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

He+ Tails from PLASTIC Observations of H and He with SWICS instruments have indicated the presence of a V/V sw -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

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) Solar Wind E/q TOF Essd deflectors Suprathermal ions E/q TOF Essd E/q TOF Suprathermal ions PLASTIC Spatial Viewing

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 Mewaldt et al Fe O He+

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/V sw = 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? B. Klecker, Eberhard Moebius

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/V sw ) -5 spectrum

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.

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

He+ Suprathermal Tail Jan-Oct 2008: SW speeds < V maxHe+, SC Frame PSD averaged over total time with SW speeds < V maxHe+ 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 n sw =-5 V/Vsw = 4.5 is: -6.4

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

Low Speeds: V sw ≤ 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

Low Speeds: V sw ≤ 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

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/V sw ≤ 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/V sw < 5 (S/C frame speed range) –has the steepest spectrum of the set: -5.8 ± 0.1 –Vsw ≤ 327 km/s for … –3 < V/V sw < 4 (S/C frame speed range) –has the steepest spectrum of the set: -9.0 ± 0.4

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 N sw = -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. -6.9

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. -5.0