Wednesday, November 30, 2011

Predicting Aurora: GOES

Okay, so I got a little distracted and never finished this series. Today I write about the Goes satellite. Two of them actually. GOES stands for Geostationary Operational Environment Satellite. I just learned that from Wikipedia actually, so I guess it doesn't make sense to keep saying "Goes satellite".

One of the very best plots I check when wanting to know if I will see any aurora on any given night is this one:

which can be found here.

If you are looking at this plot from the past two nights you will be kicking yourself for not looking outside (because there was probably some good aurora if you are at a high enough latitude).

So what does this plot mean? Along the x-axis you have three days of time, and along the vertical you have magnetic field strength. There are two traces plotted. The red trace (as seen from the label on the right) is at a longitude of W75 (oh look Mark Twain's 176th birthday, thanks google). I live at a longitude of W . The blue trace is from a satellite that is over the West coast. I guess the main purpose for these satellites is for weather measurements. We use them because they have magnetometers to measure the Earth's magnetic field.

What does measuring the Earth's magnetic field have to do with aurora? Stop getting so distracted by Mark Twain and magnetic fields.

Don't worry, the magnetic field is important. Mark Twain is important for things not related to the aurora.

So imagine the Earth's magnetic field as a perfect dipole. Like so:


If you were a satellite, orbiting around the Earth at the same rate as the Earth's rotation, measuring the just the vertical component of the magnetic field, what would you measure? If you said a straight line, you were correct. As you orbit around the Earth, the magnetic field you would measure is constant, say 90 nanotesla. 

But the Earth's magnetic field is more interesting than a perfect dipole, because it interacts with the sun's magnetic field, aka the IMF (interplanetary magnetic field), and the solar wind. Remember this post? That is what actually happens. So the Earth's magnetic field ends up looking more like this:

The field lines on the Sun-side of the Earth get shoved in, and the field lines on the opposite side get stretched out into a tail. Ignore the labels on that photo that say "acceleration region" and "auroral electrons" because they are wrong/don't make sense.

So now, as your little-satellite-self orbits around, when you are at noon, you measure a stronger magnetic field, say 100 nT. And when you are at midnight you measure a less strong (vertical component) of magnetic field, say 80 nT. If you plotted this, you would end up with a sinusoidal wave. A sinusoidal wave is a realistic trace to see on the GOES plot, and unfortunately that means there will probably not be any aurora. 

The aurora, in the simplest way of thinking of it, is the result of a giant energy release. Which means you need to somehow get energy into the magnetosphere system. This energy comes from the solar wind, which is why we look at ACE as part of predicting aurora. When a lot of energy is being put into the system that magnetic field will no longer be nice and stable, but highly variable. If you look at the plot at midnight (conveniently labeled with an 'M' (and there is an 'N' for noon)), you can sometimes see that tail getting stretched out like crazy, so you are only measuring a magnetic field of 25 nT or even less. This measurement is indicative of a huge amount of energy being put into the system, and that energy needs to get released, and the beautiful result is an amazing light show.

And since we are talking about predicting the aurora, I'll mention that if you see a sudden spike in the magnetic field measured at GOES, this is indicative of aurora to come in 30-60ish minutes.

The main idea: smooth sinusoidal curve is no good, jagged trace with big spikes and dips is very good.

I guess the last main thing I look at for predicting the aurora is the optics from the ground, so that will be up next!

Wednesday, November 23, 2011

Predicting Aurora: ACE and the solar wind

Now we are really getting into the meat of the matter. The ACE satellite is located just upstream from the Earth, so it basically measures the solar wind about an hour before it reaches us at the Earth. But why does the solar wind matter?

The aurora is essentially a result of a huge energy release. This huge amount of energy that gets pumped into Earth's geospace is coming from the sun, via the solar wind. So if we want to know when there will be a lot of geomagnetic activity, we need to know when the solar wind is going to be putting a lot of energy into the system.

This is the ACE plot I look at most often. You can follow links on that page to look at longer or shorter timescales and also at different variables. Here is how to interpret this plot:

In the top panel, the red trace shows Bz which is important. This is the vertical component of the sun's magnetic field (the sun has a magnetic field too!). The white trace shows the magnitude of the sun's magnetic field. A high magnetic field strength is good (say, over 6 nT). Generally, if Bz is high, but also flipping between north and south (large positive to large negative) this is a good thing.

The solar wind speed and density are also important. Ideally, for strong activity you would like to see a high solar wind speed and a high density. Unfortunately, if the speed is high, the density is usually lower, and vice versa. This is like cars on a road, if you let them go fast the spread out, but if they have to go slowly they get bunched up. For solar wind speed, 300 km/s is kind of a baseline. Getting up to 600 km/s is really good. For the density, 1 /cm^3 is baseline, and getting up to 10 /cm^3 is really good.

The NOAA page takes this information and tries to put it in an easy-to-understand "dial" plot, and they will have a little arrow pointing to the current value (like a car speedometer):

This plot can be found on this page.

One of my favorite plots to look at combines the data from ACE and from Stereo-B. Check it out. This shows the solar wind density, velocity, and magnetic field from both satellites, only the data is shifted appropriately along the x-axis so that they should line up in time. This is a great way to see solar wind changes that might be observed at Earth.

Next up will be the GOES satellites, which give us around a one hour predictor.

Tuesday, November 22, 2011

Predicting Aurora: Stereo B

Stereo-A and -B are a NASA satellites that are taking images of the sun. If you looked down on the Earth-Sun line from above, you would find them here:

Stereo-A is named A because it is on approximately the same orbit as Earth, except Ahead. Stereo-B is Behind. So imagine we see a new sunspot, as viewed from the Earth. The sun is rotating, so then Stereo-A will see it, and then (assuming it hasn't disappeared) Stereo-B will see it, and then it will be back in view from the Earth. With these three views, we basically have full coverage of the surface of the sun. This is useful, because if a new sunspot appears on the exact opposite side of the Earth, we still get a warning, because we can see it in Stereo-B. This gives us our ~3 day predictor. [Sidenote: These two satellites are slowly drifting further ahead and behind (while still staying on an orbit similar to Earth's), so we are probably looking at slightly more than 3 days at this point.]

Here you can find the latest images from both Stereo satellites.

The Stereo satellites are also measuring the solar wind. Next up we will talk about the ACE satellite, which is measuring the solar wind just upstream from the Earth.

Monday, November 21, 2011

Predicting Aurora: Kp index

I was recently talking with my brother, who spent his summer closer to the auroral oval than I did. He had a lot of opportunities to see really good aurora and asked what some of my methods are for knowing when the lights will be bright. So I'm going to do this blogging series, Predicting Aurora.

It will be fun.

So one thing he was checking was the Kp index. Kp index is averaged over three hours from ground magnetometers across the US and Canada. Basically, when a lot of geomagnetic activity will occur in conjunction with auroral activity. The geomagnetic fluctuations can be observed on the ground. Then they are averaged and put into this arbitrary 0-9 scale. Usually if Kp is below 3 aurora will be limited or be at very high altitudes. Kp between 4-6 will make really awesome aurora (as long as it is close to midnight over the US & Canada, and not noon). If Kp goes above 6, anyone living below latitudes of 45 degrees should go outside because you could be getting a rare auroral treat.

This is where I go to view the Kp index.

There is another great advantage of the Kp index. You can keep track of it on longer timescales than the 3 day plot linked above, and this gives you a method for predicting activity ~27 days in advance. From the Earth, we observe the sun to have a rotation period of ~27 days. So, if we see a sunspot and observe aurora to occur in conjunction with that sunspot, it is likely that sunspot will reappear again once the sun makes a full rotation. Obviously sunspots don't last forever, so we might only see each one two or three times, but it is still a good predictor.

And lucky for us, someone has already taken the time to keep an updated plot of the Kp index on this conveniently useful cycle of 27 days. So if you go here, you will find that often, spikes of high Kp are on top of each other.

Next up will be how to predict aurora ~3 days in advance.

Friday, November 18, 2011

Love of Contra

Recently I polled my facebook friends with the prompt:
Friends who love contra dancing:
Please tell me one (short) reason Why you love contra dancing.
(I am fine with repeats if your top reason is already posted.)
I didn't get a very strong response, but the responses I got were great and much appreciated. This is what they were:
 It's such a joyous thing to do!
Dancing with you and all the nice people who like dancing!
‎"There is something just so RIGHT about it" Testimonial from Ted via. Peggy Pegster
 I was particularly excited about the last one, because Ted was a great dancer who was always at all the dances. He died (unexpectedly) of a heart attack in the spring. And I totally agree with him that there is something just so Right about it. I've been trying to think of a concise way of describing why I love contra. This is what I've come up with: 
It is an incredibly enjoyable way to connect with people of all ages in a healthy and logical way.

Thursday, November 17, 2011

Trying out my Gimpy hand

I've been trying different things with Gimp more and more these days. I knew it was possible to make  a photo edit and then somehow get that same photo edit onto another photo. I figured out a way to easily do this today. Basically I just add a new layer and make the edits I want, and then click and drag the layer to another photo. If I wanted to duplicate edits on a large collection of photos I'm not so sure this would be efficient, but for now it works!

I started with this photo, which shows the snow accumulating on one side of trees on the top of Mt. Moosilauke. The photo was kind of monotonous, so I wanted to tone down some areas and leave the top right corner brightest. Then I added a nice border.

Then I applied the same edits to this photo, which was taken on the way to the top of the mountain.

These photos were just taken with my little camera, which is why they are on the Adventure blog instead of the Photo blog.

Sunday, November 13, 2011

Style Tip

Last night I went to an amazingly abundant contra dance. Lately at the dances in Norwich there is a fairly constant amount of regulars and also always a handful of newcomers. Last night was a different story. There is a dance organizers conference going on in the Upper Valley this weekend, which brought over 80 dancers who dance often to the dance. Some would call them "experienced" dancers, but that term also has a lot of negative connotations, so we'll just say they were a large group of people who are quite familiar with contra dancing. There were also almost 50 high schoolers from the Mountain School who came to the dance. They have come to the Norwich dance before, and it is tons of fun to have so many people with such high energy. Most of these people would fall into the "beginners" category. Then there was also a larger-than-normal Dartmouth contingent, many of whom were at their first-ever contra dance.

Trying to call a dance for such a varied group is really an amazing feat. You need to choose dances that are easy enough for the first-timers to not get totally frustrated, but also interesting enough to keep the regulars from getting bored. Needless to say, the caller did a fantastic job. Fantastic to the point where it was announced that we were doing the last dance of the evening, and I couldn't believe that three hours had passed in what seemed like one. Last night was an incredibly fun dance, and I particularly enjoyed seeing so many people from the dances I travel to, all at my hometown dance.

However, all of that is not what I intended to focus this post on. Often callers will give "style tips" when they walk through a dance. For example, a common one is "look at the person you are balancing" because a balance is kind of like a greeting, so it is just nice to make eye contact with the person you are greeting and dancing with. There are plenty of other style tips that have been pointed out to me (and hopefully others) at the various dances I've gone to, which I find really helpful in enriching my dance experience by making me feel more connected to the other dancers. To me, a good style tip is just like a little nugget that isn't totally necessary, but if the tip is acted upon, makes the dance exponentially better.

One style tip that I have yet to hear vocalized is regarding eye contact during a do-si-do. I was lucky enough to somehow notice that sometimes during a women's do-si-do, the other woman was sometimes making these intentional moments of eye contact. So we would step into the do-si-do, twirl, make eye contact, step, twirl, make eye contact, and if there was enough time even a third time. Once I figured that little tip out, my do-si-do experience was revolutionized. Typing it doesn't make it sound like a huge deal, but it was. Just trust me. If I encounter a woman who will do this, our dance experience is totally different than when I encounter one who doesn't. The woman and I who make eye contact are dancing with each other. We are "socializing" maybe? The point of social dance is to connect with others, right? Whereas the woman who does not know this style tip is just off twirling as many times as she can, often while admiring her twirling skirt.

Sidenote: Last night after the dance I was so exhausted, and upon trying to comment on how I really liked the twirliness of my skirt which I hadn't worn before stated, "I really liked that squirrel."

So the thing that really surprised me at the dance last night was just how many women are unaware of this do-si-do style tip. The majority of women there, including those from the conference, never danced a do-si-do with me, but rather just danced around me twirling their twirly skirts. Booooorrriiing.

Don't get me wrong. I do love to twirl a twirly skirt, but I prefer to focus my social dance experience on the social aspect. It does make me wonder what other style tips I have not yet picked up on, and that no one is really vocalizing..

Monday, November 7, 2011


Yesterday I ventured out of my comfort zone and ended up at the English country dance. English country dancing is similar to contra dancing in that there is a live band and a caller who teaches all the dances and then calls throughout the dance. I actually think contra dancing was born from English country, but that might not be true. There are also a lot of differences between the two though. The English dance seemed much more varied to me, although this could be just because it was all new to me. In contra there are usually two main formations. At the English dance there were all sorts of formations. There were also all sorts of time signatures. There were a few dances in waltz time, which is not something I've found at contra. And there was a slip jig, which I've only encountered once at a contra dance, and it could have just been an English country dance that the caller just stuck in there without us knowing the difference. The main difference I noticed at the English dance was just the feel. I know that is very vague, but it really was the main difference. I think there was more emphasis on elegance and style.

And above all, going to the English country dance gave me the experience of a dance form originating from the 1600s (I think, again, might not be true). Basically it would have been exactly like Pride & Prejudice, except that Mr. Darcy wasn't there.