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ACE News #73 - July 17, 2003

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Interstellar and Heliospheric Parameters derived from Observations of Pickup Helium in the Focusing Cone

Interstellar pickup ions (PIs) are created by ionization from interstellar atoms that drift at ~26 km/s into the inner heliosphere due to the relative motion of the Sun through the local interstellar cloud (LIC). Because He has the smallest ionization probability it is not significantly depleted even at 1 AU and He+ is by far the most abundant interstellar PI at ACE.

The gravity of the Sun focuses the beam of incoming He atoms and concentrates them in a roughly conical region (focusing cone) in the direction opposite (downwind) of the direction from which they arrive. Thus, the velocity distribution (phase space density spectrum) of pickup He+ measured with the SWICS (Solar Wind Ion Composition Spectrometer) instrument (Fig. 1a) has its highest density and the helium PI flux peaks once every year when ACE is in the downwind focusing cone as observed during 5 consecutive years (Fig. 1b). The shapes of the PI spectra and the phase, maximum intensity, and time profiles of the He PI fluxes in the yearly focusing-cone peaks depend on the physical properties of He atoms in the LIC (number density, relative velocity, temperature) and on ionization processes in the heliosphere (solar cycle and spatial dependence of ionization rates).

The downwind spectrum shown in Fig. 1a has the characteristic sharp cutoff at Wcutoff ≈ 2 - Vr/Vsw where Vr is the radial component of the local velocity of neutral He and Vsw is the solar wind He speed. The value of Wcutoff can be determined from the shape of the velocity spectrum, as shown in Fig. 1a. Its value is close to, but generally not exactly, equal to 2. It should be possible to find the average speed of interstellar He atoms using the expected gradual changes with longitude of the measured cutoffs. Combining these ACE He+ PI measurements with SWICS/Ulysses H+ PI observations, it should also be possible to determine for the first time the speed of interstellar H atoms.

The model spectrum is computed using interstellar parameters determined from Ulysses measurements of interstellar neutral and PI He, production rates for He PIs, βprod, measured at 1 AU by SOHO (photo-ionization) and ACE (electron-impact-ionization), and loss rates for the interstellar gas on its way into the inner heliosphere, βloss, adjusted to fit the spectrum. The shape of the spectrum (or βloss) thus determines the radial profile of the density of He atoms from ~ 0.03 to 1 AU. Values used for βloss were lower than the βprod values, corresponding to rates more typical of solar minimum conditions, and larger-than-expected electron-impact ionization rates had to be used inside 1 AU. Extended analysis of PI He spectra with SWICS on ACE and Ulysses over a complete solar cycle will provide knowledge of the time, latitude, and radial dependence of interstellar He atoms and, when combined with time-dependent models, of the spatial and temporal variations of ionization rates of He.

Fig. 1b shows the time variations of the average phase space density over 1.4 ≤ W ≤ 2, which is approximately proportional to the flux of PI He. The 1-day averages (light violet curve) show large day-to-day variability, most likely the result of variations in the cutoff speed. The 9-day running average (blue curve) reduces the variability and shows clearly the five He focusing cone traversals and the He density change with solar activity. The position of the peak of the He cone gives the solar ecliptic longitude of the interstellar wind flow direction. The mean value from SWICS is 74.43° ± 0.33° in beautiful agreement with direct neutral He measurements from Ulysses.

In Fig. 1c are shown the 3-point running averages of 3-day-averaged phase space densities from 0.7 to 1.0 times Wcutoff from late 1998 to early 1999, and model calculations averaged in the same fashion. The error bars take into account the accuracy in determining Wcutoff. To calculate the model curve we used measured values for βprod at 1 AU and linear (in time) representations of the electron impact and photo-ionization loss rates derived from values that gave the best fits to the velocity spectra of DOY98 340 and 367 respectively. Again, larger-than-expected electron-impact rates at <1 AU, which cannot presently be measured, and near-solar-minimum values of photo-ionization loss rates were required to fit the spectra. The model curve provides an excellent fit to the data, indicating that cross-field diffusion of He PIs is negligible, and that they faithfully map the neutral He cone.

Submitted by George Gloeckler of the University of Maryland and University of Michigan.

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