Transcript prediccs.sr.unh.edu

Modeling suprathermal proton acceleration by an ICME within 0.5 AU:
the May 13, 2005 event
K. A.
1,5
Kozarev ([email protected]),
R. M.
2
Evans ,
M. A.
3
Dayeh ,
N. A.
4
Schwadron ,
M.
2
Opher ,
K. E.
5
Korreck ,
and T. I.
6
Gombosi
2
1
Boston University, Boston, MA; George Mason University, Fairfax, VA; 3Southwest Research Institute, San Antonio, TX; 4University of New Hampshire, Durham, NH;
5
6
Smithsonian Astrophysical Observatory, Cambridge, MA; University of Michigan, Ann Arbor, MI
We modeled the acceleration of suprathermal protons to high energies by a complex traveling plasma structure between 0.05-0.5
AU. We used a global 3D MHD simulation of the May 13, 2005 coronal mass ejection event, and coupled it to a global kinetic code
for particle acceleration and propagation. We scaled pre-event suprathermal spectra observed at 1 AU to 2.5 Rs and used them for
inner boundary conditions for the kinetic simulation. We find that suprathermal protons can become the seed for solar energetic
particles, as they get strongly energized by the enhanced traveling shock and complex plasma structure in the corona and
interplanetary space.
Introduction
The kinetic model
Coronal Mass Ejection (CME)-driven interplanetary shocks are able to accelerate
charged particles to high energies, potentially dangerous to astronauts and spacecraft
electronics. This is an important problem in heliospheric physics, space weather
forecasting and space operations. Recent modeling work has shown that interplanetary
shocks can be very effective in accelerating particles up to several GeV/nuc close to
the sun, but are not very efficient as they move outward (Zank et al., 2000; Roussev et
al., 2004). We investigate how quiet-time suprathermal protons close to the sun can
become the seed population for energetic protons accelerated by a complex traveling
plasma structure in the solar corona, and transported in the inner heliosphere. We have
simulated the May 13, 2005 CME event with a global 3D MHD and coupled it to a
kinetic model.
The Energetic Particles Radiation Environment Module (EPREM; Schwadron et al.
2010, Kozarev et al. 2010) solves numerically the focused transport equation (Kota et
al., 2005) on a dynamical grid representing the inner heliosphere, in which nodes are
convected outward from the sun according to the underlying solar wind parameters.
This parallelized code solves for streaming, adiabatic cooling and focusing, convection,
pitch-angle scattering, and stochastic acceleration effects. Energetic particles are
accelerated and transported through this grid along magnetic field lines, based on
interplanetary conditions.
Fig.4. Event fluence at 0.3 and 0.5
AU, between 0.1-50 MeV, compared
with the source spectrum at 0.15 AU
(black).
Fig.3 Average shock properties as a
function of distance. The shock remained
very strong between 2 and 20 solar radii.
Fig. 2. EPREM's dynamic grid shown near the origin. Red dots represent grid nodes,
streamlines are traced in white. The green box shows the region containing the expanding
CME structure.
Coupling between kinetic and MHD models
Fig.1. Density contour plot of the CME and solar magnetic field. The red square shows the
approximate size and position of the box extracted from the MHD model, for this particular
time step.
The MHD model
We use the Space Weather Modeling Framework (Toth et al. 2005) for modeling the
global solar wind and CME. The 3D magnetohydrodynamic (MHD) code Block
Adaptive Tree Solar-Wind Roe Upwind Scheme (BATS-R-US) which serves as its core
is highly parallelized and includes adaptive mesh refinement. The initial solar magnetic
field is calculated with the Potential Field Source Surface model (Altschuler & Newkirk.
1969) and an MDI synoptic magnetogram for CR 2029 (corresponding to April-May
2005.)
After steady-state is achieved, grid resolution in the CME region is increased in order
to capture the shock and ICME properties. A modified Titov-Demoulin (TD) flux rope
(Roussev et al. 2004) is inserted out of equilibrium in an active region near the equator.
The ejecta field is poloidal. The flux rope's magnetic field orientation is anti-parallel to
the active region magnetic field, and perpendicular to the global coronal field. The
initial free energy is 2 x 1032 ergs.
We extracted a 3D box (100x100x100 cells) around the expanding CME in the MHD
simulation at two-minute increments (shown in Fig. 2). The kinetic code does trilinear
interpolation on the BATSRUS box to obtain plasma parameters. Outside the box we
extrapolate, assuming a constant solar wind speed, and an inverse square radial
dependence of density and magnetic field.
Modeling the May 13, 2010 event
The EPREM simulation radial domain is between 2.5 Rs and 0.6 AU. We set up a
computational grid dense enough close to the sun to resolve the features in the plasma
model, that becomes sparse beyond 0.1 AU. The energy range of the injected proton
spectra was between 0.1-50 MeV. We use a fixed particle mean free path of 0.1 AU,
consistent with the particle energies. Following Dayeh et al. (2009), we use quiet time
He-4 ion (0.1 – 0.5 MeV/nuc) observations from ACE/ULEIS at L1 to obtain a seed
suprathermal proton pre-event spectrum of the form dJ/dE=9.95 x 1010 x E-1.96. We
convert the spectrum to protons at 10 Rs assuming: 1) the flux is scaled to 2.5 Rs via
an inverse-square dependence; 2) a helium to proton abundance of 10%; 3) the
resulting spectral form is valid between 0.1-50 Mev. We inject the scaled spectrum
uniformly at the inner boundary of the kinetic code. The simulation spans half a day of
data time.
Fig. 5. Modeled proton fluxes over four hours at 0.5 AU for energies 0.2-10.0 MeV (left) and
5.0-50.0 MeV (right). The time-dependent behavior is caused by the traveling shock and
plasma structure.
Summary and Future Work
We have combined results from a time-dependent global MHD simulation of the
May 13, 2005 coronal mass ejection with a global kinetic code, and modeled
proton acceleration and transport between 2.5 solar radii and 0.5 AU, for energies
0.1-50.0 MeV. We used a quiet time suprathermal Helium-4 spectrum observed at
1 AU and scaled to represent protons in the solar corona. We find that the
suprathermal particles are easily energized by the transient plasma structure,
and could serve as the seed population for solar energetic particles created by
strong interplanetary shocks. In future work we will expand our simulations to 1
AU in order to verify our simulations against observations, and further refine
them. We are also improving the integration of MHD simulation results in order to
cover the entire kinetic code domain.
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