Observations of Pickup Ions and their Tails in the Heliosphere and Heliosheath

George GloecklerUniversity of Michigan, Ann Arbor, MI

Implications of Interstellar Neutral Matter

Holloway Commons Piscataqua Room

University of New HampshireNovember 15 & 16, 2011

OverviewCharge exchange plays a central role in the interaction of

neutral gas with plasmas - Pickup Ions are created from ambient slow moving neutrals by charge exchange, photoionization and electron impact ionization

- Energetic Neutrals (ENAs) are produced by charge exchange with the ambient neutral gas

- Pickup Ions are affected by solar wind compressions and expansions, and their spectra reveal these conditions along the solar wind flow direction from the sun to the location of the spacecraft

- Pickup Ion densities depend on local neutral densities and ionization rates, and their spectra reveal variations in these along the solar wind flow direction from the sun to observer

- Pickup Ions are the core population that feeds and grows suprathermal tails by the pumping acceleration mechanism

- In the heliosheath these tails grow further in energy to become the Anomalous Cosmic Rays (ACRs) near the heliopause, their modulated spectra observed throughout the heliosphere and heliosheath

- Pickup ions and their extended tails reveal the composition of their neutral source (e.g. interstellar gas, Inner Source, cometary gas)

- ENAs provide information on plasmas, spectra and composition of pickup ions and their extended tails in remote regions such as the heliosheath

Distributed Sources • Interstellar Neutrals

• Extended Inner Source - Evaporated or sputtered material

from interstellar dust and small objects (e.g. KBO) > ~1 AU

- Vaporization of dust in dust-dust

collisions (~10-200 Rs)

- Dust-desorbed atoms and

molecules (~10-200 Rs) (‘Recycled Solar Wind’)

Local Sources • Sun-Grazing Comets • Cometary Neutrals • Planetary Neutrals

Pickup ions are newly ionized atoms or molecules that are picked up by the solar wind and carried outward

0.01 101.00.1 100

Term

inat

ion

shoc

k

IP dust

Sun AU IS dust

IS atoms

IS atoms

dust - dust collisions

Pickup ions

He

H, N, O, Ne, Ar

Sources of Pickup Ions

Neutrals from LIC are detected inside the heliosphere and in the

heliosheath (HS)

• directly in the heliosphere (H, He, O, Ne )

• as pickup ions in the heliosphere

• as pickup ion Tails in the HS

• as Anomalous Cosmic Rays in the HS

Use composition measurements ofPickup Ions

Pickup Ion Tails in the HS Anomalous Cosmic Rays in the HS

to deduce the densities of atoms near the Termination Shock

LIC Compositionneutral

gas

grains

pickupions

- Bulk Solar wind- Halo solar wind- Pickup protons- Suprathermal tail

In the solar wind frame the tail spectrum has the form

dj/dE = joE –1.5exp[–(E/Eo)0.63]

Eo = 0.72 MeV

In the spacecraft frame the spectra are steeper than -5

Four-component Differential Energy Spectrum

Four-component Proton Velocity Distribution in all of 1998

- Bulk Solar wind- Halo solar wind- Pickup protons- Suprathermal tail

In the solar wind frame the tail spectrum has the form

f(v) = fov –5exp[–(v/vo)α]

The exponential rollover parameters vo and α will vary, since they depend on ambient solar wind conditions, e.g. spatial diffusion coefficient

Measured and Model Quiet-time Velocity Distributions of Protons and He+ in the High Speed (Coronal Hole) Solar Wind

The proton distribution has four components• Bulk solar wind (maxwellian)• Halo solar wind (kappa function, κ = 3)• Interstellar pickup protons• Suprathermal tail (power law, γ = -5 in solar

wind frame)

The He+ distribution has three components• Interstellar He+

• He+ from charge exchange of neutral hydrogen with solar wind He++ and some

Inner Source He+

• Suprathermal tail is not observed because the upper energy limit of SWICS in the high speed solar wind is just above W=2

Phase Space Density in the Solar Wind Frame

Different shapes below the cutoff speed (w ≈ 1) reflect the different radial profiles of interstellar neutral H and He caused by different ionization rates

The tail spectrum has the form

f(v) = fov –5exp[–(v/vo)α]

with the exponential rollover parameters vo and α determined by ambient solar wind conditions, e.g. spatial diffusion coefficient and its dependence on rigidity

~3.7 AU–50° Lat

Model Phase Space Densities of of H and Hein the Solar Wind Frame

Simple ‘hot model’ with standard interstellar parameters was used to calculate the model curves

The loss rates, βloss were selected to fit spectral shapes

βprod = βloss

Neutral hydrogen density just upstream of the Termination Shock is

All of 1998

8 years (1991- 1993, 1997-2001 and 2003)

8 years (1991- 1993, 1997-2001 and 2003)

8 years (1991- 1993, 1997-2001 and 2003)

Model and Measured Velocity Distributions of of H, N, O and Ne in the Spacecraft Frame

βprod = βloss = 8•10-7 s-1

NH(95 AU) = 0.095 cm-3

βprod = βloss = 5•10-7 s-1

NH(95 AU) = 8•10-6 cm-3

βprod = βloss = 6•10-7 s-1

NH(95 AU) = 6.6•10-5 cm-3

βprod = βloss = 4•10-7 s-1

NH(95 AU) = 7•10-6 cm-3

- Pickup ions as well as the bulk and halo solar wind are strongly heated in CIRs- Model spectra of the form dj/dE = joE –1.5exp[–(m/q)0.27(E/Ec)

0.63] with Ec = 0.72 MeV/n provide good fits to the data

Measured Velocity Distributions of H to Fe at 1AU with ACE/SWICS and ULEIS in the Spacecraft Frame

H+, He++, He+ (SWICS)H4, He, C, O, Fe (ULEIS)

H+, He++, He+ (SWICS)H4, He, C, O, Fe (ULEIS)

The ENA Hydrogen spectrum is a superposition of four distinct components produced by charge exchange with the ambient interstellar gas in the heliosheath

(a) Heliosheath Solar Wind(b) Heliosheath Pickup H+

(c) Heliospheric Pickup H+

(d) F&G suprathermal Tail

Density of (c) downstream of the TS is taken to be 6•10-4 cm-3

Sum of all four components fits the observed EHA spectrum

The fact that the SoHO EHAs fall ~ a factor of two below the model curve may be due to changes in the heliosheath thickness with time and viewing direction

ENA Hydrogen Spectrum and and Low Energy Proton Spectrum in the

Heliosheath

Model differential intensities for four heliosheath proton populations as would be measured with a large field-of-view particle detector in the heliosheath

near the termination shock at ~91 AU (solidcurve)

in the transition region with high turbulence δu2

at ~140 AU (dashed curve)

near the heliopause at ~148 AU dotted curve)

Local Tail at 110 AU (blue circles, V-1)

Modulated ACRs at 104 AU (red circles, V-1 CRS)

GCRs are not shown

Populations (b) and (c) are not measured by Voyagers.

Heliosheath Proton Spectrum at Different Distances in the

Heliosheath

Voyager 1 velocity distributions of H, He, O, N, Ne, and Ar (also C and Fe), in the heliosheath, averaged over 2.8 years

Fits to the lower energy data of F&G tail distributions (–5 power laws with gentle roll over) were obtained and tail pressures computes

Ratios (relative to He) of H, N, O, Ne and Ar pickup ion fluxes in the heliosheath were derived from the respective pressure ratios

Composition of Interstellar Gasfrom Heliosheath Tails and ACRs

ModulatedACRs

F&G Tail

ModulatedACR spectrum

Modulation function Rollover function

5%/AU 16 MeV/nuc He gradient is built inro = 100 AU; A is (m/q); λ is e-folding distance of solar wind speed decrease

Composition of Interstellar Gas fromHeliospheric Pickup Ions, Heliosheath Tails and ACRs

Densities of interstellar H, He, N, O, Ne and Ar at ~100 AU from averages of Pickup ions (all but He and Ar), Heliosheath Tails (all but H, He and Ar), and

ACRs (all except H, He, O)

Solar Wind Conditions Bulk SWpressure

Corepressure

Tailpressure

Coronal Hole (R ≈ 3 AU) 1.2•10-11 6.3•10-13 4.2•10-15

Quiet (R ≈ 1 AU) 3.5•10-12 3.7•10-12 4.6•10-14

Quiet (R ≈ 5.2 AU) 1.3•10-12 4.12•10-13 3.8•10-14

Many disturbed periods (R ≈ 5.3 AU) 3.7•10-13 4.6•10-13 9.3•10-14

Disturbed (R ≈ 46 AU)† 6.5•10-15 1.1•10-13 3.5•10-14

Quiet (R ≈ 94 AU)† 1.6•10-15 5.4•10-14 4.8•10-15

Heliosheath (R ≈ 100 AU)† 2.8•10-14 1.2•10-12 2.9•10-13

ACRs (R ≈ 140 AU)† ~7•10-13 8•10-13

†Core (Pickup proton) pressure is based on model calculations

Pressures (dyne/cm2) in Bulk Solar Wind, Pickup Ionsand Tail in Various Regions of the Solar System

Ulysses at 1.4 AU

He++ + H –––> He+ + H+He++ + He –––> He+ + He+

Two primary sources: (a) Inner Source He+, (b) Solar He+ by charge exchange

•

••

InnerSource

Source of He+ near W = 1

•Heliocentric distance (AU)

Flu

x (c

m-2 s

-1)

Crosswind

Slow Solar Wind Conditions

Sources of He+ near W = 1

Two primary sources(a) Inner Source

He+

(b) Solar He+ by charge exchange

At 1 AU Inner Source dominates

The tail just above W = 2 is much steeper in the spacecraft frame than it is just above w = 2 in the spacecraft frame

InnerSource

InterstellarPickup He+

-5 Tail

21

• Sources of “Solar Wind” He+ include (a) e.g. dust-desorbed He (inner source) and (b) charge-exchanged He+ from neutral H whose density decreases with decreasing

radial distance• At 1.4 AU the ‘Inner Source’ He+ density is larger than the ‘Solar Wind’ He+ density• At 5.4 AU the ‘Solar Wind’ He+ density exceeds the ‘Inner Source’ He+ density

Contributions from Inner Source and Solar Wind to He+ at W≈1 at 1.4and 5.4 AU

Velocity Distributions of Inner Source and Interstellar Pick up O+

Since the H/O, He/O, C/O, N/O and Ne/O ratios in the In-ecliptic Inner Source are nearly identical in the corresponding ratios in the In-ecliptic solar wind it is likely that most of the Inner Source pickup ions originate as dust-desorbed

Detection of C+,Mg+, Si+ and

molecular ions including water-

group molecules at ~ 5 AU with speeds

above W ≈ 1.25 indicates a

significant inner source that

extends at least up to 5 AU.

Inner Source Composition

• Since the H/O, He/O, C/O, N/O and Ne/O ratios in the In-ecliptic Inner Source are nearly identical in the corresponding ratios in the In-ecliptic solar wind it is likely that most of the Inner Source pickup ions originate as dust-desorbed atoms and molecules

• Because the In-ecliptic Inner Source has He and Ne it is unlikely that most of the Inner Source pickup ions come from cometary material

Inner Source and Solar Wind Compositions

Hourly Variations of Interstellar Pickup He Density and Solar Wind Bulk and Thermal Speeds

He FocusingCone

• Spikes in Pickup He density most often coincide with large rapid increases in the solar wind bulk and especially thermal speeds (compression regions)

• Broader dips (minima) in tail densities are well correlated with gradual decreases in the solar wind bulk and thermal speeds (expansion regions)

• He Focusing cone is clearly visible in the smoothed data (light blue curve)

Hourly Variations of Interstellar Pickup He Density and Solar Wind Bulk and Thermal Speeds

• Spike in Pickup He density at DOY ~357 is associated with rapid increase in solar wind thermal speed

• Spike in Pickup He density at DOY ~380 is associated with a very minor rapid increase in local solar wind thermal speed and hardly any increase in the solar wind bulk speed

This one-hour pickup He+ spectrum is at a spike in the pickup ion density which is associated with a fairly rapid increase in solar wind thermal and bulk speeds (compression region)

Pickup He 1-hour Velocity Distribution on DOY 357

Pickup He 1-hour Velocity Distribution on DOY 361

This one-hour pickup He+ spectrum is at a dip in the pickup ion density which is associated with a fairly rapid increase in solar wind thermal and bulk speeds (expansion region)

Pickup He 1-hour Velocity Distribution on DOY 14

This one-hour pickup He+ spectrum is at a spike in the pickup ion density which is associated with at best a very slight increase in the solar wind thermal speed and hardly any change in the solar wind bulk speed

2008 Helium Focusing Cone at 1 AULarge hourly density variations on top of a smoothed density profile of the He+ cone (light blue curve)

Model cone density profile (dashed red curve) is computed using standard parameters and observed photoionization rate, and then scaled by (1/12)

Model cone density profile scaled to fit observed profile (red curve)

Pickup He+ cone is wider than neutral He cone indicating transport of He+ ions

Electron impact ionization most likely contributes to reduce the neutral He density in the inner heliosphere

Hourly density of He+ computed by integrating the solar wind frame velocity distribution (assumed to be isotropic) from w = 0.2 to 1.2

Interstellar Pickup Ions Inner Source Pickup Ions

Spatial distribution of densities

Related to distribution of interstellar gas driven by time and spatial (radial, latitude and longitude) dependent ionization rates that deplete neutrals in the inner heliosphere and are influenced by local solar wind conditions (e.g. compression/expansion region)

Related to distribution of neutrals in the inner heliosphere driven by time and spatial dependent (radial, latitude and longitude) production of these neutrals, and time and spatial variations of ionization rates

Velocity distributions

Long term time averages (days to years): well represented by the Vasyliunas & Siscoe (1976) spectrum and neutral density distributions calculated using the hot model (sharp cutoff at ~2•VSW)Short term time averages (hours): Generally complex spectra with multiple peaks and cutoffs near 2•VSW

Long term time averages (months s to years): Broad peak below W=1 (at 0.95W) with tail ending at a cutoff near 2•VSW

Short term time averages (hours): not yet studied

Composition Reflects composition of neutral gas in the heliosphere (H, He, N, O, Ne, Ar) Pickup Ar has not yet been detected

Reflects composition of neutrals (elements and molecules) near the sun

Summary

Conclusions - Electron impact ionization plays an important role, particularly in the inner heliosphere (< 1A) and close to the sun

- Large temporal increases in the hourly He+ density are often associated with compression regions in the solar wind

- Low hourly He+ densities (dips) are often associated with expansion regions in the solar wind

- Complex hourly He+ spectra with multiple peaks are often observed, providing information on solar wind structure along the flow direction between the sun and spacecraft

- Velocity distributions of Inner Source pickup ions peak below W =1 (at about 0.95W) indicating anisotropies in their spectra close to the sun

- Spectral shapes of Inner Source pickup ions indicate that their neutral source peaks at about 0.04 to 0.08 AU, but extends to 1 AU and beyond (i.e. extended Inner Source)

- Pickup ions as well as the bulk and halo solar wind are strongly heated in CIRs

- Inner source elemental composition is similar to that of the solar wind but also many molecular ions are also observed consistent with the ‘recycled solar wind’ as its source

- Pickup Ions are the core population that feeds and grows suprathermal tails by the F&G pumping acceleration mechanism

END