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He+ Suprathermal Tails as Observed by STEREO/PLASTIC

He+ Suprathermal Tails as Observed by STEREO/PLASTIC. Mark Popecki (UNH) and Berndt Klecker (MPE) Kennebunkport June 10, 2010. He+ Tails from PLASTIC.

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He+ Suprathermal Tails as Observed by STEREO/PLASTIC

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  1. He+ Suprathermal Tailsas Observed by STEREO/PLASTIC Mark Popecki (UNH) and Berndt Klecker (MPE) Kennebunkport June 10, 2010

  2. He+ Tails from PLASTIC • Observations of H and He with SWICS instruments have indicated the presence of a V/Vsw-5 suprathermal tail (e.g. Gloeckler et al., 2007) • He+ suprathermal tails may be studied with STEREO/PLASTIC • PLASTIC is functionally similar to the SWICS instruments in: • Measurement technique and • Energy range • but has a unique instrument design • PLASTIC observations are independent and complementary to those of SWICS

  3. Solar Wind • E/q • TOF • Essd deflectors Suprathermal ions • E/q • TOF • Essd • E/q • TOF Suprathermal ions PLASTIC Spatial Viewing • STEREO PLASTIC has three apertures. • The solar wind aperture uses deflectors, azimuth, time-of-flight and energy measurements to obtain M, Q, E and incoming direction of solar wind ions. • Apertures for directions other than the solar wind provide M, Q, E or just M/q. • PLASTIC is primarily a solar wind instrument, but it can provide some suprathermal tail data as well. • Ion Selection: E/q • Measurements: • Time of Flight, • Residual Energy (SSD)

  4. Mewaldt et al. 2001 • He, O and Fe fluences during an active solar period (Mewaldt et al., 2001). • Energy spectrum changes at ~7 keV/nuc from one characteristic of solar wind to another characteristic of energetic particles. • He+ can be measured by PLASTIC at energies up to ~20 keV/nuc Fe O He+

  5. B. Klecker, Eberhard Moebius PLASTIC and ACE/SWICS He+: June 2007 • Phase space density for He+ from ACE/SWICS and PLASTIC • PLASTIC data incorporate best values of: • energy-dependent efficiency and • geometric factor • Data are normalized near V/Vsw = 1 • Normalization agrees to better than a factor of two with PLASTIC efficiencies and GF alone • What do we see in a long term average for V/Vsw ≥ 3?

  6. Measurement Period • Long term average of He+ phase space density collected over ~10 months of 2008 on STEREO A • Results were compared to a (V/Vsw)-5 spectrum

  7. Solar Wind Speeds • Observation period: 305 days from Jan to Oct 2008 • Several CIRs occurred • S/W speeds between 300 and 775 km/s • 68% of the 305 days have a daily average SW speed ≤ 500 km/s and can be entirely measured in the energy range of the entrance system up to V/Vsw <= 4.

  8. Effect of Maximum Entrance System E/Q • The maximum E/q for the entrance system is ~79 keV/e • Maximum He+ energy/nuc: ~20 keV/nuc • Maximum s/w speed for which VmaxHe+/Vsw = 4 (S/C frame) is 482 km/s • Periods of higher s/w speed are included, but they will not contribute to large values of V/Vsw, therefore undercounting PSD and tending to cause a spectral steepening. • An alternative approach: normalize by the number of sample periods where Vsw is small enough at a given speed ratio • At V/Vsw = 4, normalize by the number of periods with Vsw ≤ 482 km/s

  9. He+ Suprathermal Tail Jan-Oct 2008: SW speeds < VmaxHe+ , SC Frame • PSD averaged over total time with SW speeds < VmaxHe+ • SC frame • Spectral index over the speed range 3 < V/Vsw < 6 : -7.1 ± 0.1 • Expectations for the S/C frame if the index in the S/W frame is -5: • Ipavich (1974) for • 0 deg • nsw=-5 • V/Vsw = 4.5 is: -6.4

  10. He+ Suprathermal Tail Jan-Oct 2008: SW speeds < VmaxHe+ , ~SW Frame • PSD averaged over total time with SW speeds < VmaxHe+ • ~SW frame by subtracting Vsw • Spectral index over the speed range 2 < V/Vsw < 5 : -5.2 ± 0.1

  11. Low Speeds: Vsw ≤ 397 km/s • Another alternative: restrict Vsw for a selected V/Vsw maximum: look at a low speed case • Average PSD over 120 days with Vsw ≤ 397 km/s to obtain V/Vsw ≤ 5 in the S/C frame • ~SW frame: subtract Vsw • Spectral index over the speed range 2 < V/Vsw < 4: -5.8 ± 0.1

  12. Low Speeds: Vsw ≤ 327 km/s • Another alternative: restrict Vsw for a selected V/Vsw maximum: look at a low speed case • Average PSD over 45 days with Vsw ≤ 327 km/s to obtain V/Vsw ≤ 6 in the S/C frame • ~SW frame: subtract Vsw • Poor statistics above V/Vsw = 3.25 • Spectral index over the speed range 2 < V/Vsw < 3: -9.0 ± 0.4

  13. Summary: STEREO/PLASTIC observations of the He+ suprathermal tail • Spectrum in the S/W frame normalized by solar wind speed periods is -5.2 ± 0.1 for 3 ≤ V/Vsw ≤ 6 (s/c frame speed range) • Restricting the long term PSD average to low speed solar wind periods produces a steeper spectral index: • Vsw ≤ 397 km/s for … • 3 < V/Vsw < 5 (S/C frame speed range) • has the steepest spectrum of the set: -5.8 ± 0.1 • Vsw ≤ 327 km/s for … • 3 < V/Vsw < 4 (S/C frame speed range) • has the steepest spectrum of the set: -9.0 ± 0.4

  14. -6.9 He+ Suprathermal Tail Jan-Oct 2008: All SW speeds, SC Frame • PSD averaged over total time, with all SW speeds • SC frame • Spectral index over the speed range 3 < V/Vsw < 4: -7.0 ±0.1 • Expectations for the S/C frame if the index in the S/W frame is -5: • Ipavich (1974) for • 0 deg s/w direction • Nsw = -5 (power law index in s/w frame) • V/Vsw = 3.5 is: -7 • V-5 spectrum is consistent with observations for 3 < V/Vsw < 4 • At higher speeds, the phase space density falls off more steeply.

  15. -5.0 He+ Suprathermal Tail Jan-Oct 2008: All SW speeds, ~SW Frame • Average PSD over all 305 days • ~SW frame approximation: subtract Vsw • Spectral index over the speed range 2 < V/Vsw < 3: -5.0 ± 0.1 • V-5 spectrum is consistent with observations for 2 < V/Vsw < 3 • At higher speeds, the phase space density falls off more steeply.

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