010105: Motions of coronal-hole boundaries I. Supergranulation
In this nugget, we expected to find that the boundaries of the coronal hole would flutter like a Kansas curtain in a thunderstorm, as a consequence of the solar surface motions we call supergranulation. On first look, that may not be the case.
  010112: "Starburst" Images and Scattering Research
This week the SXT scientists are gathered for a small workshop dedicated to the subject of this nugget, which is another of our series on how the SXT instrument works. We have to understand all the nitty-gritty details of the instrument before we can be confident about what is real, and what is not, in our data.
  010119: A Flare in an Almost-Anemone
When a solar active region emerges in a coronal hole, the new active region may not appear to have any coronal connections to other regions. We call this an "anemone" region, after the windflower. It's noteworthy that the tiny little flare that occurred in this region is more than twice the diameter of Earth!
  010126: A coronal conveyor belt?
Normally we write these Yohkoh science nuggets based on interesting phenomena that we spot for the first time in the SXT data. Sometimes we miss something, and in this case, it's what the authors of a published paper describe as the conveyance of energy from one solar active region to another.
  010202: A dark 'S' erupted!
Bright 'S' shapes (sigmoids) are popular among solar physicists, especially those who study flares and CMEs. However, the 'S' here is invisible to SXT. A gorgeous dark 'S' shape appeared in SoHO/EIT images and produced a spectacular Coronal Mass Ejection.
  010209: A most peculiar high-energy event
Normally we write these Yohkoh science nuggets mostly about coronal soft X-ray sources. In this Nugget we talk about events that emit hard X-rays, gamma-rays, and copious "solar cosmic rays" -- which make the topics that are our bread and butter seem rather flacid.
  010216: A two-sided jet 1,000,000 km long
Soft X-ray "jets" were one of the truly new discoveries of Yohkoh/SXT, apparently one that was not anticipated by the NASA Skylab mission 30 years ago. Here we report a huge bi-directional jet that spread out across the solar surface, with a beautiful structure.
  010223: Two-stage dimming
Coronal dimming, seen in soft X-rays, should rightly be regarded as one of Yohkoh's major discoveries. This is one of those cases where a coronal disturbance does not focus entirely down to a compact origin (eg a major impulsive flare); instead, there are distinctly global effects.
  010302: Temperature of active region loops
In order to understand how the solar corona is heated, the distribution of temperature along a typical X-ray loop is essential information. To get this, you take the ratio of two images in different filters (with slightly different temperature sensitivities), and the ratio directly gives you a temperature.
  010309: A CME and an impulsive flare
This week we have had four M-class flares after several weeks of their absence. One of them was very well observed by SXT, and showed many of the fascinating optical, X-ray and radio phenomena that are associated with flares and Coronal Mass Ejections.
  010316: Twin-dimming event of 28 February 2001
Coronal dimming in X-rays and EUV is often discussed in the context of Coronal Mass Ejections (CMEs). Are these dimmings the footpoints of a large magnetic flux rope, which in turn are the central part of the CME?
  010323: "Magnificent cusp", now analyzed
We report a success story here: an item we reported in a nugget entitled "A magnificent cusp" two years ago has now been analyzed and published. Now what do we need? More theory!
Aurora photo courtesy Chris Petrich, 
Dunedin, New Zealand
  010330: What an aurora!
Spectacular auroras were seen around the world on March 31, 2001. What happened on the Sun that caused all this hullabaloo? A great big sunspot, a major X-ray flare, and an Earth-directed CME!
Two big flares at once
  010406: The most powerful solar flare in decades!
We missed the fun part of this event, as often happens, so we need another one, preferably one jointly observed with HESSI). Huge events show us more of the things we like to study.
The X2 flare
of 12 April 2001
  010413: A beautifully observed major flare
Yohkoh was flying through clear sunshine at the time of the flare, leading to great observations. It's becoming clearer and clearer that geometrical re-formation of the coronal magnetic field plays a decisive role in all kinds of phenomena.
An eruption from 
the far side of the Sun
  010420: Energetic particles from the far side of the Sun
This event is interesting because it shows that a far-side event can have effects at 1 AU due to a shock wave that has a wide longitudinal ecent. It also gives us a chance to think more seriously about the 3-D nature of Coronal Mass Ejections.
A magnetic cloud
  010427: A transient coronal hole
Today let's take a break and pick up a little-discussed subject: the nature of "transient coronal holes." The inference here is that magnetic reconnection may not be such an energetic process, and may be dictated more by global coronal conditions than by local physics.
An unfortunate situation
  010504: Hard x-ray ejection: a work in progress
This week many other nice things happened on the Sun, but the April 18 event has obsessively preoccupied attention. Why is this? We show some of the work in progress and comment here on why we have dropped everything to pursue it.
A confined eruption
  010511: Sigmoid alert and eruptive flares
When is an eruption not an eruption? When you call a sigmoid alert! Last week we executed our carefully-laid plans for an observing campaign, based upon the recognition of a new "sigmoid" coronal structure in our images.
A full-frame SXT image
  010518: Cleverly-Designed Image Buffers
This week let's turn the spotlight backstage and look the instrumentation of the Yohkoh Soft X-ray Telescope, SXT. In this nugget we discuss how the SXT takes various kinds of images of the Sun and sends them to us on the ground.
The Sun's West Limb
  010525: Two kinds of CMEs
There tend to be two kinds of CMEs -- fast ones, associated with solar flares, and slow ones, associated with the eruption of filaments. "Halo" CMEs -- directed toward or away from Earth -- are of unique importance. How do fast and slow CMEs show up in such events?
Ornate Sun
  010601: Hard-soft-hard
As we get closer to the launch of HESSI (it almost happened this week!), naturally our thoughts turn to the high-energy aspects of solar flares. Solar "hard" X-rays and gamma rays give us quite direct views of these phenomena, compared to the "soft" X-ray emission that the Yohkoh Soft X-Ray Telescope (SXT) images.
Model of the coronal magnetic field of the Sun on eclipse day
  010608: Predicting the Coronal Structure During the Upcoming Total Solar Eclipse
The first total solar eclipse of this millennium will be seen on 21st June, 2001, along a path crossing southern Africa through Madagascar. For scientists planning observations, it is helpful to know the global structure of the corona in advance -- but how?.
bouncing surface
  010615: All Loops, All the Time
When we talk about "coronal loops" in Yohkoh SXT observations, we mean long, slender structures that are seen in the corona. These loops act like hoses, channeling squirts of solar gas driven by heating or cooling.
information about an eclipse
  010622: How smooth is the Sun?
Comparatively speaking, the Sun's surface (photosphere) is really smooth. The thickness of the solar atmosphere is about 0.02% of the solar radius. This corresponds to a 2-mil tolerance on a ten-inch part, in machinist's parlance, not a bad job.
coronal loops
  010629: Where are Coronal Loops Heated?
Coronal heating is one of the Big Questions of solar physics. Simply put, this is because understanding how the energy gets transported to the upper atmosphere and deposited there is crucial to figuring out such fundamental things as how the solar wind is accelerated, and why coronal loops are hot.
An active region located unusually far from the solar equator
  010706: Breaking Records -- It's all about Latitude!
This week the Sun is showing us a pair of active regions that is unusually far from the equator. Finding the limits of size and latitudinal extent of active regions and CMEs helps us to put constraints on theories of the Sun's magnetic fields.
A model of the magnetic field of the region responsible for the Bastille Day 2000 flare
  010713: Bastille Day throughout history
This nugget is nothing more than a transparent ruse by which we get to show you more pretty pictures of solar events that took place on July 14, 2000 (the French Bastille Day). The author's tongue is squarely in his cheek!
Hard three-part harmony
  010720: Three-part harmony
All you tenors, altos, basses, sopranos, baritones and so forth: this science nugget has nothing to do with music theory. Instead we point out some nice three-part features of coronal mass ejections (CMEs), which are among the most spectacular and beautiful of solar phenomena.
A solar
  010727: A solar "Island Universe"
The romantic-sounding title of this nugget echoes astronomers' celebrated Shapley-Curtis debates regarding the nature of nebulae. The somewhat farfetched analogy here is suggested by an "island universe" of magnetism on the Sun, seen via the Yohkoh Soft X-Ray Telescope.
An inward thing!
  010803: Magnetic collapse and a flare
A solar flare consists of an extravagant collection of energetic phenomena - radiation, particle acceleration, expansion, coronal mass ejection, etc. This lavish display needs a source of energy, and this week two SXT scientists think they've found an example that shows that it's extracted from the Sun's magnetic field.
The Sun in X-rays
  010810: A gentle giant
During a recent period, the sun showed dramatic variability in its coronal structure, but very little variability in its overall X-ray brightness. The huge region that accounted for much of the restructuring is the gentle giant named in the title of this nugget.
A big flare in X-Rays
  010817: Yohkoh observations of X-class flares
Whenever a big (X-class) flare occurs, we are pleased. Such big flares are likely to be observed by many instruments covering a wide wavelength range, and thorough analysis of these data has the potential to give us an important insight into the mechanism(s) that account for these spectacular events.
  010824: Reverberations of April 7, 1997
Sometimes the initial look at a given phenomenon doesn't reveal all of its subtleties. It is nice to see the information accumulating, and this nugget reviews several pieces of work on one of the first really nice flare/CME events of the current solar maximum: April 7, 1997.
  010831: Millimeter waves, white light, and gamma rays
The X-class flare of August 25, 2001 had a huge radio flux at short wavelengths. Here is a chance for us to write a nugget showing how all parts of the solar spectrum do things simultaneously in such an event.
Magnetic field lines
  010907: Three X-ray global waves
Large-scale (global) coronal waves are launched by yet-mysterious processes in the heart of a solar flare. We now believe that these are probably akin to bow waves driven ahead of CMEs rather than free-running "blast waves" of the sort seen in the corona.
A cusp within
a cusp
  010914: Cusps Within Cusps
We love cusps. There have been several Nuggets dedicated to them (see here ) and several tens mentioning them. We have had Magnificent cusps and Cusps upon Cusps . The Sun continues to surprise us by its ingenuity, and in this Nugget we discuss cusps within cusps, prompted by a very nice example seen this week.
X-ray and optical images
  010921: One Week in the Life of a Sigmoid
This week we go back to a set of observations which were made in August of 1999, as part of `Whole Sun Month 3', where, jointly with many other instruments, SXT followed an active region for a week as it crossed the solar disk. Our observations allowed us to see how the region behaved over many days.
Time history of X-rays from the Czech-built HXRS
  010928: Czech hard X-rays fit beautifully!
This is our first opportunity to intercompare Yohkoh data with hard X-ray photometry from the new Czech-built Hard X-ray Spectrometer (HXRS for short). We're pleased to report that the HXRS and Yohkoh data fit well together and can be analyzed.
A spiny arcade
  011005: Spiny arcades repeat
Yohkoh has seen multiple spiny arcades this week. A pointy coronal streamer is a often called a "helmet streamer", but following this analogy a spiny arcade would be more like a whole platoon of Prussian soldiers wearing helmets with spikes.
A complex of activity
  011012: Life story of a super region complex
A high level of flare activity was observed from mid-August through late-September. The spectacular movies in this nugget show, over three solar rotations, the growth and decay of the active region complex which was responsible for the fireworks.
SXT, HXT, and TRACE images
  011019: Hard X-ray and UV emissions in flares
Comparisons between TRACE and Yohkoh/HESSI provide useful information about the transport of high-energy electrons in solar flares and the physical conditions of the transition region and chromosphere. This nugget shows why we hope the HESSI satellite for flare studies will be launched without further delay.
The swirling flare
  011026: A swirling X-class flare
This Yohkoh science nugget marks the beginning of the fifth year of weekly nugget-writing. This nugget describes a spectacular large flare whose swirling motion may be another hint of helicity in a flare/CME source.
There's another region here!
  011102: A veteran complex of activity, still alive and kicking!
Volcanic eruptions on Earth occur at certain "hot spots" like those underneath the Hawaiian islands. Active regions tend to show the same behavior, popping up repeatedly at sites of earlier activity -- demonstrating an effect we talked about in an earlier nugget, as well!
What are those points on his cap?
  011109: Sigmoid success: a CME predicted
More evidence here that the sigmoid signature is a real one. Now we need to figure out how to convert this sigmoid signature into a more disciplined tool for forecasting and prediction, and to learn more about the physics of the process we're watching unfold.
You can't comb a hairy ball!
  011116: A skinny but robust coronal hole
A coronal hole is a dark region of the corona from which coronal magnetic field lines open out into interplanetary space. But why should there be any open field lines coming out of the solar corona? This is the starting point for one of the mysteries of the solar magnetic field.
That's a sigmoid!
  011123: Another Sigmoid, with Partial Success
Active region NOAA 9704 displayed a nearly perfect S-shape on 20 November. Because such sigmoids seem to be more likely to erupt than non-sigmoids, the SXT team has standing orders to be alert for opportunities to observe them.
A related event on the back side of the Sun?
  011130: High-latitude Eruption with Coronal Signatures
This week's solar activity included a filament eruption that occurred at an unusually high latitude. An earlier eruption from the same vicinity event might have been seen when the region was on the backside of the Sun -- as hinted by the single image at the left.
Image of a flare at the solar limb?
  011207: Unusual Motions in a Solar Flare
It's not as simple as "what goes up must come down" when you are talking about motions seen in solar flares. In this nugget the SXT Chief Observer talks about unusual downward motions in a recent observation.
The Eclipse path across the Pacific
  011214: Annular eclipse, December 14, 2001
We are sad to report that the Yohkoh attitude control system could not handle this eclipse, which threw the spacecraft into a "safehold" state, in which it remains. Recovery from this will be difficult, but perhaps not impossible. Stay tuned!
The trajectory of the Moon as seen from Yohkoh
  011221: Yohkoh's Troubles
This science nugget explains further what we know about the present condition of Yohkoh. In a nutshell, it is a situation very similar to that experienced by SOHO in 1998, from which a nice recovery could be made.
The flare imaged by SXT and HXT
  011228: Rapid variability in a superhot source
By "superhot source" we mean a coronal soft X-ray source with an apparent temperature exceeding 30 MK or so. Lower temperatures commonly occur in flare plasmas, but sometimes we see higher temperatures, as in this event. Right now there is no concensus about what this may mean.
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