# Has Voyager 1 entered a solar radiation belt?

The Voyager 1 probe was sent out in 1977 to go where no man made object has gone before, after more than 35 years it’s still going strong. It’s now 124 AU away from Earth and many are wondering when it’s going to leave our heliossphere and set off into unknown interstellar space. The readings from the probe have been more or less as expected until a dramatic change appeared. In August 2012. Voyager 1 entered a strange region where the solar wind came to a complete stop while the amount of cosmic rays set new records:

As Voyager 1 entered the new region, it seems like the region boundary where fluctuating back and forth. The magnetic field also strengthened as Voyager 1 entered the region, but the magnetic field did not change direction and that is the reason why the Voyager team has concluded that Voyager 1 is still in our heliossphere.

Voyager 1 is now in this new unknown region within our heliossphere and we don’t know what this region is. Here is a suggestion; the new region is a solar radiation belt.

Most scientists currently believe that the Sun has no radiation belts because the magnetic field, which turns every 11 years, is not stable enough to sustain a solar radiation belt. But recent observations from Earths outer radiation belts show that the belts can be drained and refilled with particles within weeks, so it may be time to take a second look at the possible existence of stellar radiation belts.

But why is the possible solar radiation belt so far from the Sun? Earths outer solar radiation belt is compressed by the solar wind, so a solar radiation belt may experience an opposite effect and get blown far away from the Sun, the unknown interstellar wind could on the other side push it against the sun.

The readings we got when Voyager 1 entered the new region may be explained by Voyager 1 leaving the high density solar plasmasphere and entering the low density energetic solar outer radiation belt.

-The observed disappearance of the solar wind could be Voyager 1 leaving the solar plasmasphere. Earths plasmasphere is filled with an outflow of ions from the ionosphere from low and mid-latitudes, which could be similar to the solar wind filling the solar plasmasphere. What happens to the solar wind ions on the threshold to this new region seems to be unclear, are they slowing down and piling up, or are they accelerated out of there?

-The observed sharp increase in cosmic rays could be energetic ions and electrons in the solar radiation belt itself. Ions with low cosmic ray velocity have recently been found in Earths radiation belts. We know that Jupiter has much stronger and more energetic radiation belts, so size amplify radiation belts and create cosmic ray velocity particles. We could then expect cosmic ray velocities to be normal in solar radiation belts which are located in a huge magnetic heliossphere. This also gives possible solutions to other cosmic ray problems, like why cosmic rays are decreasing during solar maximum, why cosmic rays can come in showers and temporary vary in particle type. It could also explain why we have found unstable isotopes in cosmic rays which cannot survive the long journey from the stars. Collisions with interstellar medium causing spallation are a proposed explanation of the unstable isotopes and the observed anti-matter, but collisions by the radiation belt with the solar wind will also give spallation and isotopes and anti-matter.

We currently don’t know what creates cosmic rays, supernovas is one proposal, but there is not enough supernovas in the universe to count for all the cosmic rays, so an additional source is needed. Cosmic rays from the radiation belts of stars could be such a source. If we have a solar outer radiation belt which is a Cosmic Ray source, CR-particles could also gyrate along solar magnetic field flux lines and bombard the Sun, with a much higher CR intensity than Earth experience in its magnetic field shielded quiet zone.

-Voyager 1 observed an increase in the magnetic field as it entered the new region. Earths outer radiation belt is also the home of Earths ring current which induces a magnetic field. So the solar radiation belt itself could harbor a solar ring current which induces a magnetic field which can explain the observed increase in the magnetic field:

-The observed fluctuations of the region boundary could be caused by irregular solar wind causing expansion and compression of the solar radiation belt, similar to how solar wind causes Earths radiation belt to fluctuate. The fluctuations could also be caused by variations in the solar magnetic field as the Sun spins.

-The number of cosmic rays hitting Earth is inverse proportional to the number of sunspots. An explanation to this could be that a varying solar magnetic field during solar maximum cannot sustain or confine a big stable outer radiation belt which may produce cosmic rays that hit Earth. It’s also possible that solar storms deplete the solar radiation belt of particles like solar storms deplete Earth’s outer radiation belts of particles.

The Voyager team states that Voyager 1 has not left the heliossphere, the observed region then have another explanation. A solar radiation belt may be a solution, and may also explain how the majority of cosmic rays may be created in stellar radiation belts. It’s a proposal which so far seems to fit well with the observations and it could give us some great answers. Any help in the evaluation process or comment is most appreciated. If the Sun and Earth have more in common than we think, this realization may lead us to further insights in how these great machinery's works.

• +1 for very nice post (content/presentation)! If possible, please summarize question once more in last paragraph. – Asphir Dom Jul 26 '13 at 10:32
• There isn't a single, coherent dipole magnetic field defining the heliospheric magnetic field so the idea of trapped particles, like those found in the Earth's radiation belts, is not directly applicable to the heliosphere. – honeste_vivere Nov 1 '16 at 16:22

I assume the idea is that the lower-energy anomalous cosmic rays, not the galactic cosmic rays, are the charged particles from the "solar radiation belt"... According to what I've read, the anomalous cosmic rays were expected, and the standard theory is that they are a population of charged particles in the "heliosheath", but they were observed far outside their expected range.

Anyway, whether the idea of a radiation belt is reasonable may depend on how it is defined. If you define it as broadly and vaguely as possible, it could just mean any population of charged particles, persistently occupying the same volume of space, and by that broad definition, perhaps even the standard, pre-Voyager models had a radiation belt.

On the other hand, if we specifically take Earth's radiation belts as defining the concept, then perhaps it means a population of charged particles that are wandering back and forth between the poles of the magnetic field of a celestial object. In that case, I wonder whether particle populations so far from the sun would have the time and stability required to exhibit that specific behavior.

Hopefully you will get a reply from someone who actually knows about the long-range solar magnetosphere, interstellar magnetic fields, the galactic population of cosmic rays, and other relevant topics...

• Thanks for your answer! In Earths radiation belt we have measured trapped particles with an energy exceeding 100 MeV. The majority of cosmic rays are between 100 MeV and 1 GeV. We know that size and strength of the magnetic field amplify, so yes I mean that the possible solar radiation belt could be a "galactic" Cosmic ray source. Your definition of a radiation belt is good but I will add: Particles with relativistic speed confined by a magnetic field. The pole to pole is the bounce motion, the orbiting motion is the drift motion, and the particles also gyrate around magnetic flux lines. – Enos Oye Jun 26 '13 at 9:39
• The magnetic field is generally weak so far from the sun, but the smaller radius of the radiation belt torus the stronger magnetic field we need to confine the particles. So 125 AU from the sun, we might not need a very strong magnetic field. What a great tokamak reactor. I guess we could do some calculations here to see if the measured magnetic field is strong enough to confine the CR-particles. Anyone up for the task? – Enos Oye Jun 26 '13 at 9:55

Voyager 1 is now in this new unknown region within our heliossphere and we don’t know what this region is.

The current interpretation of the data from Voyager 1 is that it entered the local interstellar medium (ISM) some time near August 25, 2012. So it's not a new, unknown region, there was just some initial hesitancy about whether the spacecraft actually crossed the heliopause and whether there would be a helioshock (turns out there was not a helioshock).

Most scientists currently believe that the Sun has no radiation belts because the magnetic field, which turns every 11 years, is not stable enough to sustain a solar radiation belt. But recent observations from Earths outer radiation belts show that the belts can be drained and refilled with particles within weeks, so it may be time to take a second look at the possible existence of stellar radiation belts.

I am not sure I would characterize it in this way. The dipole term of the sun's multipole expansion of its magnetic field is rather small near the sun as the other higher order terms dominate locally, which is why SDO images during active times are so busy. The magnitude of the solar magnetic field roughly follows the dipole decay with distance, i.e., $$B \propto r^{-3}$$, but the turbulent nature of the solar wind (along with other factors) causes it to be an extremely dynamic system. Further, the tilt of the dipole axis of the sun is rather large compared to the rotation axis, which means the field at Earth flips sign roughly every ~27 days due the crossing the heliospheric current sheet.

The typical definition of a radiation belt is a region of space near a magnetized body where energetic particles are trapped in specific regions that are governed by the adiabatic invariants of charged particle motion in a (rough) magnetic dipole field. That is, to maintain a region of trapped, energetic particles one needs to maintain a consistent magnetic field geometry for multiple drift orbits of the particles about the magnetized body.

But why is the possible solar radiation belt so far from the Sun? Earths outer solar radiation belt is compressed by the solar wind, so a solar radiation belt may experience an opposite effect and get blown far away from the Sun, the unknown interstellar wind could on the other side push it against the sun.

Even if we could "shut off" the solar wind, the location of the inner radiation belt would not dramatically change. The outer belt would be stable to larger radii, but again it would not change dramatically.

The readings we got when Voyager 1 entered the new region may be explained by Voyager 1 leaving the high density solar plasmasphere and entering the low density energetic solar outer radiation belt.

So far as I know, the number density increased dramatically upon crossing the heliopause (e.g., see the following article https://science.sciencemag.org/content/341/6153/1489), not decreased. The shock-sheath-pause system is "backwards" from that of the Earth's magnetosphere in that the termination shock forms the inner boundary, followed by a lower density heliosheath, then bounded by the heliopause. That is, the heliosheath densities are ~0.001--0.003 cm-3 while those beyond the heliopause are ~0.06--0.08 cm-3, or an increase by a factor of ~80 or so.

What happens to the solar wind ions on the threshold to this new region seems to be unclear, are they slowing down and piling up, or are they accelerated out of there?

Most of the bulk thermal ions that are incident on the termination shock are decelerated and heated. There are some that are energized/accelerated by the termination shock and an even smaller fraction become energetic enough to contribute to the anomalous cosmic rays (ACRs) (other sources include pick-up ions and/or pre-energized ions from the solar wind). The basic idea is that that ACRs are accelerated locally within the heliosphere while galactic cosmic rays (GCRs) come from outside the heliosphere. This is why the ACR intensity dropped while the GCR intensity increased at the heliopause. GCRs generally have much larger energies than ACRs and that the heliopause is a gradual boundary (i.e., it's not really a sharp discontinuity) leads to the enhancement in GCR intensity ~100 days prior to crossing the heliopause (Note that the heliopause is not a static boundary relative to the sun, it can move a lot).

We know that Jupiter has much stronger and more energetic radiation belts, so size amplify radiation belts and create cosmic ray velocity particles. We could then expect cosmic ray velocities to be normal in solar radiation belts which are located in a huge magnetic heliossphere.

The Jovian radiation belts are more intense than Earth's because the Jovian magnetic field is much stronger than Earth's, thus it can trap more energetic particles. The magnetic field near the heliopause is orders of magnitude weaker than that of even Earth.

This also gives possible solutions to other cosmic ray problems, like why cosmic rays are decreasing during solar maximum, why cosmic rays can come in showers and temporary vary in particle type.

The cosmic intensity drops during solar maximum due to a well known effect called the Forbush decrease. Further, this only affects the lower energy end of the cosmic ray spectrum, i.e., below 10s to 100s of GeV. The more energetic particles do not care about the primary cause of the decreases, namely coronal mass ejections because they just "plow through" these magnetic obstacles due to their larger gyroradii.

We currently don’t know what creates cosmic rays, supernovas is one proposal, but there is not enough supernovas in the universe to count for all the cosmic rays, so an additional source is needed. Cosmic rays from the radiation belts of stars could be such a source. If we have a solar outer radiation belt which is a Cosmic Ray source, CR-particles could also gyrate along solar magnetic field flux lines and bombard the Sun, with a much higher CR intensity than Earth experience in its magnetic field shielded quiet zone.

Note that in all the radiation belts in the solar system for which we have observations, the energization is not caused by the belts themselves. That is, the particles are energized by multiple processes (e.g., see discussion at https://physics.stackexchange.com/a/142922/59023) and the belts only indicate the regions where these energetic particles are trapped.

Voyager 1 observed an increase in the magnetic field as it entered the new region. Earths outer radiation belt is also the home of Earths ring current which induces a magnetic field. So the solar radiation belt itself could harbor a solar ring current which induces a magnetic field which can explain the observed increase in the magnetic field

The terrestrial ring current tends to generate a magnetic field opposite to the Earth's dipole, thus why geomagnetic storms are indicated by a negative Dst field, i.e., it basically measures the deviation from the tilted dipole approximation of the Earth's field, which goes negative during storms due to an enhanced ring of ions with energies in the 10s to few 100s keV. Thus, the field is not really enhanced during storms, it is compressed initially by some external disturbance (e.g., CME-shock) but the magnetosphere response is often an over compensation.

The observed fluctuations of the region boundary could be caused by irregular solar wind causing expansion and compression of the solar radiation belt, similar to how solar wind causes Earths radiation belt to fluctuate. The fluctuations could also be caused by variations in the solar magnetic field as the Sun spins.

Yes, the boundary oscillates due to changes in the solar wind and ISM flow. By the time the solar magnetic field has reached the termination shock, it is so "wound up" that in the ecliptic plane it is basically orthogonal to the termination shock unit normal vector. The only significant changes are caused by CMEs hitting the boundary and the effect of these can be sensed remotely.

The number of cosmic rays hitting Earth is inverse proportional to the number of sunspots. An explanation to this could be that a varying solar magnetic field during solar maximum cannot sustain or confine a big stable outer radiation belt which may produce cosmic rays that hit Earth. It’s also possible that solar storms deplete the solar radiation belt of particles like solar storms deplete Earth’s outer radiation belts of particles.

This is just the Forbush decrease mentioned above.

The Voyager team states that Voyager 1 has not left the heliossphere, the observed region then have another explanation. A solar radiation belt may be a solution, and may also explain how the majority of cosmic rays may be created in stellar radiation belts. It’s a proposal which so far seems to fit well with the observations and it could give us some great answers. Any help in the evaluation process or comment is most appreciated. If the Sun and Earth have more in common than we think, this realization may lead us to further insights in how these great machinery's works.

The general consensus has changed in favor of Voyager 1 being in the ISM. As an aside, recent work on Voyager 2 suggests it left the heliosphere in November of 2018. There are some Nature papers on the heliopause crossing in the works by the Voyager team as of July 2019.

Summary
No, I do not think the evidence supports any solar radiation belts nor do I think this would be physically possible given the requirements for trapping coherent populations of energetic electrons and ions.