Neutrals near the Sun and the inner source pickup ions P. Mukherjee and T.H. Zurbuchen

Department of Atmospheric, Oceanic, and Space Science, The University of Michigan, Ann Arbor, MI 48109

This work is supported by award number NNG04GL44H of the Graduate Student Research Program of the National Aeronautics and Space Administration.

There is a source of neutral particles near the Sun, resulting in so-called “inner source pickup ions”. Pickup ions from this inner source have to date been treated very similarly to those from interstellar space. In particular, the assumption has been made that they are effectively motionless when picked up, and thus in the solar wind frame have a velocity of -Vsw which then isotropizes into ring or hemispheric distributions. We suggest that there are other motions or effective motions of these neutral particles in the near-solar environment that need to be considered. Many, if not most, of the neutral particles arise from dust grains spiraling into the Sun in Keplerian orbits, and thus these neutrals have a large azimuthal velocity perpendicular to the solar wind and the average heliospheric magnetic field. In addition, many of these ions should be picked up where the solar wind is sub-Alfvenic. As such, the Alfven wave velocity needs to be taken into account when finding the effective frame in which ions are picked up, and their thermal velocity should isotropize around a value dependent upon major speed contributions. This value, much larger than that of the currently accepted stationary pickup, strongly affects the cooling of pickup ions in the inner heliosphere, and hence the interpretation of inner source pickup ion measurements done to date.

Abstract Density Profiles

MotivationInterplanetary neutral atoms have two major sources: the interstellar medium and the so-called inner-source dust arising from the asteroids and comets. These populations not only have different compositions, but are ionized and picked up in significantly different environments, and yet to date have been treated almost the same for modeling purposes. As can be seen from Figure 1 below, there are velocity components near the Sun that are easily ignored farther out into the heliosphere. Since these effects, as well as the solar wind acceleration, are all nonlinear very close to the Sun, it’s worth examining where dust will be found. Most inner source papers to date make the assumption that most of the dust-source neutrals are found between 10-50 solar radii, but Krivov et al (1998) indicates that non-negligible amounts of dust survive to within 2-4 solar radii, depending on their dielectric and morphological properties.

Adiabatic Expansion Considerations

Figure 1: Solar wind and Alfven wave speeds in the near solar region, computed using the formulae below (from Hu, Kohl, Lie-Svendsen, and Sittler papers), and azimuthal dust grain speed calculated from standard circular Keplerian orbit.

The field-aligned speed of ions is the sum of Up and Va, and is thus dominated by Va, while the perpendicular velocity at injection will depend on the azimuthal speed of the source dust.

Assumed valuesSolar wind speed: 450 km/sec Proton density at 1 AU: 5 cm-3

Ratio of pickup protons to SW protons: 1E-4Ionization rate for H at 1 AU: 7.44E-7 s-1 (Rucinski et al, 1996)

ConclusionsWe considered inner source pickup ion populations throughout the inner heliosphere. Close to the Sun, the pickup process needs to account for a pair of velocity components that are negligible beyond a few dozen solar radii: azimuthal speeds of the dust grains, and the enhanced heating due to increased Alfvén wave speeds. We made a model that predicts the neutral atom and ion populations and adiabatic cooling of the ions. We showed that this model has solutions consistent with inner source number densities and thermal speeds measured at 1 AU, and that the additional velocity components require the pickup process to happen far closer to the Sun than predicted by traditional models. This provides exciting opportunities for future missions close to the Sun.

The authors would like to thank for their help Dr. Susan Lepri, Dr. Len Fisk, and Jim Raines of the University of Michigan and Dr. George Gloeckler of the University of Maryland

References• Gloeckler et al (2000), J. Geophys. Res., 105, 7459-7463• Gloeckler et al (2000), Proc. of ACE 2000 Symp, 221-228• Hu et al (1997), J. Geophys. Res., 102, 14661-14676• Isenberg (1997), J. Geophys. Res., 102, 4719• Kohl et al (1998), Astrophys. J., 501, L127-L131• Krivov et al. (1998), Icarus, 134, 311-327• Lie-Svendsen et al (2001), J. Geophys. Res., 106, 8217-8232• Leinert and Grun (1990), Physics of Inner Heliosphere Vol 1,

ed. Schwenn & Marsh, 207-275

• Ruciński et al (1996), Space Sci. Rev., 78, 73-84• Schwadron (1998), J. Geophys. Res., 103, 20643-20649• Schwadron et al (1999), Solar Wind 9, 487-490• Schwadron et al (2000), J. Geophys. Res., 105, 7465-7472• Sittler and Guhathakurta (1999), Astrophys. J., 523, 812-

826• Wilck and Mann (1996), Planet. Space Sci., 44, 493-499• Vasyliunas and Siscoe (1976), J. Geophys. Res., 81, 1247-

1252

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orbitsunV

R: radial distance in solar radiiB: magnetic field in nanoTesla (nT)U: flow speed in km/sVa: Alfven speed in km/sN: number density in cm-3Lambda: latitude in degrees (0 at equator)G: Gravitational constant

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Neutral Profiles

Dust distribution

Production ofneutrals from dust

We considered profiles for α=1 and α=2,λ=6-30 solar radii, and scaled the constantD0P0 as needed to match values measuredat 1 AU. Other parameters listed below.

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Ion density derivation

In both cases, increasing lambda resulted in a decrease of the peak density value and movement of the peak outward, as is to be expected. Note that the peak densities fell offnon-linearly while the locations moved with linear fashion as seen in Figures 4-5 below. Inaddition, the α=1 case uniformly resulted in lower peaks at further radial distances.

Figure 4 Figure 5

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Inner source H+

Figure 6: H+ distribution function and inner source fit. The 1/e width of the inner source distribution is approximately 0.33 * Solar Wind Speed. Thanks to Dr. George Gloecklerfor this data.

Given the particle densities in Figures 2 and 3 and velocity components from Figure 1, we can now take a look at the expected behavior of the particle distributions as they adiabatically expand out to 1 AU.

Figure 6 gives us a baseline thermal velocity at 1 AU for comparison of 1/3 the solar wind, or ~150 km/sec. Then, using the adiabatic relation 1

1

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AU

peakth

obsAUth

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we can solve for the thermal velocity at 1 AU for a wide variety of assumed conditions. Figure 7 demonstrates the results of these calculations. The lambda values where the model fits the measured data can be traced back to a given pickup ion peak location in Figure 5. Notice that the curves fitted to our new model require the pickup peaks to be far closer to the Sun than those using the standard model, which is the entire point of this poster.

The lambda values associated with our model fall at 15Rs and 35Rs while those of the standard model fall at 63 Rs and 101Rs. Those correspond to peak locations (from Figure 5) of 7.6, 12.8, 31.6, and 37 Rs respectively, so it’s clear that our model demands pickup far closer to the Sun than currently accepted models.

Luckily, three missions currently in planning stages will acquire data much closer to the Sun: Sentinels, Solar Orbiter, and Solar Probe. The orbital ranges of the three spacecraft are displayed on Figure 8 along with the density curves matching the lambda values listed above.

This author is currently working on nanoscale ultraviolet filters that may be of significant use on those missions (below).

Figure 7: Modeled thermal velocities at 1 AU for α=1 andα=2. Solid lines include all pickup velocity componentsfrom Figure 1, dotted lines include only the standard VSW

component.

Model Fits

MeasuredVth

Solar Orbiter (45 to155 Rs)

Solar Probe (4 Rs to 5 AU)

Sentinels (56 to167 Rs)

Figure 8: Particle density curves for the λ values found inFigure 7. Notice that the α=1 cases have higher peaksthan the α=2 cases, which does not match Figure 4, but thereason is that the λ values for the two cases do not match as in previous figures. Also displayed are orbit ranges for upcoming missions.