Chapter 4. The Ionosphere

Transcription

1 Chaptr 4 Th Ioosphr W ca dfi th ioosphr as th hight rgio of th arth s atmosphr whr th coctratio of fr lctros is so larg that it affcts radio wavs. Th ioosphr was discovrd wh it was obsrvd that radio wavs ca propagat ovr larg distacs, ad o thrfor had to assum th xistc of a lctrical coductiv layr i th uppr atmosphr which could rflct th wavs. Th lctrically coductiv rgio strtchs from about 5km to 5km abov th groud (s figur 4.3), ad th coctratio of lctros varis from 7 particls pr m 3 at 5km to a maximum of particls pr m 3 at 5-3km. Th ioosphr is formd wh rgtic lctromagtic-ad particl radiatio from th su ad spac ioiz air molculs, cratig plasma i th uppr atmosphr. This plasma is wakly ioizd; th ratio btw lctro dsity ad dsity of utral air vr bcoms largr that 7, v at th altitud wh rachs its maximum. Th rgular ioosphric layrs w will dscrib i this chaptr ar formd by xtrm ultraviolt (EUV) ad X-ray radiatio from th su, ad hav a charactristic variatio with th tim of day ad latitud. I polar rgios, i.., orth of 65, rgtic lctros ad protos prcipitat alog th magtic fild lis ad giv ris to particl impact ioizatio. Irrgular ioosphric layrs ar formd, which ar associatd with th orthr light phoma. Ths layrs ca caus strog prturbatios i radio-wav propagatio ad caus problms for commuicatio ad avigatio systms. 4. Itroductio Th Scottish physicist, Balfour Stwart, udrstood as arly as 88 that thr had to b a ioizd rgio i th atmosphr. Compass masurmts of th arth s magtic fild showd variatios, which Stward thought could oly b du to lctric currts i th uppr atmosphr. H cocludd that th uppr atmosphr was mor ioizd i th daytim tha at ight, ad mor i summrtim tha i witrtim, ad mor at suspot maxima tha miima (tim of day, saso, solar cycl dpdc). I Dcmbr 9, th Italia Marcoi st radio wavs from Corwall, Eglad to wfoudlad, Caada. British scitists Havisid ad Klly cocludd that th wavs had to follow th curvatur of th arth alog lctrically coductiv layrs i th uppr atmosphr. Thr had to b a ioosphr that actd lik a mirror for radio wavs with wav lgth λ > m. Togthr with othr scitists thy dcidd to masur th lctric proprtis of th uppr atmosphr. Th Brito Applto was amog th first who studid rflctios from th uppr atmosphr by mas of itrfrc. H usd cotiuous radio wavs ad dtctd displacmts by th Dopplr pricipl. Soo, svral ioizd layrs whr discovrd, ad Applto suggstd a subdivisio ordrd alphabtically startig with th E-layr (Havisid ad Klly) at th bottom, ad with a F-layr abov it. Masurmts showd that th F-layr was dividd i two parts, ach havig its pak. Th layrs wr amd F ad F, rspctivly. Latr, a D-layr blow th E-layr was discovrd (s figur 4.3 pag 6). Th atmosphr abov th F-layrs (>5km) is calld th magtosphr, sic th magtic fild has a domiat impact o th movmt of th lctrically chargd particls i this rgio.

2 Figur 4.: Marcoi s radio sdig from Eglad to th USA o Dcmbr th, 9 stablishd that thr had to xist a lctric coductiv layr i th atmosphr. Radio wavs hav vr sic b th mai tool i th xploratio of th ioosphr. 4. Formatio of th layrs of th ioosphr I this sctio w ar goig to dscrib brifly how th ioosphr is formd. Th grad of ioizatio dpds o th itsity ad th wavlgth of th icomig radiatio, as wll as th compositio of th atmosphr. Rfr also to th illustratio of th compositio of th atmosphr i figur 3. pag Photo ioizatio Th ioosphric layrs ar formd by photo ioizatio of atoms (X) ad molculs (XY). Ioizatio is maily owd to EUV-radiatio from th su. hν X hν XY X XY (4.) Ios ar lost by rcombiatio through svral possibl procsss: X X hν (Slow) (4.) XY X Y (Fast) (4.3) Th lattr is calld dissociativ rcombiatio, ad lads to th splittig of a molcul ito two atoms i a xcitd stat. Th procss is ffctiv bcaus impuls ad rgy ca asily b distributd amog X * ad Y *. I additio, fr lctros ca form gativ ios by attachmt: XY XY Ad th gativ ios ca b lost by photo dtachmt: XY hν XY (4.4)

3 W gt a cotiuity quatio of th form d q productio α [ XY ] loss With th otatio [XY ] w ma th coctratio of th molcul XY. Elctric utrality rquirs that [ XY ] [ ] XY With th xcptio of th lowst part of th ioosphr, whr [XY - ] ad [XY ]. W gt d q α (4.5) Th tim drivat d r u t i th mor gral cas whr u r is th vlocity of th air through th volum lmt w look at. Th simplst modls assum u r so that th quatio bcoms d q α (4.6) Th rcombiatio cofficit α dpds of what kid of io spcis ar prst. I th quatio, α may b rplacd by a ffctiv rcombiatio cofficit α ff ( αi [ XY ] i ) i (4.7) Whr α i rfrs to a crtai io typ. Typical ios aro, O, O i th E- ad F-layr, ad composit ios of th typ O (H O) i th lowr ioosphr. For O ado, α 5-7 cm 3 /s. Wh gativ ios ar prst (typical blow 75km) w ca quat th dsity of th gativ ios with - ad dfi λ. By assumig lctrical utrality, th cotiuity quatio bcoms: d ( ) q αff ( ) d If w assum λ tim idpdt w gt (4.8) ( ( λ) ) q α ( λ) ff (4.9)

4 d q (4.) αff ( λ) This quatio is similar to Eq. 4.6 xcpt for th fact that th io productio q is rplacd by a ffctiv q productio ( λ) 4.. Chapma layrs which is always lss tha q. Sydy Chapma prstd i 93 a simpl mathmatical modl for th formatio of ioizd layrs, which was basd o th fact that rgtic photos from th su split air molculs ito lctros ad positiv ios. Th modl dscribs th major charactristics of th obsrvd variatios i th diffrt layrs of th ioosphr. W will ow outli th fudamtal thory bhid th formatio of ioizd layrs i th atmosphr. Th goal is to dvlop a simpl modl for how th plasma dsity varis with hight ad th su s zith agl. W start out with th followig assumptios: Thr is oly o typ of gas prst Th atmosphr is horizotally stratifid. Radiatio is moochromatic ad paralll. kt mg Th atmosphr is isothrm (scal hight H costat ). I chaptr 3 w foud that rgy absorbd alog th radiatio path ca b giv as (Eq. 3.3) di τ sc( χ ) I σ sc( χ) (4.) dz whr σ is th absorptio cross sctio ad is th umbr of absorbig molculs/atoms pr uit volum Is this a xtra uit? (s figur 3. pag 49). Th rat of io productio should b proportioal to th rat at which radiatio is absorbd. If η lctro-io pairs ar producd pr uit of rgy absorbd, th io productio rat bcoms Sic τ σ H w gt q( χ, z) q ( χ, z) τ sc( χ ) I σ η (z) (4.) I η H τ sc( χ ) τ (4.3) dq dz Th maximum io productio q m is foud by calculatig τ wh. W fid that ad τ cos( χ) sc(χ )

5 q ( χ, z m m I η ) (4.4) H sc( χ) For χ (su i zith), I η qm(,zm ) (4.5) H otic that th altitud for maximum io productio z m i this cas is th altitud whr th optical dpth τ Chapma variatios W foud a simpl xprssio for how th io productio varis with hight z ad th su s zith agl χ. W also foud a xprssio for th maximum io productio q(,z m ) at χ ad at hight z m. It is covit to itroduc a ormalizd hight paramtr z, which masurs th hight i uit of scal hight, ad with z m as rfrc hight. z' (z z H m ) (4.6) W hav th chos a rfrc altitud z at whr vrtically icomig radiatio rachs a optical dpth of τ. W itroduc z by lookig at th hight variatio of (z): W put this i quatio 4.3 z zm (zz m ) z m ( ) H H H H z' (4.7) I η q ( χ,z) σ H H ad rmmbr that τ σ H (τ whr z z m ). Aftr som calculatio w gt or q q σ z m H H σ H sc( χ ) I η z' ( z' sc(χ ),z') ) H (χ (4.8) z' ( z' sc(χ ), z') q ) m (χ (4.9)

6 Figur 4..: Th ormalizd photo ioizatio rat q(z)/q ad th lctro dsity (z)/ accordig to Chapma s thory show th Chapma layr variatios with altitud z ad zith agl χ. otic that th horizotal axis is logarithmic. Whr q m q m (,) is th maximum io productio at χ. Figur 4. shows how io productio i such a Chapma-layr varis with z ad χ Elctro dsity i a Chapma layr Th umbr of io pairs pr uit volum cosists of productio q ad loss L, ad ca b dscribd by th cotiuity quatio d q L (4.) whr is th umbr of positiv ios pr volum uit. I th cas of charg utrality, ad if o gativly chargd ios ar prst, th lctro dsity is. Th loss trm th has to b L α whr α is a costat (α is th rcombiatio cofficit). Th cotiuity quatio ca th b writt as d q α (4.)

7 Figur 4.3.: Typical lctro dsity profil i th ormal ioosphr d At quilibrium, ad whr q α. This rsults i a pak lctro dsity mo z' ( z' sc(χ ) ) (χ,z') (4.) m m qm(,) ad qm(,) m α Ths quatios show that th lctro dsity i a Chapma layr varis with hight ad su agl as squar root of th io productio q. W gt a daily ad sasoal variatio of th layr. A Chapma layr has its highst pak lctro dsity, ad lowst hight of this pak, wh th su is at its highst i th sky. Th layr disappars at ight. α cos( χ)

8 4.3 Th layrs of th ioosphr Figur 4.3 shows a typical lctro dsity profil for a ormal ioosphr at daytim. As w ca s, it is dividd ito thr diffrt layrs: D-layr (5-95km) E-layr (95-5km) F-layr (5-5km) Th lctro coctratio i ths layrs varis with th xposur to diffrt typs of radiatio, diffrt typs of rcombiatio ad various trasport procsss, ad o of th layrs bhav xactly lik th idal Chapma layr with rspct to variatios i altitud, tim of day, ad latitud. I additio, o of th layrs disappar totally wh th su is blow th horizo, bcaus of scattrd radiatio, ad trasport mchaisms, which ca trasport plasma from a sulit rgio to a dark rgio of th atmosphr. Lt us ow look mor closly at ach of ths layrs Th F-layr I th altitud abov ca. 5km, ios ad lctros ar formd wh th atmosphr s major compots, O ad, absorb EUV (Extrm Ultraviolt) radiatio with wavlgth m < λ < 9m. Th primary ios ar O ad, but ths ract quickly with utral atoms ad molculs O hν hν O O O O O O O O O Th most importat ios ar thrfor O, O ad O. Ths rcombi to O XY XO Y (4.3) XO X O (4.4) Whr X ad Y ca b O or. Th cotiuity quatio for O is giv by d [ O ] q γ [ O ] [ XY ] q β [ O ] (4.5) whr β γ[xy]. Th factor β will i a crtai hight b approximatly costat bcaus th utral dsity [XY ] dos ot chag sigificatly durig th ioizatio procss (th ioizatio rat is small). Th cotiuity quatio bcoms: [ XO ] β [ O ] α [ XO ] d (4.6) W ca writ [O ] [XO ]. By combiig ths two quatios w gt

9 [ XO ] d q α (4.7) At high altituds ( >km) whr O is th domiat io, ad th loss rat bcoms proportioal to th lctro dsity. d [ O ] >> [ XO ] [ O ] > q β I th lowst part of th F-layr, XO ios domiat, ad w gt a loss proportioal to th squar of th lctro dsity. d [ O ] << [ XO ] [ XO ] > q α As mtiod arlir, th F-layr dviats from a idal Chapma layr. This dviatio rsults from r complicatd rcombiatio procsss, ad also from th divrgc trm u, which is usually ot gligibl. Th cotiuity quatio should thrfor b [ XO ] u q α r t (4.8) 4.3. Th E-layr Th E-layr strtchs from ~95km to 5km abov th groud ad is th layr of th ioosphr that is i closst agrmt with th Chapma dscriptio. Th io productio i th ormal E-layr is causd by X- rays (m < λ< m) ad ultraviolt radiatio (m < λ < 5m) dissociatig O ad to O ad. disappars quickly by charg xchag. so that O O (4.9) O O (4.3) O ad O ar th domiatig ios. Rcombiatio is thrfor dissociativ. ad th cotiuity quatio gts ito th stadard form O O O (4.3) O O (4.3) d q α ff Thi sporadic layrs appar i th E-layr du to, amog othr thigs, th icidc of mtal ios (i.., a, Mg,ad F ) with a log liftim, which ar affctd by dyamic procsss. Sporadic E, dotd as (Es), ca b xtrmly thi ad ds layrs. Thy ar ot associatd with th ormal E-layr, ad occur mor occasioally both day ad ight.

10 4.3.3 Th D-layr Elctro dsity i th D-layr ormally dos ot hav a distict pak (s figur 4.3). Both th io productio ad th rcombiatio procsss ar highly complicatd btw 5km ad 95km. H-Lyma α radiatio (λ.5m), which is a vry strog spctral li, ptrats dow ito th D-layr ad has ough rgy to ioiz O, which is foud i small amouts (s figur 3. Chck)). This productio mchaism domiats from about 7-95km, but th cotributio from solar X-rays ca b importat as wll. Blow about 7km, ormal ioizatio from high-rgy cosmic radiatio domiats. I th D-layr, a larg umbr of complicatd ad havy positiv ad gativ ios is formd, ad th rcombiatio procsss ar thrfor both hight- ad tmpratur-dpdt. 4.4 Th disturbd ioosphr Ergtic particls lik lctros, protos ad α - particls from th su ad th magtosphr ca ptrat th atmosphr ad cotribut to a xtraordiary productio of ios ad lctros i th ioosphr. Such particl prcipitatio is closly rlatd to th orthr light phomo, ad th layrs of th ioosphr vary, thrfor, oft vry irrgularly, i th auroral zo. Espcially importat is th io productio that taks plac i th E-ad D-layr. Hr, th icras i lctro dsity ca giv ris to grat disturbacs i radio-wav propagatio coditios (rflctio, absorptio, ad scattr). Disturbacs i th polar ioosphr ca caus a total brakdow i shortwav commuicatio ( radio blackouts ), ad th lctric currts that ar iducd i th ioosphr ca ifluc powr supply, corrosio i oil piplis, tc.