Now you see them... The sunspot group as observed by SOHO MDI today (NASA/SOHO)
[/caption]
After an extended period of calm for Solar Cycle 24, a cluster of sunspots have appeared on the disk of the Sun. Although we have observed sunspots since the beginning of this new solar cycle (which officially began on January 4th, 2008 with the observation of a high-latitude sunspot pair), this is the first time for many months “new” Cycle 24 sunspots have shown themselves. Before today, the sunspots (including occasional flares and coronal mass ejections) belonged to the previous cycle (Cycle 23). It would appear the spots have evolved into a cluster in a high-latitude location with the magnetic polarity consistent with this new cycle. But does this mean we can expect an increase in solar activity after this pretty dull period of “blank” solar disk observations? Your guess is as good as mine…
Overlapping solar cycles are natural occurrences, and extended solar minima are not unexpected, but many predictions of an extended period of solar calm have been put forward since Solar Cycle 24 appeared to shy away after the initial excitement in January. Although the Sun has been surprisingly quiet for several months, we’ve still had sporadic sunspot activity (plus the occasional flare and CME eruption), but none could be attributed to the new Cycle 24 (although I erroneously thought the August sunspot activity was due to Cycle 24, it was in fact due to the overlapping Cycle 23).
So how can we be so sure these new observations are of Cycle 24 spots and not Cycle 23 spots? After quickly glancing at the Solar and Heliospheric Observatory (SOHO) image (top), we can see a cluster of activity at a fairly high latitude. Generally speaking, one would expect sunspots at the beginning of a new cycle to appear at high latitudes. As the 11-year solar cycle progresses, sunspot activity will begin to drift equator-wards, to lower latitudes. “Old” Cycle 23 sunspots have generally appeared near the solar equator, so the sunspots observed today can be attributed to the “new” Cycle 24.
The clincher for identifying these spots as belonging to a new solar cycle is their magnetic polarity. Sunspots often appear in pairs of opposite polarity (i.e. one will be magnetic north, the other will be magnetic south), and this new cluster is consistent with the polarity expected for Cycle 24 sunspots. SOHO uses its Michelson Doppler Imager (MDI) Magnetogram instrument to observe magnetic polarity, and it would appear that the polarity of this sunspot cluster has an opposite magnetic north/south to previous Cycle 23 observations.
So does this mean we might see an increase in solar activity from here on in? Although this is an encouraging observation, the Sun could revert back to its “blank” state as quickly as it revealed these sunspots to SOHO. However, there is also a chance this could herald the beginning of accelerated solar activity, possibly still fulfilling NASA’s 2006 prediction that Solar Cycle 24 will be a “doozy.”
A Black Brant sounding rocket of the type that will carry SUMI above Earth's atmosphere (NASA)
[/caption]
Solar physicists will have the unprecedented opportunity to peer inside one of the most mysterious regions in the Sun’s atmosphere. Separating the chromosphere (at a temperature of a few thousand Kelvin) and the extended corona (at a temperature of over a million Kelvin) is a very thin layer about 5000 km above the photosphere (a.k.a. the Sun’s “surface”). The transition region dictates the characteristics of the hot plasma passing from the Sun into space and is right at the start of the solar-terrestrial chain, controlling space weather. We are unable to directly observe the transition region as it doesn’t radiate in wavelengths observable from the Earth’s surface, but it does emit UV radiation observable from space. So a group of solar researchers are packing some very sensitive instrumentation into a sounding rocket that will very briefly take some snapshots of the transition region. But they will have to be quick, from instrument deployment to re-entry, only eight minutes will be allowed to take the necessary UV spectroscopic observations…
To start us off, Jonathan Cirtain from the Marshall Space Flight Center explains this new mission in a nutshell:
“Early next year, we’re going to launch an experimental telescope that can measure vector magnetic fields in the transition region.” – Cirtain.
But why? Hasn’t the transition region been observed before? Actually, no. The surface of the Sun has been tirelessly studied, as has the solar atmosphere, but the thin layer separating the two has, so far, remained hidden from solar astronomers. “Just bad luck, really,” says Cirtain as he explains why the transition region has remained a mystery for so long. “Gas in the transition region doesn’t produce many strong spectral lines that we can see at visible wavelengths.” But it does radiate UV emission that can be observed from space, so Cirtain hopes his research group will be the first to peer right inside by pushing into space.
Coronal loops as viewed by the Transition Region and Coronal Explorer (TRACE)
The transition region is critical to the understanding of the magnetic structure of the Sun and its corona. Below this thin layer, plasma pressure dominates, above it, magnetic pressure dominates. This means that above the transition region, the characteristics of the Sun’s magnetic field overwhelm the decreasing plasma pressure. The corona becomes a highly structured entity from the transition region and upward, which can be seen in the structure of magnetic coronal loops (pictured).
But what will they be measuring? How can the magnetic structure inside the transition region be seen? The instrument to be launched is called the Solar Ultraviolet Magnetograph Investigation (SUMI) and it is designed to measure the magnetic phenomenon of “Zeeman Splitting.” The Zeeman effect occurs when radiating plasma is in the presence of a strong magnetic field. So in the case of SUMI, the instrument will observe the UV emission from the transition region and detect the fine-scale splitting of the UV spectroscopic emission lines. The stronger the magnetic field, the greater the splitting.
SUMI can also measure the polarization of the split lines, so Cirtain’s team will have all the information they’ll ever need about the magnetic field in the transition region: both magnetic filed strength and direction. So far, so good.
But how is Cirtain planning on getting SUMI into space? To possibly make it cheap, and because SUMI is a comparatively simple instrument, it won’t need to be in space long. So the plan is to blast it into the lowest reaches of space on a sub-orbital flight inside the nose cone of a Black Brant sounding rocket. 68 seconds and 300 km (185 miles) into the flight, SUMI will be jettisoned. “We’ll be above more than 99.99% of Earth’s atmosphere. From that moment, we’ve only got 8 minutes to work with. We’ll target an active region and start taking data,” Cirtain added.
Carrying out short sounding rocket missions is not new to solar physics, some of the very first space-based observations of the Sun could only come from high altitude rockets. However, Cirtain will be nervous to see SUMI disappear into the stratosphere at 5,000 mph and has dubbed the flight the “Eight minutes of terror.”
So, from one of the simplest and cheapest observation campaigns, the mysterious transition region may start to give us some answers…
[/caption]
Amateur astronomers have observed the first sunspots to appear on the solar surface for weeks. This period of extreme magnetic calm has made some scientists believe that Solar Cycle 23 might be a quiet affair. This comes in stark contrast to NASA’s 2006 forecast that this cycle would be a “doozy.” Whether or not the slow start of solar activity is indicative of things to come, we’re not sure, but it sure is great to see activity starting to churn on the solar surface once more…
To make the situation even more cloudy, back in March, we had a false alarm. Suddenly, the Sun erupted to life, only three months after the start of Cycle 24 was announced. Sunspots, flares and Coronal Mass Ejections sprung to life around the solar equator. You would have been forgiven for thinking the Sun was going to make good on the NASA 2006 forecast. But it wasn’t to be. Critically, these active sunspots were “left overs” from the previous cycle. Like revellers turning up an hour after the party had finished, these sunspots were overlapping remainders of the previous cycle.
At the root of all these observations is space weather prediction. All our activities in space are in some way influenced by solar activity, so it would be advantageous if we could predict when the next solar storm is coming. We have complex models of the Sun and our observational skills are becoming more and more sophisticated, but we still have a very basic grasp on what makes the Sun “tick.”
So today’s discovery, although a little overdue, will excite solar physicists and astronomers the world over. But will the solar activity continue? Is this just an isolated occurrence? For now, we just do not know. We have to sit back, observe and enjoy what surprises the Sun has in store for us in Cycle 24.
[/caption]
We’ve all wondered at some point or another what mysteries our Solar System holds. After all, the eight planets (plus Pluto and all those other dwarf planets) orbit within a very small volume of the heliosphere (the volume of space dominated by the influence of the Sun), what’s going on in the rest of the volume we call our home? As we push more robots into space, improve our observational capabilities and begin to experience space for ourselves, we learn more and more about the nature of where we come from and how the planets have evolved. But even with our advancing knowledge, we would be naive to think we have all the answers, so much still needs to be uncovered. So, from a personal point of view, what would I consider to be the greatest mysteries within our Solar System? Well, I’m going to tell you my top ten favourites of some more perplexing conundrums our Solar System has thrown at us. So, to get the ball rolling, I’ll start in the middle, with the Sun. (None of the following can be explained by dark matter, in case you were wondering… actually it might, but only a little…)
10. Solar Pole Temperature Mismatch
Data from Ulysses (D. McComas)
Why is the Sun’s South Pole cooler than the North Pole? For 17 years, the solar probe Ulysses has given us an unprecedented view of the Sun. After being launched on Space Shuttle Discovery way back in 1990, the intrepid explorer took an unorthodox trip through the Solar System. Using Jupiter for a gravitational slingshot, Ulysses was flung out of the ecliptic plane so it could pass over the Sun in a polar orbit (spacecraft and the planets normally orbit around the Sun’s equator). This is where the probe journeyed for nearly two decades, taking unprecedented in-situ observations of the solar wind and revealing the true nature of what happens at the poles of our star. Alas, Ulysses is dying of old age, and the mission effectively ended on July 1st (although some communication with the craft remains).
However, observing uncharted regions of the Sun can create baffling results. One such mystery result is that the South Pole of the Sun is cooler than the North Pole by 80,000 Kelvin. Scientists are confused by this discrepancy as the effect appears to be independent of the magnetic polarity of the Sun (which flips magnetic north to magnetic south every 11-years). Ulysses was able to gauge the solar temperature by sampling the ions in the solar wind at a distance of 300 million km above the North and South Poles. By measuring the ratio of oxygen ions (O6+/O7+), the plasma conditions at the base of the coronal hole could be measured.
Mars, just a normal planet. No mystery here... (NASA/Hubble)
Why are the Martian hemispheres so radically different? This is one mystery that had frustrated scientists for years. The northern hemisphere of Mars is predominantly featureless lowlands, whereas the southern hemisphere is stuffed with mountain ranges, creating vast highlands. Very early on in the study of Mars, the theory that the planet had been hit by something very large (thus creating the vast lowlands, or a huge impact basin) was thrown out. This was primarily because the lowlands didn’t feature the geography of an impact crater. For a start there is no crater “rim.” Plus the impact zone is not circular. All this pointed to some other explanation. But eagle-eyed researchers at Caltech have recently revisited the impactor theory and calculated that a huge rock between 1,600 to 2,700 km diameter can create the lowlands of the northern hemisphere (see Two Faces of Mars Explained).
Bonus mystery: Does the Mars Curse exist? According to many shows, websites and books there is something (almost paranormal) out in space eating (or tampering with) our robotic Mars explorers. If you look at the statistics, you would be forgiven for being a little shocked: Nearly two-thirds of all Mars missions have failed. Russian Mars-bound rockets have blown up, US satellites have died mid-flight, British landers have pock-marked the Red Planet’s landscape; no Mars mission is immune to the “Mars Triangle.” So is there a “Galactic Ghoul” out there messing with our ‘bots? Although this might be attractive to some of us superstitious folk, the vast majority of spacecraft lost due to The Mars Curse is mainly due to heavy losses during the pioneering missions to Mars. The recent loss rate is comparable to the losses sustained when exploring other planets in the Solar System. Although luck may have a small part to play, this mystery is more of a superstition than anything measurable (see The “Mars Curse”: Why Have So Many Missions Failed?).
8. The Tunguska Event
Artist impression of the Tunguska event (www.russianspy.org)
What caused the Tunguska impact? Forget Fox Mulder tripping through the Russian forests, this isn’t an X-Files episode. In 1908, the Solar System threw something at us… but we don’t know what. This has been an enduring mystery ever since eye witnesses described a bright flash (that could be seen hundreds of miles away) over the Podkamennaya Tunguska River in Russia. On investigation, a huge area had been decimated; some 80 million trees had been felled like match sticks and over 2,000 square kilometres had been flattened. But there was no crater. What had fallen from the sky?
This mystery is still an open case, although researchers are pinning their bets of some form of “airburst” when a comet or meteorite entered the atmosphere, exploding above the ground. A recent cosmic forensic study retraced the steps of a possible asteroid fragment in the hope of finding its origin and perhaps even finding the parent asteroid. They have their suspects, but the intriguing thing is, there is next-to-no meteorite evidence around the impact site. So far, there doesn’t appear to be much explanation for that, but I don’t think Mulder and Scully need be involved (see Tunguska Meteoroid’s Cousins Found?).
7. Uranus’ Tilt
Uranus. Does it on its side (NASA/Hubble)
Why does Uranus rotate on its side? Strange planet is Uranus. Whilst all the other planets in the Solar System more-or-less have their axis of rotation pointing “up” from the ecliptic plane, Uranus is lying on its side, with an axial tilt of 98 degrees. This means that for very long periods (42 years at a time) either its North or South Pole points directly at the Sun. The majority of the planets have a “prograde” rotation; all the planets rotate counter-clockwise when viewed from above the Solar System (i.e. above the North Pole of the Earth). However, Venus does the exact opposite, it has a retrograde rotation, leading to the theory that it was kicked off-axis early in its evolution due to a large impact. So did this happen to Uranus too? Was it hit by a massive body?
Some scientists believe that Uranus was the victim of a cosmic hit-and-run, but others believe there may be a more elegant way of describing the gas giant’s strange configuration. Early in the evolution of the Solar System, astrophysicists have run simulations that show the orbital configuration of Jupiter and Saturn may have crossed a 1:2 orbital resonance. During this period of planetary upset, the combined gravitational influence of Jupiter and Saturn transferred orbital momentum to the smaller gas giant Uranus, knocking it off-axis. More research needs to be carried out to see if it was more likely that an Earth-sized rock impacted Uranus or whether Jupiter and Saturn are to blame.
6. Titan’s Atmosphere
False colour image of Titan's atmosphere. Credit: NASA/JPL/Space Science Institute/ESA
Why does Titan have an atmosphere? Titan, one of Saturn’s moons, is the only moon in the Solar System with a significant atmosphere. It is the second biggest moon in the Solar System (second only to Jupiter’s moon Ganymede) and about 80% more massive than Earth’s Moon. Although small when compared with terrestrial standards, it is more Earth-like than we give it credit for. Mars and Venus are often cited as Earth’s siblings, but their atmospheres are 100 times thinner and 100 times thicker, respectively. Titan’s atmosphere on the other hand is only one and a half times thicker than Earth’s, plus it is mainly composed of nitrogen. Nitrogen dominates Earth’s atmosphere (at 80% composition) and it dominates Titans atmosphere (at 95% composition). But where did all this nitrogen come from? Like on Earth, it’s a mystery.
Titan is such an interesting moon and is fast becoming the prime target to search for life. Not only does it have a thick atmosphere, its surface is crammed full with hydrocarbons thought to be teeming with “tholins,” or prebiotic chemicals. Add to this the electrical activity in the Titan atmosphere and we have an incredible moon with a massive potential for life to evolve. But as to where its atmosphere came from… we just do not know.
5. Solar Coronal Heating
Coronal loops as imaged by TRACE at 171 Angstroms (1 million deg C) (NASA/TRACE)
Why is the solar atmosphere hotter than the solar surface? Now this is a question that has foxed solar physicists for over half a century. Early spectroscopic observations of the solar corona revealed something perplexing: The Sun’s atmosphere is hotter than the photosphere. In fact, it is so hot that it is comparable to the temperatures found in the core of the Sun. But how can this happen? If you switch on a light bulb, the air surrounding the glass bulb wont be hotter than the glass itself; as you get closer to a heat source, it gets warmer, not cooler. But this is exactly what the Sun is doing, the solar photosphere has a temperature of around 6000 Kelvin whereas the plasma only a few thousand kilometres above the photosphere is over 1 million Kelvin. As you can tell, all kinds of physics laws appear to be violated.
However, solar physicists are gradually closing in on what may be causing this mysterious coronal heating. As observational techniques improve and theoretical models become more sophisticated, the solar atmosphere can be studied more in-depth than ever before. It is now believed that the coronal heating mechanism may be a combination of magnetic effects in the solar atmosphere. There are two prime candidates for corona heating: nanoflares and wave heating. I for one have always been a huge advocate of wave heating theories (a large part of my research was devoted to simulating magnetohydrodynamic wave interactions along coronal loops), but there is strong evidence that nanoflares influence coronal heating too, possibly working in tandem with wave heating.
Although we are pretty certain that wave heating and/or nanoflares may be responsible, until we can insert a probe deep into the solar corona (which is currently being planned with the Solar Probe mission), taking in-situ measurements of the coronal environment, we won’t know for sure what heats the corona (see Warm Coronal Loops May Hold the Key to Hot Solar Atmosphere).
4. Comet Dust
Comets - where does their dust come from?
How did dust formed at intense temperatures appear in frozen comets? Comets are the icy, dusty nomads of the Solar System. Thought to have evolved in the outermost reaches of space, in the Kuiper Belt (around the orbit of Pluto) or in a mysterious region called the Oort Cloud, these bodies occasionally get knocked and fall under the weak gravitational pull of the Sun. As they fall toward the inner Solar System, the Sun’s heat will cause the ice to vaporize, creating a cometary tail known as the coma. Many comets fall straight into the Sun, but others are more lucky, completing a short-period (if they originated in the Kuiper Belt) or long-period (if they originated in the Oort Cloud) orbit of the Sun.
But something odd has been found in the dust collected by NASA’s 2004 Stardust mission to Comet Wild-2. Dust grains from this frozen body appeared to have been formed a high temperatures. Comet Wild-2 is believed to have originated from and evolved in the Kuiper Belt, so how could these tiny samples be formed in an environment with a temperature of over 1000 Kelvin?
The Solar System evolved from a nebula some 4.6 billion years ago and formed a large accretion disk as it cooled. The samples collected from Wild-2 could only have been formed in the central region of the accretion disk, near the young Sun, and something transported them into the far reaches of the Solar System, eventually ending up in the Kuiper Belt. But what mechanism could do this? We are not too sure (see Comet Dust is Very Similar to Asteroids).
3. The Kuiper Cliff
The bodies in the Kuiper Belt (Don Dixon)
Why does the Kuiper Belt suddenly end? The Kuiper Belt is a huge region of the Solar System forming a ring around the Sun just beyond the orbit of Neptune. It is much like the asteroid belt between Mars and Jupiter, the Kuiper Belt contains millions of small rocky and metallic bodies, but it’s 200-times more massive. It also contains a large quantity of water, methane and ammonia ices, the constituents of cometary nuclei originating from there (see #4 above). The Kuiper Belt is also known for its dwarf planet occupant, Pluto and (more recently) fellow Plutoid “Makemake”.
The Kuiper Belt is already a pretty unexplored region of the Solar System as it is (we wait impatiently for NASA’s New Horizons Pluto mission to arrive there in 2015), but it has already thrown up something of a puzzle. The population of Kuiper Belt Objects (KBOs) suddenly drops off at a distance of 50 AU from the Sun. This is rather odd as theoretical models predict an increase in number of KBOs beyond this point. The drop-off is so dramatic that this feature has been dubbed the “Kuiper Cliff.”
We currently have no explanation for the Kuiper Cliff, but there are some theories. One idea is that there are indeed a lot of KBOs beyond 50 AU, it’s just that they haven’t accreted to form larger objects for some reason (and therefore cannot be observed). Another more controversial idea is that KBOs beyond the Kuiper Cliff have been swept away by a planetary body, possibly the size of Earth or Mars. Many astronomers argue against this citing a lack of observational evidence of something that big orbiting outside the Kuiper Belt. This planetary theory however has been very useful for the doomsayers out there, providing flimsy “evidence” for the existence of Nibiru, or “Planet X.” If there is a planet out there, it certainly is not “incoming mail” and it certainly is notarriving on our doorstep in 2012.
So, in short, we have no clue why the Kuiper Cliff exists…
2. The Pioneer Anomaly
Artist impression of the Pioneer 10 probe (NASA)
Why are the Pioneer probes drifting off-course? Now this is a perplexing issue for astrophysicists, and probably the most difficult question to answer in Solar System observations. Pioneer 10 and 11 were launched back in 1972 and 1973 to explore the outer reaches of the Solar System. Along their way, NASA scientists noticed that both probes were experiencing something rather strange; they were experiencing an unexpected Sun-ward acceleration, pushing them off-course. Although this deviation wasn’t huge by astronomical standards (386,000 km off course after 10 billion km of travel), it was a deviation all the same and astrophysicists are at a loss to explain what is going on.
One main theory suspects that non-uniform infrared radiation around the probes’ bodywork (from the radioactive isotope of plutonium in its Radioisotope Thermoelectric Generators) may be emitting photons preferentially on one side, giving a small push toward the Sun. Other theories are a little more exotic. Perhaps Einstein’s general relativity needs to be modified for long treks into deep space? Or perhaps dark matter has a part to play, having a slowing effect on the Pioneer spacecraft?
How do we know the Oort Cloud even exists? As far as Solar System mysteries go, the Pioneer anomaly is a tough act to follow, but the Oort cloud (in my view) is the biggest mystery of all. Why? We have never seen it, it is a hypothetical region of space.
At least with the Kuiper Belt, we can observe the large KBOs and we know where it is, but the Oort Cloud is too far away (if it really is out there). Firstly, the Oort Cloud is predicted to be over 50,000 AU from the Sun (that’s nearly a light year away), making it about 25% of the way toward our nearest stellar neighbour, Proxima Centauri. The Oort Cloud is therefore a very long way away. The outer reaches of the Oort Cloud is pretty much the edge of the Solar System, and at this distance, the billions of Oort Cloud objects are very loosely gravitationally bound to the Sun. They can therefore be dramatically influenced by the passage of other nearby stars. It is thought that Oort Cloud disruption can lead to icy bodies falling inward periodically, creating long-period comets (such as Halley’s comet).
In fact, this is the only reason why astronomers believe the Oort Cloud exists, it is the source of long-period icy comets which have highly eccentric orbits emanating regions out of the ecliptic plane. This also suggests that the cloud surrounds the Solar System and is not confined to a belt around the ecliptic.
So, the Oort Cloud appears to be out there, but we cannot directly observe it. In my books, that is the biggest mystery in the outermost region of our Solar System…
NASA’s twin STEREO spacecraft have been studying the sun since their launch in 2006. But the mission made a surprising and unexpected discovery by detecting particles from the edge of the solar system, and for the first time, scientists have now been able to map the region where the hot solar wind meets up with the cold interstellar medium. However, this wasn’t done with optical instruments imaging in visible light, but by mapping the region by means of neutral, or uncharged, atoms. This breakthrough is a “new kind of astronomy using neutral atoms,” said Robert Lin, from the University of California Berkeley, and lead for the suprathermal electron sensor aboard STEREO. “You can’t get a global picture of this region, one of the last unexplored regions of the heliosphere, any other way because it is too tenuous to be seen by normal optical telescopes.” The findings also help clear up a discrepancy in the amount of energy in the region found by the Voyager 2 spacecraft as it passed through the edge of the solar system last year.
The heliosphere stretches from the sun to more than twice the distance of Pluto. Beyond its edge, called the heliopause, lies the relative quiet of interstellar space, at about 100 astronomical units (AU) – 100 times the Earth-sun distance. The termination shock is the region of the heliosphere where the supersonic solar wind slows to subsonic speed as it merges with the interstellar medium. The heliosheath is the region of churning plasma between the shock front and the interstellar medium.
The twin STEREO spacecraft, in Earth’s orbit about the sun, take stereo pictures of the sun’s surface and measure magnetic fields and ion fluxes associated with solar explosions.
Between June and October 2007, however, the suprathermal electron sensor in the IMPACT (In-situ Measurements of Particles and CME Transients) suite of instruments on board each STEREO spacecraft detected neutral atoms originating from both the shock front and the heliosheath beyond.
“The suprathermal electron sensors were designed to detect charged electrons, which fluctuate in intensity depending on the magnetic field,” said lead author Linghua Wang, a graduate student in UC Berkeley’s Department of Physics. “We were surprised that these particle intensities didn’t depend on the magnetic field, which meant they must be neutral atoms.”
UC Berkeley physicists concluded that these energetic neutral atoms were originally ions heated up in the termination that lost their charge to cold atoms in the interstellar medium and, no longer hindered by magnetic fields, flowed back toward the sun and into the suprathermal electron sensors on STEREO.
“This is the first mapping of energetic neutral particles from beyond the heliosphere,” Lin said. “These neutral atoms tell us about the hot ions in the heliosheath. The ions heated in the termination shock exchange charge with the cold, neutral atoms in the interstellar medium to become neutral, and then flow back in.”
According to Lin, the neutral atoms are probably hydrogen, since most of the particles in the local interstellar medium are hydrogen.
The findings from STEREO, reported in the July 3 issue of the journal Nature, clear up a discrepancy in the amount of energy dumped into space by the decelerating solar wind that was discovered last year when Voyager 2 crossed the solar system’s termination shock and entered the surrounding heliosheath.
The newly discovered population of ions in the heliosheath contains about 70 percent of the energy dissipated in the termination shock, exactly the amount unaccounted for by Voyager 2’s instruments, the UC Berkeley physicists concluded. The Voyager 2 results are reported in the same issue of Nature.
A new NASA mission, the Interstellar Boundary Explorer (IBEX), is planned for launch later this year to map more thoroughly the lower-energy energetic ions in the heliosheath by means of energetic neutral atoms to discover the structure of the termination shock and how hydrogen ions are accelerated there.
We could be in for a huge firework display in 2012. The Sun will be approaching the peak of its 11-year cycle, called “solar maximum”, so we can expect a lot of solar activity. Some predictions put the solar maximum of Solar Cycle 24 even more energetic than the last solar maximum in 2002-2003 (remember all those record breaking X-class flares?). Solar physicists are already getting excited about this next cycle and new prediction methods are being put to good use. But should we be worried?
According to one of the many Doomsday scenarios we have been presented with in the run-up to the Mayan Prophecy-fuelled “end of the world” in the year 2012, this scenario is actually based on some science. What’s more, there may be some correlation between the 11-year solar cycle and the time cycles seen in the Mayan calendar, perhaps this ancient civilization understood how the Sun’s magnetism undergoes polarity changes every decade or so? Plus, religious texts (such as the Bible) say that we are due for a day of judgement, involving a lot of fire and brimstone. So it looks like we are going to get roasted alive by our closest star on December 21st, 2012!
Before we go jumping to conclusions, take a step back and think this through. Like most of the various ways the world is going to end in 2012, the possibility of the Sun blasting out a huge, Earth-damaging solar flare is very attractive to the doomsayers out there. But let’s have a look at what really happens during an Earth-directed solar flare event, the Earth is actually very well protected. Although some satellites may not be…
The Earth has evolved in a highly radioactive environment. The Sun constantly fires high-energy particles from its magnetically dominated surface as the solar wind. During solar maximum (when the Sun is at its most active), the Earth may be unlucky enough to be staring down the barrel of an explosion with the energy of 100 billion Hiroshima-sized atomic bombs. This explosion is known as a solar flare and the effects of which can cause problems here on Earth.
Before we look at the Earth-side effects, let’s have a look at the Sun and briefly understand why it gets so angry every 11 years or so.
The Solar Cycle
First and foremost, the Sun has a natural cycle with a period of approximately 11 years. During the lifetime of each cycle, the magnetic field lines of the Sun are dragged around the solar body by differential rotation at the solar equator. This means that the equator is spinning faster than the magnetic poles. As this continues, solar plasma drags the magnetic field lines around the Sun, causing stress and a build up of energy (an illustration of this is pictured). As magnetic energy increases, kinks in the magnetic flux form, forcing them to the surface. These kinks are known as coronal loops which become more numerous during periods of high solar activity.
This is where the sunspots come in. As coronal loops continue to pop up over the surface, sunspots appear too, often located at the loop footpoints. Coronal loops have the effect of pushing the hotter surface layers of the Sun (the photosphere and chromosphere) aside, exposing the cooler convection zone (the reasons why the solar surface and atmosphere is hotter than the solar interior is down to the coronal heating phenomenon). As magnetic energy builds up, we can expect more and more magnetic flux to be forced together. This is when a phenomenon known as magnetic reconnection occurs.
Reconnection is the trigger for solar flares of various sizes. As previously reported, solar flares from “nanoflares” to “X-class flares” are very energetic events. Granted, the largest flares my generate enough energy for 100 billion atomic explosions, but don’t let this huge figure concern you. For a start, this flare occurs in the low corona, right near the solar surface. That’s nearly 100 million miles away (1AU). The Earth is nowhere close to the blast.
As the solar magnetic field lines release a huge amount of energy, solar plasma is accelerated and confined within the magnetic environment (solar plasma is superheated particles like protons, electrons and some light elements such as helium nuclei). As the plasma particles interact, X-rays may be generated if the conditions are right and bremsstrahlung is possible. (Bremsstrahlung occurs when charged particles interact, resulting in X-ray emission.) This may create an X-ray flare.
The Problem with X-ray Solar Flares
The biggest problem with an X-ray flare is that we get little warning when it is going to happen as X-rays travel at the speed of light (one of the record breaking 2003 solar flares is pictured left). X-rays from an X-class flare will reach the Earth in around eight minutes. As X-rays hit our atmosphere, they are absorbed in the outermost layer called the ionosphere. As you can guess from the name, this is a highly charged, reactive environment, full of ions (atomic nuclei, and free electrons).
During powerful solar events such as flares, rates of ionization between X-rays and atmospheric gases increase in the D and E region layers of the ionosphere. There is a sudden surge in electron production in these layers. These electrons can cause interference to the passage of radio waves through the atmosphere, absorbing short wave radio signals (in the high frequency range), possibly blocking global communications. These events are known as “Sudden Ionospheric Disturbances” (or SIDs) and they become commonplace during periods of high solar activity. Interestingly, the increase in electron density during a SID boosts the propagation of Very Low Frequency (VLF) radio, a phenomenon scientists use to measure the intensity of X-rays coming from the Sun.
Coronal Mass Ejections?
X-ray solar flare emissions are only part of the story. If the conditions are right, a coronal mass ejection (CME) might be produced at the site of the flare (although either phenomenon can occur independently). CMEs are slower than the propagation of X-rays, but their global effects here on Earth can be more problematic. They may not travel at the speed of light, but they still travel fast; they can travel at a rate of 2 million miles per hour (3.2 million km/hr), meaning they may reach us in a matter of hours.
This is where much effort is being put into space weather prediction. We have a handful of spacecraft sitting between the Earth and the Sun at the Earth-Sun Lagrangian (L1) point with sensors on board to measure the energy and intensity of the solar wind. Should a CME pass through their location, energetic particles and the interplanetary magnetic field (IMF) can be measured directly. One mission called the Advanced Composition Explorer (ACE) sits in the L1 point and provides scientists with up to an hour notice on the approach of a CME. ACE teams up with the Solar and Heliospheric Observatory (SOHO) and the Solar TErrestrial RElations Observatory (STEREO), so CMEs can be tracked from the lower corona into interplanetary space, through the L1 point toward Earth. These solar missions are actively working together to provide space agencies with advanced notice of an Earth-directed CME.
So what if a CME reaches Earth? For a start, much depends on the magnetic configuration of the IMF (from the Sun) and the geomagnetic field of the Earth (the magnetosphere). Generally speaking, if both magnetic fields are aligned with polarities pointing in the same direction, it is highly probable that the CME will be repelled by the magnetosphere. In this case, the CME will slide past the Earth, causing some pressure and distortion on the magnetosphere, but otherwise passing without a problem. However, if the magnetic field lines are in an anti-parallel configuration (i.e. magnetic polarities in opposite directions), magnetic reconnection may occur at the leading edge of the magnetosphere.
In this event, the IMF and magnetosphere will merge, connecting the Earth’s magnetic field with the Sun’s. This sets the scene for one of the most awe inspiring events in nature: the aurora.
Satellites in Peril
As the CME magnetic field connects with the Earth’s, high energy particles are injected into the magnetosphere. Due to solar wind pressure, the Sun’s magnetic field lines will fold around the Earth, sweeping behind our planet. The particles injected in the “dayside” will be funnelled into the polar regions of the Earth where they interact with our atmosphere, generating light as aurorae. During this time, the Van Allen belt will also become “super-charged”, creating a region around the Earth that could cause problems to unprotected astronauts and any unshielded satellites. For more on the damage that can be caused to astronauts and spacecraft, check out “Radiation Sickness, Cellular Damage and Increased Cancer Risk for Long-term Missions to Mars” and “New Transistor Could Side-Step Space Radiation Problem.”
As if the radiation from the Van Allen belt wasn’t enough, satellites could succumb to the threat of an expanding atmosphere. As you’d expect, as if the Sun hits the Earth with X-rays and CMEs, there will be inevitable heating and global expansion of the atmosphere, possibly encroaching into satellite orbital altitudes. If left unchecked, an aerobraking effect on satellites could cause them to slow and drop in altitude. Aerobraking has been used extensively as a space flight tool to slow spacecraft down when being inserted into orbit around another planet, but this will have an adverse effect on satellites orbiting Earth as any slowing of velocity could cause it to re-enter the atmosphere.
We Feel the Effects on the Ground Too
Although satellites are on the front line, if there is a powerful surge in energetic particles entering the atmosphere, we may feel the adverse effects down here on Earth too. Due to the X-ray generation of electrons in the ionosphere, some forms of communication may become patchy (or be removed all together), but this isn’t all that can happen. Particularly in high-latitude regions, a vast electric current, known as an “electrojet”, may form through the ionosphere by these incoming particles. With an electric current comes a magnetic field. Depending on the intensity of the solar storm, currents may be induced down here on the ground, possibly overloading national power grids. On March 13th 1989, six million people lost power in the Quebec region of Canada after a huge increase in solar activity caused a surge from ground-induced currents. Quebec was paralysed for nine hours whilst engineers worked on a solution to the problem.
Can Our Sun Produce a Killer Flare?
The short answer to this is “no”.
The longer answer is a little more involved. Whilst a solar flare from out Sun, aimed directly at us, could cause secondary problems such as satellite damage and injury to unprotected astronauts and blackouts, the flare itself is not powerful enough to destroy Earth, certainly not in 2012. I dare say, in the far future when the Sun begins to run out of fuel and swell into a red giant, it might be a bad era for life on Earth, but we have a few billion years to wait for that to happen. There could even be the possibility of several X-class flares being launched and by pure bad luck we may get hit by a series of CMEs and X-ray bursts, but none will be powerful to overcome our magnetosphere, ionosphere and thick atmosphere below.
“Killer” solar flares have been observed on other stars. In 2006, NASA’s Swift observatory saw the largest stellar flare ever observed 135 light-years away. Estimated to have unleashed an energy of 50 million trillion atomic bombs, the II Pegasi flare will have wiped out most life on Earth if our Sun fired X-rays from a flare of that energy at us. However, our Sun is not II Pegasi. II Pegasi is a violent red giant star with a binary partner in a very close orbit. It is believed the gravitational interaction with its binary partner and the fact II Pegasi is a red giant is the root cause behind this energetic flare event.
Doomsayers point to the Sun as a possible Earth-killer source, but the fact remains that our Sun is a very stable star. It does not have a binary partner (like II Pegasi), it has a predictable cycle (of approximately 11 years) and there is no evidence that our Sun contributed to any mass extinction event in the past via a huge Earth-directed flare. Very large solar flares have been observed (such as the 1859 Carrington white light flare)… but we are still here.
In an added twist, solar physicists are surprised by the lack of solar activity at the start of this 24th solar cycle, leading to some scientists to speculate we might be on the verge of another Maunder minimum and “Little Ice Age”. This is in stark contrast to NASA solar physicist’s 2006 prediction that this cycle will be a “doozy”.
This leads me to conclude that we still have a long way to go when predicting solar flare events. Although space weather prediction is improving, it will be a few years yet until we can read the Sun accurately enough to say with any certainty just how active a solar cycle is going to be. So, regardless of prophecy, prediction or myth, there is no physical way to say that the Earth will be hit by any flare, let alone a big one in 2012. Even if a big flare did hit us, it will not be an extinction event. Yes, satellites may be damaged, causing secondary problems such as a GPS loss (which might disrupt air traffic control for example) or national power grids may be overwhelmed by auroral electrojets, but nothing more extreme than that.
But hold on, to sidestep this issue, doomsayers now tell us that a large solar flare will hit us just as the Earth’s geomagnetic field weakens and reverses, leaving us unprotected from the ravages of a CME… The reasons why this is not going to happen in 2012 is worthy of its own article. So, look out for the next 2012 article “2012: No Geomagnetic Reversal“.
Leading image credits: MIT (supernova simulation), NASA/JPL (solar active region in EUV). Effects and editing: myself.
The Suns photosphere is looking particulary boring (NASA/SOHO)
So what’s up with our Sun? Is it going through a depression? It seems as if our closest star is experiencing a surprisingly uneventful couple of years. Solar minimum has supposedly passed and we should be seeing a lot more magnetic activity, and we certainly should be observing lots more sunspots. Space weather forecasts have been putting Solar Cycle 24 as a historically active cycle… but so far, nothing. So what’s the problem? Is it a ticking bomb, waiting to shock us with a huge jump in solar activity, flares and CMEs over a few months? Or could this lack of activity a prelude to a very boring few years, possibly leading the Earth toward another Ice Age?
It’s funny. Just as we begin to get worried that the next solar maximum is going to unleash all sorts of havoc on Earth (i.e. NASA’s 2006 solar storm warning), scientists begin to get concerned as to whether there is going to be a solar maximum at all. In a conference last week at Montana State University, solar physicists discussed the possibility that the Sun could be facing a long period of calm, leading to the concern that there could be another Maunder Minimum. The Maunder Minimum (named after the late 19th Century solar astronomer Edward W. Maunder, who discovered the phenomenon) was a 17th Century, 30-year period when very few sunspots were observed on the disk of the Sun. It is thought by many scientists that this period contributed to what became known as the “Little Ice Age” here on Earth. As the Sun provides Earth with all its energy, during extended periods when the solar output is lower than average, it seems possible a lack of sunspots on the Sun (i.e. low activity) may be linked with periods of cold down here.
“It continues to be dead. That’s a small concern, a very small concern.” – Saku Tsuneta, National Astronomical Observatory of Japan and program manager for the Hinode solar mission.
However, solar physicists are not too worried about this possibility, after all, it’s only been two years since solar minimum. Although activity has been low for the beginning of Cycle 24, sunspots have not been non-existent. In January of this year, a newborn spot was observed, as expected, in high latitude regions. More spots were seen in April. In March, sunspots from the previous solar cycle even made an appearance, putting on an unexpected show of flares and coronal mass ejections (CMEs).
As pointed out by David Hathaway, a solar physicist at NASA’s Marshall Space Flight Center, the fact that sunspots have already been observed in this new cycle means that it is highly unlikely we face anything as extreme as another Maunder Minimum. Hathaway says there is nothing unusual about having a relatively understated solar cycle after several particularly active cycles. Solar Cycle 23 was a very active period for the Sun with a greater than average number of sunspots observed on the solar surface.
It appears there are two different predictions for the activity level of the next solar cycle. On the one hand we have scientists that think this cycle might be below average, and on the other hand we have scientists who believe the next cycle will be the biggest yet. We certainly have a long way to go before we can begin making any accurate solar forecasts…
Coronal loops as imaged by TRACE at 171 Angstroms (1 million deg C) (NASA/TRACE)
Coronal loops, the elegant and bright structures threading through the solar surface and into the solar atmosphere, are key to understanding why the corona is so hot. Yes, it’s the Sun, and yes, it’s hot, but its atmosphere is too hot. The puzzle as to why the solar corona is hotter than the Sun’s photosphere has kept solar physicists busy since the mid-twentieth century, but with the help of modern observatories and advanced theoretical models, we now have a pretty good idea what is causing this. So is the problem solved? Not quite…
So why are solar physicists so interested in the solar corona anyway? To answer this, I’ll pull up an excerpt from my first ever Universe Today article:
…measurements of coronal particles tell us the atmosphere of the Sun is actually hotter than the Suns surface. Traditional thinking would suggest that this is wrong; all sorts of physical laws would be violated. The air around a light bulb isn’t hotter than the bulb itself, the heat from an object will decrease the further away you measure the temperature (obvious really). If you’re cold, you don’t move away from the fire, you get closer to it! – from “Hinode Discovers Sun’s Hidden Sparkle“, Universe Today, December 21st, 2007
This isn’t only an academic curiosity. Space weather originates from the lower solar corona; understanding the mechanisms behind coronal heating has wide-ranging implications for predicting energetic (and damaging) solar flares and forecasting interplanetary conditions.
So, the coronal heating problem is an interesting issue and solar physicists are hot on the trail of the answer to why the corona is so hot. Magnetic coronal loops are central to this phenomenon; they are at the base of the solar atmosphere and experience rapid heating with a temperature gradient from tens of thousands of Kelvin (in the chromosphere) to tens of millions of Kelvin (in the corona) over a very short distance. The temperature gradient acts across a thin transition region (TR), which varies in thickness, but can be only a few hundreds of kilometers thick in places.
These bright loops of hot solar plasma may be easy to see, but there are many discrepancies between the observation of the corona and coronal theory. The mechanism(s) responsible for heating the loops have proven to be hard to pin down, particularly when trying to understand the dynamics of “intermediate temperature” (a.k.a. “warm”) coronal loops with plasma heated to around one million Kelvin. We are getting closer to solving this puzzle which will aid space weather predictions from the Sun to the Earth, but we need to work out why the theory is not the same as what we are seeing.
Solar physicists have been divided on this topic for some time. Is coronal loop plasma heated by intermittent magnetic reconnection events throughout the length of a coronal loop? Or are they heated by some other steady heating very low in the corona? Or is it a bit of both?
James Klimchuk from the Goddard Space Flight Center’s Solar Physics Laboratory in Greenbelt, Md., takes a different opinion and favours the nanoflare, impulsive heating mechanism, but he is highly aware that other factors may come into play:
“It has become clear in recent years that coronal heating is a highly dynamic process, but inconsistencies between observations and theoretical models have been a major source of heartburn. We have now discovered two possible solutions to this dilemma: energy is released impulsively with the right mix of particle acceleration and direct heating, or energy is released gradually very close to the solar surface.” – James Klimchuk
Nanoflares are predicted to maintain warm coronal loops at their fairly steady 1 million Kelvin. We know the loops are this temperature as they emit radiation in the extreme ultraviolet (EUV) wavelengths, and a host of observatories have been built or sent into space with instruments sensitive to this wavelength. Space-based instruments such as the EUV Imaging Telescope (EIT; onboard the NASA/ESA Solar and Heliospheric Observatory), NASA’s Transition Region and Coronal Explorer (TRACE), and the recently operational Japanese Hinode mission have all had their successes, but many coronal loop breakthroughs occurred after the launch of TRACE back in 1998. Nanoflares are very hard to observe directly as they occur over spatial scales so small, they cannot be resolved by the current instrumentation. However, we are close, and there is a trail of coronal evidence pointing to these energetic events.
“Nanoflares can release their energy in different ways, including the acceleration of particles, and we now understand that the right mix of particle acceleration and direct heating is one way to explain the observations.” – Klimchuk.
Slowly but surely, theoretical models and observation are coming together, and it seems that after 60 years of trying, solar physicists are close to understanding the heating mechanisms behind the corona. By looking at how nanoflares and other heating mechanisms may influence each other, it is very likely that more than one coronal heating mechanism is at play…
Aside: Out of interest, nanoflares will occur at any altitude along the coronal loop. Although they may be called nanoflares, by Earth standards, they are huge explosions. Nanoflares release an energy of 1024-1026 erg (that is 1017-1019 Joules). This is the equivalent of approximately 1,600 to 160,000 Hiroshima-sized atomic bombs (with the explosive energy of 15 kilotonnes), so there is nothing nano about these coronal explosions! But on the comparison with the standard X-ray flares the Sun generates from time to time with a total energy of 6×1025 Joules (over 100 billion atomic bombs), you can see how nanoflares get their name…
Periodically our sun blasts streams of hot, ionized gas into the solar system. These eruptions, called coronal mass ejections or CMEs pose a potential threat to astronauts or satellites if aimed at Earth. On April 9, the Sun erupted with a CME, and because the eruption was located on the edge or limb of the sun, it was observed in unprecedented detail by a fleet of spacecraft, revealing new features that are predicted by computer models but are otherwise difficult to see, even for specialized sun-watching spacecraft. From these observations, astronomers have been able to create an animation of this spectacular event.
When a CME occurs, usually spacecraft watching the event need to protect themselves from the bright X-ray solar flare associate with a CME. However, since the April 9 CME occurred on the edge or limb of the Sun as viewed from Earth, the solar flare was hidden from view, which allowed spacecraft to take longer exposures and uncover fainter structures than usual.
“Observations like this are very rare,†said Smithsonian astronomer Ed DeLuca.
Using the Smithsonian-developed X-ray Telescope (XRT) aboard the Japanese Hinode sun-watching satellite, astronomers saw a spiral (helical) magnetic structure unwind as it left the Sun during the CME. Such unwinding can release energy as the magnetic field goes from a more twisted to a less twisted configuration, thereby helping to power the eruption.
Hours later, XRT revealed an inflow of material toward a feature that appears as a bright line—actually an object known as a current sheet seen edge-on. A current sheet is a thin, electrified sheet of gas where oppositely directed magnetic field lines annihilate one another in a process known as magnetic reconnection. The extended observations from XRT show that magnetic fields flow in toward the current sheet for many hours after the eruption, progressing first toward the sheet and then down to the sun’s surface.
They also determined that the temperature of the current sheet is between 5 and 18 million degrees Fahrenheit, which matches previous measurements higher up in the corona by the Ultraviolet Coronagraph Spectrometer on the SOHO spacecraft.
Astronomers study these explosions in hope of being able to predict them and provide “space weather†forecasts.
NASA’s Swift satellite picked up one of the brightest solar flares ever seen — not from our own sun, but a star 16 light-years away. This flare packed the power of thousands of solar flares combined, and a flare of this magnitude from our own sun would have stripped Earth’s atmosphere and sterilized the planet. Astronomers say the flare would have been visible to the naked eye on April 25, 2008 if the star had been easily observable in the night sky at the time. As it was, the flare’s brightness caused Swifts’ Ultraviolet/Optical Telescope to shut down for safety reasons. But Swift was able to study the flare for over 8 hours with its X-ray capabilities.
The Swift satellite normally searches for gamma ray bursts, and is surrounded with detectors that look for bursts of light. The spacecraft then “swiftly” and autonomously re-points itself to the location of the burst. However, this was no gamma ray burst, just a solar flare. But what a solar flare!
The star, EV Lacertae, is a basic red dwarf, the most common type of star in the universe. It shines with only one percent of the Sun’s light, and contains only a third of the Sun’s mass. It’s one of our closest stellar neighbors, but normally is not visible with the naked eye, as it holds a magnitude of -10.
“Here’s a small, cool star that shot off a monster flare. This star has a record of producing flares, but this one takes the cake,” says Rachel Osten, from NASA’s Goddard Space Flight Center. “Flares like this would deplete the atmospheres of life-bearing planets, sterilizing their surfaces.”
Astronomers say EV Lacertae is like an unruly child that throws frequent temper tantrums. It’s a relatively young star at a few hundred million years of age. But it’s a fast rotating star which generates a strong magnetic field, about 100 times as magnetically powerful as the Sun’s field. The energy stored in its magnetic field powers these giant flares.
The flare’s incredible brightness enabled Swift to make detailed measurements in X-ray, as the star remained bright in x-rays for about 8 hours. “This gives us a golden opportunity to study a stellar flare on a second-by-second basis to see how it evolved,” says Stephen Drake of NASA Goddard.
Flares release energy across the electromagnetic spectrum, but the extremely high gas temperatures produced by flares can only be studied with high-energy telescopes like those on Swift. Swift’s wide field and rapid repointing capabilities, designed to study gamma-ray bursts, make it ideal for studying stellar flares. Most other X-ray observatories have studied this star and others like it, but they have to be extremely lucky to catch and study powerful flares due to their much smaller fields of view.
