Solar Probe Plus to be Named the Parker Solar Probe

Howard Tutoring Blog – Supplemental – The Parker Solar Probe

NASA has announced that it is changing the name of Solar Probe Plus to the Parker Solar Probe (see the ABC report here). As a former solar physicist I thought I would share some views I have on this decision, and talk a little about my hopes and concerns about the newly-named Parker Solar Probe.

The Parker Solar Probe (Howard Tutoring Blog - Supplemental - The Parker Solar Probe)
Artist’s depiction of the newly-named Parker Solar Probe, formerly called Solar Probe Plus (courtesty of NASA)

The Parker Solar Probe is one of two solar spacecraft missions that are scheduled to be launched in 2018. The other, Solar Orbiter, will orbit the Sun with a very high incline, being the first spacecraft since Ulysses to measure the Sun and solar wind at high latitudes. The Parker Probe will be the first spacecraft to fly close to the Sun, beating the prior record holder Helios by an order of magnitude. These two missions will be the last of the large solar missions that have been funded. While it’s likely that the next one will be announced in the next round of the Heliophysics Explorer program (the announcement is imminent), it will be several years before that one flies, if it even makes it that far. Note also that the last big NASA mission, SDO, was launched over 7 years ago, so this is not unusual, particularly in the current funding environment.

(Also I don’t want to take away the importance of the DSCOVR mission, which was launched in 2015, but this was a NOAA mission and not a NASA one.)

The orbit of the Parker Solar Probe will consist of several flybys of the solar corona with increasing eccentricity, bringing it closer to the Sun with each one. At its closest point it will be within ten solar radii of the Sun. It will be fitted with an extensive heat shield to protect it and its instruments from the extreme heat that it will encounter during these close passes.

Things I hope that Solar Probe Will Find
Solar Probe will directly measure the solar corona for the first time. Leaving aside the spectacle of the technology needed to build a spacecraft that will actually survive the extremely harsh climate of the corona, here is a list of scientific advancements that Solar Probe will (hopefully) forward, that I am personally excited about.

1. The Coronal Magnetic Field.
Measuring the magnetic field of the corona remotely is almost impossible. We have a way of getting field measurements of the photosphere (via a means involving the Zeeman splitting of spectral lines that is far less reliable than solar physicist make out), and the potential of using Faraday rotation to measure the coronal field is currently being explored (a topic for another blog someday), until now we have had to rely on modeling to estimate the coronal field. The Solar Probe will provide direct measurements of the field, enabling ground-truth testing of the existing models and perhaps assisting other techniques for remote measurement.

2. The Magnetic Field of CMEs.
Coronal Mass Ejections (CMEs) are the primary cause of severe space weather at Earth. The most important parameter of CMEs is their internal magnetic field, since this is the property that governs the rate at which the CME magnetic field will merge with the Earth’s magnetic field when the CME impacts the Earth. This is done via a process known as magnetic reconnection and it is well known that if the CME has a strong magnetic field that is directed southward then it will magnetically reconnect most efficiently with the Earth, allowing the particles and energy within the CME to be dumped into the Earth. While the Solar Probe will not be able to monitor CME activity it might be fortunate enough to pass through a CME during one of its near-Sun passes. It might even pass through one that is heading towards the Earth! If so, it will provide the first measurements of the magnetic field of the CME in the corona that we can compare directly with other instruments nearer the Earth. This, once again, will provide ground truth for models of CMEs near and far from the Sun.

As a side note, I have published two books on CMEs: You can check them out here.

3. The Alfven Point.
The Alfven Point is where the corona becomes the solar wind. In physical terms, it is the point where the solar wind speed exceeds the Alfven speed. In solar terms, it is the point beyond which magnetic field cannot retract back into the Sun. In general terms if a magnetic field line crosses the Alfven point it is destined to flow outwards through the solar wind away from the Sun. It is important to know where this point lies partly because it’s cool to know where the corona becomes the solar wind, but also because we need to know the point at which the solar magnetic field becomes “open”, which assists our understanding of the energy budget of the corona and also with modeling.

Back in 2014 I published a paper with Craig DeForest and Dave McComas where we were searching for the Alfven point remotely. Our thinking was that since everything outside the Alfven point should be moving away from the Sun then we could apply a motion filter to white light coronagraph images that only sees things moving towards the Sun. The goals was to figure out how far from the Sun we got before we stopped seeing things going inwards. Our results were mixed; you can read the paper here. The Solar Probe will fly through the Alfven point several times. It will be very interesting to shed some light on the results from our paper.

4. The Beta = 1 point.
This is not regarded as the interface between the corona and solar wind, but still represents an important physical boundary. The plasma beta is the ratio between the plasma pressure and magnetic pressure of a plasma (the Wikipedia entry is here). In the corona, close to the Sun, beta is small, i.e., the magnetic pressure dominates and the corona is governed by the physics of magnetic fields. In the solar wind, far from the Sun, however, beta is large and so the solar wind is governed mostly by hydrodynamics. The beta = 1 boundary, then, is the point where the magnetic and plasma pressure are equal, marking the transition between magnetic dominance and hydrodynamic dominance.

The vast majority of studies to date work on the assumption that the beta = 1 point is quite close to the Sun, perhaps within 2 or 3 solar radii. I can’t help but wonder whether it might be much further away than this. I spent most of my career working with heliospheric imagers and noted with interest that the solar wind in the HI-1 instrument on STEREO shows a solar wind that appears to be much more turbulent than both the corona and further away. Since HI-1 observes at distances from around 15 to 75 solar radii I wonder whether this is where we might find the beta = 1 point. The Solar Probe will cross this point many times.

Concerns that I have about Solar Probe
Leaving aside the obvious concerns about whether the spacecraft and instruments will survive such harsh conditions, I have two main concerns about the ability of some of the instruments to deliver on their scientific promises.

1. Will the High Speed Mess Up the Particle Instrument?
The Solar Probe will be the fastest manmade object, reaching speeds up to 200 km/s. While we do not know for sure, the Alfven speed in the corona likely varies around this value, perhaps around 50-100 km/s. In the context of a magnetized plasma the Alfven speed is effectively the same as the sound speed, so does that mean that the Solar Probe will be supersonic (super-Alfvenic) during its passage across the corona?

Why would this be a problem? Take a look at this picture of the Solar Probe.

A forward shock may deflect particles away from the ISOIS instrument. (Howard Tutoring Blog - Supplemental - The Parker Solar Probe)
My concern is that a forward shock may form in front of the spacecraft which can deflect particles away from the ISOIS instrument.

That instrument that is circled in red is ISOIS (once called ISIS, but that name has received a bad rap lately). This instrument has cups that guide coronal particles into the detectors enabling us to measure the properties of the coronal plasma. If the Solar Probe was supersonic it would drive a shock in much the same way that there is a bow shock on the sunward side of the Earth’s magnetosphere. Shocks deflect particles around the supersonic body. Would this mean that particles will be deflected away from the collecting cups? And could particles that make their way into the cups have had their properties changed so drastically that they would not resemble the properties of the corona?

2. Will the Imagers See Anything?
One of the instruments on board the Solar Probe is WISPR, which is a widefield white light imager working in a similar way to the coronagraphs and heliospheric imagers that have flown on board other spacecraft. This instrument is supposed to observe features in the solar wind that are flowing towards and past the spacecraft.

The problem is that images of solar wind features do not get clearer by going closer to them. This seems counter-intuitive at first glance, but we have examples of this on the Earth. Have you ever been in a plane that is flying over the clouds? From a distance those clouds look almost solid, almost like you could reach down and break off a part of one. As you get closer to the cloud the image becomes less sharp and by the time you have reached it there is nothing but wind and rain. Features in the solar corona are like this. The corona is optically thin, has a 3-D extent, and the physics of the Thomson scattering by which the light from it is observed has a broad response function. In other words, it is impossible to extract any 3-D information from white light images alone, and the proximity to the image makes it even more difficult to measure. I published an article on the Journal of Space Weather and Space Climate on the difficulties of measuring features in the corona and solar wind. That article focused on CMEs but the same principle applies to all feature. You can read that here.

In short, I fear that WISPR will be a severe disappointment. It could even be a complete waste.

Thoughts on the Change in Name
Gene Parker is colloquially known as the man who discovered the solar wind. While this is not technically the case, the Parker model was one of a variety of models that were around in the mid-20th century that were competing to describe its nature. Parker was the one who got it right, which was verified by direct measurements of the solar wind (the first were provided by Mariner 2). An interesting side story is that I believe that one of his famous papers on the solar wind in 1958 was recommended by rejection by a peer who was reviewing the paper for publication. Good thing that reviewer was overruled!

I think it’s a fantastic idea to honour Gene by naming a spacecraft after him and I like the idea of doing so now since, quite frankly, he may not be around when the next large solar spacecraft flies. I must confess, however, to being somewhat baffled as to why this particular mission is to be named after him. The Solar Probe is specifically a mission to study the corona, not the solar wind. It would have been much more fitting to name the Solar Orbiter after Gene, since Orbiter will spend most of its time in the fast solar wind, which Gene predicted.

When all is said and done, I hope that the Parker Solar Probe will be a huge success and I am glad that it will now be named after the great solar physicist. Some of us think that he should have won the Nobel Prize for physics along with Hannes Alfven.

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