The solar wind-magnetosphere dynamo and the magnetospheric substorm

Planet. Space Sci. 1975, Vol. 23, pp. 817 to 823. Persamon Press. Printed in Northern Ireland

THE SOLAR WIND-~AGN~TOSPHE~ THE ~AGNETOSPHERrC

DYNAMO SUBSTOR~

AND

S.-I. AKASOPU Geophysical Institute, University of Alaska, Fairbanks, Alaska 99701, U.S.A. (Received 18 September 1974)

Abstract-In a quiet condition, the soIar wind kinetic energy is converted into electrical energy. A small part of this energy is dissipated as heat energy in the polar ionosphere. We identify at least three types of magnetospheric disturbances which are not associated with an increase of the heat production and call them reversible disturbances, while the magnetospheric substorm is an irreversible disturbance which is associated with a large increase of the heat production. The magnetosphere appears to have an inherent internal instability by which a large amount of heat energy is sporadically produced in the polar upper atmosphere at the expense of the magnetic energy in the magnetotaii. A positive feed-back process may be responsible for the growth of the instability and for the expansive phase, while therecovery phase sets in whensome process begins to suppress the positive feed-back process. 1. TIIE SOLAR WIND-MAGNETOSPHERE

INTERACTION AS AN MHD DYNAMO

A study of the quiet-time magnetosphere is of fundamental importance in understanding basic infraction processes between the solar wind and the magnetosphere. It also provides the base-lines from which various disturbed quantities are measured. The interaction between the solar wind and the magnetosphere constitutes a magnetohydrodynamic (MHD) dynamo. The solar wind blows across the polar cap geomagnetic field lines which are merged with interplanetary magnetic field lines. Figure 1 shows schematically this situation (cf. Akasofu, 1974). The dynamo process takes place near the ma~etopause (AED), resulting in a potential of the order of 40-50 kV across the magnetotail. Much of the current generated by the dynamo is discharged across the magnetotail (AFD). This portion of the current is called the cross-tail current. However, a small amount of the current is also discharged across the polar cap (ABCD); this part of the circuit is called thepolar cap circuit. It is this portion of the current circuit which is responsible for the aurora. That is to say, the aurora is a discharge phenomenon associated with the polar cap circuit. It should be pointed out that we now have at least a partial answer to the question of why auroras tend to appear along a particular belt, the aurora1 oval. In the simplest situation in which the interplanetary magnetic field has only a southward component, a neutral line surrounds the magnetosphere in the equatorial plane. Its morning half acts as the positive “terminal” of the solar wind ma~etosphere dynamo, from which the fieldaligned polar cap current ffows into the morning half of the oval, while the evening half of the neutral line acts as the negative “terminal” which is connected with the polar cap current flowing outward from the evening half of the oval. The field-aligned currents are connected via Pedersen current in the polar cap ionosphere; see Fig. 2. Since the interaction with the solar wind is a permanent feature of the magnetosphere, the MHD dynamo and the resulting cross-tail and polar cap currents are also permanent features of the planet Earth For example, the aurora had been considered to be a disturbance feature, but we know now that the aurora1 oval is a permanent feature of the Earth. The presence of the quiet day polar cap daily magnetic variation (Z&g)is another indication of the permanent feature of the polar cap circuit (Kawasaki and Akasofu, 817

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S.-I. AKASOFU

wind

FIG. 2.

THE GEOMETRY OF THE X-TYPE NEUTRAL LINE IN THE EQUATORIAL PLANE AND ITS RI?LATKONS TO THE AURORAL OVAL AND THE San CURRENT SySCEM.

1973). The magnetosphere can be considered to be a mechanical device which converts the solar wind kirretic energy into electrical energy. Most of the current generated by the dynamo is fed into the two ‘solenoids’ which constitute the magnetotail. However, a part of the electrical energy is converted into heat energy. The heat energy is generated in the polar cap ionosphere where the polar cap circuit has a significant resistance. The amount of heat energy Pa generated per unit time in both the northern and southern polar caps can be estimated to be Prr=2x(E.J)xrz!6x 101’ergjsec, where E ‘Y 20 mV/m denotes the average electric field across the polar cap and J e 5 x 106 A denotes the total current in the polar cap circuit in one hemisphere (assuming the

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SUBSTORM

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diameter r of the polar cap to be of the order of 3000 km). The above value may be compared with the energy input rate P, by the solar wind into the magnetotail (Siscoe and Cummings, 1969; see also Akasofu and Chapman, 1973). P, = F. Y = (BT2/87r) rrRr2 V = lOl@ergs/set, where F = the force acting between the Earth and the tail ~4 x loll dyn, V = solar wind speed = 300 km/set, B, = the magnetotail field = 15y, R, = the radius of the magnetotail = 20 RE and RIF = the Earth’s radius. It is quite likely that the above value of P, is overestimated and that of PH is underestimated. Nevertheless, it is interesting to note that the estimated value of P, is not far from the energy dissipation rate during an intense substorm. 2. A NEW CLASSIFICATION OF MAGNETOSPHERIC DISTURBANCES

Magnetospheric disturbances can be classified into two: reversible disturbances and (2) irreversible disturbances.

(1) reversible or quasi-

2.1 Reversible or quasi-reversible disturbances A reversible disturbance is defined to be one for which the magnetosphere returns to a quiet-time configuration after the responsible interplanetary disturbance is removed, with or without any significant increase of the heat production from the quiet-time value in the polar cap circuit. We have so far found three interplanetary disturbances which cause reversible or quasi-reversible disturbances. (a) A weak interplanetary shock wave. A weak interplanetary shock wave simply compresses the magnetosphere. After the solar wind returns to a pre-shock condition, the magnetosphere expands, returning to a quiet-time configuration. There is no significant increase of heat production by this process. (b) The east-west component of the interplanetary magnetic $eld. Svalgaard (1973) showed that there exists a counterclockwise current (observing from above the north geomagnetic pole) along the geomagnetic latitude circle of about 75-80°, when the interplanetary magnetic field has a westward component (i.e. in the “away” sector) and a clockwise current for an eastward component (i.e. in the “toward” sector). Heppner (1972) showed that the electric field distribution becomes asymmetric with respect to the Sun-Earth line when the interplanetary magnetic field has an east-west component. It may be possible that some of these observations can be understood in terms of the east-west shift of the aurora1 oval with respect to the Sun-Earth line (or the noon-midnight meridian) and thus of the polar cap circuit, instead of the growth of a new current system. If it is indeed the case, these particular disturbances can also be classified as reversible processes. The aurora1 oval and the polar cap circuit return to their symmetric configurations, without an increase of the heat production, when the east-west component is removed. For details of polar cap current systems, see Kawasaki et al. (1973). (c) The north-south component of the interplanetary magneticjield. It has been suggested that the north-south component of the interplanetary magnetic field controls the merging rate of the geomagnetic field lines with the interplanetary magnetic field lines. An increase in the merging rate results in an increase in area of the polar cap and thus of the aurora1 oval. Some of the other observed changes associated with an increased merging rate are: (i) earthward shift of the magnetopause; (ii) equatorward shift of the

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cusp; (iii) an increase of the magnetic field intensity in the high latitude lobe; (iv) an increase of the flaring angle of the magnetotail. The expansion of the aurora1 oval and thus of the polar cap circuit causes a particular type of magnetic variations at magnetic observatories in the polar region, even if there is no change in the total current intensity of the polar cap circuit. However, the actual change, identified first by Nishida (1968) as the DP-2 variations, can be understood in terms of a combination of an expansion of the circuit and of an appreciable increase of the total current intensity (Akasofu, Yasuhara and Kawasaki, 1973). Thus, the response of the magnetosphere to the southward component of the interplanetary magnetic field (and thus to an increase in the merging rate) is not strictly a reversible process. However, since the increase in heating is rather small compared with what will be described as an irreversible process in the next section, we may call it a quasi-reversible process. This dynamical feature of the aurora1 oval associated with changes of the north-south component of the interplanetary magnetic field has only recently been recognized (Akasofu et al., 1973). Since most of the aurora1 observations are conducted along the aurora1 zone (the dipole latitude of about 60-67”), it is not possible to observe the aurora1 oval when it contracts poleward, to the dipole latitude of about 70” or above; this situation occurs when the interplanetary magnetic field has a large northward component. It had thus been thought by most that the aurora and the associated magnetic disturbance did not occur when the interplanetary field had a northward component. Actually, aurora1 phenomena were present, but well beyond the field of view of aurora1 zone stations. On the other hand, when the interplanetary magnetic field has a southward component, the aurora1 oval descends to the latitude of the aurora1 zone in the midnight sector. This had been interpreted by many as an indication that the aurora could occur only when the interplanetary magnetic field had a southward component. For the same reason, typical substorm features can be observed at the standard aurora1 zone stations only when the interplanetary magnetic field has a southward component. Thus, it had incorrectly been interpreted that the southward component was a necessary condition for the occurrence of substorms. Fortunately, all these misconceptions have now been removed recently by improved aurora1 observations, particularly by a meridian chain of all-sky cameras and satellite photography. 2.2 Irreversible disturbances Here, we identify the magnetospheric substorm as an irreversible phenomenon. Indeed, there occurs a large increase of heating in the polar cap ionosphere during substorms. It should be noted that the magnetospheric storm period is defined as a period when intense substorms occur very frequently. Thus, the magnetospheric storm is also an irreversible phenomenon. In the next section, we study in detail substorm processes as a perturbation in the polar cap circuit. 3. MAGNETOSPHERIC SUBSTORMS

As mentioned above, the magnetospheric substorm is an irreversible process which appears to grow explosively as a perturbation in the polar cap circuit. Thus, it is tempting to look for a positive feed-back process in the circuit as a cause of the magnetospheric substorm. Note that it seems unnecessary to invoke a particular process which leads to the accumulation of substorm energy in the magnetotail just prior to substorms. In the

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821

quiet-time magnetosphere, a magnetic energy of about (lOas - 10ePerg) is maintained in the magnetotail, consisting of two solenoids which are constantly fed by the solar wind magnetosphere dynamo. Even without P, = lOIn erg/set, the above amount of energy is enough for at least a few intense substorms. The process we look for should be able to dissipate rapidly a part of this energy as heat energy in the polar ionosphere. Keeping these two requirements in mind, let us consider, for example, a sudden growth of ion acoustic waves along the field line portions of the polar cap circuit, particularly between A and B in Fig. 1 where an inflow of electrons carries the upward current. A possible sequence of processes which might follow is (see Fig. 3): (1) a sudden enhancement of the anomalous resistivity along the field line portion of the polar cap circuit; (2) the resulting potential drop along the field lines; (3) the acceleration of some aurora1

I:“F-_/-t

I

1 t

Te - TL______, of ion acoustic

anomalous resistivity

F10.3.

A

POSSIBLEPOSITIVEFEED-BACKPROCESSESFORTHEEXPANSIVEPHASEOFSUBSTORMSAND THE CORRESPONDING SUPPRESSION PROCESSES.

822

S.-I. AKASOFU

electrons along the potential drop; (4) an increase of ionization in the E region of the ionosphere; (5) a large reduction of the resistivity of the polar cap circuit; (6) the shortcircuiting and partial disruption of the crosstail current; (7) the resulting increase of the current intensity along the polar cap circuit. The resulting increase of the current intensity along the polar cap circuit would further enhance initially the growth of ion acoustic waves, since the growth occurs when the current density exceeds a certain value (Swift, 1965). If this is indeed the case, the chain of processes in the above would form a positive feed-back system. A part of the magnetic energy in the magnetotail is converted into heat energy by the process of short-circuiting the cross-tail current by the polar cap circuit. The above positive feed-back process should be considered only as one of the possible explanations for the expansive (or flare) phase of substorms. A crucial test of this particular proposed process is to examine the presence of ion acoustic waves along the geomagnetic field lines, which leads to the injection of aurora1 electrons into the polar upper atmosphere. The positive feed-back process should, however, not grow indefinitely. That is to say, when the positive feed-back process grows to a certain stage, it begins to be suppressed by some process. Ion acoustic waves can grow only when T, > Ti, When positive ions along field lines are sufficiently heated by the growth of ion acoustic waves, namely Ti - T,, Landau damping is known to severely damp ion acoustic waves. The decay of ion acoustic waves would lead to the following chain of processes (see Fig. 3): (1) The reduction of the anomalous resistivity; (2) the reduction of the potential along the field lines; (3) the reduction of the acceleration of aurora1 electrons; (4) the reduction of the ionization in the E region; (5) the increase of the resistivity of the polar cap circuit; (6) the cessation of the short-circuiting. Since the presence of ion acoustic waves has not been confirmed yet, this particular combination of the positive feed-back process and its suppression process described here is still speculative. It should, however, be emphasized that the correct substorm mechanism should be one in which a positive feed-back process is responsible for the expansive phase, but that it is suppressed by some other process at a certain stage. In the above example, we have used the Landau damping of ion acoustic waves as the mechanism for the transition from the expansive phase to the recovery phase. 4. WHAT IS THJI SUBSTORM? The general circulation and the magnetospheric convection

In the simplest (non-rotating Earth) situation, a net heat input in tropical regions and a net heat loss in polar regions would cause a meridional circulation of air mass. The classical convection of magnetosphere plasma, proposed by Axford and Hines (1961), might correspond to the classical meridional (general) circulation in meteorology. The polar cap circuit for a quiet condition is associated with the magnetospheric convection, since the morning half of both the neutral line and the aurora1 oval has a positive space charge and the evening half of both the neutral line and the aurora1 oval has a negative space charge; thus, in the polar ionosphere the equipotential lines consist of two cells, centered around the morning and evening halves of the oval. It is, however, known that the Earth’s atmosphere tends to maintain the heat balance between the tropical and polar regions more efficiently by a sporadic process called the cyclogenesis than by the steady meridional (general) circulation. The atmosphere generates internally cyclones which cause an extensive exchange of warm and cold air mass.

SOLAR WIND AND ~GN~TOSP~~~

SURSTORM

823

It is tempting to speculate that there is a certain analogy between cyclogenesis and the occurrence of substorms. The substorm is associated with a sporadic and large increase of the heat energy which is converted from the magnetic energy stored in the ma~eto~il. We have so far failed to find extra-magnetospheric causes of the magnetospheric substorms, except for some intense interplanetary shock waves (Kawasaki et al. 1971). Like cyclones, the direct cause of substorms may be found within the magnetosphere rather than in inte~lane~~ space. The polar cap circuit may have an inherent instability which may grow explosively. The north-south component of the interplanetary maguetic field has, however, an important effect on the intensity of substorms by regulating the efticiency of the solar and-ma~etosphere dynamo. It has been found that the ~tensity of ma~etosphe~c substorms is well regulated by the north-south component of the interplanetary magnetic field (Kamide and Akasofu, 1974). Substorms tend to be more intense when the interplanetary magnetic field has a large southward component than a large northward component. ~ontb~and (1969) classified substorms into three groups, weak, medium and intense substorms. The characteristics of weak substorms are found in substorms which occur frequently along the contracted oval. work was supported by the National Science Foundation, Atmosphere Sciences Section, Grant GA-36873X. I wouId like to thank Professor H. Kamiyama at the Geophysical Institute, Tohoku University, Sentai, where this manuscript was prepared. I would like to thank Dr. K. Kawasaki for this discussion on an early draft of this paper. Ack~wIedge~nfs-his

~FU,

S.-I. (1974). The aurora and the magnetosphere:

The Chapman Memorial Iecture. Planet.

Space Sci. 22, 885. AKASOFU, S.-I. and CHAPMAN,S. (1972). Solar-Terrestrial

Physics, p. 339. Oxford University Press, London. AKASOFU,S.-I., PERRIWULT, P. D,, Ynsrnrm, F. and MENG, C.-L (1973). Aurora1 substorms and the interplanetary magnetic field. J. geophys. Res. 78,749O. A~~soru, S.-I., ~ASUHARA, F. and KAWASAKI, K. (1973). A note on the DP-2 variation, Planet. Space Sci. 21,2232. AXFORD, W. I. and Hmss, C. 0. (1961). A unifying theory of high-latitude geophysical phenomena and g~ma~etic storms. Can. J. Phys. 39,1433.

HELPER, 1. P. (1972). Polar cap electric field distributions related to the interplanetary magnetic field direction, J. geophys. Res. 77,4877. KAMIDE, Y. and AKaso~u, S.-I. (1974). Latitudinal cross section of the aurora1 electrojet and its relation to the interplanetary magnetic tieId polarity. J. geophys. Res. 79, 3755. KAWASAKI, K. and Arxsorr~, S.-I. (1973). A possible current system associated with the S,* variation. Planet. Space Sci. 21,329. KAWASAKI, K., YASUHARA F. and AKA~~FIJS.-I. (1973). Short-period interplanetary and polar magnetic field variations. Planet. Space Sci. 21, 1743. MONTBRKAND,L. E. (1969). Morphology of aurora1 hydrogen emissions during aurora1 substorms, Ph.D. Thesis, University of Saskatchewan. Nrs~n>~, A. (1968). Coherence of geomagnetic DP-2 fluctuations with interplanetary magnetic variation. J. geophys. Res. 73, 5547. Srsco~, 0. L. and was, W. 0. (1969). On the cause of geomagnetic bays. Planet. Space Sci. 17,179s. Swrrr, D. W. (1965). A mechanism for energizing electrons in the magnetosphere. J. geophys. Res. 70, 3061. SVAIA~, L. (1973). PoIar cap magnetic variations and their relationship with the interplanetary magnetic sector structure. J. geophys. Res. 78,2064.