1,277 research outputs found
Iron-rich solar particle events measured by SOHO/ERNE during two solar cycles
We study the differences in the heavy ion composition of solar energetic
particle (SEP) events between solar cycles 23 and 24. We have surveyed the
SOHO/ERNE heavy ion data from the beginning of solar cycle 23 until the end of
June 2015, that is, well into the declining phase of cycle 24. We used this
long observation period to study the properties of heavy ions (from C to Fe)
and to compare the two solar cycles in this respect. We surveyed the data for
SEP events with enhancements in the Fe/C and Fe/O intensity ratios in the
energy range 5-15 MeV per nucleon, and associated the events with solar flare
and coronal mass ejections (CME) when possible. We studied the properties of
heavy ions in these events and compared the average relative abundances of
heavy ions between the two solar cycles. We found that fewer days had C and O
intensities higher than ~10
cmsrs(MeVn) during solar cycle 24 than
during cycle 23. For Fe this difference was clear even at lower intensities. We
also found that fewer days had Fe/(C+O) > 0.183 during cycle 24. We identified
86 SEP events with at least one Fe-rich day, 65 of which occurred during cycle
23 and only 21 during cycle 24. We found that impulsive events have been almost
completely absent during cycle 24. Mean abundances of heavy ions in the events
were found to be significantly lower during cycle 24 than in cycle 23. Our
results reflect the reduced solar activity in cycle 24 and indicate lower
efficiency of particle acceleration processes for both gradual and impulsive
SEP events in cycle 24.Comment: 14 pages, 9 figures and 3 tables. To be published in A&
Stochastic Acceleration in Relativistic Parallel Shocks
(abridged) We present results of test-particle simulations on both the first
and the second order Fermi acceleration at relativistic parallel shock waves.
We consider two scenarios for particle injection: (i) particles injected at the
shock front, then accelerated at the shock by the first order mechanism and
subsequently by the stochastic process in the downstream region; and (ii)
particles injected uniformly throughout the downstream region to the stochastic
process. We show that regardless of the injection scenario, depending on the
magnetic field strength, plasma composition, and the employed turbulence model,
the stochastic mechanism can have considerable effects on the particle spectrum
on temporal and spatial scales too short to be resolved in extragalactic jets.
Stochastic acceleration is shown to be able to produce spectra that are
significantly flatter than the limiting case of particle energy spectral index
-1 of the first order mechanism. Our study also reveals a possibility of
re-acceleration of the stochastically accelerated spectrum at the shock, as
particles at high energies become more and more mobile as their mean free path
increases with energy. Our findings suggest that the role of the second order
mechanism in the turbulent downstream of a relativistic shock with respect to
the first order mechanism at the shock front has been underestimated in the
past, and that the second order mechanism may have significant effects on the
form of the particle spectra and its evolution.Comment: 14 pages, 11 figures (9 black/white and 2 color postscripts). To be
published in the ApJ (accepted 6 Nov 2004
Solar interacting protons versus interplanetary protons in the core plus halo model of diffusive shock acceleration and stochastic re-acceleration
With the first observations of solar γ-rays from the decay of pions, the relationship of protons producing ground level enhancements (GLEs) on the Earth to those of similar energies producing the γ-rays on the Sun has been debated. These two populations may be either independent and simply coincident in large flares, or they may be, in fact, the same population stemming from a single accelerating agent and jointly distributed at the Sun and also in space. Assuming the latter, we model a scenario in which particles are accelerated near the Sun in a shock wave with a fraction transported back to the solar surface to radiate, while the remainder is detected at Earth in the form of a GLE. Interplanetary ions versus ions interacting at the Sun are studied for a spherical shock wave propagating in a radial magnetic field through a highly turbulent radial ray (the acceleration core) and surrounding weakly turbulent sector in which the accelerated particles can propagate toward or away from the Sun. The model presented here accounts for both the first-order Fermi acceleration at the shock front and the second-order, stochastic re-acceleration by the turbulence enhanced behind the shock. We find that the re-acceleration is important in generating the γ-radiation and we also find that up to 10% of the particle population can find its way to the Sun as compared to particles escaping to the interplanetary space
Dynamical effects of self-generated magnetic fields in cosmic ray modified shocks
Recent observations of greatly amplified magnetic fields () around supernova shocks are consistent with the predictions of the
non-linear theory of particle acceleration (NLT), if the field is generated
upstream of the shock by cosmic ray induced streaming instability. The high
acceleration efficiencies and large shock modifications predicted by NLT need
however to be mitigated to confront observations, and this is usually assumed
to be accomplished by some form of turbulent heating. We show here that
magnetic fields with the strength inferred from observations have an important
dynamical role on the shock, and imply a shock modification substantially
reduced with respect to the naive unmagnetized case. The effect appears as soon
as the pressure in the turbulent magnetic field becomes comparable with the
pressure of the thermal gas. The relative importance of this unavoidable effect
and of the poorly known turbulent heating is assessed. More specifically we
conclude that even in the cases in which turbulent heating may be of some
importance, the dynamical reaction of the field cannot be neglected, as instead
is usually done in most current calculations.Comment: 4 pages, 1 figure, accepted for publication in ApJ Letter
Shock-accelerated electrons during the fast expansion of a coronal mass ejection
Publisher Copyright: © D. E. Morosan et al. 2022.Context. Some of of the most prominent sources for energetic particles in our Solar System are huge eruptions of magnetised plasma from the Sun called coronal mass ejections (CMEs), which usually drive shocks that accelerate charged particles up to relativistic energies. In particular, energetic electron beams can generate radio bursts through the plasma emission mechanism. The main types of bursts associated with CME shocks are type II and herringbone bursts. However, it is currently unknown where early accelerated electrons that produce metric type II bursts and herringbones propagate and when they escape the solar atmosphere. Aims. Here, we investigate the acceleration location, escape, and propagation directions of electron beams during the early evolution of a strongly expanding CME-driven shock wave associated with herrinbgone bursts. Methods. We used ground-based radio observations from the Nançay Radioheliograph combined with space-based extreme-ultraviolet and white-light observations from the Solar Dynamics Observatory and and the Solar Terrestrial Relations Observatory. We produced a three-dimensional (3D) representation of the electron acceleration locations which, combined with results from magneto-hydrodynamic (MHD) models of the solar corona, was used to investigate the origin of the herringbone bursts observed. Results. Multiple herringbone bursts are found close to the CME flank in plane-of-sky images. Some of these herringbone bursts have unusual inverted J shapes and opposite drifting herringbones also show opposite senses of circular polarisation. By using a 3D approach combined with the radio properties of the observed bursts, we find evidence that the first radio emission in the CME eruption most likely originates from electrons that initially propagate in regions of low Alfvén speeds and along closed magnetic field lines forming a coronal streamer. The radio emission appears to propagate in the same direction as a coronal wave in three dimensions. Conclusions. The CME appears to inevitably expand into a coronal streamer where it meets ideal conditions to generate a fast shock which, in turn, can accelerate electrons. However, at low coronal heights, the streamer consists of exclusively closed field lines indicating that the early accelerated electron beams do not escape. This is in contrast with electrons which, in later stages, escape the corona so that they are detected by spacecraft.Peer reviewe
Supermagnetosonic jets behind a collisionless quasi-parallel shock
The downstream region of a collisionless quasi-parallel shock is structured
containing bulk flows with high kinetic energy density from a previously
unidentified source. We present Cluster multi-spacecraft measurements of this
type of supermagnetosonic jet as well as of a weak secondary shock front within
the sheath, that allow us to propose the following generation mechanism for the
jets: The local curvature variations inherent to quasi-parallel shocks can
create fast, deflected jets accompanied by density variations in the downstream
region. If the speed of the jet is super(magneto)sonic in the reference frame
of the obstacle, a second shock front forms in the sheath closer to the
obstacle. Our results can be applied to collisionless quasi-parallel shocks in
many plasma environments.Comment: accepted to Phys. Rev. Lett. (Nov 5, 2009
A type II solar radio burst without a coronal mass ejection
The Sun produces the most powerful explosions in the solar system, solar
flares, that can also be accompanied by large eruptions of magnetised plasma,
coronal mass ejections (CMEs). These processes can accelerate electron beams up
to relativistic energies through magnetic reconnection processes during solar
flares and CME-driven shocks. Energetic electron beams can in turn generate
radio bursts through the plasma emission mechanism. CME shocks, in particular,
are usually associated with type II solar radio bursts. However, on a few
occasions, type II bursts have been reported to occur either in the absence of
CMEs or shown to be more likely related with the flaring process. It is
currently an open question how a shock generating type II bursts forms without
the occurrence of a CME eruption. Here, we aim to determine the physical
mechanism responsible for a type II burst which occurs in the absence a CME. By
using radio imaging from the Nan{\c c}ay Radioheliograph, combined with
observations from the Solar Dynamics Observatory and the Solar Terrestrial
Relations Observatory spacecraft, we investigate the origin of a type II radio
burst that appears to have no temporal association with a white-light CME. We
identify a typical type II radio burst with band-split structure that is
associated with a C-class solar flare. The type II burst source is located
above the flaring active region and ahead of disturbed coronal loops observed
in extreme ultraviolet images. The type II is also preceded by type III radio
bursts, some of which are in fact J-bursts indicating that accelerated electron
beams do not all escape along open field lines. The type II sources show
single-frequency movement towards the flaring active region. The type II is
located ahead of a faint extreme-ultraviolet (EUV) front propagating through
the corona.Comment: 10 pages, 8 figure
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