MAGNETIC RECONNECTION

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In this page you will find:
item     Introduction
item     Reconnection on the Sun
item     Earth magnetosphere
item     Magnetars
Chapter 4

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Introduction

If in the last times it is spoken about plasma physics, most of the discussion tends to be about the reconnection of magnetic field lines. It is about a phenomenon, which helps to explain a whole range of processes in the plasma that so far are not understood. Among them belongs for example the sudden flares in the corona of the Sun, the phenomena at the boundary of Earth's magnetosphere and interplanetary space, the phenomena in the atmosphere of the magnetars, the same as phenomena in plasma laboratories. The reconnection of the magnetic field lines can be achieved for all sizes of plasma.

For this phenomenon, there is not a permanently defined phrase in Czech language. The most usual is to speak about the "switching over" of the magnetic field lines, about reconnection (which would be the original term in English) or about a magnetic "short circuit". Each of us roughly knows, what means the electric short circuit. The electrical current finds a shorter, more convenient path. In a similar way magnetic field lines structure changes its original topology into another, energetically more convenient configuration. During such process there is a release of energy, which heats up the surrounding plasma. Sometimes so much, that the plasma radiates in the X-ray region. In any case, the change of topology of the magnetic field lines implies a fundamental change in the plasma behavior.

Magnetic Reconnection

If a plasma has a finite conductivity, it can reach a transformation between magnetic, thermal and kinetic components of the total energy. The currents flowing in the plasma and their consequent Ohm (or Joule) heating contribut to such a process. Imagine that in the plasma two regions of the magnetic field get closer to each other, each one with backwardly oriented field lines. In the middle the so-called diffusion region develops. There is low value of the magnetic field. Precisely here comes the change in the topology of the magnetic field lines, which "switches over" into the new configuration with lower energy. During the reconnection a high electric current flows in the diffusion region, which heats up the plasma. The energy of the magnetic field is transformed into the thermal energy of the plasma. A hot plasma intensively radiates its surplus of energy towards its surroundings. The macroscopical motion of the plasma is also influenced during the reconnection. Before the reconnection, the plasma moves perpendicular to the field lines towards the diffusion region (red arrows in the figure above). After the reconnection the plasma is expelled in the direction of the original orientation of the magnetic field lines. According to the form of the magnetic field lines, the middle of the diffusion region, where the field is null, is sometimes called neutral point of the type X. In the three dimensional situation a whole curve is created around the zone with a null value of the magnetic field. In some situations an instability is created when there is a recursive reconnection of the magnetic field  lines with periodically repeated null points with shapes X and O.

Magnetic Reconnection

After the reconnection of the magnetic field lines a plasmoid originates (a compact plasma formation, which can carry away with itself the so-called "frozen" magnetic field) and jets or spurts – plasma zones, which take away part of the energy of the magnetic field transformed into kinetic and thermal energy.

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Reconnection in the Sun

The ejection of coronal matter from the Sun belongs to the most well known phenomena related with the reconnection of magnetic field lines (CME, Coronal Mass Ejection). In the vicinity of the surface it is found a strong local field (for example, around a Sun spot and protuberances). Not infrequently it happens that, at an elongated loop of magnetic field, two zones with fields of opposite orientation get against each other. In that moment it is possible the reconnection of field lines and the transformation of the liberated energy into thermal energy. The originated hot cloud radiates mostly in the X-ray region. The cloud may separate itself from the Sun and, in the form of a plasmoid with a "frozen up" magnetic field, it can travel through the solar system. If this object collides with some planet, the charged particles flow along shock wave and through the polar cusps (funnel like regions in the vicinity of the polar magnetic field of a planet) penetrate into the planetary magnetosphere. It generates the polar lights, magnetic storms and some other accompanying phenomena. In the year 2000, one of the plasmoids, expelled from the Sun, "flybyed" the magnetospheres of Earth, Jupiter and Saturn.

CME

Incvestigation of the coronal mass ejections is very important. The ejection of a plasmoid (whenever it flies in the direction towards Earth) may induce a magnetic storm and the emergence of electric potential difference, which is able to damage electric power networks. We classify the CMEs according to the measured intensity of its accompanying X-ray flare into B, C, M and X types.

X ray flare classification energy flow description
B < 10−6 W/m2 inexpressive X ray peak
C 10−6÷10−5 W/m2 small flare
M 10−5÷10−4 W/m2 medium flare
X >10−4 W/m2 the most intensive flare

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Earth's Magnetosphere

The reconnection of magnetic field lines happens often in the Earth's magnetosphere. In the "day" side (towards the Sun) happens a connection between the magnetic field lines of the interplanetary magnetic field and the field of the Earth itself. If the direction of the field lines is opposite and an enhanced flux of solar plasma flows  towards us from the Sun, it can lead to a reconnection between both of types of the field lines. The charged particles then penetrate into the upper layers of the atmosphere along the originated open filed lines, where they can create the polar lights. The flowing plasma disrupts the magnetosphere, and leads to a magnetic storm. The plasma reaches the plasma lobe in the night side, where it induces further reconnection of the magnetic field lines and the creation of plasmoids with "frozen upon" magnetic field. The reconnection of magnetic field lines can happen even in the case when the interplanetary field is parallel to the direction of the field lines in the day side. The magnetic reconnection of the field lines is then reached sidewards with respect to the magnetosphere, not at all at the zone of the local noon. The outcoming energy during the reconnection is transformed into low frequency electron oscillations known as whistlers.

James Dungey reflected about the possibility of the reconnection of the magnetic filed lines for the first time in the year 1961. The reconnection was observed for the first time by the satellite POLAR as far as in the year 2000. The measurement was completed with the conditions of observation in the plasma tail from the satellite Geotail.

Earth Magnetosphere


Magnetars

Magnetars are neutron stars with an extremely strong magnetic field, which can reach values up to 1011 T. It is about objects with the strongest yet known field in the Universe. The surface of the magnetar creates a crust from the neutrons and the magnetic field, in which magnetoquakes repeatedly occur. Through the magnetoquake, the disturbed and deformed lines of the magnetic field rip apart and connect again into a more convenient configuration. During this process, a significant amount of energy is liberated, which heats up the magnetic corona of the magnetar. A hot envelope of electron-positron pairs and photons (hot non-hadron matter, igneous ball or fireball) is created. Electrons and positrons are trapped and contained by a strong magnetic field, photons escape in the form of an X-ray flare or a soft gamma burst. We call these unusual X-ray stars AXP (Anomalous X ray Pulsars) and in the case of gamma bursts, SGR (Soft Gamma Repeaters). Altogether, of such sources, in our galaxy are known over ten. The flares manifest themselves at non regular intervals, we talk about the so-called active phase of the magnetar. Sometimes a catastrophic reconnection of the magnetic filed lines is reached and a gigantic flash occurs, which is a thousand times more energetic than nominal, repetitive flares. So far there were observed three of such cases (1979, 1998, 2004), in the last one there was the biggest release of energy yet, and up to 1039 J.

MHD dynamo origin Magnetar outburst SGR 1900+14 in radio band

Left: Emergent magnetar, a neutron star with an unusually strong magnetic field up to 1012 T. The crust is already unstable, explodes , a regular magnetoquake manifests, accompanied by flares in the soft gamma region. The first trustworthy detection of a magnetar was in the year 1998 by Chryssa Kouveliota from the Marshall Space Flight Center at NASA. The heat is taken out through the convection towards the surface. If during the birth of a magnetar its rotation is bigger than 200 revolutions per second, then the combination of the rotation and the convection creates so called  magnetohydrodynamic dynamo, a fluid variation of the classical dynamo, which induces the onset of a strong magnetic field.

Middle: Magnetar, on which occurred a reconnection of magnetic filed lines accompanied by X-ray or gamma flare. Such flares repeat very often.

Right: Gigantic burst SGR 1900+14 from August 27, 1998. The flare passed through all the regions of the spectrum. This animation from the NRAO observatory is in the radio band.

gigantic flare date released energy
SGR 0525-66 March 5, 1979 6×1037 J
SGR 1900+14 August 27, 1998 2×1037 J
SGR 1806-20 December 27, 2004 2×1039 J

The reconnection of magnetic field lines is an important phenomenon, which in nature occurs more often than we used to think. We observe it both in the cosmic and laboratory plasmas and in the last years the magnetic reconnection got the proper attention from astronomers and plasma physicists.


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Translation: Arturo Ortiz Tapia, 2005