020104: A superhot coronal structure leads to a CME?
The complex event that we began describing in last week's nugget had several novelties: first, a superhot source with rapid fluctuations accompanied by a partial ejection, with no coronal manifestations; second, hot on the heels of this, another event with massive coronal effects.
  020111: Perplexing definitions of coronal holes
Coronal holes are easy to spot in soft X-ray images - they are almost absolutely dark. The magnetic field lines within these dark hole-like regions are open, in the sense that they support the solar wind. A particle can trace out the field line, in principle all the way from the photosphere to Andromeda.
  020118: A suggestive before-and-after view
The before view here shows footpoints of magnetic loops that disappear during the flare. Just the pre-flare structure that suggested complicated connectivity, for example a "quadrupolar" field, seems to have disappeared, leaving simplicity in its place.
  020125: What's connected to what?
The 10th anniversary of the Yohkoh launch took place in August 2001, and we have just completed a wonderful meeting to commemorate it. As one might expect from a mature program such as Yohkoh, the quality of the presentations was excellent; what one might not have expected was the real novelty of many of them.
  020201: Spectra from the high corona
The work reported here, based on a presentation at the 10th anniversary meeting, combines Yohkoh images with spectra from the UVCS on board SOHO, a spectroscopy instrument we've never done a Yohkoh science nugget about. Since one rarely understands the physics until one does spectroscopy, it is high time.
  020208: Moreton waves and coronal waves
In this age of great interest in "space weather", people often talk about shock waves associated with CMEs as the main agent for solar energetic particle (SEP) events. How are these related to Moreton waves, known for many years, and the ones we see in the X-ray corona?
  020215: Synoptic maps -- the big picture
The principal meaning of the word, 'synoptic', according to the Merriam-Webster dictionary, is 'affording a general view of a whole'. Synoptic solar observations help us get the big picture about how the Sun generates its all-important magnetic field.
  020222: A solar Pleiades?
In this nugget we show a puzzling coincidence in the formation of a swarm of tiny active regions. There is no known physical mechanism, so this may simply be a coincidence. But, if one could find a second case, one would be tempted to seek some bizarre explanation invoking new physics.
  020301: Problems of refraction
It's every astronomer's dream environment: space, where no atmosphere gets in the way. No "seeing", no refraction, no scattered light, no dirt. Alas the Universe is not that simple. Even X-ray observatories such as Yohkoh or HESSI have their specific problems, as discussed in this nugget.
  020308: Topology of a White Light Flare
"White light" flares are those flares which are detectable in visible light, as opposed to the ultraviolet or X-ray radiation we usually discuss in these nuggets. These flares are among the most energetic of solar events. This nugget shows that by following another high-energy signature -- hard X-rays -- we gain clues about how the energy release is related to the arrangement of magnetic fields.
  020315: Flux emergence and CME events
In this followup to a previous nugget, the relationship of CME-related flares to moving magnetic features and the emergence of magnetic flux is examined. Includes an excellent movie of the magnetic field at the photosphere.
  020322: How Deep is a Coronal Hole?
A "coronal hole" is a region where the upper atmosphere of the Sun appears to be missing something. We know that these areas are sources of the high-speed solar wind, but how easy are they to detect? This nugget demonstrates that combining many images can be helpful, especially for pinning down very faint subjects.
  020329: Coronal hard X-rays and millimeter waves
The first installment in a brief series, look to this nugget for a quick look at hard X-ray imaging and the time variations of X-ray spectra, as well as a name-dropping list of solar X-ray satellites!
  020405: WBS Flare Statistics
Yohkoh carried a Wide Band Spectrometer (WBS) as its fourth scientific instrument. You haven't seen much mention of WBS in these nuggets, because the analysis of the data is extremely difficult. This nugget introduces WBS data to the nugget-reading world, with a presentation of a brand-new PhD thesis about the statistics of the most energetic solar flares, the gamma-ray flares.
  020412: Homologous Jets
We've done several nuggets on coronal jets, because they are truly fascinating: eruptions of million-degree plasma channeled by magnetic fields so that they look nearly linear. Check out this nugget for an additional view, and introduction to some new web tools for the latest solar events.
  020419: A Wonderful X-class Flare for RHESSI
On Sunday, April 21, the Sun produced a flare that was beautiful for a couple of reasons. It was a high-energy X-class flare, the first seen by the new solar observatory RHESSI; it was simultaneously observed by several telescopes around and above the world; and moreover, it was just a good-looking flare (though we may be biased about such things). Check this nugget to see data in a variety of wavelength regimes.
  020426: A Summary of the September 1997 Filament
Filaments are curious creations, collections of cool chromospheric material suspended at coronal elevations. How do they get there? How are they related to the magnetic fields? This nugget reviews some filament observations from several telescopes, and compares the filament to the magnetic field configuration.
  020503: When Do Powerful Flares Occur?
In studying the Sun -- and other stars too -- we are naturally drawn to the exciting transient events, like flares; and the bigger, more powerful flares can exert a stronger pull on our curiosity. This nugget examines the question of how many huge flares we should expect, and what does it mean when we see more/fewer than expected.
  020510: Complicated Radio Waves from an X-class Flare
A few weeks ago, we showed you images from a powerful flare on April 21, 2002. The images showed remarkable structures and motions, and the fun doesn't stop there -- this nugget introduces you to additional observations of this same flare, in radio. The data are no less mystifying.
  020517: An Invisible Hand: The X15 Flare of April 15, 2001
To call this a "reconfiguration of a magnetic structure" might be an understatement -- this was one of the biggest solar flares in recent history, and the associated coronal arcade appears to be blown away.
  020524: Comparing Coronal Dimming at Different Temperatures
When a region in the coronal images becomes dark, is it because the plasma has left the scene, or because the temperature has changed, making it the "wrong color" for the telescope's filter? This nugget shows that the way to find out is to look in as many different telescopes as you have available.
  020531: The Yohkoh Hard X-ray Flare Catalog
As part of the project to archive all the Yohkoh solar data, a catalog of HXT images from all the flares observed during the mission (several thousand of them!) has been created. Check out this nuget for an introduction to the online data catalog.
  020607: The Yohkoh Galileo Project
What do you do when you've finished taking photographs? You put them in an album! Well, if you've got ten years' worth of images, light curves, and spectra, then it's going to be a very big album, and a very big job. This week's nugget describes the task to create a lasting archive of the Yohkoh data.
  020614: Properties of Big Flares in this Cycle
All flares are not the same. Sounds obvious, but it bears some investigation to find how often a big flare is related to, say, a coronal mass ejection or to a sudden increase of energetic particles at the Earth. This nugget investigates the association of the biggest flares with some elements of space weather.
  020621: Slow Solar Wind Coming From Active Regions
Coronal holes have appeared in several previous nuggets, including some revelations of the "skinny" or "narrow" coronal holes. This week's nugget announces the discovery that these features appear to be a source of the slow component of the solar wind.
  020628: A Balloon, a Novel Detector, and Solar Hard X-rays
If you want to see the Sun's X-radiation, then you'll need to get outside of Earth's atmosphere. For many researchers over the years, balloon rides have been just the ticket. Check out this nugget for insight into how new detectors are space-tested, without risky (and costly!) rocket flights.
  020705: The "Disappearing Solar Wind" Revisited
Do you remember "the day the solar wind disappeared"? It was a pretty unusual occurrence, and a lot of research has focused on trying to understand it. This nugget recaps the circumstances of the event, and describes some of the ongoing research.
  020712: Footpoints (and ankles?)
"Footpoint" is the term we use to describe the location where coronal magnetic field lines connect to the photosphere -- they are the apparent "feet" of the coronal loops. As such, they are located in magnetically interesting places, and their behavior during flares helps us to learn about the structure in general. Read on!
  020719: Supra-arcade downflows in the April 21 flare
We see over and over again that the combination of different kinds of data helps us to look at scientific problems from new angles. This week's nugget combines imagery from TRACE and spectroscopy from SoHO/SUMER for a closer look at an enigmatic solar phenomenon.
  020726: Footpoints slipping along
As we discussed a couple of weeks ago, the footpoints of coronal loops are where the magnetic field connects to/through the photosphere. Changes in the location and in the "color" of the hard X-radiation tell us about the evolution of the magnetic field and of the population of charged particles which are responsible for the radiation.
  020802: A New Word in the English Vocabulary?
Every mission produces its own stock of jargon, and Yohkoh is no exception. Here's a bit of obscure terminology that has already found its way into a new space mission.
  020809: Setting the Record Straight on Big Flares
When we count the number of solar flares, how many big ones should we expect, and how many really big ones? Previous nuggets have pointed out an apparent shortfall of the biggest flares during some time periods, but new analysis suggests that we might have been mistaken.
  020816: RHESSI's First Big Flare -- The New and the Newer
Our current model of CME-related flares includes a description of how coronal arcades are built. But this week's nugget introduces some observational details that may help us to refine the specifics of the process.
  020823: Coronal Oscillations and Type II Bursts
Magnetic fields can exert pressure and support tension, and as a consequence they can oscillate when disturbed -- no abstract idea, these oscillations have been observed. This week's Nugget asks if the disturbances which set off the oscillations may simultaneously be responsible for another enigmatic solar phenomenon: Type II radio bursts.
  020830: Precise Hard X-ray Measurements
The highest-energy photons -- hard X-rays and gamma rays -- are very difficult to make images with, because they do not respond to lenses and mirrors in the same way as lower-energy photons. We use "modulation collimators", a kind of masking, to create images of hard X-ray sources with Yohkoh HXT and RHESSI. Look here for a cursory comparison of the resolution possible with these two telescopes.
  020906: Has RHESSI Observed Yohkoh Jets?
X-ray jets were an exciting discovery of Yohkoh -- now it appears that our newest solar observatory, RHESSI, may be able to observe them as well, and in a different energy range.
  020913: A Little Bit of (Naked Eye) Sun Spot History
Just in case you forgot, here's a reminder that the Sun has been high on the list of astronomers' targets for a very, very long time.
  020920: Mysterious Anomalies in Better Detail
Charged particles can be a nuisance to high-energy detectors like those on Yohkoh, SoHO, TRACE, and RHESSI. This week's nugget summarizes (very briefly) the source and location of some particles which interfere with observations by three of these four spacecraft.
  020927: Coronal Compton Scattering?
The first observation of the corona was in fact not in X-rays or ultraviolet, but ordinary white light. And its visibility is due not to emission from million-degree plasma but from scattering of light (that is, reflected light) from electrons. This nugget describes an extension of the original coronal observations to the regime of hard X-rays.
  021004: Possible Solar Signatures of SEP Events
Energetic particles arriving at Earth can cause big problems, and the Sun is a major source of these particles. It's easy to say that the cause is solar flares, but this nugget demonstrates that such a broadbrush description may be misleading: some ejections are more likely to send energetic particles to Earth than others.
  021011: A 'Simple Loop Flare' Up Close
We've seen many nuggets about eruptive flares -- but we must remember that many flares don't seem to "explode" and throw material out into space. Do these "confined" or "simple loop" flares have a different kind of basic physics?
  021018: Chromospheric Evaporation in Flares
The brightly emitting plasma we see in the corona arrives there from somewhere else, probably from the lower levels of the atmosphere. How is the plasma driven there, and how quickly is it lifted?
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