, S., Nose, M., Rostoker, G., Singer, H., Lui, A.T.Y. and Nakamura, M. (2001): Storm-substorm relation-109Modelling fields and currents during stormsship: Contribution of the tail current to Dst. J. Geophys. Res., 106, 21199–21209.Roederer, J.G. (1970): Dynamics of Geomagnetically Trapped Radiation. New York, Springer, 166 p.Tsyganenko, N.A. (1989): A magnetospheric magnetic field model with a warped tail current sheet. Planet. SpaceSci., 37, 5–20.Tsyganenko, N.A. (2002a): A model of the near magnetosphere with a dawn-dusk asymmetry: 1. Mathematicalstructure. J. Geophys. Res., 107, SMP 12-1, CiteID 1179, DOI 10.1029/2001JA0002192001.Tsyganenko, N.A. (2002b): A model of the near magnetosphere with a dawn-dusk asymmetry: 2. Parameterizationand fitting to observations. J. Geophys. Res., 107, SMP 10-1, CiteID 1176, DOI 10.1029/2001JA000220.Turner, N.E., Baker, D.N., Pulkkinen, T.I. and McPherron, R.L. (2000): Evaluation of the tail current contributionto Dst. J. Geophys. Res., 105, 5431–5439.110 N. Ganushkina et al.

Event-oriented modelling of magnetic fields and currents during storms

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Event-oriented modelling of magnetic fields and currents during storms

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