The Sun Playlist
The Science On a Sphere is a wonderful tool to view datasets which show current solar activity. There are certain limitations due to the 2-D nature of the projection, but it provides a very useful way to view these phenomenon. These datasets were created from data collected by the EUVI (Extreme UltraViolet Imager) instrument aboard the STEREO (Solar TErrestrial RElations Observatory) space telescope twins. EUVI images the solar atmosphere at several wavelengths, all within the far ultraviolet part of the electromagnetic spectrum. Because hotter material emits ultraviolet radiation at short wavelengths, images taken at different wavelengths show solar material at different temperatures. They are updated every five days, and so will change periodically as imagery is updated.
Here, we’re presenting images taken at two wavelengths: 195 and 304 Angstroms. In the images taken at 304 Angstroms, false-colored orange, the bright material is at 60,000 to 80,000 degrees Celsius, and 195 Angstrom images are false-colored green and correspond to about 1.5 million degrees Celsius. The false colors these images are rendered in bear no relationship to their actual appearance, as UV radiation in these wavelengths cannot be seen by the human eye.
The two datasets in this playlist are quite different in appearance, because the sun’s atmosphere actually gets hotter as you gain altitude. In each image, darker colors denote cooler temperatures and brighter images show hot, energetic regions. Both datasets show the solar surface, or photosphere, and the solar atmosphere, but because each dataset shows a different temperature, different structures are visible. While it’s difficult to use these 2D projections to ascertain the altitude of an imaged feature, in general, hotter temperatures correspond to higher altitudes in the solar atmosphere.
304A Dataset Information
At the 304A wavelength, which corresponds to cooler temperatures, the photosphere is more visible. The photosphere is the region of the sun that emits the majority of sunlight seen from Earth; below it, the sun is opaque to many wavelengths of light, and above it the solar atmosphere is transparent. It is considered the sun’s surface, though the sun’s atmosphere extends far above it. The topmost layer of the sun below the photosphere is the Convective Zone, where super-hot regions of hydrogen and helium boil from the heat generated in the sun’s core. These images show the photosphere’s granular, almost foamy, appearance. The granules on the photosphere are the upper regions of countless vast convection cells that circulate hot material up from lower regions of the Sun. The convective zone is rich in hydrogen ions, which absorb visible light easily, and the photosphere is where their concentrations drop below the level required tomake the solar material opaque. As the solar material becomes transparent, the radiation trapped within it is released, and its energy escapes the Sun entirely.
195A Dataset Information
The hotter 195A wavelength shows more of the higher solar atmosphere - the chromosphere, transition region, and corona. Given the 2D nature of Science on a Sphere projections, how much of the chromosphere is visible depends on the spacecraft’s angle of view. Seemingly paradoxically, this region is much, much hotter than the photosphere. It is theorized that magnetic reconnection, a process in highly energized plasmas where magnetic fields transfer kinetic energy from place to place, may play a role in this phenomenon, but it is poorly understood. It is difficult to distinguish at a glance between the various regions of the solar atmosphere, because the spacecraft’s angle of view and the projection may distort and flatten features of each region.
Points of Interest
STEREO’s mission is to monitor solar phenomena- solar flares, coronal mass ejections, sunspot cycles, and other features of space weather. These datasets clearly show a number of these features. As the datasets are updated regularly, this document cannot point out specific features, but some general features to note are outlined below
- Sunspots: Sunspots are darker than their surroundings (because they are cooler), but they are surrounded by very hot, energetic material that emits strongly in the ultraviolet - and so they will appear bright in these images. The imagery, when projected on the sphere, tends to hide the actual sunspot itself behind the bright features that surround them. Sunspots form when magnetic fields in the sun’s convective zone are twisted by differential rotation forming concave depressions and associated areas of complex magnetic activity on the sun’s surface. Sunspots usually occur in magnetically opposite pairs, and it’s typical to see multiple pairs of sunspots in the same general vicinity.
- Solar flares: Solar flares, like the unusually powerful one that erupted on 1/24/2012, originate from highly magnetically active areas near sunspots. They are eruptions of highly charged plasma from deep in the solar atmosphere, triggered by the snapping of unstable, twisted magnetic fields that burst through the solar corona into space. Solar flares carry enormous energy and may cause auroras and disruptions to electrical systems on Earth. These may manifest in the STEREO data as bursts of bright UV radiation that brighten and fade quite rapidly.
- Coronal Mass Ejections: CME arise from instabilities in the magnetic field of the solar corona, and unlike solar flares are mostly comprised of material from the corona. They, too, are immensely powerful, but are less localized and explosive, and are unlikely to be visible in these datasets.
http://hpde.gsfc.nasa.gov/LWS_Space_Weather/mdigraphic.gif
http://hpde.gsfc.nasa.gov/LWS_Space_Weather/SpaceWeatherOverview.html
Slide blurbs:
304A: Have you noticed the patchy pattern visible in these images? They’re called “granules,” and they’re formed as hot material from deep inside the sun circulates up to a region called the photosphere, which is considered the sun’s surface.
195A: These images were taken using only shorter wavelengths of UV light. The hotter a region of the sun is, the shorter the wavelength of the light it tends to emit - so these images show more of the solar atmosphere, which is hotter than the photosphere.