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Near infrared solar coronal observations: A perspective for future investigations?
Phys. Chem. Earth, Vol. 22, No. 5, pp. 493-494, 1997 © 1997 Published by Elsevier Science Ltd. All rights reserved
Pergamon
Printed in Great Britain 0079-1946/97 $17.00 + 0.00 PII: S0079-1946(97)00179-1
Near Infrared Solar Coronal Observations : a Perspective for Future Investigations? I. M a n n 1 and J. K u h n 2 1Max Planck Institut for Aeronomie, D-37189 Kaltenberg Lindau, Germany 2NSO/Sacramento Peak, Sunspot, N e w Mexico, U.S.A.
Received 18 July 1996; accepted 10 December 1996
A b s t r a c t . We propose a set of observations of the near infrared coronal brightness. Such a study allows unique measurements of the near solar dust dynamics and material properties. In addition, recent suggestions for a cool gas component in the corona will be investigated by observations of the infrared neutral helium lines. These observations are difficult and require an upper atmospheric or satellite platform to reduce the noise background. © 1997 Published by Elsevier Science Ltd 1
Introduction
The measurements of the SOHO satellite provide the opportunity for remote observations of the solar corona, however they don't study the near infrared brightness of the solar corona. Recent advances in IR detector technology now make it possible to attack several longstanding coronal problems such as the analysis of the near solar dust dynamics as well as properties of dust particles, the study of neutral helium components in the outer corona and the anlysis of coronal magnetic fields. We will discuss these issues, which can be approached with aircraft measurements, but should as well be included in concepts for dedicated missions. 2
D e t e c t i o n o f N e u t r a l Helium
coronal and interplanetary medium processes. We propose observations of the HeI 1.083 micron near infrared line, which has been observed already during the 1994 eclipse (see Kuhn et al. 1996). Observation from above the lower atmosphere would not only reduce the noise background, but also would give a clear separation from geocoronal emission in the 1.083 micron line.
3
Observation of the C o n t i n u u m Brightness
The brightness of the near solar corona is predominantly produced from scattering and thermal emission of dust in the solar vicinity (Mann 1992) and along the line of sight of the observation (the F-coronal brightness). The observation of the F-corona in the near infrared has several benefits: the thermal emission brightness is naturally produced especially from hot dust particles, which means particles close to the Sun and therefore especially suitable for the analysis of the spatial distribution near the Sun. The reddening of the solar F-corona in the near infrared reveals scattering and thermal emission properties of the dust particles which are related to their structure, material composition and size distribution (Mann 1993, Mann and MacQueen 1993, Davidson et al. 1995). Again, observations from ground, even under eclipse conditions (see MacQueen and Greeley 1995) are limited by atmospheric scattered light.
The existence of a cool component in the corona results for instance from coronal mass ejections that deposit cooler material as they move outward. However, Yokkoh x-ray and HAO K-coronameter observations have not yet seen this component. Field's (1965) thermal instability calculations also suggested that a cool coronal component could exist in the 10s K coronal environment.The detection of neutral helium in the solar corona and in the solar environment is of special interest for
4
D o p p l e r Shift of Fraunhofer Lines
The F-coronal brightness indicates the spatial distribution of dust in the solar environment, but gives no direct information about the dust fluxes, which would indicate the dynamical effects the particles undergo in the solar magnetic and radiation field. The best way to analyse this is to detect the Doppler shift of Fraunhofer Lines in the scattered light component of the F-coronal brightness (cf. Mukai and Mann 1993). The near infrared
Correspondence to: I. Mann 493
494
I. Mann and J. Kuhn 4x104 FeXlll
1.0747
~o oL .U
3xlO 4
E uE ~~ 2xl 04
FeXlll
L
1.0798
Fig. 1. The mean IR coronal spectrum from 43" near the solar limb observed during the 1994 solar eclipse from Putre, Chile. The intensity calibration is based on solar disk brightness of 1.2 • 10 l° erg s - l s t e r -1 ~m -L. Besides the Si, S and He lines mentioned in the text the Fe XIII lines are prominent. The high background signal is due to atmospheric effects (from Kuhn et al. 1996).
Hel 1 . 0 8 3 0
o) E
SiX 1 . 4 . 5 0 5 Ix10 4
"E m
......... 1.00
I ,qS;T,-; 1.10
....
I ......... 1.20 Wavelength
i
I , , ~";~k 1.30 [microns]
lines give a sufficient Doppler shift and at the same time can be compared to synoptic detection of the F-coronal intensity and colour.
.......
u,
__.
,,i
1.4-0
,
, 50
considered as a long term program to study the coronal magnetic field and coronal dust dynamics. References
5
Coronal Emission Lines
Knowledge of the density and temperature of the low corona is based largely upon interpretation of the behaviour of coronal atomic emission lines. Knowledge of the near infrared coronal spectrum may yield additional tools for temperature and density diagnostics. It may also be possible to measure weak coronal magnetic fields by means of Zeeman splitting, which increases in wavelength. Observations from the ground have already detected new emission lines (Penn 1995, Kuhn et al. 1996) and revealed upper limits to coronal fields on the order of 40 G, which may already be interesting for the coronal measurements near active regions, and can be improved by nearly one order of magnitude with advanced infrared detectors (see Kuhn 1995). Currently, the reduction of the scattered light level obtained by using satellite or aircraft observations are expected to lead to the measurement of even smaller Zeemann splittings and correspondingly to smaller magnetic fields. Lines expected in the near infrared regime are transitions between high Rydberg levels in ions and forbidden transitions between low excitation levels, among which the latter seem to be the more promising source for emission lines. 6
Summary
The suggested set of observations offers an interesting analysis of both the solar corona and the interplanetary medium near the Sun. These observations are suitable not only for a single observation program but should be
Davidson, W. C., MacQueen, R. M., and Mann, I., Scattering Models of the Solar Infrared Corona, Planet. Space Sci., 43, Nos. 10/11, 1395-1400, 1995. Field, G. B., Thermal Instability, Astrophys. J., 1,12, 531, 1965. Kuhn, J. R., Infrared Coronal Magnetic Field Measurements, pp. 89-93, in Infrared Tools of Solar Astrophysics, (J. R. Kuhn and M. J. Penn, eds.), World Scientific Publ., Singapore 1995. Kuhn, J. R., Penn, M. J., and Mann, I., The Near Infrared Coronal Spectrum, Astrophys. J., Lett., 456, L67-L70, 1996. MacQueen, R. M. and Greeley, B. W., Solar Corona Dust Scattering in the Infrared, Astrophys. J., 440, 361-368, 1995. Mann, I., The Solar F-Corona: Modellings of the Optical and Infrared Brightness of Near Solar Dust, Astren. Astrophys., 261,329-335, 1992. Mann, I., The Influence of Circumsolar Dust on the Whitelight Corona - Study of the Visual F-Corona Brightness, Planet. Space Sci., 41,301-305, 1993. Mann, I. and MacQueen, R. M., The Solar F-Corona at 2.12 micron: Calculations of near solar dust in comparison to 1991 eclipse observations, Astron. Astrophys., 275, 293-297, 1993. Mukai, T. and Mann, I., Analysis of Doppler Shifts in the Zodiacal Light, Astron. Astrophys., 271, 530-534, 1993. Penn, M. J., New Observations of Infrared Coronal Emission Lines, pp. 69-74, in Infrared Tools o.f Solar Astrophysics, (J. R. Kuhn and M. J. Penn, eds.), World Scientific Publ., Singapore 1995.
Pergamon
Printed in Great Britain 0079-1946/97 $17.00 + 0.00 PII: S0079-1946(97)00179-1
Near Infrared Solar Coronal Observations : a Perspective for Future Investigations? I. M a n n 1 and J. K u h n 2 1Max Planck Institut for Aeronomie, D-37189 Kaltenberg Lindau, Germany 2NSO/Sacramento Peak, Sunspot, N e w Mexico, U.S.A.
Received 18 July 1996; accepted 10 December 1996
A b s t r a c t . We propose a set of observations of the near infrared coronal brightness. Such a study allows unique measurements of the near solar dust dynamics and material properties. In addition, recent suggestions for a cool gas component in the corona will be investigated by observations of the infrared neutral helium lines. These observations are difficult and require an upper atmospheric or satellite platform to reduce the noise background. © 1997 Published by Elsevier Science Ltd 1
Introduction
The measurements of the SOHO satellite provide the opportunity for remote observations of the solar corona, however they don't study the near infrared brightness of the solar corona. Recent advances in IR detector technology now make it possible to attack several longstanding coronal problems such as the analysis of the near solar dust dynamics as well as properties of dust particles, the study of neutral helium components in the outer corona and the anlysis of coronal magnetic fields. We will discuss these issues, which can be approached with aircraft measurements, but should as well be included in concepts for dedicated missions. 2
D e t e c t i o n o f N e u t r a l Helium
coronal and interplanetary medium processes. We propose observations of the HeI 1.083 micron near infrared line, which has been observed already during the 1994 eclipse (see Kuhn et al. 1996). Observation from above the lower atmosphere would not only reduce the noise background, but also would give a clear separation from geocoronal emission in the 1.083 micron line.
3
Observation of the C o n t i n u u m Brightness
The brightness of the near solar corona is predominantly produced from scattering and thermal emission of dust in the solar vicinity (Mann 1992) and along the line of sight of the observation (the F-coronal brightness). The observation of the F-corona in the near infrared has several benefits: the thermal emission brightness is naturally produced especially from hot dust particles, which means particles close to the Sun and therefore especially suitable for the analysis of the spatial distribution near the Sun. The reddening of the solar F-corona in the near infrared reveals scattering and thermal emission properties of the dust particles which are related to their structure, material composition and size distribution (Mann 1993, Mann and MacQueen 1993, Davidson et al. 1995). Again, observations from ground, even under eclipse conditions (see MacQueen and Greeley 1995) are limited by atmospheric scattered light.
The existence of a cool component in the corona results for instance from coronal mass ejections that deposit cooler material as they move outward. However, Yokkoh x-ray and HAO K-coronameter observations have not yet seen this component. Field's (1965) thermal instability calculations also suggested that a cool coronal component could exist in the 10s K coronal environment.The detection of neutral helium in the solar corona and in the solar environment is of special interest for
4
D o p p l e r Shift of Fraunhofer Lines
The F-coronal brightness indicates the spatial distribution of dust in the solar environment, but gives no direct information about the dust fluxes, which would indicate the dynamical effects the particles undergo in the solar magnetic and radiation field. The best way to analyse this is to detect the Doppler shift of Fraunhofer Lines in the scattered light component of the F-coronal brightness (cf. Mukai and Mann 1993). The near infrared
Correspondence to: I. Mann 493
494
I. Mann and J. Kuhn 4x104 FeXlll
1.0747
~o oL .U
3xlO 4
E uE ~~ 2xl 04
FeXlll
L
1.0798
Fig. 1. The mean IR coronal spectrum from 43" near the solar limb observed during the 1994 solar eclipse from Putre, Chile. The intensity calibration is based on solar disk brightness of 1.2 • 10 l° erg s - l s t e r -1 ~m -L. Besides the Si, S and He lines mentioned in the text the Fe XIII lines are prominent. The high background signal is due to atmospheric effects (from Kuhn et al. 1996).
Hel 1 . 0 8 3 0
o) E
SiX 1 . 4 . 5 0 5 Ix10 4
"E m
......... 1.00
I ,qS;T,-; 1.10
....
I ......... 1.20 Wavelength
i
I , , ~";~k 1.30 [microns]
lines give a sufficient Doppler shift and at the same time can be compared to synoptic detection of the F-coronal intensity and colour.
.......
u,
__.
,,i
1.4-0
,
, 50
considered as a long term program to study the coronal magnetic field and coronal dust dynamics. References
5
Coronal Emission Lines
Knowledge of the density and temperature of the low corona is based largely upon interpretation of the behaviour of coronal atomic emission lines. Knowledge of the near infrared coronal spectrum may yield additional tools for temperature and density diagnostics. It may also be possible to measure weak coronal magnetic fields by means of Zeeman splitting, which increases in wavelength. Observations from the ground have already detected new emission lines (Penn 1995, Kuhn et al. 1996) and revealed upper limits to coronal fields on the order of 40 G, which may already be interesting for the coronal measurements near active regions, and can be improved by nearly one order of magnitude with advanced infrared detectors (see Kuhn 1995). Currently, the reduction of the scattered light level obtained by using satellite or aircraft observations are expected to lead to the measurement of even smaller Zeemann splittings and correspondingly to smaller magnetic fields. Lines expected in the near infrared regime are transitions between high Rydberg levels in ions and forbidden transitions between low excitation levels, among which the latter seem to be the more promising source for emission lines. 6
Summary
The suggested set of observations offers an interesting analysis of both the solar corona and the interplanetary medium near the Sun. These observations are suitable not only for a single observation program but should be
Davidson, W. C., MacQueen, R. M., and Mann, I., Scattering Models of the Solar Infrared Corona, Planet. Space Sci., 43, Nos. 10/11, 1395-1400, 1995. Field, G. B., Thermal Instability, Astrophys. J., 1,12, 531, 1965. Kuhn, J. R., Infrared Coronal Magnetic Field Measurements, pp. 89-93, in Infrared Tools of Solar Astrophysics, (J. R. Kuhn and M. J. Penn, eds.), World Scientific Publ., Singapore 1995. Kuhn, J. R., Penn, M. J., and Mann, I., The Near Infrared Coronal Spectrum, Astrophys. J., Lett., 456, L67-L70, 1996. MacQueen, R. M. and Greeley, B. W., Solar Corona Dust Scattering in the Infrared, Astrophys. J., 440, 361-368, 1995. Mann, I., The Solar F-Corona: Modellings of the Optical and Infrared Brightness of Near Solar Dust, Astren. Astrophys., 261,329-335, 1992. Mann, I., The Influence of Circumsolar Dust on the Whitelight Corona - Study of the Visual F-Corona Brightness, Planet. Space Sci., 41,301-305, 1993. Mann, I. and MacQueen, R. M., The Solar F-Corona at 2.12 micron: Calculations of near solar dust in comparison to 1991 eclipse observations, Astron. Astrophys., 275, 293-297, 1993. Mukai, T. and Mann, I., Analysis of Doppler Shifts in the Zodiacal Light, Astron. Astrophys., 271, 530-534, 1993. Penn, M. J., New Observations of Infrared Coronal Emission Lines, pp. 69-74, in Infrared Tools o.f Solar Astrophysics, (J. R. Kuhn and M. J. Penn, eds.), World Scientific Publ., Singapore 1995.