**2. The radial intensity vs. time profiles**

**Figure 1** shows radial intensity profiles for the two lowest energy electron channels, the HET A1-A2 stop channel at ~0.7 ± 0.3 MeV and the HET B1-B2-C4 stop channel with a peak response

**Figure 1.** Time history of 26 day average intensities of A-stop 0.7 MeV (in black) and 4 MeV B stop electrons (in red) from launch at earth 1977 to the end of data in 2017, at ~140 AU. These rates provide radial and temporal intensity profiles. The B-stop rate is normalized to A-stop rate of 0.21 c⋅ s−1 in LIS using the factor 0.75.

between 3.0 and 4.0 MeV, from the time of launch (1977) to the present time (2018). These energy responses are determined by extensive GEANT-4 calculations. These telescopes are described in [2, 3]. The two intensities are normalized to those observed in local interstellar space (LIS).

Then, suddenly these particles completely vanished and new and completely different spectra of particles between 1 MeV up to ~1 GeV, instantly recognizable as those for galactic cosmic rays, were observed. These LIS intensities at all energies have remained constant to within ±1% for 5 years corresponding to 20 AU beyond the HP. These low energy galactic spectra and their relation to higher energy measurements such as those made by AMS-2 will be discussed in this paper in terms of the acceleration, source distribution and propagation of cosmic rays

We begin in this paper by discussing the radial intensity profiles outward from the Earth through the heliosphere and beyond into interstellar space. We will compare these intensity profiles for electrons and protons, from the lowest energies, ~1 MeV, to the higher energies of a few hundred MeV which are normally considered as cosmic rays. The profiles provide a graphic view of the scale of the various regions of the heliosphere and the entrance into the

We will then discuss the spectra of galactic cosmic ray electrons, protons, helium and heavier nuclei that are measured for the 1st time by Voyager. Every step in this process provides a new insight into the features of these galactic cosmic rays that could never be studied previ-

**Figure 1** shows radial intensity profiles for the two lowest energy electron channels, the HET A1-A2 stop channel at ~0.7 ± 0.3 MeV and the HET B1-B2-C4 stop channel with a peak response

**Figure 1.** Time history of 26 day average intensities of A-stop 0.7 MeV (in black) and 4 MeV B stop electrons (in red) from launch at earth 1977 to the end of data in 2017, at ~140 AU. These rates provide radial and temporal intensity profiles.

The B-stop rate is normalized to A-stop rate of 0.21 c⋅ s−1 in LIS using the factor 0.75.

uncharted regions of interstellar space and the realm of galactic cosmic rays.

ously because of our location within the heliosphere.

**2. The radial intensity vs. time profiles**

in the galaxy.

62 Cosmic Rays

One can define from **Figure 1** four temporal or spatial regions that apply to all the components and energies discussed here. The 1st region is from launch in 1977 to 1983.0 when V1 was at about 20AU and also near its maximum latitude of +30°. This region (nearest the Sun) is punctuated by many abrupt increases related to interplanetary propagating electron events of energy <1 MeV of solar origin. These are closely coordinated with observations inside the Earth's orbit from HELIOS during the 1977–1978 time period. The individual events are separated temporally at Voyager and HELIOS. Also included is the Jupiter encounter occurring at ~1979.2.

The second region (time period) is from 1983.0 to 2005.0 at which time V1 crossed the HTS and entered the heliosheath at ~95.0 AU and at +30°. The effects of the solar 11 year modulation cycle are rather weakly seen in this region. These variations are similar temporally to what was observed for higher energy protons [4] and shown later in **Figure 3**. Two massive solar induced interplanetary electron events are seen in the A stop rate in 1989 and 1991 when V1 was at between 40 and 50 AU. These events are related to large shocks moving outward in the heliosphere with speeds close to 1000 km/s.

During the time V1 is in the heliosheath region from 2005.0 to 2012.65 between 95 and 121.7AU (Region 3) there is a factor of 3–4 increase in the sub-MeV electron intensities which, during most of this time period, have even higher intensities than those observed beyond the HP, in the local interstellar medium. These are electrons accelerated in the heliosheath. Meanwhile the intensity of the 4 MeV electrons increases in the heliosheath from the HTS crossing to the HP by a factor ~30. This is believed to be a result of modulation effects in the heliosheath which reduce the LIS intensity of these higher energy electrons.

Beyond the heliopause (HP) in LIS the electron intensities have remained constant to within ±1% at both energies for 5 years (~18 AU of outward travel for V1). Notice that the intensities of these electrons in LIS are higher than those at the Earth by a factor ~4 at 0.7 MeV and by a factor ~20 at 4 MeV. These differences are due in part to solar modulation effects and in part to the local acceleration of 0.7 MeV electrons.

In **Figure 2** we show the 1.8–2.6 MeV proton intensity, again normalized to the LIS value that is measured for 0.7 MeV electrons. This is the lowest energy part of the interstellar proton spectrum that can be measured on Voyager in a 2-D, dE/dx, telescope mode. It represents a limit to the characteristics of the lower energy proton propagation in the galaxy from the nearest sources in the galaxy since it corresponds to only ~35μ of equivalent Si traversed during the galactic propagation.

In region 1, between launch and 1983 and inside ~20 AU, the Earth and the inner heliosphere are bathed in an almost continuous flux of 2 MeV protons which exceeds the possible background galactic cosmic rays at this energy and location by a factor ~1000 and is comparable to the intensities observed further out in the heliosheath. This continuously high intensity results from the 1.8 to 2.6 MeV protons from many individual solar events, with the intensities piling up due to the large longitudinal diffusion.

**Figure 2.** Same as **Figure 1**. The L1-L2 stop 1.8–2.6 MeV proton rate (in red) is normalized to the A-stop 0.7 MeV LIS electron rate using the factor ×750.

In region 2, between ~20 AU and the HTS at 95 AU, these 2 MeV proton intensities reach a level which is less than that in LIS. This corresponds to a solar activity minimum and also to solar modulation minimum. This intensity, which is up to a factor ~12 times less than the LIS intensity, could be a representation of the level of solar modulation of these particles at this low energy of ~2 MeV.

Moving outward into region 3, we find that the intensity of these 250 MeV protons increases by a factor ~4 in the heliosheath. The large modulation beyond the HTS at this higher energy was previously unrecognized and amounts to ~1/3 of the total solar modulation of these par-

Galactic Cosmic Rays from 1 MeV to 1 GeV as Measured by Voyager beyond the Heliopause

⋅sr⋅s⋅MeV (in red) is normalized to A-stop 0.7 MeV LIS

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65

This solar modulation of these higher energy protons can be well described with a single modulation parameter φ corresponding to an energy loss resulting from a potential difference, φ in MV, between the observation point and the LIS spectrum [8]. This simple description arises from the fact that the overall spherically symmetric solar modulation in the heliosphere appears to follow the description provided by Louville's Theorem relating to the constancy for the particle density and momentum in phase space. Of course there are deviations from this simple picture due to structural features in the heliosphere such as the heliospheric current sheet, and also for the solar polarity changes which induce a 22 year cycle in the solar modulation process, but these other processes do not appear to dominate at these energies and above, where, in fact, the same value of φ derived from these protons also gives a good description of the historical neutron monitor observations of cosmic ray modulation effects at

Here we summarize the most general features of all of the radial intensity profiles. The most prominent feature is the sharpness and effectiveness of the heliospheric boundary, the heliopause. For low energy protons the reduction of intensity is a factor ~500, taking place in only

The effects of this boundary on electrons are even more astounding. A radial intensity gradient ~130%/AU just inside the HP for 4 MeV electrons changes to a 0.1%/AU radial gradient in

the Earth that have been carried out over the last 70 years.

**Figure 3.** Same as **Figure 1**. The 250 MeV proton intensity in p/m2

1–26 day interval corresponding to less than 0.1 AU in distance.

ticles in the heliosphere.

electron rate using the factor ÷60.

In region 3, the intensity of these low energy protons increases by a factor 1000 beyond the HTS. It remains at this high level for a distance ~30 AU as a result of heliosheath accelerated protons. Then suddenly the intensity decreases by a factor ~500 corresponding to a distance of ~0.1 AU near the HP. This intensity change over such a small radial interval requires a remarkably effective particle barrier at the HP.

In region 4, beyond the HP, the proton intensity represents the low energy tail of the galactic proton spectrum with an intensity ~0.5 of that at the intensity peak of the differential spectrum which occurs at ~30 MeV.

In **Figure 3** we show the higher energy proton (250 MeV) intensity time profile from the solar modulation study in [4]. This proton intensity is also normalized to the 0.7 MeV A1-A2 stop LIS electron intensity beyond the HP. The solar modulation effects are seen in region 1 inside 20 AU where the Voyager observations for 250 MeV protons blend in well with spacecraft observations of this modulation near the Earth and also neutron monitor observations [5–7] at the same time. The intensity at the Earth at this energy is a factor ~8 below the LIS values.

In region 2, between about 20 and 95 AU, solar 11 year modulation effects are observed at 250 MeV which are in synch in both time and magnitude with those observed at the Earth, but with a time delay corresponding to outward moving structural features with a speed ~600 km⋅s−1.

Galactic Cosmic Rays from 1 MeV to 1 GeV as Measured by Voyager beyond the Heliopause http://dx.doi.org/10.5772/intechopen.75877 65

**Figure 3.** Same as **Figure 1**. The 250 MeV proton intensity in p/m2 ⋅sr⋅s⋅MeV (in red) is normalized to A-stop 0.7 MeV LIS electron rate using the factor ÷60.

In region 2, between ~20 AU and the HTS at 95 AU, these 2 MeV proton intensities reach a level which is less than that in LIS. This corresponds to a solar activity minimum and also to solar modulation minimum. This intensity, which is up to a factor ~12 times less than the LIS intensity, could be a representation of the level of solar modulation of these particles at this

**Figure 2.** Same as **Figure 1**. The L1-L2 stop 1.8–2.6 MeV proton rate (in red) is normalized to the A-stop 0.7 MeV LIS

In region 3, the intensity of these low energy protons increases by a factor 1000 beyond the HTS. It remains at this high level for a distance ~30 AU as a result of heliosheath accelerated protons. Then suddenly the intensity decreases by a factor ~500 corresponding to a distance of ~0.1 AU near the HP. This intensity change over such a small radial interval requires a

In region 4, beyond the HP, the proton intensity represents the low energy tail of the galactic proton spectrum with an intensity ~0.5 of that at the intensity peak of the differential spec-

In **Figure 3** we show the higher energy proton (250 MeV) intensity time profile from the solar modulation study in [4]. This proton intensity is also normalized to the 0.7 MeV A1-A2 stop LIS electron intensity beyond the HP. The solar modulation effects are seen in region 1 inside 20 AU where the Voyager observations for 250 MeV protons blend in well with spacecraft observations of this modulation near the Earth and also neutron monitor observations [5–7] at the same time. The intensity at the Earth at this energy is a factor ~8 below

In region 2, between about 20 and 95 AU, solar 11 year modulation effects are observed at 250 MeV which are in synch in both time and magnitude with those observed at the Earth, but with a time delay corresponding to outward moving structural features with a speed ~600 km⋅s−1.

low energy of ~2 MeV.

electron rate using the factor ×750.

64 Cosmic Rays

trum which occurs at ~30 MeV.

the LIS values.

remarkably effective particle barrier at the HP.

Moving outward into region 3, we find that the intensity of these 250 MeV protons increases by a factor ~4 in the heliosheath. The large modulation beyond the HTS at this higher energy was previously unrecognized and amounts to ~1/3 of the total solar modulation of these particles in the heliosphere.

This solar modulation of these higher energy protons can be well described with a single modulation parameter φ corresponding to an energy loss resulting from a potential difference, φ in MV, between the observation point and the LIS spectrum [8]. This simple description arises from the fact that the overall spherically symmetric solar modulation in the heliosphere appears to follow the description provided by Louville's Theorem relating to the constancy for the particle density and momentum in phase space. Of course there are deviations from this simple picture due to structural features in the heliosphere such as the heliospheric current sheet, and also for the solar polarity changes which induce a 22 year cycle in the solar modulation process, but these other processes do not appear to dominate at these energies and above, where, in fact, the same value of φ derived from these protons also gives a good description of the historical neutron monitor observations of cosmic ray modulation effects at the Earth that have been carried out over the last 70 years.

Here we summarize the most general features of all of the radial intensity profiles. The most prominent feature is the sharpness and effectiveness of the heliospheric boundary, the heliopause. For low energy protons the reduction of intensity is a factor ~500, taking place in only 1–26 day interval corresponding to less than 0.1 AU in distance.

The effects of this boundary on electrons are even more astounding. A radial intensity gradient ~130%/AU just inside the HP for 4 MeV electrons changes to a 0.1%/AU radial gradient in the LIM in just 1–26 day interval of ~0.1 AU in radius. Essentially whatever intensities exist in the heliosphere apparently stay in the heliosphere as a result of an almost impenetrable heliopause at these lower energies. As far as energetic particles go, the interstellar medium at this location has little recognition of the nearby heliosphere.

Other features of the heliosphere newly recognized from this study include: (1) The heliosheath is a very interesting and important region both in terms of accelerating protons and also (more weakly) electrons as well as for large solar modulation effects. The solar modulation effects on the intensity in this region range from a factor ~4 for ~1 GV protons to a factor ~100 for 15 MV electrons and then decrease to a factor ~30 for 4 MV electrons and much less for 1 MeV electrons which appear to be mostly locally produced.

The massive acceleration of nuclei, e.g., protons and He and O nuclei in the heliosheath, extending down to ~1 MeV and up to ~100 MeV/nuc, which was previously known and believed to be fueled by high ionization potential IS ions, is joined by the newly found acceleration of sub-MeV electrons.

The 11 year and longer solar activity cycles studied now for over 70 years using neutron monitors, balloon and spacecraft borne instruments at higher energies, and so important for geophysical studies [9], are still significant beyond the HTS. Even for protons at 250 MeV the intensity increase in the heliosheath region is a factor ~4; for electrons, this increase due to solar modulation reaches a maximum of a factor ~100 at ~15 MeV. These effects were previously unrecognized.

**Figure 4** shows the electron spectrum measured at Voyager and also in the higher energies

**Figure 4.** The electron spectrum measured at Voyager using the TET, B stop and A stop telescopes and the measurements

Galactic Cosmic Rays from 1 MeV to 1 GeV as Measured by Voyager beyond the Heliopause

which amounts to a factor ~500 below 100 MeV and is still ~20% between10–20 GeV/nuc. Also note the V1 spectrum which is ~E−1.3 between about 1–60 MeV. The black curve shows a Monte Carlo calculation with an electron source spectrum ~E−2.25 and a path length ~20.6 β P−0.45 above 0.562 GV and with a diffusion coefficient ~P−1.0 below 0.562 GV.

low and high energy differences. There is a large gap in the intermediate energy range for LIS electrons from ~100 MeV to several GeV. At energies above a few GeV the electron spectrum measured by PAMELA and AMS-2 steepens rapidly. Part of this increase in spectral index is

Monte Carlo propagation calculations of the electron spectrum using various source spectral

We have shown from these calculations that, in order to fit both low energy and high energy data, the source spectral exponent for electrons changes from being ~P−2.24 below about 8 GV

At ~400 MeV, which is ~1.0 GV for protons there are corresponding measurements of the proton and electron intensities at the Earth by PAMELA and also at Voyager. The ratio of Voyager to PAMELA proton intensities at the two locations is ~4.0 which is caused by the amount of solar modulation between the LIS and the Earth at 1 GV. The electron intensity is also measured at this time by PAMELA and at 1 GV it is 2.5 × 10−2 × the proton intensity. We believe from solar modulation theory [8] that at 1 GV the total modulation of electrons and protons should be nearly the same. So the LIS electron intensity at 1 GV could be estimated

format. The AMS-2 data on electrons [11] is shown along with

⋅s⋅sr/MeV. This intensity is very close to the value

presentation which shows both

format. Note the severe solar modulation of electrons

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67

. This high energy region

where solar modulation effects are small. The plot is a x E<sup>2</sup>

of PAMELA and AMS-2 at higher energies. The figure is in a x E<sup>2</sup>

to one ~P−2.4 at higher energies extending up to ~1 TV [12, 13].

from this approach to be 1.4–1.6 × 10−1e/m2

is shown in **Figure 5** in a x E<sup>3</sup>

indices [12, 13].

due to synchrotron and inverse Compton energy loss which are ~E<sup>2</sup>

And finally the new observations reported here have also extended the V1 measurements of the local interstellar electron spectrum by a factor ~10 lower in energy (e.g., down to 0.5 MeV) as compared with even the initial Voyager measurements of [2] beyond the HP. For protons the minimum energy measured is now ~1.8 MeV, a factor ~2 times lower than the initial measurement for these particles. The intensities obtained at these lowest energies appear to be consistent with a smooth extension of those measured at higher energies as would be expected from propagation calculations. The range of the lowest energy electrons and protons is only ~50 μ of Si. This places severe constraints on the distribution of nearby sources of these particles but illustrates that, even at these lower energies, the spectra are still a compendium of many sources with an individuality obscured by diffusion in the galactic magnetic fields.
