He+ Plots



Singly charged helium (He+) observed in interplanetary space may be of solar (ICME), magnetospheric, or interstellar origin (pickup ions).  One way to establish the origin of the He+ ions is from their energy spectrum and the direction of flow.  


The first direct observations of He+ of interstellar origin were made with the AMPTE/IRM spacecraft and reported by Moebius et al. (1985), and subsequently have been observed by instruments on Ulysses, ACE, SOHO, and Nozomi.  The pickup ions have energy spectra with an energy cutoff (in the spacecraft frame) of about twice the solar wind speed, in the sunward facing direction.


STEREO now joins this constellation of spacecraft in providing interstellar helium pickup ion observations:




Figure 24 from Galvin et al., 2008:  a) Mass per charge spectrum for Helium, showing separation of the two charge states.

b) Two-hour accumulation count spectrum for pickup He+, showing the expected cutoff at twice the solar wind speed (four times the solar wind energy).




Online monthly He+ - time series plots are available on the STEREO PLASTIC Data Plots website.  These overview plots are meant to assist in event selection for He+ studies, including multi-longitudinal measurements of the focusing cone (for previous s/c measurements see Gloeckler et al., 2004), the variation of pickup ion intensity as a function of solar wind parameters, the injection and acceleration at shocks, and development of suprathermal tails in corotating interaction regions (CIRs).  These plots are created using the PLASTIC Level 1 data files (CDF) and the SPLAT analysis program (PLASTIC online Resources).





General Information:

-- Title: Spacecraft, instrument, and species identification.

--  First (top) Panel:  Solar wind protons bulk speed (black) and number density (red)

--  Next five panels:  Energy spectra of He+ rates.  Count rate intensity is color-coded.

--  Last (bottom) Panel:  total He+ count rate, summed over all energies and directions.


Directional Information:

--  Second and third panels:  Solar Wind Sector observations (Forty degrees centered on the spacecraft-sun line. The sunward direction is 0o.)

--  Fourth and fifth panels:  Adjacent to the sunward sector.  Nominal Parker spiral angle is about 45o.

--  Sixth panel is anti-sunward sectors.


Composition Information:

--   The rates with a tcr label have a Helium mass and mass-per-charge identification (triple-coincidence rate using time-of-flight plus measured energy). 

-- The rates with a dcr label have M/Q identification only (double coincidence rate, using time-of-flight measurement).


Other Information:

-- Purple traces in the energy spectra panels show the nominal energy-per-charge (E/q) value of He+ corresponding to twice the solar wind speed (2*Vsw).

-- Black lines in the second and third panels indicate the E/q value of a switch in the geometrical factor that takes place in the Solar Wind Sector (Main vs. Small Channels).

-- Some rates exhibit a trace at lowest energies that are partially an instrumental effect (spillover)

-- Note suprathermal tail events at CIR compression regions, seen as peaks in the Total Counts panel, and as energy spectra exceeding the 2*Vsw trace.