04 gennaio 2009

La ionosfera si abbassa

Ho letto solo oggi, con qualche settimana di ritardo una notizia che riguarda i primi risultati ottenuti da un satellite lanciato lo scorso aprile per studiare la ionosfera e i suoi fenomeni. Il satellite CNOF/S, Communication/Navigation Outage Forecast System, fa parte di una missione dell'Air Force americana con la collaborazione di NASA e università il cui obiettivo è fornire dati sulle possibili alterazioni nelle comunicazioni alle alte frequenze, in virtù di fenomeni perturbativi come le "scintillazioni", che disturbano le operazioni terra-satellite anche nelle UHF e in banda L.
Secondo le ultime analisi effettuate con gli strumenti della missione CINDI, Coupled Ion-Neutral Dynamics Investigations (qui trovate delle interessanti animazioni), la ionosfera si starebbe abbassando di quota. La regione di transizione tra ionosfera e spazio, teoricamente collocata intorno a quota 950 chilometri durante il giorno è stata misurata a 800 chilometri. Nelle ore notturne il limite si dimezza, scendendo a 420 chilometri di quota contro i 640 misurati da terra.
I due articoli che seguono, ricavati dal sito Space Mart e dal sito della University of Texas Dallas (cui è affidata la direzione scientifica della missione CINDI) riassumono questo e altri risultati, tra cui per esempio il rilevamento di un livello termico ionosferico assai più basso del previsto. La ionosfera è più fredda e meno alta. In teoria, una quota più bassa implicherebbe una riduzione dei percorsi propagativi per chi ascolta in onde medie e corte. Ma in realtà sospetto che i dati raccolti in questi mesi dalla missione C/NOFS-CINDI siano piuttosto uno strumento di conoscenza aggiuntivo, più preciso di quelli, molto circoscritti e limitati, di cui disponevamo finora. Il ritratto della ionosfera che sta emergendo non è "diverso", ma "nuovo". Il che non lo rende meno interessante per noi.

Contraction Of Boundary Between The Earth's Ionosphere And Space

The C/NOFS mission gives scientists a new tool for forecasting space weather. The CINDI instrument aboard C/NOFS specifically studies the major elements that influence space weather near Earths equator.

by Staff Writers (Credit: NASA) Washington DC (SPX) Dec 19, 2008

Observations made by NASA instruments onboard an Air Force satellite have shown that the boundary between the Earth's upper atmosphere and space has moved to extraordinarily low altitudes. These observations were made by the Coupled Ion Neutral Dynamics Investigation (CINDI) instrument suite, which was launched aboard the U.S. Air Force's Communication/Navigation Outage Forecast System (C/NOFS) satellite on April 16, 2008.
The CINDI suite, which was built under the direction Principal Investigator Rod Heelis of the University of Texas at Dallas, includes both ion and neutral sensors and makes measurements of the variations in neutral and ion densities and drifts.
CINDI and C/NOFS were designed to study disturbances in Earth's ionosphere that can result in a disruption of navigation and communication signals. The ionosphere is a gaseous envelope of electrically charged particles that surrounds our planet and it is important because Radar, radio waves, and global positioning system signals can be disrupted by ionospheric disturbances.
CINDI's first discovery was, however, that the ionosphere was not where it had been expected to be. During the first months of CINDI operations the transition between the ionosphere and space was found to be at about 260 miles (420 km) altitude during the nighttime, barely rising above 500 miles (800 km) during the day.
These altitudes were extraordinarily low compared with the more typical values of 400 miles (640 km) during the nighttime and 600 miles (960 km) during the day.
The height of the ionosphere/space transition is controlled in part by the amount of extreme ultraviolet energy emitted by the Sun and a somewhat contracted ionosphere could have been expected because C/NOFS was launched during a minimum in the 11-year cycle of solar activity. However, the size of the actual contraction caught investigators by surprise.
In fact, when they looked back over records of solar activity, they found that C/NOFS had been launched during the quietest solar minimum since the space age began.
This extraordinary circumstance is providing an unparalleled opportunity to study the connection between the interior dynamics of the Sun and the response of the Earth's space environment.
CINDI is a NASA sponsored Mission of Opportunity conducted by the University of Texas at Dallas. NASA's Explorer Program at Goddard Space Flight Center, Greenbelt, Md., managed the CINDI mission. The Explorer Program provides frequent flight opportunities for world-class scientific investigations from space within heliophysics and astrophysics.
The CINDI investigation is carried out as an enhancement to the science objectives of the C/NOFS satellite undertaken by the Air Force Research Laboratory and the Space and Missile Command Test and Evaluation Directorate.

***

UT Dallas Project Helps Fill Out Picture of Earth’s Ionosphere

Researchers’ Instruments Reveal Surprises About Size and Shape of Electrically Charged Layer

Dec. 16, 2008

A space weather satellite with Coupled Ion-Neutral Dynamics Investigation (CINDI) instruments aboard have for the first time revealed the size and shape of the gaseous envelope of electrically charged particles that surrounds the globe.

Called the ionosphere, this essential border between Earth and space has now been mapped at its upper boundary and shown to occupy less height than expected.
Measurements from CINDI instrumentation have made the first map of the ionosphere’s upper surface. It expands and contracts from day to night but has much less height than expected.
A collaboration among NASA, the U.S. Air Force Research Laboratory (AFRL) and the University of Texas at Dallas, CINDI is revealing what happens during periods of low sunspot activity, when the upper atmosphere cools off.
The ionosphere plays a particularly important role in satellite communication and any type of technology that uses space-based communications. GPS systems used by ships, trucking companies, and airplanes depend on reliable, uninterrupted streams of information from satellites, which must punch a signal cleanly through the ionosphere.
“Any radio or location system signal that utilizes space-based communication has to go through the ionosphere,” said CINDI Principal Investigator Rod Heelis, director of the Hanson Center for Space Sciences at UT Dallas. “On its best day, the ionosphere just bends that signal rather like water bends light. On its worst day it can completely distort that signal so that it doesn’t make it out the other side.”
Predicting when these disturbances might occur is a key goal of the CINDI project and its satellite, the Communication/Navigation Outage Forecast System (C/NOFS).

CINDI reveals for the first time:

  • A view of the ionosphere never seen before, during solar “quiet” times.
  • A map showing the size and shape of the Earth’s ionosphere.
  • That the ionosphere is up to 100 degrees cooler than previously thought.
  • That the effective thickness of the ionospheric shell is less than expected.
  • A link between the extent of the ionosphere and solar activity levels observed at solar minimum.
  • A view of the daily expansion and contraction of the ionosphere around the equator.
“The ionosphere is extremely cold at night, leading to a much thinner altitude and less dense layer than we expected,” Heelis said. “We have found that it is up to 100 degrees cooler than we expected and the effective thickness of the ionospheric shell is smaller than we expected.”
Heelis said CINDI revealed that the ionosphere expands during the day, when the upper surface rises, but not as high as the team thought it might. Further, the daily expansion and contraction of the ionosphere has been observed continuously around the equator for the first time. Had sunspot activity not dropped off—with associated cooling of the ionosphere—scientists would not have been able to watch the ionosphere expand and contract.
The so-called “quiet time” view of the ionosphere, when sunspot activity is low, allows Heelis and Greg Earle, another UT Dallas physics professor and CINIDI team member, to study the region of the ionosphere that is hazardous to radio communications.

Heelis and Earle built the two instruments that comprise CINDI:

The Ion Velocity Meter, which measures the direction and speed of ions as well as their density, temperature and chemical composition.

The Neutral Wind Meter, which measures the speed and direction of the neutral atoms and molecules in the near vacuum of space.

The 20-pound package of sensors and electronic equipment was fabricated in Heelis’ UT Dallas laboratory with Earle’s assistance and in collaboration with Paul Mahaffey of NASA’s Goddard Space Flight Center.

CINDI was carried on the C/NOFS satellite that was launched on April 16, 2008 on a Pegasus XL rocket aboard Orbital Science Corporation’s L-1011 “Stargazer” jet. The C/NOFS mission was launched to explore ways to forecast disturbances in the Earth’s ionosphere that can result in a disruption of navigation and communication signals.
“Years ago, my basic question began as, ‘How does our space environment interact with the sun?’” Heelis said. “I was intellectually curious about that. But now, as we become more dependent on assets in space, answering that question has real importance to everyday commerce, to military and commercial communications and navigation. NASA and the Air Force want to know the answers, and it’s enlightening to see major agencies working with us at UT Dallas to share resources and work on these problems together.”
Heelis and representatives from NASA and the Air Force presented the recent results from CINDI at the 2008 fall meeting of the American Geophysical Union.

2 commenti:

Giobatta P. ha detto...

Last week's Propagation Forecast Bulletin ARLP001 reported that the ionosphere is now at a lower elevation than in the past, but Carl Luetzelschwab, K9LA wrote in suggesting that this isn't really true, and our misunderstanding is probably due to some poor science reporting. The data reported is only accurate for equatorial latitudes.

An explanation is on Carl's web site,
http://mysite. verizon.net/ k9la/. Just click on "Timely Topics" toward the top, then the January 3 report, titled "Is the ionosphere really lower?"

Andrea ha detto...

Potete seguire il link segnalato da Giobatta qui. Sono molto d'accordo con quanto scritto.