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NOAA operates a series of meteorology observing satellites known
as Geosynchronous Operational Environmental Satellites (GOES). Even
though the weather
pictures from GOES are seen nightly in our living rooms via
the local weather broadcast, few people know that GOES also monitors
space weather via its onboard Space Environment Monitor (SEM) system.
The three main components of space weather monitored by GOES at
35,000 Km altitude are: X-rays, energetic particles, and magnetic
field.
In 2001 GOES-12 was launched with a new X-ray instrument onboard
-- the Solar X-ray Imager (SXI). This instrument creates images
of the Sun, whereas the original XRS instrument only generated whole
disk flux measurements. See the SXI
pages for more information.
The GOES X-ray detector's primary function is to provide a sensitive
means of detecting the beginning of solar flares--explosive events
on the Sun's surface that are fueled by the intense magnetic fields
that accompany sunspots. The larger solar flares can cause massive
ejections of solar matter which reach all points in the solar system
and are measured by GOES energetic particle sensors. Solar activity
can also cause disturbances in the solar wind which can propagate
to Earth and disturb our local magnetic field. The GOES on board
magnetometer measures fluctuations near the boundary of that field
which are used to correlate with the world-wide system of ground-based
magnetometers.
Space Weather is dominated by the Sun which supplies "seasons"
in the form of the solar cycles--cycles of solar storms that erupt
with varying force and frequency over an 11-year period. Ever present
"trade winds" come in the form of galactic cosmic rays,
a wind of atomic nuclei that blows steadily from all points in the
galaxy and moves with such velocity that the nuclei penetrate everything
in their path and come to rest deep within the Earth's crust. Earth's
atmosphere and magnetosphere interact with these greater forces
in a complex manner that in some ways provides a measure of protection
from them and in others carries their effects to all forms of space-borne
and ground based technologies and biological systems.
NGDC archives most of the key parameters needed to study space
weather and to increase our understanding of its physical dynamics.
The GOES SEM archive is the corner stone of that understanding and
is critical to the newly initiated National
Space Weather Program --a interagency program to provide timely
and reliable space environment observations and forecasts. Current
Space Weather forecasts are inadequate because techniques are based
on statistics rather than an understanding of how Space Weather
really works. Meteorologists have demonstrated that significant
improvements in forecasting come when timely and relevant measurements
are used in numerical models based on sound physical principles.
US business partners in the National Space Weather Program include
operators and manufacturers of satellite systems, electrical power
systems, navigation systems, communication systems and manned space
flight systems.
The data are transmitted via direct telemetry to the Space Environment
Center (SEC) in Boulder, Colorado where they are use in real-time
alerts and space weather forecasts. At the end of each month
these data are transferred to the Solar-Terrestrial Physics Division
of the National Geophysical Data Center, an organization known internationally
as World Data Center A for Solar-Terrestrial Physics.
A New GOES Platform
NOAA began making geosynchronous weather observations in July 1974.
The initial series of satellites maintained attitude control by
spinning. With the advent of GOES-8, launched in 1995, the basic
platform design was changed to one called "3-axis stabilized."
This required changes in all of the GOES data collection systems.
The data descriptions below mention details of the "spinning"
SEM because it applies to data collected prior to GOES-8.
X-ray Sensor (XRS)
Ion chamber detectors provide whole-sun X-ray fluxes for the 0.5-to-4
and 1-to-8 Angstrom wavelength bands. These observations provide
a sensitive means of detecting the start of solar flares. Two bands
are measured to allow the hardness of the solar spectrum to be estimated.
X-ray photons pass through a collimator which defines the view aperture,
followed by a thin metallic window which defines the low energy
threshold, before entering the ion chamber. The XRS viewing direction
is in the meridian of the spacecraft spin axis. Dynamic positioning
of the XRS elevation provides for maintaining the sun in the swept
field. The X-ray emission of the sun is determined once during each
spin. The spin period is 0.6 seconds and the data for both bands
are given in Watts/cm2 sec. The full archive has 3.06 second time
resolution.
Particle Sensor (EPS)
Solid-state detectors with pulse-height discrimination measure
proton, alpha-particle, and electron fluxes. The look direction
of the EPS is perpendicular to the GOES spin axis which is approximately
aligned with Earth's rotation axis. Since the satellite spin period,
0.6 seconds, is much shorter than the accumulation times, the EPS
provides a spin-averaged estimate of the local high-pitch-angle
particle fluxes. The integral electron channel is given in units
of count/cm2 sec sr while the other channels are given in count/cm2
sec sr MeV at the average energy. Because GOES spacecraft travel
in a geostationary orbit, the E1 and P1 channels are responding
primarily to trapped outer-zone particles. The P2 channel may occasionally
respond to trapped particles during magnetically disturbed conditions.
The geomagnetic cutoff at geostationary orbit is typically of the
order of a few MeV as indicated by the lack of trapped P2 response
except as noted above. Therefore, the remaining proton and alpha
particle channels measure fluxes originating outside the magnetosphere
-- from the sun or the heart of the Galaxy.
Magnetometer
A twin-fluxgate spinning sensor allows Earth's magnetic field to
be described by three mutually perpendicular components: HP, HE
and HN. HP is parallel to the satellite spin axis, which is itself
perpendicular to the satellite's orbital plane. HE lies parallel
to the satellite-Earth center line and points earthward. HN is perpendicular
to both HP and HE, and points westward for SMS-1, SMS-2, GOES-1,
GOES-2, GOES-3, and GOES-4, and eastward for later spacecraft. HE
and HN are deconvoluted from the transverse component HT. Field
strength changes as small as 0.2 nanoTesla can be measured.
The magnetometer samples the field every 0.75 seconds. Four of
these values constitute a frame and are sent to the ground station
together. For data from GOES-3 or earlier spacecraft, the high and
low values in the frame were thrown out and the remaining value
closest to the previous frame's value is recorded. For data from
GOES-5 and later spacecraft the high and low values in the frame
are thrown out and the average of the two remaining values is recorded.
No record is kept of which of the four values are used in the archive.
Data availability
In addition to the online data plotting capabilities, we offer
these data in a variety of resolutions and media. Full-resolution
data can be prepared on custom CD-ROMs for any period of interest
between 1974 and the present. Five and one-minute averages are also
available on CD-ROM or diskette but the time coverage is limited
to 1986 to the present.
If you have questions about ordering data offline please contact
Dan Wilkinson at dwilkinson@ngdc.noaa.gov
Acronymns:
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