Background on Human Observations
Humans have studied the Sun with telescopes for 400 years, but our understanding of our nearest star has improved dramatically since the 1960s, which began the era of space-based technology and observations of the Sun. While there have been more than 40 missions since 1960, that have observed or will observe conditions on the Sun—such as the Parker Solar Probe or DSCOVR—there are three key missions that have helped scientists pinpoint the highs and lows of the Sun’s activity, which follows an 11-year cycle that includes the solar minimum— the Solar and Heliospheric Observatory (SOHO), the Solar Dynamics Observatory (SDO), and Geostationary Operational Environmental Satellites-R Series (GOES-R).
Currently, the science behind studying the Sun focuses on something called the solar wind, and the solar cycle (aka solar maximum and solar minimum). The current solar minimum, which is a period characterized by the least amount of activity on the Sun during its natural 11-year solar activity cycle, is determined after many careful observations from the ground along with supplemental data from satellites monitoring the number and location of sunspots. Sunspots are dark areas of high magnetic concentration, a part of the Sun’s surface that helps generate the energy to make solar flares and coronal mass ejections (or CMEs).
How to Monitor Space Weather?
The Sun’s most common influence on Earth and other planets, known as space weather, is primarily produced via the solar wind, which is outflowing material from the Sun that fills nearly every corner of the solar system and travels around the same speed as the upper limit of hypersonic flight. Space weather events can also be triggered by coronal mass ejections (CMEs), which are giant explosions that travel outward from the Sun; most of which are much weaker than the Carrington event.
The frequency of CMEs is tied to the solar cycle, they occur more frequently near solar maximum. In order to determine when the solar minimum at the end of Solar Cycle 24 occurred and when to expect activity to begin to rise again, scientists at NOAA and NASA work together to look at the number of sunspots that appear over a 13-month time period, using hand counted observations from scientists on the ground. For supplemental information supporting the solar minimum timing, they also use several specialized telescopes and other instruments aboard environmental satellites, which are detailed below.
Long-term monitoring of sunspots helps pinpoint the timeframes when the Sun’s activity is at its lowest or highest, or what scientists call the solar minimum/maximum. Several different satellites and missions, profiled below, help scientists better understand, model, and eventually forecast space weather.
Solar and Heliospheric Observatory (SOHO)
NASA and the European Space Agency’s SOHO, which launched in 1995, was one of the groundbreaking missions that allows us to monitor solar activity that could affect the Earth, such as coronal mass ejections (or CMEs) defined by NOAA as “large expulsions of plasma and magnetic field from the Sun’s corona.”
An instrument called a coronagraph helped scientists from SOHO observe the Sun by taking pictures of the faint upper atmosphere, or corona by blocking the light from the sun’s surface and creating an artificial eclipse inside the telescope. The SOHO satellite is around one million miles from Earth and carries several instruments, including a coronagraph. SOHO’s ground-breaking Large-Angle Spectrometric Coronagraph (or LASCO instrument) was able to help researchers study how solar wind accelerates, study the causes of CMEs, and find clues on the relationship between the development of the Sun’s magnetic field and what’s happening in the corona.
Also, SOHO had the Extreme Ultraviolet Imaging Telescope (or EIT), which, together with the Soft X-ray Telescope (SXT) from JAXA’s Yohkoh satellite, provided additional evidence linking sunspots and the changing of the Sun's magnetic poles and the solar cycle.
Solar Dynamics Observatory (SDO)
While SOHO has lasted far beyond its initially planned two-year mission, scientists worked hard to come up with improved replacements for vital instruments on board. NASA’s Solar Dynamics Observatory (or SDO) mission, launched in 2010, became the primary scientific data source for some of the instruments that had started to degrade aboard the SOHO mission.
The Helioseismic and Magnetic Imager (HMI), provides higher-resolution continuous full-disk (the entire half of the sun that faces us) coverage of several features of the solar surface: visible brightness, Doppler motions, and magnetic field strength.
Finally, the Extreme Ultraviolet Variability Experiment (EVE) instrument measures the changing total brightness of the Sun in many wavelengths.
Geostationary Operational Environmental Satellites-R Series (GOES-R)
NOAA’s geostationary satellites, GOES-16 (GOES-East) and GOES-17 (GOES-West) also provide important solar measurement capabilities, and are designed specifically for monitoring and predicting space weather here on Earth. The Solar Ultraviolet Imager (or SUVI) and Extreme Ultraviolet and X-ray Irradiance Sensors (EXIS) replace earlier GOES satellite series instruments.
SUVI is a telescope that takes images of the Sun in six extreme ultraviolet channels and compiles full-disk solar images around the clock. Various elements in the atmosphere of the Sun release light at specific wavelengths depending on their temperature, so by observing in several different wavelengths, it can give us a more complete picture of the Sun’s upper atmospheric and elemental structure.
EXIS detects solar flares and monitors solar irradiance, or the output of light energy from the entire disk of the Sun. Data from EXIS gives the first indication that a flare is occurring on the Sun, the strength of the flare, how long it lasts, the location of the flare on the Sun, and the potential impact here on Earth. Flares and CMEs are related, but one doesn’t always come with the other.
A great deal of collaboration goes on between NASA and NOAA in order to devise and interpret the readings from mission instruments; the SDO mission, which is responsible for the AIA instrument, works closely with the GOES-R SUVI project; additionally, NASA’s EVE system and NOAA’s EXIS instrument were built by the same team at the University of Colorado at Boulder.
The Future- NOAA's CCOR on GOES-U
With the planned launch of GOES-U in 2024, a new instrument, the Compact Coronagraph (CCOR) will become the primary source for coronagraph data and it’s advanced capabilities will supplement the long-running LASCO data. LASCO, despite being launched more than twenty years ago, is still a critically important instrument since it is still the best way for researchers on earth to track CMEs, making the race to replace it with CCOR on SWFO and GOES-U especially important.
Ultimately, CCOR will be able to better image the evolving solar activity of the corona and CMEs, enabling NOAA to better predict space weather that could affect technology and weather patterns on Earth.
LASCO: a coronagraph, which launched on the NASA-ESA SOHO Mission in 1995. Designed to track CMEs.
EIT: a telescope on SOHO that helped researchers link sunspot activity, the Sun’s magnetic pole change, and the Sun’s solar cycle.
HMI: a camera on NASA’s SDO 2010 mission that takes visible light pictures of the entire Earth-facing side of the Sun. It looks at the Sun’s brightness and measures magnetic field strength.
AIA: a telescope on the SDO mission that sees the Sun’s extreme ultraviolet light in 10 different wavelengths.
EVE: an instrument on SDO that measures the change in the Sun's brightness over time.
SUVI: a telescope on NOAA’s GOES-R Series mission that measures the Sun’s UV activity.
EXIS: an instrument on GOES-R used to measure X-rays and UV rays that helps measure the brightness of the Sun.
CCOR: a coronagraph set to launch in 2024 on NOAA’s GOES-U satellite. Will replace LASCO.