- Email: [email protected]
Electron pressure in an active region observed by SERTS
A& Space Res. Vol. 17, No. 4/5, pp. (4/5)2C9+/5)21 I, 1996 copyright @ 1995 COSPAR Printedin Great Britain All rights reserved. 0273-t 177/96 $9.50 + 0.00
Pergamon
0273-l 177(95)00570-6
ELECTRON PRESSURE IN AN ACTIVE REGION OBSERVED BY SERTS B. N. Dwivedi* Max-Planck-Institutfr
Aeronomie, D-37189 Katlenburg-Linduu, Germany
ABSTRACT The electron pressure in the active region AR 5464, at S18 W45, has been studied with the help of emission lines from the Si VIII ion observed by the Solar EUV Rocket Telescope and Spectrograph (SERTS) on 5 May 1989. The EUV line emissivity ratios have been computed as a function of electron density and also at, a.11appropriate elect,ron pressure in the emitsting region. The electron pressure in the active region, from theoretical line ratio curves and wit,11 the help of t,he SERTS observat,ion, is discussed.
INTRODUCTION ObservaGons in the EUV spectral region present a rich source of information on t,he physical conditions of t,he emitting sources. Such observat,ions have clearly shown that, the solar a.tmosphere has a complex st,ructure, having no spherical symmetry. The inhomogeneous solar atmosphere covering wide ranges in density and temperature, therefore, poses a serious problem in the ii~ter~retation of observational data. Density-sensitive and temperature-sensitive line pairs give inforrnatioil on the avera.ge density and temperature structure independent of t,he emitt,ing volume. The observed intensit,y of emission lines supposedly comes from severa. emitting layers, each layer having different density and temperature but the electron pressure being almost constant in each emit,ting layer or change insignificantly from layer to layer. It is, therefore, physically meaningful to st,udy the variation of theoretical line ratios with electron density (and thus temperature) at8 constant electron pressure. The comparison of theoretical line ratio curves with the observed line intensity ratios will then be indicative of the effective values of elect,ron density and temperature in the emitting source. Thomas and Neupert /l/ have presented wavelengths and absolute line int,ensit,ies, averaged over an active region, observed by the Solar EUV Rocket Telescope and Spectrogra,ph (SERTS) on 5 May 1989. For this initial catalogue, the spectra have been spatially averaged over a field-of-view of 7 QTCS~Cx 4.6 arcmin cutting through the active region AR 5464 at Sf8 W45. Wavelengtl~ coverage was 170-450 A with a spectral resolving power approaching 10000. The absolute line int,ensities reported by Thomas and Neupert /I/ h ave been used to study t,he density and t,emperature st,ructure in the active region /2,3/. The investigation of EUV line diagnostics of several ions and their relative element abundances, observed by the SERTS, is underway. We present here only the Si VIII (which has its peak ionic concentration at a telIlperature of 8 x lo5 li according to the ionisation equilibrium calculations of Arnaud and Rothen~ug /4/) line emissivit.y rat,ios as a function of electron density and also at an appropriate elect,ron pressure paramet,er and study t,he electron density and temperature structure in t,he active region using its EUV spectrum from t,he SERTS. *Permanent
Address:
Dept.
of Applied
Physics,
Banaras
Hindu University,
(4/5)209
Varanasi-221005,
India
(4/5)210
B.N.Dwivedi
LINE EMISSION
AND ATOMIC
MODEL
Taking account of atomic excitation mechanisms, the line emissivity, time, for an optically thin spectral line is given by the expression:
E(Xij) = Nj
Aji;
per unit volume,
per unit
(j > i)
*3
where Aji is the spontaneous radiative transition probability, h is Pla,nck’s constant, c is the velocity of light and &j is the wavelength for the tra.nsition i - j. Nj is the number density of the upper level j which is parametrised as Nj(X+‘)
Nj(X+‘)
=
N(Xtp)
l\'(X)
N(H)
*N(X)-N(H).lV,.N’
N(X+p)
Here N(X+P)/N(X) is the ionisation ratio of the ion Xtn relative to the total number density of the element X and is primarily a function of the electron temperature (for low electron densities); N(X)/N(H) is the element abundance which may or may not be constant in the solar atmosphere; N(H)/Ne is the hydrogen abundance which is usually assumed to be 0.8 for a fully ionised plasma; N, is the electron number density and Nj(X+P)/N(X+P) is the population of level j relative to the total number density of the ion X +P which is a function of electron density and tempera,ture and is determined by taking account of atomic excitation mechanisms, and solving the detailed balance equations for the ion. In the case of two lines emitted from the same ion, the line emissivity ratio can be expressed as -4&j) c(Akl)
=
Aji
/\kl
Nj(X+P)
Xij
Nl(X+P)
(3)
_.A.
Alk
The first 13 lowermost energy levels of the Si VIII ion have been considered. The radiative probabilities and collision strengths have been taken from Bhatia. a.nd Ma.son /5/. RESULTS
transition
AND DISCUSSION
Density-sensitive Si VIII line emissivity ratios that are suitable for a.ctive region diagnostics and well observed by the SERTS are 276.850/314.345, 276.850/316.220, and 276.850/319.839 /2,3/. In Figure 1, we have plotted line ratios as a function of electron density at T,,, = 8 x lo5 K. Also plotted on the theoretical curves in Figure 1 are the line intensity ratios in the active region spectrum together with error limits in the value of the ratios. The inferred densities from these line ratios are found to be 3 x lOlo, 4.5 x lOlo and 8 x 10” c,mm3. At I’,,, = 8 x lo5 li, these densities correspond to electron pressures of 2.4 x 1016, 3.6 x 10m and 6.4 x 1Om cmm3 K, respectively. It is, therefore, more appropriate to study these line ratios under the assumption of an appropriate electron pressure in the emitting source. Accordingly, we have computed these line emissivity ratios at N, T, = 4 x 10m cmb3 K, under the consta.nt electron pressure assumption. They are shown in Figure 2. The circles and error bars in this figure refer to the line intensity ratios in the active region spectrum observed by SERTS. The inferred electron densities a,nd temperatures from this analysis are presented in Table 1. TABLE
1 Electron densities and tempera.tures inferred from Si VIII line ratios under a constant electron pressure N, T, = 4 x 10m cm13 A’ (cf. Figure 2). Line pair A
Observed line intensity ratio (R)
276.8501314.345
1.21
3.2 x 10”
1.3 x lo6
276.850/316.220
0.74
3.0 x 10’0
1.3 x 106
276.850/319.839
0.58
8.9 x lOlo
4.5 x lo5
N,(cm-3)
T, (Ii)
Electron Pressure in an Active Region Observed by SERTS
(4/5)211
a l.O-
0.8 0.6-
0.6
0.6-
9.8
10.6
10.2
log Ne
11.0
log Ne
line ratio curves Fig. 1. Si VIII theoretical as a function of electron density at T,,,,, = 8 x lo5 Ir’. The SERTS observed intensity ratios in the active region spect,rum are indicated with circles and error bars.
Fig. 2. Si VIII theoretical line ratio curves as a funct,ion of elect,ron density while keeping the elect,ron pressure constant, N, T, = 4 x lOI CTJL-~ Ii. The SERTS observed intensity rat,ios in t,he active region spect,rum are indicated with circles and error bars.
Within the error limits of about 30 % in the theoretical curves and error limits on t,he observed line intensity ratios shown on the respective curves, no definit,e conclusion can be drawn on t,he electron pressure. However, this analysis is indicat,ive of density inhomogeneity in the active region under investigation. One would, t,herefore, tend to believe that, electron pressure must va.ry to explain the observation. This small scale pressure variation could well be due t,o filamentat,ion of the plasma in the emitting volume while t,he whole plasma is in overall equilibrium including the magnetic pressure /6/. These EUV lines will be excellent,ly observed by the Coronal Diagnostic Spectrometer (CDS) instrument (in ba.nd GI 261-346 a) to be flown on boa.rd the SOHO mis-ion. ACKNOWLEDGEMENTS It is my pleasure to thank Drs. E. Marsch a.nd 1~. Wilhelm Dr. R.D. Bentley (referee) for helpful comment,s.
for their help a.nd encoura.getnents
and
REFERENCES 1. R.J. Thomas
and W.M. Neupert,
2.
B.N. Dwivedi,
3.
B.N. Dwivedi
4.
A. Arnaud
5.
A.K. Bhatia
Ast@~ys.
J. Sup$.,
91, 461 (1994).
Solar Whys., in press (1994). and A. Mohan,
and R. Rothenflug, a,nd H.E. Mason,
6. K.P. Dere, J.-D.F. (1987).
Bartoe,
Solur Phys., submitt,ed Ash-on.
Astrophys.
(1994). Suppl., 60, 425 (1985).
Mon. Not. Roy. Astron.
G.E. Brueckner,
Sot.,
190, 925 (1980).
J.W. Cook and D.G. Sacker: Solar Phys., 114, 223
Pergamon
0273-l 177(95)00570-6
ELECTRON PRESSURE IN AN ACTIVE REGION OBSERVED BY SERTS B. N. Dwivedi* Max-Planck-Institutfr
Aeronomie, D-37189 Katlenburg-Linduu, Germany
ABSTRACT The electron pressure in the active region AR 5464, at S18 W45, has been studied with the help of emission lines from the Si VIII ion observed by the Solar EUV Rocket Telescope and Spectrograph (SERTS) on 5 May 1989. The EUV line emissivity ratios have been computed as a function of electron density and also at, a.11appropriate elect,ron pressure in the emitsting region. The electron pressure in the active region, from theoretical line ratio curves and wit,11 the help of t,he SERTS observat,ion, is discussed.
INTRODUCTION ObservaGons in the EUV spectral region present a rich source of information on t,he physical conditions of t,he emitting sources. Such observat,ions have clearly shown that, the solar a.tmosphere has a complex st,ructure, having no spherical symmetry. The inhomogeneous solar atmosphere covering wide ranges in density and temperature, therefore, poses a serious problem in the ii~ter~retation of observational data. Density-sensitive and temperature-sensitive line pairs give inforrnatioil on the avera.ge density and temperature structure independent of t,he emitt,ing volume. The observed intensit,y of emission lines supposedly comes from severa. emitting layers, each layer having different density and temperature but the electron pressure being almost constant in each emit,ting layer or change insignificantly from layer to layer. It is, therefore, physically meaningful to st,udy the variation of theoretical line ratios with electron density (and thus temperature) at8 constant electron pressure. The comparison of theoretical line ratio curves with the observed line intensity ratios will then be indicative of the effective values of elect,ron density and temperature in the emitting source. Thomas and Neupert /l/ have presented wavelengths and absolute line int,ensit,ies, averaged over an active region, observed by the Solar EUV Rocket Telescope and Spectrogra,ph (SERTS) on 5 May 1989. For this initial catalogue, the spectra have been spatially averaged over a field-of-view of 7 QTCS~Cx 4.6 arcmin cutting through the active region AR 5464 at Sf8 W45. Wavelengtl~ coverage was 170-450 A with a spectral resolving power approaching 10000. The absolute line int,ensities reported by Thomas and Neupert /I/ h ave been used to study t,he density and t,emperature st,ructure in the active region /2,3/. The investigation of EUV line diagnostics of several ions and their relative element abundances, observed by the SERTS, is underway. We present here only the Si VIII (which has its peak ionic concentration at a telIlperature of 8 x lo5 li according to the ionisation equilibrium calculations of Arnaud and Rothen~ug /4/) line emissivit.y rat,ios as a function of electron density and also at an appropriate elect,ron pressure paramet,er and study t,he electron density and temperature structure in t,he active region using its EUV spectrum from t,he SERTS. *Permanent
Address:
Dept.
of Applied
Physics,
Banaras
Hindu University,
(4/5)209
Varanasi-221005,
India
(4/5)210
B.N.Dwivedi
LINE EMISSION
AND ATOMIC
MODEL
Taking account of atomic excitation mechanisms, the line emissivity, time, for an optically thin spectral line is given by the expression:
E(Xij) = Nj
Aji;
per unit volume,
per unit
(j > i)
*3
where Aji is the spontaneous radiative transition probability, h is Pla,nck’s constant, c is the velocity of light and &j is the wavelength for the tra.nsition i - j. Nj is the number density of the upper level j which is parametrised as Nj(X+‘)
Nj(X+‘)
=
N(Xtp)
l\'(X)
N(H)
*N(X)-N(H).lV,.N’
N(X+p)
Here N(X+P)/N(X) is the ionisation ratio of the ion Xtn relative to the total number density of the element X and is primarily a function of the electron temperature (for low electron densities); N(X)/N(H) is the element abundance which may or may not be constant in the solar atmosphere; N(H)/Ne is the hydrogen abundance which is usually assumed to be 0.8 for a fully ionised plasma; N, is the electron number density and Nj(X+P)/N(X+P) is the population of level j relative to the total number density of the ion X +P which is a function of electron density and tempera,ture and is determined by taking account of atomic excitation mechanisms, and solving the detailed balance equations for the ion. In the case of two lines emitted from the same ion, the line emissivity ratio can be expressed as -4&j) c(Akl)
=
Aji
/\kl
Nj(X+P)
Xij
Nl(X+P)
(3)
_.A.
Alk
The first 13 lowermost energy levels of the Si VIII ion have been considered. The radiative probabilities and collision strengths have been taken from Bhatia. a.nd Ma.son /5/. RESULTS
transition
AND DISCUSSION
Density-sensitive Si VIII line emissivity ratios that are suitable for a.ctive region diagnostics and well observed by the SERTS are 276.850/314.345, 276.850/316.220, and 276.850/319.839 /2,3/. In Figure 1, we have plotted line ratios as a function of electron density at T,,, = 8 x lo5 K. Also plotted on the theoretical curves in Figure 1 are the line intensity ratios in the active region spectrum together with error limits in the value of the ratios. The inferred densities from these line ratios are found to be 3 x lOlo, 4.5 x lOlo and 8 x 10” c,mm3. At I’,,, = 8 x lo5 li, these densities correspond to electron pressures of 2.4 x 1016, 3.6 x 10m and 6.4 x 1Om cmm3 K, respectively. It is, therefore, more appropriate to study these line ratios under the assumption of an appropriate electron pressure in the emitting source. Accordingly, we have computed these line emissivity ratios at N, T, = 4 x 10m cmb3 K, under the consta.nt electron pressure assumption. They are shown in Figure 2. The circles and error bars in this figure refer to the line intensity ratios in the active region spectrum observed by SERTS. The inferred electron densities a,nd temperatures from this analysis are presented in Table 1. TABLE
1 Electron densities and tempera.tures inferred from Si VIII line ratios under a constant electron pressure N, T, = 4 x 10m cm13 A’ (cf. Figure 2). Line pair A
Observed line intensity ratio (R)
276.8501314.345
1.21
3.2 x 10”
1.3 x lo6
276.850/316.220
0.74
3.0 x 10’0
1.3 x 106
276.850/319.839
0.58
8.9 x lOlo
4.5 x lo5
N,(cm-3)
T, (Ii)
Electron Pressure in an Active Region Observed by SERTS
(4/5)211
a l.O-
0.8 0.6-
0.6
0.6-
9.8
10.6
10.2
log Ne
11.0
log Ne
line ratio curves Fig. 1. Si VIII theoretical as a function of electron density at T,,,,, = 8 x lo5 Ir’. The SERTS observed intensity ratios in the active region spect,rum are indicated with circles and error bars.
Fig. 2. Si VIII theoretical line ratio curves as a funct,ion of elect,ron density while keeping the elect,ron pressure constant, N, T, = 4 x lOI CTJL-~ Ii. The SERTS observed intensity rat,ios in t,he active region spect,rum are indicated with circles and error bars.
Within the error limits of about 30 % in the theoretical curves and error limits on t,he observed line intensity ratios shown on the respective curves, no definit,e conclusion can be drawn on t,he electron pressure. However, this analysis is indicat,ive of density inhomogeneity in the active region under investigation. One would, t,herefore, tend to believe that, electron pressure must va.ry to explain the observation. This small scale pressure variation could well be due t,o filamentat,ion of the plasma in the emitting volume while t,he whole plasma is in overall equilibrium including the magnetic pressure /6/. These EUV lines will be excellent,ly observed by the Coronal Diagnostic Spectrometer (CDS) instrument (in ba.nd GI 261-346 a) to be flown on boa.rd the SOHO mis-ion. ACKNOWLEDGEMENTS It is my pleasure to thank Drs. E. Marsch a.nd 1~. Wilhelm Dr. R.D. Bentley (referee) for helpful comment,s.
for their help a.nd encoura.getnents
and
REFERENCES 1. R.J. Thomas
and W.M. Neupert,
2.
B.N. Dwivedi,
3.
B.N. Dwivedi
4.
A. Arnaud
5.
A.K. Bhatia
Ast@~ys.
J. Sup$.,
91, 461 (1994).
Solar Whys., in press (1994). and A. Mohan,
and R. Rothenflug, a,nd H.E. Mason,
6. K.P. Dere, J.-D.F. (1987).
Bartoe,
Solur Phys., submitt,ed Ash-on.
Astrophys.
(1994). Suppl., 60, 425 (1985).
Mon. Not. Roy. Astron.
G.E. Brueckner,
Sot.,
190, 925 (1980).
J.W. Cook and D.G. Sacker: Solar Phys., 114, 223