Propagation

What's Wrong with the Sun? (Nothing)

2008-09-08T13:06:00.000-07:00

July 11, 2008: Stop the presses! The sun is behaving normally.

So says NASA solar physicist David Hathaway. "There have been some reports lately that Solar Minimum is lasting longer than it should. That's not true. The ongoing lull in sunspot number is well within historic norms for the solar cycle."

This report, that there's nothing to report, is newsworthy because of a growing buzz in lay and academic circles that something is wrong with the sun. Sun Goes Longer Than Normal Without Producing Sunspots declared one recent press release. A careful look at the data, however, suggests otherwise.

But first, a status report: "The sun is now near the low point of its 11-year activity cycle," says Hathaway. "We call this 'Solar Minimum.' It is the period of quiet that separates one Solar Max from another."

Above: The solar cycle, 1995-2015. The "noisy" curve traces measured sunspot numbers; the smoothed curves are predictions. Credit: D. Hathaway/NASA/MSFC. [more]

During Solar Max, huge sunspots and intense solar flares are a daily occurrence. Auroras appear in Florida. Radiation storms knock out satellites. Radio blackouts frustrate hams. The last such episode took place in the years around 2000-2001.

During Solar Minimum, the opposite occurs. Solar flares are almost nonexistent while whole weeks go by without a single, tiny sunspot to break the monotony of the blank sun. This is what we are experiencing now.

Although minima are a normal aspect of the solar cycle, some observers are questioning the length of the ongoing minimum, now slogging through its 3rd year.

"It does seem like it's taking a long time," allows Hathaway, "but I think we're just forgetting how long a solar minimum can last." In the early 20th century there were periods of quiet lasting almost twice as long as the current spell. (See the end notes for an example.) Most researchers weren't even born then.

Hathaway has studied international sunspot counts stretching all the way back to 1749 and he offers these statistics: "The average period of a solar cycle is 131 months with a standard deviation of 14 months. Decaying solar cycle 23 (the one we are experiencing now) has so far lasted 142 months--well within the first standard deviation and thus not at all abnormal. The last available 13-month smoothed sunspot number was 5.70. This is bigger than 12 of the last 23 solar minimum values."

In summary, "the current minimum is not abnormally low or long."

The longest minimum on record, the Maunder Minimum of 1645-1715, lasted an incredible 70 years. Sunspots were rarely observed and the solar cycle seemed to have broken down completely. The period of quiet coincided with the Little Ice Age, a series of extraordinarily bitter winters in Earth's northern hemisphere. Many researchers are convinced that low solar activity, acting in concert with increased volcanism and possible changes in ocean current patterns, played a role in that 17th century cooling.

For reasons no one understands, the sunspot cycle revived itself in the early 18th century and has carried on since with the familiar 11-year period. Because solar physicists do not understand what triggered the Maunder Minimum or exactly how it influenced Earth's climate, they are always on the look-out for signs that it might be happening again.

The quiet of 2008 is not the second coming of the Maunder Minimum, believes Hathaway. "We have already observed a few sunspots from the next solar cycle," he says. (See Solar Cycle 24 Begins.) "This suggests the solar cycle is progressing normally."

What's next? Hathaway anticipates more spotless days¹, maybe even hundreds, followed by a return to Solar Max conditions in the years around 2012.

SOLAR CYCLE UPDATE

2008-03-20T17:35:00.000-07:00

2008 has been a year of few sunspots and mostly blank suns. A solar cycle update just released by NASA solar physicist David Hathaway shows why. We are experiencing the lowest ebb of solar minimum:

In the plot, the noisy curve is the International Sunspot Number measured by a worldwide network of solar observers. The smoothed curves are predictions for the future. We see that sunspot numbers may remain low for many months to come before beginning a rapid ascent in early 2009 toward the next solar maximum. It's something to look forward to. Meanwhile, stay tuned for quiet.

2008-03-20T17:26:00.000-07:00

THREE RED SPOTS: How many red spots does Jupiter have? On March 17th, Mike Salway of Australia looked through his 12-inch telescope and counted three:

Red spot #1 is the Great Red Spot you've heard about, hundreds of years old and twice as wide as Earth. Red spot #2 is Oval BA, which formed white in 2000 and turned red in 2006. Red spot #3 is a newcomer, "the Little Red Spot," says Salway, possibly only weeks old.

All these spots are storms--anticyclones big enough to swallow a rocky planet. What makes them red? Curiously, no one knows why the Great Red Spot itself is red. A favorite idea is that the storm dredges "chromophores" (color-changing compounds) from deep inside Jupiter up to the cloudtops where sunlight triggers a chemical reaction with red by-products. But what are the chromosphores and what is the chemical reaction? It's a mystery--now multiplied by three.

Jupiter is emerging from the glare of the sun as a bright morning star, visible in the southeast before sunrise: sky map. "I'm still waiting for some 'excellent' morning to deliver the best resolution and detail," says Salway, "but as Jupiter keeps climbing I'm sure it will come soon."

The aurora oval

2008-01-18T13:29:00.000-08:00

The auroral zones represent the places on earth where auroras occur most often and with greatest intensity. It was the Swiss physicist Herman Fritz (1829-1902), in the 1881 book "Das Polarlicht." who first showed that the northern lights have a maximum zone close to 67 degrees north. He called this belt the auroral zone. Thus, the auroral zones encompass the statistical distributions in latitude of all visible, night side auroras. The more detailed location of the auroral zones is based on professor Størmer's extensive auroral observations between 1910 and 1950.

The momentary, instantenous distribution of the auroras as a function of both latitude and local time were mapped by ground, rocket and satellite measurements in the 1960s. The best overview was obtained by satellite photos of the earth. Then it was discovered that the auroras display a continous oval zone around the magnetic pole in both hemispheres. Thus the auroral ovals are the regions on earth where the auroras are seen most often and with the greatest intensity.

The auroral oval is nearly twice as wide and twice as far from the magnetic pole at midnight as at midday, about 23 degrees and 12 degrees, respectively. On the night side the oval is roughly 10 degrees (about 1100 kilometres) closer to the equator than at the day side.

The auroral oval can be regarded as fixed in space with reference to the sun. As the earth revolves underneath, the daily variations in the aurora's position occur. In the Scandinavian sector you find that Andøya Rocket Range is located under the oval at night, while the oval lies across Svalbard during daytime. Halfway between northern Norway and Svalbard, northern lights can be observed in zenith both morning (around 0600) and evening (around 1800).

Modern studies have clearly shown that the shapes and locations of the ovals vary greatly with solar activity. With increasing activity on the sun, the oval widens and spreads, mainly towards the equator.

Current Auroral Oval:

2008-01-18T13:26:00.000-08:00

Switch to: Europe, USA, New Zealand, Antarctica
Credit: NOAA/POES
Updated:
What is the auroral oval?

2008-01-18T13:24:00.001-08:00

Coronal Holes

Earth is inside a solar wind stream flowing from the indicated coronal hole.

IS A NEW SOLAR CYCLE BEGINNING?

2007-12-15T16:42:00.001-08:00

Dec. 14, 2007: The solar physics community is abuzz this week. No, there haven't been any great eruptions or solar storms. The source of the excitement is a modest knot of magnetism that popped over the sun's eastern limb on Dec. 11th, pictured below in a pair of images from the orbiting Solar and Heliospheric Observatory (SOHO).

It may not look like much, but "this patch of magnetism could be a sign of the next solar cycle," says solar physicist David Hathaway of the Marshall Space Flight Center.

Above: From SOHO, a UV-wavelength image of the sun and a map showing positive (white) and negative (black) magnetic polarities. The new high-latitude active region is magnetically reversed, marking it as a harbinger of a new solar cycle.

For more than a year, the sun has been experiencing a lull in activity, marking the end of Solar Cycle 23, which peaked with many furious storms in 2000--2003. "Solar minimum is upon us," he says.

The big question now is, when will the next solar cycle begin?

It could be starting now.

"New solar cycles always begin with a high-latitude, reversed polarity sunspot," explains Hathaway. "Reversed polarity " means a sunspot with opposite magnetic polarity compared to sunspots from the previous solar cycle. "High-latitude" refers to the sun's grid of latitude and longitude. Old cycle spots congregate near the sun's equator. New cycle spots appear higher, around 25 or 30 degrees latitude.

The region that appeared on Dec. 11th fits both these criteria. It is high latitude (24 degrees N) and magnetically reversed. Just one problem: There is no sunspot. So far the region is just a bright knot of magnetic fields. If, however, these fields coalesce into a dark sunspot, scientists are ready to announce that Solar Cycle 24 has officially begun.

2007-12-15T16:36:00.001-08:00

Solar Cycle 23 is coming to an end. What's next? Image credit: NOAA/Space Weather Prediction Center.

Many forecasters believe Solar Cycle 24 will be big and intense. Peaking in 2011 or 2012, the cycle to come could have significant impacts on telecommunications, air traffic, power grids and GPS systems. (And don't forget the Northern Lights!) In this age of satellites and cell phones, the next solar cycle could make itself felt as never before.

The furious storms won't start right away, however. Solar cycles usually take a few years to build to a frenzy and Cycle 24 will be no exception. "We still have some quiet times ahead," says Hathaway.

Meanwhile, all eyes are on a promising little active region. Will it become the first sunspot of a new solar cycle? Stay tuned for updates from Science@NASA.

SUNSPOT 978

2007-12-13T18:25:00.001-08:00

Giant sunspot 978 hasn't exploded yet, but it is seething with activity. Witness this video recorded by Gary Palmer of Los Angeles on Dec. 11th:

"There is a magnetic filament that seems to leapfrog over the leading spot," he points out. "Isn't Mother Nature wonderful!"

Sunspot 978 continues to grow: movie. It now covers an expanse of Sun about as wide as the planet Jupiter, making it a fine target for backyard solar telescopes (Palmer used a Coronado SolarMax90). It has also developed a "beta-gamma" magnetic field that harbors energy for M-class solar flares. Will it erupt? Stay tuned!

more images: from John Nassr of Baguio, Philippines; from Malcolm Park of London, UK; from Pete Lawrence of Selsey, West Sussex, UK; from Paul Haese of Blackwoo, South Australia;

2007-09-22T03:37:00.000-07:00

Earth's Conjugate Aurora

2007-08-20T12:03:00.000-07:00

EARTH'S AURORAS MAKE RARE JOINT APPEARANCE IN A FEATURE FILM
Scientists using NASA's Polar spacecraft have captured the first-ever movie of auroras dancing simultaneously around both of Earth's polar regions. During a space weather storm on October 22, Polar's Visible Imaging System observed the aurora borealis and aurora australis (northern and southern lights) expanding and brightening in parallel at opposite ends of the world. The images confirm the three-century old theory that auroras in the northern and southern hemispheres are nearly mirror images -- conjugates - of each other.
"This is the first time that we have seen both auroral ovals simultaneously with such clarity," says Dr. Nicola Fox, the science operations manager for the Polar spacecraft, based at NASA Goddard Space Flight Center. "With these images, we have the ability to see the dynamics of conjugate auroras."
Auroras occur when fast-moving particles trapped in Earth's magnetic field come crashing down into the gases of Earth's upper atmosphere. Those particles (electrons and protons) can only move along the invisible magnetic field lines, which are connected to Earth near the North and South poles. When a space weather event pours energy into the space around Earth and energizes the magnetic field, those particles travel to both ends of the field lines, creating auroral displays in approximately 2500 mile diameter rings encircling each pole.
"For the first time, the northern and southern auroral ovals were observed simultaneously with enough resolution to confirm that the northern and southern aurora are mirror images of each other on a global scale," says Dr. John Sigwarth, a space physicist at the University of Iowa who helped design and operate the VIS cameras. "Further analysis of these images should help us determine if the all of the auroral features are exactly mirrored down to the finest detail." Preliminary research suggests that while the auroras mimic each other on broad scales, there are also some fine features that do not match.
The first recorded sighting of conjugate auroras occurred in September 1770, during the expeditions of Captain James Cook. While exploring Australia and the South Pacific on the HMS Endeavour, Cook's crew noted "a phenomenon appeared in the heavens in many things resembling the Aurora Borealis." Later studies of the Qing-shigao, a draft history of the Chinese Qing Dynasty, revealed that an aurora was observed on the same night - September 16, 1770 - in the northern hemisphere.
In the years since then, scientists have conducted ground- and aircraft-based studies of simultaneous auroras in both hemispheres. In the 1980s, NASA's Dynamics Explorer spacecraft snapped three images of auroral crowns around both poles, but those images were taken on different days and times and did not allow researchers to study the variations of the ovals.
Polar was launched by NASA in 1996 to study the aurora, the radiation belts, and other phenomena in the space around Earth.

CORONAL HOLES:

2007-07-12T20:46:00.000-07:00

A solar wind stream flowing from the indicated coronal hole should reach Earth on July 14th or 15th.

DAILY SUN: 12 JULY 07

2007-07-12T20:42:00.000-07:00

Sunspot 963 is decaying and poses a diminishing threat for M-class solar flares. Credit: SOHO/MDI

Great Perseids

2007-07-12T20:39:00.000-07:00

July 11, 2007: Got a calendar? Circle this date: Sunday, August 12th. Next to the circle write "all night" and "Meteors!" Attach the above to your refrigerator in plain view so you won't miss the 2007 Perseid meteor shower.
"It's going to be a great show," says Bill Cooke of NASA's Meteoroid Environment Office at the Marshall Space Flight Center. "The Moon is new on August 12th--which means no moonlight, dark skies and plenty of meteors." How many? Cooke estimates one or two Perseids per minute at the shower's peak.

Above: A Perseid fireball photographed August 12, 2006, by Pierre Martin of Arnprior, Ontario, Canada. [Larger image]
The source of the shower is Comet Swift-Tuttle. Although the comet is nowhere near Earth, the comet's tail does intersect Earth's orbit. We glide through it every year in August. Tiny bits of comet dust hit Earth's atmosphere traveling 132,000 mph. At that speed, even a smidgen of dust makes a vivid streak of light--a meteor--when it disintegrates. Because Swift-Tuttle's meteors fly out of the constellation Perseus, they are called "Perseids."
Note: In the narrative that follows, all times are local. For instance, 9:00 pm means 9:00 pm in your time zone, where you live.
Sign up for EXPRESS SCIENCE NEWS delivery The show begins between 9:00 and 10:00 pm on Sunday, August 12th, when Perseus rises in the northeast. This is the time to look for Perseid Earthgrazers--meteors that approach from the horizon and skim the atmosphere overhead like a stone skipping the surface of a pond.
"Earthgrazers are long, slow and colorful; they are among the most beautiful of meteors," says Cooke. He cautions that an hour of watching may net only a few of these--"at most"--but seeing even one makes the long night worthwhile.
As the night unfolds, Perseus climbs higher and the meteor rate will increase many-fold. "By 2 am on Monday morning, August 13th, dozens of Perseids may be flitting across the sky every hour." The crescendo comes before dawn when rates could exceed a meteor a minute.
For maximum effect, Cooke advises, "get away from city lights." The brightest Perseids can be seen from cities, he allows, but the greater flurry of faint, delicate meteors is visible only from the countryside. Scouts, this is a good time to go camping.

2007-07-12T20:37:00.000-07:00

Above: The eastern sky, viewed during the hours before sunrise on Monday, Aug. 13, 2007.
And there's a bonus: Mars. In the constellation Taurus, just below Perseus, Mars shines like a bright red star. Many of the Perseids you see on August 12th and 13th will flit right past it. Instead of following the meteor, you may find you have a hard time taking your eyes off Mars. There's something bewitching about it, maybe the red color or perhaps the fact that it doesn't twinkle like a true star. You stare at Mars and it stares right back.
Earth and Mars are converging for a close encounter in December 2007. NASA is taking advantage by launching a new mission to Mars--the Phoenix Lander. Phoenix will touch down on an arctic plain where it can dig into the ground and investigate layers of soil and ice, searching for, among other things, a habitable zone for primitive microbes. The launch window opens on August 3rd, so by the time the Perseids arrive Phoenix may be hurtling toward the Red Planet. Landing: late Spring 2008.
It's something to think about at four in the morning, with Mars rising in the east, meteors flitting across the sky, and a summer breeze rustling the legs of your pajamas.
Maybe you should go circle your calendar again.

2007-04-25T22:59:00.000-07:00

SUNSPOT ALERT: A new sunspot is emerging over the sun's east limb, and it is a big one. John Nassr took this picture a few hours ago from his home in Baguio, the Philippines. The spot is not only photogenic, but also harbors energy for M-class solar flares.

2007-03-12T22:07:00.000-07:00

What is a Solar Flare?
A flare is defined as a sudden, rapid, and intense variation in brightness. A solar flare occurs when magnetic energy that has built up in the solar atmosphere is suddenly released. Radiation is emitted across virtually the entire electromagnetic spectrum, from radio waves at the long wavelength end, through optical emission to x-rays and gamma rays at the short wavelength end. The amount of energy released is the equivalent of millions of 100-megaton hydrogen bombs exploding at the same time! The first solar flare recorded in astronomical literature was on September 1, 1859. Two scientists, Richard C. Carrington and Richard Hodgson, were independently observing sunspots at the time, when they viewed a large flare in white light.
Soft x-ray image of a solar flare on the Sun
As the magnetic energy is being released, particles, including electrons, protons, and heavy nuclei, are heated and accelerated in the solar atmosphere. The energy released during a flare is typically on the order of 1027 ergs per second. Large flares can emit up to 1032 ergs of energy. This energy is ten million times greater than the energy released from a volcanic explosion. On the other hand, it is less than one-tenth of the total energy emitted by the Sun every second.
There are typically three stages to a solar flare. First is the precursor stage, where the release of magnetic energy is triggered. Soft x-ray emission is detected in this stage. In the second or impulsive stage, protons and electrons are accelerated to energies exceeding 1 MeV. During the impulsive stage, radio waves, hard x-rays, and gamma rays are emitted. The gradual build up and decay of soft x-rays can be detected in the third, decay stage. The duration of these stages can be as short as a few seconds or as long as an hour.
Solar flares extend out to the layer of the Sun called the corona. The corona is the outermost atmosphere of the Sun, consisting of highly rarefied gas. This gas normally has a temperature of a few million degrees Kelvin. Inside a flare, the temperature typically reaches 10 or 20 million degrees Kelvin, and can be as high as 100 million degrees Kelvin. The corona is visible in soft x-rays, as in the above image. Notice that the corona is not uniformly bright, but is concentrated around the solar equator in loop-shaped features. These bright loops are located within and connect areas of strong magnetic field called active regions. Sunspots are located within these active regions. Solar flares occur in active regions.
The frequency of flares coincides with the Sun's eleven year cycle. When the solar cycle is at a minimum, active regions are small and rare and few solar flares are detected. These increase in number as the Sun approaches the maximum part of its cycle. The Sun will reach its next maximum in the year 2011, give or take one year.
A person cannot view a solar flare by simply staring at the Sun. (NEVER LOOK DIRECTLY AT THE SUN! EYE DAMAGE CAN RESULT.) Flares are in fact difficult to see against the bright emission from the photosphere. Instead, specialized scientific instruments are used to detect the radiation signatures emitted during a flare. The radio and optical emissions from flares can be observed with telescopes on the Earth. Energetic emissions such as x-rays and gamma rays require telescopes located in space, since these emissions do not penetrate the Earth's atmosphere.

2007-01-22T21:48:00.000-08:00

2007-01-07T16:15:00.000-08:00

Cycle 23 could bottom out this summer (Jan 4, 2007) -- Radio wave propagation could be looking up after this summer, according to past and predicted sunspot and solar (radio) flux statistics this week from the National Oceanic and Atmospheric Administration's (NOAA) Space Environment Center. Those numbers indicate that current Cycle 23 will bottom out in July. As for Cycle 24, which should peak in approximately five years, the predictions are all over the map, with some saying it could be one of the most intense cycles in history and others calling for a weak or average cycle. This week's data predict a smoothed sunspot number of 9.5 and a 10.7 cm solar (radio) flux of 72.0 for July. More information on radio wave propagation is available on the ARRL Technical Information Service Propagation page....

2007-01-03T20:11:00.000-08:00

North America
T Index Map

This image shows the DIFFERENCE between current OBSERVED HOURLY conditions and PREDICTED MONTHLY conditions for the Australian region. The colours blue, green, yellow, red, correspond to "enhanced", "normal", "mildly depressed" and "depressed" conditions respectively. Depressions and enhancements are with respect to the IPS predicted monthly T index for that month.

2006-12-27T00:17:00.000-08:00

Sound Propagation

Sound propagates through air as a longitudinal wave. The speed of sound is determined by the properties of the air, and not by the frequency or amplitude of the sound. Sound waves, as well as most other types of waves, can be described in terms of the following basic wave phenomena.

2006-12-17T12:45:00.000-08:00

THE STORM IS OVER: A coronal mass ejection hit Earth on Dec. 16th, but the glancing impact failed to re-energize geomagnetic activity. The severe magnetic storm of Dec. 14th is truly finished.

GOODBYE... and thanks for the X-flares. Sunspot 930 announced itself on Dec. 5th with one of the strongest flares in years--an X9, followed by an X6 on Dec. 6th, an X3 on Dec. 13th and an X1 on Dec. 14th. Not bad for solar minimum!
Now the spot is departing. The sun's rotation is carrying it toward the western limb where it disappear from view in a day or so.

As soon as sunspot 930 is out of sight, solar activity will return to low levels. Stay tuned for quiet.

2006-12-09T12:44:00.000-08:00

SOLAR TSUNAMI: When sunspot 930 exploded on Dec. 6th, producing an X6-category flare, it also created a tsunami-like shock wave that rolled across the face of the sun, wiping out filaments and other structures in its path. An H-alpha telescope in New Mexico operated by the National Solar Observatory (NSO) recorded the action:

"These large scale blast waves occur infrequently, however, are very powerful," says Dr. K. S. Balasubramaniam of the National Solar Observatory. "They quickly propagate in a matter of minutes covering the whole sun and apparently sweeping away filamentary material." Researchers are unsure whether the filaments were blown off or were compressed so they were temporarily invisible. Get the full story from the NSO.

2006-12-08T21:05:00.000-08:00

Current Sun View of the EarthThe current position of the Sun over the Earth is a primary factor determining current radio propagation conditions between points on Earth, because energy from the Sun ionizes the ionosphere. Just as the part of the Earth the Sun is directly over tends to receive the most heat, it also tends to receive the most ionizing energy.
Current Solar High-Noon This is how the Earth would appear through a telescope from the Sun right now if the clouds currently around the Earth were removed. The Sun was directly over the point on Earth that is in the exact center of this image at the time it loaded in your web browser. The current gray-line transition between daylight and darkness extends around the extreme periphery of this globe. (Allow a few seconds for current-position calculations, image creation, and image loading.)

2006-12-08T20:46:00.000-08:00

Current Moon View of the EarthThe current position of the Moon over the Earth is important to Earth-Moon-Earth (EME) moon-bounce communications, because two or more stations on Earth must be able to see the moon simultaneously to communicate.
Current View of Earth from the Moon This is how the Earth would appear through a telescope from the Moon right now if the clouds currently around the Earth were removed. The Moon was directly over the point on Earth that is in the exact center of this image at the time it loaded in your browser. (Allow a few seconds for current-position calculations, image creation, and image loading.)