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.

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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

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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

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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

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(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

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(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

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(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.

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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

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(V-Vsw)/Vsw

(Cts/s)/(tot hrs*V/Vsw^4)

PSD

-5.0