MESSENGER Solves Solar Flare Mystery


In a case of being in the right place at the right time, the MESSENGER spacecraft was able to capture a average-sized solar flare, allowing astronomers to study high-energy solar neutrons at less than 1 astronomical unit (AU) from the sun for the first time. When the flare erupted on Dec. 31, 2007, MESSENGER – on course for entering orbit around Mercury — was flying at about half an AU, said William C. Feldman, a scientist at the Planetary Science Institute. Previously, only the neutron bursts from the most powerful solar flares have been recorded on neutron spectrometers on Earth or in near-Earth orbit. The MESSENGER results help solve a mystery of why some coronal mass ejections produce almost no energetic protons that reach the Earth, while others produce huge amounts.

Solar flares spew high-energy neutrons into interplanetary space. Typically, these bursts last about 50 to 60 seconds at the sun. But MESSENGER’s Neutron Spectrometer was able to record neutrons from this flare over a period of six to ten hours. “What that’s telling us is that at least some moderate-sized flares continuously produce high-energy neutrons in the solar corona.” Said Feldman. “From this fact, we inferred the continuous production of protons in the 30-to-100-MeV (million electron volt) range due to the flare.”
About 90 percent of all ions produced by a solar flare remain locked to the sun on closed magnetic lines, but another population results from the decay of the neutrons near the sun. This second population of decayed neutrons forms an extended seed population in interplanetary space that can be further accelerated by the massive shock waves produced by the flares, Feldman said.

“So the important results are that perhaps after many flare events two things may occur: continuous production of neutrons over an extended period of time and creation of seed populations of neutrons near the sun that have decayed into protons,” Feldman said. “When coronal mass ejections (nuclear explosions in the corona) send shock waves into space, these feedstock protons are accelerated into interplanetary space.”

“There has always been the question of why some coronal mass ejections produce almost no energetic protons that reach the Earth, while others produce huge amounts,” he added. “It appears that these seed populations of energetic protons near the sun could provide the answer, because it’s easier to accelerate a proton that already has an energy of 1 MeV than a proton that is at 1 keV (the solar wind).”

The seed populations are not evenly distributed, Feldman said. Sometimes they’re in the right place for the shock waves to send them toward Earth, while at other times they’re in locations where the protons are accelerated in directions that don’t take them near Earth.

The radiation produced by solar flares is of more than academic interest to NASA, Feldman added. Energetic protons from solar flares can damage Earth-orbiting satellites and endanger astronauts on the International Space Station or on missions to the Moon and Mars.

“People in the manned spaceflight program are very interested in being able to predict when a coronal mass ejection is going to be effective in generating dangerous levels of high-energy protons that produce a radiation hazard for astronauts,” he said.

To do this, scientists need to know a lot more about the mechanisms that produce flares and which flare events are likely to be dangerous. At some point they hope to be able to predict space weather — where precipitation is in the form of radiation — with the same accuracy that forecasters predict rain or snow on Earth.

MESSENGER could provide significant data toward this goal, Feldman observed. “What we saw and published is what we hope will be the first of many flares we’ll be able to follow through 2012,” he said. “The beauty of MESSENGER is that it’s going to be active from the minimum to the maximum solar activity during Solar Cycle 24, allowing us to observe the rise of a solar cycle much closer to the sun than ever before.”

MESSENGER is currently orbiting the sun between 0.3 and 0.6 AU — (an AU is the average distance between the Earth and the sun, or about 150,000 km) — on its way to orbit insertion around Mercury in March 2011. At Mercury, it will be within 0.45 AU of the sun for one Earth year.

Read the team’s paper: Evidence for Extended Acceleration of Solar Flare Ions from 1-8-MeV Solar Neutrons Detected with the MESSENGER Neutron Spectrometer.

Source: PSI

15 Replies to “MESSENGER Solves Solar Flare Mystery”

  1. If my memory serves me correctly, coronal mass ejections are caused by magnetic fields. It starts as a loop of magnetic field lines associated with a sunspot. The loop gradually takes on a more complicated shape that traps some of the coronal gas inside it, and there is a lot of energy bound up in that complicated shape. Eventually the field lines snap back into a more simple shape and the gas is propelled away from the Sun; the stored energy in the twisted field lines is converted into the kinetic energy of the escaping gas. Not a very lucid explanation, I know, but it’s hard to describe without pictures.

  2. CME are magnetohydrodynamical (MHD)events. Knots of magnetic field in a plasma release that energy by pulling free of the photosphere of the sun and throwing the material out in an arc. This article though raises to my mind some questions. Neutrons will move along magnetic field lines, contrary to protons and electrons which will orbit or cycle around them. So if there are neutrons in the material near the photosphere they will move along the field lines, or more really a gradient in the magnetic field.

    To my mind the question is where do these neutrons come from. Protons have a mass of 938MeV and neutrons an additional MeV. That means there has to be energy to generate neutrons from protons when they absorb a high energy electron. But these are nuclear energy scaled processes. The photosphere is 5800K in temperature, which is a number of magnitude lower in energy per particle for nuclear processes.

    The paper quotes:
    Said Feldman. “From this fact, we inferred the continuous production of protons in the 30-to-100-MeV (million electron volt) range due to the flare.”

    Yet these are on an energy scale I should think is far below what is expected for the solar surface


  3. “In the fields of observation, chance favors only the prepared spacecraft” – Louis Pasteur, University of Lille, 7th December 1854.

    At least that’s what I think he said.

  4. I just hoping that proton or neutron fluxes near MESSENGER don’t cripple any instrumentation onboard. I know shielding and spacecraft orientation will help mitigate the problem, but getting hit by a sufficiently powerful and prolonged proton/neutron ‘storm’ might play havoc with some of the onboard electronics.

  5. Lawrence Crowell, in your earlier post concerning the source of energetic neutrons, you mention the inadequacy of the photosphere (in terms of temperature) to create /maintain neutron flux, if I understand you correctly. Might the energy required for for these processess reside in the solar corona? Temperatures there can range from 3-10 million K. (See wiki page on solar corona and theories for its’ creation and extreme temps here: . The above article mentions ” at least some moderate-sized flares continuously produce high-energy neutrons in the solar corona “. Would not the temperatures encountered in the scorching solar corona as opposed to the much cooler photosphere possibly possibly be the energy source needed to sustain this neutron population, at least initially? Just a guess on my part 🙂

  6. The solar corona is kept hot by Alfven waves which oscillate charge particles, electrons and protons. This appears to be the source as I understand it for the extreme heat of the corona. Now let us assume that the temperature is 10^7K. If I use the equipartition theorem kinetic energy 1/2mv^2 = E_k = (3/2)kT, for k = 1.4e^{-23}J/K. So the energy is ~ 5e^{-16}J, per particle. This is then about 3000 eV (3KeV) . Well lets throw that into the Boltzmann distribution exp(-E/kT) and set E as ~1Mev for the energy difference between a proton and neutron. So the probability estimate is P ~ 1 – exp(-10^{-3}) ~ 10^{-3} (Taylor theorem). Not bad, and this is how the nuclear cycle is maintained in the solar interior at comparable temperatures. So energetically it is plausible that a thermal generated bath of neutrons could be produced.

    Of course the next question is whether there is sufficient density of protons and electrons to maintain this process. I would have to look up stuff on plasma physics, an area I am not any expert on, to see if this could be maintained. Maybe a pulse of protons and electrons into the corona has a delayed response in the production of neutrons. This process would have to be robust enough to keep producing neutrons at a reasonable rate, since they do have a ~ 1000 sec half life.

    Lawrence B. Crowell

  7. LBC, thanks for your cogent and informed answer to my previous post. Of course the wiki entry for the solar corona mention the likelyhood that magnetic reconnection as well as wave heating may both be responsible for the extreme coronal heating. And, of course, proton and neutron density are also major factors in this question of baryonic density. Also , fermionic densities would also have a bearing on this discussion.

  8. “Of course the next question is whether there is sufficient density of protons and electrons to maintain this process.”

    Your calculations seem to suggest the effect increases with both density and temperature. CMEs are both hotter and denser than average for the corona, so it’s plausible that CMEs could produce larger numbers of neutrons after they have been launched away from the Sun. Some of these neutrons decay into protons which are able to escape because they are not trapped by the magnetic field lines/

  9. The material in a CME is basically plasma dragged out of the photophere by the intense magnetic field heaving out of the surface as it uncoils and releases energy. I am not very well informed on these matters I have to admit, but I would suspect the temperature of the material is not significantly different from that on the solar surface. I am sure this data exists, all it requires is to measure the blackbody spectrum of the hot material.

    Now maybe this material gets rapidly heated by oscillating magnetic fields, and maybe pockets or strands of the stuff reach 10^6 to 10^7K temperatures. I would think X-ray and gamma ray data could support or refute this.

    I still am a bit perplexed over where these neutrons come from.

    Cheers LC

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