48 research outputs found
The electron distribution function downstream of the solar-wind termination shock: Where are the hot electrons?
In the majority of the literature on plasma shock waves, electrons play the
role of "ghost particles," since their contribution to mass and momentum flows
is negligible, and they have been treated as only taking care of the electric
plasma neutrality. In some more recent papers, however, electrons play a new
important role in the shock dynamics and thermodynamics, especially at the
solar-wind termination shock. They react on the shock electric field in a very
specific way, leading to suprathermal nonequilibrium distributions of the
downstream electrons, which can be represented by a kappa distribution
function. In this paper, we discuss why this anticipated hot electron
population has not been seen by the plasma detectors of the Voyager spacecraft
downstream of the solar-wind termination shock. We show that hot nonequilibrium
electrons induce a strong negative electric charge-up of any spacecraft
cruising through this downstream plasma environment. This charge reduces
electron fluxes at the spacecraft detectors to nondetectable intensities.
Furthermore, we show that the Debye length
grows to values of about compared to the classical value in this
hot-electron environment. This unusual condition allows for the propagation of
a certain type of electrostatic plasma waves that, at very large wavelengths,
allow us to determine the effective temperature of the suprathermal electrons
directly by means of the phase velocity of these waves. At moderate
wavelengths, the electron-acoustic dispersion relation leads to nonpropagating
oscillations with the ion-plasma frequency , instead of
the traditional electron plasma frequency.Comment: 6 pages, 2 figure
Traveling solar-wind bulk-velocity fluctuations and their effects on electron heating in the inner heliosphere
Ambient plasma electrons undergo strong heating in regions associated with
compressive traveling interplanetary solar-wind bulk-velocity jumps due to
their specific interactions with the jump-inherent electric fields. After
thermalization of this energy gain per shock passage through the operation of
the Buneman instability, strong electron heating occurs that substantially
influences the radial electron temperature profile. We describe the reduction
of the jump amplitude due to energy expended by the traveling jump structure.
We consider three effects; namely energy loss due to heating of electrons,
energy loss due to work done against the pick-up-ion pressure gradient, and an
energy gain due to nonlinear jump steepening. Taking these effects into
account, we show that the decrease in jump amplitude with solar distance is
more pronounced when the initial jump amplitude is higher in the inner solar
system. Independent of the initial jump amplitude, it eventually decreases with
increasing distance to a value of the order of at the
position of the heliospheric termination shock, where is the jump
amplitude, and is the average solar-wind bulk velocity.The electron
temperature, on the other hand, is strongly correlated with the initial jump
amplitude, leading to electron temperatures between 6000 K and 20 000 K at
distances beyond 50 AU. We compare our results with in-situ measurements of the
electron-core temperature from the Ulysses spacecraft in the plane of the
ecliptic for , where is the
distance from the Sun. We find a very good agreement between our results and
these observations, which corroborates our extrapolated predictions beyond
.Comment: 7 pages, 4 figures, accepted for publication in Astron. Astrophy
Diamagnetic solar wind ions changing the MHD conditions at the heliospheric termination shock
The electron distribution function downstream of the solar-wind termination shock: Where are the hot electrons?
Differential (2+1) Jet Event Rates and Determination of alpha_s in Deep Inelastic Scattering at HERA
Events with a (2+1) jet topology in deep-inelastic scattering at HERA are
studied in the kinematic range 200 < Q^2< 10,000 GeV^2. The rate of (2+1) jet
events has been determined with the modified JADE jet algorithm as a function
of the jet resolution parameter and is compared with the predictions of Monte
Carlo models. In addition, the event rate is corrected for both hadronization
and detector effects and is compared with next-to-leading order QCD
calculations. A value of the strong coupling constant of alpha_s(M_Z^2)=
0.118+- 0.002 (stat.)^(+0.007)_(-0.008) (syst.)^(+0.007)_(-0.006) (theory) is
extracted. The systematic error includes uncertainties in the calorimeter
energy calibration, in the description of the data by current Monte Carlo
models, and in the knowledge of the parton densities. The theoretical error is
dominated by the renormalization scale ambiguity.Comment: 25 pages, 6 figures, 3 tables, submitted to Eur. Phys.
Multiplicity Structure of the Hadronic Final State in Diffractive Deep-Inelastic Scattering at HERA
The multiplicity structure of the hadronic system X produced in
deep-inelastic processes at HERA of the type ep -> eXY, where Y is a hadronic
system with mass M_Y< 1.6 GeV and where the squared momentum transfer at the pY
vertex, t, is limited to |t|<1 GeV^2, is studied as a function of the invariant
mass M_X of the system X. Results are presented on multiplicity distributions
and multiplicity moments, rapidity spectra and forward-backward correlations in
the centre-of-mass system of X. The data are compared to results in e+e-
annihilation, fixed-target lepton-nucleon collisions, hadro-produced
diffractive final states and to non-diffractive hadron-hadron collisions. The
comparison suggests a production mechanism of virtual photon dissociation which
involves a mixture of partonic states and a significant gluon content. The data
are well described by a model, based on a QCD-Regge analysis of the diffractive
structure function, which assumes a large hard gluonic component of the
colourless exchange at low Q^2. A model with soft colour interactions is also
successful.Comment: 22 pages, 4 figures, submitted to Eur. Phys. J., error in first
submission - omitted bibliograph
Measurements of Transverse Energy Flow in Deep-Inelastic Scattering at HERA
Measurements of transverse energy flow are presented for neutral current
deep-inelastic scattering events produced in positron-proton collisions at
HERA. The kinematic range covers squared momentum transfers Q^2 from 3.2 to
2,200 GeV^2, the Bjorken scaling variable x from 8.10^{-5} to 0.11 and the
hadronic mass W from 66 to 233 GeV. The transverse energy flow is measured in
the hadronic centre of mass frame and is studied as a function of Q^2, x, W and
pseudorapidity. A comparison is made with QCD based models. The behaviour of
the mean transverse energy in the central pseudorapidity region and an interval
corresponding to the photon fragmentation region are analysed as a function of
Q^2 and W.Comment: 26 pages, 8 figures, submitted to Eur. Phys.
Forward Jet and Particle Production at HERA
Single particles and jets in deeply inelastic scattering at low x are measured with the H1 detector in the region away from the current jet and towards the proton remnant, known as the forward region. Hadronic final state measurements in this region are expected to be particularly sensitive to QCD evolution effects. Jet cross sections are presented as a function of Bjorken- x for forward jets produced with a polar angle to the proton direction, θ jet , in the range 7° < θ jet < 20°. Azimuthal correlations are studied between the forward jet and the scattered lepton. Charged and neutral single particle production in the forward region are measured as a function of Bjorken- x , in the range 5° < θ < 25°, for particle transverse momenta larger than 1 GeV. QCD based Monte Carlo predictions and analytical calculations based on BFKL, CCFM and DGLAP evolution are compared to the data. Predictions based on the DGLAP approach fail to describe the data, except for those which allow for a resolved photon contribution
Diffractive dijet production at HERA
Interactions of the type ep -> eXY are studied, where the component X of the hadronic final state contains two jets and is well separated in rapidity from a leading baryonic system Y. Analyses are performed of both resolved and direct photoproduction and of deep-inelastic scattering with photon virtualities in the range 7.5 5 GeV relative to the photon direction in the rest frame of X. Models based on a factorisable diffractive exchange with a gluon dominated structure, evolved to a scale set by the transverse momentum p^hat_T of the outgoing partons from the hard interaction, give good descriptions of the data. Exclusive qqbar production, as calculated in perturbative QCD using the squared proton gluon density, represents at most a small fraction of the measured cross section. The compatibility of the data with a breaking of diffractive factorisation due to spectator interactions in resolved photoproduction is investigated