# Why does the Solar Wind consist of charged particles?

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The Wikipedia article on Solar Wind gives the following explanation:

The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between 0.5 and 10 keV.

If the solar wind consists of both electrons and protons, why don't they combine and give Hydrogen or any other elements? Then, why would Solar Wind be charged? Is that because of the high kinetic energy of the ejected particles? If yes, let us consider a proton and an electon. They must be of almost the same energy if so there will be no relative kinetic energy, so I think they must combine in the same way as if they were at rest or in motion at small velocities.

You are right about the kinetic energy (ie the fast motion) of the particles being the reason, but wrong when you say

They must be of almost the same energy if so there will be no relative kinetic energy,

The individual particles are moving fast, but there is huge variation between them in just how fast and in what direction. In other words, the solar wind is very hot. The extent of disorderly motion in a gas is measured by its temperature. The wikipedia article you link gives temperatures of 100 000 to 800 000 Kelvins. So if an electron and proton did happen to collide gently enough to form an atom of hydrogen, another particle would probably smash into them before very long and knock the electron off again.

You might think they are all moving away from the Sun, but you have to allow for the effects of magnetic fields. Charged particles moving in a magnetic field curve around in all sorts of ways, and, additionally, moving charged particles create a magnetic field which affects other particles. So the movement is extremely turbulent.

Expanding on @Steve Lintons answer: In physics no quantity is just large, a quantity can only be large relative to some other quantity.

So here we want to compare the kinetic energy to something, and that must be the binding energy of i.e. a hydrogen atom. The binding energy of the hydrogen atom is 13.6 eV. And this is much smaller than the keV energies that the solar wind possesses. So the particles are much too fast w.r.t. each other, they don't get the chance to recombine.

## Solar wind study explains why the northern lights are so spectacular

This may also explain why there are more blackouts in the Northern Hemisphere.

Throughout its 11-year cycle, the Sun occasionally sends explosive streams of charged particles our way. As they travel towards Earth, these particles interact with our planet's atmosphere. In the aftermath of the collision, a beautiful display of green, purple, and red lights may twist across the sky.

Near the Earth's North Pole, these lights are known as aurora borealis, or the famous northern lights. At the South Pole, the same process is called the aurora australis, or the less famous southern lights. In a new study, astronomers reveal the reason why the northern lights may be so much better known than their southern counterparts may be to do with the Sun itself.

What they discovered — In a new study published this week in the journal Nature, a team of scientists show how solar wind emitted by the Sun favors the Earth's magnetic North Pole rather than the South Pole. In other words, space weather may be worse in the Northern Hemisphere than in the south.

The findings have implications for how space weather affects us down here on Earth, with the Northern Hemisphere being subjected to harsher solar winds — and more potential damage.

Here's the background — Solar activity largely depends on the Sun's magnetic field.

The Sun’s magnetic field goes through a periodic cycle in which the south and north poles essentially switch spots, and it takes another 11 years or so for them to switch back. We're currently in Solar Cycle 25.

Solar flares are intense, bright bursts of radiation linked to the star's magnetic energy. As these flareups shoot out into space, lots of that energy, in the form of streams of charged particles, travels towards Earth.

Space weather is controlled by the flareups of the Sun that are ejected into outer space.

Earth's magnetic field shields us from most of these particles — like an umbrella protects from Earth's bad weather, so too does our magnetic field fend off the ill-affects of solar wind. But this shield is only so strong, and sometime the charged particles can travel along our planet's magnetic field lines at the North and South Poles and seep into Earth’s atmosphere.

As the particles interact with the different gases found in our planet's atmosphere, namely oxygen and nitrogen, the atoms are excited by the interaction and emit light. This process is what causes the auroras that we see at the North and South Poles

What's new — Scientists had assumed solar wind travels evenly to both the Southern and Northern Hemispheres, with an even distribution of electromagnetic radiation to the North and South Poles.

But the new study suggests solar wind tends to favor the north, with more charged particles traveling towards the Northern Hemisphere.

The team of scientists behind the new study believe that this asymmetry may be caused by the fact that Earth's magnetic South Pole is further away from Earth’s spin axis than the North Pole, affecting how much energy makes its way towards the Northern and Southern Hemispheres.

“We are not yet sure what the effects of this asymmetry might be, but it could also indicate a possible asymmetry in space weather and perhaps also between the aurora australis in the south and the aurora borealis in the north," Ivan Pakhotin, a researcher at University of Alberta's physics department and lead author behind the new study, said in a statement.

How they did it — The discovery was made using the European Space Agency's Swarm satellites, a group of three identical satellites named Alpha, Bravo, and Charlie that launched into space in November, 2013 to provide precise measurements of Earth's magnetic field and interactions in the planet's atmosphere.

Why it matters — Space weather is no joke. Coronal mass ejections are highly energetic eruptions from the Sun and the main source of major space weather events.

Essentially, these eruptions are giant bubbles of gas and magnetic flux released from the Sun with up to a billion tons of charged particles, traveling at high speeds that can reach several million miles per hour. These clouds, and the shock waves they cause, can sometimes reach Earth and cause geomagnetic storms.

These storms, if they are especially powerful, can wreak havoc on the electronics within satellites, pose a threat to astronauts with their radiation, and disrupt power grids on Earth enough to black-out a city.

Back in 1859, an outburst of plasma from the Sun's corona, now known as the Carrington Event, knocked out telegraph systems all over the world. As some telegraph operators tried to send and receive messages, they received electric shocks.

What's next — Scientists are working towards being able to better forecast space weather so that they can predict when these powerful solar storms may occur.

“The Sun’s activity, such as mass coronal ejections, can have potentially serious consequences for our modern way of living," Ian Mann, a researcher from the University of Alberta, and co-author of the study, said in a statement.

"Studying the underlying physics of space weather and the complexities of our magnetic field is very important to building up early warning systems and designing electrical grids better able to withstand the disturbances the Sun throws at us," Mann says.

Scientists will need to investigate the Sun's activity further in order to find out how much this asymmetrical distribution of its electromagnetic radiation affects our planet, and whether the Northern Hemisphere may be more vulnerable to damage during a solar storm than the Southern Hemisphere.

#### BU astrophysicist and collaborators reveal a new model of our heliosphere that’s shaped somewhere between a croissant and a beach ball

You are living in a bubble. Not a metaphorical bubble—a real, literal bubble. But don’t worry, it’s not just you. The whole planet, and every other planet in the solar system, for that matter, is in the bubble too. And we may just owe our very existence to it.

Space physicists call this bubble the heliosphere. It is a vast region, extending more than twice as far as Pluto, that casts a magnetic “force field” around all the planets, deflecting charged particles that would otherwise muscle into the solar system and even tear through your DNA, should you be unlucky enough to get in their way.

The heliosphere owes its existence to the interplay of charged particles flowing out of the sun (the so-called solar wind) and particles from outside the solar system. Although we think of the space between the stars as being perfectly empty, it is actually occupied by a thin broth of dust and gas from other stars—living stars, dead stars, and stars not yet born. Averaged across the whole galaxy, every sugar-cube-sized volume of space holds just a single atom, and the area around our solar system is even less dense.

The solar wind is constantly pushing out against this interstellar stuff. But the farther you get from the sun, the weaker that push becomes. After tens of billions of miles, the interstellar stuff starts to push back. The heliosphere ends where the two pushes balance out. But where is this boundary, exactly, and what does it look like?

Merav Opher, BU professor of astronomy. Photo by Cydney Scott

Merav Opher, a professor of astronomy at Boston University’s College of Arts & Sciences and the Center for Space Physics, has been examining those questions for almost 20 years. And lately, her answers have been causing a stir.

Because our whole solar system is in motion through interstellar space, the heliosphere, despite its name, is not actually a sphere. Space physicists have long compared its shape to a comet, with a round “nose” on one side and a long tail extending in the opposite direction. Search the web for images of the heliosphere, and this is the picture you’re sure to find.

But in 2015, using a new computer model and data from the Voyager 1 spacecraft, Opher and her coauthor, James Drake of the University of Maryland, came to a different conclusion: they proposed that the heliosphere is actually shaped like a crescent—not unlike a freshly baked croissant, in fact. In this “croissant” model, two jets extend downstream from the nose rather than a single fade-away tail. “That started the conversation about the global structure of the heliosphere,” says Opher.

Hers wasn’t the first paper to suggest that the heliosphere was something other than comet-shaped, she points out, but it gave focus to a newly energized debate. “It was very contentious,” she says. “I was getting bashed at every conference! But I stuck to my guns.”

Then, two years after the “croissant” debate began, readings from the Cassini spacecraft, which orbited Saturn from 2004 until 2017, suggested yet another vision of the heliosphere. By timing particles echoing off the boundary of the heliosphere and correlating them with ions measured by the twin Voyager spacecraft, Cassini scientists concluded that the heliosphere is actually very nearly round and symmetrical: neither a comet nor a croissant, but more like a beach ball. Their result was just as controversial as the croissant. “You don’t accept that kind of change easily,” says Tom Krimigis, who led experiments on both Cassini and Voyager. “The whole scientific community that works in this area had assumed for over 55 years that the heliosphere had a comet tail.”

Now, Opher, Drake, and colleagues Avi Loeb of Harvard University and Gabor Toth of the University of Michigan have devised a new three-dimensional model of the heliosphere that could reconcile the “croissant” with the beach ball. Their work was published in Nature Astronomy on March 16.

Unlike most previous models, which assumed that charged particles within the solar system all hover around the same average temperature, the new model breaks the particles down into two groups. First are charged particles coming directly from the solar wind. Second are what space physicists call “pickup” ions. These are particles that drifted into the solar system in an electrically neutral form—because they aren’t deflected by magnetic fields, neutral particles can “just walk right in,” says Opher—but then had their electrons knocked off.

The New Horizons spacecraft, which is now exploring space beyond Pluto, has revealed that these particles become hundreds or thousands of times hotter than ordinary solar wind ions as they are carried along by the solar wind and sped up by its electric field. But it was only by modeling the temperature, density, and speed of the two groups of particles separately that the researchers discovered their outsized influence on the shape of the heliosphere.

That shape, according to the new model, actually splits the difference between a croissant and a sphere. Call it a deflated beach ball, or a bulbous croissant: either way, it seems to be something that both Opher’s team and the Cassini researchers can agree on.

The new model looks very different from that classic comet model. But the two may actually be more similar than they appear, says Opher, depending on exactly how you define the edge of the heliosphere. Think of transforming a gray scale photo to black-and-white: the final image depends a lot on exactly which shade of gray you pick as the dividing line between black and white.

So why worry about the shape of the heliosphere, anyway? Researchers studying exoplanets—planets around other stars—are keenly interested in comparing our heliosphere with those around other stars. Could the solar wind and the heliosphere be key ingredients in the recipe for life? “If we want to understand our environment we’d better understand all the way through this heliosphere,” says Loeb, Opher’s collaborator from Harvard.

And then there’s the matter of those DNA-shredding interstellar particles. Researchers are still working on what, exactly, they mean for life on Earth and on other planets. Some think that they actually could have helped drive the genetic mutations that led to life like us, says Loeb. “At the right amount, they introduce changes, mutations that allow an organism to evolve and become more complex,” he says. But the dose makes the poison, as the saying goes. “There is always a delicate balance when dealing with life as we know it. Too much of a good thing is a bad thing,” Loeb says.

When it comes to data, though, there’s rarely too much of a good thing. And while the models seem to be converging, they are still limited by a dearth of data from the solar system’s outer reaches. That is why researchers like Opher are hoping to stir NASA to launch a next-generation interstellar probe that will cut a path through the heliosphere and directly detect pickup ions near the heliosphere’s periphery. So far, only the Voyager 1 and Voyager 2 spacecrafts have passed that boundary, and they launched more than 40 years ago, carrying instruments of an older era that were designed to do a different job. Mission advocates based at Johns Hopkins University Applied Physics Laboratory say that a new probe could launch some time in the 2030s and start exploring the edge of the heliosphere 10 or 15 years after that.

“With the Interstellar Probe we hope to solve at least some of the innumerous mysteries that Voyagers started uncovering,” says Opher. And that, she thinks, is worth the wait.

The researchers thank the staff at NASA Ames Research Center for the use of the Pleiades supercomputer. This work was also supported by NASA and by the Breakthrough Prize Foundation.

The Solar wind does indeed exert a force on the planets, however it turns out that the force is so small that it has no measurable effect.

The force can be calculated using the fact that force is equal to the rate of change of momentum. Suppose the total mass of all the Solar wind particles hitting the Earth per second is $M$, and the average velocity of the particles is $v$, then the force the solar wind exerts on the Earth is simply:

Off-hand I don't know what the mass flux and velocity are, but the Wikipedia article on the solar wind reports the pressure, $P$, produced by the wind at the Sun-Earth distance to be 1 to 6 nano-Pascals. The total force on the Earth is this pressure multiplied by the cross sectional area $pi r^2$. The radius of the Earth is about 6,371,000 metres, so we get:

$F = P imes pi r^2 approx 130 , ext, 800 , ext$

To see why this is negligible, let's compare it with the gravitational force between the Sun and the Earth. This is given by Newton's law of gravity:

$F approx 3.54 imes 10^ <22>, ext$

so the force from the Solar wind is only about 0.000000000000001% ($10^<-15>\%$) of the gravitational force.

The solar wind does disrupt the planets.

If a planet does not have a magnetic field (for reasons described later), the solar wind can strip an atmosphere through a process called sputtering. Without a magnetic field, the solar wind is able to hit the planet's atmosphere directly. The high-energy solar wind ions can accelerate atmosphere particles at high altitudes to great enough speeds to escape.

The relative importance of this effect compared to other forms of atmospheric escape is a topic of active research. The NASA Maven probe is one of the latest tools to answer this question:

Scientists have thought that Mars lost much of its atmosphere through a process known as stripping, when the solar wind pushed a lighter isotope (type) of hydrogen out into space, leaving a heavier isotope called deuterium behind. As hydrogen escaped, the atmosphere thinned. This could account for why water stopped flowing on the Martian surface billions of years ago.

So, although not all scientifics agree (Wikipedia contains an unsourced claim that those NASA Maven scientists are in error!), to claim that the solar wind does not disrupt the planets, is currently premature.

The solar wind is a flow of charged particles, called a plasma, constantly eminating from the sun. Plasmas can exhibit an interesting property called frozen-in whereby the magnetic field and bulk flow are locked together (well, technically it's a flux freezing condition but. ). The magnetic field does not really move, rather the sources move, but that's a rather nuanced point. The point is that a magnetized plasma is constantly bombarding planetary magnetospheres.

The answer to your question is that the solar wind does in fact affect planetary atmospheres and certainly their magnetospheres. Below I will describe one example for how this can affect our atmosphere. John already explained that they dynamic pressure is quite low, thus the "wind" itself is not really an issue. So I will focus on the electromagnetic effects.

The Aurora
Through a process called magnetic reconnection, energy and momentum can transport across the magnetopause into the magnetosphere. The reconnection process results in a reconfiguration of the magnetic field topology and new stress/strains are imposed on the field in different places within the magnetosphere.

One of those places is the geomagnetic tail (i.e., anti-sunward side of the planet), where enhanced solar wind flows and dayside reconnection can lead to "stretching" of the topology. The stretched fields in the tail can also reconnect. One of the consequences of the reconnection process is similar to that of a slingshot. When magnetic field lines are stretched and then released, they can react in manner similar to a relaxing rubber band. Since charged particles generally do not like to move across the magnetic field, due to the Lorentz force, the relaxation of the magnetic field can result in significant particle acceleration/energization.

So when the tail fields relax, they can accelerate particles toward the planet. As the particles near the planet, encountering different plasma densities and field intensities, they can be further accelerated by several other processes. I discussed one of those processes here. Some of these particles will gain enough energy and have a small enough pitch-angle that they can enter the planetary atmosphere. The deposition of energetic particles can excite neutral atoms and lead to the emission of light, called the aurora.

Other Effects
There are several other effects of the solar wind on planets including the rate of polar outflow (i.e., process by which charge particles "leak" out of an atmosphere along the magnetic field), ground induced currents, effects on total electron content, etc.

So in general, there are many ways in which the solar wind can affect a planet, whether it is internally magnetized or not.

## Energy from solar wind favors the north

Using information from ESA’s Swarm satellite constellation, scientists have made a discovery about how energy generated by electrically-charged particles in the solar wind flows into Earth’s atmosphere – surprisingly, more of it heads towards the magnetic north pole than towards the magnetic south pole. Credit: ESA/Planetary Visions

Using information from ESA's Swarm satellite constellation, scientists have made a discovery about how energy generated by electrically-charged particles in the solar wind flows into Earth's atmosphere—surprisingly, more of it heads towards the magnetic north pole than towards the magnetic south pole.

The sun bathes our planet with the light and heat to sustain life, but it also bombards us with dangerous charged particles in the solar wind. These charged particles have the potential to damage communication networks, navigations systems such as GPS and satellites. Severe solar storms can even cause power outages, such as the major blackout that Quebec in Canada suffered in 1989.

Our magnetic field largely shields us from this onslaught.

Generated mainly by an ocean of superheated, swirling liquid iron that makes up the outer core around 3000 km beneath our feet, Earth's magnetic field is like a huge bubble protecting us from cosmic radiation and the charged particles carried by powerful winds that escape the sun's gravitational pull and sweep across the Solar System.

Like a bar magnet, Earth's magnetic field at the surface is defined by the north and south poles that align loosely with the axis of rotation.

The aurorae offer visual displays of the consequences of charged particles from the sun interacting with Earth's magnetic field.

Until now, it was assumed the same amount of electromagnetic energy would reach both hemispheres. However, a paper, published in Nature Communications, describes how research led by scientists from the University of Alberta in Canada used data from ESA's Swarm mission to discover, unexpectedly, that the electromagnetic energy transported by space weather clearly prefers the north.

These new findings suggest that in addition to shielding Earth from incoming solar radiation, the magnetic field also actively controls how the energy is distributed and channeled into the upper atmosphere.

The paper's lead author, Ivan Pakhotin who is carrying out this research as part of ESA's Living Planet Fellowship, explains, "Because the south magnetic pole is further away from Earth's spin axis than the north magnetic pole, an asymmetry is imposed on how much energy makes its way down towards Earth in the north and south. There seems to be a differential reflection of electromagnetic plasma waves, known as Alfven waves.

"We are not yet sure what the effects of this asymmetry might be, but it could also indicate a possible asymmetry in space weather and perhaps also between the Aurora Australis in the south and the Aurora Borealis in the north. Our findings also suggest that the dynamics of upper-atmospheric chemistry may vary between the hemispheres, especially during times of strong geomagnetic activity."

Swarm is ESA’s first constellation of Earth observation satellites designed to measure the magnetic signals from Earth’s core, mantle, crust, oceans, ionosphere and magnetosphere, providing data that will allow scientists to study the complexities of our protective magnetic field. Credit: ESA/AOES Medialab

Ian Mann from the University of Alberta said, "The sun's activity, such as mass coronal ejections, can have potentially serious consequences for our modern way of living. Therefore, studying the underlying physics of space weather and the complexities of our magnetic field is very important to building up early warning systems and designing electrical grids better able to withstand the disturbances the sun throws at us.

"We are fortunate that we have ESA's three Swarm satellites in orbit, delivering key information that is not only vital for our scientific research, but can also lead to some very practical solutions for our daily lives."

In orbit since 2013, the three identical Swarm satellites have not only return information about how our magnetic field protects us from the dangerous particles in solar wind, but about how the field is generated, how it varies and how the position of magnetic north is changing.

## What is the speed of Solar Wind?

The solar wind radiates in all directions from the Sun. The speed at which it is radiated differs depending on where the particles originated. The average speed of the wind as it is radiated from the Sun’s surface is approximately 300 – 400 kilometers per second. Coronal holes, or large regions on the Sun’s surface that are cooler than surrounding areas, eject these particles at speeds up to 800 kilometers per second.

The reason for the above differences is counterintuitive. The solar wind is actually radiated faster from cooler parts of the Sun than hotter parts of the Sun. This is because the strength of the Sun’s magnetic field over these holes is lower than the magnetic fields over the surrounding hotter areas. The charged particles radiating from the cooler areas then have less resistance to overcome on their way to the solar system.

## The solar wind and Earth

Beautiful aurorae are caused when charged particles, like protons and electrons, stream into Earth ’ s atmosphere and excite the nitrogen and oxygen atoms in the upper atmosphere. When these atoms return to their normal, non-excited state, they emit the shimmering, green or red curtains of light (the Northern Lights, or aurora borealis) familiar to individuals living parts of Canada or the northern United States.

If the solar wind is continuous, why do humans not see aurorae all the time? Earth is surrounded by a magnetic field, generated by its rotation and the presence of molten, conducting iron deep in its interior. This magnetic field extends far into space and deflects most particles that encounter it. Most of the solar wind therefore streams around Earth before continuing on its way into space. Some particles get through, however, and they eventually find their way into two great rings of charged particles that surround the entire Earth. These are called the Van Allen belts, and they lie well outside the atmosphere, several thousand kilometers out.

Besides the gentle, continuous generation of the solar wind, however, the sun also periodically injects large quantities of protons and electrons into the solar wind. This happens after a flare, a violent eruption in the sun ’ s atmosphere. When the burst of particles reaches Earth, the magnetic field is not sufficient to deflect all the particles, and the Van Allen belts are not sufficient to trap them all above the atmosphere. Like water overflowing a bucket, the excess particles stream along the Earth ’ s magnetic field lines and flow into the upper atmosphere near the poles. This is why aurorae typically appear in extreme northern or southern latitudes, though after particularly intense solar flares, aurorae may be seen in middle latitudes as well.

## Auroras in History

There is no doubt that some spectacular auroral displays have occurred throughout Earth’s history, but in recorded history, the displays that occurred on August 28 th , and again on September 2 nd in 1895 must surely take pride of place as being the most powerful such event ever recorded.

On this occasion, the aurora was caused by an enormous and tremendously violent coronal mass ejection event on the Sun, and provided the first unambiguous evidence that auroral displays and electricity are inextricably linked.

On this occasion, parts of the telegraph network across the USA were of an appropriate length and orientation to conduct the electricity caused by the aurora, and some telegraph operators were able to communicate over long distances across the network solely through the electricity provided by the aurora. Below is a partial transcript of one such conversation-

Boston operator (to Portland operator): “Please cut off your battery [power source] entirely for fifteen minutes”.

Portland operator: “Will do so. It is now disconnected.”
Boston: “Mine is disconnected, and we are working with the auroral current. How do you receive my writing?”
Portland: “Better than with our batteries on. – Current comes and goes gradually.”
Boston: “My current is very strong at times, and we can work better without the batteries, as the aurora seems to neutralize and augment our batteries alternately, making current too strong at times for our relay magnets. Suppose we work without batteries while we are affected by this trouble.”

## Solar wind

The sun&rsquos super-hot atmosphere, the corona, emits the solar wind, a stream of extremely fast (250 to 750 km/s) charged particles. This wind pushes against the particles in comet tails, causing them to point away from the sun. Fortunately we are protected from the particles in the solar wind by earth&rsquos strong (albeit decaying) magnetic field, which traps them. We see the results as the aurorae, which are most prevalent near the poles in the wintertime.

Some might ask, why would God make such a dangerous sun? But in reality, our sun is ideal for life here on earth. First of all, it is remarkably stable. It is very quiescent compared to most stars of its type. And the latest Voyager 2 findings show that the powerful solar wind produced by our sun is an important design feature.

## Radiation & Health: Humans in Space

Once we leave the atmosphere and travel beyond the cocoon of Earth’s magnetic field, the radiation environment changes dramatically. With talk increasingly turned towards a return to the Moon and manned trips to Mars, what radiation problems will our astronauts encounter?

Those of us living on Earth are in a privileged location with regards to radiation. Space’s radiation environment includes charged particles from the Sun and elsewhere in the galaxy and high-energy photons in the form of x-rays, UV, and gamma radiation. But we see virtually none of that at the surface of our planet – the Earth’s magnetic field envelopes our planet, diverting the charged particles. The van Allen belts are like two nested donuts, one filled with positively charged particles and the other filled with electrons and other particles with a negative charge. Photons lacking an electrical charge are not affected by our magnetic field, but their intensity is reduced by a factor of over a billion as they plow through the 100 km of our atmosphere.

After leaving our atmosphere, the X-rays, gamma rays, and the Sun’s ultraviolet are unattenuated. But these are not very damaging, and they’re relatively easy to control – more critical and more problematic (not to mention much more interesting) are the high-energy particles from the Sun and beyond the Solar System.

Particles from the Sun

The Sun emits thousands of tons of gas every second – the solar wind. This “wind” consists of ionized gas – mostly protons, electrons, and helium nuclei. According to NASA, the solar wind involves about 400 million protons, moving at 400 km/second, passing through every square cm of the space near Earth every second. The proton’s speed seems fast, but its “negligible” weight means they have little energy, and such particles can penetrate only a relatively short distance into matter. This is the primary reason why charged particles from the Sun cannot penetrate to the Earth’s surface and why the Sun provides very little sea-level radiation exposure.

In space, outside the protection of Earth’s magnetic field, solar charged particles are a bigger concern. High-energy solar flares (containing photons) or coronal mass ejections (containing charged particles) directed toward the spacecraft pose the greatest risk. In such cases, the crew can receive a fatal dose of radiation in a short time unless they can shelter in a “storm cellar” surrounded by water, plastic, or other forms of radiation shielding.

Particles from other stars

The highest cosmic radiation dose to the Earthbound is from heavy ions blasted off from exploding stars elsewhere in our galaxy. This sounds improbable – except that scientists have found “live” radioactive iron and heavy isotopes of plutonium in deep-sea sediments that could only have been synthesized in a supernova. These supernova particles tend to have a higher mass and electrical charge, making them far more damaging to human tissue. Luckily, our astronauts don’t necessarily have to worry about a wave of supernova-produced high-energy particles slamming unexpectedly into their spacecraft the galactic magnetic field and the great distance make this highly improbable.

The effect of heavier particles, those that astronauts are likely to experience, are not well understood. An iron nucleus passing through the body, for example, is likely to deliver its high energy to only a handful of cells and none to neighboring cells. On the other hand, an iron nucleus that strikes an atom in a spaceship's skin can splinter into smaller pieces emerging like a shotgun spray of particles rather than the single particle that struck the spacecraft. This small shower of high-mass charged particles, each of which can damage a handful of cells, can cause more damage and risk of future cancer.

What we don’t know

Each type of tissue in the human body reacts differently to radiation exposure our central nervous system is remarkably resistant to radiation, while the cells that line our stomachs are quite sensitive. We have a reasonably good understanding of how DNA is damaged by beta and gamma radiation and how the body repairs it. But when it comes to understanding the damage caused by an iron nucleus blasted out of a dying star by unimaginable forces, we’re not quite as certain.

The nearest analog is an alpha particle, but alphas are much lighter and cannot penetrate through nearly as much tissue. And, of course, we have never seen humans who have been exposed to the full brunt of a coronal mass ejection or a high-energy solar flare. Even with a shielded “storm cellar” in which to take refuge, we simply don’t know if there might be unanticipated effects.

One thing that we can and have done to mitigate the risks of travel beyond Earth’s magnetic field is to assign dose limits to astronauts the same as we do to workers at hospitals, nuclear power plants, and other jobs involving radiation exposure. NASA’s limits for astronauts are quite a bit higher than for Earthbound radiation workers and change over one’s career to account for an increased number of years in space as well as a lower risk of developing cancer from radiation exposure as we age. [1]

With long-haul space flights apparently on the horizon, NASA is contemplating raising these dose limits to allow their more experienced astronauts to participate and benefit long-duration missions. The question is, what’s a reasonable dose limit that balances omitting a valuable crew member with putting that crew member at risk of developing cancer? The National Academies of Science is currently studying this at the request of NASA.

Putting risks in perspective

There’s one other factor to consider as well – what exposure risk astronauts are willing to accept. About a decade or so ago, when baseball was embroiled in a steroid controversy, the Tour de France suffering another doping scandal, and aspersions were cast upon several Olympics sports, I was listening to an interview with an Olympic athlete. The interviewer mentioned all of the adverse health effects associated with the use of performance-enhancing drugs and asked if the athlete was aware of the risks he was taking. The athlete replied that he was – but that if it improved his chances of winning Olympic Gold, it would be worth it.

Those of us who are not Olympic athletes might question his judgment – not to mention the ethics of doping. We cannot doubt that the athlete was fully aware of the risks and found them acceptable to achieve something hugely important to him. Similarly, for an astronaut facing the challenge and dreams of a lifetime, the risks from radiation exposure en route to Mars and back are also likely to be acceptable.

This shouldn’t surprise us – risk has always attended exploration, yet there have always been explorers. Exploration, by definition, launches one into the unknown to see what’s there. A prudent explorer does what they can to minimize the risks, but they know that risks may not be contemplated and cannot be banished. While exposure to high-mass, high-energy cosmic rays is a bit different from uncharted reefs, being becalmed, or fleeing a hungry predator, the decision-making remains the same – is the astronaut willing to accept the risk, knowing what it means, in exchange for being able to make the trip.

[1] In particular, a male astronaut just beginning his career will not be permitted to receive more than 150 rem of cumulative radiation exposure. Over the following decades, his allowable lifetime exposure will increase to 250 and as much as 400 rem of cumulative exposure if he continues going into space until age 55. Women have similar limits, albeit somewhat lower, to account for some tissues' greater sensitivity to radiation exposure.

## Prominences

prominences: see chromosphere.
See more Encyclopedia articles on: Astronomy: General

Introduction to the Sun Solar Structure Size, Mass Flares, Solar Wind, Prominences Sun's Birth Solar Eclipses Activities,
Web Links Solar Rotation Sunspots Sun's Death
Solar Flares, Prominences, the Solar Wind, and Coronal Mass Ejections
SOLAR FLARES .

But as you point out, anyone can see prominences any day with the aid of a telescope fitted with a hydrogen-alpha (Hα) filter. This special filter blocks all light from the Sun except for the red light emitted by excited hydrogen atoms at a wavelength of 656.3 nanometers (6563 angstroms).

The Sun is also an active star that displays sunspots, solar flares, erupting

, and coronal mass ejections. These phenomena, which are all related to the Sun's magnetic field, impact our near-Earth space environment and determine our "space weather".

and Filaments
When a prominence is viewed from a different perspective so that it is against the sun instead of against space, it appears darker than the surrounding background. This formation is instead called a solar filament.

(when seen near the solar limb) and plages (when seen superimposed on the solar disk) are corona regions that appear very bright in the visible part of the spectrum. These features often appear as streamers or filaments, suggesting a structure related to magnetic fields.

Huge columns of gas arcing out over the sun's limb or horizon. When the same structures are seen against the backdrop of the sun, they are called filaments. They are made of cooler solar material, or plasma, supported in the sun's atmosphere by magnetic fields.

appear as huge arching columns of gas above the limb (edge) of the Sun.

are anchored to the Sun's surface in the photosphere and extend outwards into the Sun's corona.

. A mass of glowing gas, mainly hydrogen, that rises from the surface of the Sun.
Proper motion. The motion of the stars relative to each other, caused by their actual motion in different directions at different speeds through space.

bright areas in the Sun's atmosphere from which hot gases shoot out.
solar system .

with Na and Mg emissions and centrally reversed Balmer lines p. 1069
G. Stellmacher and E. Wiehr
DOI: .

Gases trapped at the edge of the Sun which appear to shoot outward from the Sun's surface.
SOLAR SYSTEM
The Sun and all of the planets, comets, etc. which revolve around it.

are seen projecting out above the limb, or edge, of the Sun.

are giant arching columns of gas in the corona that often form just before a sunspot appears below them in the photosphere. They are a result of the interactions of the gas in the corona with the magnetic fields of the Sun.

occur mainly in two zones in either hemisphere, chiefly away from active regions.

15.3 Solar Activity above the Photosphere
proper motion17.4 Using Spectra to Measure Stellar Radius, Composition, and Motion
Proteins30.2 Astrobiology .

are large flames erupting from the burning surface of the Sun. (Hint)
13. Positrons are the antiparticles of electrons. (Hint)
14. Nuclei are held together by the strong force. (Hint) .

Filaments are dark, thread-like features that are seen in red light (H-&alpha). They are dense, somewhat cooler, clouds of material. They are suspended above the surface of the sun by loops of magnetic fields.

One feature shown in such pictures are

, large clouds of denser and cooler gas, rising high above the photosphere. Some of them stand out against the dark background sky on the visible edge ("limb") of the Sun, and if one watches them for a while, one can see material falling back towards the Sun.

Sometimes the Sun releases built-up energy (presumably from the magnetic fields) in the form of

, solar flares, and coronal mass ejections. These alterations in the Sun's magnetic field can affect communications on Earth.

CMEs are often but not always associated with erupting

, disappearing solar filaments, and flares. coronal rain (CRN). Material condensing in the corona and appearing to rain down into the chromosphere as observed in H alpha at the solar limb above strong sunspots. coronal streamer.

Filaments are dark structures when seen against the bright solar disk, but appear bright when seen over the solar limb, Filaments seen over the limb are also known as

can be seen along the limb (in red) as well as extensive coronal filaments.The Earth orbits the Sun once a year, and the Moon orbits the Earth once a month it turns out that the planes of the Moon's and Earth's orbits are almost, but not quite, aligned (the offset is about 5 degrees).

The corona is the seat of the solar wind

are threads of cool gas that lie in the corona and are supported by magnetic fields. (From Stars, J. B. Kaler, Scientific American Library, Freeman, NY, 1992.)
After 4.

Some of the most startling details of the sun's outer edge are the

, magnetically directed arcs of plasma on the limb of the solar disk. Two such features were visible on the sun's right limb..

When there are many sunspots visible on the surface of the Sun (at the time of solar maximum), other features such as solar flares and

are also visible. These are eruptions from the surface, thought to be associated with sunspot activity, but what actually causes them is not completely understood.

Solar flares are eruptions more powerful than surge

(a flare is shown in the Sun + planets montage above). They will last only a few minutes to a few hours. A lot of ionized material is ejected in a flare.

74k gif
solar magnetic fields 170k gif
X-ray images of the Sun from nascom ftp directory
Flare in H-Alpha 160k gif
Hedgerow Prominence 49k gif
Loops Prominence 76k gif
Prominenece 78k gif .

are often visible on the Sun's limb (left).
The Sun's output is not entirely constant. Nor is the amount of sunspot activity. There was a period of very low sunspot activity in the latter half of the 17th century called the Maunder Minimum.

He initiated in 1866 the spectroscopic observation of sunspots, and in 1868 he found that solar

are upheavals in a layer around the Sun, which he named the chromosphere.

The Sun's faint corona will be visible, and even the chromosphere, solar

, and possibly even a solar flare may be visible.

Mini documentary: How big are solar flares'

as compared to Earth.
The Most Powerful Solar Flares Ever Recorded - spaceweather.com (X9+ summary)
Most Energetic Flares since 1976 (X5.7+ details)
Davis, Chris. "Tracking the X Flare".

After 38 seconds of chromosphere,

and coronal streamers - with time to shoot a sequence of magical photos - you'll observe another long-lasting diamond ring and the wide arc of Baily's beads as daylight returns.

I'm talking about the solar flares/

. You could see them using telescope with an Hα filter. Such filters are quite expensive, and the cheapest "solar telescopes" (regular telescope + Hα filter mounted on the objective lens) are around \$500.

Prominence is a structure in the Sun's corona consisting of cool plasma supported by magnetic fields.

seen on the disk also are known as filaments.

SOHO satellite images of the Sun, highlighting huge clouds of cool, dense plasma and

suspended in the hot, thin corona (the outermost layer). Fusion takes
place deep within the inner core, hidden from our sight. [NASA]
Thanks to Mike Bolte (UC Santa Cruz) for the base contents of this slide.

How Solar Flares Affect Communication
How Do

Affect the Earth?
How Do Solar Flares Affect the Earth?
Does the Moon Have Solar Wind Storms?
The History of Solar Flares on Earth
How Does the Sun Affect the Earth?

Mass of hot, hydrogen rising from the Sun's chromosphere, best observed indirectly during a total eclipse. Eruptive

are relatively pacific but may last for months. [A84]
Solar Rotation .

Rapid rotation also drives increased levels of stellar activity such as starspots, flares and

, producing X-ray emission over 4,000 times more intense than the peak emission from the Sun.
KSw 71 is thought to have recently formed following the merger of two Sun-like stars in a close binary system.

An explosion of hot gas that erupts from the Sun's surface. Solar

are usually associated with sunspot activity and can cause interference with communications on Earth due to their electromagnetic effects on the atmosphere.
Proper Motion .

A bright eruption of hot gas in the Sun's photosphere. Solar

are usually only detectable by specialised instruments but can be visible during a total solar eclipse.
Solar Wind
A flow of charged particles that travels from the Sun out into the Solar System.

ACTIVE PERIOD - Lots of solar activity including sunspots, flares,

, and coronal mass ejections. Our Sun was most recently active during the late 1980's and early 1990's.
Photograph September 28, 1991
by Yohkoh Satellite .

Prominence - A region of cool gas embedded in the corona.

are bright when seen above the Sun's limb, but appear as dark filaments when seen against the Sun's disk
Proper Motion - The rate at which a star appears to move across the celestial sphere with respect to very distant objects .

The 11- or 22-year cycle with which such solar activity as sunspots, flares, and

varies
solar cell - (n.)
A device used for converting sunlight into electricity a photoelectric cell.

An elongated dark region on the surface of the Sun. They are solar

seen silhouetted against the photosphere.

As the starship approached the star, Data reported there was an unusual number of sunspots and eruptive

. As well, he noted that the magnetic field was extremely irregular.

Prominence: Hot gas hanging just above the solar surface, usually appearing as a red-colored arc or filament hovering in the lower part of the corona.

are quickly covered by the Moon after second contact and revealed just prior to third.

All eclipses of the Sun are interesting, but for sheer grandeur total eclipses are unrivalled only then can the solar chromosphere, the

and the corona be seen with the naked eye.

First discovered in 1892, the nebula complex IC 405 was eloquently described by Max Wolf in 1903 as "a burning body from which several enormous curved flames seem to break out like gigantic

". Eventually "The Flaming Star Nebula" became adopted as the popular name for IC 405. .

One fine Fall evening, Galileo pointed his telescope towards the one thing that people thought was perfectly smooth and as polished as a gemstone - the Moon. Imagine his surprise when found that it, in his own words, was "uneven, rough, full of cavities and

Sol is the star that the planets & comets in our solar system orbit around. The Sun is spectacular when viewed using a solar filter or dedicated solar scope for sunspots &

In the seven years after graduating from the Massachusetts Institute of Technology in 1890, he revolutionized solar observations with the invention of the spectroheliograph: an instrument that made it possible to photograph the Sun's

"Well, with my little refractor, the blackness of the maria, the brilliant white of the mountains, and the peppering of tiny craters down to the limit of visibility make it a feast for the senses that is very satisfying! Solar

, faculae, and what looks to me like cat hair on the Sun.

approximately 100 to 115 km wide where the Sun appears to be completely covered by the Moon for a short period of time (between 2 and 3 minutes). This is the most spectacular part of the eclipse, as those who are lucky enough to be in this corridor are able to see the Sun's corona, the chromosphere,

These dedicated solar scopes use a "hydrogen-alpha" filter which narrowly restricts the light to a wavelength of red light emitted by hydrogen (656.28 nm). Since the sun is mostly hydrogen, these filters do an excellent job of showing detailed solar activity such as flares,

of the Sun thus only when the eclipse is total can it be seen if even a tiny fraction of the solar surface is still visible it drowns out the light of the corona. At this point the sky is sufficiently dark that planets and brighter stars are visible, and if the Sun is active one can typically see solar

This allows us to see features of the Sun that would otherwise be invisible, except from outer space. These include the glorious corona, which stretches outwards from the Sun in all directions, and solar

- large arch-shaped structures observable in the corona.

The corona interacts with many of the more dynamic aspects of the solar atmosphere. Examples include

, which are great arcs of gas that extend outwards from the Sun, and solar flares which are great explosions and jets of gas from the solar surface.

are most easily visible close to the limb of the Sun, but some are also visible as bright streamers on the photosphere. promontorium A cape. pseudocrater A generally circular crater produced by a phreatic eruption resulting from emplacement of a lava flow over wet ground.

coronagraph An instrument for photographing the corona and

of the sun at times other than at solar eclipse. An occulting disk is used to block out the image of the body of the sun in the focal plane of the objective lens.

are usually only detectable by specialized instruments but can be visible during a total solar eclipse.
SOLAR MASS: The mass of the sun it is used as a standard weight against which other celestial objects can be compared.

The advantage of such a filter is that when used to observe the Sun, certain features (such as

) can be much more readily seen. hatches Hatches are lines on the edge of the screen showing intervals of right ascension and declination.