Suprathermal Tails in Coronal Proton Velocity Distributions

Suprathermal Tails in Coronal Proton Velocity DistributionsJ. L. Kohl, A. Panasyuk, S. Cranmer, S. Fineschi, L. D. Gardner, D.H. Phillips, J. C. Raymond, and M. Uzzo

Theory of shock acceleration of SEPs M. A. Lee (1983, 2005) developed atheory of coupled turbulent wave excitation and proton accelerationat shocks.

Theory of shock acceleration of SEPs In this theory, in order to produce large SEP events, it is necessary for a suprathermal seed particle population to exist after the first encounter of the coronal plasma with a CME shock. The theory requires that .001 to .01 of this proton velocity distribution have an injection speed higher than 2 times the difference between the shock speed and the wave phase speed (~VA in corona).Alternatively, there could be a pre-existing suprathermal population in the corona that would help to satisfy this requirement. Gopalswamy et al. (2004) found higher SEP intensities when there was a preceding CME within ~24 hours that perhaps left behind suprathermals.

Theory of shock acceleration of SEPs

The threshold velocity for particle injection is extremely uncertain. There are several lines of evidence that particles of 1000 to 2000 km/s (i.e., 5.2 20.7 keV) are preferentially accelerated. From the experimental side, that includes creation of anomalous cosmic rays from pick-up ions. Theories such as the transparency function of Gieseler et al. give similar results.

Density of Suprathermal Seed ParticlesPreshock fi (v): Resonantly scattered Ly constrains the seed particle distribution

fe(v): Thomson-scattered Ly

kappa needed to provide various population fractions with velocities above the injection velocity

Injection speeds of 940 1460 km/s, are 6.1 to 9.7 in units of V1/e

A kappa distribution where .01 - .001 of the population has speeds beyond 6.1 to 9.7 V1/e has a kappa between 4 and 2.

Hence, proton velocity distributions resembling kappa distributions with kappa values in this range and lower are of interest.

Proton velocity distribution in a diffuse coronal regionKappa = 21.8 7.0

Observation of a proton velocity distribution with kappa = 3.5Kappa = 3.5 0.34

20 Jan 2005 Event

Pseudo-kappa functionData clearly show asymmetrical wings.Kappa-function cannot model asymmetrical line profiles.Idea is to create a function that is close to a kappa-function when the line is symmetrical but allows for a shift of the wings relative to the core.Some theoreticians use a power-law, so a sum of Gaussian and power-law function seems natural.For example the plot on the right shows that a kappa-function does not fit the observation. +

Pseudo-kappa functionWe empirically determine the dependency of all parameters (G,A,, and ) of the k- value of approximated function. The only additional parameter (vs kappa-function) is the shift which allow us to fit asymmetrical profiles.

Best fit to data

20 Jan 2005 Event

Effect of Stray Light

10 Feb 2006

Conclusions UVCS/SOHO is able to measure proton velocity distributions including departures from Maxwellians.These observations may lead to testing and refining theories of SEP production.Work is in progress: line of sight, more observations and archival data analyses to be done.

Simulation of observation for kappa = 4Simulation includes coronal emission assuming kappa = 4, Poisson noise, Binning to UVCS sampling, Random flat field uncertainty, Detector background, Fitting with and without error in instrument profile

Fit with no profile error yields kappa = 3.92 +/- 0.66Fit with profile error yields kappa = 3.66 +/- 0.59

20 Jan 2005 Event

20 Jan 2005 Event

Kappa is large

19 Jan 2005, 22:01 + 2:00

20 Jan 2005 Event

20 Jan 2005 Event

Kappa = 3.43 1.0

19 Jan 2005, 22:01 + 3:30

20 Jan 2005 Event

20 Jan 2005 Event

19 Jan 05, 22:01 + 5:3019 Jan 05, 22:01 + 6:00

InterpretationPhil Isenberg suggested that the non-Maxwellian tails might be associated with heat flux along the magnetic field.He speculates that the appearance and disappearance of these tails could be due to rotations of the field direction into and out of the line of sight.He points out that this interpretation probably would not be consistent with a symmetric LOS velocity distribution.

Interpretation Next week Gang Li of UC, Riverside will give an SSP seminar describing his recent theoretical finding that a predecessor CME can greatly enhance turbulence upstream of a second shock. This decreases the acceleration time scale at the second shock allowing fast particle acceleration to occur.To explain the result of Gopalswamy et al., the turbulence would need to be present for several hours after the first shock.It is not clear if our observations indicate any increase in turbulence following a predecessor shock.

LASCO C2 image of CME region on 23 & 24 Dec 1996

Simulation of observation for kappa = 4

UVCS Determinations of Pre-CME Corona UVCS routinely obtains the densities, temperatures, outflow speeds, ionization states and elemental abundances in the pre-CME coronaDensities obtained by UVCS can be combined with Type II radio burst drift rates to obtain shock speedsThe angle between the shock front and the magnetic field requires the pre-shock field direction, which can be determined from the streamer morphology

Testing and Guiding Theoretical Models of SEP AccelerationThe measured and derived parameters allow shock acceleration and current sheet models to be tailored to a specific event.The theoretical models can then predict SEP acceleration, transport and energy spectra for those events.In situ measurements of SEP energy spectra near the Sun (e.g., by Inner Heliospheric Sentinels) can then be used to test and guide the theoretical models.

Key Parameters in Theories of SEP Acceleration by shocksPre-shock plasma conditions (including the supra-thermal seed particle population)The shock speedThe compression ratio (which yields the Mach number)The angle between the magnetic field and the shock motion

20 Jan 2005 Event

Density of Suprathermal Seed ParticlesPreshock fe(v): Thomson-scattered Ly

fi (v): Resonantly scattered Ly constrains the seed particle distribution

Lin and Forbes Unified Model of a Flare and CMEIn the Lin and Forbes model, a stressed magnetic arcade begins to rise. A current sheet develops as external pressure forces oppositely directed magnetic field lines to reconnect. The liberated energy heats and drives the CME and drives energetic particles downward producing the flare.

SEP source regionsThe source regions of solar energetic particles (SEPs) are not well established and the physical processes associated with their acceleration are not well understood.Gradual phase SEPs are believed to be produced by CME shocks, while impulsive phase SEPs are believed to be produced in the current sheet or other sites closely associated with the solar flare.

UVCS observations of CME shocks

Broad lines appear when shock forms

Shock speeds can be determined from detection of shock arrival at different heights

At onset radius, Mach number = 1, so Vshock gives VAlfven .

Unshocked foreground/ backgroundshocked O+5 (T > 108 K)

The required insertion speed for producing observed SEP events

UVCS observations of CME shocks yielded an upper limit for the Alfven speed VA = 540 km/s at 2.3 Rsun

Assuming a shock speed of 1000 km/s and 270 < VA < 540 km/s at 3.5 Rsun , we derive an injection speed of 920 to 1460 km/s (4.4 10.1 keV).

Proton V1/e in a streamer at 3.5 Rsun is 150 km/s.

Unshocked foreground/ backgroundshocked O+5 (T > 108 K)

UVCS Instrument ProfileLeft: UVCS instrument profile. Right: High spectral resolution scan of Ne-Pt hollow cathode spectrum convolved with UVCS instrument profile (black) and UVCS scan of lamp spectrum (red).

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Suprathermal Tails in Coronal Proton Velocity Distributions