21 research outputs found
Dual spacecraft determinations of magnetopause motion
We examine the motion of Earth's magnetopause for 16 dawnside traversals of this boundary by the sister spacecraft AMPTE/UKS and IRM in December, 1984, when their separation was 400-900 km. We compare magnetopause normal vectors, n, and speeds of motion, u(n), obtained separately from each spacecraft by use of three different methods, and also compare those u(n) to corresponding speeds, u(n)*, obtained from observed time lags between the two spacecraft. Agreement between u, values and n vectors determined from the three methods ranges from poor to excellent. Comparing u(n)* and u(n) values, we find a clear tendency for \ u(n)*\ to be larger than \ u(n)\: While slightly less than half of the results show reasonable agreement (0.5 < u(n)/u(n)* < 2), there are about as many results in the range 0 < u(n)/u(n)* < 0.5, and a few cases give the wrong sign of u(n)
In situ evidence for the structure of the magnetic null in a 3D reconnection event in the Earth's magnetotail
Magnetic reconnection is one of the most important processes in
astrophysical, space and laboratory plasmas. Identifying the structure around
the point at which the magnetic field lines break and subsequently reform,
known as the magnetic null point, is crucial to improving our understanding
reconnection. But owing to the inherently three-dimensional nature of this
process, magnetic nulls are only detectable through measurements obtained
simultaneously from at least four points in space. Using data collected by the
four spacecraft of the Cluster constellation as they traversed a diffusion
region in the Earth's magnetotail on 15 September, 2001, we report here the
first in situ evidence for the structure of an isolated magnetic null. The
results indicate that it has a positive-spiral structure whose spatial extent
is of the same order as the local ion inertial length scale, suggesting that
the Hall effect could play an important role in 3D reconnection dynamics.Comment: 14 pages, 4 figure
Wind anisotropies and GRB progenitors
We study the effect of wind anisotropies on the stellar evolution leading to
collapsars. Rotating models of a 60 M star with on the ZAMS, accounting for shellular rotation and a magnetic
field, with and without wind anisotropies, are computed at =0.002 until the
end of the core He-burning phase. Only the models accounting for the effects of
the wind anisotropies retain enough angular momentum in their core to produce a
Gamma Ray Burst (GRB). The chemical composition is such that a type Ic
supernova event occurs. Wind anisotropies appear to be a key physical
ingredient in the scenario leading to long GRBs.Comment: 5 pages, 4 figures, accepted for publication in A&A Lette