Geoscience Reference
In-Depth Information
TS; (3) those from ACR in HS. Each group of ENA is a direct sample of the
ion population of its place of origin, while ions of that region cannot reach
us and be detected. The ballistic trajectories of ENA across magnetic field
afford imaging of remote ion populations. Group 3 has the most circuitous
journey in the heliosphere and carries most information concerning particle
acceleration at TS, since ACR are believed to be PUI accelerated at TS,
and PUI are penetrating LIC neutrals ionized in and picked up by the
supersonic SW. Also represented in Fig. 1 are ENA from ions accelerated
by interplanetary shocks.
7
This group of ENA may mask the detection of
ENA from the boundaries.
8
The detection of ENA requires dedicated instruments and opportuni-
ties, in order to take the unique advantage of imaging space plasmas in
regions not easily accessible to space missions; e.g., heliospheric bound-
aries. The first detection of ENA, in 1951, was in the aurora by ground-
based optical means in the blue shifted H-Balmer lines.
9
First detection
of ENA (1-20 keV H) in space was performed on a rocket flight, by an ion
mass spectrometer using a thin carbon foil to ionize penetrating incident
ENA, so they could be analyzed by an electrostatic ion mass analyser.
10
Earnest testing and development of space-borne ENA detectors revived in
the 1970s. Thin carbon foils are used as filter against intense EUV radia-
tion as well as generator of secondary electrons for “start” signals needed
for the newly evolving time-of-flight (TOF) mass spectrometers.
11
-
15
At
lower energies, say, ENA from LIC neutrals, the technique of surface con-
version is employed and yielded the first direct detection of LIC He atom
on the Ulysses mission.
16
A thorough review of techniques in ENA imaging
of space plasma, including the use of transmission grating as EUV filter is
given by Gruntman.
17
For a more recent exposition, see Ref. 18.
2. Current Status
Indeed, the study of the heliospheric boundaries via ENA began with
modeling,
19
,
20
and modeling continues to lead the way. We shall mention
observations and their role in modeling. Page limitation prohibits more
coverage of many worthy works.
2.1.
Observation
Of the three groups of ENA from heliospheric boundaries, only ENA of
ACR origin can claim to have been observed. Even that is only the begin-
ning and the results from HSTOF of CELIAS on SoHO (55-88 keV H and
