Total solar irradiance (shown in color) over the past three solar cycles
since 1978 adjusted to a ground-based cryogenic instrument funded by
NASA in collaboration with the National Institute of Standards and
Technology (NIST). Image Credit: Greg Kopp, LASP, University of Colorado / NASA
Total Irradiance Monitor used to measure solar irradiance on the TCTE and SORCE missions. Image Credit: LASP, University of Colorado
The SORCE satellite prior to its launch in January 2003. The satellite has
three instruments designed to measure solar irradiance and the solar
spectrum to better understand the sun's role in climate change and
another instrument to measure high-energy radiation from the sun. Image Credit: NASA
Maintaining a record of solar measurements is important in
understanding the sun's effect on Earth and the National Oceanic and
Atmospheric Administration's (NOAA), Total solar irradiance Calibration
Transfer Experiment, or TCTE, is now providing that information.
Many natural conditions on Earth such as the surface temperature or
air temperature depend on energy that comes from the sun in the form of
electromagnetic radiation. A solar cycle lasts about 11 years and
typically has modest changes in solar radiation. There are also dramatic
solar events that eject solar material, but the energy variation caused
by these particle emissions, when averaged over a year or longer, is
small compared to variations in the sun's electromagnetic radiation.
Scientists have noted these changes in the sun's energy by observing
from Earth's surface for more than a hundred years, but were only able
to begin to determine their magnitude and impact on Earth's climate with
more accurate measurements from space, starting in 1978 with
measurements of the "total solar irradiance," or TSI, made by NASA's
Nimbus 7 satellite.
It is important to continue this TSI measurement record without a
break in the data. The TCTE is designed to prevent such a break by
continuing measurements from space to determine how solar changes are
influencing Earth's climate.
A NOAA, Joint Polar Satellite System-sponsored mission, TCTE launched
aboard U.S. Air Force Space Test Program Satellite-3, Tuesday, Nov. 19,
2013, from NASA's Wallops Flight Facility in Virginia. Since launch,
TCTE successfully turned on and is transmitting information.
Solar irradiance is currently measured by the Total Irradiance
Monitor or TIM deployed in 2003 on NASA's Solar Radiation and Climate
Experiment or SORCE mission. The TIM on TCTE is one of three nearly
identical instruments built as part of NASA's investment in the Total
Irradiance Monitor deployed in 2003 on the SORCE mission.
While SORCE was designed to last for five years, it is still recording
data more than 10 years later, but the aging satellite is nearing the
end of its battery life. It is critical for the continuity of the data
stream to have both instruments overlap to allow for syncing the
measurements, allowing the new more accurate TCTE calibration to be
transferred to SORCE TIM, and then to earlier overlapping measurements,
so greatly increasing the accuracy and value of the overall TSI record.
Mission scientists hope for an overlap period of about ten days for
the two instruments in space. After that period, because there are other
instruments on the Air Force satellite, TCTE will be turned on once a
week for a few orbits of the Earth, typically lasting about an
hour-and-a-half, with a view of the sun for about 45 minutes during each
orbit.
"The basic input to the climate system is the sun. We need that basic
measurement before we can do other science," said Jeffrey Privette,
chief of Climate Services and Monitoring Division for NOAA's National
Climatic Data Center in Asheville, N.C. "The important thing is to not
lose this record. I'm very optimistic about the quality of the data and
the ability to use it," he said.
The solar irradiance measurements achieved by the TIM deployed in
2003, and its near clone as part of TCTE, are significantly more
accurate than their previous space-based predecessors, said Robert
Cahalan, project scientist for SORCE and TCTE at NASA's Goddard Space
Flight Center in Greenbelt, Md.
Prior to SORCE, the space-based solar irradiance measurements located
their precision apertures, that control exposure to the sun's light,
inside the instrument, requiring the light to travel past baffles and
other instrument surfaces before reaching that aperture, allowing some
unknown amount of sunlight to be reflected from those surfaces, and then
to enter the aperture, resulting in lost precision. "The big innovation
with the SORCE TIM is that the precision aperture is right out in
front, so the exposure is known very precisely," said Cahalan.
Even though the TIM on TCTE was built at the same time as the TIM
currently flying on SORCE, scientists now have had the advantage with
the current TIM of calibrating it with a ground-based cryogenic system
operating at very cold temperatures to establish a more accurate
measurement. This cryogenic system did not exist when the instrument on
SORCE was launched.
This innovation will benefit the previous space-based solar data
recorded since 1978. "Because we have a calibrated instrument, we will
transfer that information to the data from SORCE, allowing us to correct
the last 11 years of TIM data. And all the earlier instruments can
potentially be corrected by their instrument teams and principal
investigators," Cahalan said.
Scientists acknowledge that there is evidence that Earth's
relationship to the sun, including changes in Earth's orbit, is a cause
of some changes to the climate.. Yet over the last few decades, there
has been no significant trend in the sun's energy output, its TSI, and
over a 1,000 year timescale, Earth's orbit is remains essentially fixed
and unchanged. Known orbital changes require more than 10,000 years to
occur.
"In recent decades Earth has experienced a dramatic rise in
temperature over the planet as a whole, and as the temperature has
risen, ice has melted and the ocean has acidified (become less basic).
These changes are traceable to the same cause, namely the increasing
concentration of carbon dioxide and other greenhouse gases being emitted
from fossil fuel use, trapping heat near Earth's surface, and being
absorbed in the oceans," Cahalan said.
"During these decades, the sun's brightness (energy) has undergone
several up-and-down cycles, in sync with the sunspot cycle, but with no
overall trend that could explain Earth's temperature trend," Cahalan
said. "That doesn't mean we should stop measuring the sun. Just because
the sun hasn't significantly brightened or dimmed since 1978, doesn't
mean it won't brighten or dim between now and 2050. Even a very small
trend in the sun would either enhance the warming, if the sun were
brightening, or partially offset it, if the sun were dimming."
Related Links:
For more information about JPSS and its precursor, Suomi NPP, please visit: http://www.jpss.noaa.gov/ and http://www.nasa.gov/npp
For more information about TCTE please visit: http://lasp.colorado.edu/home/missions-projects/quick-facts-tcte/
For more information on the Total Irradiance Monitor please visit: http://lasp.colorado.edu/home/sorce/instruments/tim/
For more information on SORCE, please visit: http://science1.nasa.gov/missions/sorce/ and http://lasp.colorado.edu/home/sorce/
NASA Goddard: Audrey Haar
NASA's Goddard Space Flight Center, Greenbelt, Md.
240-684-0808
audrey.j.haar@nasa.gov
NOAA: John Leslie
NOAA Office of Communications and External Affairs
301-713-0214
john.leslie@noaa.gov
