Slide 1

1 Voyager Observations of Energetic Particles in the Distant Heliosheath A. C. Cummings and E. C. Stone, Caltech N. Lal and B. Heikkila, Goddard Space Flight Center 12 th Annual International Astrophysics Conference Myrtle Beach, SC 14-19 April 2013

Slide 2

2 V1 123.9 AU, 34.6N V2 101.4 AU, 30.3S ? X (McComas et al., Science, 2012) X X X X X X

Slide 3

3 Three episodes In each case, GCRs increased while TSPs and ACR decreased. So, galactic particles increased; heliospheric particles decreased. CRS team GCRs >70 MeV/nuc GCR electrons ~5-100 MeV ACRs -> GCRs ~7-60 MeV H TSPs -> background ~0.5-30 MeV H

Slide 4

Voyager Cosmic Ray Telescopes HET 2 B HET 1 B TET LET D boresight is approximately perpendicular to magnetic field direction; Bmag

Slide 5

Strong anisotropies in H with 3.3-7.8 MeV following last drop at day 238 until ~day 264 (2012.72). Intensity ~level and spectral index near zero after day 264 (2012.72). Day 264 is ~0.27 AU after boundary at day 238. Gyroradius of 5 MeV H in 0.4 nT field is 0.0054 AU (so anisotropies strong for at least 50 gyroradii). 5 Day 238, Aug. 25 Bmag 264

Slide 6

6 DOY 342.0 Choice to construct GCR H, He energy spectra is from 2012/342 forward to minimize ACR contributions. For H just after day 238, LET B j ~ LET C j and both lower than LET D, which is consistent with LECP observations at similar energy. Pattern different for He and O, with B much higher than A & C. LET D O intensity high until ~day 342.

Slide 7

7 V1 H spectra, 13-day intervals Intensities shown separately in various telescopes, which have different view directions. After day 264, appear to be observing GCR H spectrum down to ~2-3 MeV. First time this part of GCR spectrum below ~100 MeV has been revealed.

Slide 8

8 Pitch angle distributions of H with 3-7.8 MeV in 13-d intervals based on estimated directions of magnetic field (upper right panel). Note transition to near isotropic distribution of GCRs at the end. More analysis of these anistropies is in the works using different time averages, including down to 1-day averages, and for different elements.

Slide 9

V1 H, He, C, and O spectra for 2012/342- 2013/60. Also shown is H spectrum for 2012/274- 2012/121. Believe we are observing GCRs down to ~3 MeV/nuc for H and He; C & O down to ~10 MeV/nuc. GCR H, He spectra peak at ~20-40 MeV/nuc and are in good agreement at higher energies with leaky box model from Webber & Higbie 2009 -- as is GCR C. GCR C/O ratio ~1. 9 Moskalenko et al. 2002 DC Ip & Axford 1985 model a Fisk & Gloeckler 2012 -- pump Webber & Higbie 2009 -- LB

Slide 10

Preliminary V1 spectra for 2012/342-2013/60 for 8 elements (red). Also shown are spectra from a reference period (blue, 2011/1-365), which is dominated by ACRs below ~50 MeV/nuc. With more accumulation time spectra will be better defined and more spectra will become available. 10

Slide 11

11 H, He, e spectra from Ip & Axford, 1985 with and without energy losses taken into account. Factor of 400 different for protons at 1 MeV and factor 50 different at 10 MeV. It is possible that the energy spectrum we are observing is still a modulated one either from residual solar modulation or perhaps there is a gradient in the interstellar medium (Scherer et al. 2011, Herbst et al. 2012). Ip & Axford, 1985

Slide 12

12 V1 H intensities vs time. No measurable gradients yet, but will be something to monitor in the coming years.

Slide 13

13 H &He spectra for 2012/342-2013/60. H/He ratio ~12.6 from ~3-350 MeV/nuc -> observing GCR H and He. Ratio 12.6 is recommended abundance in solar photosphere (Lodders, 2003)

Slide 14

14 V1 >70 MeV rate showing some small (few per cent) anisotropy. Low intensities are when S/C was rolled 70 degrees off nominal.

Slide 15

15 A small step up in intensity observed in several GCR rates near day 80 of 2013 for a couple of weeks. Variations still occurring -> probably not yet in interstellar space.

Slide 16

16 V1 entered new region on day 238 of 2012 GCRs have good access to spacecraft; heliospheric particles abruptly decline. Spectral evolution from different LET telescopes indicates that heliospheric particles persist a significant distance beyond the regions boundary, with those having 90 degree pitch angle persisting the longest. Can distinguish co-existing ACR and GCR populations based on isotropy. By 50 gyroradii beyond the boundary (if boundary stationary) H intensities are essentially isotropic, consistent with GCRs. Heliospheric He and O persisting longer at 90 degree pitch angle, likely because pitch angle scattering is less efficient at higher rigidities. Observing GCR H and He spectra down to ~3 MeV/nuc and GCR C and O down to ~10 MeV/nuc. GCR C/O ~1; GCR H/He ~12.6 GCRs recently showing some small intensity changes and anisotropies Future: Will there be gradients? Other structures? More elemental and some isotopic GCR spectra for Zs up through Fe Ion Summary

Slide 17

17 GCR Electrons

Slide 18

Voyager Cosmic Ray Telescopes HET 2 B HET 1 B TET The Electron Telescope

Slide 19

GCR Electrons Two ways of getting electron measurements from CRS HET BSe mode: No dead material Total energy is measured (~2.5-10 MeV), so straight forward in principle Plan to do GEANT4 simulation to determine efficiency as a function of energy Will not discuss today The Electron Telescope (TET) (~5-100 MeV) Stack of silicon detectors with interleaved W absorbers Energy measured in first two detectors only Range in stack determined Was calibrated prior to launch with electron beams from 6 72 MeV (Space Radiation Internal Report 79, J Zmuidzinas and N. Gehrels, and other internal reports) Recently, simulated response functions using GEANT4 have also been done Necessary to correct for GCR proton background 19

Slide 20

20 TET telescope diagram

Slide 21

R i = 21 Counting rate from response function and incident energy spectrum Response functions measured at accelerator and simulated using GEANT4 Parameterize energy spectrum with 3 parameters to make a smooth curve: dJ/dE = A*(E^g)*exp(-E/Eo)) Do least-squares fit to observed counting rates

Slide 22

22 Response functions from accelerator calibration D1D3 D1D4 D1D5 D1D6 Energy bins will overlap

Slide 23

23 Comparison of response functions from accelerator calibration (solid lines) with GEANT4 simulation (dotted lines). Not as different as they may look because its a linear scale and its the integral thats important. GEANT4 simulations still being refined.

Slide 24

Background possibilities from high energy protons 24

Slide 25

25 Estimate of background for D1D5 GCR electrons emerged from background from 2005 forward Background ~8% at present for D1D5 Background ranges from 2% to 34% from D1D3 to D1D7 Other ways to estimate background are being explored

Slide 26

26 Background corrected rates. Intensities reasonably steady with time after the day 238 jump.

Slide 27

27 Response functions convolved with energy spectrum. Emin, Emax, and Eplot selection. Emin and Emax chosen by moving horizontal line down from peak and stopping when 68% of distribution is between Emin and Emax. Eplot = sqrt(Emin*Emax).

Slide 28

28 V1 electron spectrum for 2012/342-2013/60 using: response functions from calibration (open) and response functions from GEANT4 simulation (solid)

Slide 29

29 New analysis of TET electron data Parameterize energy spectrum and calculate rates from response functions Use response functions (four) that were determined at accelerator calibration and response functions (five) from GEANT4 simulation Least-squares fit to determine parameters of energy spectrum Energy bins not contiguous (previously they were) Energy ranges depend somewhat on which response functions are used and also vary slightly with energy spectrum but are approximately (GEANT4 simulation in MeV): 5-13, 9-24, 15-43, 21-70, and 20-100. Correct for background based on 225-646 MeV proton rate using a linear relationship for periods when background is thought to account for all of the rate Other methods for correcting for background are being explored Deduced energy spectrum is between LISM estimates of Langner et al. 2001 and Webber and Higbie 2008 GCR electron intensity steady since the 2012/238 abrupt increase GCR Electron Summary

Slide 30

30 The End

Slide 31

31

Slide 32

32 LIS GCR H, He, C, e spectra from Ip & Axford, 1985 Energy losses in interstellar medium cause flattening at low energies Ip & Axford, 1985

Slide 33

Intensity ratio LET D/C directions divided by beta vs rigidity Higher rigidity particles have more beamed pitch-angle distribution 33

Slide 34

34 V1 O spectra, 13-day intervals. Intensities shown separately in various telescopes, which have different view directions. LET D O intensity declines throughout but may still have some ACR contribution at lowest energies.

Slide 35

35 V1 H spectra, 3-day intervals.

Slide 36

36 Comparison of response functions from accelerator calibration (solid lines) with GEANT4 simulation (dotted lines)

Slide 37

37 H &He spectra for 2012/342-2013/60. H/He ratio ~12.6 from ~3-350 MeV/nuc -> observing GCR H and He. Ratio 12.6 is recommended abundance in solar photosphere (Lodders, 2003)