[Cover of Sullivan's book 2009, Cosmic Noise]
Sullivan's Cosmic Noise, Cambridge University Press, 2009


NATIONAL RADIO ASTRONOMY OBSERVATORY ARCHIVES

Papers of Woodruff T. Sullivan III: Tapes Series

Interview with John P. Hagen
At Grenoble
August 27, 1976
Interview Time: 1 hour, 49 minutes
Transcribed by Pamela M. Jernegan

Note: The interview listed below was either transcribed as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009) or was transcribed in the NRAO Archives by Sierra Smith in 2012-2013. The transcription may have been read and edited for clarity by Sullivan, and may have also been read and edited by the interviewee. Any notes added in the reading/editing process by Sullivan, the interviewee, or others who read the transcript have been included in brackets. If the interview was transcribed for Sullivan, the original typescript of the interview is available in the NRAO Archives. Sullivan's notes about each interview are available on the individual interviewee's Web page. During processing, full names of institutions and people were added in brackets and if especially long the interview was split into parts reflecting the sides of the original audio cassette tapes. We are grateful for the 2011 Herbert C. Pollock Award from Dudley Observatory which funded digitization of the original cassette tapes, and for a 2012 grant from American Institute of Physics, Center for the History of Physics, which funded the work of posting these interviews to the Web.

Please bear in mind that: 1) This material is a transcript of the spoken word rather than a literary product; 2) An interview must be read with the awareness that different people's memories about an event will often differ, and that memories can change with time for many reasons including subsequent experiences, interactions with others, and one's feelings about an event.

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Sullivan

OK, this is talking with John Hagen on 27 August ’76 at Grenoble. So, before we get to radio astronomy per se, can you tell me about your background at NRL [Naval Research Laboratory] before then?

Hagen

All the dates I'm going to give you, of course, are from memory and approximate, but I think I came to NRL in 1935 with the expressed purpose of developing centimeter wave techniques because we were just at that time at NRL beginning to work on radar. The radar in the United States was originated, or invented at NRL.

Sullivan

The [Gregory] Breit and [Merle] Tuve thing, you're referring to?

Hagen

No, no. The Breit and Tuve thing was just an observation, the actual development of the techniques for detecting airplanes was started at NRL under Dr. [Albert Hoyt] Taylor.

Sullivan

Taylor, I see.

Hagen

And it was in that group that I was brought in to see whether centimeter wave technique, which was just then beginning, could be developed to the point where they could be used for radar. So we spent some years on making magnetrons and crystal detectors and things like that to attempt to get centimeter radar going. That continued up to the war, but in 1939 [Sullivan: 1940] the British came over and we had a conference with them on radar techniques. It was quite astounding to see how the radar development in England, which, of course, was a secret development as was the radar development at NRL, how much there was in common.

Sullivan

You didn't know about each other's work?

Hagen

No, not until 1939.

Sullivan

But they were working at lower frequencies, weren't they in general?

Hagen

In general, so was NRL. The main thrust at NRL, at that time, was at 200 megahertz. The British also had their main radar at 40 and somewhat higher frequencies. But the British also developed the Boot magnetron, which was a real breakthrough for centimeter radar.

Sullivan

Right. But you said you came to develop centimeter techniques, but you didn't do too much on that in the first few years?

Hagen

The first few years were spent on doing that - on working on developing receivers and on developing magnetrons.

Sullivan

I'm a little confused. But it hadn't gotten to an operational point like the longer wavelengths?

Hagen

No it hadn't. We had in the centimeter region, we worked with- at that time, because of the power limitations on the transmitters, we worked with CW radar rather than pulse and demonstrated the use of CW radar against ships from the laboratory. And also from at sea from a destroyer escort vessel where the meter radar was first tried out at sea.

Sullivan

I see. About when was this?

Hagen

1938 or thereabouts. This was before the British came over and we started the joint venture. And of course, out of the British trip to the States then the Radiation Laboratory developed under the overall committee, whose name I've now forgotten [Sullivan: NDRC]. And then the principal work on the centimeter radar was done in the Radiation Laboratory where they had so many people.

Sullivan

Now let me just ask, this British delegation was in 1939 and, of course, when England got in the war and was the purpose essentially to see if a combined effort would prove fruitful?

Hagen

Yes. Obviously there’d been a high level decision at that time to cooperate with the British so they came over to find out what we knew and to see whether there could be some joint effort.

Sullivan

And, in fact, during the war, it's been my impression that most of the centimeter radar was built in the States and the British kept to somewhat longer wavelengths.

Hagen

More or less that's true, yes. The British did develop their own centimeter radar, which they had on ships. So you can't really write it off with that generality. You'd have to say the British continued their work on centimeter radar during the war.

Sullivan

Okay. But now, what specifically did you work one during the war?

Hagen

During the war, I worked on several projects. One of them was the development of a radar for use in submarines where you couldn't use conventional antennas, of course. And so we built an antenna that was enclosed in a periscope. That radar was used by the submarines who- they don't like to become exposed. I also worked on countermeasures for centimeter radar during the war. And then, one of the long standing projects, which we had going before the war and carried on through the war, was a device for the automatic navigation of aircraft to get a ground speed indicator using centimeter techniques.

Sullivan

How would you differentiate the effort the Rad [Radiation] Lab versus NRL? Of course, it was much larger at the Rad Lab.

Hagen

Much, much larger at the Rad Lab. At NRL, our main thrust during the war was for radar for the Navy.

Sullivan

Was the Army developing its radar through its own lab somewhere?

Hagen

Yes. The Army was doing their own up at Fort Monmouth. And that was quite a separate effort from what was going on at NRL.

Sullivan

During the war, did you learn about [James] Hey’s accidental discovery of solar emission?

Hagen

Yes. Somewhat late in the war- I've forgotten, of course, time doesn't mean anything it's hard to say- but we were aware of it.

Sullivan

Do you know of any experiments that might have been done in the U.S. radar labs that might never have gotten published but you might have heard about? There was [George] Southworth's thing which I know about but of any others?

Hagen

No, I think those that were done were published. There's work on detecting the Moon and work on atmospheric absorption.

Sullivan

By [Robert] Dicke. That was right at the end of the war. So, given the end of the war, what direction was your own work going to take?

Hagen

At the end of the war, we had a large group of people trained in these centimeter techniques and so we cast about for some useful scientific thing to do and radio astronomy looked like the right thing, and we then organized a group dedicated to radio astronomy. Our first thrust was to work with the solar radiation.

Sullivan

You were reading in Nature about the Australians and the British observations, I assume?

Hagen

Oh, yes. There were not many people involved in that kind of work at that time and everybody knew everybody else.

Sullivan

But was this immediately after the war?

Hagen

Oh yes, yes.

Sullivan

Because the first publication I have that you're involved in in any way is 1948, so that's a three year gap in there somehow.

Hagen

Well, the first thing that we did and published was to observe an eclipse of the Sun, which I think was done in either 1946 or 1947.

Sullivan

Yes, I have it here - May, 1947 eclipse and it was published actually in 1949 as part of a Symposium on Microwave Astronomy which I think the AAS [American Astronomical Society] organized. I'd like to hear about that.

Hagen

It was the results of that work were delivered at an URSI [International Union of Radio Science] meeting a couple of years prior to that time.

Sullivan

But that's still May, 1947. Did it take two years to sort of get things rolling?

Hagen

Oh yes. It takes time to get things rolling, to do the experiments, to write it - it takes a couple of years.

Sullivan

Can you tell me what was required in getting things rolling?

Hagen

What we had to do, of course, was to look about and to determine how much of the equipment that was available as a residue of the war could be used and how much we had to construct. And instead of using radar mounts, which many people did, the group at NRL constructed its own mounts specifically designed for solar observation.

Sullivan

Equatorial, therefore.

Hagen

Equatorial, right. One thing about NRL, which you know since you were there, they have an excellent shop. We felt we would have a much better experimental set-up if we started from scratch.

Sullivan

Okay, that helps to explain right away a couple of years. You weren't slopping together stuff in order to do some measurements. You decided to really build specifically the astronomical.

Hagen

That's right. And at that time, too, we also decided that we would do some work at millimeter wavelengths. So at the same time we were developing this 10 centimeter and 3 centimeter effort in radiometers, we were developing receivers and antennas to be used at millimeter wavelengths.

Sullivan

Right. 8 millimeters was, I guess, the shortest that you went, which, of course, is incredibly short for those days - it's still a short wavelength. But before we get up to that, that was 1949 when you gave a talk at an AAS meeting about that. Could you tell me a little bit more about what you needed to do during those two years after the war to get things rolling? You had to get this dish built...

Hagen

Well, that period, it was a transition period - we had a group of 80 people to begin with when the war ended and out of those, we selected the group that would stay on and work at radio astronomy. And then after that, we had, of course, to organize the laboratory and to get the equipment built and installed on the roof of the lab - all of which takes time. Then, concurrently with that, we were getting equipment ready to build for the eclipse.

Sullivan

But you were working 100% on this. It wasn't a matter of one project versus another.

Hagen

Oh yes. Well, part of my group was working 100% on it.

Sullivan

Now you said radio astronomy group- it wasn't called that at that time, I suspect.

Hagen

I believe it was called "Centimeter Wave Research," if I'm not mistaken [Sullivan: R. F. Research Branch, I believe]. I don’t know when radio astronomy appellation was put on, but it was somewhere around 1950 that it began to be called radio astronomy, but I’m not sure of the date [Sullivan: ~1953-4].

Sullivan

Can I just ask- was this a Branch [Sullivan: NRL organizational branch]?

Hagen

It was a Branch.

Sullivan

What the other groups in the Branch were doing?

Hagen

Some of them were doing things I can't tell you. The group was still working on this ground speed indicator, which used centimeter and millimeter readings. But a good deal of the work was being done by the rest of the group was classified, and unfortunately, I don't always remember what was and what wasn't. So I better just not talk about it.

Sullivan

Okay. Can you tell me who was in the radio astronomy group?

Hagen

Yes. And I hope I don't forget someone.

Sullivan

You can check the transcript and think about it later.

Hagen

[Fred T.] Haddock and [Cornell H.] Mayer, [Russell M.] Sloanaker, McCullough, a fellow named [E.] Sees...

Sullivan

[T.] Decker, I see here.

Hagen

Decker, [M.] Schulkin.

Sullivan

[Edward F.] McClain, was he there?

Hagen

McClain. This was immediately after the war. Now Schulkin was not there immediately after the war; I've forgotten when he came into the group. That was a little bit later, but it was soon after the war. [J. G.] Gibson, McKuen- I think those were the principal people in that group.

Sullivan

Okay, and another question that comes to mind is why was the Navy willing to support this kind of research? What was their interest in it?

Hagen

Well, NRL from the beginning was always interested in promoting basic scientific work. A certain portion of the NRL budget was set aside for basic scientific work, of course, that might possibly at some time benefit the Navy. But there was no pressure to show a direct use.

Sullivan

And this came under that heading?

Hagen

Yes. And we were able to convince them that radio astronomy was just a good effort to get into. Principally because it led to the development of the useful radio techniques that would not otherwise have been that early developed.

Sullivan

Can you name a couple of examples?

Hagen

Well yes, antennas. The whole matter of antennas for use at very short wavelengths really developed out of radio astronomy. The first big antenna that was useful at the centimeter and millimeter wavelengths was that 50 foot dish that we built at NRL. So that what was learned there made it much easier- not only there, but of course, don’t misunderstand me, in other radio installations- made it much easier for the communications satellite people, for example, to get on with their [?]. And then the development of receiver techniques, that's the most important part. In radio astronomy you must deal with the most sensitive and the most stable receivers. And so, the thrust for the development of very stable and very sensitive receivers was in those days pretty nearly all from radio astronomy.

Sullivan

I see, there wasn't really a commercial need for them.

Hagen

No, no commercial need at all.

Sullivan

That has developed since. That's an interesting comment.

Hagen

Another point is in the matter of precise calibration. I mentioned a fellow named Sees in this list of people we worked with. Of course, we understood that we had to have very accurate measurements for flux of sources. And initially, we used essentially thermometers and oven techniques or so-called "hot-loads" to calibrate our receiver. But at about that time, which was in the late ‘40s, the use of discharge tubes was introduced and Sees, I think, made the first precise calibration of a discharge tube. Now those things are used all over. So there again, that's another one of the reasons why it was worth the Navy's while to back radio astronomy.

Sullivan

This is a very interesting list, can you think of any others? I was just wondering if interferometry- of course, NRL didn't get into interferometry until really VLBI in the 1960s, I don't think.

Hagen

No. We had a Michaelson interferometer operating in the laboratory in 1936 at 10 centimeters. We never made great use of it; one of the first things that we did do, and we did that internally and never did publish it, was to measure the dielectric constant of material, some materials that we were interested in.

Sullivan

I see, this is before the war now.

Hagen

Before the war when all this stuff was classified.

Sullivan

I was thinking of radio astronomy.

Hagen

No, in radio astronomy, no. We did not use interferometry at that time.

Sullivan

Which is actually sort of an interesting contrast to virtually every other radio astronomy group in the ‘40s and ‘50s. Was this because you were concentrating on the higher frequencies where resolution wasn't such a problem?

Hagen

Well, two reasons: 1) we were concentrating entirely on the higher frequencies. Until we started work on the hydrogen line, anything lower in frequency than 3,000 megahertz. The other reason why we didn't do interferometry was geographic. Our equipment was all on the roof of the building at NRL and had we gone into interferometry, we would have had to go out and find land somewhere where we could build an interferometer we would have to go out and find land somewhere to build interferometers. There was more of a problem with that than it looked to be worth. And furthermore, at that time, there was not a great push to develop interferometry techniques at centimeter wavelengths.

Sullivan

Why?

Hagen

Well, because the facilities for making very large antenna were not yet with us. It wasn't until 1950 or so that we got the 50 foot dish; and it's only with the very large antennas that you can see the weaker sources where you need the higher resolution.

Sullivan

I see.

Hagen

That statement is not entirely correct. There are some things on the Sun of interferometry techniques would have been useful. And we could have, of course, done that with the antenna that we had available, but we didn't.

Sullivan

Well, one can't do everything. Let's go back to the first experiment that was published anyway, is this the first one you did, in fact? There is a report of a burst in July of 1948 in Physical Review by Schulkin, Haddock, Decker and Mayer, and Hagen, at 9500 megahertz using a 10 foot dish. Was this your first real thing?

Hagen

Possibly that's right, yes.

Sullivan

When you got this system working, what did you do with it? Just monitor the Sun and this burst just happened to come along?

Hagen

That equipment we kept in daily operation so that we were able to amass data on the flux of the Sun and analyze that as a function of the sun spot activity. At that time, of course, not so much was known about the origin of the enhanced solar radiation and this was part of the early work in demonstrating that the excess solar radiation, is a function of, or has a very close relationship to the solar activity as evidenced by sunspots.

Sullivan

Right. Did you know about the work of [Arthur E.] Covington that was going on almost simultaneously with yours?

Hagen

I'm not sure when we first became aware of what Covington was doing. I can't give you the exact date, about that time - somewhere around then.

Sullivan

What about the [Sullivan: AAS] Symposium on Microwave Astronomy - it’s one of the earliest such meetings, I think. Where and when was this held? [Sullivan: New Haven- December 1948]

Hagen

Is this the one that was held under the National Academy [of Sciences]? It was held in Washington?

Sullivan

I'm not sure - a few papers were published in the Astronomical Journal and undoubtedly it says there where and when. That was in ‘49, I'm not sure of the meeting date.

Hagen

There was a separate publication on that thing.

Sullivan

Oh, no, you're thinking of the 1954 meeting that was in JGR [the Journal of Geophysical Research].

Hagen

Was it that late?

Sullivan

Yes, the one where virtually all the U.S. radio astronomers got together.

Hagen

Yes.

Sullivan

Yes, that's 1954. This is one that's in AJ in 1949 - the Symposium on Microwave Astronomy. I think it was part of a AAS meeting.

Hagen

Well, my memory's gone - I don't recall that. I'd have to go back and look at the journal.

Sullivan

And you had a paper on your eclipse, May '47 eclipse.

Hagen

Yeah. I'd have to go back and look that up.

Sullivan

Well, can you tell me about this eclipse?

Hagen

Well, the eclipse was a lot of fun. We had a fairly long series of eclipses that we went to and observed at centimeter and millimeter wavelengths. The principal purpose for the eclipse observations later on was to demonstrate the truth of the existence of limb brightening, but in this first eclipse, we had two- well we didn't have a preconceived notion, we hadn't worked up the theory by that time. We had no real prior knowledge of limb brightening, but we were aware of the need to correlate the excess flux with regions on the Sun. So we went to that eclipse to demonstrate that or to investigate whether the excess radiation was confined to small areas and whether they would be eclipsed. And that, in fact, was demonstrated.

Sullivan

In particular, these short wavelengths - this had been demonstrated in long wavelengths by that time. [Martin] Ryle and the Australians had found that spots were smaller than the Sun in the radio as well.

Hagen

Possibly again, I'd have to check the records. The other thing, of course, that came out of this was an indication that it was limb brightening.

Sullivan

When you say you hadn't worked out the theory, were you aware that, for instance, [David] Martyn had a paper in Nature in which he at several different frequencies showed [Sullivan: predicted] how limb brightening sets in as you go to higher frequency, on his theory anyway. That was ’46.

Hagen

Well, I was aware of Martyn's work, yes.

Sullivan

But that was just the results, he didn't go through the whole theory in the paper in Nature. I'm just wondering if your observations were in response to that same thing.

Hagen

Not specifically in response to that, no. Again, I'm not too clear in my memory about the events at that time and the particular sequence of events that led up to that. We were aware of Martyn's effort. That combined with the work of [Thomas George] Cowling in England really led to the theoretical work that was done by [Jean-François] Denisse and by me. Denisse put his effort mainly on the corona, the other part of the Sun; and I put the emphasis on the chromosphere. But the two things fitted together quite well, and my part of the work, at least, leaned heavily on the work of Cowling and the work of Martyn.

Sullivan

Now, was Cowling using radio data?

Hagen

No, no, no. Cowling was purely theoretical on the theory of the diffusion and the theory of impact, which was the basic part of this [?] radiation.

Sullivan

So he wasn't working on the Sun at all?

Hagen

Oh, no, no.

Sullivan

The name didn't ring a bell. Well that's right, finally you published this in 1951 in ApJ and I think also that this is your Ph.D. thesis at Georgetown, if I remember correctly.

Hagen

That's right, it was.

Sullivan

Did you go take time off to become a student or did you do it all part time?

Hagen

I did it part-time and at night.

Sullivan

This is rather interesting, because this is one of the earliest Ph.Ds in radio astronomy, I think.

Hagen

I'm told it is the earliest.

Sullivan

I'm not sure I can think of a counter example, now that you bring it up.

Hagen

I'd be interested if you could. [Donald H.] Menzel, who was on my committee, thinks that it was the first.

Sullivan

Okay. I just talked to him a couple of days ago, by the way. But what I wanted to ask was that this probably meant that you took a lot of courses in optical astronomy and so forth.

Hagen

Well, you see, I started out in physics and astronomy and then the Depression came along and then the war. The Depression led me to NRL and then I stayed on when the war came on and I had previously done graduate work at Yale, so that I didn't have too much to finish off- the graduate work I did at Yale was in Physics, but I didn't have too much to finish off at Georgetown for a Ph.D.

Sullivan

I see.

Hagen

I had, I did have quite a bit of course work.

Sullivan

When you say astronomy as an undergraduate - you took a number of astronomy courses?

Hagen

Yes, and at that time, my intention was to go into astronomy.

Sullivan

Oh really?

Hagen

Oh, yes, after I got into graduate school, but I really entered graduate school in physics.

Sullivan

This explains a little bit more about why you were eager to latch on to this new radio astronomy.

Hagen

Oh, yes.

Sullivan

It combined two of your real interests.

Hagen

That's right. It got me back on the track.

Sullivan

May I ask what undergraduate school you attended?

Hagen

Boston University. And then I did a Master's at Wesleyan working on the properties of quartz by using radio techniques- looking at the decay of oscillation.

Sullivan

Well, to go back to your thesis work on the theory of the solar radiation and radiative transfer, as it’s called now anyway, and so forth, there was similar work going on by, I think, [Stefan F.] Smerd and so forth, but you probably weren't aware of this at the time - this was all just brand new as you were working on it?

Hagen

It was brand new and what his name- [Joseph L.] Pawsey visited the laboratory at one time just as I was about ready to publish this- not to publish it, to put the thesis into Georgetown, this was before I condensed it for publication. I told him at that time of the contents of the theory and what it led to and subsequent to that time, [John "Jack" Hobart] Piddington down in Australia published a paper, which I think actually was published before my paper, dealing with the same topic, but in much more general ways - he just assumed an exponential decay and pressure, which is easy to do. The Australians were working at that time. Also, Smerd and I forget.

Sullivan

[Kevin C.] Westfold, was it?

Hagen

Westfold and there was another fellow, but Smerd and Westfold in particular had been working in this area, but their interest was in the other part of the Sun’s atmosphere.

Sullivan

Because they lower frequencies, basically?

Hagen

Yes. And they did some very excellent work. The work that they did on synchrotron radiation is still good.

Sullivan

Synchrotron you say? That would not be the quiet corona then?

Hagen

No, no.

Sullivan

You weren't dealing with bursts....

Hagen

No, no, I was dealing with the quiet Sun. And the process that I dealt with specifically was [?] radiation.

Sullivan

Right. I notice, also in 1949, that you gave a AAS talk about the microwave observations of the Sun and what brightness temperatures implied at the various frequencies, going from 6700 to two million and so forth. But the important thing I see here is that you're talking at a AAS meeting - in other words, you're talking with astronomers. Does this come out of the fact that you were a student at Georgetown and really felt being part of the astronomical community as opposed to going to an IRE [the Institute of Radio Engineers] meeting?

Hagen

Yes. The important thing here is that at this period in time radio astronomy was a nasty word in the astronomical community. And...

Sullivan

Why was this?

Hagen

Well, astronomers are, don't quote me on this, they’re a peculiar breed of people - they resist what is new. They weren't ready to accept the fact that radio astronomy was telling them things which they couldn't find out by any other means. This is the American Astronomical Society. This is not true so much in Europe nor was it true so much in Australia. But in the United States, this was true. One well known American astronomer, whose name I won't give you, said one time when some work was being considered here on developing equipment for observation of the hydrogen line...

Sullivan

Being considered where?

Hagen

In the United States, "You don't need to do that, why do it, the Dutch have got it all done." So the point - I'm getting back to your question, is that I felt that what we were doing in radio astronomy was astronomy and therefore it ought to be reported to the American Astronomical Society. That’s why I gave the paper there.

Sullivan

That was really an exception, I think.

Hagen

At that time, yes.

Sullivan

And for quite a while, actually. Or maybe I should be careful.

Hagen

Well, I think it's something that gradually came on; most of the time the people in the AAS didn't know what you were talking about. They just didn't know. But they gradually became more receptive.

Sullivan

What you're saying is that you don't mind so much that they don't know what you're talking about, but that they didn't really care to make the effort either.

Hagen

Yeah. At that time.

Sullivan

Can you tell me a little bit about the model that you worked out for your thesis work? What light it shed as opposed to what was known before?

Hagen

One of the big problems in working with the Sun is that optically you cut off at about a height of about 1-2,000 kilometers. Above that the optical depth is so small that observations are just impossible, and so the principal information about the Sun gained through optical observation at that time cut off at about 1,000 kilometers or so. What I did was to take the everybody's radio observations, not only my own, and put them together to show the variation in flux or in effective temperature of the Sun as a function of frequency and then set about using the [?] radiation process as the efficient radiation process to put together an atmosphere that would predict the observed flux as a function of wavelength. And it was questionable in the beginning whether this has any uniqueness to it, but I think by testing the model, I satisfied myself that it is unique. There are not a hundred different ways you can put an atmosphere together and still satisfy the criteria you set up. This also explains our great interest in eclipses. The eclipse measurement gives you a means of establishing height and once you put that measurement in, then the whole question of uniqueness is set to rest. You know that if your heights are right then everything else is okay. So the outcome of this was a model of the sun's atmosphere giving temperature as a function of height and electron density as a function of height. And there have been other models since then. I still, up until very recently, used that same model and it worked quite well. I am on the verge of publishing a much more up-to-date version of this which shows a unique feature, but that is not a part of this story. But based on the same assumptions there are much better observations now on flux, at least 20 or 30 years.

Sullivan

Yes, but what you're saying is that after 25 years, this model basically still holds.

Hagen

It still holds except for the lower part of it. There's a problem here - the radio evidence tends to show the temperature of the Sun's atmosphere falls very steeply as you go in the transition region between the corona and the chromosphere. The solar observations from way back when, back in the ‘30's and then again some made in the ‘40's, show that perhaps without question, that the height is not awfully sure- somewhere 2,000-3,000 kilometers above the limb, the temperature has to be on the order of, depending on the person you talked with and the...

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Modified on Wednesday, 17-Dec-2014 16:28:16 EST by Ellen Bouton, Archivist (Questions or feedback)