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)