Evidence of a long-term trend in total solar irradiance

Abstract

Aims. During the solar minimum of 2008, the value of total solar irradiance at 1 AU (TSI) was more than 0.2 Wm −2 lower than during the last minimum in 1996, indicating for the first time a directly observed long-term change. On the other hand, chromospheric indices and hence solar UV irradiance do not exhibit a similar change. Methods. Comparison of TSI with other activity parameters indicates that only the open solar magnetic field, B R , observed from satellites at 1 AU show a similar long-term behaviour. The values at the minima correlate well and the linear fit provides a direct physical relationship between TSI and B R during the minimum times.Results. This correlation allows an unambiguous reconstruction of TSI back in time, provided the open solar magnetic field can be determined from e.g. geomagnetic indices or cosmogenic radionucleides. Since the solar UV irradiance has no long-term trend, the mechanism for the secular change of TSI must differ from the effect of surface magnetism, as manifested by sunspots, faculae, and network which indeed explain well the intra-cycle variability of both total and spectral irradiance. Conclusions. The long-term trend of TSI is most probably caused by a global temperature change of the Sun that does not influence the UV irradiance in the same way as the surface magnetic fields.

Total solar irradiance since 1996: is there a long-term variation unrelated to solar surface magnetic phenomena?

“…The PMOD composite includes three periods of high solar activity called solar maxima (around the years 1980, 1991, 2002) and low solar activity called solar minima (around the years 1986, 1996, 2009). While TSI during the first two solar minima (1986 and 1996) was practically constant, it dropped significantly from the minimum level as observed in 1996 by about 0.2 W m 2 (140 ppm) in the solar minimum of 2009 (Fröhlich 2009a).…”

“…IRMB composite (Institut Royal Météorologique Belgique) (Dewitte et al 2004), 3. PMOD composite (PhysikalischMeteorologisches Observatorium Davos) (Fröhlich 2006) As is discussed in Fröhlich (2006Fröhlich ( , 2009a, the ACRIM and the IRMB composites show unreasonable behavior in time, whereas the PMOD composite does not. Therefore we restrict ourselves in this study on the PMOD composite.…”

“…Because VIRGO has been in space over a relative long period, the observed drop in the TSI could be due to aging of the VIRGO instrument. However, this is not the case as emphasized in Fröhlich (2009a). The author compares the ratio of TSI measured by VIRGO with TSI measured by TIM (Kopp & Lawrence 2005) on-board the NASA Earth Observing System Solar Radiation and Climate Experiment.…”

Solar Influence on Global and Regional Climates

“…This relationship is more indirect than that with GCR flux in that F S is a small, and potentially variable, fraction of the field that threads the photosphere and thereby modulates the visible/IR emission of Sun (via the darkening effects of sunspots and the brightening effect of smaller magnetic flux tubes called faculae: Solanki et al 2005;Wenzler et al 2006). F S correlates well with TSI over the solar cycle (Lockwood and Stamper 1999;Lockwood 2002), and recent studies indicate that observed TSI does also reflect some of the longer-term drift in F S (Fröhlich 2009). Modelling of the long-term variations (since the Maunder Minimum) in F S and TSI Wang et al 2005) provides support for the broad correlation between the two.…”

Comparison of the Past Two Solar Minima from the Perspective of the Interior Dynamics and Dynamo of the Sun

“…It has also been a deep minimum; the lowest ssn was 1.7, whereas the lowest ssn at the end of cycle 22 was 12.6. M. Dikpati ( ) HAO/NCAR, 3080 Center Green, Boulder, CO 80301, USA e-mail: dikpati@hao.ucar.edu The effect of this unusually long, deep minimum has been observed in various other solar cycle indices, such as in f10.7 radio flux, total solar irradiance (TSI) (Fröhlich 2009;Domingo et al 2009), atmospheric neutral density (Solomon et al 2010), as well as in the trend of solar p-mode frequency (Tripathy et al 2010) and torsional oscillation (Howe et al 2009). These unusual features observed in cycle 23 are most likely due to the dynamo operating in a somewhat different fashion compared to the previous cycles.…”