Force free magnetic flux rope with a high current density on the axis

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Аннотация

A new model of a force-free magnetic flux rope with a high concentration of electric current on the axis is presented. The general property of axisymmetric force-free magnetic ropes is that with the exit of the top of the magnetic loop-rope into the corona, the external pressure that keeps it from lateral expansion steadily decreases, and with some critical decrease in this pressure, the longitudinal magnetic field of the rope becomes zero on the surface where the electric current changes its sign (it is current inversion surface — CIS). In this case, the force-free parameter α(r) and the azimuthal electric current experience a second-order discontinuity on this surface, so that in the vicinity of CIS their values begin to increase without limit. The current (drift) speed of electrons here will inevitably exceed the speed of ion sound. This serves as a trigger for the heating of non-isothermal plasma (so it turns out Te >> Ti) and the excitation of plasma ion-acoustic instability of the plasma not only near the CIS, but also in the central region of the rope, on its axis, where the current density is especially high. The appearance of anomalous resistance leads to rapid dissipation of the magnetic field and the generation of a super-Dreicer electric field. The Parker effect, associated with the equalization (with some delay) of the torque along the axis of the rope due to the transfer of the azimuthal field to the region of energy release, leads to quasiperiodic pulsations of hard flare radiation and, ultimately, ensures the flare release of the most part of free magnetic energy accumulated in the rope.

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Авторлар туралы

A. Solov’ev

Main (Pulkovo) Astronomical Observatory of the Russian Academy of Sciences

Хат алмасуға жауапты Автор.
Email: astrojourn@pran.ru
Ресей, St. Petersburg

Әдебиет тізімі

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Қосымша файлдар

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Әрекет
1. JATS XML
Жүктеу
2. Fig. 1. A section of a magnetic cylinder with a cross-section radius a, as the top of a slightly curved magnetic loop with bases fixed far in the photosphere (Parker model). Since the total current in the rope is screened and equal to zero, the rope inevitably has a magnetic surface of radius r0 on which the current changes sign (CIS — current inversion surface). Cartesian (x, y, z) and cylindrical (r, ϕ, z) coordinate systems are shown. The blue line is the helical magnetic field, Bex is the external longitudinal field providing lateral equilibrium of the magnetic rope.

Жүктеу (91KB)
Метадеректер
3. Fig. 2. Function (5), green line, is expressed in units . Purple line is the density of longitudinal current , in units . Its value on the axis is 11, the current changes sign at . Blue line is the azimuthal field .

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4. Fig. 3. Blue line is the field. Purple dotted line is the longitudinal field at external pressure above critical, red line is the field, which vanishes on the CIS surface with coordinate at .

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Метадеректер
5. Fig. 4. a. Here the quantities have a discontinuity at G = Gcr = 2. The purple line is the total current in units (maximum +11 on the axis), the blue line is the azimuthal current. The green line is the force-free parameter α(r), on the axis it is equal to 3k. b. The behavior of electric currents and the force-free parameter α(r) at an external pressure slightly above the critical pressure: G = 2.03 > Gcr = 1.99999 = 2.0.

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Метадеректер
6. Fig. 5. The form of the integrand in expression (23) for different values of the external pressure G. For G = Gcr, the area under the curves, i.e. the integral 2W, takes on a minimum value. For magnetic fields in the integrand (23), formulas (7) and (8) are used.

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Метадеректер
7. Fig. 6. Typical X-ray profile of a flare loop. The intervals between peaks are about 10 s.

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Метадеректер

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