TWO NEW EFFECTS:

IMF Variations:

When the IMF vector, B || Vsw, the flux density in the antisunward direction (especially for freshly created ions with V ~ 2V sw) decreases as a result of inhibited pitch angle scattering.

Solar Wind Speed Variations:

In the upwind direction, the cut-off in the pickup spectra is observed to be > 2 V sw due to a nonzero inflow speed of interstellar neutrals. The effect is more pronounced when V sw is small:

V cut-off ~ 2 V sw + V neutral

Effects of the IMF Variations:

  1. The antisunward flux of the most recently created pickup He+ (w ~ 2) [see top figure] decreases when the field turns radial. (See right figure below)
  2. The pickup ion flux at lower speeds (w ~ 1.3) [see top figure] is independent of the field direction. (See left figure below)
  3. Pickup ion spectra have a smooth decrease rather than a sharp cut-off at high speeds when the field is radial. (See bottom figure below)

Related Figures:

SOHO CELIAS CTOF and MTOF/PM: Average pickup He+ spectrum:

OHO CELIAS CTOF and MTOF/PM: Average pickup He+ spectrum

Average pickup He+ spectrum. Two velocity bins are selected:
1.3 < w <1.4 and 1.9 < w < 2.0

SOHO CELIAS CTOF and WIND MAG: Pickup He+ flux for 1.3 < w < 1.4 versus IMF direction:

SOHO CELIAS CTOF and WIND MAG: Pickup He+ flux for 1.3 < w < 1.4 versus IMF direction

Pickup He+ flux for 1.3 < w <1.4 versus IMF direction. In the plateau regime, the distribution is effectively unaffected.

SOHO CELIAS CTOF and WIND MAG: Pickup He+ flux for 1.9 < w < 2.0 as a function of IMF direction:

SOHO CELIAS CTOF and WIND MAG: Pickup He+ flux for 1.9 < w < 2.0 as a function of IMF direction

Pickup He+ fluxes for 1.9 < w < 2.0 as a function of IMF direction. Strong dependence of the pickup He+ flux close to the cutoff.

SOHO CELIAS CTOF and WIND MAG: Pickup ion spectra averaged over 30-degree bins in IMF angle:

SOHO CELIAS CTOF and WIND MAG: Pickup ion spectra averaged over 30 bins in IMF angle

Pickup ion spectra averaged over 30-degree bins in IMF angle.
As the angle decreases, the high-energy cut-off of the pickup distribution becomes a smooth decrease.

Effects of the Solar Wind Speed Variations and the Interstellar Gas Flow

  1. The cut-off velocity of the pickup He+ distribution shifts to velocities > 2 V sw for the SOHO measurements in the upwind region. (See right figure below)
  2. The cut-off velocity effectively coincides with or is somewhat lower than 2V sw for the AMPTE measurements in the focusing cone on the downward side. (See left figure below)
  3. The relative shift of the cut-off appears to scale inversely with V sw. (See bottom figure below)
  4. This shift of the cut-off can be described correctly, if is V sw is replaced by V sw + V neutral in the solar wind frame. (See top figure below)

Related Figures:

Sensitivity of Pickup Ion Distribution to the Interstellar Gas Flow:

Sensitivity of Pickup Ion Distribution to the Interstellar Gas Flow

AMPTE SULEICA, 15 Nov. 1985:

AMPTE SULEICA, 15 Nov. 1985

The cut-off energy of the pickup ions as observed by AMPTE SULEICA on the downwind side is consistent with no or a slight shift to lower energies.

SOHO CELIAS CTOF and MTOF/PM:

SOHO CELIAS CTOF and MTOF/PM: Pickup ion spectra for two solar wind speed ranges

Comparison of the pickup ion spectrum as observed by SOHO CTOF with a model based on the injection of He+ at V = 0 and V = V neutral

SOHO CELIAS CTOF and MTOF/PM: Pickup ion spectra for two solar wind speed ranges:

SOHO CELIAS CTOF and MTOF/PM

Pickup ion spectra for two solar wind speed ranges. The relative shift in the cut-off velocity of the pickup ion distribution increases as the solar wind speed decreases.