The intensity and timescale of nonthermal Doppler-shifted hydrogen L alpha photon emission as diagnostics of 10 keV to 10 MeV protons bombarding the solar chromosphere during flares are investigated. The steady-state excitation and ionization balance of the proton beam are determined, taking into account all important atomic interactions with the ambient chromosphere. For a proton energy flux comparable to the electron energy flux commonly inferred for large flares, L alpha wing intensities orders of magnitude larger than observed nonflaring values were found. Investigation of timescales for ionization and charge exchange leads researchers to conclude that over a wide range of values of mean proton energy and beam parameters, Doppler-shifted nonthermal L alpha emission is a useful observational diagnostic of the presence of 10 keV to 10 MeV superthermal proton beams in the solar flare chromosphere

Canfield, R. C.

Chang, C. R.

English

NASA Technical Reports Server

86
SH1.3-5
ON THE DETECTABILITY OF KEV-MEV SOLAR PROTONS
THROUGH THEIR NONTHERMAL LYMAN-ALPHA EMISSION
RichardC. Canfmeldand Chmng-RayChang
Centerfor Astrophyslcsand SpaceSciences,C-011
Unmversltyof Callfornla,San Diego
La Jolla,Callforn_a92093,USA
ABSTRACT
We anvestmgate the mntensity and tamescale of nonthermal
Doppler-shlftedhydrogen L_ photon emassaon as dlagnostacs
of i0 keV-lO MeV protons bombardlng the solar chromosphere
durang flares. We determane the steady-state excatatlon and
aonazatlon balance of the proton beam, takang into account
all amportant atomic interactions wath the ambient chromo-
sphere. For a proton energy flux comparable to the electron
energy flux commonly inferred for large flares, we flnd L_
wlng antensltles orders of magnitude larger than observed
nonflaring values, our anvestigatmon of tamescales for
ionlzataon and charge exchange leads us to conclude that
over a wade range of values of mean proton energy and beam
parameters, Doppler-shafted nonthermal L_ emisslon as a
useful observatmonal dlagnostic of the presence of i0 keV-
10 MeV superthermalproton beams an the solar flare
chromosphere.
!. Introductlon. Gamma-rayand hard X-ray observatlonsof solar flares
implythat,at energaes_ IOMeV, the numberofof energetacprotonsexceeds
that of energetlcelectronsby severalordersof magnltude[6]. This as
curren%lyunderstood[4]robe the consequenceof an acceleratlonmechanism
(e.g., a stochastacFermi mechanism)mn whlch the accelerationrate is
proportaonalto momentum,acceleratlontakesplacein a tenuousmedlum(the
corona),and the emmss_onis producedwhen the energeticparticlesare
stopped fully in a thick target (the chromosphereand photosphere).
However,thas mechanismoperateseffectivelyonly if some partlclesare
alreadyacceleratedabove a thresholdenergy by some other mechanism.
Orrall and Zlrker[5]have shown that charge-exchangeL_ emissionmay be
usefulforthe detectiono£ relativelylow energy(E < 1 MeV) superthermal
protons,whichdo not producea sagniflcantlevelof nuclearemission.In
this paper we extend the work of Orrall and Zirker to energetically
slgniflcantvaluesof beam flux,and addressthe questionof the rangeof
valuesof beam flux for whi:h the intensatyand timescaleof LG charge-
exchangeemlssmonmayfallIntoobservationallyrelevantranges.
2. Atomic Physics. To simplifyour study of the emissionof LG by
superthermal protons we represent the protons Injected into the
chromosphereby a verticalbeam darectedaway from the observer.Some
superthermalprotonsbecomesuperthermalhydrogenatomsin excitedstates,
and subsequentlyradiatebefore they stop. Potentiallyrelevantatomic
processesthat we have examined include charge exchange between beam
protonsand ambaen+hydrogenatoms,excitationof beam hydrogenatoms by
collisionswithamblentelectrons,hydrogenatomsandprotons,spontaneous
radiativede-excitatlonof beam hydrogenatoms,stimulatedra_iatlvede-
excitationof beamhydrogenatoms,collisaonalionizationof beamhydrogen
86 
ON THE DETECTABILITY OF KEV-MEV SOLAR PROTONS 
THROUGH THEIR NONTHERMAL LYMAN-ALPHA EMISSION 
R~chard C. Canf~eld and Ch~ng-Ray Chang 
center for Astrophys~cs and Space Sc~ences, C-Oll 
Un1vers1ty of Ca11forn~a, san D~ego 
La Jolla, Cal~forn~a 92093, USA 
ABSTRACT 
S8 1.3-5 
We ~nvest1gate the 1ntensity and t~mescale of onthermal 
Doppler-sh1fted hydrogen La photon em1ss~on as d~agnost~cs 
of 10 keV-IO MeV pr tons bombard~ng the solar chr mosphere 
dur~ng flares. We d term~ne the steady-state exc~ at~on and 
10n~zat1on balance of the pr ton beam, t k~ng 1nto account 
all 1mportant atom~c 1nteractions w~th the ambient chromo-
sphere. For a proton energy flux comparable to the electron 
nergy flux commonly ~nferred for large flares, we f~nd La 
w~ng 1ntens~t~es orders of magnitude larger than observed 
non flaring values. Our ~nvestigat10n of t~mescales for 
ion~z t10n and charge exchange leads us to conclude that 
over a w~de range of values of mean pr ton energy and beam 
parameters, Doppler-sh~fted non thermal La emiss~on 1S a 
useful observat10nal d1agnostic of the pr sence of 10 keV-
10 MeV superthermal proton beams 1n the solar flare 
chr mosphere. 
1. Introduct1on. Gamma-ray and hard X-ray observat10ns of solar flares 
unply that, at energ1es ~ 10 MeV, the number of of energet1c protons exceeds 
that of nerget1c lectrons by s veral orders of magn1tude [6]. This 1S 
currently understood [4] to be the consequence of an accelerat10n mechanis  
(e. g., a stochast1.c Fermi mechanism) 1.n wh1ch the acceleration rate is 
pr port1onal to omentum, acc lerat10n tall.es place 1n a tenuous med1um (the 
c rona), and th  em1.SS10n is produced when th  nergetic part1cles are 
stopped fully in a thick target ( the chromosphere and ph tosphere) • 
However, th1s mechanism operates effect1vely only 1f some part1cles are 
already acc lerated above a threshold nergy by some other mechanism. 
Orrall and Z~rker [5] have shown that charge-exchange La emiSS10n may be 
useful for the d tect10n of relatively low nergy (E < 1 MeV) superthermal 
pr tons, which do not produce a s1gnif1cant l vel of nuclear emiss1on. In 
this aper w  extend the work of Orrall and Z1rker to nergetica ly 
s1gnif1cant values of beam flux, and address the question of the range of 
values of beam flux for whizh the intens1ty and timescale of La charge-
exchange em1SS10n may fall 1nto observationall  rel t ranges. 
1,. Atom1c Phys1cs. To s1mplify our study of th  em1ssion of La by 
superthermal pr tons we represent the pr tons 1njected into the 
chromosphere by a vertical beam d1rected way from the observer. Some 
superthermal pr tons become superthermal hydrogen atoms in excited states, 
and subsequently rad1ate before they stop. Potentially r levant atomic 
processes that we have examined include charg  exchange between beam 
pr tons and amb1en+ hydrogen atoms, excitat10n of beam hydrogen atoms by 
collisions with amb1ent lectrons, hydrogen atoms and pr ton , spontaneous 
rad1at1ve de-excitat10n of beam hydrogen atoms, st1mulated ra1iat1ve de-
exc1tation of beam hydrogen atoms, col11s1onal ionization of beam hydrogen 
https://ntrs.nasa.gov/search.jsp?R=19850026439 2020-03-20T16:53:08+00:00Z
8 7 se z.3-s
atoms by ambient electrons, protons and hydrogen atoms, and spontaneous
radlatmve recomb_natlonof beam hydrogen atoms.
We have examlned the relatlve mmportance of these various competing
atomlc processes, with the following conclusions.The dominant destruction
process for superthermal beam protons is charge exchange with ambient
chromosphermc hydrogen atoms. Ionazlng collaslonsof superthermalhydrogen
atoms wlth ambient electrons, protons, and neutral hydrogen atoms are of
comparable importance in creating superthermal protons. The maln source of
LG emasslon as charge exchange from the proton beam. The dominant slowing
process as Coulomb collaslonswith ambient electrons [7].
3. Computed La Spectra. Our chosen value of the hydrogen aonazed fraction
an our uniform chromospheric model atmosphere is guided by Orrall and
Zlrker, who used a detailed empirical model [8] to show that i0-I000 keY
proton nonthermal LG emasslon originates in the chromosphere.Our assume@
10% Ionized fractaon as an appropraatemean value. We have adopted a smmple
monoenergetmc proton beam at input. The values of the input energy E0 and
the total anput energy flux FE(O) are free parameters.
In Fagure 1 we show the computed nonthermal La spectrum generat@dby
monoenergetlc proton beams of anput energy flux FE(O) - i0_i erg cm-Z s-I
(an upper iimat to plausible values), for values of E0 ranglng from _ key to
_0 MeV If we examlne a s_ngle curve, say for EO = 30 keV, we see a sharp
Proton Energy
I keV I0 keV I00 keV IMeV
i i T i--
8[ F(Eo,O)O:8(E-Eo) FE(O):lO"erg cm-2 s"
Fig. I. - Theoretical nonthermal L_ spectra for protons of initial energy
E0•
87 SH 1 -5 
ato s  a b1e t l trons, r to s a  ro en atoms, a  s taneous 
1 t1ve ~ at10n  m  tom . 
e  ~ ed t  r lat1ve ~ tance  t  ~ s pet1  
atom1c r cesses, it  t e follow1ng c cl sions. he do inant estruct10n 
r  f r rtherm l  r t  1S r  nge it  bi t 
r s r1c r en to s. Ion1z1ng l11s10ns f s rtherm l r en 
ato s 1th a bjent electrons, protons, and e tral hydrogen atoms are f 
par le ~ nce ~  ~  s rtherm l r to . e a1  s r e f 
a e 1SS10  ~  r   from t  r ton ea . e 1na t slow1 g 
r ss 1S oulomb c l11s1ons it  a bie t le trons [ ]. 
1.. co puted a ectra. ur chosen value f t e hydrogen 10n1zed fract10n 
1  r 1f rm r s r1c odel t re i  U1   rrall  
1r ,    t 1led p1r1 l odel 8]  ow  1 1000 V 
to  t r l a 1SS10n 1 1 tes i  t  o e. ur ~ 
 10 1  act10n 1S  1 te e  l . e  ted  1 le 
monoenerget1c proton beam at ~nput. The values of the 1nput energy EO and 
the total 1nput energy flux F E( 0) are free parameters. 
 P1    ow  t d therm l  trum ted by 
monoenerget1c proton beams of 1nput energy flux PE(O) - loll erg cm-2 s-
(an upper l1m1 t to plaus1ble values), for values of Eo rang1ng from ~ keV to 
10 M~V If we examln~ a slnqle curv~, say for Eo = 30 keV, we see a sharp 
r t n rg  
 eV 1  V 1  " " -'-___ --'-'1 eV 
,----...:...:..:',-'---------'-=- 1 ,- 1 
  ( o, 0) a: 8(E - Eo) 
7 
r-;:::::"' 
o~ 
, .... 6 
(J) 
"" '" 'E 
u 
C'I 
.... 
~ 
-< 
<l 
-.. 
---'----' 
C'I 
..2 - FLARE 
ACTIVE REGION 
---
~~L_ _____ ~ _________ ~ ___________ ~_L___ ____ _L~ 
I 
log [1l>'(A)] 
F1g. 1. - Theoret1cal nonthermal La spectra for protons of in~t~al energy 
o' 
88
SH 1.3-5
peak in the spectrum at the wavelength shlft Ak appropriateto the veloclty
of protons of inltlal energy E0, correspondlngto emlsslon from near the top
of the chromosphere.At lesser values of AX the emisslon comes from slower
protons that have penetrated some dlstance z into the target, where their
velocity (and hence Doppler-shlft) has dropped. If we consider the spectra
in Figure 1 for E0 = 300 keY and 3 MeV, we see that the curves peak somewhato
below E = I00 key (somewhat above AX = i0 A). This peak L_ intenslty
corresponds to the peak in the charge-exchange cross-section In th&s same
energy range. It is Interestlng to compare the computed spectra to an
observed solar active region La spectrum [3] and an observed (presumably
thermal) solar flare L_ spectrum [2]. Throughout much of the energy range
consldered in Figure 1 the intensityof the computed nonthermalL_ emlssaon
exceeds the observed active reglon emisslon by several orders of magnltude.
4. T_mescales. Our equ111briumcomputationsof La emissionwill not be
appllcableunless the lonlzatlonstructureof the chromosphereremalns
substantlallyunchangedwithlnthe tlme requlredfor chargeexchange.If
the chromospherebecomeshighlyionizedwithinthecollisionalrangeofthe
protons,littlechargeexchangebetweensuperthermalprotonsand amblent
hydrogenatomscan takeplace.BecauserecomblnatlonIs severalof orders
of magnltudeless efficientthan chargeexchange,per ambientpartlcle,
much lessnonthermalLa emlsslonwlllbe createdIfthe atmospherebecomes
lonlzed.
Fig. 2. The range of relevance at the equlllbrlumLG spectra in Figure i.
 
SH 1. 3-5 
ea  ~n the spectrum t the a elen th shl.ft 6}" appropr~ate to the eloc~ty 
of protons of ~n~t~al energy EO' correspondl.ng to em1SSl.0n from near the top 
 the rom e.  esser ues  6}" the e ssl.on om s from slower 
r tons th t have trated some 1stance z into the target, ere thel.r 
velocity (and hence oppler-shl.ft) has dropped. If e consider the spectra 
in Fl.gure 1 for EO ... 300 keV and 3 MeV I we see t.,hat the curves peak somewhat 
below  = 100 keV (somewhat above 11)" = 10 ). his peak a l.ntensl.ty 
rresponds to the  l.n the rge-exchange cross-section 1n thl.S same 
rgy range,  is 1 terest1ng to com re the o ted sp tra to an 
served s lar actl.ve region a s ectrum [3] and an served (presumably 
thermal) solar flare a spectrum [2J, Throughout uch of the energy range 
l. red ~  ~ e  the tens~ty  the co ted therma1 a m~SSl.on 
e cee s the served tive regl.on e issl.on  s r l rd rs f agn~ tude, 
!. l. es les,  ul.l~rium ations   ssl.on i l   
appll.cable unless the J.onl.Zat10n structure of the chromosphere remal.ns 
s stantl.ally unchanged ith1n the t1me requ1red for charge exchange. If 
the chromosphere becomes highly 10nized ithin the collisional range of the 
rotons, ittle arge change tween uperthermal rotons nd 1ent 
rogen toms an take lace. ause recombl.nat10n 1S everal  ders 
f a 1tude less fficient than arge xchange, r ient rtl.cle, 
uch less nonthermal La e l.SS10n 1ll be created 1f the atmosphere becomes 
10 1ze . 
5.-------r-------
/ 
4 
16A <active region.! / / 
- 3 / / > / Q) / ..::.:: 
- / / 0 
w / / 
01 / / 0 2 
/ 
. 
o~----~------~----~~~~~~~--~----~ 
4 6 8 10 12 14 
log FE(O) [erg cm-2 sec-~ 
P'1g. 2. The range of relevance ot the equ1ll.brl.um La spectra 1n F1gure 1. 
89
sbi.3-5
The rangeof relevanceof the La spectraof Figure1 is exploredin
Figure 2.The thicksolidcurve isthat forwhlchthe Ionizationtlmescale
Ti equalsthe chargeexchangetlmescaleTc.The thinlinesindlcatevarious
valuesof 71. On the long-dashedcurvethe peak intensityof nonthermalLa
emisslonequalsthatof observedflareLa-wlngemission.On the dot-dashed
curvethe peakcomputedintensltyof nonthermalLa emissionmatchesthatof
observed actlve-reglonLG-wing emission.We see that at an energy of
interest(say100 keV),theintensityof nonthermalLa emlsslonexceed_the
observedactive-regionbackgroundfor allFE(O) valuesaboveabout10" erg
cm-2 s-1,andthe charge-exchangemechanismiseffectiveupto aroundFE(O)
- 1013 erg cm-2 s-I. Over this range of FE(O)valuesthe ionizationtlme
ranges from about 1000 s to very small values.Obviouslyat the lowest
detectable intensities the equilibrium calculation given above is
physicallyrelevantthroughoutthe duratlonof typicalflares;at the high
end of the FE(0) range,the durationof validitybecomesinconsequentially
small.Figure2 also servesto show that at Inputprotonenergiesabove
aboutI0 MeV andbelowabout1 keV the equll_briumcalculationis not both
physicallyand observationallyrelevanta% any value of the total energy
flux.
_. Conclusions. We concludethat if the sun'schromosphereIs bombarded
suddenlyfromaboveby superthermalprotonsof energyl0 keV - 1 MeV, with
energy fluxes consistentwith the hypothesisthat such protons are a
significantcomponentof the populationof superthermalparticles in
flares,chargeexchangewouldleadto an intensebut briefburstof Doppler-
shiftedL_ emisslon.At much lowerproton fluxlevelsthe emissionwould
stillexceedpreflarebackground,but lastmuchlonger.Inotherwork[i]we
considerpower-lawand thermalformsof the inputprotonenergyspectrum,
the generationof emissionin the hydrogenHa llne,and the detailsof the
atomicphysics.
!. Acknowledqements. We thankEdwardShoub,WilliamThompson,and Jack
Zirker for stimulatingdiscussions.This researchhas been supportedby
NASAgrantNSG-7406andNationalSclenceFoundationgrantAST 83-20306.
References
1. Canfield,R.C., andChang,C.-R.1985,AstroDhvs._., 295,inpress.
2.Canfield,R.C., andVanHoosier,M. E. 1980,SolarPhl., 67, 339.
3. Cohen,L. 1981,AllAtlas of the Solar SPectrumBetween1175 and 1950
AnustromeRecordedonSkvlabwithNRL'sADolloTelescoDeMountExDeriment,
NASA ReferencePublication1069 (WashingtonzU. S. GovernmentPrinting
Office).
4. Heyvaerts,J. 1981, in Solar Flare Maunetohvdrodvnamics,ed. E. R.
Priest(NewYorkzGordonandBreachPubl.).
5.Orrall,F. Q., andZirker,J. B.1976,Astrophvs._., 208,618.
6. Ramaty,R. et al. 1980, in Solar Flares,ed. P. A. Sturrock(Boulder:
Universityof ColoradoPress),p. 117.
7. Trubnikov,B. A. 1965, Reviewsof PlasmaPhysics,consultantsBureau,
NewYork,I, 105.
8.Vernazza,J. E., Avrett,E. H., and Loeser,R. 1981,AD.J. SUDDI.Set.,
45,619.
89 
S8 1.3-5 
The range of relevance of the La spectra of Figure 1 is explored in 
Pigure 2. The thick solid curve is that for Wh1Ch the 1onization t1mescale 
T i equals the charge exchange t1mescale T c' The thin lines ind1cate various 
values of T l' On the long-dashed curve the peak 1ntensity of nonthermal La 
emiss10n equals that of observed flare La-w1ng emission. On the dot-dashed 
curve the peak computed intens1ty of nonthermal La emission matches that of 
observed act1ve-reg10n La-wing emission. We see that at an energy of 
interest (say 100 keV), the intens1 ty of nonthermal La em1SS10n exceed, the 
observed active-region background for all PE(O) values above about 10 erg 
cm-2 s-1, and the charge-exchange mechanism 1S effective up to around FE(O) 
- 1013 erg cm-2 s-1. OVer this range of FE(O) values the ionization t1me 
ranges from about 1000 s to very small values. Obviously at the lowest 
detectable intensities the equ11~r1um calculat10n given above is 
physically relevant throughout the durat10n of typical flares; at the high 
end of the FE(O) range, the duration of validity becomes inconsequentially 
small. Figure 2 also serves to show that at 1nput proton energies above 
about 10 MeV and below about 1 keY the equ111.brium calculation is not both 
phYSically and observationally relevant at any value of the total energy 
flux. 
~. ConclUSions. We conclude that if the sun's chromosphere 1S bombarded 
suddenly from above by superthermal protons of energy 10 keY - 1 MeV, with 
energy fluxes consistent with the hypothesis that such protons are a 
significant component of the population of superthermal particles in 
flares, charge exchange would lead to an intense but brief burst of Doppler-
shifted La emiss1on. At much lower proton flux levels the emission would 
still exceed pre flare background, but last much longer. In other work [1] we 
consider power-law and thermal forms of the input proton energy spectrum, 
the generation of emission in the hydrogen Ba 11ne, and the details of the 
atomic physics. 
§.. Acknowledgements. We thank Edward Shoub, William Thompson, and Jack 
Zirker for stimulating discussions. This research has been supported by 
NASA grant NSG-7406 and National SC1ence Foundation grant AST 83-20306. 
References 
1. Canfield, R. C., and Chang, C.-R. 1985, AStrophys. i1.., 295, in press. 
2. Canfield, R. C., and VanBoosier, M. E. 1980, Solar ~., 67, 339. 
3. Cohen, L. 1981, An Atlas of the Solar Spectrum Between 1175 and 1950 
Angstroms Recorded Q!l Skylab with NRL' s Apollo Telescope ~ Experiment, 
NASA Reference publication 1069 (Washington: U. S. Government printing 
Office ). 
4. Heyvaerts, J. 1981, in Solar Plare Magnetohydrodynamics, ed. E. R. 
Priest (New York: Gordon and Breach Publ. ). 
5. Orrall, F. Q.. and Zirker, J. B. 1976, Astrophys. i1.., 208, 618. 
6. Ramaty, R. et al. 1980, in ~ FlareS, ed. P. A. sturrock (Boulder: 
University of Colorado Press), p. 117. 
7. Trubnikov, B. A. 1965, Reviews of Plasma Physics, Consultants Bureau, 
New York, 1, 105. 
8. Vernazza, J. E., Avrett, E. H., and Loeser, R. 1981, ~. il.. suppl. Ser., 
45, 619. 


On the detectability of key-MeV solar protons through their nonthermal Lyman-alpha emission

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