Selected Questions- Answers From All Experts Astronomy Forum (General Astronomy Information)

Question: i need information on astronomy, anything...what it is, it's history,
etc... thank you. Wolfie, Tacoma Park, Md

Answer:   'Astronomy' is actually quite a vast area to try to explain at one time
because it includes a number of large sub-disciplines:

astrometry - the study of star positions and how these change over time
with celestial coordinates

celestial mechanics - the prediction of the future positions of the
planets, Moon, etc. using orbital elements

 stellar astrophysics - the study of the physics of the stars, and their
evolution, changing properties

 solar physics - the study of the Sun, its physics and properties, as well
as terrestrial effects

radio astronomy - the study of celestial objects using sensitive antennas designed

 to receive signals  in the radio spectrum

galactic dynamics  - the study of the dynamics and physics of the galaxies, galaxy 
clusters, and their origin, evolution, 

cosmology - the study of the large scale structure, origin and evolution of the universe.

Each one of these - particularly the respective history of each sub discipline - 

would probably take several large books to even survey, far less 'explain'. 

What this shows is that like many other scientific disciplines, astronomy has grown
and developed beyond simple description or being one simple science. When
people become astronomers today, in fact, they have to specialize in one
area, say solar physics - they can't just do astronomy. It's too much!

As for the history, or any one part of it, as you can gather from the
above it's far beyond the scope of any practical (and even partially
complete) response here.  Hence, rather than expect a concise set of 

information it is better to go out and get basic books to learn the 

knowledge and master the principles on your own.

Many of the basic books are somewhat dated, but the essential (foundational)
 knowledge  and physical principles remain intact. One of the best is Sir Fred
 Hoyle's 'Frontiers of Astronomy' which  is  available online:

https://archive.org/details/frontiersofastro031522mbp

His textbook, 'Astronomy and Cosmology - A Modern Course' is also excellent

and very readable.  Both of these books provide astronomy insights across a wide

spectrum of subjects. 

As for a concise history of astronomy as an overall subject, an excellent book

 to read - which should be available at any public library- is:

'A Short History of Astronomy: From Earliest Times Through the Nineteenth
Century' by Arthur Berry, Dover Publications, 1961.


If you are serious about garnering more information about astronomy and its
 many facets, the  approach I've outlined here will be a major help - as well as
 the books cited.  You can, of course, also find numerous other good texts on
your own simply by use of appropriate Google inputs.

Solar and Space Physics Students Being Left Out In The Cold

Giving a course in basic space physics ca. 1988.

The report appearing in Eos - Earth and Space Science News (15 July) was distressing to say the least. It noted the current extreme difficulty of newly minted Ph.D.s in solar and space physics getting full time academic research positions.  And despite the critical role space and solar physics plays in our daily lives - from the interference of large solar flares with airplane navigation systems to the solar wind's interaction with the magnetosphere (see image) to coronal mass ejections, solar and space physics remain relatively tiny enterprises.

Indeed, according to the report, only 50 to 70 students in these disciplines graduate per year in North America. This is literally pathetic given that we need many more of such specialists, over say more investment bankers or Wall Street brokers.

The cruel facts? Despite the National Research Council's finding that nearly all of these grads desire research positions, only about a third find them - and even those don't last for long (especially if the person fails to get tenure.)

A lot of this has to do with the rise of temporary faculty in many universities, These adjunct professors are now approaching nearly half of the total faculty population. They are only paid per credit hour, have zero financial security and other benefits. Instead they  must cobble together enough hours to forge a  decent career. Even then they must often fall back on food stamps, e.g.

http://www.salon.com/2014/09/21/professors_on_food_stamps_the_shocking_true_story_of_academia_in_2014/

This has to make any old line  or retired academician wince given the entire purpose of getting to graduate school and hopefully gaining the coveted Ph.D. was to finally ensure financial security. Not necessarily to get rich, but just be financially secure to the point you don't have to depend on government handouts.

The other aspect, of course, is grant funding for research projects. If such money isn't available, and it generally comes from the government, then the 'well' of that academic department will dry up .

The sad fact today for the aspiring Ph.D. is that even if they make it through the sundry  hurdles there are no assurances of glory...or academic jobs. As I noted in earlier posts, the newly minted Ph.D. must compete with postdocs as well as adjunct lecturers and already ensconced TAs (and RAs!). Since most universities (especially after the credit meltdown) are on tight academic dept.  budgets (most of the $$ going for jazzed up dorms and athletic centers), it makes more sense to hire more adjuncts or keep postdocs longer than to bring in full time Ph.D.s. A potential to generate research funding is the main item which may elicit a university to look more closely at a prospect, but even here those prospects suffer because of the current impetus toward congressional budget cuts - given the size of the deficit.


Still, two authors cited in the report - Moldwin and Morrow ('Space Weather', 2016) found that success was more likely for a grad who: a) published a full paper within a year of submitting their dissertation, and b) who continued to publish for at least 4 more years.  They also found that 'there was no relationship between department size and a Ph.D.'s likelihood of finding a research career".

Nonetheless the pair still found "large differences between individual institutions" This is understandable because in many schools space and solar physics will have relatively small or poorly funded departments and the subjects may be viewed as too irrelevant or obscure to attach importance.  Indeed, the authors recommend "investigating financial support models, admissions policies and qualifying exams as well as department culture"..

I was fortunate in my solar physics research as the Barbados government paid for a full four years of it, since the importance of solar flares and their geo-effective influence was grasped. Others without such funding didn't fare as well. It was that funding that enabled me to publish multiple papers even before the dissertation was completed, see e.g.

http://brane-space.blogspot.com/2016/06/establishing-physics-identity.html

One hopes that these recent graduates will eventually be able to find needed funding, grant money to allow them to achieve successful careers in their chosen fields. A necessary component of that is not attaining a position that translates into becoming food stamp dependents.

In the meantime, those of us fortunate to have had such careers - even for a decade or so - must now share our insights into solar and space physics to make the taxpaying public more aware of their importance.

Establishing A Physics Identity

Front cover of my 400+ page thesis, and me (ca. 1981) recording magnetic classifications of sunspots, estimating their areas using a solar graticule.

In a recent issue of Physics Today (May, p. 47) we learn ('Developing Physics Identities') of the various tracks to establishing a "physics identity".  The one that most caught my attention was "When you have your own ideas and your own project. Basically you're working on something not having been spurred by someone else telling you to do it."

After passing two 3-hour physics qualifying exams for the M. Sc. degree,  I was asked by my adviser whether I needed "ideas for the thesis". I told him 'no, thanks', as I had already been working on a problem of my own for the past two years. This concerned how one might use morphological and magnetic features of sunspots to predict geo-effective events, specifically flares that incepted sudden ionospheric disturbances or SIDs.  Thus, my thesis was born, leading to no fewer than six published papers in peer-reviewed journals- twice as many as any other staff members in the time.

The title of the thesis became: 'Can Sunspot Morphology Be Used To Predict The Occurrence of SID Flares?'  and had been subsequently advanced for  Ph.D. submission at my advisor's request after the publication of the first 3 papers. Exemption from the oral exam followed but the conferral of the Ph..D.  was delayed by one outside examiner (from Sweden) to "enable further observational work via the Solar Optical Telescope" (Not the Hinode one!).

Alas, after 1 1/2 years the SOT was terminated in the NASA budget under GOP pressure - and with Reagan's military expenditures already reaching $1. 7 trillion, leaving the Ph.D. stillborn. See e.g.

http://brane-space.blogspot.com/2011/06/telescope-that-never-was.html

An M.Phil. (Master of Philosophy) higher research degree, next thing to a Doctor of Philosophy or Ph.D.,  was awarded instead.  This despite the fact my thesis had produced no fewer than five papers based on original work and insights. And at 401 pages (including 68 pages of Appendices) the thesis was more substantial than most doctoral dissertations - and with more published papers engendered.

The strategic basis and motivation for my work was originally published in a paper appearing in The Journal Of the Royal Astronomical Society of Canada, e.g.


http://adsabs.harvard.edu/full/1983JRASC..77..203S

The stand-alone authorship is especially important because it discloses the person's ability to write a full scientific paper on his own, and have it published professionally. This proof alone qualified me for Full Membership  (as opposed to 'Associate') in the American Astronomical Society, despite not having the Ph.D. (My 2011 monograph, 'Selected Analyses in Solar Flare Plasma Dynamics' is the equivalent of the doctoral dissertation that never was, i.e. had the thesis been so accepted without the need for the added SOT inputs).

Not long after writing the first four chapters of my thesis, they appeared in distilled form, in two lengthy papers published in Solar Physics, e.g.

http://adsabs.harvard.edu/full/1983SoPh...88..137A

And a somewhat shorter paper that followed, e.g

http://adsabs.harvard.edu/full/1984SoPh...92..259A

Given all the material in the papers was basically from my thesis, the only elements not prepared by me (in the 2 co-authored papers) were the assorted final (publication ready) graphs and line diagrams. These were prepared by my thesis advisor (Dr. A. Achong) since he had the special technical implements to do the quality figures required. (This was for Solar Physics. ) The JRAS - for my original paper given above - didn't have these technical drawbacks and could convert ordinary black ink line drawings-figures to page compatible ones, hence I drew all those myself).

At times it was amusing leading up to the papers' submission, because I had to bring Achong up to speed on the statistical basis for the papers (e.g. Poisson distributions in the 2nd) and the physical basis for the interpretation in the first, i.e. using vector magnetograms  to support a mutual polarity intrusion leading to enhanced magnetic field gradient and higher probability of solar flare. However, Achong insisted his name go first because he was my advisor and "How would it look otherwise?"

Never mind, the ability to conceive and forge a project of my own - an extended, original work with numerous data sets and parameters - showed me I irrevocably qualified as a solar physicist. This project also sealed my identity along with my contribution to perhaps the most monumental space and solar physics project ever undertaken, The Solar -Terrestrial Predictions Workshop e.g.


The massive effort saw the input from over two  hundred solar and space physicists covering every aspect of the problem of solar-terrestrial interactions, including: long, medium and short term solar  forecasting, associated  geomagnetic activity and auroral (substorm) forecasts, as well as ionospheric predictions.

 My paper on the statistical limits to solar flare prognostication appeared on pp. 276-284 of the Proceedings and received much attention from the other contributors - since of course it impacted in multiple ways on their work as well.  The paper followed from the last  two chapters of my thesis and was based on the concept of a Poisson-based “delay time” for build up of magnetic free energy,  first postulated by me in early 1984,  for application to “SID” (sudden ionospheric disturbance-generating) flares, with the release attendant on a change in initial free magnetic energy (E _m = B²/2m ) given by[i]:

¶ / ¶  t  {ò _v  B²/2m  dV} = 1/m  ò _v div[(v  X B) X B] dV 

 -   ò _v  {h_an | J_ms |² }dV       

where the first term on the right side embodies footpoint motion, and the second, joule dissipation, but with J_ms the current density at marginal stability – since the marginal stability hypothesis is required for a driven process, and h _an  is the anomalous resistivity. In the same paper, it was shown how the flare distribution corresponds to a Poisson process of the form P(t) =    =   exp (- l)   l^t  / t!, where theoretically the Poisson mean rate of occurrence is: l_m =   l Dt, , with Dt  = t,  assuming the time interval Dt  = 1d. In reality, measuring constraints (say achieving uniformly equal time intervals between successive Mt. Wilson magnetograms), will usually ensure  Dt ¹ 1d, so  Dt  ¹  t  thereby introducing a selection effect variability, complicating computation of P(t).

 It was also suggested, but not proven, that variability in l arises from  variability in vertical magnetic gradients (B_z) and critical changes in the associated current density at marginal stability (Jz_ms) such that:  d(Ñ(+ B_z) ) Þ d Jz_msÞ dJ_z  ( dt)/ dt where J_z  is the vertical current density associated with putative footpoints magnetic induction (+ B_z ) and rate of change in |B_z| modulated by significant evolutionary changes (dt) in the lifetime of the magnetic field, especially critical if   dt < Dt.

Since magnetic gradients and associated scale lengths (ℓ_B) also will change in time, there would be scope for accepting a Poisson process of form P(t) =  =   exp (- l)   l^t  / t!  which would embody an energy modulation with some inbuilt variance, with the latter having to be known to determine how much energy might be released and when. In other words, the differing scale factors inevitably introduced variabilities that were difficult to account for. The Poisson statistics therefore had to be able to take these differing modalities into account.

Below: Photo taken after being awarded the M.Phil. in Physics:

-------------------------------------

[i] Stahl, P.A. and  Achong, A.: 1984a, Proceedings of the Second Caribbean Physics Conference, Ed. L.L. Moseley, pp. 1-11.

Looking at Bessel Functions & Applications

Plot of Bessel function of the first kind, J_α(x), for integer orders α = 0, 1, 2.

Among the most important special functions is the Bessel function. In the field of solar physics, for example, it's of inestimable importance in the analysis of solar magnetic fields and their evolution. One very key equation is the Helmholtz, viz.

1/ r  [¶/ ¶r  ( r  ¶/ ¶r)] B  +  (a)² B = 0

where r is the radial coordinate, B  the magnetic field intensity, and a a quantity called the "force free parameter". Then the axially symmetric (i.e.- in cylindrical coordinates r, z, q) Bessel function solutions are

B _z (r)    =   B_o J_o(a r)  

B _q (r)  =  B_o J₁(ar)

where the axial (top) and azimuthal magnetic field components are given, respectively, and  J_o(a r) is a Bessel function of the first kind, order zero and J₁(ar) is a Bessel function of the first kind, order unity. (See graphs at the top for Bessel functions of the orders 0, 1 and 2).

The Bessel functions are mathematically defined (cf. Menzel, 'Mathematical Physics', 1961, p. 204):

J_m (x) = (1/ 2^m m!) x^m [1 -  x ²/ 2² 1! (m + 1)  +  x⁴/ 2⁴2! (m + 1) (m + 2) -  ….(-1)^j x^2j / 2 ^2j j! (m + 1) (m + 2)…(m + j) +  …]

which we terminate with second order terms.

For m = 0 and m = 1 forms one gets:

J_o(x) = 1 - x²/ 2² (1!)² + x⁴/ 2⁴ (2!)² - x⁶/ 2⁶ (3!)² + ......

and:

J₁(x) = x/ 2 - x³/ 2³ ·1! 2! + x⁵/ 2⁵ ·2!3! - x⁷/ 2⁷ ·3!4! - .....

The equations in B _z (r),  B _q (r),  with the special Bessel functions at root, are critical in describing the respective magnetic fields for a magnetic tube. For a cylindrical magnetic flux tube (such as a sunspot represents viewed in cross-section) the “twist” is defined:

T(r)  =  (L B _q(r))/  (r  B _z (r))

Where L denotes the length of the sunspot-flux tube dipole and r, the radius. If the twist exceeds 2p then the magnetic configuration may be approaching instability and a solar flare.

Problems for the Math Maven:

1) Compute the Bessel functions for J_o(x)   and  J₁(x)  with x = 1 and then compare with the values obtained from the graph shown at top.

2) Find the twist in a solar loop (take it to be a magnetic tube) if: B _q(r) = 0. 1T and B _z (r) = 0.2T. Take the radius of the tube to be r = 10 ⁴  km and the length L = 10 ⁸  m.   Is the tube kink unstable or not? (Kink instability is said to obtain when: T(r) > 2p)

3) Compute the intensity for the azimuthal magnetic field component (i.e. B _q (r) ) of a large sunspot, if its equilibrium magnetic field B_o = 0.01 T and the value of  J₁(ar) conforms to a = 0.4 and r = 40.