Solar activity

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Recent visual sun

The Sun seen on 25 June 2019. Photo by courtesy of SOHO/MDI/spaceweather.com. Click here and here to read the about the last long solar minimum. Click here, here, here and here to read about predictions on the current and the next sunspot cycle, cycle 24 and 25, respectively.  

 

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Recent sunspot activity

Daily observations of the number of sunspots since 1 January 1900 according to Solar Influences Data Analysis Center (SIDC). The thin blue line indicates the daily sunspot number, while the dark blue line indicates the running annual average. The recent low sunspot activity is clearly reflected in the recent low values for the total solar irradiance. Data source: WDC-SILSO, Royal Observatory of Belgium, Brussels. Last day shown: 31 May 2019. Last diagram update: 1 June 2019.

 

 

Daily observations of the number of sunspots since 1 January 1977 according to Solar Influences Data Analysis Center (SIDC). The thin blue line indicates the daily sunspot number, while the dark blue line indicates the running annual average. The recent low sunspot activity is clearly reflected in the recent low values for the total solar irradiance. Compare also with the geomagnetic Ap-index.  Data source: WDC-SILSO, Royal Observatory of Belgium, Brussels. Last day shown: 31 May 2019. Last diagram update: 1 June 2019.

 

In 1848 the Swiss astronomer Johann Rudolph Wolf introduced a daily measurement of sunspot number. His method, which is still used today, counts the total number of spots visible on the face of the sun and the number of groups into which they cluster, because neither quantity alone satisfactorily measures sunspot activity (NOAA's National Geophysical Data Center; NGDC) .

An observer computes a daily sunspot number by multiplying the number of groups he sees by ten and then adding this product to his total count of individual spots. Results, however, vary greatly, since the measurement strongly depends on observer interpretation and experience and on the stability of the Earth's atmosphere above the observing site. Moreover, the use of Earth as a platform from which to record these numbers contributes to their variability, too, because the sun rotates and the evolving spot groups are distributed unevenly across solar longitudes. To compensate for these limitations, each daily international number is computed as a weighted average of measurements made from a network of cooperating observatories. Today, much more sophisticated measurements of solar activity are made routinely, but none has the link with the past that sunspot numbers have (NOAA's National Geophysical Data Center; NGDC) .

 

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Recent solar irradiance

Total solar irradiance since 25 February 2003, according to the Laboratory of Atmospheric and Space Physics (LASP). These data are obtained using the SORCE Total Irradiance Monitor (TIM). The thin line indicates daily TSI values, while the thick line represents the running simple 183 day (c. 6 months) average. The measured values for the total solar irradiation (TSI) are somewhat lower than reported by most other TSI-measuring instruments, which typically report TSI values around 1366 W/m2. Click here to read about the background for this difference, or see the text paragraphs below this diagram. However, the low sunspot activity 2006-2010 is clearly reflected in the TIM data provided by LASP. Last day shown: 18 June 2019. Last diagram update: 25 June 2019.

  • Click here to download the entire series of TSI data from the Laboratory of Atmospheric and Space Physics (LASP).

  • Click here to make use of an online data plot facility kindly made available by LASP.

  • Click here to read about data smoothing.

 

The SORCE Total Irradiance Monitor (TIM) measures the Total Solar Irradiance (TSI), a measure of the absolute intensity of solar radiation, integrated over the entire solar irradiance spectrum.

The TIM's measured value of TSI at 1 AU is lower than that reported by other TSI-measuring instruments; an upcoming solar minimum value of 1361 W/m2 is estimated from the above TIM data. This is due to unresolved differences between the various TSI instruments in operation. The TIM measures TSI values 4.7 W/m2 lower than the VIRGO and 5.1 W/m2 lower than ACRIM III.

This difference exceeds the ~0.1% stated uncertainties on both the ACRIM and VIRGO instruments. Differences between the various data sets are solely instrumental and will only be resolved by careful and detailed analyses of each instrument's uncertainty budget. LASP report only the TSI measurements from the TIM, and make no attempt to adjust these to other TSI data records.

The TIM TSI data available are based on fundamental ground calibrations done at CU/LASP, NIST, and NASA. On-orbit calibrations measure the effects of background thermal emission, instrument sensitivity changes, and electronic gain. The TIM TSI data products have been corrected for instrument sensitivity and degradation, background thermal emission, instrument position and velocity, and electronic gain. The TIM relies on several component-level calibrations, as no calibration source or detector is available with the level of accuracy desired for this instrument -- a level of accuracy nearly 10 times better than that previously attempted for space-based radiometry.

 

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Sunspot activity since 1700

Annual sunspot activity since 1700 according to the Solar Influences Data Analysis Center (SIDC). The blue line shows annual values, red line shows the running 11 yr average. Last year shown: 2014. Last diagram update: 4 May 2015.

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Solar irradiance and sunspot number

 

Solar irradiance and sunspot number since January 1979 according to NOAA's National Geophysical Data Center; NGDC. The thin lines indicate the daily irradiance (red) and sunspot number (blue), while the thick lines indicate the running annual average for these two parametres. The total variation in solar irradiance is about 1.3 W/m2 during one sunspot cycle, as an order of magnitude.

 

The number of sunspots correlates with the intensity of solar radiation over the period (since 1979) when satellite measurements of absolute radiative flux were available. Since sunspots are darker than the surrounding photosphere it might be expected that more sunspots would lead to less solar radiation and a decreased solar constant. However, the surrounding margins of sunspots are hotter than the average, and so are brighter; overall, more sunspots increase the sun's solar constant or brightness. 

The variation caused by the sunspot cycle to solar output is relatively small, on the order of 0.1% of the solar constant (a peak-to-trough range of 1.3 W/m2 compared to 1366 W/m2 for the average solar constant). This number refers to the projected area of planet Earth, as seen from the Sun. However, the total surface area of the planet is four times the projected area, and the variation of 1.3 W/m2 therefore corresponds to about 0.325 W/m2 for the entire planet surface. This value might be compared with the IPCC 2007 estimate of 1.6 W/m2 for the total effect of all recognized climatic drivers 1750-2006, including release of greenhouse gasses from the burning of fossil fuels.

During the Maunder Minimum in the 17th Century (c.1650-1720) there were hardly any sunspots at all, and in all likelihood, the intensity of solar radiation was low. This event coincides with a documented period of maximum cooling within the Little Ice Age.

Irradiance is the radiometry term for the power of electromagnetic radiation at a surface, per unit area. Irradiance due to solar radiation is also called insolation. Total solar irradiance describes the radiant energy emitted by the sun over all wavelengths that falls each second on 1 square meter outside the earth's atmosphere, a quantity proportional to the "solar constant" observed earlier in this century. It measures the solar energy flux in Watts/square meter.

 

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