274 research outputs found
Conditions for substorm onset by the fast reconnection mechanism
The fast reconnection mechanism, involving slow shocks and Alfvénic fast plasma jets, is most responsible for the explosive conversion of magnetic energy associated with geomagnetic substorms and solar flares. In this paper, the spontaneous fast reconnection model is applied to well-known phenomena of substorms. When the east-west width of the tail current sheet becomes 3–4 times larger than its north-south thickness, the fast reconnection mechanism can fully be established, which may lead to substorm onset. The resulting Alfvénic jet can exactly explain, both qualitatively and quantitatively, the in-situ satellite observations of the traveling compression regions (TCRs) associated with large-scale plasmoids propagating down the tail. Also, the earthward fast reconnection jet causes drastic magnetic field dipolarization, so that the sheet current ahead of the magnetic loop of closed field lines suddenly turns its direction toward the loop footpoint and a large-scale current wedge is formed according to the growth of field-aligned currents. It is demonstrated that an MHD generator arises ahead of the magnetic loop and drives the current wedge to distinctly enhance the current density in a pair of thin layers of the loop footpoint, giving rise to drastic heating in the form of two ribbons
Self-similar solution of fast magnetic reconnection: Semi-analytic study of inflow region
An evolutionary process of the fast magnetic reconnection in ``free space''
which is free from any influence of outer circumstance has been studied
semi-analytically, and a self-similarly expanding solution has been obtained.
The semi-analytic solution is consistent with the results of our numerical
simulations performed in our previous paper (see Nitta et al. 2001). This
semi-analytic study confirms the existence of self-similar growth. On the other
hand, the numerical study by time dependent computer simulation clarifies the
stability of the self-similar growth with respect to any MHD mode. These
results confirm the stable self-similar evolution of the fast magnetic
reconnection system.Comment: 15 pages, 7 figure
Fast magnetic reconnection in free space: self-similar evolution process
We present a new model for time evolution of fast magnetic reconnection in
free space, which is characterized by self-similarity. Reconnection triggered
by locally enhanced resistivity assumed at the center of the current sheet can
self-similarly and unlimitedly evolve until external factors affect the
evolution. The possibility and stability of this type of evolution are verified
by numerical simulations in a very wide spatial dynamic range. Actual
astrophysical reconnection in solar flares and geomagnetospheric substorms can
be treated as an evolutionary process in free space, because the resultant
scale is much larger than the initial scale. In spite of this fact, most of the
previous numerical works focused on the evolutionary characters strongly
affected by artificial boundary conditions on the simulation boundary. Our new
model clarifies a realistic evolution for such cases. The characteristic
structure around the diffusion region is quite similar to the Petschek model
which is characterized by a pair of slow-mode shocks and the fast-mode
rarefaction-dominated inflow. However, in the outer region, a vortex-like
return flow driven by the fast-mode compression caused by the piston effect of
the plasmoid takes place. The entire reconnection system expands
self-similarly.Comment: 17 Pages, 17 Figure
Patchy Reconnection in a Y-Type Current Sheet
We study the evolution of the magnetic field in a Y-type current sheet
subject to a brief, localized magnetic reconnection event. The reconnection
produces up- and down-flowing reconnected flux tubes which rapidly decelerate
when they hit the Y-lines and underlying magnetic arcade loops at the ends of
the current sheet. This localized reconnection outflow followed by a rapid
deceleration reproduces the observed behavior of post-CME downflowing coronal
voids. These simulations support the hypothesis that these observed coronal
downflows are the retraction of magnetic fields reconnected in localized
patches in the high corona.Comment: 4 pages, 3 figure
Self-Regulation of Solar Coronal Heating Process via Collisionless Reconnection Condition
I propose a new paradigm for solar coronal heating viewed as a
self-regulating process keeping the plasma marginally collisionless. The
mechanism is based on the coupling between two effects. First, coronal density
controls the plasma collisionality and hence the transition between the slow
collisional Sweet-Parker and the fast collisionless reconnection regimes. In
turn, coronal energy release leads to chromospheric evaporation, increasing the
density and thus inhibiting subsequent reconnection of the newly-reconnected
loops. As a result, statistically, the density fluctuates around some critical
level, comparable to that observed in the corona. In the long run, coronal
heating can be represented by repeating cycles of fast reconnection events
(nano-flares), evaporation episodes, and long periods of slow magnetic stress
build-up and radiative cooling of the coronal plasma.Comment: 4 pages; Phys. Rev. Lett., in pres
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