Polar ionospheric disturbances and solar corpuscular emissions

Planet. Space Sci.

Pergamon Press, 1961. Vol. 5, pp. 59-69.

Printed in Great Britain.

POLAR IONOSPHERIC DISTURBANCES AND SOLAR CORPUSCULAR EMISSIONS T. OBAYASHI and Y. HAKURA Radio Research Laboratories, Kokubunji, Tokyo (Received

3 November

1959)

Abstract-It has been found that the study of polar radio blackouts due to abnormal ionization in the lower ionosphere yields considerable evidence indicating the existence of energetic solar particles associated with solar flares. Polar radio blackouts are classified into two characteristic types, one is the polar cap blackout and the other is the aurora1 zone blackout. It is shown that the polar cap blackout appears with some hours delay after a major solar radio outburst of type IV, and the blackout is confined within the geomagnetic latitude of 60”65’. The estimated energies of particles causing this are of about l&100 MeV. The aurora1 zone blackout then follows, being accompanied with geomagnetic storms and aurorae, and it may be caused by the so-called aurora1 particles of 1 MeV or less. The energy spectrum of solar particles associated with solar flares is revealed from the present result together with all information from various observations related to solar and terrestrial disturbances. It is concluded that solar particles have a conspicuous suprathermal non-Maxwellian tail extending from a few keV up to relativistic energy range, though the bulk of corpuscular clouds consists of rather low energy particles and hence likely to be in the

Maxwellian distribution. Some discussions on the nature of solar corpuscular clouds and their effect upon the terrestrial ionosphere are also given. 1. INTRODUCTION

which would produce geomagnetic and ionospheric storms. At the time of a great solar flare, the sun also ejects corpuscles of relativistic energies, and occasionally they can be observed as an abrupt increase of the cosmic ray intensity, though only a few cases are known until at present. Recently, Bailey”) suggested the existence of new solar particles in the sub-cosmic ray energy range from his study of the polar ionospheric absorptions during the great solar event of 23 February 1956. From their analysis of the IGY data, the present authors@, 31have also found considerable evidence indicating that energetic charged particles of l&100 MeV reach the earth with some delay from the solar flare, yet well before the onset of a geomagnetic storm. They impinge on the lower ionosphere at high latitudes and cause the polar cap blackouts. During geomagnetic storms, however, the particles precipitate along the auroral zone producing aurorae and also the aurora1 zone blackouts. Energy of these so-called aurora1

One of the important projects aimed at in the Third International Geophysical Year 19571958 was to establish a better understanding of the physical aspects of disturbances in the Since the greatest earth’s upper atmosphere. solar activity was attained during this period, considerable interest has been stimulated on the research concerning solar disturbances and their effects upon the terrestrial ionosphere. Some new results are forthcoming, and comprehensive studies are now being concentrated to solve this attractive problem. For the study of “earth storms”, the transient radiative and corpuscular emissions associated with solar flares are the most important and prominent subjects. Solar flares produce the excess of ultraviolet radiation which extends into the X-ray range, causing the effect so-called sudden ionospheric disturbances. It is generally believed that the corpuscular emissions are followed by the ionized jet plasma travelling with velocities of the order of 1000 km/set, 59

T. OBAYASHI

60

particles is presumably lower, being of about 1 MeV or less. Independent confirmations of the existence of these new particles have been given by the discovery of abnormal absorptions of cosmic radio noise observed in the polar cap regions,@’ 5, also by floating balloon observations of cosmic rays at high latitudes@ and by satellite, Pioneer IV.(‘) Since these evidences may provide a new way to investigate the nature of solar corpuscular emissions and the mechanism of geomagnetic storms, further studies are very promising. In the present paper, the characteristics of enhanced ionizations in the polar ionosphere associated with solar flares are briefly reviewed. The statistical relationship between solar and terrestrial disturbances is re-examined, and it is confirmed that major radio outbursts of type IV are closely related to the polar cap blackouts preceding geomagnetic storms. From these results together with all other information from various observations related to solar and terrestrial disturbances, a tentative model of the energy spectrum of solar particles is proposed. It is shown that the ejected particles associated with solar flares have a considerably wide energy spectrum extending up to relativistic range, and that these particles may mostly be generated in the solar corona during flares by

and Y. HAKURA

the Fermi acceleration mechanism. The effect of invading solar particles upon the upper atmosphere and also the mechanism of earth storms are discussed briefly. 2. TERRZSTRIAL DISTURBANCES ASSOCIATED WITH SOLAR FLARE23

It is well known that, at the solar flare, an outstanding radio emission from the solar corona, which is called the solar radio outburst, An investigation of dynamic is observed. spectra of radio outbursts reveals some important nature of the disturbance in the solar atmosphere. As has been pointed out by one of the authorsC8), intense solar radio outbursts, whose maximum power spectrum lies in the SHF band, cause sudden ionospheric disturbances. They are explained as the effect of the enhanced ionization in the ionosphere due to On the excess of the soft X-ray radiation. other hand, major radio outbursts predominant in UHF and VHF bands, especially those of continuum radiation named type IV,@’ are closely correlated with severe geomagnetic storms. It has been revealed, recently, that an enhanced ionization in the ionosphere, which is indicated by the increased fmin or the radio blackout of echoes in the ionograms, occurs in

Polar bk~4~~~ts associated with Geonqnatic

12th

13th

Storm

Sept. 1957

Latitude effect of polar blackouts associated with a geomagnetic storm Fig. I. of 13. Sept. 1957 (hatched region shows abnormal increase of fmin).

U.T.

POLAR

Fig. 2.

IONOSPHERIC

DISTURBANCES

AND

SOLAR

EMISSIONS

f-plots of ionospheric vertical soundings on Sept. I t-14, 1957, at Thule (polar cap), Fort Belvolr (sub-aurora1 zone) and Grand Bahma (middle latitude),

61

T. OBAYASHI

62

the polar region several hours after a major radio outburst of type IV and, yet, well before the onset of a geomagnetic storm.(2) Since it is thought that polar radio blackouts are caused by the abnormal ionizations due to bombardment of energetic particles, this fact would suggest the invasion of solar particles presumably of higher energies than those producing geomagnetic storms. In order to show the general aspect of this phenomenon, a typical one observed on 12-13 September 1957 is described. A major type-IV solar radio outburst was observed at 2 hr 45 min U.T. on 11 September, associated with an intense solar flare. The variation of fmin vs. geomagnetic latitude following this solar event is illustrated in Fig. 1, using data of about twenty ionospheric stations in the North America Zone and with the geomagnetic record at Kakioka, Japan. The hatched regions indicate increased fminor complete blackouts (dense hatching). The regions enclosed by dotted lines are those of increased fmincaused by SD’s and they are confined in the middle and low latitudes of the sunlit hemisphere. An outstanding increase of fmin appeared at high latitudes more than 10 hr before the onset of geomagnetic storm. Intense polar blackouts

and Y. HAKURA were then observed along the aurora1 zones in the earlier stages of the storm, while in the last phase any systematic wide-spread blackout was not observed. The original f-plots obtained at Thule (geomagnetic latitude 87*0”), Fort Belvoir (50.1”) and Grand Bahma (37-O”) are also reproduced in Fig. 2. It is noted that an abrupt increase of fminwas noticed at 9 hr U.T. on 12 September at Thule (polar cap station), while no appreciable change appeared at Fort Belvoir (sub-auroral zone station) until at the main phase of the geomagnetic storm. The world-wide patterns of the abnormal ionization for respective stages of geomagnetic storm are characteristic. As shown in Fig. 3, the one which appears before the onset of geomagnetic storms is that the enhanced ionization (radio blackouts) is confined within the polar cap, while during geomagnetic storms it spreads along the aurora1 zone forming a spiral shaped ionization pattern. The former is termed the polar cap blackout and apparently no marked magnetic activity is associated, while the latter is the aurord zone blackout which is for the most part a night-time effect and its occurrence is correlated with that of visible aurorae and local geomagnetic disturbances. It has been shown elsewhereC3) that the polar cap blackout

0

270

180 Fig. 3. Typical patterns of the polar cap blackout and the aurora1 zone blackout (viewed from above the geomagnetic north pole) at the stage before the onset of the geogmagnetic storm (pre-s.c.), 00 hr U.T., and the main phase, IO hr U.T.. on 13 Sept., 1957.

POLAR

IONOSPHERIC

DISTURBANCES

is caused by the invasion of charged particles of about 10-100 MeV and those particles would impinge on the polar ionosphere confined within the geomagnetic latitude of 60”-65”, providing that they follow the Stormerian orbits. The aurora1 zone blackout is related to aurora1 displays and it is presumably due to aurora1 particles of 1 MeV or less. It is noted that the precipitation of these aurora1 particles on the earth is strongly affected by the effect of distortion of the outer geomagnetic field during geomagnetic storms. As has already been emphasized, it is very important that major solar radio outbursts are closely related to those disturbances which indicate the invasion of energetic particles into the earth. In order to confirm this, a further statistical analysis is made using the IGY data of solar radiometry, polar blackouts and geomagnetism. During the period July 1957 to December 1958, there were fifty-nine S.S.C. magnetic storms, among them twenty were followed by major radio outbursts with type IV. It has been found that prolonged severe polar cup b~u~k~t~ were always associated with those of the type-IV outbursts except three rather doubtful cases. The occurrence probability of the polar cap blackouts following type IV outbursts is about 75 per cent, while without type IV it is only 8 per cent. The result is summarized in Table 1, and more details of this analysis is given elsewhere.(s*lO) Table 1.

Occurrence of Magnetic Storms and Polar Cap Blackouts

Sun Major outburst with type IV Major outburst without type IV

Magnetic storms

Polar cap blackouts

20

15

39

3

Since a type-IV outburst is caused by synchrotron radiation due to relativistic electrons spiralling in magnetic fields, the generation of high-energy particles in the solar corona

AND

SOLAR

EMISSIONS

63

would likely be expected. A dynamical process creating “suprathermal particles” associated with solar flares has been suggested by Hayakawa and Kitao(‘l) and by Parker.@‘) They showed that an agitated solar plasma bearing magnetic fields may be capable of accelerating protons from thermal IO relativistic Therefore energies by Fermi’s mechanism. corpuscular clouds ejected from the sun may contain a considerable amount of energetic protons, and they will arrive at the earth with some delay after the type-IV outburst. It is likely that ejected clouds would trap the magnetic fields of sunspots or the corona and travel outward through the solar atmosphere into interplanetary space. High-energy particles may be partly trapped in such magnetic clouds during the earlier stage of their passage through ~terplaneta~ space. However, as the cloud undergoes an expansion and accordingly magnetic fields of the cloud dilute, H2 - n4j3, the high-energy particles may gradually escape from the cloud and reach the earth causing p&r cap blackouts before the time of arrival of the cloud, which is indicated by the onset of a magnetic storm. This implies that high-energy particles may not come directly straight from the sun to the earth, and the observed delay of the polar cap blackouts after solar flares at least of the order of a few hours is explainable on this basis. Although it has been a puzzling problem that the incoming speed of aurora1 particles estimated from the observations is several times the speed of corpuscular, clouds inferred from solar terrestrial relationships, it can now be understood that a considerable amount of high-energy particles exists in the corpuscular cloud, which could provide the energy of aurora1 displays or polar blackouts, even though the speed of the cloud itself is only of the order of 1000 km/set. Summarizing the present results together with other knowledge presently at hand, a general scheme of solar and terrestrial distances is depicted in Table 2. High-energy protons of the order of 10100 MeV reach the earth several hours after a solar flare accompanied with major radio out-

T. OBAYASHI

64

Table 2.

Solar Flares and Disturbances

e.m. Radiations

Relativistic electrons Relativistic protons High energy particles

. .. ...

Low energy particles

,

and Y. HAKURA in the Earth’s Upper Atmosphere

Soft X-rays HQ Enhancement and Major Radio Outbursts

SID and Crochet

Synchrotron radiation .. . .. . Solar cosmic ray . . . ... ... Solar cosmic ray . . . . .. ... Auroral particles . . . . .. .. . (trapped in the cloud) .. . . .. Magnetic cloud . . .. ... Corpuscular cavity formation . . . Magnetic barrier for cosmic rays

Type IV outbursts Unusual increase of cosmic ray Polar cap blackouts Am-oral zone blackouts Aurorae, polar magnetic storms

,

bursts of type IV, while occasionally solar cosmic ray particles well above relativistic energies are observed associated with particular great solar flares. (~1 Those high-energy particles invade into the polar regions following nearly Stijrmer orbits and cause prolonged polar cap blackouts or abnormal VHF absorption at high latitudes, so-called type III absorption.‘4* 51 On the other hand, the bulk of a corpuscular cloud consisting mainly of low-energy particles arrives after a day or so, corresponding to a speed of the order of 1000 km/set. The particle density of the cloud is considerably high and hence the cloud behaves like a conducting fluid. The interaction of such a conducting cloud with the geomagnetic field is exactly the same as reported by Chapman and Ferraro.(l”) As has been pointed out by one of the authors,(15) the geomagnetic field is confined within a cavity surrounding the earth, and the outside of this is exposed to violently agitated solar plasma. The size of the cavity is variable according to the kinetic energy density of the incoming corpuscular cloud, and the storm-time variation Dst of geomagnetic storms may in some way be related to this cavity formation. It is noted that aurora1 particles, trapped by magnetic fields (order of 10-3G) of the corpuscular cloud, reach the earth after the arrival of the cloud. They impinge on the polar ionosphere through the distorted geomagnetic field and cause auraral zone blackouts, aurorae and polar magnetic

Dst of magnetic storms Forbush-type. decrease

I storms. An equatorward shift of the aurora1 zone during the main phase of magnetic storms is explained by the distortion of the geomagnetic field due to the change of the geomagnetic cavity.@’ The corpuscular clouds advance further into outer space until to a distance of a few astronomical units, where they form magnetic barriers for cosmic ray particles coming from the interstellar space.(13,15) 3. THE ENERGY SPECTRUM OF SOLAR PARTICLES

Although it has been shown in the preceding section that the sun radiates an enormous amount of corpuscles-ranging from thermal to relativistic energies-in order to have further insight into the problem, it is of special importance to know the energy spectrum of particles ejected from the sun. Together with the present result, some other information is now available, and it is possible to delineate the energy spectrum of particles associated with solar flares at least consistent with some principal facts of terrestrial disturbances. To unify the data of various observations into the same scale, every information is converted in terms of the integral energy spectrum, i.e. the particle flux J cm-‘set-’ having energies in excess of the specified value E is defined as J(>E)=

rj(E’)dE &

(1)

POLAR IONOSPHERIC

DISTURBANCE

where j is the differential energy spectrum. Results obtained by various inves~gators are summarized in Table 3, in which some data are given in terms of magnetic rigidity instead of energy. Magnetic rigidity P is given as P=+-$

'Ea+2m,c?E Iii

(3

where ze is the charge (e.s.u.) and m,c2 is a rest energy. Table 3. Energy range 2-15BV 0.5 BV@5BV 100-4OOMeV 304OOMeV 1OMeV 10MeV lO-1OOKeV 30-5OOKeV lo-1OOKeV fMeV 2OOOkm/sec 5~15~km/s~ lOeV-1 KeV 2-ISBV 30eV

AND SOLAR

particles. The abnormal ionization in the polar ionosphere began gradually and reached a maximum a few hours after the flare. Baiiey’” showed that the energetic protons of lO-1000 MeV are responsible for this. His estimation (2) is based mainly on the results of balloon observations by Van Allen and Winckler, which show the rigidity spectrum varying with P-5 at a range of 1000 MeV. The spectrum in the energy range between 1 and- 100 MeV

List of Solar Particles

Integraispectrum

Investigators

I, (P/PJ7---

Meyer, Parker, Simpson Van Allen, Winckler I, (P/P,)-5 J,= l*3/cm2/sec-sterad. Pa=1.2 BV Bailey J, V/P,,-I J,=(E/E,)-* Winckler et al I@= 1*5/cm2/sec, E, = 100 MeV I*- 5~/cm2/sec Rothwetl, McIIwain l~rv10zfcm2fsec Hakura, Obayashi J, = @8 x 102/cma/sec Reid, Collins ^I J, = 107-10scmz/sec Chamberlain Meredith et al 3, ezp (--E/E,) I,==1.8x 105/cm2/sec-sterad. E,=72KeV J, - 106-l~/cmz/sec Van Allen JO- 105/cm2/sec Hakura, Obayashi J 0 =2x 1010/cm2/sec JO- 10~“-lO1z/cmz/s~ JON 103-1012/cme/sec

65

EMISSIONS

Chapman Biermann Obayashi

J, {P/P&-l’= Van Allen, Singer J,=@S/cm*/sec-sterad. P0 = 1 BV Jo=+ nvl09/cmz/sec 1Chapman

Combining all the results, the integral energy spectrum of solar particles is ihustrated in Fig. 4. The ~fo~ation of solar particles of the highest energy is given by the unusual increase of cosmic ray intensity associated with the great solar flare on 23 February 1956, which yields the spectrum of relativistic protons P-‘.i2s) This is indicated by (1) in the Figure, and for comparison the energy spectrum of normal cosmic rays is also shown. The late effect of the solar flare on 23 February 1956 gives further information about the spectrum for the non-relativistic

year

Remarks

-.._.1956 5 hr Feb. 23 1956 1957 21 hr Feb. 23 1956 (Balloon Obs.) 1959 21 hr Feb. 23 1956 1959 11 hr Aug. 23 1958 (Balloon Ohs.) 19.59 Satellites, Aug. 1958 1959 PoIar cap blackouts 1959 Type III absorption

A&oral doppler shift G& 1958 Rocket obs. (Ion counter) 1958 Ion electrons 1959 Aurora1 zone blackouts 1950 Magnetic storms 1957 Comet tail 1958 Storm pulsations 1950

Zosmic rays

1957

Extension of corona

-

can be obtained from various phenomena, such as polar cap blackouts, type III absorptions of VI-IF cosmic radio waves and high altitude observations of cosmic radiation by satellite. Curve (3) is based on results of Winckler et al *w from their balloon observations of solar cosmic rays at Churchill. Most other values mentioned are ranging between curve (2) and (3). Below 1 MeV, Chamberlain’s result (curve 4) from the observations of Doppler shift of the aurora1 spectral lines is available.(16) Direct

T. OBAYASHI

66

p-

, to’

I

IO2

I 103

I lo6

I

I

104

Id

Proton

Fig. 4.

and Y. HAKURA

kinetic

I

107

energy,

I

108

I 109

,

IO"

I

to”

eV

The integral energy spectrum of solar particles associated with intense solar flares.

rocket observations of aurora1 particles have been made;(“* l*) however, there seems to Corpuscular remain still some ambiguities. streams consist mainly of rather low energy particles and travel interplanetary space with a speed of the order of 1000 km/set. Chapman@’ has given an estimate of the density of a neutral ionized gas stream-100 protons and electrons/ cm3-sufficient to cause a moderate magnetic storm resulting in a velocity of 2000 km/set. On the other hand, BiermamP) has pointed out that the motion of comet tails indicates the existence of corpuscular clouds streaming away from the sun with velocities of 500-1500 km/set. These observations lead to a value of the particle flux near the earth of some lOlo cmW2 set-l under quiet conditions, but up to 10’” cm+ set-’ in magnetic storms. These values are in good agreement with the estimates from the theory of formation of geomagnetic cavities surrounded by corpuscles during magnetic storms.(15) The minimum flux of solar corpuscles may be indicated by the density of

the interplanetary gas. In his theory of the solar outer corona, Chapman’21’ showed that the temperature of the gas is about 2 x 105”K (30 eV) near the earth, and the density may be of the order of 600/cm3 according to the observations of zodiacal lights.(22) These values yield a flux of particles of about lo9 cnP see-I. Little is known about the spectrum of the solar corpuscles of lower energies. However, there is an evidence that they might be in a state of kinetic equilibrium. This condition can be studied by examining a characteristic collision time of particles. According to SpitzeP3’, a measure of the time required for the distribution of kinetic energies of particles to approach the Maxwellian distribution is given by 7 _ 11.42=‘2 cnlnh for proton gas, where T is in degrees K and In A= 20 at most conditions in the solar atmosphere. Inserting numerical values of

POLAR

IONOSPHERIC

DISTURBANCES

T - 106-10’ “K and n - lO’/cnY in the corona, it can be shown that the Maxwellian distribution is attained within a few hours. For electron gas this characteristic time is less than that for protons by a factor of l/43. Therefore one may conclude that solar corpuscles in the lower energy range (say less than 1 keV) are in kinetic equilibrium and hence Maxwellian distribution is attained.* This result has an interesting consequence. Since the corpuscular cloud undergoes an expansion, the particles in the cloud suffer adiabatic cooling. If the expansion is slow compared to 7,, then T a

J(>E)&‘dE E

Assuming

a Maxwellian

distribution,

2

J(=+=n,V[l-

&[exp(-x2)&+

$

0

x exp (-3)

1

.X=

where

n, is the total number

volume and v= J(>E)=

5

67

n2ia

nr-l=

* If the particle density n/cm3 is given with mean ra.ndom velocity v and the translational velocity perpendicular upon a unit fixed surface with V, then the integrated flux is

v < V.

EMISSIONS

1 keV undergo adiabatic cooling, high energy particles would not suffer this because of their high velocities and low particle density, though they might be accelerated in the corona by Fermi mechanism. Recently Parker and TidmarW have discussed the generation of suprathermal particles in a violently agitated plasma carrying magnetic fields. According to their analysis, in order that thermal particles can be raised to suprathermal ones, the characteristic time for the Fermi process, r,, must be shorter than that of the Coulomb interaction time, r,, i.e.

(4) Therefore the mean kinetic energy of particles would decrease from 1 keV to 10 eV as the particle density decrease from lO’/cm” to loA/ cm3. It is to be noted that, though the speed of the bulk of corpuscular cloud is about 1000 km/set, energies of individual particles or their thermal velocities become considerably low. Although most low-energy particles less than

when then

AND SOLAR

of particles

JC > E kT,

E* > [IT e4n~hth-J ‘Ia - 0.3 keV

(6)

Therefore most thermal particles with energy exceeding above 1 keV would undergo Fermi acceleration to form a high-velocity nonFor lower Maxwellian suprathermal tail. energy particles, however, the slowing down by Coulomb interaction is so effective that they remain in the Maxwellian distribution. The energy spectrum of such suprathermal particles can be computed theoretically assuming that particles will undergo N, collisions in average with moving magnetic inhomogeneities before being expelled from the cloud. According to ParkeruQ, if the Fermi acceleration is operative, the integral energy spectrum J (> E) near the relativistic range is given by

in a unit

. For the case when v > V, (1 +x2) exp (-x2):

where L is the mean free path for the collision of a particle within magnetic clouds and V is the velocity of hydromagnetic waves which are usually comparable to the thermal velocity. Then the threshold energy E* is given, putting L - lad km and n - 107/cm3,

1 I/El

where y = C2/(4N,V2) and m,,c2 is the rest energy of a proton. For extreme relativistic particles, J - E-+. As has been shown, the observed spectrum at the relativistic range is about E-“, yielding the value y N 2.5. In Fie. 4. the vertical A nenetration denths and L

T. OBAYASHI and Y. HAKURA

68

magnetic cut-offs are also indicated in order to have some idea of the range-vs.-energy characteristics of typical particles penetrating into the lower ionosphere. The penetration depths are due to Bailey(l) and the cut-off magnetic rigidities for the indicated geomagnetic latitudes are those for an ideal geomagnetic dipole field given by the equation P= 14.7 cos4 a., (in BV) (8) Another very interesting criterion for charged particles is the one which gives the condition of whether the particles entering the geomagnetic field behave like a single particle or a conducting plasma. This criterion has been investigated by Ferraro(25), who showed that below a certain critical density of particles the electro-magnetic interaction between particles becomes negligible and their motion will, then, be very nearly Stbrmerian. The condition can approximately be obtained by equating the shielding depth of a plasma A, and the impact parameter of a particle in the geomagnetic field (Stormer’s unit length), i.e.

‘D=

J(z)

2

,/(s)

(9)

where n is the particle density and M is the geomagnetic moment. Then, the critical flux of particles is given by J,=

1

4ncw=

&

P

(

3oo

a

>

such as cosmic rays, solar radio emissions, cosmic radio waves, aurorae, ionosphere and geomagnetism can be co-ordinated. Though the present aspects about solar corpuscular emissions and their effects in the earth’s upper atmosphere are consistent with the results obtained in various fields, the investigation is still crude and further observational and theoretical analyses are desired. In conclusion, the authors wish to express their thanks to the members of the Ionosphere Research Committee for valuable discussions. REFERENCES 1.

2. 3. 4. 5. 6.

where P is the rigidity in volts. As is indicated by a dotted line, this limit crosses over the spectrum of solar particles at a range of about 10-100 MeV. Therefore above this energy range, solar particles can be treated as independent particles and the scale of the geomagnetic cut-offs shown in the figure is applicable, while lower energy particles below this limit must be regarded as a conducting gas. 4. CONCLUSIONS Important evidence indicating the existence of energetic solar particles associated with solar flares is given. It has now become possible to investigate the whole spectrum of solar particles, and many individual observations

absorption in northern latitude and solar particle emissions. Private communication, May (1959). J. R. WINCKLER,R. ARNOLDY, R. HOFFMAN, L. PETERSONand K. A. ANDERSON, Bull. Amer.

Phys. Sot. 4, 238

7. 8.

(10)

D. K. BAIL,EY,Proc. Inst. Radio Engrs., N.Y. 47, 255 (1959). Y. HAKURA,Y. TAKENOSHITA and T. OTSUKI,Rep. Zones. Res., Japan 12,459 (1958). T. OBAYASHIand Y. HAKURA. Z. Atmos. Terr. Phys., in press; and J. Radio Res. Lab. Japan, 7, 17 (1960). G. C. REID and C. COLLINS.J. Atmos. Terr. Phys. 14, 63 (1959). H. LEINBACHand G. C. REID, VHF radio wave

9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

P.

(1959).

ROTHWELL and

C. MCJLWNN, Satellite Observation of Solar Cosmic Rays, Zowa Univ. Rep. SUI-59-12(1959). K. SINNOand Y. HAKURA, Rep. Zonos. Res.. Japan 12,285 (1958). A. Bo~sc~or and J. F. DENISSE, C.R. Acad. Sci.. Paris 245,2194 (1957). Y. HAKURA and T. GOH, J. Radio Res. Lab., Japan 6, No. 28 (1959). S. HAYAKAWAand K. KITAO, Prog. Theor. Phys., Osaka 16, 139 (1956). E. N. PARKER,Phys. Rev. 107,830 (1957). P. MEYER, E. N. PARKERand J. A. SIMPSON,Phys. Rev. 108, 1563(1957). S. CHAPMANand V. C. A. FERRARO,Terr. Magn. Atmos. Elect. 37, 147 (1932). T. OBAYASHI, Rep. Zonos. Res., Japan 12, 301 (1958). J. W. CHAMBERLAIN, Astrophys. J. 120, 360 (1954). J. A. VANALLEN, ZGY Rocket Rep. Ser. No. 1, 159 (1958). L. H. MERELXIIJ,L. R. DAVIS, J. P. HEPPNER and 0. E. BERG,ZGY Rocket Rep. Ser. No. 1, 169 (1958).

POLAR 19. 20. 21.

22.

IONOSPHERIC

DISTURBANCES

S. CHAPMAN,J. Geophys. Res. 55,361 (1950). L. B~ZMANN, Observatory 77, 109 (1957). S. CHAPMAN,Smithson: Cktr. ‘Astriphys. 2, No. 1 (1957). H. SIEIYPNTOPF, A. BEHR and H. ELSASSER, Nature, Land. 171, 1066 (1953).

AND

SOLAR

69

EMISSIONS

23.

L. SPY, Physics of Fully Ionized Interscience Publishers, New York (1956).

24.

E. N. PARKERand D. A. T~DMAN,Phys. Rev. 1026 (1958).

25.

V. C. A. FERRARO,J. Geophys.

Res.

Gases.

111,

57,15 (1952).