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1 Introduction

The largest known disturbances in thesolar wind-magnetosphere-ionosphere coupledsystem, geomagnetic storms are characterizedby a prolonged period in which the horizontalcomponent of geomagnetic fields is depressedin the mid to low latitudes on a global scale,with such periods lasting from one-half to sev-eral days. This paper examines the character-istics of a geomagnetic storm and the cause ofits development, as well as prospects forfuture research.

2 Features of Geomagnetic Vari-ations

The Earth has an internal magnetic fieldgenerated by the dynamo effects of electric

currents in its fluid outer core. Couplingsbetween this internal magnetic field and thesolar wind form the magnetosphere, which hasa comet-like shape, with a tail extending awayfrom the Sun. Variations in solar wind-mag-netosphere couplings serve as the drivingforce behind various characteristic geomag-netic disturbances. Although components ofthe geomagnetic field variation can be attrib-uted to the internal dynamo effect, their peri-ods are much longer than the periods of varia-tions caused by solar wind-magnetospherecouplings, and can be regarded as a separatephenomenon.

During quiet periods free of geomagneticdisturbances, the geomagnetic field displays arelatively regular pattern of variation. Theseregular variations are caused by ionosphericcurrents generated by the dynamo effect aris-

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3-5 Geomagnetic StormsNAGATSUMA Tsutomu

Geomagnetic storms, in which the global geomagnetic field intensity decreases on theorder of tens to hundreds nT, are phenomena that occur on the largest scale in the solarwind-magnetosphere-ionosphere coupled system. Geomagnetic storms develop whensolar wind-magnetosphere couplings are intensified by solar wind disturbances (coronalholes and CME phenomena) that accompany southward IMF. Perturbations in the magneticfield are caused by geomagnetic storms and can be explained by the westward electric cur-rent along the geomagnetic equator (ring current). Such perturbations on the scale of 1015-1016 J occur when the magnetosphere responds to the injections of energy during geomag-netic storms.

Geomagnetic storms are generally believed to develop in association with an increase inmagnetospheric convection. However, in contrast to magnetospheric convection develop-ment (which is saturated with strong solar wind electric fields), analysis of the correlation ofsolar wind parameters to magnetospheric convection and to geomagnetic storms hasrevealed that geomagnetic storm growth is not saturated with such electric fields. This indi-cates that geomagnetic field growth and magnetospheric convection growth may not corre-late perfectly.

Keywords Geomagnetic storms, Solar wind-magnetosphere coupling, Magnetospheric convec-tion, Ring current, Substorm

140

ing from the tidal motion of the thermosphere. The tidal motion of the thermosphere is

driven by heat energy from the Sun.Geomagnetic disturbances generally fall

into two broad categories. The first involvesvariations in the horizontal geomagnetic fieldcomponent in the polar region in the range ofseveral hundred to one thousand and severalhundred nT, occurring on time scales of 30minutes to 2 hours. These variations corre-spond to geomagnetic field perturbationscalled "substorms." Optically, a substorm isdefined as a phenomenon that begins with anexplosive illumination of the aurora near themidnight region on the nightside that gradual-ly expands both in the longitudinal and latitu-dinal directions. The changes observed in thegeomagnetic field accompanying substormsdiffer significantly for different magnetic localtimes and magnetic latitudes. Furthermore,the effects of a substorm may appear as bay-like geomagnetic field variations in the low tomiddle latitudes on the nightside.

The second type of geomagnetic distur-bance involves a prolonged depression of thehorizontal geomagnetic field component in themid to low latitudes in the range of severaltens to several hundred nT that lasts from one-half to several days. This type of disturbanceis called a "geomagnetic storm." The period ofprogressive depression of field strength iscalled the "main phase." The period of restora-tion to original field strength is called the"recovery phase." A geomagnetic storm mayaccompany a "sudden commencement (SC),"characterized by a sudden increase in the mag-netic field intensity shortly before the mainphase. The period between the sudden com-mencement and main phase is called the "ini-tial phase." A geomagnetic storm not accom-panied by a SC is called a "gradual geomag-netic storm (SG)." Fig.1 shows an example ofSC- and SG-type geomagnetic stormsobserved at the Kakioka Geomagnetic Obser-vatory. The SC is a geomagnetic perturbationcaused by an increase in magnetopause cur-rents due to the rapid compression of the mag-netosphere by the dynamic pressure of the

solar wind, which is intensified by interplane-tary shock.

However, in certain cases, interplanetaryshock does not trigger geomagnetic storms. Inother words, geomagnetic perturbations duringthe main and recovery phases are the essentialfeatures of a geomagnetic storm. These twophases are believed to be essentially identicalin both SC and SG storm types. Substormsalso frequently occur during geomagneticstorms, and the superposition of correspon-ding short-period geomagnetic perturbationsonto those of geomagnetic storms results in acomplex pattern of variations.

The degree of depression of the horizontalgeomagnetic field component observed duringgeomagnetic storms differs, depending on themagnetic local time. The maximum depres-sion of the geomagnetic field strength is seenon the night to dusk side, while the minimumdepression is seen on the day to dawn side.This is due to the asymmetrical flow of thering current, which will be explained in a latersection.

3 Solar Wind Variations and Geo-magnetic Storms

Geomagnetic disturbances are driven bysolar wind-magnetosphere couplings. Solarwind energy is injected into the magnetos-phere through field line merging of the inter-

Journal of the Communications Research Laboratory Vol.49 No.3 2002

Examples of geomagnetic storms forsudden commencement (SC) type(top panel) and gradual (SG) type(bottom panel)

Fig.1

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planetary magnetic field (IMF) and geomag-netic field. This energy injection is most effi-cient during southward IMF (Bs component).Prolonged periods of strong southward IMFwill trigger geomagnetic storms. Observa-tions have shown that a state of Bs >_ 10 nTlasting for over 3 hours will always generate ageomagnetic storm[1]. Solar wind velocity(V) is another important factor. Geomagneticstorm development is known to demonstrate astrong positive correlation with the product ofthese two physical quantities, VBs.

Two types of solar surface phenomena arebelieved to generate high VBs conditions:

coronal mass ejection (CME) and coronalholes. CMEs are a phenomenon in whichlarge amounts of solar coronal plasma isreleased into interplanetary space (Fig.2).CMEs appear simply as a region of high plas-ma density, but may contain a magnetic fluxrope structure[2].

The magnetic flux rope has a stable mag-netic field structure. When the structure con-tains a stable southward magnetic field com-ponent, it is a significant driver of geomagnet-ic storms.

This structure also generates a interplane-tary shock before the CME, helping to devel-

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CME observed by coronagraph (LASCO) aboard the SOHO satellite (ESA & NASA)Fig.2

op geomagnetic storms when the magneticfield in the sheath region has a southwardcomponent.

Coronal holes are areas in which high-speed solar winds stream out from the Sun.

Coronal holes have weak solar magneticfields compared to active regions and form inregions having the same polarity over a widearea (unipolar magnetic fields). Such regionshave lower coronal plasma temperatures thansurrounding regions, and hence, appearing forthis reason to be relatively dark (Fig.3). Thehigh-speed solar winds ejected from coronalholes overtake and interact with low-speedsolar wind in its path, creating a co-rotatinginteraction region (CIR). Within the CIR,plasma pressure is increased by compression.

Magnetic field strength also increases.When the southward component is dominantin the strengthened magnetic field, its interac-tions with the geomagnetic field are strength-ened, driving a geomagnetic storm.

Since CMEs are sporadic, geomagneticstorms associated with them are sometimescalled "sporadic geomagnetic storms." On theother hand, the structure of a coronal hole maysometimes remain relatively stable throughseveral periods of solar rotation. In suchcases, a recurrent CIR is observed from theEarth, with cycles equivalent to the rotationalperiods of the Sun. If the CIR has a dominantsouthward magnetic field component, "recur-rent geomagnetic storms" are generated. Inaddition to above case, recurrent geomagnetic

storms may also be generated when CMEsoccur often in a specific active region on theSun over several rotational periods. If astrong southward magnetic field is producedby interactions between a high-speed solarwind and CME, even stronger geomagneticstorms are triggered.

4 Geomagnetic Storm Indices

Used as an indicator of the magnitude of ageomagnetic storm, the Dst index is based ongeomagnetic data at middle latitudes, withranges stretching in the meridional directioncollected at four stations (Kakioka, Hermanus,San Juan, and Honolulu), and is expressed asthe hourly value indicating the degree of vari-ation in the symmetrical component[3]. Thelocations of the geomagnetic observatories areshown in Fig.4. The Dst index is calculatedby assuming that the intensity of a geomagnet-ic storm can be represented equivalently bythe scale of the symmetric current flowingwestward at the equator, called the ring cur-rent. The details of the ring current will bediscussed in the next section. The actual Dstindex is known to contain components of geo-magnetic field variations in the magnetopausecurrent and in the tail currents, which areimposed on variations of the ring current. TheDst index corrected for the effects of magne-topause current is called the pressure correctedDst index (Dst* index), and is defined by thefollowing equation[4]:

(1)

where PSW is the solar wind dynamic pressure(ρV2) [nPa]. Values for b and c are deter-mined from analysis. Various values havebeen proposed according to different models.However, all proposed values are approxi-mately equal. (For example, b = 7.26[nT(nPa)-1/2] and c = 11[nT][5].) The extent ofthe contribution from the tail current is stillbeing disputed, and no correction method hasbeen established.

The relationship between the Dst* indexand the total energy of the ring current parti-

142 Journal of the Communications Research Laboratory Vol.49 No.3 2002

Coronal hole observed by the Yohkohsatellite (courtesy of ISAS)

Fig.3

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cles can also be approximated by the Dessler-Parker-Scopke relation below[6][7].

(2)

where B0 is the geomagnetic field strength atthe Earth's surface and 2E⊥ and EM, are thetotal energy of the ring current particles andthe Earth's external magnetic field. This equa-tion shows that the Dst* index is an indicatorfor the total energy stored within the magne-tosphere in the form of the ring current. Thus,a geomagnetic storm can be considered a statein which a large amount of energy accumu-lates within the magnetosphere. This equationindicates that total energy during a geomag-netic storm (energy of the geomagnetic storm)is on the order of 1015-1016 J.

5 The Ring Current

The depression of the geomagnetic fieldduring geomagnetic storms can be explainedby the effects of the dominating symmetricwestward electric current at the equator, whichflows in the region of 2-9 Re (Earth radii),called the ring current. Development of thiscurrent is thought to be promoted by solarwind-magnetosphere couplings. Convectiondriven by the dayside merging of the solarwind and geomagnetic field lines transportsplasma and magnetic flux to the magnetotail.

The plasma and magnetic flux are then trans-ported from the tail to the interior region ofthe magnetosphere (inner magnetosphere)through magnetic reconnection of the distant-tail neutral lines. As convection accelerates,the plasma is transported further inwards. In asteady state (dv/dt=0), the pressure gradient ofthe plasma and the Lorentz force (J×B) is inequilibrium in the magnetospheric convec-tion[8]. In the inner magnetosphere, the plas-ma pressure gradient and the Lorentz force bythe ring current are balanced.

The pressure distribution within the plas-ma depends on state of geomagnetic activity.

On average, the plasma pressure peaks atapproximately 3 Re, with gradients decreasingin the Earthward and anti-Earthward direc-tions away from the peak. Therefore, if it isassumed that the plasma pressure gradient isin balance with J×B, the electric currentflows westward in the anti-Earthward regionfrom the peak, and eastward in the Earthwardregion from the peak. As a result, the decreasein total geomagnetic force is more markednear the peak of the plasma pressure than atthe Earth's surface.

In Fig.5, the changes in Dst indicesobserved for a geomagnetic storm on June 4-6, 1991 are shown as a solid line, whilechanges inΔB at the geomagnetic equator atapproximately 2.5 Re observed by the Ake-bono satellite during the corresponding timeperiod are shown as solid black dots. TheAkebono satellite, a magnetosphere explo-ration satellite launched in Feb. 1989 by theISAS, has made observations during the timeperiod indicated in Fig.5 at the geomagneticequator at approximately 2.5 Re at MLT nearmidnight. ΔB is the difference in total mag-netic force observed by satellite and estimatedby the International Geomagnetic ReferenceField (IGRF), one of the standard models rep-resenting the Earth's internal magnetic field.In approximation, it corresponds to the changein magnetic field caused by the ring current atthe position of the satellite. The most obviousfeature of this plot is the difference of approx-imately 50 nT betweenΔB and Dst indices in

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Locations of the geomagnetic obser-vatories used to determine the Dstindex[3]

Fig.4

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the quiet period, which is believed to resultfrom the effect of the ring current flowing dur-ing the quiet period. Results of past satelliteobservations have shown that even when geo-magnetic fields are quiet, magnetic field issuppressed near 2.5 Re by approximately －30nT to －50 nT, compared to the model mag-netic field. This suppression is greatest on thedusk side and weakest on the dawn side[9].When the effects of the ring current during thequiet period are subtracted, the trend ofchanges in ΔB generally corresponds to thatof Dst indices.

However, near 6:00 UT and 18:00 UT,when the main phase of the geomagneticstorm progresses rapidly, it can be seen thattheΔB is more than 200 nT lower than the Dstindices.

The large decrease observed inΔB relativeto Dst implies that the peak position of theplasma pressure in the inner magnetospherehad reached the vicinity of the Akebono satel-lite during the development of the main phase.

Since ions are the dominant sources indetermining plasma pressure, ring currents canbe said to consist mainly of ions. Normally,the magnetosphere consists predominantly ofprotons (hydrogen ions). However, the ratioof oxygen ions within the magnetosphere mayincrease significantly during large geomagnet-ic storms[10]. These oxygen ions are believedto originate in the ionosphere. However, theenergy of ions in the ionosphere is only on theorder of several eV, while that of the ions inthe ring current is on the order of several keVto several ten keV. Clearly, some mechanismnot clearly understood at present heats andaccelerates the ionospheric ions to energy lev-els seen in the ring current during geomagnet-ic storms.

The dissipation of the ring current isthought to be caused by charge-exchangeprocess with the geocorona, Coulomb scatter-ing with the thermal plasma of the plasmas-phere, and pitch angle scattering due to inter-actions with the plasma wave. It is knownthat in some cases, the magnetic field recoversin two stages: rapidly at the beginning of the

recovery phase of a geomagnetic storm, andgradually thereafter. It has been suggestedthat this may be due to the effects of a shorterdecay constant for oxygen ions than protonsfor charge-exchange reaction, or due to theeffects of loss at the dayside magnetopause byconvection.

6 Particle Drift

Plasma motion in the external magnetos-phere can be modeled as a magnetohydrody-namics (MHD), after which the phenomenacan be reconstructed by global MHD simula-tions. On the other hand, the inner magnetos-phere in which the ring current develops has asmallβ value (p/(B2/2μ0)) and large gradientsin both the geomagnetic field strength and cur-vature. In this region, the effects upon particletransport of gradient drift and curvature driftby the magnetic field becomes dominant,along with that of the E×B drift.

Since ideal MHD can only be applied toplasma motion by the E×B drift, it cannot beused to express plasma motion accurately inthe inner magnetosphere[11]. The previoussection discussed the ring current within aMHD framework. For a more precise exami-nation, particle kinetic effects must also beconsidered. Here, we will examine particlemotion in the inner magnetosphere.

Journal of the Communications Research Laboratory Vol.49 No.3 2002

The changes in Dst indices during ageomagnetic storm on June 4-6, 1991and changes in ΔB observed at thegeomagnetic equator (L = 2.5 Re)near midnight region by the Akebonosatellite

Fig.5

The gradient drift WG and curvature driftWC can be described as follows:

(3)

(4)Here, v⊥ and v// are velocities perpendicu-

lar and parallel to magnetic field lines, while qis electric charge. Gradient drift is enhancedcloser to the Earth. Plasma transported fromthe magnetotail by the E×B drift is morestrongly affected by gradient drift as itapproaches the Earth, as a result of which ionsare transported duskward, while electrons aretransported dawnward, both being kept fromfurther approaching the Earth. The point atwhich the E×B drift velocity equals gradientdrift velocity is considered to indicate the par-ticle penetration limit (Fig.6). It can be seenthat particles penetrate deeper into the Earth'svicinity when the applied electric field isstronger. The penetration limit also dependson particle energy. The region further withinthe particles' penetration limit is called theAlfvén layer.

Since the centers of gyration (guiding cen-ter) of electrons and ions move in the oppositedirection in the cases of gradient and curvaturedrifts, a drift current is generated.

The gradient drift current JG and the curva-ture drift current JC are expressed as follows:

(5)(6)

where P⊥ and P// are plasma pressures perpen-dicular and parallel to the magnetic field lines,respectively.

A magnetization current Jm generated bynon-uniform spatial distribution of the parti-cles' magnetic moment is given by the follow-ing equation[12]:

(7)

The current perpendicular to the magneticfield line J⊥ can be expressed as the sum of thedrift current and the magnetization current.

(8)

When the pressures parallel and perpendi-cular to the magnetic field lines are equal,only the effect of the magnetization currentremains, and J⊥ is equivalent to that obtainedfrom the MHD formula. If an isotropic pres-sure distribution can be assumed, the balanceof force can be discussed within a MHDframework. We also see that the gradient driftand part of the magnetization current canceleach other out, so that the effect of the formeris not apparent. Gradient drift transports ionsto the dusk side and electrons to the dawn sidein the inner magnetosphere, but these particlemotions do not generate an electric current.But since ions contribute significantly to plas-ma pressure, the asymmetrical transport ofelectrons and ions results in a dawn-duskasymmetry in the pressure gradient, which inturn creates a dawn-dusk asymmetry in thering current. When the magnetospheric elec-tric field is increasing, an asymmetry is creat-ed in the ring current. When the developmentof the electric field terminates and particleinjection stops, particle motion in the innermagnetosphere assumes a closed orbit, and thering current recovers its symmetry.

Although the opposing motions of ionsand electrons may appear to result in the accu-mulation of positive and negative charges onthe dusk and dawn sides, respectively, thecharges are quickly canceled out by plasmasupplied from the ionosphere. Thus, they donot accumulate. Except in an unsteady state,such charge accumulation is not considered toplay an essential role in the development ofconvection and geomagnetic storms. If theydo not cancel out, the polarization electricfield produced by the motions of ions andelectrons promotes changes in particle driftmotion, and the state of particle distributionactually observed in the magnetosphere isunstable. Note that the above equations fordrift current and magnetization current assumea condition of motion within a static magneticfield; the effects of the secondary electric fieldgenerated by the motions of ions and electronsare not considered.

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7 Geomagnetic Storm (Dst Index)Prediction

If the Dst* index is considered to be anindicator for total particle energy stored withinthe magnetosphere, the growth and decay inDst* values can be expressed by an equationof balance between energy injection anddecay[4]:

(9)

Here, Q(t) is the rate of energy injection,whileτ is the decay constant. The secondterm on the right-hand side of the equation(decay term) takes this form because the Dsttends to decay exponentially during the recov-ery phase.

Numerous past studies have attempted topredict the Dst index by defining Q(t) as afunction of the solar wind parameter and byassuming a value forτ. Although variousinput parameters for solar wind have been pro-posed, results of statistical analysis of long-term solar wind data have shown that VBs isthe parameter that most closely reconstructsobserved variations in Dst[5][13]. Modelsassuming a constant decay constant or onesdependent on the value of Dst index have beenthe ones most widely used[14]. For example,Burton's model[4] assumesτ= 7.7 [hour].

A model recently proposed defines thedecay constant as a function of VBs when Q(t)is defined as a function of VBs, based on the

assumption that the main process of decay isthe charge exchange between the ring currentions and the geocorona[5]. Since the densityof the geocorona increases exponentiallytowards the Earth, the decay constant for thecharge exchange becomes shorter nearer to theEarth. If the magnetospheric plasma is trans-ported closer to the Earth according to thevalue of VBs, the shortening of the decay con-stant ofτ can be interpreted to be dependenton VBs as well. But note that the estimatedmagnitude of geomagnetic storms tends to belower than the actual observed magnitude forstorms with Dst <_－150 nT when using thisdecay constant model.

Fig.7 compares the Dst* indices predictedbased on the O'Brien and McPherron model[5](OM model) to those determined from obser-vations, as well as Dst* indices predictedbased on the Burton model[4] (B model) tothose determined from observations.Although the OM model has a high coefficientof correlation, predicted Dst* values tend tobe lower than the observed values for Dst* < －150 nT. For larger geomagnetic storms, theB model sometimes appears to give moreaccurate predictions. To reconstruct large geo-magnetic storms, we must increase the energyinjection rate accompanying an increase inVBs, or to use longer decay constants thanproposed by the OM model. As will be elabo-rated further below, it is difficult to imagine acondition in which only the energy injectionrate increases, since the development of mag-netospheric convection are saturated by theintense solar wind electric fields. On the otherhand, the OM model definesτ, assuming thatdecay occurs only by the charge exchangeprocess by the geocorona. In reality, Coulombscattering and wave-particle interaction alsocontribute. The model adopted forτmust beexamined further.

The energy injection rate due to increasednumbers of particles injected into the magne-tosphere cannot be ignored. However, it isdifficult to determine a valid and universaldecay constant, since the number of large geo-magnetic storms occurrences is limited.

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Penetration limit of particles transport-ed from the magnetotail by convec-tion

Fig.6

147

A different approach to Dst index predic-tion uses neutral networks[15]. The correla-tions between solar wind data and Dst indicesare catalogued and the results used to con-struct a neutral network model. This model isthen used to predict Dst indices. The resultshave been found to be relatively accurate.However, certain difficulties arise in predic-tions with the arrival of solar wind variationsof unknown patterns. The validation of resultsand the stability of the model need to be fur-ther examined.

8 Magnetospheric ConvectionDuring a Geomagnetic Storm

As stated earlier, a correlation has beenconfirmed by observation between geomag-netic storms and solar wind VBs. Since suchpositive correlation is also present during thegrowth of magnetospheric convection, it hasbeen suggested that the growth of a magnetos-pheric convection may be the major factorbehind geomagnetic storm growth[5][16].

While this may be true under normal solarwind conditions, the two have been found tobehave differently when the solar wind elec-tric field is intensified.

It has long been suggested that the magne-tospheric convection driven by the solar wind-magnetosphere coupling may be a non-linear

coupling. One such possibility is the suppres-sion of the reconnection efficiency of magnet-ic field lines when magnetic field strengthsdiffer for the dayside magnetosphere and theIMF. In this case, the development of magne-tospheric convection would be suppressed forstrong IMF[17][18]. Another possibility is adecrease in magnetic reconnection efficiencydue to changes in the dayside magnetopausemagnetic field caused by the magnetic fieldgenerated by Region 1 field-aligned currents(FAC), which transfer the convection motionfrom the magnetosphere to the ionosphere.The development of magnetospheric convec-tion can be expressed as the polar cap poten-tials calculated from the width of the convect-ing region and the electric field of the magne-tosphere.

If the ionosphere has uniform conductivi-ty, the current intensity of the Region 1 FAC isproportional to the polar cap potential. Thus,we can predict that the growth of magnetos-pheric convection will be suppressed forstrong solar wind electric fields[19][20].

The PC index is an indicator representingthe variation in the horizontal component ofthe geomagnetic field near the magnetic pole.It is known to correlate well with the solarwind electric field (Em = VBTsin2(θ/2)), andcan be used as an indicator for both the polarcap potential and the magnetospheric convec-

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Observed and predicted Dst* indices (Left: Burton's model [4]; Right: O'Brien and McPherronmodel [5].)

Fig.7

148

tion[21][22]. PC index and solar wind parame-ters (Em) have been used to investigate theresponse of the magnetospheric convection tostrong solar wind electric fields. Fig.8 showsthe results of statistical analysis of the PCindex (PCN) of Qaanaaq (Thule), according tovarying ranges of solar wind parameters (Em).According to this analysis, the magnetosphericconvection displays a clear non-linear effect,including a peak for Em >_ 5 mV/m. Further-more, it was found that this non-linear effectdepends only on the intensity of the solar windelectric field, not on IMF strength. This sup-ports the theory that the development ofRegion 1 currents suppresses magnetosphericconvection[23].

Are such non-linear effects seen for theDst* index? According to Burton's model[4],when an electric field with a maximumstrength of 5 mV/m is applied to the magne-tosphere, the maximum Dst* (geomagneticstorm) predicted assuming a steady field of 5mV/m is around－190 nT. However, numer-ous geomagnetic storms on far larger scaleshave been observed in the past. As stated inthe previous section, the Burton[4] and theO'Brien and McPherron[5] models, whichassume that the energy influx from the solarwind to the magnetosphere increases linearly,have predicted geomagnetic storms with rela-tive accuracy, including those with Dst* < －190 nT, although the problem of decay con-stants does remain.

This implies that the energy influx fromthe solar wind to the ring current increases lin-early, behaving differently from the magnetos-pheric convection, which grows non-linearly.This throws some doubt on the theory thatmagnetospheric convection growth drivesgeomagnetic storm development.

Several observations suggest that the plas-ma is transported to the vicinity of the Earthduring geomagnetic storms. The CRRESsatellite has made direct observations of theelectric field of the inner magnetosphere dur-ing the main phase of a geomagnetic storm,reporting electric fields of several mV/m inregions closer to the Earth than L = 4[24].

However, the mechanism that generatessuch large electric fields (plasma flow) withinthe inner magnetosphere remains unclear.Given the limited numbers of satellites capa-ble of making direct observations of the innermagnetosphere at this time, electric fieldmeasurements of the inner magnetosphereduring geomagnetic storms remains an impor-tant research theme for the future.

Observations have also confirmed that thepeak position of particle flux in the outer radi-ation belt moves inwards with the growth of ageomagnetic storm[25]. The most widely sup-ported theory at this time to explain theincrease in flux near the peak in the outer radi-ation belt invokes the acceleration of electronswith energies of several 10 keV injected intothe inner magnetosphere (internal accelerationtheory). According to this theory, the plasmaof several 10 keV must be transported furtherinwards, depending on the size of the geomag-netic storm. Such plasma transport wouldrequire a very large electric field.

If magnetospheric convection is not satu-rated, in contrast to saturated ionospheric con-vection, a gap will exist in the values of thepolar cap potential at the ionosphere/magne-tosphere boundary. As long as the magnetos-phere and ionosphere are coupled, the differ-ence in polar cap potentials on the ionosphereand the magnetosphere sides must be relievedas the difference in FAC potential. Calcula-tions indicate that a FAC potential exceedingseveral 10 keV will be required. However,observations have yet to confirm the existenceof such large FAC potential differences. Fur-ther study is required to clarify the mutualeffects of plasma motions in the ionosphereand the magnetosphere.

On the other hand, perhaps the increase insolar wind density during geomagnetic stormsleads to increased numbers of particles enter-ing the magnetosphere, even when the rate ofenergy injection into the magnetosphere peaksand begins to decline due to the saturation ofconvection, increasing the energy accumulatedin the form of the ring current.

Observations have demonstrated a correla-

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tion between plasma sheet density and solarwind density[26][27], supporting this hypothe-sis. Furthermore, Dst indices calculated byparticle tracing simulations that take plasmasheet density variations into considerationhave been shown to agree quite well withobservation[28]. However, note that particletracing simulations can only reconstruct geo-magnetic storms with Dst of－100 nT. It hasbeen statistically shown that solar wind densi-ty variations have no dependency on geomag-netic storm developments[29].

9 Geomagnetic Storms and Sub-storms

A substorm was originally defined as abasic element of a geomagnetic storm (thus,termed "sub" storm)[30]. When substorms hadfirst been defined, a working hypothesis thatfrequent occurrences of substorms leads to the

generation and development of a geomagneticstorm was proposed. It was generallybelieved that an understanding of substormsshould lead to an understanding of geomag-netic storms. Now, it is clear that the relation-ship between substorms and geomagneticstorms is not so simple, and that they need tobe considered as independent phenomena.

The frequent occurrence of substorms doesnot necessarily lead to geomagnetic stormdevelopment. Results of statistical compar-isons of variations in geomagnetic storms andsubstorms using geomagnetic indices haveshown that the growth of Dst index is sup-pressed during substorm development, or evenconverted to a recovery phase[31]. This sug-gests the possibility that substorms actuallysuppress the growth of Dst. On the otherhand, some scientists view the recovery of theDst index as only an apparent one arising fromthe decrease in the tail current, arguing that

BT dependence of PCN index for Em of 1-2 mV/m (top left), 03-04 mV/m (top right), 05-06mV/m (bottom left), and 07-08 mV/m (bottom right) [23]

Fig.8

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the ring current itself may be developing,alongside the substorm[32].

The hypothesis of frequent substorms trig-gering geomagnetic storms attributes ring cur-rent growth to the injection of particles intothe inner magnetosphere during substorms.

However, the results of imaging observa-tions of the inner magnetosphere using ener-getic neutral atoms (ENA) demonstrate thatparticle injection accompanying substormsalone cannot account for the creation of thering current[33].

Based on results of particle tracing simula-tions, it has also been reported that thedecrease in Dst caused by particles injected byinduced electric fields during a single sub-storm event is, at most, several nT[34].

Substorms occur frequently, even whengeomagnetic storms are absent. The differ-ence between substorms that trigger geomag-netic storms and those that do not are notclear. However, some researchers do claim tohave found a difference between substormsassociated with geomagnetic storms and thosenot so associated[35].

10 Prospects for Future Geomag-netic Storm Research (TheImportance of Inner Magne-tosphere Research)

The deployment of global observation net-works both on ground and by satellite, togeth-er with the emergence of global MHD simula-tions, has advanced our understanding of theconvection in the Earth's magnetosphere andsubstorms[8]. However, given the limitednumber of satellites that have made directobservations of the inner magnetosphere,many problems remain to be resolved byobservation, especially with respect to thedynamics of the outer radiation belt. Sinceparticle motion due to gradient and curvaturedrift in the magnetic field cannot be ignored,the extent of the understanding that may beachieved through ideal MHD simulationsalone is limited. Numerous attempts havebeen made to reconstruct geomagnetic storms

through particle tracing simulations based onstatic magnetic field models. However, in thecase of large geomagnetic storms with Dstdropping below－100 nT, the electric currentsgenerated by injected particles would signifi-cantly affect the structure of the magneticfields in their vicinity. Thus, the results ofparticle tracing simulations using a static mag-netic field model are unlikely to reconstructthe actual state of geomagnetic storms.Although we must consider the non-lineareffects of changes in magnetic field structurescaused by injected particles, simulations havenot yet been able to incorporate such effects.Additionally, the results of such simulationsdiffer significantly depending on the magneticand electric field models adopted. The authorbelieves that a quantitative reconstruction ofgeomagnetic storms must ultimately await afull particle simulation capable of incorporat-ing particle kinetic effects.

Direct satellite measurements of the innermagnetosphere are also important and must bemade by multiple satellites (constellations),since temporal and spatial variations are noteasily distinguishable in data from singlesatellite observations. In recent years,advances in ENA imaging technology haveenabled 2-D imaging of the ring current.

Snapshots of plasma (ion) spatial distribu-tions are now becoming available. In thefuture, satellite observation networks such asconstellations for direct observation of theinner magnetosphere and ENA imaging con-stellations should be deployed to reveal,through observations, variations in plasmapressure distribution and changes in plasmaflow (electric field distribution) in the innermagnetosphere and their various causal fac-tors.Appendix: The Effect of GeomagneticStorms on Society

Geomagnetic storms are the largest-scaledisturbances in the magnetosphere.

Large-scale variations produced in thespace environment by this disturbance gener-ate numerous hazards. One of the mostimportant themes in space weather forecasting

151

is to gain a solid understanding of geomagnet-ic storms, to make accurate predictions and toprovide advance warning of its effects. Thefollowing is an introduction to the space envi-ronment variations that accompany geomag-netic storms and the potential hazards thatthey present.Ionospheric Storms and ThermosphericExpansion

During the growth of geomagnetic storms,convection in the magnetosphere is enhancedand substorms occur frequently. The ionos-pheric current in the polar region also gathersstrength. The energy supplied to the polarregion heats the thermosphere, which changeschemical compositions and generates large-scale motion in the thermosphere. Such con-ditions may lead to the generation of an ionos-pheric storm, which in turn may causes com-munication failures[36]. The heating and sub-sequent expansion of the thermosphere maysignificantly disturb the attitudes and orbits ofsatellites in low Earth orbit.Geomagnetic Induced Currents

Geomagnetic storms may accompany sud-den, short-period geomagnetic field variations.Sudden geomagnetic changes induce currentsin power lines and pipelines. These inducedcurrents may destroy power transmission sys-tems and corrode pipelines. The threats posedby geomagnetic induced currents are widelyacknowledged in polar regions, where strongionospheric currents have been observed.However, recent observations have confirmedcases of strong currents induced in power linesduring geomagnetic storms, even in middle tolow latitude regions such as Japan[37].Increase in Outer Radiation Belts Elec-tron Flux

Variations in the relativistic electron fluxin the outer radiation belt are deeply related togeomagnetic storms. Normally, during thegrowth of the main phase of the geomagneticstorm, the relativistic electron flux in the outerradiation belt decreases. Later in the recoveryphase, the relativistic electron flux sometimesdisplays a significant increase relative tobefore the onset of the geomagnetic storm[38].

Relativistic electrons cause deep dielectriccharges in electronic circuits within satellites.When flux increases, the probability of deepdielectric charges also increases, potentiallyleading to malfunctions and the failure ofsatellite instruments.Expansion of the Region Affected bythe Proton Event

The arrival of energetic protons accelerat-ed by solar flares at the Earth is called a pro-ton event. Proton events damage solar cellpanels and increase radiation exposure of crewaboard spacecrafts and space stations. Theprecipitation of energetic protons in a protonevent in the polar ionosphere may cause ion-ization anomalies leading to communicationfailures in the HF band, called polar capabsorptions (PCA). The precipitated protonsalso generate secondary cosmic rays, increas-ing the exposure of aircraft crew to radiationwhile traveling through polar regions. Duringquiet periods, the penetration of energetic par-ticles are limited to polar caps by the shieldingeffect of the geomagnetic field. But duringgeomagnetic storms, the magnetic field struc-ture of the inner magnetosphere changes,expanding the region penetrated by energeticparticles to lower latitudes. When a protonevent occurs simultaneously with geomagneticstorms, communication failures in the HFband and radiation exposure of crew membersaboard space stations and aircrafts occuracross a wider region.Acknowledgements

The author wishes to thank Prof. Tanaka ofKyushu University, Dr. Obara, the leader ofthe Space Simulation Group at CRL, Dr.Kikuchi of the Space Weather Group at CRL,and Dr. Kunitake, senior researcher at CRL,for valuable advice. The LASCO image dataacquired by the SOHO satellite was providedby ESA and NASA; SXT image data of theYohkoh satellite was provided by ISAS; dataon PCN indices was provided by the DanishMeteorological Institute. Dst indices wereprovided by the World Data Center for Geo-magnetism, Kyoto University. Solar winddata was taken by NASA Wind and ACE

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152 Journal of the Communications Research Laboratory Vol.49 No.3 2002

satellites (MAG, SWEPAM). The authorwishes to express his gratitude to all of theabove organizations.

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154 Journal of the Communications Research Laboratory Vol.49 No.3 2002

NAGATSUMA Tsutomu, Ph. D.

Senior Researcher, Space WeatherGroup, Applied Research and Stand-ards Division

Solar-Terrestrial Physics