Updated: November 19, 2009
How intense will the next solar cycle be? Can we predict when a violent solar storm will blast Earth with energetic particles? Could a prolonged period of inactivity on the Sun plunge Earth into a prolonged winter? These are a few of the questions that scientists anticipate the new Solar Dynamics Observatory (SDO) will help to answer.
Scheduled to launch this winter on an Atlas V rocket, SDO will peer into the Sun's atmosphere and probe the Sun's inner workings. SDO is the first mission of NASA's Living With a Star program, which seeks to reveal how solar activity is generated and to understand the causes of solar variability and its impact on Earth.
"Contrary to popular belief, the Sun is a magnetic variable star," Says Madhulika Guhathakurta, lead program scientist for Living With a Star at NASA Headquarters, Washington. "SDO will show us just how variable the Sun really is and will reveal the underlying physics of solar variability."
To accomplish the ambitious goals of the science team, "SDO will take full-disk, high definition images of the Sun all of the time," says project manager Liz Citrin at NASA's Goddard Space Flight Center, Greenbelt, Md. Previous missions could not capture images at as rapid a cadence as SDO will, nor did they have the bandwidth to transmit all of the data back to Earth for processing. "These advances will provide the data to better understand how the Sun works and will allow us to develop the tools to predict its behavior."
Where Do Magnetic Fields Come From?
The Sun's magnetic field powers all solar activity. Flows of hot, ionized gases in the Sun's convection zone — the region inside the Sun where hot gas parcels rise and transport energy toward the surface — act as electrical currents to generate the Sun's powerful magnetic fields.
SDO will examine these fields at the surface and infer from where they originate inside the Sun to where they are expressed as active regions, sunspots, and coronal loops, to when they eject particles into space as coronal mass ejections and solar flares.
The goal is to determine how the Sun's magnetic field is generated and to study how stored magnetic energy changes into kinetic energy (the energy of motion) in the form of the solar wind, energetic particles, and variations in the solar irradiance. If solar physicists can get a better understanding of the Sun's magnetic field, they can predict how the effects of that field move into the solar system and near-Earth space to create space weather.
Cycling with the Sun
Solar activity builds from the passive and quiet to the explosively volatile and falls off again in a cycle lasting about 11 years. Throughout the solar cycle, the number of sunspots varies from day to day and year to year. A solar cycle is declared to be at its "maximum" when the greatest number of sunspots is counted in a year — a declaration that can only be made after the peak has been reached. The last maximum occurred in 2001-2002, and scientists predict the next maximum will occur in 2013 or even 2014.
Predicting the intensity of a solar cycle and the timing of a solar maximum or minimum isn't as straightforward as counting 11 years. Sunspots from Solar Cycle 24, which form near the Sun's equator, began to appear in mid-2008. But scientists have observed sunspots from both Cycle 23 — formed at higher latitudes — and Cycle 24 in 2009. Solar minimum probably occurred in late 2008, when the number of sunspots from Solar Cycle 24 finally passed those from the earlier cycle, though the Sun has remained very quiet late into 2009.
Scientists disagree about whether Solar Cycle 24 will be weaker or stronger than the previous cycle. Some scientists maintain that the prolonged solar minimum at the start of Solar Cycle 24 signals a weak solar maximum will follow. Others suggest that Solar Cycle 24's late start means we are in store for a more active solar maximum.
And there are exceptions to the typical 11-year solar cycle. The most famous exception was the Maunder Minimum, a time between 1645 and 1715 when sunspots were rarely observed and Europe and North America experienced bitterly cold winters. This "Little Ice Age" suggests there may be a connection between a prolonged lack of solar activity and Earth's climate.
SDO will use three science instruments to investigate the connections between the internal workings of the Sun and the external effects of our nearest star. The Helioseismic and Magnetic Imager (HMI) will peer into the Sun and map the plasma flows that generate magnetic fields. HMI will also map the surface of the magnetic field.
The Atmospheric Imaging Assembly (AIA) will image the solar atmosphere in multiple wavelengths that cannot be seen from the ground. Together, HMI and AIA will link changes on the solar surface to the Sun's interior.
The Extreme Ultraviolet Variability Experiment (EVE) will measure how much radiant energy the Sun emits at extreme ultraviolet wavelengths — light that is so completely absorbed by our atmosphere it can only be measured from space. While our atmosphere protects us from nearly all of these harmful rays, humans in space are vulnerable to excessive amounts of EUV light.
A Special Orbit
SDO will orbit Earth once in 24 hours at an angle offset, or inclined, from our planet's equator. Unlike a geostationary orbit, which would keep the spacecraft above the same area of Earth all the time, SDO will trace a figure-eight path above Earth.
An inclined geosynchronous orbit will allow SDO to watch the Sun almost 24 hours a day, seven days a week, for at least five years with only brief interruptions as Earth passes between SDO and the sun. The orbit permits a dedicated ground station near Las Cruces, New Mexico, to maintain continuous contact with the observatory.
Science on a Prolific Scale
SDO will return a voluminous amount data in a continuous stream. Throughout its five-year mission, SDO will take images of the sun every few seconds — images that approach the visual quality of an IMAX™ movie. Every 10 seconds, SDO will take full-disk solar images with 10 times the resolution of high-definition TV. The spacecraft will send about 1.5 terabytes of data back to Earth each day — the equivalent of streaming 380 full-length movies.
The data SDO collects will give scientists the most detailed and in-depth glimpse of the Sun and its inner workings to date. With this new insight, they will have the information they need to unlock the mysteries of the solar cycle and better protect us from the effects of space weather.