University of Kansas

Space Physics and
Plasma Astrophysics

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Proton Studies and Interplanetary Processes

What comes to mind when the term "proton" is mentioned? You probably recall from a past science class that it is an elementary particle with a positive electrical charge. You also probably think that only scientists come into contact with such things. Well, did you realize that the environment of our earth is populated with low energy protons? Two University of Kansas (KU) Department of Physics and Astronomy researchers, Dr. Thomas Armstrong and Dr. Moncef Boufaida, have examined these particles primarily because the explanation for the presence of these particles and accounting for their variations has been a problem for some time in space physics. They now will share their findings with us.

In order to observe the global characteristics of protons of high energy (1.8-5 MeV) and low energy (0.2-0.5 MeV), observations were taken simultaneously by several spacecraft at various locations. This decision was made because various processes that produce certain characteristics occur over large volumes of the interplanetary medium. Single point observations would have been limited in value. The chosen paired spacecraft were IMP 8 and Voyager 1, IMP 8 and Voyager 2, and IMP 8 and Ulysses. IMP 8 is at fixed earth-orbit which stays within 1 Astronomical Unit (AU) (an Astronomical Unit is the distance from the Earth to the Sun) as compared with the other spacecraft that are beyond 1 AU. The KU researchers' goal was to establish universal and general characteristics of the behavior of low and high (0.3-5 MeV) energy protons.

The researchers also reported monthly sunspot numbers, flare counts, and geomagnetic storm sudden commencements (SSC's) which were compared to monthly 0.3 to 0.5 MeV proton fluxes with IMP 8 during 2 solar cycles. Observations show a similar cycle increase and decrease of flare number, sunspot number, and particle flux. An additional observation was made regarding the monthly total storm sudden commencements (SSC's). (These geomagnetic effects are recorded by ground-level magnetometers based in different areas of the Earth.) Studies show that there is a correlation between shocks and SSC's. Forty-one of 48 disturbances correspond to interplanetary shocks. Nearly all SSC's are accompanied by shocks. Therefore, SSC's can serve as indicators for the occurring interplanetary shocks, and the number of SSC's is larger during high solar activity than during low solar activity. Interplanetary shock waves are known to produce increases in 0.3 to 0.5 MeV proton fluxes.

Observations suggest that the sun is probably the most important source of particles of 0.5 to several MeV within 1 AU, but that beyond 1 AU these particles are energized and have their intensity enhanced. Simultaneous paired spacecraft evaluation of proton flux intensity and energy with the 1-5 AU region has been done for different periods of solar flares and the modulation of particle fluxes and spectra by interplanetary, most notably shock acceleration due to either flare or stream interaction-generated shocks. Results confirm the description of the interplanetary medium as a locale of particle energization, mainly during high solar activity where interplanetary shocks are almost always present based on SSC indicators.

The simultaneously paired spacecraft reports show that about 65% of the Ulysses, Voyager 1, and Voyager 2 low energy (0.2 to 0.5 MeV) proton intensities exceed IMP 8 intensities. However, for high energy (1.8 to 5 MeV) protons, the situation was reversed. IMP 8 fluxes exceed Ulysses, Voyager 1 and Voyager 2 by about 60% of the time. These observations prove that lower energy protons observed at Ulysses and Voyagers 1 and 2 have their energy enhanced relative to the ones observed at 1 AU by IMP 8, whereas higher energy protons show the opposite trend.

Let's review. The 2 principal observations were:

  1. propagation of low energy protons (0.3-0.5 MeV) and high energy protons (1.8-5 MeV) in the interplanetary medium between 1 and 5 AU, using simultaneous observations at 1 AU (IMP 8) and at other locations (Ulysses, Voyager 1, and Voyager 2) and
  2. intensity enhancements of 0.3-0.5 MeV due to the action of interplanetary shock waves beyond 1 AU, between IMP 8 and Ulysses or Voyagers 1 and 2.

The Sun emits energetic particles over a wide range of species and energies into interplanetary space. These particle emissions appear to be related to the solar cycle represented through solar flares, sunspot number, SSC's and low energy particles. Therefore, let's discuss the nature of particle fluxes from solar flare events that have been measured at 1 AU by Earth-orbiting IMP 8.

The reports made for the low energy protons show that the average of particle fluxes seen at Ulysses, Voyager 1, and Voyager 2 are higher than the ones recorded by IMP 8. This proves that the lower energy proton fluxes (0.2 to 0.5 MeV) are enhanced between 1 and 2 AU while the higher energy fluxes (2 to 5 MeV) are weakened. From 2 to 4 AU low energy fluxes diminished while the higher energy fluxes increased. Observations show a shorter time delay as the energy range increases, and the delay appears to be longer as the radial distance increases and starts to disappear for larger radial distances.

For 0.3 to 0.6 MeV protons, the flux reaches its peak at about 2 AU and starts to decrease as the radial distance increases. However, higher energy protons with 2 to 5 MeV range have the lowest peak at about 2 AU and start to increase as the radial distance increases. This suggests that between 1 to 5 AU, low energy protons of 0.3 to 0.6 MeV that originate within 1 AU are energized within a few AU, and this acceleration (mainly shocks) becomes less and less efficient as the radial distance increases for this particular range of energy. In contrast, protons with energy higher than 2 MeV are more affected with the energization process at greater radial distance.

Apparently, an acceleration mechanism exists. This energization seems to take place in the 1 to 2 AU region, where it is highest at 1 AU and decreases as the radial distance increases. The researchers conclude that there is a high probability that lower energy particles undergo energy changes due to the propagation of interplanetary shocks based on SSC numbers, which serve as interplanetary shock signatures. Observational evidence proves that the flux of an energetic charged particle often peaks at or close to the passage of an interplanetary quasi-perpendicular shock. These shocks are common in solar wind, especially beyond 1 AU. Therefore, these shocks are significant accelerators of low energy ions in the heliosphere.

From these results, researchers establish that the lower proton energy fluxes are more affected than the higher ones. There must be an interplanetary process or interplanetary acceleration which maintains, redistributes, and recirculates the lower energy particles in the interplanetary medium in contrast to the higher energy particles which are less affected by this process and originate within 1 AU, probably from flares or coronal ejection events. The researchers observed an increase or build-up in intensity for the 0.3 to 0.5 MeV protons. Two to 5 MeV protons observed by each spacecraft have their source within 1 AU. In several different studies, it was suggested that there is a mechanism whereby interplanetary acceleration is occurring, leading to an enhancement of intensities of low energy particles beyond 1 AU.

Protons with energy between 2 to 5 MeV are not energized in the 1 to 2 AU region.

In summary, Drs. Boufaida and Armstrong interpret the results to establish that 0.2 to 0.5 protons are affected more rapidly by interplanetary processes beyond 1 AU and are observed beyond 1 AU than are 1.8 to 5 MeV protons. Solar flare generated particles of high energy pass 1 AU and are observed beyond 1 AU with less interplanetary processing than lower energy particles. 0.3 to 0.5 MeV protons are more subject to interplanetary acceleration arising mainly from interplanetary shocks, primarily during high solar activity where shocks are present sometimes.

Why should the average person be concerned about protons? These particles do impact our Earth's environment; therefore it benefits us to be aware of their characteristics. But we must also recognize the importance of exploring the whole picture. We educate ourselves by focusing on the large as well as the small scale, because one side of the continuum reveals information about the other.

Last modified May 26, 2015
C. Graves and T. Hunt-Ward