Solar Cosmic Rays, Flares, and Coronal Mass Ejections

The Sun is a quiet star when one looks at its overall emission, which is largely dominated by visible light from its photosphere. But it displays intense activity visible in emissions from the corona, particularly in X-rays, EUV, and radio waves. The corona is the dilute gas around the Sun we see during a solar eclipse. Its structures, like its activity, is governed by magnetic fields.

The explosive conversion of energy in solar eruptive events is at the origin of hot plasma and accelerated particles, at energies well above the mean average thermal energy in the corona of about 100 eV. Some of these eruptive events accelerate particles to very high energies: solar cosmic rays. They are rare events: 70 have been observed between their discovery in 1942 and the year 2009.


The solar corona: a dynamical environment structured by magnetic fields

An eclipse photograph (here: 26 February 1998, Guadeloupe; © Christian Viladrich, SAF http://www2.saf-lastronomie.com/accueil.html) shows that the corona has an irregular shape - unlike the visible sphere of the Sun, its photosphere, which is nearly spherical. This is because the photosphere is governed by gravity. Gravity attracts every particle to the centre of the mass concentration, and thereby creates spherical bodies like planets, the moon, and the body of the Sun itself. The corona, on the other hand, is a 1 million degrees hot, ionised gas, composed of electrically charged particles, electrons, protons, nuclei of helium and of heavier elements. This gas is structured by magnetic fields, besides gravity, which are rooted in the solar interior.

Magnetic fields and the structuring of the corona

The situation is similar to the well-known experiment through which we learn at school that magnetic field lines join the two poles of a magnet: take a magnet, put a sheet of paper on top of it, and spread some iron powder on the paper. The tiny pieces of iron will align along the field lines, and make the field lines visible that way.



What has this to do with the solar corona ?

Let us have a new look, using emission that comes from the hot coronal gas. The picture on the right shows a detail of the corona in extreme ultraviolet (EUV), emitted by gaseous iron at about 1 million degrees (photo: Transition Region and Coronal Explorer satellite, TRACE; NASA). The photosphere, which emits nearly all the visible and infrared light of the Sun, is dark here, because at about 6000 degrees it is not hot enough for EUV emission. The hot coronal gas is confined in loops. This is because the emitting iron ions are trapped in magnetic field structures: like the pieces of iron powder, which are forced by the magnetic field to align along the field lines, the charged particles can only move freely along field lines, not in the perpendicular direction. We therefore “see” field lines in the corona where they confine material – like we “see” field lines of a magnet with the help of the iron powder. The magnetic field lines structuring the corona are rooted in the body of the Sun.

The dynamic Sun

There is a big difference between the magnet and the Sun: the magnet is essentially a static configuration, and so are its field lines. But the Sun’s interior is a turbulent gas, and the flows of this gas modify continually the embedded magnetic fields and their extension into the corona. Thus, unlike a familiar magnet, the magnetic field of the Sun is not static.

As a consequence, the large-scale coronal structures that we see during an eclipse or with a spacecraft at EUV wavelengths are not stable! The eclipse view is just a snapshot of a dynamical situation. The corona expels these structures in spectacular coronal mass ejections, and it explosively heats gas and accelerates charged particles to high energies in flares.


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The violent solar corona: coronal mass ejections and flares

The most spectacular manifestation of eruptive solar activity are coronal mass ejections (abbreviation: CMEs). The series of snapshots above was taken by the LASCO coronagraph aboard the Solar and Heliospheric Observatory spacecraft (SoHO; ESA/NASA). In a coronagraph the bright visible disk of the Sun is occulted, which makes the faint corona visible like during a natural solar eclipse.

The first snapshot shows the corona before the mass ejection event. The structure emerging above the occulting disk on the lower right is called a streamer – a feature that is familiar from eclipse photographs. In the following snapshots one sees the gas as it is propelled into the high corona with the confining magnetic field. It eventually leaves the Sun and propagates through the Heliosphere. Here again the gas makes the magnetic field structures visible. It is in fact not primarily the gas which is ejected, but the coronal magnetic field structure. The magnetic field takes the gas with it. This is different from the eruption of a volcano on Earth, where matter is explosively expelled, and then falls down under the action of gravity.

A solar flare manifests itself by a sudden brightening in different ranges of the electromagnetic spectrum. Such brightenings are particularly prominent in the typical coronal emissions: extreme ultraviolet (EUV), X rays, and radio waves. Or even at gamma rays, where the Sun is usually invisible to our instruments outside flares. The three pictures above show different snapshots of the Sun taken on 14 July 2000 by the Extreme Ultraviolet Telescope (EIT; wavelength 19.5 nm) aboard SoHO. Note the bright “active region” slightly above the centre of the solar disk: the middle panel shows that it suddenly brightens. The brightening persists in the next snapshot, taken more than 1 hour later. This is a solar flare. Solar flares and coronal mass ejections are not independent. The coronagraphs aboard SoHO also observed a CME with this flare.



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Solar energetic particle events

The white dots in the rightmost EIT picture are traces of high energy particles, protons and ions at energies of tens to hundreds of MeV that impact on the instrument – clear evidence that particles are accelerated to high energies during this solar event and escape to interplanetary space. This picture illustrates the impact of solar energetic particles on space technology.

Protons of still higher energies were detected by neutron monitors on the Earth. The figure shows the time profiles observed by several neutron monitors, extracted from the NMDB data pool. The acceleration of these particles is clearly associated in time with the solar flare and the coronal mass ejection that occurred at the Sun. Events like this, where the Sun accelerates charged particles to such energies that they can be detected by neutron monitors or other particle detectors on the Earth, are called Ground Level Enhancements (GLE). It is these high energy particles that we also call solar cosmic rays .

If you want to look at other GLEs, go to the NMDB event search tool. Choose the number of the GLE and the station whose observations you want to plot, and press "Submit".


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How do coronal mass ejections and flares arise ?

Let us have a closer look on what happens in an active region when a flare and a coronal mass ejection occur: the NASA spacecraft TRACE observed the 14 July 2000 flare at EUV, like SoHO/EIT, but with a smaller field of view, and with a higher time cadence.

The two snapshots on the left show the initial phase of the eruption: (1) a dark filament that was suspended above the bright active region (top panel) lifts off and is ejected. (2) As the filament rises through the corona, part of it is still seen in the snapshot shown in the bottom panel. The filament will become part of the coronal mass ejection.

The underlying region brightens, as seen in the first snapshot on the right (3). Subsequently an increasing number of loop-shaped features form, and fade away from view after several hours. The snapshot at the bottom (4) shows this.

Find movies of these and other TRACE observations at http://trace.lmsal.com/POD/.

Magetic reconnection: a key process in solar eruptive events

The processes that occurred during this event can be envisioned in a simple cartoon scenario depicted in the following Figure. This is a two-dimensional cut through the filament, which is a dense gas maintained against gravity by the coronal magnetic field.

(a) Electric currents flow in this gas, and create a magnetic field around the filament, as shown by the green circular field line in the Figure. At the same time, the filament is surrounded by magnetic field lines rooted below the solar photosphere – they emerge from the solar interior.

(b) If, under the influence of the turbulent gas motions in and below the photosphere, the filament magnetic field, together with the confined matter, rises to greater altitude, the region where it was originally has less matter than before, hence a lower pressure than its surroundings: the surrounding matter will flow into this region, and take the magnetic field with it. Oppositely oriented magnetic field lines approach each other in a region indicated by a yellow rectangle. This region is called a current sheet, because the abrupt change of the magnetic field there implies intense electric currents.

(c) Magnetic field lines can reconnect in the current sheet: the single red field line in (b) then forms two new field lines – one is closed around the rising filament, the other is part of a new loop below the filament.

(d) The process of magnetic reconnection affects successively field lines with increasing distance from the filament. When field lines reconnect that are rooted with one end in the Sun, and with the other end somewhere else in the solar system (not shown in the Figure), the filament can disconnect from magnetic field rooted in the solar interior. It is then expelled to the high corona and interplanetary space. This sequence is exactly what we saw in the TRACE images above: the filament rises and eventually disappears, while underneath new magnetic loops form, are filled with hot gas, and radiate for some time, for instance in EUV.

Particle acceleration

When magnetic fields reconnect, energy is converted to heating of the gas and to the acceleration of some particles to high speeds and energies. This creates various radiative signatures at different places, as shown in (d). Particles accelerated during the reconnection process can also escape to interplanetary space.

Particles are not only accelerated in the reconnection region below the filament. When the filament is expelled at high speed, it can generate a shock wave in front of it – like an air plane that flies faster than sound creates a shock wave in the air that we perceive as a sudden violent sound. In the solar corona, where the gas consists of charged particles, a shock wave involves electric fields, which can accelerate particles to high energies.

We do not know for sure how the cosmic rays which reach the earth after some large flares and coronal mass ejections are accelerated. What we know is that such particle events are always accompanied by large flares and fast and broad coronal mass ejections. Intense research activites address the respective role of reconnection and shock waves in particle acceleration in the solar corona.

Researchers employ various tools to elucidate the origin of large solar energetic particle events, and attempt to develop models to predict their occurrence, peak intensity and temporal evolution. Neutron monitors are key instruments for research on the fastest solar energetic particles.


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