X-ray and UV X-ray and UV spectroscopy of spectroscopy of

the Sun and Flare the Sun and Flare StarsStars

Ken PhillipsKen PhillipsPostgraduate LecturePostgraduate LectureDecember 18, 2008December 18, 2008

Recommended readingRecommended readingIntroduction to Stellar AstrophysicsIntroduction to Stellar Astrophysics, vol. 2 , vol. 2

(Stellar Atmospheres). E. B(Stellar Atmospheres). E. Bööhme-Vitense hme-Vitense (CUP, 1989)(CUP, 1989)

The Solar Transition RegionThe Solar Transition Region. . J. Mariska (CUP, J. Mariska (CUP, 1992)1992)

Ultraviolet and X-ray Spectroscopy of the Ultraviolet and X-ray Spectroscopy of the Solar Atmosphere. Solar Atmosphere. K.J.H. Phillips, U. K.J.H. Phillips, U. Feldman, E. Landi (CUP, 2008) Feldman, E. Landi (CUP, 2008)

Space Science Space Science (eds L. Harra and K. Mason: (eds L. Harra and K. Mason: Imp. College Press, 2004) esp. chapter 8Imp. College Press, 2004) esp. chapter 8

Atomic Spectra.Atomic Spectra. 22ndnd ed. H. G. Kuhn (Longman ed. H. G. Kuhn (Longman 1969)1969)

Atomic Spectra and Atomic Structure. Atomic Spectra and Atomic Structure. G. G. Herzberg (tr. Spinks) (Dover 1944)Herzberg (tr. Spinks) (Dover 1944)

Atmospheres of “active” Atmospheres of “active” stars and the Sunstars and the Sun

Flare stars and the Sun have hot Flare stars and the Sun have hot atmospheres, usually aatmospheres, usually a coronacorona (temperature ~ 10(temperature ~ 1066 K) plus a K) plus a chromospherechromosphere (~10,000 K) and (~10,000 K) and “transition region”“transition region” (~10(~1055 K). K).

These temperatures are generally much These temperatures are generally much hotter than their hotter than their surfacesurface temperatures.temperatures.

E.g. The Sun has surface (photospheric) T E.g. The Sun has surface (photospheric) T ~ 6000K but its corona has T~10~ 6000K but its corona has T~1066KK

Why are some stellar Why are some stellar atmospheres hot? atmospheres hot?

Some stellar atmospheres are hot because Some stellar atmospheres are hot because there is a there is a non-radiativenon-radiative energy source.energy source.

This is associated with aThis is associated with a magnetic fieldmagnetic field present in the convective zone of its interior. present in the convective zone of its interior.

Heating may occur by either dissipation of Heating may occur by either dissipation of MHD wavesMHD waves or numerous or numerous tiny flarestiny flares..

The magnetic field continually re-created by The magnetic field continually re-created by an an αα--ωω dynamodynamo action in the stellar interior action in the stellar interior – differential rotation (– differential rotation (ωω) + convection () + convection (αα). ).

HR diagram for nearby HR diagram for nearby starsstars

MS stars liable to have coronae

RS, XC

NC

RS = RS CVn binaries.

XC = X-ray coronae

NC = no coronae, just cool winds

Some preliminariesSome preliminaries

In visible, UV and X-rays, In visible, UV and X-rays, nm is SI nm is SI unit unit of wavelength but in solar of wavelength but in solar physics still generally use physics still generally use wavelengths in wavelengths in Ångströms: Ångströms: 1Å = 1Å = 0.1nm0.1nm

For X-rays, sometimes use energies For X-rays, sometimes use energies in keV rather than wavelengths:in keV rather than wavelengths: E(keV) = 12.4 / E(keV) = 12.4 / λλ (Å) (Å)

Spectral unitsSpectral units

““Spectral flux” – Spectral flux” – spectral irradiancespectral irradiance – measured – measured in in erg cmerg cm-2-2 s s-1-1 ÅÅ-1-1 (cgs) or (cgs) or W mW m-2-2 nm nm-1-1 (SI) or (SI) or W mW m-2-2 nm nm-1-1 Hz Hz-1-1 (SI frequency units). (SI frequency units).

For X-rays or UV radiation, units often in For X-rays or UV radiation, units often in photonsphotons cmcm-2-2 s s-1-1 ÅÅ-1-1 (cgs) or (cgs) or photons mphotons m-2-2 s s-1-1nmnm-1-1 (SI)(SI)

““Spectral intensity” – Spectral intensity” – spectral radiancespectral radiance – units: – units:

erg cmerg cm-2-2 s s-1-1 Å Å-1-1 sr sr-1-1 (cgs) or (cgs) or W mW m-2-2 nm nm-1-1 sr sr-1-1 (SI).(SI).

Example of a spectrumExample of a spectrum

-- but ideally spectral irradiance is in SI units (W m-2 nm-1) and wavelength in nm.

Emission from solar and Emission from solar and stellar atmospheresstellar atmospheres

Chromospheres and transition regions: Chromospheres and transition regions: fromfrom EUVEUV (100-1000 (100-1000 Å) Å) to to UVUV (>1000(>1000Å)Å)

Coronae: from Coronae: from EUVEUV toto soft X-rays soft X-rays (<100(<100Å)Å)

Active regions: from Active regions: from EUVEUV toto soft X-rayssoft X-rays

Flares: from Flares: from EUVEUV toto hard X-rays hard X-rays (<1(<1Å)Å)

Line emissionLine emissionLine emission from abundant elements Line emission from abundant elements

– – H, He, C, N, O, Ne, Mg, Al, Si, S, Ar, H, He, C, N, O, Ne, Mg, Al, Si, S, Ar, Ca, FeCa, Fe..

Elements Elements normally ionizednormally ionized, e.g. in , e.g. in stellar coronae: C in form of Cstellar coronae: C in form of C+3+3, C, C+4+4, , CC+5+5; Fe is in form of Fe; Fe is in form of Fe+9+9, Fe, Fe+10+10… Fe… Fe+16+16..

Often useOften use iso-electronic series iso-electronic series to to describe an ion, e.g. Cdescribe an ion, e.g. C+4+4 is “He-like” (2 is “He-like” (2 electrons), Celectrons), C+5+5 is “H-like” (1 electron). is “H-like” (1 electron).

Only ions with at least one electron can Only ions with at least one electron can emit lines – e.g. Cemit lines – e.g. C+6+6 is fully stripped C, is fully stripped C, so cannot emit lines. so cannot emit lines.

Ion and spectrum Ion and spectrum notationnotation

Atom or IonAtom or Ion produces spectrum:produces spectrum:

Neutral HNeutral H H I (first spectrum)H I (first spectrum)

Neutral HeNeutral He He I (1He I (1stst spectrum) spectrum)

HeHe+1+1 He II (2He II (2ndnd spectrum) spectrum)

Neutral CNeutral C C IC I

CC+4+4 (He-like)(He-like) C VC V

FeFe+16+16 (Ne-like) (Ne-like) Fe XVIIFe XVII

FeFe+24+24 (He-like) (He-like) Fe XXVFe XXV

Note: “H II regions” doesn’t make sense!Note: “H II regions” doesn’t make sense!

Electron configurationsElectron configurationsElectrons in an atom have 4 Electrons in an atom have 4 quantum quantum

numbersnumbers, ,

nn (principle q.n. related to distance from (principle q.n. related to distance from nucleus), nucleus),

l l (orbital or azimuthal q.n. related to (orbital or azimuthal q.n. related to angular momentum and thus shape of angular momentum and thus shape of orbit), orbit),

mml l (orientation w.r.t. mag. field) of orbital (orientation w.r.t. mag. field) of orbital plane), plane),

mmss (orientation of electron spin). (orientation of electron spin).

No two electrons can have same set of 4 No two electrons can have same set of 4 q.n.’s q.n.’s ((Pauli’s exclusion principlePauli’s exclusion principle))..

Notation for Notation for configurationsconfigurations

Principal quantum number Principal quantum number nn = 1, 2, 3,... = 1, 2, 3,...

Orbital quantum number Orbital quantum number ll for a given for a given nn can have values 0, 1, ..., can have values 0, 1, ..., nn - 1 - 1

Notation used is Notation used is ss ( (l l =0), =0), pp ( (l l =1), =1), dd ( (l l =2), =2), ff ( (l l =3) .... =3) ....

For given For given nn, orbit with largest , orbit with largest ll is is circular, those with smaller circular, those with smaller ll’’s ’’s progressively more elliptical. progressively more elliptical.

Electron configurationsElectron configurationsH-like ionH-like ion: one electron, configuration : one electron, configuration

in ground state is in ground state is 1s1s ( (nn=1, =1, ll=0, spin =0, spin either up or down)either up or down)

He-like ionHe-like ion: 2 electrons, g.s. config. is : 2 electrons, g.s. config. is 1s1s22 (spins up and down)(spins up and down)

Li-like ionLi-like ion: 3 electrons, g.s. config. is : 3 electrons, g.s. config. is 1s1s22 2s 2s

Ne-like ionNe-like ion: 10 electrons, g.s. config. is : 10 electrons, g.s. config. is 1s1s22 2s 2s22 2p 2p6 6 (2s e’s elliptical orbits, 2p (2s e’s elliptical orbits, 2p

e’s in circular orbits)e’s in circular orbits)Ar-like ionAr-like ion: 18 electrons, g.s. config. is : 18 electrons, g.s. config. is

1s1s22 2s 2s22 2p 2p66 3s 3s22 3p 3p66

Pauli’s exclusion principle Pauli’s exclusion principle explains “chemistry”explains “chemistry”

He, Ne, Ar, Kr He, Ne, Ar, Kr have closed subshells have closed subshells (1s(1s22, 2p, 2p66, 3p, 3p66 etc.) – inert (“noble”) etc.) – inert (“noble”) gases gases

Na, K Na, K have single outer electron (3s, 4s) have single outer electron (3s, 4s) – highly reactive– highly reactive

F, Cl F, Cl have subshells “missing” an have subshells “missing” an electron in outer shell.electron in outer shell.

SoSo Na, Cl Na, Cl have strong affinity for each have strong affinity for each other –other – NaCl NaCl a common moleculea common molecule

Pauli’s principle and the Pauli’s principle and the Periodic TablePeriodic Table Filled

sub-shells

1 outer e

Atomic transitionsAtomic transitionsIn a stellar atmosphere, an atom or ion is In a stellar atmosphere, an atom or ion is

normally in its normally in its ground stateground state, but can be , but can be excited (by excited (by ee-- collisions) to an upper collisions) to an upper level. level.

A A radiative transition radiative transition back to the ground back to the ground state or some lower state may follow, state or some lower state may follow, resulting in resulting in line emission, line emission, i.e. emission i.e. emission of a of a quantum hquantum hνν

E.g. H-like ions may undergo excitation E.g. H-like ions may undergo excitation from from 1s to 2p1s to 2p, followed by a , followed by a 2p 2p →→1s 1s transitiontransition, resulting in a , resulting in a “Lyman-“Lyman-αα” ” line line photon (photon (1216Å1216Å for H, for H, 304Å304Å for He II etc.) for He II etc.)

Ionization conditionsIonization conditionsStellar atmospheres are low-density, Stellar atmospheres are low-density,

hot plasmas.hot plasmas.Generally, for Generally, for ionizationionization, only , only

collisionalcollisional processes important (e processes important (e-- = = a free electron): a free electron):

XX+m+m + e + e--11 -> X -> X+m+1+m+1 + e + e--

1 1 + e+ e--22

RecombinationRecombination processes are either processes are either radiative radiative (h(hνν = photon) = photon) ::

XX+m+1+m+1 + e + e-- -> X -> X+m+m +h +hννor or dielectronicdielectronic::XX+m+1+m+1 + e + e-- -> (X -> (X+m+m)** )** (doubly excited)(doubly excited)

Ionization equilibriumIonization equilibriumIn the In the quiet solar coronaquiet solar corona, and to a 1, and to a 1stst

approximation in active regions (or maybe approximation in active regions (or maybe even flares), there is even flares), there is ionization ionization equilibriumequilibrium: :

Number of ionizations/unit vol. and time = No. Number of ionizations/unit vol. and time = No. of recombinations/unit vol. and timeof recombinations/unit vol. and time: :

NNee N(X N(X+m+m) Q(T) = N) Q(T) = Nee N(X N(X+m+1+m+1) ) αα(T)(T)

where Q(T) = rate coefft. of ionization, where Q(T) = rate coefft. of ionization, αα(T) = (T) = rate coefft. of recombination. rate coefft. of recombination.

From this one can calculate all the From this one can calculate all the ion fractions ion fractions for a particular element as a function of T. for a particular element as a function of T.

[Note: ionization equilibrium is NOT the same [Note: ionization equilibrium is NOT the same as LTE! LTE holds in the solar photosphere.]as LTE! LTE holds in the solar photosphere.]

Fe ion fractions as a Fe ion fractions as a function of temperature function of temperature

TTee

9 = fractional abundance of Fe+9 ions: N(Fe+9)/N(Fe)

Line excitation from stellar Line excitation from stellar atmospheresatmospheres

UV and X-ray lines excited by UV and X-ray lines excited by collisions collisions of e’s with ionsof e’s with ions, followed by , followed by spontaneous spontaneous radiative de-excitationradiative de-excitation::

XX+m+m + e + e-- -> (X -> (X+m+m)* + e)* + e--

(X(X+m+m)* -> X)* -> X+m+m + h + hννlineline

[h[hννlineline = line photon; = line photon; asterix * means asterix * means ion is excited]ion is excited]

Line radiant fluxesLine radiant fluxesLine emission Line emission FF (photons s (photons s-1-1) ) from an emitting volume from an emitting volume

VV, electron density , electron density NNee, ion density , ion density NN(X(X+m+m) is:) is:

FF = = NNee N N (X(X+m+m) ) CCijij((T T ) ) VV

= = NNee22 VV×[(×[(NN(X(X+m+m)/)/NN(X)]×[(X)]×[NN(X)/(X)/NN(H)]×[(H)]×[NN(H)/(H)/NNee]×]×CCijij ( (TT))

where where CCijij = = collisional rate coefficient collisional rate coefficient (cm(cm33 s s-1-1) )

FF = = NNee22 VV ×× f f ((T T ) ) ×× Ab Ab (X) × 0.8 × (X) × 0.8 × CCijij ( (T T ))

NNee22 VV is the (volume) is the (volume) emission measure emission measure (cm(cm-3-3))

f f ((T T ) = ) = ionization fraction ionization fraction = N(X= N(X+m+m)/N(X))/N(X)

Ab Ab (X) = (X) = abundance of element abundance of element (X)(X) relative to H.relative to H.

Line irradiance (flux) at Line irradiance (flux) at Earth or SOHO etc. Earth or SOHO etc.

Line radiant flux Line radiant flux (or power) (or power) FF is number of is number of photons (or ergs or J) from an emitting photons (or ergs or J) from an emitting volume volume VV..

At Earth, theAt Earth, the line irradiance line irradiance (or (or fluxflux) is ) is

F F / (1 A.U.)/ (1 A.U.)2 2 where 1 A.U. = 1 astronomical where 1 A.U. = 1 astronomical unit = 150 × 10unit = 150 × 1066 km. km.

Units are Units are photons cmphotons cm-2-2 s s-1-1..

Note Note SOHOSOHO is at inner Lagrangian (L1) point is at inner Lagrangian (L1) point which is 148.5 × 10which is 148.5 × 1066 km from Sun ~ 1 A.U. km from Sun ~ 1 A.U. to ~1% (therefore fluxes to within ~2%).to ~1% (therefore fluxes to within ~2%).

Lagrangian points: an Lagrangian points: an aside!aside!

There are 5 Lagrangian points for the There are 5 Lagrangian points for the Earth’s motion round the Sun.Earth’s motion round the Sun.

3 of these points are on the Sun—3 of these points are on the Sun—Earth line, L1 (inner), L2 and L3 are Earth line, L1 (inner), L2 and L3 are beyond the Earth and Sun.beyond the Earth and Sun.

L1 is L1 is NOTNOT the centre-of-mass point!! the centre-of-mass point!!

It is defined by the balance of the It is defined by the balance of the centripetal acceleration (vcentripetal acceleration (v22/r) and the /r) and the net gravitational acceln. of the Sun net gravitational acceln. of the Sun and Earth.and Earth.

Hinode/EIS spectrum from quiet Hinode/EIS spectrum from quiet SunSun

Fe XII

Fe XI

He II

CHIANTI: spectral synthesisCHIANTI: spectral synthesis

Some strong lines in EIS Some strong lines in EIS spectraspectra

Fe IX 171.0 Fe IX 171.0 ÅÅ Transition 3pTransition 3p66 – 3p – 3p55 3d. 3d. A “resonance” line. Near edge of EIS A “resonance” line. Near edge of EIS channel 1, so always weak.channel 1, so always weak.

Fe X 174.8 ÅFe X 174.8 Å Transition 3pTransition 3p55 – 3p – 3p44 3d. 3d.

Fe XI 180.4 Å Fe XI 180.4 Å Transition 3pTransition 3p44 – 3p – 3p33 3d. 3d.

Fe XII 195.1 Å Fe XII 195.1 Å Transition 3pTransition 3p33 – 3p – 3p22 3d. 3d. Main contributor to Main contributor to TRACE/EIT/STEREO “195” channel.TRACE/EIT/STEREO “195” channel.

He II 256.3 Å He II 256.3 Å Transition 1s – 3p (Ly-Transition 1s – 3p (Ly-ββ), ), emitted in chromosphere. emitted in chromosphere.

Typical X-ray stellar Typical X-ray stellar spectraspectra

Sun-like stars with hot coronae

Age

Degree of activity

Solar flare X-ray Solar flare X-ray spectrumspectrum

What do UV and X-ray What do UV and X-ray spectra tell us?spectra tell us?

TemperaturesTemperatures ((TTee) or distribution ) or distribution of material with of material with TTe e differential differential emission measureemission measure

DensitiesDensities ((NNee))

PlasmaPlasma flowsflows andand turbulenceturbulence

ElementElement abundancesabundances

Temperatures from spectral Temperatures from spectral line ratiosline ratios

Several temperature “diagnostics”, Several temperature “diagnostics”, including including lines from different ions of lines from different ions of same element same element (e.g. Fe XXVI/Fe XXV (e.g. Fe XXVI/Fe XXV lines in solar flare X-ray spectra)lines in solar flare X-ray spectra)

Lines of same ion with different Lines of same ion with different excitation energies excitation energies (e.g. Ly-(e.g. Ly-αα/Ly-/Ly-ββ of of H-like ions)H-like ions)

For solar flares, “For solar flares, “dielectronic satellitedielectronic satellite” ” lines/nearby resonance X-ray lines lines/nearby resonance X-ray lines often used.often used.

P78-1 Fe XXV solar flare P78-1 Fe XXV solar flare spectraspectra

Satellite j / res. line w = f(Te)

w = 1s2 – 1s2p

j = 1s2 2p – 1s2p2

Doschek et al. (1980)

Flare peak

Flare decay

Electron densities from line Electron densities from line emission: O VII linesemission: O VII lines

OVII Spectrum line ratio

R = I(22.1Å)/I(21.8Å)

Level diagram

R = line m-1 / line 3-1

O VII ratios in a solar O VII ratios in a solar flareflare

SOLEX spectra from P76-1 spacecraft: Doschek et al. (1981)

O VII lines in AB DoradusO VII lines in AB Doradus

XMM-Newton RGS spectra of rapidly rotating K main sequence star

AB Dor (Güdel et al. 2001)

Case of CapellaCase of Capella

CapellaCapella: spectroscopic binary - two G-type : spectroscopic binary - two G-type giants, separation 157 solar radii, period giants, separation 157 solar radii, period ~100 days.~100 days.

O VII triplet in Chandra spectra – O VII triplet in Chandra spectra – NNee = 2 x 10 = 2 x 101010 cmcm-3-3. .

EM (for both stars) is EM (for both stars) is NNee2 2 V ~ 10V ~ 105252 cm cm-3-3..

Volume ~Volume ~ 2.5 x 102.5 x 103131 cm cm33, x100 less than solar , x100 less than solar corona, 10corona, 1055 less than the vol. of each star. less than the vol. of each star.

If uniform, coronae of two stars have depth If uniform, coronae of two stars have depth ~ ~ 30 km30 km

Maybe coronae are made up of lots of small Maybe coronae are made up of lots of small loops.loops.

Capella’s “Quiescent” X-ray Capella’s “Quiescent” X-ray Spectrum (8 – 20 Spectrum (8 – 20 Å)Å)

Capella spectrum is like a low-temperature (4-6MK) solar flare

Solar flare in 1980

Quiescent Capella spectrum

FeFe+13+13 level diagram level diagram

Density-dependent line Density-dependent line ratios in the UV, EUV & X-ratios in the UV, EUV & X-

ray regionsray regions

4d level

3p levels

59.0 Å59.6 Å

5303 Å

Ratio of Fe XIV X-ray lines,I (59.6)/I (59.0) = function of Ne

Turbulence and flowsTurbulence and flows

Turbulent events (TE) and jets observed by HRTS in the UV from solar transition region lines. Brueckner & Bartoe (1983)

Note: SUMER results suggest they are the same. Velocities up to 400 km/s.

X-ray lines at the start of X-ray lines at the start of solar flares: upflows and solar flares: upflows and

turbulenceturbulence

Ca XIX lines formed at ~10 MK, seen at the “impulsive” stage of a solar flare in 1980 with the BCS on SMM.

Upflow vels. ~ 200 km/s

Spectrum from stationary plasma

Spectrum from upflowing plasma

Element abundances: the Element abundances: the “FIP” effect“FIP” effect

In the solar atmosphere, the abundances of In the solar atmosphere, the abundances of Mg, Al, Si, K, Ca, Fe are Mg, Al, Si, K, Ca, Fe are enhancedenhanced relative relative to the photosphere – up to x 4.to the photosphere – up to x 4.

These elements all haveThese elements all have low first ionization low first ionization potentialspotentials, FIPs (< 10 eV), FIPs (< 10 eV)

Possibly aPossibly a magnetic/electric field magnetic/electric field mechanism mechanism which takes the partially which takes the partially ionized material of photosphere into the ionized material of photosphere into the corona as it rises.corona as it rises.

For some or even most active stars, there is For some or even most active stars, there is an an “inverse” “inverse” FIP effect (O is v. abundant). FIP effect (O is v. abundant).

FIP bias = FIP bias = coronal/photospheric coronal/photospheric abundances vs. FIPabundances vs. FIP

Element AbundancesElement Abundances

K abundance from He-like K (K XVII) line in RESIK X-ray solar flare spectra (Sylwester et al. 2004)

With atomic parameters describing the line, the element abundance can be deduced from line flux measurements

SummarySummary

The Sun and flare (“active”) stars have The Sun and flare (“active”) stars have hot coronaehot coronae and produce and produce flaresflares

Their energetic atmospheres are the Their energetic atmospheres are the source of source of X-ray and UV emissionX-ray and UV emission

UV and X-ray spectroscopy tells us UV and X-ray spectroscopy tells us some parameters describing their some parameters describing their atmospheres: atmospheres: temperatures, temperatures, densities, flows, and element densities, flows, and element abundances abundances