Astronomy

Relative brightness of the sun's corona

Relative brightness of the sun's corona


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In preperation for the upcoming solar eclipse I'd like to practice my photography to be able to take some (hopefully) good photos of the corona during totallity.

What is the brightness of the sun's corona? How bright is it compared to a full moon (or other celestial phenomena)?

At what point during the "diamond ring" does it become too bright to look at safely without safety equipment, and what is the relative brightness of the diamond?


The corona varies dramatically in brightness from the inner portion near the sun to the outer portion a few solar radii away. The innermost portion is the brightest; at ISO 100 and f5.6, an exposure of ~1/125 second will capture it. So it's roughly the same brightness as the full moon. The outer corona will require much longer exposures of perhaps a half second, but note that during this time the sun will move noticeably with a long lens if you're not using a tracking mount, so some blurring will likely occur. Overall, practicing on the crescent moon will give you a pretty good idea of the dynamic range of the corona.

The diamond ring is dangerous to look at directly, as is any unobscured part of the sun's disk; you should look away at first glimpse, or earlier.

There's lots more detail scattered around the web, which you've probably found by now. A good overview is available at http://www.mreclipse.com/Totality2/TotalityCh12-3.html#SEExpoTab


This is not an answer, but an update on how the photography of the eclipse went.

I ended up on ISO 400 and f/5.6 (limited by my 300mm lens) doing a series of nine 0.7 stop bracketing, ranging from 1/3200 to 1/80 second exposures, which turned out to be just about bang on for corona and prominences as you should be able to see on the attached images.


Relative brightness of the sun's corona - Astronomy

Click image for full-size GIF

Total solar eclipse images of 1980 February (above) and 1988 March (below) taken from sites located in India (1980) and the Philippines (1988) by expeditions from the High Altitude Observatory of Boulder, Colorado. Note that the 1980 image, taken near the maximum of the solar activity cycle shows many streamers located at all azimuths around the occulted disk of the Sun. Taken later in the cycle, about a year past the minimum, the 1988 image shows several large (bottle-shaped) helmet streamers which are restricted to latitudes between N45 and S45. The helmet streamers, which are large scale, dense structures, have measured lifetimes from less than one to more than several solar rotations.

A special telescope, known as the White Light Coronal Camera, was used for both of these observations. Half of the diameter of the dark central image of the moon is equal to a distance of one solar radius.

Click image for full-size JPEG

Through most of history, coronal research has been dominated by the simple fact that observation was possible only during the special astronomical circumstance of a total solar eclipse. There are between two and five solar eclipses each year, but many occur over the oceans and are not easily documented. Some are not total, being only partial or annular, and a good opportunity for eclipse observation comes along every only two or three years. Solar eclipses are also brief, the average duration of totality being only two to three minutes, limiting efforts to study evolution of the corona to following changes in the corona from one eclipse observation to another.

There are a number of natural timescales operating on the Sun. The Sun rotates on its own axis once every 27 days (as viewed from the earth), and the period of the magnetic variation most often detected using sunspots is an 11-year fluctuation. Other types of changes in the structure of the corona take place on a variety of time scales ranging from minutes to a fraction of a day. Thus progress in investigating the solar corona was paced by the availability to investigate the changes of the solar corona by following ground-based observations of a series of total solar eclipses. The white light corona seen at the time of a total eclipse is the result of scattering of sunlight by electrons in the corona.

Click image for full-size JPEG

A combination of two coronal images, one taken from the ground and one from space. The central image was made in soft X-rays by an instrument on the Yohkoh ("Sunbeam") satellite (Japan) it shows the very hot plasma in primarily closed magnetic structures in the magnetically dominated lower corona. The blue-white image was made at the same time with a white light (electron scattering) coronagraph based at Mauna Loa, Hawaii, and operated by the High Altitude Observatory of Boulder, Colorado. In this case the large scale, relatively weak magnetic field structures of the solar corona are seen extending upward for roughly a solar radius in altitude. In the 1930's, a French astronomer, Bernard Lyot, solved the technical problem of creating an artificial eclipse of the Sun within a telescope system, and since that time it has been possible to view the solar corona on regular basis. Even with this development, there are practical limitations to ground-based observing of the solar corona imposed by the scattering of light by both dust and molecules in the earth's atmosphere, as the brightness of the white light corona ranges from one millionth to one billionth of the central solar disk brightness. The coronagraph flown on the 1973 Skylab mission solved this problem by observing from a location on the Apollo Telescope Mount, a cluster of instruments used to view the solar atmosphere from this early version of a space station. By using a coronagraph in space, it became possible to made eclipse like observations as often as one wished for an extended period of time. In the case of Skylab, the mission lasted almost nine months, about nine solar rotations, but only one fifteenth of the duration of the solar magnetic variability cycle. The Solar Maximum Mission spacecraft, launched in 1980 and operated until 1989, represented a further refinement of the use of a coronagraph on a satellite observatory platform for the investigation of the nature of the solar corona since it was possible to accumulate thousands of images of the solar corona over this nine-year period. Click image for full-size GIF

The lower solar corona as seen in soft X-rays on 1993 February 25. The bright regions of this image indicate the magnetic complexity found in the corona above sunspots and active regions. The base of a helmet streamer structure is seen in the lower right, and the dark lane at the lower, central portion of the disk is a coronal hole structure. Coronal holes are large scale features of reduced density (and are therefore dark in soft X-ray images, since the soft X-ray intensity is proportional to the square of the electron density in the emitting region) and are identified as being open magnetic field regions which are sources for high speed streams of solar particles (electrons, protons, and ions). By using a combination of eclipse and coronagraph observations, a picture of the solar corona has emerged which suggests that the solar corona is a place where unique physical conditions and processes exist. Spectroscopy of the corona suggests that, by some not fully understood mechanism, the Sun has the ability to create very high temperature material in the corona. Radiation characteristic of one to two million degrees are regularly observed with coronagraph instruments. Images of the corona made from satellites in low earth orbit in the soft X-ray region of the spectrum demonstrate a highly structured corona where besides the forces of pressure and gravity, magnetic fields play a role in the determination of the Sun's outer atmosphere. Occasionally observations of flare regions in the corona demonstrate radiation which is interpreted to originate at very high temperatures between 10 to 40 million degrees C. These situations arise in areas where coronal magnetic fields are relatively strong and it is believed that the Sun has an effective mechanism for converting magnetic field energy into thermal energy. Current research indicates that in regions of relatively high magnetic field strength in the solar corona, corresponding to structures of small scale size (a few hundredths of a solar radius in length), some of the most energetic radiative processes originate in these small scale, high magnetic field regions of the corona. Click image for full-size GIF

A coronal mass ejection (CME) event in progress. These two images were made with the coronagraph flown on the Solar Maximum mission spacecraft and demonstrate the scale and speed of a CME event. The occulting disk image is about 1.8 solar radii in diameter and the images are taken a few minutes apart. The large loop-shaped CME structure is roughly the size of the Sun in the second image, and the velocities estimated for this type of event range from several hundred to a thousand kilometers per second (well over a million miles an hour), a velocity that would take a space traveler from the earth to the Moon in twenty minutes. In contrast to solar flares, which occur in small scale structures with relatively high magnetic field strength, there is a second kind of energetic phenomenon detected in the solar corona. These are the huge mass ejection events which were discovered and first studied in detail in the early 1970's with data collected with the Skylab and OSO-7 coronagraphs a much larger data set was amassed with the later P78-1 and Solar Maximum Mission instruments. Evidently, some of the largest scale structures of the corona, which are governed by large scale, weak magnetic fields, become unstable and huge amounts of mass are occasionally discharged from the solar atmosphere out into the heliosphere. Particle detectors carried on research satellites operating between Venus and Jupiter have confirmed that these ejections are detected far from the Sun, and must sometimes impact the earth. At the time of peak solar magnetic activity near the maximum in the sunspot cycle, there are two or three such events per day. Near the minimum of the magnetic activity cycle this rate falls to approximately one or two mass ejection events every ten days. The size scales of such events are typically seen to be a large fraction of a solar radius, and the speed of ejection averages to a value of about 400 km/s. The detection, analysis, physical mechanisms and consequences of coronal mass ejections remains a topic of concentrated scientific research at this time. Click image for full-size GIF

Composite of a SPARTAN 201, ground-based coronagraph, and Yohkoh soft X-ray image obtained during the first flight of the SPARTAN 201 system. Images such as this have been used to:

    construct models of the distribution of temperature and density for the large scale structures of the white light solar corona,

Three forces are active in the solar corona at the base of the heliosphere these are gas pressure and gravity forces similar to those experienced by humans near the earth's surface, and a third force produced by solar magnetic fields. As a consequence of these forces, a continuous flux of material is ejected from the Sun and blows outward through the heliosphere: the solar wind of charged particles.

Within a few solar radii of the Sun's visible surface, magnetic forces are thought to be the cause of the structuring seen at the times of total solar eclipse, such as helmet streamers and coronal holes. Coronal holes are now known to be regions where the density of the corona is considerably reduced, causing a relatively dark region to appear in soft X-ray and EUV (extreme ultraviolet) images. During much of the magnetic activity cycle there are semi-permanent polar coronal holes, and it has been known since the Skylab era that coronal hole structures seen in the solar corona are associated with the detection of high speed solar wind streams which sweep past the earth. The physical mechanisms for the acceleration of the solar wind and the conditions of interplanetary space, which slowly evolve in step with the change of the Sun's periodic variation of magnetic field, are also the subject of intense interest to the international research community. Text provided by Dr. Richard R. Fisher, NASA Goddard Space Flight Center This is the access to this page since July 10, 1995. Return to the SPARTAN 201 home page.


Coronal Mass Ejection (CME)

A Coronal Mass Ejection (CME) is an explosive outburst of solar wind plasma from the Sun. The blast of a CME typically carries roughly a billion tons of material outward from the Sun at speeds on the order of hundreds of kilometers per second. A CME contains particle radiation (mostly protons and electrons) and powerful magnetic fields. These blasts originate in magnetically disturbed regions of the corona, the Sun's upper atmosphere - hence the name.

Most CMEs form over magnetically active regions on the "surface" of the Sun in the vicinity of sunspots. CMEs are often associated with solar flares, another type of explosive "solar storm". However, CMEs and solar flares don't always go together, and scientists aren't completely sure how the two phenomena are related. CMEs are much more common during the "solar max" phase of the sunspot cycle, when sunspots and magnetic disturbances on the Sun are plentiful.

CMEs travel outward through the Solar System. Some are directed towards Earth, though many others miss our planet completely. The radiation storms which are a part of CMEs can be hazardous to spacecraft and astronauts. If a strong CME collides with Earth's magnetosphere, the disturbance can trigger a series of events that sends a burst of particle radiation into Earth's upper atmosphere. As the radiation crashes into gas molecules in Earth's atmosphere, it causes them to glow. creating the magnificent light shows of the auroras (the Northern Lights and Southern Lights).


Brightness in Radio Astronomy

where I &nu = specific intensity [W m -2 Hz -1 sr -1 ] ,
T b = brightness temperature [K] ,
T = physical temperature [K] ,
&nu = frequency [Hz] ,
c = the speed of light = 2.998 × 10 8 m s -1 ,
k = Boltzmann's constant = 1.381 × 10 -23 J K -1 ,
and h = Planck's constant = 6.626 × 10 -34 J s .

S &nu = 2 &nu 2 k T b &Omega / c 2 ( h &nu k T ) ,

where the effective solid angle of an elliptical Gaussian beam is

How Much is a Jansky?

FM radio broadcast stations in the United States typically have 100 kilowatts of effective radiated power (ERP), which includes gain factors from the transmitting antenna design (most radiation goes out horizontally but is equally distributed in azimuth, with a gain of typically 5 to 10 times that of an isotropic radiator, so the actual equivalent isotropic radiated power is only 10-20 kW). Such stations have a usual range of 50 miles = 80 km. The broadcast power will be reduced by some form of inverse-square law, even though it isn't isotropic. For simplicity, let's ignore any propagation effects and assume the arriving power density (APD) is given by an isotropic pattern modified by the gain, with the receiver in the direction of maximum gain. In this case,

where d = the distance from transmitter to receiver. The bandwidth (BW) allocated to 1 FM station is 200 kHz. Let's assume the signal strength is uniform across this bandwidth. For the above parameters, the flux density at a receiver 80 km from the station -- near the edge of its effective range -- in the optimal-gain path will be:

S &nu = APD / BW
= ERP / (4 &pi d 2 BW)
= 10 5 W / [4 × 3.14 × (8 × 10 4 m) 2 × (2 × 10 5 Hz)]
= 6.2 × 10 -12 W m -2 Hz -1
= 6.2 × 10 14 Jy

where 1 Jy = 10 -26 W m -2 Hz -1 as noted above.

For comparison, most radio astronomy sources have signal strengths of a few Jy or less. The Sun, which is the brightest celestial source at most frequencies, has a flux density of about 10 6 - 10 8 Jy at 1 GHz, depending on whether there is surface activity (flares, etc.) or not. The brightest supernova remnant, Cassiopeia A, is about 3000 Jy at 1 GHz but a whopping 20,000 Jy at 100 MHz (the FM broadcast band), because it's a highly nonthermal (synchrotron) source -- as is Solar activity at these frequencies (Cas A is intrinsically much brighter than the Sun but appears fainter because it's a lot farther away). The faintest 1.4 GHz sources in recent large-scale radio surveys like the NRAO-VLA Sky Survey are a few milli-janskys. Newer, deeper surveys like the Evolutionary Map of the Universe project are targeting sources at the 50 &muJy (50 micro-janskys) level, which is about 100 times fainter than the NVSS, or 60 million times fainter than Cas A. As you might surmise, such detections require that there is no significant interference from nearby radio broadcast stations!

It's also noteworthy that the brightness contrast between radio-frequency interference and radio astronomical sources is much greater than that between optical light pollution and most optical astronomical sources! Inner-city skies (when clear) can be up to 100 times (5 magnitudes) brighter than the darkest night skies far from any artificial light sources, reducing the number of visible stars from thousands to dozens. But as indicated above, a stray radio broadcast can easily be a million times brighter than the Sun at radio wavelengths, and a trillion times brighter than more "ordinary" radio sources! The contrast in the latter case is similar to that between the optical brightness of the Sun and the 3rd-magnitude stars that fill in many of the fainter parts of prominent constellations in the night sky. The total radio energy collected from all celestial sources since the beginning of radio astronomy is less than the kinetic energy of one snowflake . This is not strictly true if the brightest few sources are included, but it's not bad as a loose illustration of the faintness of radio sources. Check: assume 1 snowflake has mass of drop of water = 1/25 gram = 4e-5 kg and falls at 1 m/s, so KE = (1/2) * (1 m/s)^2 * (4e-5 kg) = 2e-5 J. Choose: quiet Sun flux at 1 GHz = 1 MJy = 1e-20 J s^-1 m^-2 Hz^-1. time = 70 yr * 3e+07 s/yr = 2.1e+09 s area = (30 m)^2

10^3 m^2 BW = 1 GHz = 10^9 Hz => E

1e-20 J s^-1 m^-2 Hz^-1 * 2e+09 s * 1e+03 m^2 * 1e+09 Hz = 20 J Cas A is 1/300 of this, or 0.06 Jy, still much more than snowflake! In fact have to reduce flux to 1 Jy to match snowflake KE, and this is just for one 30-m telescope. Of course, all this assumes continuous observation of these bright sources, which is not realistic -- at least for large dishes, which will spend most of their time on faint sources. If Cas A is observed only, say, 1e-4 of the time (

53 min/yr), then its collected energy would be 6e-6 Jy, or

1/3 snowflake. More likely, this is an upper limit, few telescopes will observe Cas A this often (other fainter sources are used for calibration standards). An alternative (http://www.astronomytoday.com/astronomy/radioastro2.html) comparison is to the energy required to *melt* one snowflake. The enthalpy (latent heat) of fusion of H2O is 333.55 J/g, which for a 0.04 g snowflake would be 13.34 J -- about 6.7e+05 times the kinetic energy! This is almost as much solar energy as a 30m dish would collect in 70 yr, and hundreds of times what would be collected from Cas A. Thus, if Solar radiation is excluded, this limit seems about right when applied to all the radio telescopes in the world. --->

First, most FM transmissions are isotropic in azimuth it is only in elevation that they are not. FM and TV broadcast antennas have gain that yields a very flat radiation pattern, so that most of the energy goes out horizontally..

Second, by the time you combine a monaural (L+R) main channel signal with a multiplexed stereo (L-R) signal on top of it, and then on top of that add SCA sub-carrier broadcasts (e.g. elevator music for retail stores), you may be modulating with an audio signal that exceeds 50 kHz. Also, frequency modulation produces extensive sidebands, so frequency modulation really does fill the 200 kHz bandwidth allocated to commercial FM broadcasters. A good approximation to FM bandwidth is given Carson's Rule, which I am sure you can turn up on an internet search.

Third, my experience is that, unless you have a directional receiving antenna on a very tall tower, it would be unusual to receive FM or TV broadcasts at distances much over 50 miles. --->

How "Hot" is the Sky?

At radio frequencies, the major types of radiation are:

So how "hot" the sky appears varies, and at low frequencies, it has nothing to do with real temperature, except in special cases like the CMB. Below a few hundred MHz, the brightness temperature of the sky is very warm indeed, but above a GHz or so, where one can see the CMB, the sky is truly "cold" by human standards -- much colder than the ground in fact, or any person who happens to step in front of a radio telescope!


Relative brightness of the sun's corona - Astronomy

Coronal Mass Ejections

Coronal mass ejections (or CMEs) are huge bubbles of gas threaded with magnetic field lines that are ejected from the Sun over the course of several hours. Although the Sun's corona has been observed during total eclipses of the Sun for thousands of years, the existence of coronal mass ejections was unrealized until the space age. The earliest evidence of these dynamical events came from observations made with a coronagraph on the 7th Orbiting Solar Observatory (OSO 7) from 1971 to 1973. A coronagraph produces an artificial eclipse of the Sun by placing an "occulting disk" over the image of the Sun. During a natural eclipse of the Sun the corona is only visible for a few minutes at most, too short a period of time to notice any changes in coronal features. With ground based coronagraphs only the innermost corona is visible above the brightness of the sky. From space the corona is visible out to large distances from the Sun and can be viewed continuously. The animated sequence of images at the top of this page were obtained with the High Altitude Observatory's coronagraph on the Solar Maximum Mission in April of 1980.


Check out the first-ever map of the solar corona’s magnetic field

The colored fringes of this image show the strength of the corona’s magnetic field, from lower strength (blue) to higher (yellow). The whole field’s strength is just a fraction that of a refrigerator magnet. The image of the sun in the middle was taken by NASA’s Solar Dynamics Observatory.

Z.-H. Yang et al/Science 2020

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The sun’s wispy upper atmosphere, called the corona, is an ever-changing jungle of sizzling plasma. But mapping the strength of the magnetic fields that largely control that behavior has proved elusive. The fields are weak and the brightness of the sun outshines its corona.

Now though, observations taken using a specialized instrument called a coronagraph to block out the sun’s bright disk have allowed solar physicists to measure the speed and intensity of waves rippling through coronal plasma (SN: 3/19/09). “This is the first time we’ve mapped the coronal magnetic field on a large scale,” says Steven Tomczyk, a solar physicist at the High Altitude Observatory in Boulder, Colo., who designed the coronagraph.

In 2017, Tomczyk had been part of a team that took advantage of a total solar eclipse crisscrossing North America to take measurements of the corona’s magnetic field (SN: 8/16/17). He trekked to a mountaintop in Wyoming with a special camera to snap polarized pictures of the corona just as the moon blocked the sun. (I was there with them, reporting on the team’s efforts to help explain why the corona is so much hotter than the sun’s surface (SN: 8/21/17).) The team observed a tiny slice of the corona to test whether a particular wavelength of light could carry signatures of the corona’s magnetic field. It can (SN: 8/21/18).

But it’s the observations from the coronagraph, made in 2016, that allowed researchers to look at the whole corona all at once. Theorists had shown decades ago that coronal waves’ velocities can be used to infer the strength of the magnetic field. Such waves might also help carry heat from the sun’s surface into the corona (SN: 11/14/19). But no one had measured them across the whole corona before.

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The corona’s magnetic field strength is mostly between 1 and 4 gauss, a few times the strength of the Earth’s magnetic field at the planet’s surface, the researchers report in the Aug. 7 Science.

Making a map is a big step, the team says. But what solar physicists would really like to do is track the corona’s magnetic field continuously, at least once a day.

“The solar magnetic field is evolving all the time,” says solar physicist Zihao Yang of Peking University in Beijing. Sometimes the sun releases magnetic energy explosively, sending bursts of plasma can shooting out into space (SN: 3/7/19). Those ejections can wreak havoc on satellites or power grids when they strike Earth. Continuously monitoring coronal magnetism can help predict those outbursts. “Our work demonstrated that we can use this technique to map the global distribution of coronal magnetic field, but we only showed one map from a single dataset,” Yang says.

Measuring the strength of the corona’s magnetic field is “a really big deal,” says solar physicist Jenna Samra of the Smithsonian Astrophysical Observatory in Cambridge, Mass. “Making global maps of the coronal magnetic field strength … is what’s going to allow us to eventually get better predictions of space weather events,” she says. “This is a really nice step in that direction.”

Tomczyk and colleagues are working on an upgraded version of the coronagraph, called COSMO, for Coronal Solar Magnetism Observatory, that would use the same technique repeatedly with the ultimate goal of predicting the sun’s behavior.

“It’s a milestone to do it,” Tomczyk says. “The goal is to do it regularly, do it all the time.”

Questions or comments on this article? E-mail us at [email protected]

A version of this article appears in the September 26, 2020 issue of Science News.

Citations

Z. Yang et al. Global maps of the magnetic field in the solar corona. Science. Vol. 369, August 7, 2020, p. 694. doi:10.112/science.abb4462.

About Lisa Grossman

Lisa Grossman is the astronomy writer. She has a degree in astronomy from Cornell University and a graduate certificate in science writing from University of California, Santa Cruz. She lives near Boston.


Mystery of sun’s corona solved? It’s nanoflares, scientists say

One of the greatest mysteries of how stars behave has been right in our own backyard: the sun’s corona. Scientists have long wondered what heats this thin, ethereal shell of particles to roughly 300 times the temperature of the surface of the sun itself.

Now, after combining evidence from a sounding rocket and a black-hole-hunting telescope and computer modeling, researchers say they’ve found the cause: nanoflares.

“We have for the first time direct proof that nanoflares exist and heat the corona,” said Jim Klimchuk, a solar scientist at NASA’s Goddard Space Flight Center in Greenbelt, Md. “This proof takes the form of superhot plasma … it’s a real breakthrough.”

The findings, described at the first Triennial Earth-Sun Summit meeting underway in Indianapolis, may help solve the decades-old mystery of what powers the corona and help scientists better predict the effects of space weather on Earth.

The solar corona, the sun’s outer atmosphere, is so incredibly faint that it can only be seen with the naked eye during a solar eclipse, when the moon completely blocks out the sun’s bright body, leaving only the corona’s ghostly glow.

While the sun’s surface is around 10,340 degrees Fahrenheit, the corona, which extends high above the sun’s surface and into space, sports temperatures of around 4 million degrees, and can even hit 18 million degrees in some spots. Scientists have been stumped when it comes to explaining how this wispy shell of gas so far away from the sun’s blazing core can get superheated to such extremes.

Researchers have long suspected that nanoflares exist and might account for the corona’s mysterious heating source, but they haven’t been able to prove it. Nanoflares, so called because they’re one-billionth the size of typical solar flares, are still powerful, packing the equivalent energy of a 10-megaton hydrogen bomb. While they’re small by the sun’s standards, there are so many of them — millions going off each second on the sun’s surface — that they have the potential to heat the corona to its incredible temperatures.

The problem for researchers is that nanoflares are so small and brief that they’re hard to pick out against the overwhelming brightness of the sun. But now, researchers working on different lines of inquiry each say they’ve found strong evidence that nanoflares exist.

To get a better look at the sun, scientists flew a sounding rocket equipped with an instrument called the Extreme Ultraviolet Normal Incidence Spectrograph for 15 minutes, looking for signs of super-heated gas (around 18 million degrees Fahrenheit). Using this instrument, lead scientist Adrian Daw, a solar scientist at Goddard, was able to find those bits of gas, which scientists say are heated to those extreme temperatures by the nanoflares.

“That superhot plasma emission that we’re seeing there is the smoking gun of nanoflares,” Daw said.

Scientists also used NASA’s NuSTAR telescope to look for evidence of nanoflares. NuSTAR is used to study the X-rays coming from black holes, among other high-energy phenomena, but it can also be used to study X-ray emissions coming from regions of the sun where normal-sized flares could not be detected. These regions were popping with X-ray energy, a sign that nanoflares were at work, Iain Hannah, an astrophysicist at the University of Glasgow in Scotland, said in a media briefing in Indianapolis.

The scientists think these nanoflares are caused by the twisting and breaking of magnetic field lines around the sun, Klimchuk said, though it will be a while before they can probe exactly how the nanoflares work.

Tracking how nanoflares might contribute to the space weather that reaches Earth is very important, he added, because such solar radiation can disrupt terrestrial technology, including weapons guidance systems, navigation systems and anything that involves radio transmissions.

“We need to understand how these hot plasmas are created and produce these X-rays and UV radiation so we can better understand and prepare for their effects here on Earth,” Klimchuk said.

Follow @aminawrite for more stellar science news.

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Eclipse data illuminate mysteries of Sun's corona

Researchers at the University of Hawaiʻi Institute for Astronomy (IfA) have been hard at work studying the solar corona, the outermost atmosphere of the sun that expands into interplanetary space. The properties of the solar corona are a consequence of the Sun's complex magnetic field, which is produced in the solar interior and extends outward into space.

IfA graduate student Benjamin Boe conducted a new study that used total solar eclipse observations to measure the shape of the coronal magnetic field with higher spatial resolution and over a larger area than ever before. The results were published in the Astrophysical Journal on June 3.

The corona is most easily seen during a total solar eclipse -- when the moon is directly between the Earth and Sun, blocking sunlight. Significant technological advances in recent decades have shifted a majority of analysis to space-based observations at wavelengths of light not accessible from the ground, or to large ground-based telescopes such as the Daniel K. Inouye Solar Telescope on Maui. Despite these advances, some aspects of the corona can only be studied during total solar eclipses.

Boe was advised by UH Mānoa Astronomy Professor Shadia Habbal, a coronal research expert. Habbal has led a group of eclipse chasers, the Solar Wind Sherpas making scientific observations during solar eclipses for more than 20 years. These observations have led to breakthroughs in unveiling some of the secrets of the physical processes defining the corona.

"The corona has been observed with total solar eclipses for well over a century, but never before had eclipse images been used to quantify its magnetic field structure," explained Boe. "I knew it would be possible to extract a lot more information by applying modern image processing techniques to solar eclipse data."

Boe traced the pattern of the distribution of magnetic field lines in the corona, using an automatic tracing method applied to images of the corona taken during 14 eclipses the past two decades. This data provided the chance to study changes in the corona over two 11-year magnetic cycles of the Sun.

Boe found that there were very fine-scale structures throughout the corona. Higher resolution images showed smaller-scale structures, implying that the corona is even more structured than what was previously reported. To quantify these changes, Boe measured the magnetic field angle relative to the Sun's surface.

During periods of minimum solar activity, the corona's field emanated almost straight out of the Sun near the equator and poles, while it came out at a variety of angles at mid-latitudes. During periods of maximum, the coronal magnetic field was far less organized and more radial.

"We knew there would be changes over the solar cycle but we never expected how extended and structured the coronal field would be," Boe explained. "Future models will have to explain these features in order to fully understand the coronal magnetic field."

These results challenge the current assumptions used in coronal modeling, which often assume that the coronal magnetic field is radial beyond 2.5 solar radii. Instead, this work found that the coronal field was often non-radial to at least 4 solar radii.

This work has further implications in other areas of solar research -- including the formation of the solar wind, which impacts the Earth's magnetic field and can have effects on the ground, such as power outages.

"These results are of particular interest for solar wind formation. It indicates that the leading ideas for how to model the formation of the solar wind are not complete, and so our ability to predict and defend against space weather can be improved," Boe said.

Boe is already planning to be part of his team's next eclipse expeditions. The next one is slated for South America in December 2020.


Eclipses of the Sun

The apparent or angular sizes of both the Sun and Moon vary slightly from time to time as their distances from Earth vary. (Figure (PageIndex<1>) shows the distance of the observer varying at points A&ndashD, but the idea is the same.) Much of the time, the Moon looks slightly smaller than the Sun and cannot cover it completely, even if the two are perfectly aligned. In this type of &ldquoannular eclipse,&rdquo there is a ring of light around the dark sphere of the Moon.

However, if an eclipse of the Sun occurs when the Moon is somewhat nearer than its average distance, the Moon can completely hide the Sun, producing a total solar eclipse. Another way to say it is that a total eclipse of the Sun occurs at those times when the umbra of the Moon&rsquos shadow reaches the surface of Earth.

The geometry of a total solar eclipse is illustrated in Figure (PageIndex<2>). If the Sun and Moon are properly aligned, then the Moon&rsquos darkest shadow intersects the ground at a small point on Earth&rsquos surface. Anyone on Earth within the small area covered by the tip of the Moon&rsquos shadow will, for a few minutes, be unable to see the Sun and will witness a total eclipse. At the same time, observers on a larger area of Earth&rsquos surface who are in the penumbra will see only a part of the Sun eclipsed by the Moon: we call this a partial solar eclipse.

Between Earth&rsquos rotation and the motion of the Moon in its orbit, the tip of the Moon&rsquos shadow sweeps eastward at about 1500 kilometers per hour along a thin band across the surface of Earth. The thin zone across Earth within which a total solar eclipse is visible (weather permitting) is called the eclipse path. Within a region about 3000 kilometers on either side of the eclipse path, a partial solar eclipse is visible. It does not take long for the Moon&rsquos shadow to sweep past a given point on Earth. The duration of totality may be only a brief instant it can never exceed about 7 minutes.

Figure (PageIndex<2>) Geometry of a Total Solar Eclipse. Note that our diagram is not to scale. The Moon blocks the Sun during new moon phase as seen from some parts of Earth and casts a shadow on our planet.

Because a total eclipse of the Sun is so spectacular, it is well worth trying to see one if you can. There are some people whose hobby is &ldquoeclipse chasing&rdquo and who brag about how many they have seen in their lifetimes. Because much of Earth&rsquos surface is water, eclipse chasing can involve lengthy boat trips (and often requires air travel as well). As a result, eclipse chasing is rarely within the budget of a typical college student. Nevertheless, a list of future eclipses is given for your reference in Appendix H, just in case you strike it rich early. (And, as you can see in the Appendix, there will be total eclipses visible in the United States in 2017 and 2024, to which even college students may be able to afford travel.)


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This book contains over 200 problems spanning over 70 specific topic areas covered in a typical Algebra II course. The content areas have been extracted from the McDougal-Littell Algebra II textbook according to the sequence used therein. A selection of application problems featuring astronomy, earth science and space exploration were then designed to support each specific topic, often with more than one example in a specific category.


Scientists Take Temperatures of Sun's Corona, Yellowstone's Geysers

Harvard's astronomers are looking up in the sky, and for geologists are looking down in the ground. Both are looking for the same thing.

In each department, men are interested in the temperatures of things they can't touch, Professor Donald H. Menzel wants to explain the temperature of the corona, a collar of thin gas around the sun. Professor Louis C. Graton wants to find the temperature of geyser holes.

Menzel, whose thermometers must reach 93,000,000 miles, seems to have the tougher job. The temperature of the sun is 6000 Centigrade, but the temperature of the corona, which the naked eye can see only during total eclipses, appears to be 1,000,000. That's what Menzel is trying to explain, but it's only one of his worries.

The Sun Edits a Telegram

His chief problem is how the sun affects the early--why radios go on the blink when sunspots are heavy, for instance, and why a big tongue of fire on the sun will change the words on a telegram.

To solve all this, Menzel directs a cone-roofed observatory in Colorado, and a new station in New Mexico, close to the site of the first atom bomb. The observatories are equipped with spectrohelioscopes-- astronomical X-ray machines that penetrate to the inner layers of the sun--and with coronoscopes, which blot out the sun like an eclipse, so that the other corona can be watched. Menzel went west a few months ago to spend all his time at the solar stations, on the Astronomy Department's biggest project.

Professor Graton also goes west for his work--out to Yellowstone National Park and the geyser country. Last summer he wired up a cable with six electric thermometers, all recording simultaneously on a remote sheet of graph paper. He carried the device all over Yellowstone, and lowered it down the gullet of every geyser he could find.

Graton wanted to see how temperatures changed at various depths as the geyser went off. But once his experimenting brought up more than a handful of pen-line graphs.

It happened last summer, when Graton had dropped his cable into Old Faithful, to study the temperature of the world's most famous geyser. Suddenly his instruments tripped some unknown underground trigger, and Old Faithful-- which had faithfully erupted every 63 minutes since the Indians found it--blew its top 15 minutes too soon. Graton and his party didn't know the geyser was loaded, but they backed out of the way before anyone was hurt.

From the information gathered in Yellowstone, the geologists have prepared a Walt Disneyish movie, caricaturing a geyser and pointing out its temperature shifts. By these methods they hope to solve the mysteries of where the heat and the water comes from.

Graton works in hot ground. His fellow Department member, Professor Kirk Bryan, is an expert in cold ground. Bryan is doing research in the permanently frozen soil of Alaska, which presents problems to men building things like the Alcan highway.

But the best-known projects in the Geology Department are the seismographs of Professor L. Don Leet. Last Month Leet wrote earthquake history by picking up a dynamite explosion in South Holston, Tennessee--the farthest distance a man-made noise has ever been "heard."

The Tennessee Valley Authority had to touch off 681 tons of TNT before Lect's instruments could feel it, though. The blast ripped out one side of a mountain to supply crushed rock for a TVA dam. Present seismographs, says Leet, have never recorded an atom bomb explosion.

Working in an abandoned garage, Leet has developed a new labor-saving seismograph, which frees geologists from darkrooms and sub-cellar laboratories. Old seismographs recorded on photographic plates the new one relays earth tremors to a pen-and-paper graph on Leet's desk.

The Department's last big project is its X-ray lab, where scientists study the insides of crystals, learning how the molecules are put together.

Geologists are studying the shape of some of the smallest things in the Universe and astronomers are studying the shape of one of the largest--the Milky Way galaxy.

Road-Map of the Universe

Professors Bart J. Bok and Harlow Shapley are trying to map this huge disc-shaped "island universe," which includes the earth and every star that the naked eye can see. Both of them are measuring the distance to far-away suns, to determine their relative positions in the galaxy and thus the shape of the galaxy itself.

Bok measures the distance of a star by studying its color, which changes as the light passes through the dust clouds of space. Shapley looks at variable stars, which grow brighter and dimmer with a regular period. This period often depends on the absolute brightness of the star when Shapley knows the absolute brightness and the brightness as seen from the earth, he can easily determine the star's distance.

Professor Armin Deutsch is investigating another kind of variable star, which regularly changes color. Only 20 of them are known, and to astronomers the varying spectrum suggests that millions of tons of calcium are changing into other chemicals. So far Deutsch has not found much--only that these stars are surrounded by strong magnetic fields 5000 times greater than the earth's.

From the biggest astronomical bodies to the smallest--that takes one to the work Professor Fred L. Whipple, who studies the miner bodies of the solar

This is the third in a series of four articles on Harvard's scientists and what they are doing. It covers the Geology and Astronomy Departments. system meters, comets, and dust. A caravan of trucks--"Whipple's Wagon Train"--is now touring the Southwest, snapping pictures of meteors every night to discover their evolution and habits.

From the cosmic dust, Whipple has drawn a theory on the origin of the earth-now probably the ranking theory among astronomers. He hypothesizes that the solar system was once all dust, and that the dust collected to form planets. And what, at first, drove the dust together? Not gravity, says Whipple, and not molecular attraction--but the seemingly insignificant push exerted by light beams, streaming out from the sun

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Watch the video: Relative brightness. Optics Trade Debates (May 2022).