IONIZATION ABOVE THE F2-PEAK, AS AFFECTED BY THE I N T E R P L A N E T A R Y GAS (*)

by SYDNEY CHAPMAN (**)

S u mma ry - - The a tmospheres of the ear th , the sun and other bodies are sur rounded by gas t h a t is near ly un i fo rm in n u m b e r densi ty u and kinet ic t empe ra tu r e T, over spaces much greater t h a n those oeeupied by the a tmospheres . This gas may be called the ambium of the a tmosphere . In to ta l i t is much more massive t h a n tile a tmosphere i t encloses. The condit ions in the a m b i u m mus t powerfully affect the s ta te of the outer a tmosphere . In par t icular , there mus t be a cont inuous t r ans i t ion of the values of rt and T be tween the a m b i u m and the a tmosphere . In the ease of the ear th , the sun and other ho t stars, b o t h the a m b i u m and the ou te rmos t pa r t of the a tmosphere will consist of a tomic hydrogen. The t em pe r a t u r e of the a m b i u m will de termine whe ther this hydrogen and t h a t of the ou te rmos t a tmosphere are mainly ionized or neutra l .

The na tu re of the terres t r ia l a m b i u m depends on the extension of the sun 's a tmo- sphere. This a tmosphere is ho t and highly ionized i'n i ts inner par ts . At some radius not yet known, the solar a tmospher ic gas mus t become cool and neutra l . The s ta te of the ea r th ' s ou te rmos t a tmosphere depends great ly on whe the r the ea r th lies in the ioni- zed or the neu t ra l pa r t of the sun 's a tmosphere , or in the solar ambium.

Evidenee will be presented favor ing the view t h a t the ea r th ' s a m b i u m consists of ionized solar a tmospher ic hydrogen. I f this be so, the ou te rmos t pa r t of the ea r th ' s a tmosphere is likewise hot and ionized. I t mus t enelose an extensive layer of main ly neu t r a l a tomic hydrogen.

i . In troduct ion.

T h e s t a t e of t h e a i r in t h e t r o p o s p h e r e is d e t e r m i n e d m a i n l y b y t h e a m o u n t of so la r r a d i a t i o n fa l l ing on t h e subsu r face , a n d b y t h e ref lec t ing , s c a t t e r i n g , con- d u c t i n g a n d r a d i a t i v e p r o p e r t i e s of t h e l a n d a n d sea. T h e p r o p e r t i e s of t h e lower a n d m i d d l e i o n o s p h e r e , a r o u n d a n d be low t h e F 2 p e a k , are l a rge ly d e t e r m i n e d b y t h e s u n ' s u l t r a v i o l e t ( a n d X ) r a d i a t i o n a b s o r b e d the re . T h e s t a t e of t h e o u t e r m o s t a t m o s p h e r e , h o w e v e r , m u s t d e p e n d c o n s i d e r a b l y on t h e s t a t e of t h e m e d i u m b y w h i c h i t is s u r r o u n d e d .

Th i s is a gene ra l p r o p e r t y of a t m o s p h e r e s - - n o t o n l y of t h e e a r t h , b u t also of t h e o t h e r p l a n e t s , t h e s u n a n d t h e s tars . E a c h such b o d y is e n v e l o p e d b y a

(*) The Par t of this work done at High Al t i tude Observa tory was suppor ted by the Na t iona l Bureau of S t anda rds and the Air Force Cambridge I /eseareh Center.

(**) Geophysical Ins t i tu te , Univers i ty of Alaska & High Al t i tude Observa tory , Boulder, Colorado, USA.

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medium tha t in the aggregate is more massive than the outer atmospheres them- selves. I t is convenient to have a general name for this ambien t medium. The word ambittm is proposed and here used.

For many stars the ambium is interstel lar gas consisting main ly of atomic hydrogen, cool and almost ent irely neutral . The (kinetic) t empera tu re T is of order i00oK. The gas contains a small propor t ion of other elements, of which only the metal atoms, forming a fraction about i0 -~ of the whole, are pa r t l y ionized. The value of the number densi ty n suggested b y COWLING (1) is i00/cc; according to A L L ~ (2) (pp. 227, 225) i t is of order 1 except in inters te l lar clouds, where i t m a y be of order 10.

The influence of super-hot stars extends to great distances. Their intense u l t rav io le t rad ia t ion ionizes the atomic hydrogen over a large surrounding volume and mainta ins there a t empera ture of order i04K. Beyond a certain distance the t empera tu re and degree of ionizat ion drop ra ther sharply, to those character is t ic of normal inters te l lar gas. This distance m a y be regarded as mar ldng the boun- da ry between the atmosphere of such a s tar and its ambium. Some stars m a y lie within the extensive atmosphere of a superhot star. For them the ambium may be a region of ionized atomic hydrogen at about 104K.

The atmosphere may be expected to be the more extensive, the hot te r i ts outer layers. This cannot necessarily be judged from its color temperature , as is shown by the example of the sun. The outer pa r t of the sun's a tmosphere depends grea t ly on the propert ies of the corona, whose tempera ture - - of order one or two million degrees - - vas t ly exceeds t ha t of the photosphere. For this reason i t is difficult to judge the extent of the atmospheres of stars whose coronas (if any) we cannot observe.

The ex ten t of a p lane ta ry or stellar a tmosphere m a y be defined in more than one way. To me i t seems na tu ra l to place the bounda ry a t the l imi t of the influence of the pa ren t body. This is the basis of the suggestion above, t ha t the inner bomx- da ry of tke ambium of a super-hot s tar is where the tempera ture and ionizat ion begin to rise toward the star, above the values character is t ic of normal inters te l lar gas. I f the pressure varies l i t t le across the boundary , the number dens i ty mus t there begin to decline inward. Stars and planets exer t other influences than b y radia t ion, on the surrounding mat te r , namely b y gravi ta t ion, conduction, ro ta t - ion, t rans la t ional motion, and viscosity. The atmospheric bounda ry m a y be placed at the l imit of the most far-reaching influence.

2. The transition from atmosphere to ambium.

The m a i n propert ies of an a tmosphere are:

a) composi t ion; b) t empera tu re T(r), at distance r from the center of the pa ren t body; c) pressure p(r ) ; d) number densi ty n(r); e) veloci ty v(r), for example relat ive to the pa ren t body ; f ) s ta te of dissociation and ionization. These, of course, are not all inde-

pendent ; for example, p ~ knT.

There will be a continuous t rans i t ion from the propert ies of the a tmosphere to those of the ambium. As the t ransi t ional densities will be low, and the mean

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free paths long, the merging of the atmosphere into the ambium must be gradual, and extend over a great distance. I t seems unlikely tha t any collective property of the gas (e.g., n, p, T) will in general change by more than a small fraction in a distance of a mean free path. Violent actions like those exerted on the earth by streams or clouds of fast-moving solar gas may at times modify- this conclusion.

I n considering the processes tha t go on across the boundary, the ordinary formulae of gas theory may not be adequate, because of the low density of the gas. Higher terms (~) in the expressions for heat transfer and the pressure tensor may become important . I have recently considered (4) the influence of such terms in the calculation of the heat flux in the solar and terrestrial atmospheres.

At their lower levels, atmospheres share the rotat ion of their parent body. Wi th increasing distance r there must be a transi t ion of the angular velocity to tha t of the ambium. I f the parent body has a t ranslat ional motion relative to the ambium, this also will affect the t ransi t ional region. The solar motion through the galaxy, and the earth 's orbital motion, are exampIes. There will be a trm~sfer of momen tum through the transi t ional region.

There may also be a transfer of heat energy from the atmosphere to the am- bium or vice versa. Likewise there may be a transfer of matter , as, for example, i~_ the case of the earth's atmosphere. This appears to suffer a continual slow loss of helium to the ambium. There may be a two way flow of helium atoms between the t~,o regions, the outward flow being slightly greater than the inward flow.

3. The interplanetary gas: temperature and density.

At present there are differences of opinion as to the in terplanetary gas, and the outer extension of the sun's atmosphere. 0PIK (~) has given an interest ing discussion of the subject, based par t ly on information drawn from data for the zodiacal light (8). For the distance r from the sun at 1 a. u. (astronomical un i t : 1.5 X 1013 cm) BEnR & SIEDE~TOrr gave n = 600 for the electron density, and smaller values at slightly greater distances. From these values of n 0PIx inferred tha t T(r) at 3[ a .u . is 1060~ decreasing to 410~ at 1.115 a .u . He remarked tha t the in terpre ta t ion of the zodiacal light data is uncertain, and that the scat- tering and polarization of the light a t t r ibuted to electrons may be partly, perhaps even mainly, due to the dust tha t is known be present. He remarked also (p. 48): (( However, the enormous var iat ion of the temperature over a moderate range of heliocentric distance is itself a problem .. . . There remains to be explained why the temperatures are so low. Cooling through contact with dust is out of the quest- ion, at least at r ~ 1 a. u . . . . . The riddle of the enormous var iat ion of temperat- ures remains ~. The inferred temperatures would of course lose their meaning if n, for the electrons as inferred from the zodiacal light is quite wrong. BLACK- WELL (7) has suggested tha t n is far less than 600.

The solar corona is another source of information about the in terplanetary gas. BLACKWELL (S) has given estimates of the electron density n up to 20 R. where R denotes the sun's radius. They were inferred from his measurements of the in tens i ty and polarization of the coronal light. However, these also are un- certain, because of the difficulty of separating scattering effects of electrons and dust. His measurements of the coronal light were made from an aircraft at 30,CG0 ft., during the eclipse of 2954 June 30, which occurred in a period of extreme abs- ence of solar disturbance.

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I have at tempted to infer (9) the temperature distr ibution T(r) from BLACK- ~WELL's values of n, which, however uncertain, seem to be the best tha t are at present available. The inference as to T(r) was made on the (also uncertain) as- sumpt ion that the atmosphere is in static equilibrium up to 20 R. The results indicated that the maximum temperature T o of the corona is approximately 2 X i06 - - in fair agreement with many other estimates (though not with all) by methods and from data tha t may be more reliable. From T 0 the temperature decreased outwards at a rate between the conductive and adiabatic gradients. Previously (10) I had shown that conductive equilibrium in a static atmosphere of ionized hydrogen implied that T varies as r -2/7. This would indicate a very slow fall of T from the sun to the earth. The adiabatic gradient of T would be much steeper at near distances, though beyond 45 R along the polar axis of the sun the adiabatic gradient would become the smaller of the two. In the sun's equatorial plane, or the ecliptic, the Uncertainty as to the rotat ion of the gas makes it not possible, at present, to calculate the adiabatic gradient - - or the densi ty gradient beyond about 20 R.

The value of T inferred for 20 R is 280,000K. This is materially less than the value, 430,000% calculated on the basis of thermal conductive static equilibrium and T O = 106. I t is, however, a very high temperature, and if the curve of T(r) up to 20 R is extrapolated to i a. u. (2i5 R) - - a doubtful procedure - - the value suggested there is of order 100,000% If the value of n given by BL_~CKWELL for 20 R, namely 2.6 X 103, is excessive, then T will decrease more rapidly.

I t may in this connection be noted that values of n of similar magnitude are suggested by quite independent evidence, namely by the scattering of radio waves by the solar corona. HEWISH (11) is one of those who have observed such scatter- ing when the Crab nebula is ~( seen ~) from the earth by radio waves that pass through the solar corona. He concludes tha t the scattering, which he has observed up to 20 R, is due to irregularities of electron density in tl~e corona. Supposing them to be aligned along the lines of magnetic force, he finds evidence (1. c., pp . 543, 545) t ha t these lines, at distances of 15 or 20 R, are nearly radial, and do not show the curvature corresponding to a dipole field. He considers tha t at 15 R the size of the irregularities may reasonably be estimated to be 165 kin. In an earlier paper (12), whose results are confirmed by his further observations (11), he inferred tha t the deviations of electron density in irregularities of this size at 15 R must be about 1.8 • 103/cc. This, he writes (p. 251), (( may represent an appreciable fraction of the total electron density at tha t distance )). BLACKWELL'S value of n at i5 R is 4 . i X 103, which seems fairly accordant with HEwIs~'s estimate of the deviations

Thus, though it may prove that BLACKWELL'S values of n are somewhat too large, their excess may n o t be great. Smaller values of n would lead to lower values of T(r) - - on the assumption of static equilibrium - - and indicate tha t at i a. u. T is less than 100,000o; bu t values such as 50,000 ~ 30,000 ~ or 20,000 ~ seem possible. Any of these would ensure tha t the atomic hydrogen, however low its density, would be largely ionized.

Despite the scantiness of the data and the uncer ta in ty in the interpretat ion, the possibility seems at least worthy of consideration that the ambium of the earth is very hot ionized atomic hydrogen, perhaps of density decidedly less than 600/cc. This possibility would carry the implication that in the earth's outermost atmos- phere the temperature T tends to the high value met in the ambium: and tha t the

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.outermost layer of our a tmosphere is mainly ionized atomic hydrogen. This would i m p l y a very great extension of our atmosphere, as the scale height of the layer, of mainly neutra l hydrogen, below the outer layer t ha t is ionized, would be in- creasing upward, because of the decreasing grav i ty and the increasing tempera ture . As an example of the ve ry low rate of decline of number densi ty of a tomic hydro- gen, a pure ly hypothe t ica l case m a y be ci ted; i f a t i0 ea r th radi i T is 5000oK, the decimal scale height of neut ra l hydrogen is about 150 ear th radii , if one supposes tha t a t t ha t distance the gas no longer ro ta tes wi th the earth. For n to decline by one or two powers of 10 to a t t a in the densi ty of the ambium, the a tmosphere might well have to extend beyond the moon.

4. Other conceptions of the state of interplanetary space.

Observat ions of the sun at eclipse show tha t the corona changes considerably from one eclipse to another , and coronagraph observat ions confirm this from day to day for the inner corona. Thus a s ta t ic model is a t bes t only a first approx imat - ion to reali ty. According to BIERMArr (13) i t is far from the t ru th . He conceives in t e rp lane ta ry space as the scene of a s trong solar wind of gas proceeding continu- ously from all over the sun, wi th a speed of 500 to 2000 km/sec, and a normal dens i ty of order 100/cc, sometimes rising to greater values. PAI~KEI~ (14) has pro- posed a mechanism for the product ion of such a wind; he places i ts origin in the outer corona. I f there be such a continual wind, and i f i t were steady, a quasi- :steady d is t r ibut ion of the outflowing gas would form the ambium of the earth. Actual ly , ins tead or in addi t ion, the sun sends out l imited s treams and clouds of gas from t ime to t ime; thei r upward progress from levels below the coronal max- imum tempera ture level can be followed by solar radio observations. Thus the condit ions in the ear th ' s ambium are not steady.

0r IK, on the basis of the electron dens i ty and t empera tu re inferred from the zodiacal l ight observat ions by BEnR & SIEDENTOPF, considered the effect of the solar mot ion on the in te rp lane ta ry gas. He concluded t ha t a t 1 a. u. the gas pres- sure would exceed the momen tum pressure of the inters te l lar gas ( taken to have number densi ty - - of neutra l a tomic hydrogen - - about 3/cc), bu t t ha t not far outside the ear th ' s orbit the in te rp lane ta ry gas would be blown away. Compared with the speed of the solar wind and solar streams, t ha t of the solar system ( taken to be 20 km/sec) and of the ear th ' s orbi ta l mot ion is small. The solar gas, during periods of impac t of s treams or clouds on the earth, seems l ikely to dominate condi- t ions in the e a r t h ' s ambimn. But its influence may be ra ther short- l ived, and i t may react ra ther wi th the ear th 's magnet ic field than pr imar i ly wi th the outer a tmospher ic gas. I f there is an average residual in te rp lane ta ry s ta t ic gas, i ts long-continued influence on the outermost a tmosphere m a y be the chief deter- minan t of the condit ions there, a t least during periods of more than usual ly pro- longed solar calm. An example of such an intel~cal is the spring and summer of i954, when between Apri l 17 and August 22 the three-hour geomagnetic Kp index never once rose to 50. The solar corona as observed during t ha t interval , a t the eclipse of i954 June 30, was except ional ly regular. A similar bu t shorter in terva l occurred in 1945, during the previous solar cycle, between Apri l 14 and Ju ly 1. At such t imes the in te rp lane ta ry gas may remain sufficiently l i t t le d is turbed to app roach a s ta t ic condition.

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5. The Lyman alpha radiation.

Rocket observations of Lyman alpha l ight a t n ight have been repor ted b y KUPPERIAN, BYRAM, CtIUBB • FRIEDMAN (15), and discussed b y several writers (is, 17, IS). The details will not be given here, bu t a divergence of view m a y be noted, as to the locat ion of the hydrogen t ha t main ly resonant ly scatters solar L y m a n alpha photons on to t h e night side of the earth. According to the views of FRIED- MAN (16) and SHI(LOVSKY (17) the locat ion is in terp lanetary . Some of the l ight there scat tered (beyond the earth) penet ra tes a t night to a re la t ively dense l aye r of a tomic hydrogen down to about 80 k m height in the atmosphere. There some of i t is scat tered upward again. Thus the rocket sensors were able to record the l ight from below as well as from above. SHKLOVSKY's view is tha t , though t h e in te rp lane ta ry hydrogen is main ly in the ionized state , there is about 1 to 3 per - cent of neut ra l hydrogen.

JOHNSON (lS), on the contrary , infers from line width and line shape considerat- ions t ha t the solar photons t ha t fall on the night side of the ear th are scat tered by cool hydrogen gas moving with the earth, and forming par t of the ear th ' s atmos- phere. The scat ter ing hydrogen gas cannot have high optical density. He estim- ates the number of n ight- t ime p r imary scat ter ing hydrogen atoms lying above the E layer to be about 7 X 1012/cm2: and tha t most of the rescat ter ing occurs below ]20 km. He infers a d is t r ibut ion of neutra l atomic hydrogen in the uppe r atmosphere, tak ing the number densi ty to be 4 • 104/cc at the base of the iso- thermal exosphere, which in his view forms the outermost pa r t of our a tmosphere. He places the base of the exosphere at 550 km, and takes its t empera tu re to be 1250oK. I t m a y be added t h a t SRKLOYSKY does no t exclude the possibi l i ty t ha t pa r t of the n ight - t ime solar photons may be scat tered in the ear th ' s outermost atmosphere.

I have favored the same view, t ha t the night- t ime scat tered L y m a n a lpha l ight reaching the dark hemisphere of the ear th is scat tered in the ear th ' s atmos- phere and not in in te rp lane ta ry space. This was because i t seemed to me tha t t h e in te rp lane ta ry gas was p robab ly too hot tO provide the necessary number of hy- drogen atoms in the space beyond the earth. This conclusion was and is, however, only a ten ta t ive one, pa r t ly based on idealized assumptions t ha t are cer ta inly n o t completely fulfilled, and m a y at t imes be far from true. While still th inking i t l ikely t ha t the gas in in te rp lane ta ry space is ve ry hot, I now consider tha t m y es t imate of i ts value at i a. u. was too big, judging from BLACKWELL's values o f the coronal electron densi ty up to 20 R. A reduct ion from m y original es t imate of 200,000 ~ to as low as 30,000 ~ would still give too few hydrogen neut ra l a toms to give the observed night - t ime L y m a n alpha light. However, in papers read at the American Physical Society meet ing at New York in J a n u a r y 1959, and at a conference on The Realit ies of Space Research at the California Ins t i tu te o f Technology in March, I have pointed out the impor tance of direct observat ion of L y m a n alpha l ight from directions oblique to tha t to the sun, from rockets r ight outside the atmosphere. This seems to me to be a crucial observat ion, t ha t should settle the differences of view as to where the main scat ter ing takes place, in or outside our atmosphere. I t m a y not b y i tself suffice, however, to de termine the tempera ture of the in te rp lane ta ry gas. I f the scat ter ing of the solar L y m a n alpha photons in in te rp lane ta ry space is small, i t might be ei ther because the gas is too cool or of too low densi ty, or a combinat ion of bo th these causes.

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6. The hydrogen distribution in our atmosphere.

In June 1956, a t the conference on Chemical Aeronomy at Cambridge (19), I pointed out the predominance of a tomic hydrogen in the composit ion of the outermost a tmosphere, and made some ve ry ten ta t ive calculations i l lus t ra t ive of possible densities. The calculations gave the value 3 X 1013/cm 2 for the n u m b e r of neutra l hydrogen atoms above 520 km; this es t imate was based on an assumed number densi ty (*) 1.6 X 105/cc at 520 km, a t empera tu re of 2000~ and a scale height of 1850 km. The value 3 X i013 now seems too large b y a factor of about 4, according to JOHNSOn'S inferences as to the L y m a n alpha n ight - t ime light. This order-of-magnitude agreement wi th his value is main ly coincidental, because in i.956 there was no clear evidence of a direct k ind as to the amount of hydrogen at any level. Indeed such evidence is still lacking. Fur the r s tudy of rocket da t a for the L y m a n alpha l ight and other hydrogen lines seems l ikely to be our best method of gaining cer ta in ty as to the d is t r ibut ion of the hydrogen.

7. Escape of gases from the atmosphere, and the isothermal exosphere.

At present there is a divergence of opinion as to the tempera ture distributiorL in the outermost a tmosphere, and as to the escape of gases from it. One view (is. 2~) takes the outermost a tmosphere to consist of an isothermal layer, called the exos- phere, in which the part icles t ha t are moving upward suffer no collisions, and ei ther fall back along a free orbi t or escape from the ear th ' s gravi ta t ional influence. 1: have urged a different view, t ha t lays stress on the ambium as the de t e rminan t of the outermost condit ions of our atmosphere. I f the ambium is ve ry hot, as seems to me possible though not certain, then there is no isothermal exosphere; the t empera tu re mus t rise t i l l i t a t ta ins the value in the ambium. Collisions in the outermost layer will be rare bu t not negligible, especially because the atmos- phere mus t be ext remely extensive i f the ambium is ve ry hot. There will be par t i - cles escaping from our a tmosphere , bu t the traffic will not be one-way, because most of the escaping part icles are still held by the sun's gravi ty. They or o the r in te rp lane ta ry part icles will from t ime to t ime enter our atmosphere, and be ab- sorbed and become members of it.

I f the ambium is hot enough to be largely ionized, the outermost layer of the hydrogen in our a tmosphere will also be hot and largely ionized. In this case there is a considerable difference between the two views of the o u t e r a tmosphere.

However, there is also great unce r t a in ty as to the dis t r ibut ion of the neutra l hydrogen, and considerable difference of opinion about i t even between those who share the belief in the isothermal exosphere. Thus Jo~Nso~ (is) and SINGER (21) bo th adopt the exospherie view, wi th not ve ry different tempera tures and base levels (1250 ~ and 1500~ 550 and 530 k m respectively). They also agree fair ly closely as to the level a t which neut ra l hydrogen begins to be the most a b u n d a n t k ind of part icle , namely about 1200 k m and 1000 km. But a t 1000 km JoH~sor~'s es t imate of the number densi ty of a tomic hydrogen is 2.4 • 104/cc, whereas SIn- GEe gives i ts valu~ as 3 X 106/cc - - differing b y a factor of i25, comparable wi th the apparen t factor of unce r t a in ty as to n in the in te rp lane ta ry gas at 1 a . u . The

(*) This was a far extrapolation from values given by BATES &: iNIEOLET (20) for the layer 64 to 96 km.

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l a t t e r va lue would be m u c h too g rea t~ to- f i t t he L y m a n a lpha da ta according to t h e a rgumen t s g iven by J o H n s o n .

I t is na tu r a l t h a t at the p resen t t i m e the re should be differences of opinion on the stil l l i t t l e -explored regions of the ou te r a tmosphe re and i n t e r p l a n e t a r y space. The discussions t h a t t h e y give rise to will ce r ta in ly aid grea t ly , a long wi th f a r t he r observa t ions (par t ly suggested b y such discussions), in a t t a in ing rel iable knowledge on m a n y poin ts now at issue.

R E F E R E N C E S

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(Received 18 December 1961)