Magnetic reconnection: a prominent mystery, part 2…
June 11, 2012 Leave a comment
So let’s continue our discussion about the universal mystery of magnetic reconnection. In my previous post “Magnetic reconnection: a prominent mystery, part 1…” we learned about magnetic fields, particularly Earth’s magnetic field. But Earth isn’t the only magnetic field in the universe, in fact most of space is permeated by magnetic fields. The largest and strongest magnetic field near Earth is the Sun’s magnetic field, but every star has a magnetic field and most of the planets do as well. Even the largest moon in the solar system, Jupiter’s moon Ganymede, has a magnetic field– a very intriguing oddity for scientists.
But now back to our real topic of discussion: magnetic reconnection. So what is magnetic reconnection? It’s our leading theory for how a lot of phenomena occur in our universe. We’ve known for decades that large solar flares and blobs of particles called coronal mass ejections (CMEs) come streaming off the Sun. We also have a pretty good understanding that energetic particles, mostly from the solar wind, hit our atmosphere and cause the aurora. We also know that some phenomenon causes odd magnetic field signatures near black holes and certain neutron stars called pulsars. Scientists believe that magnetic reconnection may be the physical process that ties all of these universal phenomena together.
Magnetic field lines aren’t real, they’re just a representation of the force that a positively-charged particle would experience. Generally, the closer together field lines are, the higher the magnitude- or strength- of the field. A basic principle of physics is that things like to be at what we call the lowest energy state. Like a spring for instance, if a spring had a choice between being stretched out or being relaxed, it will choose to be relaxed, that’s its natural state. Most things in the universe choose to be relaxed. It’s the same for magnetic fields. If magnetic fields have the option of being stretched out and really long or finding a shortcut to relax, they’ll take the shortcut. That’s where magnetic reconnection can occur.
The image below is a simple, not-to-scale depiction of the Sun-Earth system and each body’s magnetic field. Remember that the Earth is tilted with respect to its orbital plane (which consequently causes the seasons on Earth).
If it were isolated, the Earth’s magnetic field would look like the Sun’s, but the constant stream of high-speed energetic particles coming off the Sun– the solar wind— actually exerts a pressure on the Earth’s field and causes it to distort. Now take note of the direction of the arrows associated with each body’s magnetic field. In the last post, we noted that Earth’s magnetic north pole is actually in the southern hemisphere, so the field runs from the south pole to the north. The magnetic field that permeates from the Sun, is known as the interplanetary magnetic field (IMF). Now the direction of the Sun’s magnetic field actually changes pretty regularly, but here I’m showing what we call a southward IMF, so it’s opposite to the direction of the Earth’s magnetic field*. Scientists believe that an environment like this, where oppositely directed magnetic field lines come into close contact with each other, is where magnetic reconnection can occur. The diagram below shows an up-close view of what we think occurs. This is what we believe occurs at the boundary between the Earth’s magnetic field and the IMF, a boundary known as the magnetopause. The blue field lines represent Earth’s magnetic field, the field keeping us safe from the highly energetic particles and plasma that are associated with the Sun’s magnetic field (orange). Those orange lines extend all the way back to the poles of the Sun, just as the blue lines “attach” to the Earth’s poles. But as you know, opposites attract, so the field lines make a quick U-turn and “reconnect”. We don’t fully understand the details behind the processes that cause the reconnection, but there are some intense mathematical theories out there.
So why do we care, you might ask? Well look at those newly formed field lines, they’re now basically connecting the Earth to the Sun, and inviting the energetic particles and plasma from the Sun into our safe little magnetospheric bubble. It’s those particles streaming into our atmosphere that cause geomagnetic storms and the aurora. And geomagnetic storms can wreak havoc on our power grid, not to mention bombard our satellites with radiation. You might also notice that in the first diagram there’s a region in the back part of Earth’s magnetic field– called the magnetotail— where reconnection might also occur. Here’s a nifty animated movie from NASA that shows three different possible types of reconnection. First, reconnection causes the magnetic field of the Sun to release, loosing a coronal masse ejection (CME). Then the CME hits Earth’s magnetic field and you get reconnection again– that’s the “peeling back” of the field lines that you see there. And then finally you get reconnection a third time in the magnetotail and particles are accelerated into the atmosphere causing the aurora. This isn’t always the way it happens, but it helps you to get a visual idea.
So that’s why the Magnetospheric Multiscale (MMS) mission that I’m working on at Goddard is relavent. Not only do we want to find out more about this process, that might be one of the most prominent phenomena in the universe, but we also want to better understand how and when Earth is exposed to solar radiation. Learning more about the environment necessary for magnetic reconnection may help organizations like NASA and the National Oceanic and Atmospheric Administration (NOAA) better warn us of impending solar weather.
*Note: The north-south orientation of Earth’s magnetic field is known to flip every 250,000 years or so, but I’m only focusing on the current orientation. It’s a well-documented, but as of yet, very poorly understood phenomenon referred to as magnetic pole reversal. We’re actually way overdue for such a flip, estimates showing that the last one occurred some 800,000 years ago.
- Into the belly of NASA…
- Magnetic reconnection: a prominent mystery, part 1…
- A lifetime goal accomplished: astroian, the NASA intern…