Geoscience Reference
In-Depth Information
85-141 keV He) remain singular.
21
-
24
High background dashed the hope of
harvesting from INA of MIMI
25
on
Cassini
and from HENA
26
on IMAGE
(D. Mitchell, private communication, 2005). Whether ASPERA-3
27
on
Mars Express, which has two ENA instruments, NPI (
∼
0
.
1-60 keV) and
NPD (
0
.
1-10 keV), could yield any results in heliospheric ENA, including
imaging the He cone, remains to be seen. Meanwhile, we pin our hopes on
Interstellar Boundary Explorer (IBEX), which is to be launched in 2008
to image the heliosphere in 0.01-6 keV H.
28
The SoHO results and IBEX
expectations are best seen in light of existing models.
∼
2.2.
Modeling
There is a host of good modeling work on ENA in the outer heliosphere; we
will, however, only highlight two that make use of the only available ENA
observations, i.e., that of SoHO's, and one that predicts what IBEX may
expect. Most models use H and ENA interchangeably, because the predom-
inance of H and the lack of mass resolution in some ENA instruments. Up
to date, HSTOF of CELIAS on SoHO is the only ENA instrument that can
identify H and He separately.
23
,
24
2.2.1.
Learning from SoHO observation
By assuming ACR's mode of propagation in a HS of certain size and shape
and ACR's spectral form at TS, with its parameters constrained by the
demodulated ACR spectrum based on observations of Voyagers 1 and 2, a
self-consistent model fits SoHO's measured H and He fluxes independently
and yields a common power index of the differential spectra, and a shock
strength of 2.89.
29
Figure 2 is a sample of their results.
In a comprehensive model studying the “injection problem” in acceler-
ation of superthermal PUI to become ACR at TS and in HS, the energetic
H fluxes from the apex and the heliotail direction measured by SoHO are
compared with H spectra resulting from stochastic acceleration by different
magnitudes of turbulence in each region.
30
The five curves in Fig. 3 arise from different degrees of stochastic accel-
erationinHS.
30
Curve 1 stands for the flux coming from outside TS in the
absence of stochastic acceleration. Curve 2 stands for the flux coming from
both sides of TS, also without stochastic acceleration. Curves 3-5 stand
for fluxes from PUI stochastically accelerated in different degrees, char-
acterized by a turbulence parameter of 0.012, 0.12 and 1.2, respectively.
