Wednesday, August 21, 2013
The JSOC Data Capture Computers are being replaced as I type and will be brought online as rapidly as possible. Although the replacement was planned for the next few weeks, the outage means it being done now.
Friday, August 16, 2013
X-ray images of the Sun often show the large dark regions that we call coronal holes. They may extend from the Sun's equator to its poles, a few from pole to pole. In the 1960's they were seen in X-ray images taken by sounding rockets and detected with some radio telescopes. They were first seen clearly in images taken by astronauts on board the Skylab space station in 1973 and 1974. Waldmeier had seen coronal holes in the 1950’s with a green line (Fe XIV 5303) coronagraph that allows the corona to be seen along the limb of the Sun but the X-ray images of the disk of the Sun really showed what they were.
The solar corona is the outer atmosphere of the sun, extending from the solar "surface" out into space. It is difficult to observe from the ground, being seen only during solar eclipses or with special equipment. A coronal hole is a large region in the corona that has a lower density than its surroundings but with about the same temperature. It really is a hole in the corona! You can see the surface of the Sun, which is dark because it is cool next to the coronal temperatures of several million Kelvin. Although coronal holes can appear at any time of the solar cycle, they are most common and last the longest during the declining phase of the cycle.
The magnetic field in a coronal hole opens to interplanetary space. This is unlike the active Sun where the solar magnetic fields loop back to the Sun’s surface and form the bright loops and arches seen in X-ray and UV images. The rest of the Sun is covered with the quiet Sun magnetic carpet.
During solar minimum the coronal holes near the poles can last for several years. Holes near the equator may open and close in as little as a day but often remain visible for more than a month. Other magnetic features, such as sunspots and filaments, rarely last more than a rotation of the Sun (27 days). This means coronal holes may be the longest lasting magnetic feature on the Sun.
The presence of open magnetic field lines in coronal holes allows the plasma to escape, meaning that coronal holes have a lower density then the surrounding corona. The escaping particles mean that coronal holes are sources of high-speed solar wind streams. Particles in these streams can move at speeds up to 800 km/s (1.8 million mph). When the particles from these streams hit the Earth they may cause geomagnetic storms.
At times of high solar activity, geomagnetic storms are usually caused by coronal mass ejections (CME's) striking the Earth's magnetosphere. During times of low solar activity, coronal holes are the most common source of geomagnetic storms. Because coronal holes can last for many months, it is often possible to predict the occurrence of this type of geomagnetic disturbance, as the high-speed stream sweeps past the Earth with each solar rotation (like a rotating garden sprinkler). It may be possible to predict solar activity by the size of the coronal holes that form over the poles of the Sun.
In the SDO image from August 14, 2013 shown above the bright regions indicate hotter areas of the solar corona, mainly above active regions. A large dark coronal hole extends across the northern hemisphere. The stringy dark areas are filaments, cooler plasma held above the surface of the Sun by magnetic fields.
This picture shows two sources of geomagnetic storms, coronal holes, a source of high-speed streams, and filaments, whose eruption causes coronal mass ejections, in one picture. The filament in the lower right erupted as a coronal mass ejection a few hours after this image was recorded. Aurora caused by the high-speed stream emitted by the coronal hole were seen last night.
Edit 8/19/2013: It was pointed out on Facebook that the leftmost filament is actually another coronal hole.
Wednesday, August 7, 2013
Comet ISON is a sungrazing comet whose closest approach to the Sun (perihelion) will happen on November 28, 2013. Comet ISON was discovered last year by two Russian observers when it was further from the Sun than Jupiter. This usually means the comet is large and will become bright. Comet ISON's nucleus is thought to be about 2 miles across (no more than 4 km from HST observations). It is currently approaching the frost line at 2-3 AU where water ice will start to evaporate and the comet will brighten. We had a Comet ISON workshop last week at APL in Maryland. You can go to that site for information about Comet ISON and the many ways people will be observing it.
Comet ISON may become a spectacular comet in the night sky but SDO cannot see a comet until it is passing very close to the Sun. This graphic was developed to show what we learn from Comet ISON that is different from the previous comets that were seen by the EUV telescopes on SDO and STEREO.
The numbers on the left are the distances from the center of the Sun, with curves drawn at each integer distance from 1-4 solar radii (1-4 Rsun).
Next are the perihelion distances of the two sungrazing comets seen in the EUV and Comet ISON, also extended as curves across the diagram. Sungrazing comets do not dive into the Sun, they appear to skim along the surface of the Sun. That allows for a lot of heating from the hot Sun but also means they don't burn up from friction like a meteor in the Earth's atmosphere. Many things change as you adjust the perihelion distance. For example, Comet Lovejoy was moving at 575 km/s (1.3 million mph) at a perihelion distance of 1.15 Rsun. Comet ISON will be sailing along at a mere 375 km/s (840,000 mph) at its perihelion. Comet Lovejoy took about 1 hour to go last part of its orbit (from 2.3 Rsun to 1.15 Rsun). Comet ISON will take over 3 hours to do the same, but this time moving from 5.4 Rsun to 2.7 Rsun.
"Breaking Up" shows the locations of the Roche limits for fluid and solid spheres. If a fluid ball flies closer to the Sun then the top of this bar it will break up into fragments, if a solid ball flies closer than the bottom of the bar it will break up. The first two comets were likely to fall apart because they flew so close, well below the Roche limit for a solid ball, but Comet ISON will fly through a region where it may or may not break up.
"Ice Evaporation" is an indication of the heating by solar radiation compared to the heating at the photosphere (or 1 Rsun). The heating of the comet by the Sun increases as the comet gets closer to the surface, but another effect that I call the two-pulley problem increases the heating even more. The first is the proximity effect but the second only happens when a small body gets close to a large body. More than half of the small body gets heated at once. That means the heating rate at 4 Rsun is 30 times less than the rate at the Sun's surface rather then the 16 ([1/4]4) times less you would think from the proximity effect alone. For comparison, the heating rate at the Earth is 50000 times smaller than the heating just above the surface of the Sun!
"Melting Sand" shows the region below 4 Rsun where micrometeoroids about 1 micron in size rapidly evaporate. Micrometeoroids are small sand particles in the comet ice. As the ice evaporates the particles are released and they also evaporate as they are heated by the Sun. The location of this boundary varies a great deal with the model of the micrometeoroid and will be tested by ISON where it leaves the material behind.
The "Visibility" grey bar, is the focus of our work on ISON into what makes the comet debris bright in the EUV. It is our primary known unknown at this time.
The "Escape" bar shows the solar wind acceleration region extending from 1.1 to above 4 Rsun. Debris from Comet ISON may be swept out into the heliosphere by the same acceleration that removes the solar wind from the Sun. Observations of Comet ISON may allow us to watch this happen. A lot of debris from the earlier two comets was left in the coronal loops and will slowly become part of the solar wind.
The coronal loops and helmet streamers show the types of magnetic fields the comets will fly through. At Comet ISON the magnetic field is mostly up and down, only the helmet streamers stick up this high in the corona. Closer to the Sun the field forms coronal loops and the debris is trapped near the surface. This is why we saw the tail of Comet Lovejoy wiggling. The tail of Comet ISON will probably look like swaying stalks as it will leave debris in a much simpler magnetic field.
Have a comet for Thanksgiving!