195 research outputs found
Low energy particle fluxes in the geomagnetic tail
Low energy particle fluxes in geomagnetic tail, and plasma sheet relation to auroral ova
Low energy electrons in the magnetosphere as observed by OGO-1 and OGO-3
Low energy electrons in magnetosphere as observed by OGO satellite
Magnetic reconnection with anomalous resistivity in two-and-a-half dimensions I: Quasi-stationary case
In this paper quasi-stationary, two-and-a-half-dimensional magnetic
reconnection is studied in the framework of incompressible resistive
magnetohydrodynamics (MHD). A new theoretical approach for calculation of the
reconnection rate is presented. This approach is based on local analytical
derivations in a thin reconnection layer, and it is applicable to the case when
resistivity is anomalous and is an arbitrary function of the electric current
and the spatial coordinates. It is found that a quasi-stationary reconnection
rate is fully determined by a particular functional form of the anomalous
resistivity and by the local configuration of the magnetic field just outside
the reconnection layer. It is also found that in the special case of constant
resistivity reconnection is Sweet-Parker and not Petschek.Comment: 15 pages, 4 figures, minor changes as compared to the 1st versio
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
Magnetic Reynolds number dependence of reconnection rate and flow structure of the self-similar evolution model of fast magnetic reconnection
This paper investigates Magnetic Reynolds number dependence of the
``self-similar evolution model'' (Nitta et al. 2001) of fast magnetic
reconnection. I focused my attention on the flow structure inside and around
the reconnection outflow, which is essential to determine the entire
reconnection system (Nitta et al. 2002). The outflow is consist of several
regions divided by discontinuities, e.g., shocks, and it can be treated by a
shock-tube approximation (Nitta 2004). By solving the junction conditions
(e.g., Rankine-Hugoniot condition), the structure of the reconnection outflow
is obtained. Magnetic reconnection in most astrophysical problems is
characterized by a huge dynamic range of its expansion ( for typical
solar flares) in a free space which is free from any influence of external
circumstances. Such evolution results in a spontaneous self-similar expansion
which is controlled by two intrinsic parameters: the plasma- and the
magnetic Reynolds number. The plasma- dependence had been investigated in
our previous paper. This paper newly clarifies the relation between the
reconnection rate and the inflow structure just outside the Petschek-like slow
shock: As the magnetic Reynolds number increases, strongly converging inflow
toward the Petschek-like slow shock forms, and it significantly reduces the
reconnection rate.Comment: 16 pages. to appear in ApJ (2006 Jan. 20 issue
Continuous transition from fast magnetic reconnection to slow reconnection and change of the reconnection system structure
This paper analytically investigates a series of two-dimensional MHD
reconnection solutions over a wide variation of magnetic Reynolds number
(). A new series of solutions explains a continuous transition from
Petschek-like fast regime to a Sweet-Parker-like slow regime. The inflow region
is obtained from a Grad-Shafranov analysis used by Nitta et al. 2002 and the
outflow region from a shock-tube approximation used by Nitta 2004, 2006. A
single X-point (Petschek-like) solution forms for a sufficiently small
. As gradually increases, the solutions shifts to an X-O-X
solution with a magnetic island between two X-points. When increases
further, the island collapses to a new elongated current sheet with Y-points at
both ends (Sweet-Parker-like). These reconnection structures expand
self-similarly as time proceeds. As increases, the reconnection rate
and the reducible fraction of the initial magnetic energy of the system
decrease as power-law functions of .Comment: 19 pages, 12 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
Winds, B-Fields, and Magnetotails of Pulsars
We investigate the emission of rotating magnetized neutron stars due to the
acceleration and radiation of particles in the relativistic wind and in the
magnetotail of the star. We consider that the charged particles are accelerated
by driven collisionless reconnection. Outside of the light cylinder, the star's
rotation acts to wind up the magnetic field to form a predominantly azimuthal,
slowly decreasing with distance, magnetic field of opposite polarity on either
side of the equatorial plane normal to the star's rotation axis. The magnetic
field annihilates across the equatorial plane with the magnetic energy going to
accelerate the charged particles to relativistic energies. For a typical
supersonically moving pulsar, the star's wind extends outward to the standoff
distance with the interstellar medium. At larger distances, the power output of
pulsar's wind of electromagnetic field and relativistic particles
is {\it redirected and collimated into the magnetotail} of the star. In the
magnetotail it is proposed that equipartition is reached between the magnetic
energy and the relativistic particle energy. For such conditions, synchrotron
radiation from the magnetotails may be a significant fraction of
for high velocity pulsars. An equation is derived for the radius of the
magnetotail as a function of distance from the star.
For large distances , of the order of the distance travelled by the
star, we argue that the magnetotail has a `trumpet' shape owing to the slowing
down of the magnetotail flow.Comment: 11 pages, 4 figures, accepted for publication in Ap
On Turbulent Reconnection
We examine the dynamics of turbulent reconnection in 2D and 3D reduced MHD by
calculating the effective dissipation due to coupling between small-scale
fluctuations and large-scale magnetic fields. Sweet-Parker type balance
relations are then used to calculate the global reconnection rate. Two
approaches are employed -- quasi-linear closure and an eddy-damped fluid model.
Results indicate that despite the presence of turbulence, the reconnection rate
remains inversely proportional to , as in the Sweet-Parker
analysis. In 2D, the global reconnection rate is shown to be enhanced over the
Sweet-Parker result by a factor of magnetic Mach number. These results are the
consequences of the constraint imposed on the global reconnection rate by the
requirement of mean square magnetic potential balance. The incompatibility of
turbulent fluid-magnetic energy equipartition and stationarity of mean square
magnetic potential is demonstrated.Comment: 37 pages, 2 figure
Flux and field line conservation in 3--D nonideal MHD flows: Remarks about criteria for 3--D reconnection without magnetic neutral points
We make some remarks on reconnection in plasmas and want to present some
calculations related to the problem of finding velocity fields which conserve
magnetic flux or at least magnetic field lines. Hereby we start from views and
definitions of ideal and non-ideal flows on one hand, and of reconnective and
non-reconnective plasma dynamics on the other hand. Our considerations give
additional insights into the discussion on violations of the frozen--in field
concept which started recently with the papers by Baranov & Fahr (2003a;
2003b). We find a correlation between the nonidealness which is given by a
generalized form of the Ohm's law and a general transporting velocity, which is
field line conserving.Comment: 9 pages, 2 figures, submitted to Solar Physic
- âŠ