The measurements are sufficiently detailed that the researchers can pick out signals from the separate processes that build to form an atmospheric discharge, providing a new, high-quality tool for deepening understanding of the physical processes governing lightning.
The researchers also are linking FORTE recordings with ground-based measurements from the National Lightning Detection Network and from a network of sensors that measure low-frequency electromagnetic emissions, or "sferics," and changes in the direction of the electric field.
Matching FORTE's readings with others could identify those radio signatures useful for remotely identifying important storm systems, convective cells that spawn tornadoes, for example, or unleash damaging hail.
Scientists from the U.S. Department of Energy's Los Alamos National Laboratory present a slew of papers based on the FORTE data at this week's annual fall meeting of the American Geophysical Union.
FORTE -- which stands for Fast On-orbit Recording of Transient Events -- was developed jointly by Los Alamos and Sandia National Laboratories. Launched August 1997, the satellite began acquiring radio data almost immediately, and kicked into high gear when it unfolded a 30-foot-long antenna that had been stowed in a foot-high canister for launch.
The sophisticated radio receiver samples a wide range of radio frequencies at a very high rate, essential since the impulsive radio emissions associated with lightning last less than a thousandth of a second. The receiver can make a recording and reset its trigger almost instantaneously.
A large onboard data storage capacity lets FORTE hold hundreds of measurements it can ship to Earth when it circles over the ground station.
FORTE also has two sensors that detect and measure the optical emissions from lightning.
FORTE was developed to demonstrate advanced technologies for detecting nuclear weapon detonations and thus help curb nuclear proliferation.
"Lightning is of interest to Los Alamos' national security missions because there is a parallel between the spectrum of lightning emissions and the electromagnetic pulse created by a nuclear weapon detonation," said Los Alamos physicist Abe Jacobson, who heads the FORTE science team.
"FORTE's measurements will allow us not only to build a better 'trigger' to discriminate between natural and human-caused events of interest, but we get tons more data about natural phenomena. We can see changes in the pattern of data as the satellite passes over a storm," Jacobson said, which will help increase understanding of storm processes.
FORTE's circular, low-Earth orbit is inclined 70 degrees to the equator, so it makes several passes per day over lightning-prone tropical regions, notably South America, Africa, and Southeast Asia. Due to its global coverage and its large data-acquisition and archiving capability, FORTE provides a tremendously improved opportunity to gather statistics on the characteristics of lightning radio-frequency emissions, Jacobson said.
"Global warming could drive the hydrologic cycle with increasing vigor. We're getting a handle on how to use radio frequency and optical signals for remote sensing of storm processes," Jacobson said.
FORTE's radio technology is a follow-on to the successful Blackbeard experiment carried aboard Los Alamos' ALEXIS satellite, launched in 1993. Blackbeard added to the scientific literature observations of TIPPs, Trans-Ionospheric Pulse Pairs. These were extremely short, intense, paired radio pulses known to originate from near storm systems, but otherwise of unknown origin.
FORTE has conclusively shown that TIPPs are generated by lightning, the second pulse produced by a reflection of the first pulse off Earth's surface. Blackbeard's radio technology could detect only the strongest lightning-related events, and TIPPs are generated by extremely strong discharges.
FORTE's more sophisticated technology can distinguish all the lightning signals that are generally buried in the noise of manmade radio emissions and show that TIPPs are at the tail end of the distribution of events. FORTE can also see signals from the individual events that develop and radiate from the buildup and discharge of a lightning stroke, whereas Blackbeard looked only at the integrated signal.
AGU SESSIONS FEATURING FORTE-RELATED PRESENTATIONS
Tuesday, 1:30 p.m. PST
A22A-02, "Observation and Geolocation of Lightning from Space Using the FORTE Lightning Location System (LLS), Dave Suszcynsky, presenter
A22A-15, "Worldwide Rates of Lightning Generated Radio Emissions as Detected by FORTE," R.S. Zuelsdorf, presenter.
A22A-16, "Observations of Short Duration, Isolated, Transionospheric Pulses with the FORTE Satellite," Joe Fitzgerald, presenter
A22A-20, "FORTE Observations of Simultaneous RF and Optical Emissions From Lightning," Dave Suszcynsky, presenter.
A22A-21, "Comparison of FORTE RF and field-change array observations," Bob Massey, presenter.
A22A-22, "Optical and RF Emissions From Superbolt-class Lightning Events as Observed by the FORTE satellite," Matt Kirkland, presenter.
A22A-23, "Temporal characteristics of radio-frequency pulses emitted by lightning, as observed by the FORTÉ satellite," Abe Jacobson, presenter.
Wednesday, 9 a.m. PST
A31D-01, "Observations of Lightning From Space Using the FORTE Photodiode Detector," Matt Kirkland, presenter.
A31D-02, "Correlation of FORTÉ satellite radio-frequency lightning observations with NLDN stroke reports," Abe Jacobson, presenter
A31D-03, "Initial Comparisons of Optical and Radio Frequency Satellite Observations of Thunderstorms with Ground Based Sferics, " Paul Argo, presenter.
A31D-04, "The Los Alamos Electric-Field-Change Sensor Array," Kyle Wiens, presenter
A31D-05, "Observations of Compact Intracloud Discharges, Dave Smith, presenter
Los Alamos National Laboratory, a multidisciplinary research institution engaged in strategic science on behalf of national security, is operated by Los Alamos National Security, LLC, a team composed of Bechtel National, the University of California, The Babcock & Wilcox Company, and Washington Group International for the Department of Energy's National Nuclear Security Administration.
Los Alamos enhances national security by ensuring the safety and reliability of the U.S. nuclear stockpile, developing technologies to reduce threats from weapons of mass destruction, and solving problems related to energy, environment, infrastructure, health, and global security concerns.