He+ Suprathermal Tailsas Observed by
STEREO/PLASTIC
Mark Popecki (UNH) and
Berndt Klecker (MPE)KennebunkportJune 10, 2010
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
• 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. 2001
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/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?
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/Vsw)-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.
0
5
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200 250 300 350 400 450 500 550 600 650 700 750 800 850 900STEREO A S/W Speed (Daily Average)
Frequency
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DOY 2008
STA speed (km/s)
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
0
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VmaxHe+/Vsw
Max S/W speed
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
0.00001
0.0001
0.001
0.01
0.1
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0 1 2 3 4 5 6 7 8
V/Vsw
(Cts/s)/(tot hrs*V/Vsw^4)
PSD
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
0.00001
0.0001
0.001
0.01
0.1
1
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10000
0 1 2 3 4 5 6 7
(V-Vsw)/Vsw
(Cts/s)/(tot hrs*V/Vsw^4)
PSD
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
0.001
0.01
0.1
1
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10000
0 1 2 3 4 5 6
(V-Vsw)/Vsw
(Cts/s)/(tot hrs*V/Vsw^4)
PSD
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
0.001
0.01
0.1
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10000
0 1 2 3 4
(V-Vsw)/Vsw
(Cts/s)/(tot hrs*V/Vsw^4)
PSD
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
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.
0.00001
0.0001
0.001
0.01
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V/Vsw
(Cts/s)/(tot hrs*V/Vsw^4)
PSD-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.
0.00001
0.0001
0.001
0.01
0.1
1
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10000
0 1 2 3 4 5 6 7
(V-Vsw)/Vsw
(Cts/s)/(tot hrs*V/Vsw^4)
PSD
-5.0