36 research outputs found
Towards a long-term record of solar total and spectral irradiance
The variation of total solar irradiance (TSI) has been measured since 1978
and that of the spectral irradiance for an even shorter amount of time.
Semi-empirical models are now available that reproduce over 80% of the measured
irradiance variations. An extension of these models into the more distant past
is needed in order to serve as input to climate simulations. Here we review our
most recent efforts to model solar total and spectral irradiance on time scales
from days to centuries and even longer. Solar spectral irradiance has been
reconstructed since 1947. Reconstruction of solar total irradiance goes back to
1610 and suggests a value of about 1-1.5 Wm for the increase in the
cycle-averaged TSI since the end of the Maunder minimum, which is significantly
lower than previously assumed but agrees with other modern models. First steps
have also been made towards reconstructions of solar total and spectral
irradiance on time scales of millennia
Eclipses observed by LYRA - a sensitive tool to test the models for the solar irradiance
We analyze the light curves of the recent solar eclipses measured by the
Herzberg channel (200-220 nm) of the Large Yield RAdiometer (LYRA) onboard
PROBA-2. The measurements allow us to accurately retrieve the center- to-limb
variations (CLV) of the solar brightness. The formation height of the radiation
depends on the observing angle so the examination of the CLV provide
information about a broad range of heights in the solar atmosphere. We employ
the 1D NLTE radiative transfer COde for Solar Irradiance (COSI) to model the
measured light curves and corresponding CLV dependencies. The modeling is used
to test and constrain the existing 1D models of the solar atmosphere, e.g. the
temperature structure of the photosphere and the treatment of the pseudo-
continuum opacities in the Herzberg continuum range. We show that COSI can
accurately reproduce not only the irradiance from the entire solar disk, but
also the measured CLV. It hence can be used as a reliable tool for modeling the
variability of the spectral solar irradiance.Comment: 19 pages, 9 figures, Solar Physic
Search for Short-Term Periodicities in the Sun's Surface Rotation: A Revisit
The power spectral analyses of the Sun's surface equatorial rotation rate
determined from the Mt. Wilson daily Doppler velocity measurements during the
period 3 December 1985 to 5 March 2007 suggests the existence of 7.6 year, 2.8
year, 1.47 year, 245 day, 182 day and 158 day periodicities in the surface
equatorial rotation rate during the period before 1996.
However, there is no variation of any kind in the more accurately measured
data during the period after 1995. That is, the aforementioned periodicities in
the data during the period before the year 1996 may be artifacts of the
uncertainties of those data due to the frequent changes in the instrumentation
of the Mt. Wilson spectrograph. On the other hand, the temporal behavior of
most of the activity phenomena during cycles 22 (1986-1996) and 23 (after 1997)
is considerably different. Therefore, the presence of the aforementioned
short-term periodicities during the last cycle and absence of them in the
current cycle may, in principle, be real temporal behavior of the solar
rotation during these cycles.Comment: 11 pages, 6 figures, accepted for publication in Solar Physic
Solar cycle variations in the growth and decay of sunspot groups
We analysed the combined Greenwich (1874-1976) and Solar Optical
Observatories Network (1977-2011) data on sunspot groups. The daily rate of
change of the area of a spot group is computed using the differences between
the epochs of the spot group observation on any two consecutive days during its
life-time and between the corrected whole spot areas of the spot group at these
epochs. Positive/negative value of the daily rate of change of the area of a
spot group represents the growth/decay rate of the spot group. We found that
the total amounts of growth and decay of spot groups whose life times > or = 2
days in a given time interval (say one-year) well correlate to the amount of
activity in the same interval. We have also found that there exists a
reasonably good correlation and an approximate linear relationship between the
logarithmic values of the decay rate and area of the spot group at the first
day of the corresponding consecutive days, largely suggesting that a
large/small area (magnetic flux) decreases in a faster/slower rate. There
exists a long-term variation (about 90-year) in the slope of the linear
relationship. The solar cycle variation in the decay of spot groups may have a
strong relationship with the corresponding variations in solar energetic
phenomena such as solar flare activity. The decay of spot groups may also
substantially contribute to the coherence relationship between the total solar
irradiance and the solar activity variations.Comment: 12 pages, 7 figures, Accepted for publication in Astrophysics & Space
Science. arXiv admin note: substantial text overlap with arXiv:1105.106
Tests of sunspot number sequences: 1. Using ionosonde data
More than 70 years ago it was recognised that ionospheric F2-layer critical frequencies [foF2] had a strong relationship to sunspot number. Using historic datasets from the Slough and Washington ionosondes, we evaluate the best statistical fits of foF2 to sunspot numbers (at each Universal Time [UT] separately) in order to search for drifts and abrupt changes in the fit residuals over Solar Cycles 17-21. This test is carried out for the original composite of the Wolf/Zürich/International sunspot number [R], the new “backbone” group sunspot number [RBB] and the proposed “corrected sunspot number” [RC]. Polynomial fits are made both with and without allowance for the white-light facular area, which has been reported as being associated with cycle-to-cycle changes in the sunspot number - foF2 relationship. Over the interval studied here, R, RBB, and RC largely differ in their allowance for the “Waldmeier discontinuity” around 1945 (the correction factor for which for R, RBB and RC is, respectively, zero, effectively over 20 %, and explicitly 11.6 %). It is shown that for Solar Cycles 18-21, all three sunspot data sequences perform well, but that the fit residuals are lowest and most uniform for RBB. We here use foF2 for those UTs for which R, RBB, and RC all give correlations exceeding 0.99 for intervals both before and after the Waldmeier discontinuity. The error introduced by the Waldmeier discontinuity causes R to underestimate the fitted values based on the foF2 data for 1932-1945 but RBB overestimates them by almost the same factor, implying that the correction for the Waldmeier discontinuity inherent in RBB is too large by a factor of two. Fit residuals are smallest and most uniform for RC and the ionospheric data support the optimum discontinuity multiplicative correction factor derived from the independent Royal Greenwich Observatory (RGO) sunspot group data for the same interval
Detection of Solar Rotational Variability in the LYRA 190 - 222 nm Spectral Band
We analyze the variability of the spectral solar irradiance during the period
from 7 January, 2010 until 20 January, 2010 as measured by the Herzberg channel
(190-222 nm) of the Large Yield RAdiometer (LYRA) onboard PROBA2. In this
period of time observations by the LYRA nominal unit experienced degradation
and the signal produced by the Herzberg channel frequently jumped from one
level to another. Both these factors significantly complicates the analysis. We
present the algorithm which allowed us to extract the solar variability from
the LYRA data and compare the results with SORCE/SOLSTICE measurements and with
modeling based on the Code for the Solar Irradiance (COSI)