Interview with Jean-François Denisse
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The interview listed below was originally transcribed as part of Sullivan's research for his book, Cosmic Noise: A History of Early Radio Astronomy (Cambridge University Press, 2009). The original transcription was retyped to digitize in 2017, then reviewed, edited/corrected, and posted to the Web in 2017 by Ellen N. Bouton. Places where we are uncertain about what was said are indicated with parentheses and question mark (?).
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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Begin Tape 50B
Sullivan
This is talking with Jean-François Denisse at Meudon on 9 July 1976. Well, can you tell me a little bit about your educational background?
Denisse
Es-que je veux parlais en français ou en anglais?
Sullivan
En anglais, s’il vous plaît.
Denisse
Anglais. That’s not easy for me, but that’s ok. I will try, anyway. Yes, I was a student of the École Normale in France. It’s kind of an advanced school. And, so when I was a soldier during the War, and after the War, I come back in France and at that time I started radio astronomy with Steinberg. We were both at the Laboratory of École Normale and there were Rocard -
Sullivan
Yes, he’d told me about that -
Denisse
Yes, that was the same story that Steinberg had told you. Rocard was one of the men that was in charge of looking on German development and he got there an old radar for following the V2 in London. And so we were both interested in using this. And at that time we were just receiving the first papers of Reber, and so we saw that there was something to do with this. And we installed this old Würzburg mirror and with the French Navy and we use it first for solar observations. And so I was interested from the beginning. I was a physicist of formation; I was no radio electrician nor an astronomer. And I was more looking at the things in the physicist’s way. So I started working in this field and helping Steinberg, and more in charge of the theoretical study of all this -
Sullivan
The interpretation of -
Denisse
Yes, the interpretation of it. So, that was our first work and there’s where I (?) my first very crude paper about what could be the origin of solar radio emission. At that time, we were most interested by solar emission; it was easier to get. The wavelength that we had gave more results on this.
Sullivan
Can we talk about your Revue Scientifique article in 1946? What were your ideas as to what the emission might be?
Denisse
Well, my idea was actually to try to compute what we call now the cyclotronic emission. The idea was to look at how could the cyclotronic emission be seen with electron gyrating in the magnetic field and, well, the idea was rather crude at that time and if you, from the physics and the thermodynamics of the problem, I suspect that if you want to get more than the thermal radiation, even with cyclotronic emission, you have to get this radiation out of the thing. Because we did not have, at that time, much idea about plasma oscillation and so we – I thought that the gradient of density and the magnetic field might help to get the radiation out of the sun. So you see, that was the general idea of the cyclotronic - After all, this stuff was taken up much later on, maybe 20 years after, by Roberts in Australia. But it was more or less the same idea, at the time, but I did not have the real mathematic tools.
Sullivan
Did you know about the magneto-ionic theory of Appleton and this sort of thing?
Denisse
Exactly. That was the only thing that I had at that time. And actually, soon afterwards, I go to the United States for 2 years. I was at the National Bureau of Standards working in ionospheric work mainly, and then - (?)
Sullivan
Which years were those?
Denisse
It was in ’47 and ’48. And then I met Grote Reber who was there, and Grote Reber showed me about this first record of the radio sun, and then I was working on this and I saw some people passing across, it was Pawsey and Van de Hulst -
Sullivan
So while you were at the NBS, you -
Denisse
Yes. And the third one that I met was Fred Haddock was at NRL at that time – one of my best friends there. And so I looked at the record of Reber and then I think (?) I met Covington also and compared with Covington’s records, and I think I was the first to recognize that there were at least two components in the radio burst: and slowly-varying component that I gave the name to this and although the bursting component – solar noise. The solar radio noise was observed already by Australian and British. But actually I saw that under 400 megacycles, it seemed that the quality of the record was changing and the emission was not at all of the same nature.
Sullivan
At higher frequencies?
Denisse
At high frequencies, yes.
Sullivan
Than 400?
Denisse
400, yes. And I saw that there were completely different emissions. Because I had the record of Covington – it was on 10cm. The record of Reber on 50cm.
Sullivan
Now, was this NBS data of Reber’s?
Denisse
Yes.
Sullivan
Or was this back in Wheaton, Illinois?
Denisse
Maybe in Illinois, maybe NBS at that time. He had a daily recording at NBS. Yes. And, of course, I had the recorder to help here on long wavelength, and that was the British, Ryle. So I compared all this and I saw that there were really two types of emission. And I remember Pawsey came across me and discussed with me this problem and he was very interested and then he went to Australia. And paper of Pawsey and Yabsley a very early paper confirming this type of (?)
Sullivan
And when did you publish your idea?
Denisse
I don’t know if you have it here.
Sullivan
In Comptes Rendus in ’49 you talk about Covington’s observations of sunspot numbers.
Denisse
Yes, that’s right.
Sullivan
Oh, yes. So there are three components: the thermal component, the slowly varying component and the bursts.
Denisse
That’s right.
Sullivan
So it must have been when you came back to France?
Denisse
Yes. I came back to France, I wrote this down, but I actually studied this in ’47-’48. I discussed this with Pawsey and with Haddock. I think it was - of course I study this relation – I thought there was a close relation with the slowly-varying component with the magnetic field of the sunspots, and I tried to figure out what the area and the magnetic field came about, and all this stuff. Of course, I forgot about Covington was one of the people I met there -
Sullivan
What else did you learn when you were in the States from that visit?
Denisse
From that visit? I was also very much working on ionospheric work somewhat, and there was McNish that were there working on magnetism also and - NBS was very exciting at that time because the first time standards with NH3 -
Sullivan
Oh yes, the ammonia maser -
Denisse
Not the maser, but the first - And Townes was around, I think. It was the first paper. So I got interested in the theory of this. So I make my thesis there and I work on the thermal radiation, the theory of thermal radiation on this study.
Sullivan
In the corona?
Denisse
In the corona, yes. And so I wrote my thesis there, I think. And this actually was not published and it was only published by Fred Haddock in the translation – I don’t know if you got this?
Sullivan
I have seen the translation. It was an NRL report.
Denisse
Yes, an NRL report. Yes, I only published part of my thesis because the other one was just typewritten – it was in French, somewhat disordered and -
Sullivan
I think that is the first thesis in radio astronomy, actually. I haven’t been able to find an earlier one.
Denisse
Yes. And this is published in America! The first one that was published in America.
Sullivan
Now, what did you find when you looked into the theory of the coronal emission, and so forth?
Denisse
What I found, well, first I found that the slowly-varying component could very well be explained by thermal affair. But that the bursting things need some other mechanism to - but I did not know. But I actually, you know, the idea about the thermal radiation study of plasma was not at all well-known at that time, and I remember that (?) I study this the absorption and emission and the equivalence and all (?)
Sullivan
Radiative transfer?
Denisse
Yes, all this was not known. I remember a paper by Shklovsky that probably discovered most of the things in radio astronomy. Most of the ideas – the new ideas. But he made a very big mistake. (?) I remember he made a paper in Nature where he did not realize that the coefficient of absorption, that in the Appleton-Hartree equation, is actually the equivalent of the stimulated emission in the -
Sullivan
Right, sort of with a negative sign.
Denisse
Yes, negative. So he was adding two things actually in this paper to compute the optical depth. Things were not clear at all about all those fundamental concepts. So one part of my thesis was to try to clarify this fundamental concept on the nature of the ionized gas. There was a similar paper by Townes at that time that was published, but more on galactic radiation.
Sullivan
Yeah, that was in ’46, I think. That was a little but earlier.
Denisse
Yeah, that’s right – a bit earlier. But Townes was discussing also the coefficient of absorption, but not the general Boltzmann equilibrium in the plasma.
Sullivan
Where does your work fit in with Smerd’s work, which came along a little bit later, I think?
Denisse
Probably pretty close, but completely independently.
Sullivan
It was completely independent?
Denisse
Oh yes, completely independent!
Sullivan
Did you reach similar results on the nature of the transfer in the corona, and so forth?
Denisse
Oh yes, yes, yes, yes. Yes, exactly similar results so that I was I think the first to see that it should be a bright ring at short wavelengths. It was discovered afterwards. That the sun should be very oval, and not round, most of the models were (?). And that temperature should be – it was reflecting on long wavelength instead of – so the temperature should be equivalent(?) temperature should be less than the electron temperature on long wavelength. This was the type of thing that one can get out of the study, you see. I give some idea about what could give the bursts, but I don’t think they are of real value now. You know I tried to figure out the complicated mechanism [laughter] to give the bursts, but I don’t think they are really of value now.
Sullivan
I notice a paper here in Comptes Rendus in ’49 when you talk about the expected brightness is 5 times less than people had said before. Is this what you are referring to – is this relative to what Shklovsky had predicted? I don’t have it noted here.
Denisse
No, I don’t remember.
Sullivan
The expected specific intensity or brightness temperature should be 5 times less than what people had said before. Do you remember what that was relevant to? This came from an abstract.
Denisse
Ah yes, because when you write a Boltzmann equation to get the thermal emission, you have the index of refraction that plays a role. And this index of refraction was not taken into account at that time. And when you are – you use the Appleton-Hartree equation in some region (?) refraction goes to zero, so that makes -
Sullivan
I see, the frequency dependence of the index of refraction?
Denisse
Yes, that’s right.
Sullivan
That was not considered.
Denisse
That’s right – not considered properly.
Sullivan
I also noticed that you gave a paper at the IAU meeting in Zurich in 1948, which is the first one after the War.
Denisse
(Yes)
Sullivan
I was wondering if you could remember what the reception for radio astronomy was at that meeting? Was it sort of something that was a minor little thing at the meeting?
Denisse
What? ’48?
Sullivan
’48! This was talked about in an article in Observatory magazine, and apparently you gave a paper on bursts and burst theory. Did you go to that meeting?
Denisse
Yes, in Zurich, I was there. I don’t remember that paper.
Sullivan
I was just remembering – can you remember though how radio astronomy was looked upon at that time?
Denisse
Yes, no, I don’t remember really. Zurich, I was just coming back from the U.S. and I was about to leave for Dakar and I don’t remember. I could try to remember. If I could write it to you -
Sullivan
Ok.
Denisse
If I remember something I could write.
Sullivan
Ok. Now, you gave a paper at the meeting of the American Astronomical Society in 1948, I guess? Where was that?
Denisse
Pittsburgh maybe, something like that. I don’t remember exactly.
Sullivan
And how was this -
Denisse
I thought it was well received by the physicists mainly – physicists, cosmic ray physicists, and people like that were interested by this problem.
Sullivan
And were they at this meeting of the AAS?
Denisse
Yes, there were several physicists interested in this problem of radio emission, yes. I remember there were several questions, I cannot tell you – but I remember. I was presenting more as a study on relation with magnetic field and with sunspot and things like that. And it was rather well received.
Sullivan
What about the solar optical astronomers? Were they listening to this paper? Or was it sort of just a different camp?
Denisse
Hmm. I would say I did not have much connection with the solar astronomers. Maybe from McMath, but not much. The reception was very, very active. I remember meeting physicists in a corner, meeting physicists in a Chicago meeting, meeting all the people at NRL. Even the NBS people were well interested. But I don’t remember that the astronomers were really - It was like in France, more or less. There was not much response in French astronomy about all these things.
Sullivan
Was there in the French physics community though?
Denisse
A little bit, a little bit. But we had mainly contact with the British -
Sullivan
With the foreign radio astronomers -
Denisse
With the foreign radio astronomers, yes.
Sullivan
Speaking of cosmic rays, you mentioned in Comptes Rendus in ’49, you talk about how the electrons that originate in the solar bursts may contribute to the cosmic rays detected at the earth. Although at that time, of course, the electrons had not actually been detected in the cosmic ray flux. And you talked about how the radio stars may contribute to the constant component of the cosmic rays. You said there was a solar component and a constant component.
Denisse
Yes.
Sullivan
So what were you thinking about these radio stars – there were a few of them known at that time?
Denisse
Actually I was thinking at that time of what was proved afterwards, and that was the idea I think is correctly given to Shklovsky, that background radio emission could be a background of cosmic ray. That was my idea. I was pretty much certain of this. But I could not, I had not enough things to publish this because I have made no calculation or things like that. But I figure out that could be quite possible. I remember I wrote a paper in an African review, but I could not even find it now certainly. But I remember I give this idea as a possibility. But it was just a possibility. I could not say that is a real – what we call in France a ballon d’essai – but it is not a really – it is an idea, but it is not really -
Sullivan
Not a solid, well-worked out -
Denisse
Yes. I think Shklovsky makes the point – I think he was the first to make this point.
Sullivan
And Ginzburg also.
Denisse
And Ginzburg. And I think they are correctly – they should be correctly (laughter) given this credit.
Sullivan
Now, what mechanism of emission were you thinking of for the cosmic rays?
Denisse
I was thinking of a kind of synchrotron, but I did not really – because it was the same idea for magnetic field – you know. I was thinking of cyclotron and well -
Sullivan
If you have higher energies, then you’d have higher frequencies?
Denisse
Yes, that’s right. But I think we had to wait here to learn the work of Le Roux that was already calculating the synchrotron radiation. He should be given credit for this, I think in the research paper because – And but as I did not calculate this I consider that those ideas were really just ideas. But I remember in my thesis that got published in ’49, I did try to figure out that there was a relation between the radio emission and cosmic rays, and by that time the cosmic rays were these I think – what was the result? A neutron monitor? I think it was just a neutron monitor of Simpson.
Sullivan
Which result are you referring to?
Denisse
The relation between cosmic rays and the radio emission. Between Covington data -
Sullivan
At that time, I think it was known that there were particles that came a couple of days later after a burst. Right?
Denisse
Yes, the Forbush decrease and things like that. I remember discussing with Forbush about this. Yes. But there were also neutron monitors.
Sullivan
Oh I see. I don’t know about that.
[Interruption here in French.]
Denisse
So, there was this neutron monitor that was from Simpson.
Sullivan
Oh yes, Simpson.
Denisse
Simpson was doing this. That’s why the cosmic ray physicists were interested in my talk when I pass across all those people. Because I thought there might be a connection between the Covington data and the neutron monitor that kept going back and forth. So, that was the idea at the time, I think.
Sullivan
could you make a note about that African paper that you wrote? Maybe if you can find it, because that would be interesting to me.
Denisse
(laughter) Yes. We have to write to Dakar on that.
Sullivan
Here’s an interesting paper: Auger, Denisse, and Daudin in Comptes Rendus in ’49 in which you talk about how the high energy cosmic rays show a sidereal variation which correlates with the galactic background at 64 megahertz. Now, what was going on there?
Denisse
You know people were looking at large showers and they did find that there were a maximum of the showers at the passage of the Milky Way.
Sullivan
I see. In sidereal time?
Denisse
Sidereal time. So it was my idea always that this cosmic ray – this radio background was part of the (regime? region?) of the cosmic rays. So I tried to link the two things, you know.
Sullivan
So you were not involved in the cosmic ray work?
Denisse
No. I was involved. I was just doing this idea that it might be interesting to look for more cosmic ray relations and radio.
Sullivan
Now, did this effect hold up in later years in the cosmic rays?
Denisse
I cannot tell you. Those things are so much (?) in cosmic rays.
Sullivan
I’m not familiar with -
Denisse
I don’t think they relate.
Sullivan
I’ve never heard it before.
Denisse
I know that people looked afterwards – I could try to figure out what is the situation now -
Sullivan
So, the next paper I see here is in 1950 where in Annales d’Astrophysique you give an overall review where you say that the coronal theory seems to be established pretty well. That the chromosphere, however, seems to be showing a temperature at the base of the corona which is much less than the optical people had derived, and there was some problem there.
Denisse
(yes)
Sullivan
And can you tell me a little bit about that?
Denisse
Well, that was largely a result of my thesis that was published in France. The usual paper as I told you, my thesis was not published, so (?) calculations from the temperature and I show how we can deduce the electronic temperature down in the chromosphere by just using short wavelength. But the idea at that time was – Piddington was doing more or less the same job, I would say. You see I published it late – ’50 – and the idea was from when I was in the States actually – the calculations. But it was more or less the same idea. I could not say now – you know the model – it was one of the first models that was done and this model was refined several times. I don’t know where it stands now, but it was certainly the first model of the corona and the chromosphere that was done, together with Smerd’s model and Piddington’s.
Sullivan
Now the solar theorists who were working mainly with optical measurements – did this bother them at all? Did they pay any attention to this?
Denisse
At that time, they start paying attention. I remember people in Holland for instance, you know, de Jaeger and people like that.
Sullivan
Oh yes, the Utrecht group.
Denisse
Utrecht people were interested. I would say, off hand, I would say the Utrecht people were really interested in this and to see if there were other people – maybe interested.
Sullivan
Maybe Unsöld? Possibly?
Denisse
Maybe Unsöld, yes. But this was Giovanelli probably, in Australia. Maybe in England people are doing some work on this. But mainly radio astronomers were doing the job I would say – mainly, mainly.
Sullivan
So once you got back to France, what was the observations that you were working with to try and interpret? It looks like you’ve been, in general, working on the theory of the coronal and the sun’s atmosphere in general. But there were some experiments which you did with Blum and Steinberg, right, which you worked on? For instance, I have one here in which you had very short time bursts – very short duration bursts, and so forth.
Denisse
Yes.
Sullivan
And another one here – this is ’51 – where you talked about the explanation for those bursts. Namely a shock entering the corona with a magnetic field which was enhanced due to the sunspot and so forth. I gather that you were the theoretician for this group, more or less.
Denisse
Yes, that’s right. Yes, so at that time we try any idea, (laughter) and I could not say those papers are really of value. There was a paper that I wrote with Kundu – that was of more value.
Sullivan
That must have been a good bit later though?
Denisse
That was a good bit later, yes.
Sullivan
Yes, I have that one in ’57.
Denisse
I see. Ah yes, that’s right. On the E-layer - ’67 – No – ’57?
Sullivan
Yes.
Denisse
Ah yes.
Sullivan
Also, I see in the Journal of the Physics of Radium that you wrote an article about the general free-free emission and so forth.
Denisse
Yes.
Sullivan
It looks like you were going into the basic physics?
Denisse
Yes, that was the basic physics of the ionized medium. Yes, exactly.
Sullivan
Were there some new results in that paper?
Denisse
No, that was my thesis written for the physicists, you see. That was my thesis written for the astrophysicists.
Sullivan
Right, then you had one written for the Americans in English?
Denisse
Yes, that’s right. [laughter]
Sullivan
At NRL too?
Denisse
Yes, that’s right. Oh, there are some variations, but there were not really new ideas.
Sullivan
Can you tell me a little bit about what the modus operandi was more or less for this group? It seems like – I see Blum, Denisse and Steinberg over and over here. How did you work as a group? Who did what, and so forth? How did you decide what to do next?
Denisse
Yes, the group started at the École Normale, as probably Steinberg told you with Blum and Steinberg. And I came here from Africa, because I spent a long time in Africa. So I came there. I was more or less related to this group and I start working on the earlier papers. Then I go back to the United States. Then when I come back, I stay less than one year – I think, ’50-’51, where we look at this burst and things like that, and Steinberg and Blum were still working together. And I go to Africa again, and I spend three years there in Africa. I was doing ionosphere and also brought with me a radio receiver for solar observation on metric wavelength, and I was trying to look if there were variations of slowly varying component on long wavelengths and so, you know outside the bursts.
Sullivan
Why did you go to Africa?
End Tape 50B
Begin Tape 51A
Sullivan
This is continuing to talk to Denisse at Meudon on 9 July 76. Now why was it that you went to Africa?
Denisse
Well, I went to Africa because I like it there, because I was there during the War. [laughter] I spent three years there during the War. After the War, I came back to the United States – I come back in France, but I was still interested in living in Africa because I like the country.
Sullivan
You just like the country?
Denisse
I liked the country, yes. So I came there. You know we move with an eclipse expedition there, so I keep the receiver that we are moving there for the eclipse expedition, so I go on using this receiver in Dakar, I make some observations there.
Sullivan
Did you know about the eclipse when you went to Africa initially?
Denisse
No, no. We were here in Africa under the eclipse because – it was around ’50 or something like that.
Sullivan
It was - Well, there was one in ’49. That was in France that you observed that one.
Denisse
Ah yes, with Steinberg. I was in the United States at that time.
Sullivan
And when was that one in Africa?
Denisse
There was a publication with Steinberg and Blum on solar shape.
Sullivan
It was ’51 or ’52 -
Denisse
Yes, ’51 or ’52.
Sullivan
Oh yes, September of ’51.
Denisse
Yes, that’s right. So I knew the place where the eclipse was, because I was in Africa at that time.
Sullivan
This was in French Sudan?
Denisse
French Sudan, yes, that’s right. I travelled around during the War, so I knew the place and I said there’s a good place to observe it with radio telescopes. So we moved there to see the eclipse.
Sullivan
I see, so you were there and they came down from France.
Denisse
No, no. I was back in France. I was in Africa during the War. So I knew the place was good for radio telescopes. So we moved there. But when we moved there, we left the thing there because I moved to Africa, you see. And so I used this receiver but it was not really interesting or valuable work. It’s difficult to -
Sullivan
You didn’t publish any work from -
Denisse
No, no. I did not publish except this paper in Africa. Of course, when I came back I go to the Observatory of Meudon. Danjon, who was Director of the Observatory, asked me to lead a group of radio astronomy in Meudon, and then there I called for Steinberg and Blum to move there – all the École Normale group moved there with me in Meudon, and then I became head of this group.
Sullivan
That was what year?
Denisse
Probably ’53-’54. At the time I was (?) About at that time. And so we move all to Meudon and so we were together. When I was in Paris before, Steinberg was head of the group and I worked with him. And when I was here, we changed that a bit, but we were always friends. There were never any problems between us and we had a good time.
Sullivan
So, once you began heading this group, was your first objective to establish a station at Nançay?
Denisse
Yes, our first objective was to establish the station at Nançay. Steinberg started actually before I came, and then I came as the (?) I became head of the station because I was head of the group at that time. But I could say that Steinberg and Blum did all the work in all this stuff and I was just heading the thing. And thinking and moving around.
Sullivan
You were doing the thinking and they were doing the working? Let’s see, if I look now at your papers again, it looks like the next – these are all joint papers with Steinberg in ’52 and ’53 and so forth. Oh yes, tell me about the business about the equatorial diameter of the corona being brighter than the poles – of being larger than the poles.
Denisse
That was the result of the eclipse expedition.
Sullivan
Was this surprising to you?
Denisse
Not exactly surprising. We did not expect so large an ellipticity, no. Actually all the models that were made by the Sun at that time were all circular, you see. It was the first time that we noticed this was really different from circular – remember that Ryle was working with interferometry at that time on the sun, and he confirmed this rapidly and so we were right, actually.
Sullivan
And what did you think of it in terms of theory of the corona?
Denisse
Well, we can deduce from this the temperature of the pole as compared to the temperature of the edge (?) the density and the temperature (?) It was not well known at that time. It was the first result in the course of our corona were deduced by Van de Hulst at that time. It was a model of Van de Hulst that was rather crude, and I don’t even know if it made a difference between the pole and the -
Sullivan
I don’t know either.
Denisse
No, I don’t think so at that time. It was the very beginning of all these things. Well, we were suspecting that there were burst activities at the pole, but we did not suspect that the quiet sun was so elliptic that -
Sullivan
Even in the corona?
Denisse
Even in the corona, yes.
Sullivan
Now what about if we change wavelengths now to the short wavelengths where there was a controversy, I believe, in the early 1950s about limb brightening and whether it existed or not and Steinberg was telling me that he thinks it’s still a controversy, that it’s still not clear whether it exists. But anyway, back in the early ‘50s your observations – and you published with Denisse and Blum in Nature for instance, that there was limb brightening on the sun at 3cm. Is this correct now? Is this what your theory had predicted earlier – that there should be?
Denisse
Yes, there should be, yes.
Sullivan
Right. And so that was exactly what you expected then?
Denisse
Yes, that’s right.
Sullivan
Now, who were the people that were saying that there should be no limb brightening? Or am I wrong. Was everybody agreed on that point?
Denisse
I guess so because that was - I don’t remember. Maybe, oh, maybe there were people, but I don’t remember. Well, there was controversy, I remember, but I don’t see -
Sullivan
What was their reasoning – do you remember that?
Denisse
No. Simply they did not find it – they thought they didn’t find it in their results, but the eclipse showed that it should be some limb brightening. We did not have enough resolution at that time to show limb brightening. I know the people on the interferometer did not have enough resolution, but we had more resolution limb brightening -
Sullivan
Now in ’53 together with – that’s right you weren’t involved in that – it was (Alon, Arsac?) and Steinberg used a variable baseline interferometer – the same as the Cambridge group -
Denisse
Yes, that’s right.
Sullivan
- to once again look at the limb brightening (?) Now, here’s a paper in ’51 you published with Rocard which is rather interesting because he had not published many papers in radio astronomy up until then. But he got interested in this problem, I guess. This was on treating a shock in plasma, that you might get plasma waves, and so forth.
Denisse
Yes, that’s right.
Sullivan
I don’t really know enough about the subject. Is this the general outline of how plasma waves are believed to operate in solar bursts today? Or what was the idea basically?
Denisse
The idea was that when you have a shock, you have two mixing media amount with different temperatures and that gives you a mixture of both – of two distribution functions of the electron. So you might have a two-bump and that gives the right distribution to have an amplification of radio waves. I would not say that nobody now can’t say that doesn’t work. I think that nobody knows really what happens in shock waves, now. For instance, of a Type II burst, what is the mechanism of radio emission? I don’t think nobody knows the real theory for this. So, it was a theory – another one – but I could not say that it was disproved or proved. And Rocard was a good specialist of shock waves. I remember I discussed this with him.
Sullivan
And this also apparently explained the polarization that was observed somehow. Is that correct?
Denisse
I would not say so, no.
Sullivan
Continuing on – we’ve talked about the limb brightening. Now what about this business of R spots and Q spots?
Denisse
That I think, from my idea, that is the first time that we observe what is called now – it’s seen in the X-ray sun, you know, the “black” holes.
Sullivan
The coronal holes?
Denisse
The coronal holes, yes. I think it was the same general idea. You know, I tried to relate the magnetic activity to the solar activity, you know. The first idea of this was Allen’s work.
Sullivan
You mean geomagnetic?
Denisse
Geomagnetic activity with radio activity from the sun. And because I thought that geomagnetic activity was the reflection of the particle emission of the sun. And so it was obvious that there were spots that were very active and had very large radio emission. And I think at that time I did not realize that there were the spots giving type four bursts.
Sullivan
This was before Type IV bursts were recognized?
Denisse
Before, yes, that’s right. So I saw a very large radio emission and I see that was a very active spot with the flares and things like that and a very large radio emission. And so I thought that was a typical spot. And then there were other spots where the magnetic activity decreased instead of increasing. And so I tried to find a “scale” in radio activity where above the given scale of radio emission, I had activity. Under the scale I have no activity, less than there was no spot. And now the thing is clear for me – it was not clear at that time but now with coronal hole it is for me clear. That is, when you have an optical spot active with magnetic field, but with no Type IV burst, there is not sudden geomagnetic activity but you don’t have (?) of the geomagnetic activity – that comes from the coronal hole.
Sullivan
I see.
Denisse
You see? So, you can divide on the sun three places if you like: the coronal hole, where there is no spot and that give the recurrent magnetic activity; the recurrent flux, the spot that just gives no coronal hole but not very strong geomagnetic activity; and the spot where there is the big flare – there is an activity of geomagnetic (?) activity. So I think I actually at that time, [laughter]-
Sullivan
So you think that this idea may come back a little bit?
Denisse
Oh, maybe – Oh, not now, we see the things much more clearly. But I did not explain this at that time. Why were there quiet spots? There were quiet spots because the spots were just implying that it was not a hole. So there were no recurrent activities. But that was the general idea. And the active spots, of course were while there were Type IV -
Sullivan
Looking – now we’ve come up to the mid-50s and by this time the Nançay Station had been established for a year or two and you continued to be the head of the Radio Astronomy group.
Denisse
Yes.
Sullivan
All the way until the big reflector, I think.
Denisse
Yes, yes. I started -
Sullivan
Now, for you, was most of your time being spent with trying to get the government to give you money and things like this?
Denisse
Yes. In some way, but at that time I had time to work in science because we had a very good team, very active with Steinberg, Blum, oh, (?), the young people like Lequeux. We really had time to work. And it is at that time radio interferometry was getting on the sun the first results and we had at that time a really worthwhile experimental results with the Type IV bursts. I think that was really of value and I knew something that was important.
Sullivan
That’s something you did together with (?)
Denisse
Yes, that’s right. We discussed it together, and I can’t know what (?) say I (?) but I could say that we were getting this – he was first of course to spot that there was something unnormal in measuring the record, and we discuss the theory of this and suspect that it could be electron radiation by synchrotron.
Sullivan
And was that the first time that the synchrotron mechanism had been used for the sun?
Denisse
I would say so. You can probably find a paper by Kiepenhauer likewise mentioning this as a possibility. But it was just as I say (?) as I was mentioning the cosmic ray source of radio noise in the Galaxy. I think it was the first time we give good evidence that it should be this and I think this evidence was very well received and interested people.
Sullivan
What were the other important results that you think that came out of the first years at Nançay with – as the arrays got larger and so forth?
Denisse
Well, I think that was the main results of this Type IV.
Sullivan
Type IV?
Denisse
Type IV, yes. And the different type of storms, of noise storm – it was a little bit of meteorology but anyway, I think we had a pretty much thing at that time.
?Sullivan
A bit of what did you say?
Denisse
Meteorology, because it was descriptive but we really, I had the idea of explaining the Type III bursts by plasma oscillation also but I think it is why Paul White published first. I was and we had at that time a very, very good enthusiastic collaboration with all of the people working in this field, like Paul White, for instance, and we had Pawsey. We had extremely good description and I remember I give – I don’t remember what URSI meeting, a general invited paper to talk on the radio possibility of emission on the sun and different type of emission that we could expect due to the structure of the corona and – well, it was a very lively -
Sullivan
Are you saying that there was not that much competition between the groups or that there was competition but it was healthy competition rather than - ?
Denisse
Yes, it was very healthy. I am – I remember with this phrase I would – should not say that – but it was said by (?) who told me, “Well, you really like to discuss your ideas because you have ideas.” (laughter) You know I was always open and I did not try to keep the things I was – I like to discuss. Well, I think we had very good contact at that time. There were no problems between -
Sullivan
Of course, the thing about Nançay was that – it seems to me anyway – you were continually going to higher and higher resolution – longer baselines and yet you haven’t mentioned that as being an important result – the size of the bursts or thing like that. Do you think that really didn’t pay off as much as you thought it might?
Denisse
Well, yes, oh yes, it paid to see the motion of the spot – to see where they are, etc. Yes, that paid pretty much. But it did not pay so much with the Type IV that was a really big phenomenon, you know. It was already new big phenomena introducing the cosmic ray production by the sun, (?) the proton and making a lot of people interested in this. I would say that the year after we discovered the Type IV there were a hundred papers on this, you know. In Japan, in the USA, - hundreds, something like that!
Sullivan
Amazing.
Denisse
Yes, it was really – I think it was interesting.
Sullivan
Can you expand a little bit on these other aspects of it? You mention the polar cap and so forth. I’m not familiar.
Denisse
This polar cap is the effect of the cosmic ray coming through to the polar cap and give a damping of the radio waves. The polar cap absorption.
Sullivan
You mean the earth’s polar cap?
Denisse
The earth’s polar cap, yes. When you have an emission of cosmic ray from the sun it gets to the earth to the pole and then by collision of proton with the atmosphere it ionizes the atmosphere and it give an absorption of the radio wave that pass by the pole.
Sullivan
Now which radio waves do you mean from the galactic background?
Denisse
Yes, from the galactic background. But also for the radio communication for the plane spots.
Sullivan
Oh yes.
Denisse
That was interesting for the planes, you know, using the polar way, and things like that. And so all these radio bursts were very interesting for forecasting activity, you see. All this polar – well at that time we had a big problem of radio communication because most of the radio communication was ionospheric communication. So there were no satellites. So they were interested for forecasting the good or bad communication and obviously that was a thing of importance. And then the radio – the Type IV also was outside (?) – cosmic ray was very well related with large magnetic activity also, big geomagnetic storm. So all this stuff was really -
Sullivan
Are you saying it is more related than the Types I, II, and III?
Denisse
Much more. Yes, there is no question about that. Much more, yes. Maybe not like Type II but anyway, anytime you have a Type IV you really have something big and something significant.
Sullivan
I see. So it really gave the best link in solar activity and fade outs.
Denisse
The best link, yes. It directed the attention to what the right flares, you know because it was difficult. No, the Type III is not a good indicator (?) because it (?) electrons and there are many of them – they are not good indicators. But the Type IV was really proved to be a very good indicator. And furthermore, it gave an interesting insight to look at the corona to see what was the corona as far as magnetic bottle and thing like that – magnetic (?) and how it keeps electrons there moving, and back and forth and what they do. And even now it is not so well thought(?) That first we have to wait for the last space station – American station -
Sullivan
The Space Shuttle?
Denisse
No, the space station before the shuttle launched about 3 or 4 years ago.
Sullivan
Skylab?
Denisse
Skylab to really see the Type IV. Because after a while otherwise they were not really observed.
Sullivan
Now, it just came to mind. Did you try or were you able to get any money from the French military because of the communications value of all this or were all your finances from the civilian government?
Denisse
No, no. But at that time I think we had a good reputation in France and I could not say that we had any difficulty in getting money.
Sullivan
I see. Because, of course, a lot of the American effort in solar astronomy was funded by the Navy and the Air Force -
Denisse
Yes, exactly. But no, not in France.
Sullivan
(Line missing??)
Denisse
Well, we might go inside -
Sullivan
Now I notice that at the Paris symposium in 1958 you gave a sort of summarizing talk about the state of solar radio astronomy. And what would you say was the main theme as you saw it at that time? What were the main problems that were not understood?
Denisse
I think I can answer this now. I have to – I really have to read again this book because I could not say now. I don’t remember.
Sullivan
Do you have any memories from the Paris Symposium?
Denisse
Yes, yes, I remember. It was in ’60, eh?
Sullivan
’58. I mean as to the nature of the meeting and so forth? It seems to me that it was sort of the last time that radio astronomy could be the subject of a meeting.
Denisse
Yes, that’s right. That’s true. After a while, fortunately, they explode in all kinds of way and -
Sullivan
Now what about when you talked at the COSPAR in Nice in 1960? Were you trying to communicate with the space scientists, I guess by going to this meeting, was that the idea?
Denisse
Yes, but my idea, I think at that time, was that we – it was at what year?
Sullivan
’60.
Denisse
’60. Yes, my idea was that we could observe – with space we could observe the earth as the radio emission.
Sullivan
I see!
Denisse
That was the idea, you know.
Sullivan
Which has been done recently.
Denisse
Just has been done, yes, that’s right.
Sullivan
And what sort of frequencies were you thinking of?
Denisse
Oh, ionospheric frequencies. Ionospheric could only be some kind of F2-layer frequency, yes. That was the idea.
Sullivan
Very interesting. And were you trying to get someone to put a satellite up, essentially to - ?
Denisse
Yes. I thought there were these people which had rank (?) would be interesting to do that. And that was the first idea when I engaged Steinberg in doing radio, space research actually.
Sullivan
I see, that’s where that began.
Denisse
Yes.
Sullivan
In ’57 you published a paper in Annales d’Astrophysique about radio source fluxes which you’d measured at 900 megahertz. And this is the first time that you’d gotten involved in radio sources directly. How did that come about?
Denisse
Yes, because Lovell was working on radio source. Lequeux at that time - Lovell mainly, and Scheuer was interested in all this great business and I discussed with Lovell at that time I was trying – we were trying to understand how the Crab emission – Taurus A emission was made up and I think that Lovell was one of the first that was trying to figure out what was the effect of the ultra-violet emission of synchrotron emission and excitation of the nebula. That was the idea. We discuss… And I think – I think – I don’t remember if it is in this paper or another paper – when I have an idea that the – at that time Baade, I remember from Mount Palomar observed some kind of emission from the Crab.
Sullivan
With a polarimeter.
Denisse
Yes, some kind of clouds getting out.
Sullivan
Right.
Denisse
And I thought it might be like the Type IV, you know. I think I wonder where I publish this – but I did publish this – I think not in Comptes Rendus. And that proved to be right after all. It was not so -
Sullivan
When you were thinking of the mechanism for the Crab emission were you accepting that it was synchrotron which Shklovsky had suggested?
Denisse
Yes, yes, that’s right.
Sullivan
So it was more a matter of the details of how the synchrotron worked?
Denisse
Yes, that’s right. My idea was to – that always synchrotron should be kind of quiet emission and that plasma emission should erratic because it was not only a condition of - and so. Another idea that I had at that time for the sun was that emission could be maser emission. But I never wrote anything because I -
Sullivan
Now which emission do you mean, the bursts?
Denisse
The bursts, yes. And I think, I still think that’s probably true. A stimulated… a beam of electrons -
Sullivan
Coherent?
Denisse
Coherently excite the whistler modes. I think that’s probably true. I never had anything – but I never tried to do the calculation – but I think it probably – it might be an idea – this could be true – even now because nobody really knows what the bursts are.
Sullivan
Now, when you were working on the Crab around ’57 or so, I guess it was, were you puzzled at all about what was accelerating these electrons to give you the synchrotron emission?
Denisse
No, I didn’t (?) the pulsar, I would say. (laughter) Well, yes, oh yes, oh I remember we had an excellent idea with Townes – Townes was working at the École Normale at that time and we discussed together – he had the idea -
Sullivan
How do you spell his name?
Denisse
Townes – the Nobel Prize -
Sullivan
Oh, Townes.
Denisse
And Townes was interested in this problem of Crab Nebula emission. And he had the idea that it might be - I was trying to explain the emission of the Crab Nebula by a kind of Fermi process, a Fermi process inside the Crab Nebula and I discuss this with Townes and Townes in discussion go farther and he was saying that we might find to get this Fermi process moving - considered the Crab as a cavity for magneto-hydrodynamic waves – long magneto-hydrodynamic waves inside the cavity.
Sullivan
A resonant cavity?
Denisse
A resonant cavity – more or less resonant cavity, you see. At least a cavity where the wave would be reflected back and forth. And so this waves could be amplified – no, those waves could give by Fermi process that energy to the electrons, just like collision. That was the idea that we discuss. Nobody probably told you about this idea.
Sullivan
I’ve never heard of that, no.
Denisse
No, no, that was never published but I remember – it is interesting because Townes was always interested in radio astronomy – he published one of the first papers of course, he did a lot of very good things. The maser. But this idea was one that was – he was thinking of in Paris and we discussed together about the possibility of -
Sullivan
What was the energy source of the magneto-hydrodynamic wave?
Denisse
Well, in a kind of turbulence inside -
Sullivan
A general turbulence -
Denisse
Yes, turbulence inside the cavity, yes that’s right.
Sullivan
Very interesting. Now you mention Le Roux and of course Le Roux is famous for his paper on synchrotron emission.
Denisse
Yes, yes.
Sullivan
Now did you work with him at all on that – were you his advisor?
Denisse
No, no. Yes, I was advisor but I think – no, no that was a paper of Le Roux I did not – we discussed this but this was his work. I did not -
Sullivan
Now what about this paper with Kundu that you mentioned earlier? Where you found that the 10 cm compared nicely with the critical frequency of the E layer and so forth. You said you thought that was a pretty good -
Denisse
Yes, it was a pretty good theory because that idea was the 10 cm was a source of X-ray emission. But that was exciting ionospheric ultra-violet X-ray emission exciting the ionosphere E layer. And when we observe the sun with space with X-ray, the sun was not observed with X-ray at that time. We did observe that the X-ray emission actually coming from the slowly varying component -
Sullivan
Which correlates with the 10 cm?
Denisse
Which correlates with the 10 cm. So that was an idea that proved to be right at that time.
Sullivan
My bibliography stops at 1960 here but I want to take this through the building of the Nançay telescope.
Denisse
I do not think I wrote very much because – ’60 – yes, because soon afterward I was in charge of the Paris Observatory, and I think my last work here was on plasma oscillation. I was working with a student of Paul Wild – (McLean?) that works now in Australia and has spent here one or two years, I think. And we were working on theory of plasma oscillation.
Sullivan
I see, and when was this?
Denisse
That probably was in ’60-’61 or something like that.
Sullivan
But what about – can you tell me about the early plans for the Nançay telescope? When did you first think about having a large reflector and why did you choose a Kraus type and so forth?
Denisse
We start – Steinberg told me as I told you, because it was at that time – by nature I am not at all inclined to build a large telescope. I like much better to work and do things on little things. I don’t like the big thing. But I thought at that time if we had to provide for the future generation a tool for radio astronomy the obvious tool was to move throughout the galactic work and so the galactic work was starting with 21cm line. The thing at that time. Van de Hulst with (?) I don’t know what year it was probably ’55 or -
Sullivan
The line was discovered in ’51. And then it was exploited years after that.
Denisse
Yes, after that. So we started to thinking about that probably in ’55 or (?) And so our first idea was to build an interferometer. A large interferometer – 30 meter mirror I think on the railroad.
Sullivan
Two 30 meter dishes?
Denisse
On a railroad. I would say so – Steinberg should recall it better than I do. And because we had experience with the two Würzburgs that we had in Nançay and we had good results – I don’t know – one thing that I should say for history that in Nançay, it is an addition I did not work on this – in Nançay the first work of Lequeux shows that probably the radio source were double – almost double. I think it was the place where it was discovered the first.
Sullivan
Yeah. It’s very close to the Caltech work. I’m not sure.
Denisse
Yes, but exactly – you know the Cygnus was proved to be double.
Sullivan
Right, in ’54.
Denisse
’54. Then Lequeux was working here and find 2 more double. (?) So I went to Caltech and I met Moffet – you probably remember and I say well, you know we have two more so probably they are all double. They are multiple. This is (?) So they did the experiment.
Sullivan
They got them interested.
Denisse
They were interested but all this was (?) But I remember this trip to Caltech and I -
Sullivan
When was that, this trip?
Denisse
Ah, Moffet – good to remember, but I don’t know. It is the time that Lequeux probably published his first paper on the use of the interferometer. I don’t remember the year exactly. But it was before the publication of Caltech.
Sullivan
Of Moffet and Maltby.
Denisse
Moffet, yes that’s right, yes. Yes, exactly. And they did not know that at that time. So there were – it’s more or less like Pawsey for the components – slowly varying components in the noise, you know I discussed with him at that time. And that was I think it was a digression. We were -
Sullivan
You were talking about Nançay and choosing what to have.
Denisse
Ah yes, and so we thought it was time to build a large telescope so we tried to build an interferometer more or less like Caltech picked afterwards. It was our general idea to pick this. And – but unfortunately the ground in Nançay was not strong enough to support the railroad. It was too expensive to build there a strong railroad – strong enough. So we have to choose something else. And to choose something else it was a – one solution was to make a large mirror like -
Sullivan
Dwingeloo?
Denisse
Dwingeloo, yes. But well, we are trying to do something larger and it was at that time the Kraus type stuff that give a solution, and we would have been better to use a 2-mirror because it was cleaner to use. So we choose this type of structure that is a little bit awkward but - Well, that gives a large surface, anyway a large collecting area. That proved to be useable and so that was, the reason was – the idea was to build the large stuff at least useable on the 21cm. And to have – and we could not build a moveable interferometer - I think now you had better to build it – at that time we will have – if we had a 2-mirror interferometer moveable we would have a very, very nice tool. Because we had the first idea of the synthesis antenna.
Sullivan
The which, the synthesis?
Denisse
The aperture synthesis. We had it with two little mirror on 3 cm on the sun. And we make our purchase and this was Kundu at that time.
Sullivan
Right, I remembered you talked about the changing hour angle - but now were you actually thinking of the same principles of aperture synthesis at that time?
Denisse
Yes, yes. I was thinking of this because we had different fringe and it was possible to make a starting Fourier analysis. It was obvious with the sun record. But well, we did not pursue this because the sun was a variable spot so it was not possible to use it properly. And we did not have enough power to use it on stars and so we did not pursue this idea. But actually we see the process at work. And so that was the idea when we tried to make a 2-mirror. So we make this – not exactly cross type. We prefer to use the one that we used. Although the cross type, I don’t remember why but - cross type a little bit different but I don’t remember -
Sullivan
What do you mean now? Are you talking about the large reflector or the -
Denisse
The large reflector.
Sullivan
Oh you mean as opposed to a Mills Cross?
Denisse
Yes, yes.
Sullivan
Well, there are a lot of differences.
Denisse
Ah yes, no no, I mean Nançay is not exactly like Kraus.
Sullivan
Oh, like Kraus, I thought you said Cross.
Denisse
No, no. Yes, Kraus, yes. With a minor difference.
Sullivan
Well Boischot was telling me about the difference in design and so forth. Now did the construction and design of Nançay proceed rather smoothly – was it pretty straightforward as to how to go ahead and do it?
Denisse
No, we were lacking of some managesterial support in France for this type of work. We certainly did not have the mastering of this type of work that we would have now in France. The industry was somewhat backward – with respect to what they were in Germany and in England and in USA. And it didn’t improve very much now – now it is the same.
Sullivan
You mean in terms of actually building large structures?
Denisse
Building large structures and of a special device for driving the things, you see all this type. And for electronics too, you know we did not have a special team making good receiver for instance. There were none at that time. Now with space research we have this and the French industry is up to the point now. But at that time it was not the case. So we had difficulties of that sort.
Sullivan
So it slowed things down?
Denisse
No, but we probably could have a better design if we had more experience and engineers from the field. We had to do with a firm that’s making bridge and things like that. We could have better design with firm making planes or -
Sullivan
Or antennas?
Denisse
Ah yes – at that time nobody was making good antennas. So that was the difficulties. I think it was the main difficulties. We could have made something more precise, more handy, if we had a better engineer from the industry.
Sullivan
Well, this takes it to about the point where I’m stopping. Can you think of anything else that we’ve forgotten along the way that you’d like to mention? Any general comments on the development of radio astronomy?
Denisse
No, I don’t see any now. If I ever get something I would write to you.
Sullivan
Well, thank you very much. [Notes from Sullivan, recorded at the end of the tape: After the recorder was off Denisse told me about the excitement that all the groups at that time had – for instance NRL and Wild’s group – when they were always considering what was the next experiment to do – what instrument should be built and everything was so new that you could so many discoveries that it was an extremely exciting time.] That ends the interview with Denisse on 9th July 1976.
End Tape 51A