Spaceweather


... conditions on the sun and in the solar wind, magnetosphere, ionosphere and thermosphere that can influence the performance and reliability of space-borne and ground-based technological systems and can endanger human life or health. (NSWP)


Contents

Introduction

Solar activity

Geomagnetic activity


Introduction

Because of the Internet, today everybody can predict the chance on a major solar-eruption and evaluate the condition of the geomagnetic field. You can then compare your results with other, professional, sites:

More information on Spaceweather can be found on:

When watching all the images, please make sure to keep an eye on the date the image was made. Getting images of the same timeframe facilitates spaceweather-predictions.

Return to Contents.


Solar activity

Photosphere

The observation of the photosphere still provides the basic data for solar research. Pictures can be found on the following websites:

Mid-term (7 to 10 days) predictions of potentially active groups can be based on helioseismological data from SOHO's solar backside.

The chance on solar-eruptions increases:

Return to Contents.


Magnetogram

Magnetograms give an idea on the magnetic distribution within a sunspotgroup and all over the solar surface. Pictures can be found on the following sites:

Sunspots are areas on the solar surface where the local magnetic field is very strong, prohibiting the rise of the hot solar plasma from the interior. Sunspots are really cool! These magnetic fields can have a positive or negative polarity. The closer these fields are from each other, the bigger the chance on a "short-circuit", which translates on the solar surface as an enormous explosion.

So, chances on a solar eruption increase:

Return to Contents.


H-alpha

The red light of the hydrogen-alpha line allows us to see the chromosphere, the layer of gas just above the photosphere. In the chromosphere, dark, almost serpentlike structures can be seen. These are called filaments. One sees actually prominences (relatively cool solar gas imprisoned by areas of opposite magnetic polarity), but against the hot background of the solar disc, they appear darkish. Filaments can not only be found within bipolar sunspotgroups (the so-called groupfilament), but they can appear everywhere on the solar surface.

The best site for H-alpha pictures is the Global H-alpha Network, where images can be found from 6 different locations from all over the world! Moreover, this site also contains archives with pictures and movies of recent and not-so-recent activity.

Other sites for H-alpha-images are:

Chances on a solar-eruption increase:

Erupting prominences, always occuring at the solar edge, are always directed away from earth and thus are hardly a threat to the geomagnetic field.

Return to Contents.


Corona

The corona is the superhot low-density outer atmosphere of the sun. Because this layer radiates especially on short wavelengths like e.g. the ultraviolet, and this energetic radiation is stopped by the earth's atmosphere (lucky us!), we need to use satellites to observe the corona. We have only a few satellites making global pictures of the sun in these wavelengths:

If SOHO-images are temporarily unavailable, one can sometimes find at SDAC a collage of highresolution images made by the TRACE-satellite. Also, at the sites of Sacramento Peak and Kitt Peak (NSO), images of the corona along the solar edge as well as synoptic maps can be found.

Chances on a solar-eruption increase:

Bright areas at the eastern edge of the sun indicate the potential appearance of possibly active groups; at the western edge, it means that such groups have returned to the backside of the sun.

Also look for dark patches in the corona. These are especially well visible in SOHO's EIT-images. These are the famous coronal holes. They send showers of highenergetic particles into space. If these coronal holes reach the central portion of the solar disc, they may influence the geomagnetic field.

If energetic eruptions have occured, they can be recognized in EIT-images:

An example in which all effects are clearly visible, can be found at theTRACE-website (Movie 100).

Return to Contents.


Flare activity

Crucial data in the determination of the Spaceweatherconditions are the measurements of the GOES-satellites in Röntgen (X-rays). These data are so important that NOAA always has 2 satellites circling the earth and monitoring the sun. The GOES-data show for the last 2 to 3 days how many flares have occured, when these eruptions took place, and what their intensity was. The data do not say by which sunspotgroup (or filament) they were generated. To do that, the eruptions have to be correlated with H-alpha-, EIT- or röntgen-images. Up-to-date lists can be found at the events-page of NOAA's Space Environment Center. Relating flares to sunspotgroups can be done using the BBSO-archive (H-alpha), via SOHO (EIT) and - especially - via SXI (röntgen).

In practice, the C- and especially the energetic M- and X-flares are important. With these energetic eruptions, chances are much higher that the sun billows matter into space. These "Coronal Mass Ejections (CME's)" can disturb the geomagnetic field severely. If the CME is directed away from the earth, one sees it in profile and nothing will happen to earth. If the CME is directed to earth, one can see a few hours after the explosion an ever-expanding ring around the sun (a halo-CME). If the CME is only partially directed to the earth, one will see an expanding "off-center" ellips. In the latter case, a potential impact on earth is much harder to predict.

Once more, the best site to detect CME's is the SOHO-website (LASCO-movies). SOHO has a coronagraphe with which the bright solar disc can be obscured, thus revealing the immediate surroundings of the sun. In the LASCO-movies, no ejection of solar matter goes on undetected. The faster a CME occurs after an explosion, the higher its speed, and the faster it can reach earth. If a CME is seen without a flare, it may be because the eruption occured at the sun's backside. Such a CME will have no influence on earth.

If no SOHO-images are available, radiograms can be used to determine if matter was ejected. The ejected material causes vibrations in the corona between 20 and 1000 MHz. Recent radiograms can be found at the site of the Australian Spaceweather Agency (IPS) (under "Spectrographs"), and more specific data in the lists of the events-page of NOAA's Space Environment Center.

A protonflare is an eruption that produces a lot of energetic protons. These are very dangerous for satellites and astronautes. The GOES-satellites continually monitor this protonactivity.

Return to Contents.


Geomagnetic activity

Now we have an idea on solar activity, we can determine the potential impacts on the geomagnetic field. For this, we can use another satellite, the Advanced Composition Explorer (ACE). ACE is situated close to SOHO, at one and a half million kms in between earth and sun. From there, it provides scientists on earth the first signals of the approaching storm (CME), about 45 minutes before this storm will actually slam into the geomagnetic field.

It is the solar wind, a stream of charged particles from the sun, that changes the symmetric magnetic field of the earth ("apple-core" shape) into the geomagnetic field (een " waterdrop"-shape, impressed at the side facing the sun). In the vicinity of the earth, the solar wind's average speed is about 400 km/s, its density about 7 particles/cm3, its temperature a few onehundredthousands degrees and its magnetic field +/- 6 nT (nanoTesla). CME's and coronal holes can significantly alter these values. Pending the conditions of the solar wind and the location of the eruption on the sun, a CME needs 1,5 to 2,5 days and a coronal hole 2 to 3 days to bridge the distance sun-earth. Faster "chrono's" are possible, especially if the CME's are produced by highenergetic flares.

To determine if the earth is under the influence of a CME or a coronal hole, this diagram may be of use. The magnetic field of the passing storm needs to be parallel to earth's (southerly directed, negative values) for a couple of hours before a significant disturbance can occur. Only then, particles out of the CME have a good chance to attach themselves to the magnetic fieldlines of the earth. They then can glide along these lines until they collide with particles of the earth's atmosphere. There, they can produce aurorae, disturb radio-communication or cause satellites to malfunction.

Statistics have shown that earth is more vulnerable to solar storms during spring and autumn than it is during the other seasons. This may be due to the geometric configuration sun-earth-geomagnetic field.

The disturbance of the geomagnetic field is mostly represented by the Ap- or the Kp-index. In order to have aurorae being visible over Belgium, Kp-values of 8 or 9 (out of a maximum of 9) need to be reached, at night and during clear weather...

Data on geomagnetic activity can be found at the following sites:

There exists also a spaceweather discussion forum where you can address all your questions.

Return to Contents.