Detalles de publicación
PP 07001
Properties of sunspots in cycle 23 I. Dependence of brightness on sunspot size and cycle phase
(1) Udaipur Solar Observatory, P. O. Box 198, Udaipur - 313004, India
(2) Instituto de Astrofisica de Canarias, La Laguna, Tenerife, Spain
(3) Max-Planck-Institute for Solar System Research, 37191 Katlenburg-Lindau, Germany
In this paper we investigate the dependence of umbral core brightness, as well as
the mean umbral and penumbral brightness on the phase of the solar cycle and on the
size of the sunspot. Albregtsen & Maltby (1978) reported an increase in umbral core brightness from the
early to the late phase of solar cycle from the analysis of 13 sunspots which cover
solar cycles 20 and 21. Here we revisit this topic by analysing continuum
images of more than 160 sunspots observed by the MDI instrument on board the
SOHO spacecraft for the period between 1996 June to 2004 March, i.e. a sizable
part of solar cycle 23. The advantage of this data set is its homogeneity,
with no seeing fluctuations. A careful stray light correction, which is validated
using the Mercury transit of 7th May, 2003, is carried out before the umbral and
penumbral intensities are determined. The influence of the Zeeman splitting of the
nearby NiI spectral line on the measured ``continuum''
intensity is also taken into account.
We did not observe any significant variation
in umbral core, mean umbral and mean penumbral intensities with solar cycle,
which is in contrast to earlier findings for the umbral
core intensity. We do find a strong and clear dependence of the umbral brightness
on sunspot size, however. The penumbral brightness also displays a weak dependence.
The brightness-radius relationship has numerous implications, some of which, such as those for
the energy transport in umbrae, are pointed out.
the mean umbral and penumbral brightness on the phase of the solar cycle and on the
size of the sunspot. Albregtsen & Maltby (1978) reported an increase in umbral core brightness from the
early to the late phase of solar cycle from the analysis of 13 sunspots which cover
solar cycles 20 and 21. Here we revisit this topic by analysing continuum
images of more than 160 sunspots observed by the MDI instrument on board the
SOHO spacecraft for the period between 1996 June to 2004 March, i.e. a sizable
part of solar cycle 23. The advantage of this data set is its homogeneity,
with no seeing fluctuations. A careful stray light correction, which is validated
using the Mercury transit of 7th May, 2003, is carried out before the umbral and
penumbral intensities are determined. The influence of the Zeeman splitting of the
nearby NiI spectral line on the measured ``continuum''
intensity is also taken into account.
We did not observe any significant variation
in umbral core, mean umbral and mean penumbral intensities with solar cycle,
which is in contrast to earlier findings for the umbral
core intensity. We do find a strong and clear dependence of the umbral brightness
on sunspot size, however. The penumbral brightness also displays a weak dependence.
The brightness-radius relationship has numerous implications, some of which, such as those for
the energy transport in umbrae, are pointed out.