592 research outputs found
ACRIM-gap and total solar irradiance revisited: Is there a secular trend between 1986 and 1996?
A gap in the total solar irradiance (TSI) measurements between ACRIM-1 and
ACRIM-2 led to the ongoing debate on the presence or not of a secular trend
between the minima preceding cycles 22 (in 1986) and 23 (1996). It was recently
proposed to use the SATIRE model of solar irradiance variations to bridge this
gap. When doing this, it is important to use the appropriate SATIRE-based
reconstruction, which we do here, employing a reconstruction based on
magnetograms. The accuracy of this model on months to years timescales is
significantly higher than that of a model developed for long-term
reconstructions used by the ACRIM team for such an analysis. The constructed
`mixed' ACRIM - SATIRE composite shows no increase in the TSI from 1986 to
1996, in contrast to the ACRIM TSI composite.Comment: 4 figure
Solar total irradiance in cycle 23
The apparently unusual behaviour of the TSI during the most recent minimum of
solar activity has been interpreted as evidence against solar surface magnetism
as the main driver of the secular change in the TSI. We test claims that the
evolution of the solar surface magnetic field does not reproduce the observed
TSI in cycle 23. We use sensitive, 60-minute averaged MDI magnetograms and
quasi-simultaneous continuum images as an input to our SATIRE-S model and
calculate the TSI variation over cycle 23, sampled roughly twice-monthly. The
computed TSI is then compared to the PMOD composite of TSI measurements and to
the data from two individual instruments, SORCE/TIM and UARS/ACRIM II, that
monitored the TSI during the declining phase of cycle 23 and over the previous
minimum in 1996, respectively. Excellent agreement is found between the trends
shown by the model and almost all sets of measurements. The only exception is
the early, i.e. 1996 to 1998, PMOD data. Whereas the agreement between the
model and the PMOD composite over the period 1999-2009 is almost perfect, the
modelled TSI shows a steeper increase between 1996 and 1999 than implied by the
PMOD composite. On the other hand, the steeper trend in the model agrees
remarkably well with the ACRIM II data. A closer look at the VIRGO data, that
make the basis of the PMOD composite after 1996, reveals that only one of the
two VIRGO instruments, the PMO6V, shows the shallower trend present in the
composite, whereas the DIARAD measurements indicate a steeper trend. We
conclude that (1) the sensitivity changes of the PMO6V radiometers within VIRGO
during the first two years have very likely not been correctly evaluated, and
that (2) the TSI variations over cycle 23 and the change in the TSI levels
between the minima in 1996 and 2008 are consistent with the solar surface
magnetism mechanism
A new approach to long-term reconstruction of the solar irradiance leads to large historical solar forcing
The variable Sun is the most likely candidate for natural forcing of past
climate change on time scales of 50 to 1000 years. Evidence for this
understanding is that the terrestrial climate correlates positively with solar
activity. During the past 10,000 years, the Sun has experienced substantial
variations in activity and there have been numerous attempts to reconstruct
solar irradiance. While there is general agreement on how solar forcing varied
during the last several hundred years --- all reconstructions are proportional
to the solar activity --- there is scientific controversy on the magnitude of
solar forcing. We present a reconstruction of the Total and Spectral Solar
Irradiance covering 130 nm--10 m from 1610 to the present with annual
resolution and for the Holocene with 22-year resolution. We assume that the
minimum state of the quiet Sun in time corresponds to the observed quietest
area on the present Sun. Then we use available long-term proxies of the solar
activity, which are Be isotope concentrations in ice cores and 22-year
smoothed neutron monitor data, to interpolate between the present quiet Sun and
the minimum state of the quiet Sun. This determines the long-term trend in the
solar variability which is then superposed with the 11-year activity cycle
calculated from the sunspot number. The time-dependent solar spectral
irradiance from about 7000 BC to the present is then derived using a
state-of-the-art radiation code. We derive a total and spectral solar
irradiance that was substantially lower during the Maunder minimum than
observed today. The difference is remarkably larger than other estimations
published in the recent literature. The magnitude of the solar UV variability,
which indirectly affects climate is also found to exceed previous estimates. We
discuss in details the assumptions which leaded us to this conclusion.Comment: 9 pages, 5 figures, accepted for publication in
Astronomy&Astrophysic
Solar cycle variation in solar irradiance
The correlation between solar irradiance and the 11-year solar activity cycle
is evident in the body of measurements made from space, which extend over the
past four decades. Models relating variation in solar irradiance to
photospheric magnetism have made significant progress in explaining most of the
apparent trends in these observations. There are, however, persistent
discrepancies between different measurements and models in terms of the
absolute radiometry, secular variation and the spectral dependence of the solar
cycle variability. We present an overview of solar irradiance measurements and
models, and discuss the key challenges in reconciling the divergence between
the two
EMPIRE: A robust empirical reconstruction of solar irradiance variability
We present a new empirical model of total and spectral solar irradiance (TSI
and SSI) variability entitled EMPirical Irradiance REconstruction (EMPIRE). As
with existing empirical models, TSI and SSI variability is given by the linear
combination of solar activity indices. In empirical models, UV SSI variability
is usually determined by fitting the rotational variability in activity indices
to that in measurements. Such models have to date relied on ordinary least
squares regression, which ignores the uncertainty in the activity indices. In
an advance from earlier efforts, the uncertainty in the activity indices is
accounted for in EMPIRE by the application of an error-in-variables regression
scheme, making the resultant UV SSI variability more robust. The result is
consistent with observations and unprecedentedly, with that from other
modelling approaches, resolving the long-standing controversy between existing
empirical models and other types of models. We demonstrate that earlier
empirical models, by neglecting the uncertainty in activity indices,
underestimate UV SSI variability. The reconstruction of TSI and visible and IR
SSI from EMPIRE is also shown to be consistent with observations. The EMPIRE
reconstruction is of utility to climate studies as a more robust alternative to
earlier empirical reconstructions.Comment: J. Geophys. Res. (2017
Solar Irradiance Variability and Climate
The brightness of the Sun varies on all time scales on which it has been
observed, and there is increasing evidence that it has an influence on climate.
The amplitudes of such variations depend on the wavelength and possibly on the
time scale. Although many aspects of this variability are well established, the
exact magnitude of secular variations (going beyond a solar cycle) and the
spectral dependence of variations are under discussion. The main drivers of
solar variability are thought to be magnetic features at the solar surface. The
climate reponse can be, on a global scale, largely accounted for by simple
energetic considerations, but understanding the regional climate effects is
more difficult. Promising mechanisms for such a driving have been identified,
including through the influence of UV irradiance on the stratosphere and
dynamical coupling to the surface. Here we provide an overview of the current
state of our knowledge, as well as of the main open questions
Solar irradiance models and measurements: a comparison in the 220 nm to 240 nm wavelength band
Solar irradiance models that assume solar irradiance variations to be due to
changes in the solar surface magnetic flux have been successfully used to
reconstruct total solar irradiance on rotational as well as cyclical and
secular time scales. Modelling spectral solar irradiance is not yet as
advanced, and also suffers from a lack of comparison data, in particular on
solar-cycle time scales. Here we compare solar irradiance in the 220 nm to 240
nm band as modelled with SATIRE-S and measured by different instruments on the
UARS and SORCE satellites.
We find good agreement between the model and measurements on rotational time
scales. The long-term trends, however, show significant differences. Both SORCE
instruments, in particular, show a much steeper gradient over the decaying part
of cycle 23 than the modelled irradiance or that measured by UARS/SUSIM.Comment: 8 pages, 2 figures, conference proceedings to appear in Surveys in
Geophysic
Reconstruction of solar UV irradiance since 1974
Variations of the solar UV irradiance are an important driver of chemical and
physical processes in the Earth's upper atmosphere and may also influence
global climate. Here we reconstruct solar UV irradiance in the range 115-400 nm
over the period 1974-2007 by making use of the recently developed empirical
extension of the SATIRE models employing SUSIM data. The evolution of the solar
photospheric magnetic flux, which is a central input to the model, is described
by the magnetograms and continuum images recorded at the Kitt Peak National
Solar Observatory between 1974 and 2003 and by the MDI instrument on SoHO since
1996. The reconstruction extends the available observational record by 1.5
solar cycles. The reconstructed Ly-alpha irradiance agrees well with the
composite time series by Woods et al (2000). The amplitude of the irradiance
variations grows with decreasing wavelength and in the wavelength regions of
special interest for studies of the Earth's climate (Ly-alpha and oxygen
absorption continuum and bands between 130 and 350 nm) is one to two orders of
magnitude stronger than in the visible or if integrated over all wavelengths
(total solar irradiance)
Ca II K spectroheliograms for studies of long-term changes in solar irradiance
We address the importance of historical full disc Ca II K spectroheliograms
for solar activity and irradiance reconstruction studies. We review our work on
processing such data to enable them to be used in irradiance reconstructions.
We also present our preliminary estimates of the plage areas from five of the
longest available historical Ca II K archives.Comment: 5 pages, 3 figure
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