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Recurrent geomagnetic storms, solar M-regions and the solar wind
Planet. Space Sci. 1964. Vol. 12. pp. 113 to 118. Pergamon Press Ltd. Printed in Northern Ireland
RECURRENT
GEOMAGNETIC STORMS, SOLAR M-REGIONS AND THE SOLAR WIND J. H. PIDDINGTON Radiophysics Laboratory, CSIRO, Sydney (Received 18 October 1963)
Abstract-Evidence is reviewed indicating that the main features of geomagnetic storms of both types are caused by frictional interaction with the solar wind rather than by mere pressure on the geomagnetic cavity. If this is so then recurrent or M-region storms are caused by a property of the (steady) solar wind which controls the frictional interaction. This property is the component of the interplanetary magnetic field in the geomagnetic equatorial plane. Hence M-regions are regions above which (projected along the streamlines) the magnetic field is unusually weak or lies at an angle MT/~ to this plane.
1.INTRODUCTION
In a recent examination of geomagnetic storms Akasofu and Chapman(l) found that some have large and prolonged first or sudden commencement (SC) phases, from 3 to 10 hr, with no significant main phases (MP). Others have small SC’s (20-30 min) with large MP’s. Furthermore, large storm-time (&) main phase components are accompanied by large longitude-dependent (DS) components. They conclude that these differences indicate some additional property of the simple solar plasma stream. It is difficult to avoid the conclusion of Akasofu and Chapman, and while their analysis was confined to SC-type of storms the additional property of the solar wind is also likely to affect the other type of geomagnetic storm which starts slowly, has a 27-day recurrence frequency and is ascribed to hypothetical corpuscular streams originating in solar M-regions. We will consider the possibility that these M-storms are due entirely to this additional property of the solar wind. A recent theory of recurrent geomagnetic storms t2) has also invoked a special quality in the solar wind: hydromagnetic turbulence or irregularities which are generated by the collision of a region of high wind velocity with a region of low velocity. The two theories differ in the assumed nature of the interaction between the solar wind and the magnetosphere. In the above theory the interaction is assumed to be limited to a hydromagnetic pressure on the cavity; in the present paper it is argued that recurrent storms are due mainly to a frictional interaction. However, apart from these different points of view, a possible objection to the turbulence theory is that it requires the region of high wind velocity to lie eastward of the region of low wind velocity and so to have a later central meridian passage. This does not appear consistent with the results of the analyses of Allent3) and Saemundsson(4) (who find a “cone of avoidance” above an active region), unless the wind velocity above active regions is generally lower than that above non-active regions. The principal M-regions lie eastward of the active regions and are not identified with any visible feature; there seems no reason to believe that the wind velocity should be higher above these regions than elsewhere, as required by the turbulence theory. 2. INTERACTION
BETWEEN THE SOLAR WIND GEOMAGNETIC CAVITY
AND THE
Following the early work of Chapman and Ferraro (5), it is generally accepted that the early phase of a geomagnetic storm, DJSC), is caused by a compression of the geomagnetic cavity by an impinging stream of ions. There is no such agreement about the cause of the 2
113
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J. H. PIDDINGTON
main phase of a storm. One school of thought stresses the importance of geomagnetically trapped ions, another the formation of a geomagnetic tail and of magnetospheric motions following this development. For the present purpose (a discussion of M-region storms) the important difference between these schools of thought reduces to the significance attached to frictional interaction between the solar wind and the geomagnetic cavity. In a recent discussiorP of a large amount of sea-level geomagnetic data analysed by Suguira and Chapman (‘) it was found difficult to explain much of the complex morphology of a typical storm without invoking the frictional effect and the consequent magnetospheric motions. In particular, the rapidly changing pattern of the DS-field requires a varying frictional interaction. The analysis also indicated that a ring current might exist, at least during a part of the lifetime of large and great storms and that this might explain the very different decay rates observed during successive periods of the main phase’s). However, even the formation of the ring current seemed dependent on the magnetospheric motions and hence on the frictional interaction. It was further suggested t6) that the additional property of the simple solar plasma stream, inferred by Akasofu and Chapman(l), was the strength and direction of the interplanetary magnetic field. The effect of such a field would be to reduce the frictional interaction. The presence of Kelvin-Helmholtz instability (and hence of frictional interaction) at the interface between two magnetized plasmas in relative motion has been considered@-11) for some special and rather restrictive conditions. The results, insofar as they are representative of conditions at the surface of the geomagnetic cavity, show that the external field exerts a stabilizing influence except when it is parallel (or antiparallel) with the internal field. At first sight this requirement might appear too stringent to give important effects. However, space probe measurements (12)show that the direction of the distant interplanetary field varies considerably in space and time and the field adjacent to the cavity varies even more sou3). It is likely, therefore, that at any particular moment the fields will be parallel somewhere over the sides of the cavity and that friction will occur. Once this happens the field lines in the surface layers of the geomagnetic field are rearranged and the instability may spread. The stability condition determined by FejeP has been appliedc2) for some situations observed during the tights of Explorer 10 and Explorer 12 with the conclusion that stability was likely in the region towards the Sun and possible in the region away from the Sun. However, these conclusions do not appear relevant to the question of whether instability is responsible for the main storm features. According to the theory@), friction leads almost immediately to polar DS disturbances which are, therefore, a measure of the amount of friction at a given time. Even during large storms these individual polar sub-storms are intermittent and of short duration (4-2 hr)(14)so that the frictional force is also presumably intermittent. The enduring features of a storm, notably the main phase decrease, may be due to a tail or a ring current which, once established decays only slowly. Hence the only valid test of the existence of friction is a test made over the sides of the geomagnetic cavity during a polar sub-storm. We retain the earlier conclusion that the interaction between the solar wind and the geomagnetic cavity includes a frictional term which is important in contributing to both Dst and DS. Fejer’s(rl) formula for the onset of instability shows that it tends to be inhibited by an external field, except when the two fields are nearly parallel. Hence, the notable variations in the morphologies of individual storms (l) is attributed to the variation
RECURRENT
GEOMAGNETIC
STORMS
115
in the strength and direction of the external field. When the latter is weak or parallel the main phase is large; when the field is strong and transverse the SC phase may be large with little or no main phase. 3. CONDITIONS IMMEDIATELY
OUTSIDE THE CAVITY
The solar wind is now known (15)to be supersonic (400-700 km set-l) compared with the hydromagnetic wave velocity within the wind (~100 km set-I). For this reason and by analogy with the corresponding hydrodynamic case it has been assumed that a shock front must form outside the geomagnetic cavity (lG1*). There appears, however, to be an alternative possibility (le) which might better be described as a “strong” hydromagnetic wave. According to the non-shock model, magnetic field lines are draped over the solar side of the cavity so as to increase the strength of the field outside the cavity. If the strength can be increased sufficiently to raise the local value of the Alfven velocity to equal the velocity required for streamline flow, then the need for a shock is removed. As a field line approaches the cavity its speed is reduced according to the equation of continuity of flow and because the field and plasma are compressed. The velocity is also changed because the field lines are deflected away from the cavity so as to sweep past the side of the cavity. Thus the magnetic stresses built up ahead of the cavity are sufficient to guide the plasma around the obstacle without the necessity of randomizing the ion velocities. In support of this hypothesis is the fact that coulomb collisions between ions cannot be invoked to account for the randomizing of velocities if a shock were formed. Hence their velocities must be controlled in any case by the electromagnetic fieldr20) and it is at least possible that this field will develop in such a way as to reduce or remove the shock effect. Suppose the solar wind has ion mass, density and velocity M, N and u and magnetic field B. Near the subsolar point the field is compressed so that, neglecting the gas pressure, the strength becomes B, N 8rrNMv2.
(1)
The justification for neglecting the gas pressure is that either the gas is not heated appreciably or, if it is heated it escapes along the field lines. If the gas is not heated appreciably but is compressed together with the field, then BN, = B&
(2)
V,a = &v,
(3)
from which where V and V, are the appropriate AlfvCn velocities. If V = 0*3u, then V, = 0*65u, which should be sufficiently large to avert a shock”?l). The magnetometer measurementso3) made in Explorer 10 appear consistent with a draped tail rather than a shock region. Likewise the absence of protons with random velocities of about 300 km set-l (thermal energy ~300 eV) immediately outside the cavity t2~)indicates that the ion velocities have not been randomized in a shock. The development of a magnetic sheath outside the geomagnetic cavity, as outlined above, would be likely to almost entirely inhibit friction when the external field lines lay parallel to geomagnetic equatorial plane. Their effect would be much less, however, when perpendicular to this plane(ll). It may be of interest to note that a similar draped magnetic tail should develop around the Moon and the other planets.
116
J. H. PIDDINGTON 4. RECURRENT
GEOMAGNETIC
STORMS
Recurrent geomagnetic storms have been investigated by many workers with some divergence of opinion. For the present purposes the detailed statistical analyses of Alien(3) and Saemundsson(4) will be referred to. The former made use of the geomagnetic data associated with the passage across the face of the Sun of large spot groups; the latter used much more numerous calcium flocculi data for a 35year period. The conclusions were in good agreement. Saemundsson used the superposed epoch method and reached the following conclusions : (a) The particles which are responsible for the recurrent storms travel from Sun to Earth in an average time of three days. (b) The most notable feature of the superposed epoch curves is a minimum incidence of storms three days after the central meridian passage (CMP) of active areas. (c) A maximum of incidence occurs from about four to eight days after CMP of an active area and a smaller maximum at the time of CMP of an active area. A model of the interplanetary medium has been built up from numerous observational data and from theoretical considerations. It is now known that a stream of solar ions moves past the Earth at all or most times 05), having a velocity within the range -300-700 km se&. This stream carries a magnetic field which apparently tends to lie in the ecliptic plane(12) but shows frequent large deviations in direction so that at times it must lie parallel (or antiparallel) with the Earth’s dipole axis (12*23).Above an “active” region on the surface of the Sun(24), magnetic field lines are often drawn out more or less along the streamlines of the wind*. The wind strength and magnetic field strength within this region are generally greater than above quiet regions. The region of strong wind and field is shown by the dashed lines of Fig. 1 which are drawn to correspond roughly with the average time delay of both recurrent and non-recurrent starms of three days t4). The main M-region lies about 40” in heliographic longitude away from the active region as shown. A weaker M-region lies on the other side of the active region and neither of these is identified with any visible feature on the solar disk. Another possible feature of the magnetized solar stream above an active region is that the field may be twisted into the form of a helix, as shown in Fig. 1. There is some theoretical justification for this feature in the flare model of Gold and Hoyle(25). According to their model the original fields are in two tightly twisted bundles so that the interplanetary field should comprise several separate helices; we have only sketched one of these. The present suggestion for the origin of M-storms is that the frictional interaction between the solar wind and geomagnetic cavity is different in the three regions A, B and C of Fig. 1 because of the differences in strength and directions of the magnetic fields in these regions. One might speculate on other possible effects of the interplanetary magnetic field in its control of geomagnetic activity. First we might consider the relative degree of friction between the solar wind and the geomagnetic field when the two magnetic fields are parallel and antiparallel. In the latter case the fields will tend to destroy one another (Sweet’s mechanism) and this might be expected to increase friction. Again, the relative directions of the two fields might be expected to change systematically with the season and with the sunspot cycle. These changes might introduce corresponding variations in storminess. * The strongly “wound-up” interplanetary field model proposed earlier(‘*) must be modified in view of the established outflow of plasma with velocity ~400 km see-l near the Earth. The strong winding up may occur far beyond the Earth’s orbit.
RECURRENT
GEOMAGNETIC
STORMS
117
Finally the interplanetary field direction may show some systematic change with heliographic latitude and longitude, thus accounting for the curious asymmetric effects found statistically by Bell(26)and by Jenkins and Paghisf2’). ,,-----,c 04 // F$a?;tE;’ ..\
f&l%
f
\
‘\
i’
‘\
__--0# M.- region
/
+-r. ‘\
l
‘\
Sun Rotationi
*\ 1.
B
Earth
A\ \
d
FIG. 1. A SCHEMATIC DIAGRAM
OF THE SUNANDINTERPLANETARY MEDIUMIN THEEQUATORIALPLANE. An active region and its stream of magnetized plasma is shown with the main M-region following. 5. AN M-REGION
HYPOTHESIS
The most notable feature of most geomagnetic storms is the large decrease &(MP) Neither of these could result from an increase (or and the associated ~~-component. decrease) in pressure on the geomagnetic cavity. They could, however, result from one or more effects due to frictional interaction and this interaction is likely to be controlled very strongly by the strength and direction of the interplanetary magnetic field. When this is ~r~ndicular to the geomagnetic field in the surface layer, friction is i~ibit~; when the fields are parallel (or the interplanetary field is very small) friction is not inhibited. It is suggested, therefore, that recurrent or M-region storms tend to occur when the interplanetary field is weak or else is nearly perpendicular to the geomagnetic equatorial plane. In Fig. 1 the field and plasma above an active region sweep around like a swinging garden hose. It would tend to sweep away any irregular fields ahead of it and so leave the region behind it free of magnetic fields. Such an effect would explain the major Mregion shown. Alternatively M-storms may be due not so much to a change in strength of the interplanetary field but to a change in direction-generally away from the ecliptic. A consistent change would occur if the field possessed the helical form shown. Furthe~ore the change in direction would tend to be greater on the following side of the stream (concave) than on the leading side (convex), explaining the presence of the major effect on the following side. It may be signi~cant that in the limited data available from Mariner 2(12) the main storminess occurred during the two days (5 and 6 September) during which field direction fluctuated most from the ecliptic plane. FtEFJBENCES 1. S. I. AKASO~Jand S. CHAPMAN, J.Geophys. Res. 68,125 (1963). 2. A.J.DBsLER~~~ J. A. FEIER,Z%W~.SpuceSci.11,505 (1963). 3. C. W. ALLEN,Mon. Not. Roy. Astr. Soi X04, 13 (1944). 4. TH. SAB~+KJND~~ON. Mon. Not. Rov. Astr. Sot. 123,299 (1962). 5. S. CHAPMANand $. C. A. FE&O, Terr. Mirg. ktmos.‘Elec~ 36, 171 (1931). 6. J.H.PIDDINGToN,P~~~~.Space Sci. 11, 1277 (1963).
7. M. SUGUIRAand S. CHAPMAN, AbhandL Akad. Wiss GiSttingen, Math Phys. Kl, (1960).
118
10. 11. 12. 13. 14. 15. 16. 17. ::* 20: 21. 22. 23. f:: 26. 27.
J. H. PIDDINGTON S. I.AKASOFU,S.CHAPMAN and D. VENKATESAN,J. Geophys. Res. 68,3345 (1963). S. I. SYROVATSKII,Electrodynamics of Continuous Media (Eds. L. D. Landan and E. M. Liishitz). Pergamon Press, Oxford (1960). W. I. Axroan, Canad. J. Phvs. 40, 654 (1962). J. A. FEJER, Phys. Fluids 6,508 (1563). . P. J. COLEMAN.L. DAVIS.E. J. Sm and C. P. So~err. Science 138.1099 (1962). J. P. HBPIWER:N. F. NE&X, C. S. SCEARCEand T. L. S&LMAN, J. Geophvs.‘Res. 68, 1 (1963). T. NAGATA a& N. F~KUS&MA, Rep. Zonosph. Res. Japan 6, 85.(1952): I M. NEUGEBAU~R and C. W. SUYDER.Science 138.1095 (1962). V. N. ZHIGULEV and E. A. ROMISHE&~~~, Sov. Fik Doki. 4, i59 (1960). W. I. Axroan, J. Geophys. Res. 67, 3791 (1962). P. J. KBLWG, J. Geophys. Z&s. 67, 3805 (1962). J. H. PIDDINGT~N, Planet Space Sci. 9, 305 (1962). F. J. FIWMAN, A. R. IbNTRowrrZ and H. E. PiWiWHBK,Rev. Mod. Phys. 32,959 (1960). C. S. MORA~ETZ, Phys. Fluids 4,998 (1961). H. S. BRIDGE, C. DILWORTH, A.J.LAzARus,E.L.LYoN, B.Ross~andF. SCHEBB,J.Phys. Soc.Japan 17, Suppl. A-II, 553 (1962). L. J. CAHILL,Space Research ZZZ(Ed. W. Priester) p. 324. North Holland, Amsterdam (1963). J. H. PIDDINGTON, Radio Astronomy. Hutchinson, London; Harper, New York (1961). T. GOLD and F. HOYLE,Mon. Not. Roy. Astr. Sot. 120,89 (1960). B. BELL,Smithson. Contr. Astrophys. 5, 69 (1961). R. W. JENKINSand I. PAGHI~,Canad.J. Phys.41,1056(1963). Pesionre-PaccMaTpKBaIoTCR AaHHhIe,yKasnBa~~ae Ha ~0, YT~ rvrannble xapanTepHCTllKMMarAHTHhIX6ypbo6oeroTH~a~pO~CXO~HTOT(PpMK~EIOHHOrO BaaUMO~etCTBHR C COJIHe=iHblMBeTpOM,a He JIHUIb OT AaBJIeHkiH Ha reOMarHHTHyl0IIOJIOCTL. Ecnu ~TOT~K,TO~~I~~E~HO~IIOBTO~~H)~IIXCFI~~~~B~~~~~T~:M~BJ~R~TCKCBO~CTBO(~CTO~~HBoro) conHeqHor0 BeTpa, peryjrnpyIouer0(PpkiKqHoHKoeBsanMo~eIkcTBKe.3TO CBOtiCTBO-COCTaBHOti WeMeAT Me?KllJlaHeTHOrO MarHIITHOrOIIOJIHK reOMarHElTHOtt aKBaTOpHaJlbHOii IIJIOCKOCTEI. TaKHM 06paaoM, o6nacTK M-BTO 06Jrac'rH, IlOBepX KOTOpbIX (IIpOeKTKpOBaHHOeBAOJIb JIHHHB BOaAyIIIHOrO IIOTOKI)MarHKTHOe IIOJIe HeOCjsatiHO cna6oe,Unume HaXOHHTCR IIOn-12 JTJlOM K 3TOtiIIJIOCKOCTH.
RECURRENT
GEOMAGNETIC STORMS, SOLAR M-REGIONS AND THE SOLAR WIND J. H. PIDDINGTON Radiophysics Laboratory, CSIRO, Sydney (Received 18 October 1963)
Abstract-Evidence is reviewed indicating that the main features of geomagnetic storms of both types are caused by frictional interaction with the solar wind rather than by mere pressure on the geomagnetic cavity. If this is so then recurrent or M-region storms are caused by a property of the (steady) solar wind which controls the frictional interaction. This property is the component of the interplanetary magnetic field in the geomagnetic equatorial plane. Hence M-regions are regions above which (projected along the streamlines) the magnetic field is unusually weak or lies at an angle MT/~ to this plane.
1.INTRODUCTION
In a recent examination of geomagnetic storms Akasofu and Chapman(l) found that some have large and prolonged first or sudden commencement (SC) phases, from 3 to 10 hr, with no significant main phases (MP). Others have small SC’s (20-30 min) with large MP’s. Furthermore, large storm-time (&) main phase components are accompanied by large longitude-dependent (DS) components. They conclude that these differences indicate some additional property of the simple solar plasma stream. It is difficult to avoid the conclusion of Akasofu and Chapman, and while their analysis was confined to SC-type of storms the additional property of the solar wind is also likely to affect the other type of geomagnetic storm which starts slowly, has a 27-day recurrence frequency and is ascribed to hypothetical corpuscular streams originating in solar M-regions. We will consider the possibility that these M-storms are due entirely to this additional property of the solar wind. A recent theory of recurrent geomagnetic storms t2) has also invoked a special quality in the solar wind: hydromagnetic turbulence or irregularities which are generated by the collision of a region of high wind velocity with a region of low velocity. The two theories differ in the assumed nature of the interaction between the solar wind and the magnetosphere. In the above theory the interaction is assumed to be limited to a hydromagnetic pressure on the cavity; in the present paper it is argued that recurrent storms are due mainly to a frictional interaction. However, apart from these different points of view, a possible objection to the turbulence theory is that it requires the region of high wind velocity to lie eastward of the region of low wind velocity and so to have a later central meridian passage. This does not appear consistent with the results of the analyses of Allent3) and Saemundsson(4) (who find a “cone of avoidance” above an active region), unless the wind velocity above active regions is generally lower than that above non-active regions. The principal M-regions lie eastward of the active regions and are not identified with any visible feature; there seems no reason to believe that the wind velocity should be higher above these regions than elsewhere, as required by the turbulence theory. 2. INTERACTION
BETWEEN THE SOLAR WIND GEOMAGNETIC CAVITY
AND THE
Following the early work of Chapman and Ferraro (5), it is generally accepted that the early phase of a geomagnetic storm, DJSC), is caused by a compression of the geomagnetic cavity by an impinging stream of ions. There is no such agreement about the cause of the 2
113
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J. H. PIDDINGTON
main phase of a storm. One school of thought stresses the importance of geomagnetically trapped ions, another the formation of a geomagnetic tail and of magnetospheric motions following this development. For the present purpose (a discussion of M-region storms) the important difference between these schools of thought reduces to the significance attached to frictional interaction between the solar wind and the geomagnetic cavity. In a recent discussiorP of a large amount of sea-level geomagnetic data analysed by Suguira and Chapman (‘) it was found difficult to explain much of the complex morphology of a typical storm without invoking the frictional effect and the consequent magnetospheric motions. In particular, the rapidly changing pattern of the DS-field requires a varying frictional interaction. The analysis also indicated that a ring current might exist, at least during a part of the lifetime of large and great storms and that this might explain the very different decay rates observed during successive periods of the main phase’s). However, even the formation of the ring current seemed dependent on the magnetospheric motions and hence on the frictional interaction. It was further suggested t6) that the additional property of the simple solar plasma stream, inferred by Akasofu and Chapman(l), was the strength and direction of the interplanetary magnetic field. The effect of such a field would be to reduce the frictional interaction. The presence of Kelvin-Helmholtz instability (and hence of frictional interaction) at the interface between two magnetized plasmas in relative motion has been considered@-11) for some special and rather restrictive conditions. The results, insofar as they are representative of conditions at the surface of the geomagnetic cavity, show that the external field exerts a stabilizing influence except when it is parallel (or antiparallel) with the internal field. At first sight this requirement might appear too stringent to give important effects. However, space probe measurements (12)show that the direction of the distant interplanetary field varies considerably in space and time and the field adjacent to the cavity varies even more sou3). It is likely, therefore, that at any particular moment the fields will be parallel somewhere over the sides of the cavity and that friction will occur. Once this happens the field lines in the surface layers of the geomagnetic field are rearranged and the instability may spread. The stability condition determined by FejeP has been appliedc2) for some situations observed during the tights of Explorer 10 and Explorer 12 with the conclusion that stability was likely in the region towards the Sun and possible in the region away from the Sun. However, these conclusions do not appear relevant to the question of whether instability is responsible for the main storm features. According to the theory@), friction leads almost immediately to polar DS disturbances which are, therefore, a measure of the amount of friction at a given time. Even during large storms these individual polar sub-storms are intermittent and of short duration (4-2 hr)(14)so that the frictional force is also presumably intermittent. The enduring features of a storm, notably the main phase decrease, may be due to a tail or a ring current which, once established decays only slowly. Hence the only valid test of the existence of friction is a test made over the sides of the geomagnetic cavity during a polar sub-storm. We retain the earlier conclusion that the interaction between the solar wind and the geomagnetic cavity includes a frictional term which is important in contributing to both Dst and DS. Fejer’s(rl) formula for the onset of instability shows that it tends to be inhibited by an external field, except when the two fields are nearly parallel. Hence, the notable variations in the morphologies of individual storms (l) is attributed to the variation
RECURRENT
GEOMAGNETIC
STORMS
115
in the strength and direction of the external field. When the latter is weak or parallel the main phase is large; when the field is strong and transverse the SC phase may be large with little or no main phase. 3. CONDITIONS IMMEDIATELY
OUTSIDE THE CAVITY
The solar wind is now known (15)to be supersonic (400-700 km set-l) compared with the hydromagnetic wave velocity within the wind (~100 km set-I). For this reason and by analogy with the corresponding hydrodynamic case it has been assumed that a shock front must form outside the geomagnetic cavity (lG1*). There appears, however, to be an alternative possibility (le) which might better be described as a “strong” hydromagnetic wave. According to the non-shock model, magnetic field lines are draped over the solar side of the cavity so as to increase the strength of the field outside the cavity. If the strength can be increased sufficiently to raise the local value of the Alfven velocity to equal the velocity required for streamline flow, then the need for a shock is removed. As a field line approaches the cavity its speed is reduced according to the equation of continuity of flow and because the field and plasma are compressed. The velocity is also changed because the field lines are deflected away from the cavity so as to sweep past the side of the cavity. Thus the magnetic stresses built up ahead of the cavity are sufficient to guide the plasma around the obstacle without the necessity of randomizing the ion velocities. In support of this hypothesis is the fact that coulomb collisions between ions cannot be invoked to account for the randomizing of velocities if a shock were formed. Hence their velocities must be controlled in any case by the electromagnetic fieldr20) and it is at least possible that this field will develop in such a way as to reduce or remove the shock effect. Suppose the solar wind has ion mass, density and velocity M, N and u and magnetic field B. Near the subsolar point the field is compressed so that, neglecting the gas pressure, the strength becomes B, N 8rrNMv2.
(1)
The justification for neglecting the gas pressure is that either the gas is not heated appreciably or, if it is heated it escapes along the field lines. If the gas is not heated appreciably but is compressed together with the field, then BN, = B&
(2)
V,a = &v,
(3)
from which where V and V, are the appropriate AlfvCn velocities. If V = 0*3u, then V, = 0*65u, which should be sufficiently large to avert a shock”?l). The magnetometer measurementso3) made in Explorer 10 appear consistent with a draped tail rather than a shock region. Likewise the absence of protons with random velocities of about 300 km set-l (thermal energy ~300 eV) immediately outside the cavity t2~)indicates that the ion velocities have not been randomized in a shock. The development of a magnetic sheath outside the geomagnetic cavity, as outlined above, would be likely to almost entirely inhibit friction when the external field lines lay parallel to geomagnetic equatorial plane. Their effect would be much less, however, when perpendicular to this plane(ll). It may be of interest to note that a similar draped magnetic tail should develop around the Moon and the other planets.
116
J. H. PIDDINGTON 4. RECURRENT
GEOMAGNETIC
STORMS
Recurrent geomagnetic storms have been investigated by many workers with some divergence of opinion. For the present purposes the detailed statistical analyses of Alien(3) and Saemundsson(4) will be referred to. The former made use of the geomagnetic data associated with the passage across the face of the Sun of large spot groups; the latter used much more numerous calcium flocculi data for a 35year period. The conclusions were in good agreement. Saemundsson used the superposed epoch method and reached the following conclusions : (a) The particles which are responsible for the recurrent storms travel from Sun to Earth in an average time of three days. (b) The most notable feature of the superposed epoch curves is a minimum incidence of storms three days after the central meridian passage (CMP) of active areas. (c) A maximum of incidence occurs from about four to eight days after CMP of an active area and a smaller maximum at the time of CMP of an active area. A model of the interplanetary medium has been built up from numerous observational data and from theoretical considerations. It is now known that a stream of solar ions moves past the Earth at all or most times 05), having a velocity within the range -300-700 km se&. This stream carries a magnetic field which apparently tends to lie in the ecliptic plane(12) but shows frequent large deviations in direction so that at times it must lie parallel (or antiparallel) with the Earth’s dipole axis (12*23).Above an “active” region on the surface of the Sun(24), magnetic field lines are often drawn out more or less along the streamlines of the wind*. The wind strength and magnetic field strength within this region are generally greater than above quiet regions. The region of strong wind and field is shown by the dashed lines of Fig. 1 which are drawn to correspond roughly with the average time delay of both recurrent and non-recurrent starms of three days t4). The main M-region lies about 40” in heliographic longitude away from the active region as shown. A weaker M-region lies on the other side of the active region and neither of these is identified with any visible feature on the solar disk. Another possible feature of the magnetized solar stream above an active region is that the field may be twisted into the form of a helix, as shown in Fig. 1. There is some theoretical justification for this feature in the flare model of Gold and Hoyle(25). According to their model the original fields are in two tightly twisted bundles so that the interplanetary field should comprise several separate helices; we have only sketched one of these. The present suggestion for the origin of M-storms is that the frictional interaction between the solar wind and geomagnetic cavity is different in the three regions A, B and C of Fig. 1 because of the differences in strength and directions of the magnetic fields in these regions. One might speculate on other possible effects of the interplanetary magnetic field in its control of geomagnetic activity. First we might consider the relative degree of friction between the solar wind and the geomagnetic field when the two magnetic fields are parallel and antiparallel. In the latter case the fields will tend to destroy one another (Sweet’s mechanism) and this might be expected to increase friction. Again, the relative directions of the two fields might be expected to change systematically with the season and with the sunspot cycle. These changes might introduce corresponding variations in storminess. * The strongly “wound-up” interplanetary field model proposed earlier(‘*) must be modified in view of the established outflow of plasma with velocity ~400 km see-l near the Earth. The strong winding up may occur far beyond the Earth’s orbit.
RECURRENT
GEOMAGNETIC
STORMS
117
Finally the interplanetary field direction may show some systematic change with heliographic latitude and longitude, thus accounting for the curious asymmetric effects found statistically by Bell(26)and by Jenkins and Paghisf2’). ,,-----,c 04 // F$a?;tE;’ ..\
f&l%
f
\
‘\
i’
‘\
__--0# M.- region
/
+-r. ‘\
l
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Sun Rotationi
*\ 1.
B
Earth
A\ \
d
FIG. 1. A SCHEMATIC DIAGRAM
OF THE SUNANDINTERPLANETARY MEDIUMIN THEEQUATORIALPLANE. An active region and its stream of magnetized plasma is shown with the main M-region following. 5. AN M-REGION
HYPOTHESIS
The most notable feature of most geomagnetic storms is the large decrease &(MP) Neither of these could result from an increase (or and the associated ~~-component. decrease) in pressure on the geomagnetic cavity. They could, however, result from one or more effects due to frictional interaction and this interaction is likely to be controlled very strongly by the strength and direction of the interplanetary magnetic field. When this is ~r~ndicular to the geomagnetic field in the surface layer, friction is i~ibit~; when the fields are parallel (or the interplanetary field is very small) friction is not inhibited. It is suggested, therefore, that recurrent or M-region storms tend to occur when the interplanetary field is weak or else is nearly perpendicular to the geomagnetic equatorial plane. In Fig. 1 the field and plasma above an active region sweep around like a swinging garden hose. It would tend to sweep away any irregular fields ahead of it and so leave the region behind it free of magnetic fields. Such an effect would explain the major Mregion shown. Alternatively M-storms may be due not so much to a change in strength of the interplanetary field but to a change in direction-generally away from the ecliptic. A consistent change would occur if the field possessed the helical form shown. Furthe~ore the change in direction would tend to be greater on the following side of the stream (concave) than on the leading side (convex), explaining the presence of the major effect on the following side. It may be signi~cant that in the limited data available from Mariner 2(12) the main storminess occurred during the two days (5 and 6 September) during which field direction fluctuated most from the ecliptic plane. FtEFJBENCES 1. S. I. AKASO~Jand S. CHAPMAN, J.Geophys. Res. 68,125 (1963). 2. A.J.DBsLER~~~ J. A. FEIER,Z%W~.SpuceSci.11,505 (1963). 3. C. W. ALLEN,Mon. Not. Roy. Astr. Soi X04, 13 (1944). 4. TH. SAB~+KJND~~ON. Mon. Not. Rov. Astr. Sot. 123,299 (1962). 5. S. CHAPMANand $. C. A. FE&O, Terr. Mirg. ktmos.‘Elec~ 36, 171 (1931). 6. J.H.PIDDINGToN,P~~~~.Space Sci. 11, 1277 (1963).
7. M. SUGUIRAand S. CHAPMAN, AbhandL Akad. Wiss GiSttingen, Math Phys. Kl, (1960).
118
10. 11. 12. 13. 14. 15. 16. 17. ::* 20: 21. 22. 23. f:: 26. 27.
J. H. PIDDINGTON S. I.AKASOFU,S.CHAPMAN and D. VENKATESAN,J. Geophys. Res. 68,3345 (1963). S. I. SYROVATSKII,Electrodynamics of Continuous Media (Eds. L. D. Landan and E. M. Liishitz). Pergamon Press, Oxford (1960). W. I. Axroan, Canad. J. Phvs. 40, 654 (1962). J. A. FEJER, Phys. Fluids 6,508 (1563). . P. J. COLEMAN.L. DAVIS.E. J. Sm and C. P. So~err. Science 138.1099 (1962). J. P. HBPIWER:N. F. NE&X, C. S. SCEARCEand T. L. S&LMAN, J. Geophvs.‘Res. 68, 1 (1963). T. NAGATA a& N. F~KUS&MA, Rep. Zonosph. Res. Japan 6, 85.(1952): I M. NEUGEBAU~R and C. W. SUYDER.Science 138.1095 (1962). V. N. ZHIGULEV and E. A. ROMISHE&~~~, Sov. Fik Doki. 4, i59 (1960). W. I. Axroan, J. Geophys. Res. 67, 3791 (1962). P. J. KBLWG, J. Geophys. Z&s. 67, 3805 (1962). J. H. PIDDINGT~N, Planet Space Sci. 9, 305 (1962). F. J. FIWMAN, A. R. IbNTRowrrZ and H. E. PiWiWHBK,Rev. Mod. Phys. 32,959 (1960). C. S. MORA~ETZ, Phys. Fluids 4,998 (1961). H. S. BRIDGE, C. DILWORTH, A.J.LAzARus,E.L.LYoN, B.Ross~andF. SCHEBB,J.Phys. Soc.Japan 17, Suppl. A-II, 553 (1962). L. J. CAHILL,Space Research ZZZ(Ed. W. Priester) p. 324. North Holland, Amsterdam (1963). J. H. PIDDINGTON, Radio Astronomy. Hutchinson, London; Harper, New York (1961). T. GOLD and F. HOYLE,Mon. Not. Roy. Astr. Sot. 120,89 (1960). B. BELL,Smithson. Contr. Astrophys. 5, 69 (1961). R. W. JENKINSand I. PAGHI~,Canad.J. Phys.41,1056(1963). Pesionre-PaccMaTpKBaIoTCR AaHHhIe,yKasnBa~~ae Ha ~0, YT~ rvrannble xapanTepHCTllKMMarAHTHhIX6ypbo6oeroTH~a~pO~CXO~HTOT(PpMK~EIOHHOrO BaaUMO~etCTBHR C COJIHe=iHblMBeTpOM,a He JIHUIb OT AaBJIeHkiH Ha reOMarHHTHyl0IIOJIOCTL. Ecnu ~TOT~K,TO~~I~~E~HO~IIOBTO~~H)~IIXCFI~~~~B~~~~~T~:M~BJ~R~TCKCBO~CTBO(~CTO~~HBoro) conHeqHor0 BeTpa, peryjrnpyIouer0(PpkiKqHoHKoeBsanMo~eIkcTBKe.3TO CBOtiCTBO-COCTaBHOti WeMeAT Me?KllJlaHeTHOrO MarHIITHOrOIIOJIHK reOMarHElTHOtt aKBaTOpHaJlbHOii IIJIOCKOCTEI. TaKHM 06paaoM, o6nacTK M-BTO 06Jrac'rH, IlOBepX KOTOpbIX (IIpOeKTKpOBaHHOeBAOJIb JIHHHB BOaAyIIIHOrO IIOTOKI)MarHKTHOe IIOJIe HeOCjsatiHO cna6oe,Unume HaXOHHTCR IIOn-12 JTJlOM K 3TOtiIIJIOCKOCTH.