Transcript ISSI Meeting Berne, Switzerland, 19

Particle acceleration
above the pick-up energy
at Comet 1P/Halley
Susan McKenna-Lawlor
Space Technology Ireland,
National University of Ireland,
Maynooth, Co. Kildare
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
page 1
Outline
• Ion pickup at comets (theory)
• In situ energetic particle measurements at comets
• Particle acceleration above the pickup energy
• Analysis of the data
• Conclusions
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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Ion pickup at Comets (Theory)
An understanding of how comets interact with the solar wind has been
developed through theoretical analysis, backed up by in situ
measurements made aboard spacecraft at various comets.
According to the scenario thereby developed, as a cometary body
approaches the Sun, it begins to loose mass due to the sublimation
process so that, under the pertaining low gravity conditions, an
expanding atmosphere composed of dust, molecules, radicals and
molecular ions is formed which escapes from the collision dominated
inner region of the coma (velocity ~ 1 - 8 km/s).
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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Ion pickup at Comets (Contd.)
The radially expanding molecules undergo complicated chemical
reactions in the inner reaches of the coma. Further out, at
distances of several million km ( ≥ 7.5 x 106 km from 1P/Halley)
ions are created through photoionization by solar EUV
radiation; charge exchange with solar wind ions; electron impact
and other processes.
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Berne, Switzerland, 19-23 November 2012
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Ion pickup at Comets (Contd.)
It is assumed that the solar wind is a fully ionized, highly
conducting plasma with the interplanetary magnetic field frozen
into its flow due to the high conductivity that pertains.
A freshly ionized cometary particle, which is initially practically at
rest with respect to the solar wind flow, is accelerated along the
ambient motional electric field, while also gyrating about the
magnetic field and undergoing E x B drift so that, overall, it
follows a cycloidal trajectory.
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In the frame of the comet, the energy of the particle along this
trajectory varies between ~ 1 km/s and a maximum value:
Emax = 4A sin2α Esw
where A is the ion mass in a.m.u., α is the angle between the
interplanetary magnetic field and the solar wind and Esw is the kinetic
energy of a proton traveling with the solar wind velocity. The peak
energy attained thus depends on the magnetic field direction and on
the solar wind velocity.
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Berne, Switzerland, 19-23 November 2012
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In the solar wind reference frame ions are created with a speed equal
to the solar wind speed in the comet’s rest frame and they form a
ring in velocity space as they gyrate about the magnetic field lines.
Excess free energy in the ring distribution renders it highly unstable
and low frequency Alfvén waves are generated via the ion cyclotron
instability. These waves were predicted by Wu and Davidson, 1972,
Wallis, 1973, Gary et al. 1986 and Sagdeev et al. 1986) to scatter the
ions in pitch angle so that they form a shell distribution in velocity
space (Galeev and Sagdeev, 1988). More recent studies/observations
have indicated that the actual configuration more closely resembles a
bispherical distribution than a single shell (Johnstone, 1995, Coates,
1997).
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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If efficient pitch angle scattering occurs in the flow rest frame the
peak energy of the ions is:
Emax =4A Esw
so that the mean direction of motion depends only on the solar
wind flow.
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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Upstream measurements aboard
Giotto
Measurements made inbound aboard Giotto by the
Johnstone Plasma Instrument Implanted Ion Sensor JPA/IIS)
showed that, far from comet 1P/Halley the ion distributions
detected were ring-like. They became shell-like upstream of
the bow shock and the shell thickened substantially
downstream (Coates, 2004).
Also, very high levels of magnetohydrodynamic turbulence
interpreted to be generated by the implanted cometary ion
pickup process was detected (Neubauer et al., 1986).
ISSI Meeting
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In situ energetic particle
measurements at comets
At the time of the encounter, the estimated maximum pickup energy of
water group ions under the prevailing solar wind conditions was < 60
keV. The energetic particle (EPONA) instrument aboard Giotto,
however, detected water group ions with energies > 0.5 MeV (McKennaLawlor et al., 1987).
This observation was in accord with measurements made by the Tunde
Instrument aboard Vega 1 which encountered 1P/Halley from 4-7
March, 1986 and by the EPAS Instrument aboard the ICE spacecraft
which encountered comet 21P/Giacobini-Zinner (10-13 September,
1985) [Somogyi et al. 1990, Hynds et al., 1986, Ipavich et al. 1986].
Also particles with greater than the local pickup energies were recorded
by EPONA at comet 26P/Grigg-Skjellerup during the Giotto Extended
Mission (McKenna-Lawlor et al. (1993).
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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Candidate Acceleration Processes
Several mechanisms have been proposed to explain the presence at
comets of ions with energies that significantly exceed the maximum
local pickup energy including:
Fermi Type I Process: whereby scattering centers move at different
speeds on either side of a boundary in the flow (the bow shock),
thereby causing particle acceleration (Amata and Formisano, 1985).
Fermi Type II Process: particles interact with up-stream and downstream counter propagating wave “scattering centers” so that a
particular particle may undergo acceleration or deceleration as it
either takes energy from, or gives energy to, the waves (Terasawa
and Scholer, 1989). This process results in broadening the ion shell
distribution to both higher and lower energies.
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Berne, Switzerland, 19-23 November 2012
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Theory of particle acceleration
above the pickup energy at comets
In early studies, Ip and Axford (1986) estimated the relative
strengths at comets of diffuse shock acceleration and
stochastic acceleration from Alfvén wave scattering and
concluded that, under conditions of strong scattering such
that the ratio (f) of the scattering mean free path to the ion
gyroradius was ~ 1, the stochastic process should be more
effective in accelerating particles than the First Order Fermi
process.
This was predicted to occur because the shock in the vicinity
of comets was inferred to be weak (M~2) due to continuous
mass loading of the solar wind.
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The process of stochastic acceleration is described by a Fokker-Planck equation
(1)
where Do = 3ZeBVA2 /2f represents the energy diffusion coefficient resulting from
Alfvén scattering. For an initial delta-function distribution, the time evolution of the
particle distribution function is represented by
(2)
where I½ (x) = sin h (x) /x½
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The effects of adiabatic compression were not taken into
account in (1). Later Isenberg (1987) presented a solution of
the full Fokker-Planck equation under the condition that the
plasma parameters (flow velocity, source terms, plasma
density and magnetic field strength) follow a power law
dependency on the radial distance from the nucleus.
In this model it was assumed that ions are immediately
isotropized at pickup and the effects of adiabatic acceleration
in the slowing solar wind as well as consideration of the
continuous pickup of ions as the comet is approached are
taken into account.
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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In situ measurements
To characterize particle fluxes measured close to 1P/Halley aboard
Giotto by the EPONA instrument (E > 60 keV) and by the Johnstone
Plasma Instrument Implanted Ion Sensor (JPA/IIS) in the energy
range 2-86 keV, various energy spectra (phase space densities as a
function of particle energy) were calculated (McKenna-Lawlor et al.
1987) and transformed into the solar wind frame using:
where dj/dE is the differential flux in a moving frame; K is a constant;
W is the solar wind velocity, V is the particle velocity; β is the angle
between the solar wind and proton propagation directions and γ is the
spectral index (Ipavich, 1974).
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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Comparison between measured
and theoretical spectra
Composite ion measurements made in the SW frame at
different distances (inbound) from the Halley nucleus were
then compared with theoretical distributions calculated
following Ip and Axford (1986) while including adiabatic
compression.
These theoretical distributions were estimated for water
group ions traveling along a stream line that would intersect
with the Giotto spacecraft during time intervals in close
accordance with the particle measurement times.
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The figure shows a comparison
between measurements made
upstream of the bow shock (at
1.5 x 106 km from the 1P/Halley
nucleus) on the assumption of
a value of f ~30.
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LHS: Corresponding composite measurements made at 1.1 x 106 km from the
Halley nucleus during the inbound bow shock transition compared with a
theoretical distribution calculated for f (~5).
RHS: Composite measurements made at < 106 km compared with a theoretical
distribution again for f (~5).
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It can be inferred from these comparisons that a relatively
weak scattering limit characterized by f ~ 30 is more
appropriate to represent conditions upstream of the comet
than the initially predicted value of f ~ 1.
Near the shock surface where enhanced levels of turbulence
were present, a value of f ~ 5 is sufficient to result in rapid
isotropization and thermalization of the ions.
ISSI Meeting
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Gombosi et al. (1989) suggested a scenario in which the second
order Fermi mechanism efficiently accelerates ions to moderate
energies in the cometary upstream region.
In the foreshock, where the solar wind is substantially slowed,
the super-thermal implanted ions are further energized by a
diffusive, compressive shock acceleration process. It was argued
that the entire foreshock region can serve as a region of
diffusive, compressive ion acceleration so that, in effect, a strong
shock (M ~ 13) is available to energize particles rather than the
weak (M~2) shock envisioned by Ip and Axford (1986). The
diffusion coefficient should be sufficiently large to allow the
entire foreshock region to act as a shock but small enough that
the acceleration time scales are sufficiently short to be relevant.
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The energy spectra
calculated by Gombosi et
al. (1989a) using their
(quasi-linear) model were
demonstrated to show a
satisfactory fit to a
spectrum obtained
upstream of the shock by
combining the EPONA, IIS
and Tunde observations.
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It was inferred that, initially velocity diffusion can accelerate
pickup ions at comets to moderate energies, thereby creating a
seed population for the, more efficient, diffusive compressive
shock acceleration.
Solar wind convection limits the time available for diffusive
compressive acceleration and it was predicted by the model of
Gombosi et al. (1989b) that, above 100 keV, power law type
energy spectra with spectral indices of 5-6 would be observed.
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In order to investigate the ion distributions more fully, composite
EPONA+JPA spectra were plotted at 8 positions along the Giotto
trajectory (Kirsch et al., 1991). The indices of the power law spectra
thereby obtained are presented in Table 1.
Power Law Spectra
Inbound
Outbound
1. Foreshock
 = 3.34
5. Cometosheath
 = 4.82
2. Minimum
 = 4.13
6. Bow Shock
 = 5.87
3. Bow Shock
 = 4.10
7. Wavefield
 = 5.34
4. Cometosheath
 = 3.49
8. End Wavefield
 = 3.71
Overall the energy spectra measured were somewhat harder on the
inbound than on the outbound side.
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A relatively hard spectrum was observed in the inbound
foreshock (γ = 3.3).
Inside the bow shock itself the spectrum was somewhat softer
(γ = 4.1) due to an increase in fluxes in the low energy
channels.
Further particle acceleration
cometosheath (γ = 3.5)
took
place
inside
the
Outbound, similar, although somewhat steeper spectra, were
observed.
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Transit Time Damping
Magnetometer observations made aboard Giotto indicated that the
interaction region between the solar wind and comet Halley was
characterized by large amplitude, low frequency magnetic field
fluctuations in the upstream region as well as in the cometosheath
both inbound and outbound.
These compressional magnetic field fluctuations, which had a 3-4
minute quasi-periodicity, were argued by Glassmeier et al. 1987 and
Ip and Axford (1987) to have the possibility to contribute to particle
acceleration via the Transit Time Damping Mechanism (Fisk, 1976).
In this scenario, ions are scattered and accelerated in randomly
moving magnetic gradients (a Fermi process with magnetic
scattering centers).
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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The measured spectra suggest that a combination of the First
Order and Second Order Fermi processes and the transit time
damping effect were the most likely candidates to stimulate the
particle acceleration measured by EPONA+ IIS at the inbound
and outbound bow shock.
TTD would have the effect of lowering the pitch angles of the
energetic particles (Fisk, 1976), thereby potentially causing the
escape of particles along the magnetic field vector on the
outbound side. This is in line with observations by EPONA of
an outbound flux anisotropy.
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Berne, Switzerland, 19-23 November 2012
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Comet 26P/Grigg Skjellerup
Spectral studies were made by McKenna-Lawlor et al. (1997,
1999) using EPONA data recorded at comet 26P/GriggSkjellerup during the Extended Giotto Mission.
Because of the pertaining flyby geometry, the inbound
/outbound
passes
at
26P/G-S
corresponded
to
the
outbound/inbound passes at 1P/Halley. Regular magnetic
fluctuations were observed along the inbound spacecraft
trajectory and Neubauer et al. (1993) concluded that the
large
amplitude,
compressional
magnetic
fluctuations
observed indicated the presence of a thick pulsation shock.
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Comet 26P/Grigg Skjellerup contd.
The spectrum
obtained upstream
(1 x 105 km) of the
(inbound) bow wave
indicated that the
mean free path
corresponding to
random scattering
by ambient waves
was ~ 30 gyroradii.
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Left: The energy spectrum obtained immediately
downstream of the inbound shock can be attributed to
2nd order Fermi acceleration, adiabatic compression
and the TTD effect (McKenna-Lawlor et al. 1999).
ISSI Meeting
Right: The energy
spectrum just
upstream of the
outbound Bow
Shock/Foreshock
transition had a
spectral exponent of
γ = 5.09 and is
interpreted to
indicate the
influence of diffusive
compressive
acceleration
supplemented by
transit time
damping.
Berne, Switzerland, 19-23 November 2012
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Differences and Similarities
When the ion distributions recorded at P/Halley, P/Grigg-Skjellerup
and P/Giacobini-Zinner are intercompared, certain differences, as
well as similarities, between the observational data can be identified.
When considering how particles are accelerated close to cometary
shocks, individual cases should be considered (taking into account
both inbound and outbound shock transitions) since the prevailing
interplanetary conditions, as well as the circumstances of whether a
particular shock is quasiperpendicular or quasiparallel will
influence the outcome, as will the presence or absence of large, local
fluctuating wave fields, the production of which, in turn, depends on
the gas production rate of the comet concerned (McKenna-Lawlor
et al., 1999, 2000).
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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Conclusions
Spacecraft observations by EPONA and IIS aboard Giotto and
by Tunde aboard Vega-1 at 1P/Halley; by EPAS aboard ICE at
21P/Giacobini-Zinner and by EPONA at 26P/Grigg-Skjellerup
indicate that, in each case, cometary ions were accelerated
significantly above their highest available pickup energies.
Energy spectra obtained using the same instrument (EPONA) at
1P/Halley and at 26P/Grigg-Skjellerup indicate that the mean
free path corresponding to random scattering by Alfvén waves
was of the order of 30 ion gyroradii upstream of the bow shock,
indicating that, at this location, energy diffusion was weak.
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Berne, Switzerland, 19-23 November 2012
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Conclusions
Close to the inbound and outbound shock surfaces of 1P/Halley
and 26P/Grigg-Skjellerup the most likely candidates for
accelerating
ions
are
compressive
shock
acceleration
supplemented by transit time damping.
When considering how particles are accelerated close to
particular comets, the outgassing rate of the body as well as the
pertaining interplanetary conditions should be taken into
account.
ISSI Meeting
Berne, Switzerland, 19-23 November 2012
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10th Ann.
Internat. Astrophys. Conference
Maui,19-23
Hawai,
13-18 March
2011 page
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