Space & Astronomy/Space and Astronomy (October 1993).iso/pc/text/spacedig/v13_6/v13_618.txt

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Subject: SPACE Digest V13 #618

SPACE Digest                                     Volume 13 : Issue 618

Today's Topics:
         POTENTIAL GEOMAGNETIC STORM WARNING
             Re: Gravity vs. Mass
          Re: vacuum energies for propolsion
         Re: New Subject--Solar Collectors (Dual Use)

Administrivia:

    Submissions to the SPACE Digest/sci.space should be mailed to
  [e-mail].  Other mail, esp. [un]subscription requests,
  should be sent to [e-mail], or, if urgent, to
             [e-mail]

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

Date: Thu, 23 May 91 01:23:30 MDT
From: oler <@BITNET.CC.CMU.EDU:[e-mail]> (CARY OLER)
Subject: POTENTIAL GEOMAGNETIC STORM WARNING
X-St-Vmsmail-To: st%"[e-mail]"

                /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

                     POTENTIAL GEOMAGNETIC STORM WARNING

                          ISSUED: 06:30 UT, 23 MAY

                /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\


WARNINGS ISSUED:

  - POTENTIAL MINOR GEOMAGNETIC STORM WARNING
  - LOW LATITUDE AURORAL ACTIVITY WATCH
  - POTENTIAL GIC ANOMALY WARNING


ATTENTION:

     A minor to major geomagnetic storm is expected to begin between 24-28
May. The high risk period extends from 25 to 27 May, while the moderate risk
period extends from 24 to 28 May.  The most recent data indicates that
moderate to strong geomagnetic activity may occur on these dates, caused by a
well placed solar coronal hole.  This coronal hole is the same one which
produced the major geomagnetic storm of late April.  Similar conditions are
forecasted for 25 to 27 May, with increasingly disturbed conditions being
observed from 23 May until 25 or 27 May.  Thereafter, a gradual decline in
activity is expected.  Generally unsettled to active conditions should be
observed by 01 or 02 June.

     Middle latitude geomagnetic activity will reach minor storm levels with
a risk for major storming, particularly on 25 to 27 May.  Periods of major
storming are expected over the middle latitudes.  High latitudes are
expected to experience major geomagnetic storming with a risk for isolated
severe geomagnetic storming over the auroral zone latitudes.

     Planetary A-index values between 35 and 60 are possible, particularly
between 25 and 27 May.  High latitude A-index values could range from 45 to
100 on these dates.  Planetary K-index values are expected to reach 5 and 6.
Middle and low latitudes should witness K values of 5 and 6, particularly
between 25 and 27 May.  High latitude K-indices could range between 5 and 8.
The least active periods during these disturbed days for the middle latitudes
are forecasted to occur between 11 am and 3 pm, local time.  During these
less active periods, geomagnetic activity is expected to range from unsettled
to very active levels (depending on location).

     Auroral activity will become moderate to very high over the northerly
middle and high latitude regions between 25 and 27 May.  This activity could
become visible over the lower latitudes during a fairly brief window several
hours before sunrise.  Lunar phase will interfere with attempts to view
auroral activity until the Moon sets, several hours before sunrise.  This
period of decreasing lunar luminosity (as the Moon sets) will provide
enhanced conditions and darker skies for observing auroral activity, but the
window between Moonset and sunrise (or morning twilight) will become
increasingly narrow with each passing day.  By 27 or 28 May, this window will
close.  It should be noted that the probability of observing auroral activity
during this brief window period over the lower latitudes is not high, but is
possible nonetheless.  For this reason, a Low Latitude Auroral Activity Watch
has been issued for the southerly middle and low latitude regions.

     Auroral activity over the middle latitudes is expected to be moderate to
high.  Areas from the central to northern U.S. may be able to observe periods
of auroral activity on the evenings of 25 - 27 May, particularly after the
Moon sets.  Whether the low latitude regions (equatorward of about 40N (or
35S)) observe any activity is dependent on several factors, including the
intensity of geomagnetic and auroral storming, atmospheric transparency,
availability of a flat northern horizon for observations, etc. If a good
observing site is found, good atmospheric conditions exist, and observing is
done between 10 pm and 5:30 am local time, auroral activity could be spotted
over the lower latitudes.  Use binoculars or a small telescope (with a wide
field of view) to increase your chances of observing activity over the middle
and lower latitudes.  A camera may provide the best overall chance for
spotting auroral activity over the lower latitudes, particularly during the
dark sky periods near the time when the moon sets and shortly thereafter.

     The auroral oval will expand equatorward and poleward during this
disturbed period.  Northerly middle latitudes are expected to penetrate the
auroral zone boundary during the peak in geomagnetic activity between 25-27
May.  High to very high auroral activity could be witnessed during these
periods.

     Southern hemisphere observers in Australia and New Zealand will have
good opportunities to view auroral activity on 24-28 May, with decreasing
probabilities after 26 May due to lunar interference.  Auroral activity
could become visible over the southern sections of Australia (particularly
south of the Australian Alps), and the southern or southwestern regions of
New Zealand if activity intensifies to predicted levels.

     HF propagation conditions will become increasingly disturbed between 24
May and 27 May, with a slow and gradual improvement thereafter.  Conditions
are expected to range from Good to Very Poor.  On 25-27 May, conditions will
become disturbed with generally fair to very poor propagation conditions over
most areas.  Geomagnetic activity is expected to be least active between
approximately 11 am and 3 or 4 pm local time.  During this period, HF
propagation conditions should stabilize somewhat, although ionospheric
conditions will be weaker and less stable on these dates.  Night-time
propagation conditions will not be impressive.  Noise, distortion and periods
of severe fading and flutter will exist, particularly on 25-27 May during the
evenings and early morning hours.  DX will still be possible and no
significant blackout periods are expected (except possibly over the high
latitudes during intense auroral activity), but ionospheric conditions will
not be favorable for long-distance communications.  Signal qualities will
be poor during the evening and early morning hours.  Some stabilization can
be expected during the daylight hours.

     There is a high probability for middle and high latitude VHF auroral
backscatter communications on 25 through 27 May, with a moderate probability
from 24 to 28 May.  Conditions are expected to become favorable for possibly
widespread VHF auroral backscatter communications on these dates.  For best
results, use directive antennas pointed northward (or southward for the
southern hemisphere) at low elevation angles (below 10 degrees).  Some
limited auroral backscatter communications may become possible over isolated
low latitude regions (near the middle/low latitude boundary) if geomagnetic
activity surpasses predicted levels.  Signal qualities will be poor for
backscattered VHF signals.  Use CW for best chances at longer-distance VHF
communications via auroral backscatter.

     Improvements in HF radio communications are expected to occur on 28 or
29 May.  A return to prestorm conditions is not expected until early June.
If storming reaches levels forecasted, the ionosphere will require several
days to recover and strengthen.

     A geomagnetic storm alert will be issued if middle latitude geomagnetic
activity surpasses minor storm thresholds.

PLEASE SEND ANY REPORTS OF DEGRADED RADIO PROPAGATION CONDITIONS, SIGHTINGS
OF AURORAL ACTIVITY, VHF BACKSCATTER COMMUNICATIONS OR OTHER ANOMALIES TO:
[e-mail] (INTERNET), [e-mail] (USENET), OR (SNAIL-MAIL)
TO: SOLAR TERRESTRIAL DISPATCH, P.O. BOX 357, STIRLING, ALBERTA, CANADA,
T0K 2E0.  PLEASE INCLUDE THE DATE AND TIME (LOCAL AND UT) OF THE OBSERVATION,
THE APPROXIMATE LATITUDE/LONGITUDE OF THE OBSERVATION LOCATION, AND A BRIEF
DESCRIPTION OF THE PHENOMENA OBSERVED.  WE THANK ALL THOSE WHO TAKE THE TIME
TO SEND IN OBSERVATIONAL REPORTS.


                /\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\

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

Date: 21 May 91 21:32:23 GMT
From: agate!spool.mu.edu!samsung!munnari.oz.au!comp.vuw.ac.nz!kauri.vuw.ac.nz![e-mail]  (Timothy Banks)
Subject: Re: Gravity vs. Mass

re: Discussion of Gravity at the Centre of the Earth.

Consider a spherical shell. At very large distances compared to the
shell radius R, the gravitational field will look like that of a
point mass M. The lines of force are therefore radially inward, and
equally spaced at all distances from the shell due to spherical
symmetry. Hence the field outside of a shell can be expressed as

       g = - ( G * M ) / r     , where G is the Gravitational constant,
                                 and r > R (r is the distance from the
                                 centre of the shell).

The lines must end on the shell. There are no lines inside, which can be
seen as if there were, they would converge at the sphere's centre - where
there is no mass. Hence, inside a shell, it can be seen that

     g = 0                       r < R

Another way of looking at it is this....if you are at the centre, one
"portion" of the sphere's pull is exactly balanced by a portion on the
opposite side. The "pulls" cancel out. If you move slightly to the side
of the origin, then while you are closer to one side and so experience
greater graviational attraction from there, this is balanced by there
being more mass on the opposite side - which is slightly more distant now.
However, despite each "portion" on this  side now having less individual
pull, collectively this larger region now exerts an equal and opposite
pull to the side nearer you. Oh, how I wish I could draw some diagrams
here! I'm sure my English makes this look much harder than it is. Anyway,
all I can suggest is that you scribble some diagrams yourself, to try
to convince yourself.

OK, now onto a solid sphere, which we will treat as a set of spherical
shells nestled inside each other. Outside the sphere we can write the
field (just like before) as :

        g = - ( G * M ) / r         r > R

where M is now the total mass of the sphere. This can be seen as each shell
now "behaves" as if it were a point mass at the sphere's centre. Inside
the sphere is a little more complicated. We assume a constant density
(which is not true of the Earth),i.e.

       p = M / V = M / ( 4 * pi * R^3)  where V is the Earth's volume
                                        p is the density
                                        pi is pi!
                                        and R^3 means R cubed, (R * R * R)

We now what to find the field at a distance r within the Earth. The shells
beyond radius r have no effect, as we saw above. So, we need only worry
about the mass inside radius r (call this M'), i.e

       M' = p * ( 4 * pi * r^3)

          = ( M / ( ( 4 / 3 ) * pi * R^3)) * ((4/3) * pi * r^3))

          = ( M * r^3) / R^3

and so the field at a point within the sphere is

       g = - ( G * M' ) / r^2

         = - ( G * M * r) / R^3

after substitution. Hence the field's magnitude increases with r inside the
sphere. I'll try some ascii graphics

   --------------------------------------> r
   |\               _________/
   | \         ____/
   |  \     __/
   |   \  _/
   |    \/
   g    R

Actually, beyond r it is a nice smooth line - but I can't draw that here!
:-)

Now for a fun bit: Consider a frictionless straight tunnel right through
the Earth. and not necessarily through the centre. How long to get to the
other side?

Let the x axis be along the tunnel, while the y axis cuts through the
mid way point of the tunnel, and the centre of the Earth. Then, a
particle at point x will experience the gravitational force

      f = m * g = - (m * G * M * r~) / R^2

where R is the earth's radius, r~ is the object's position vector, and M
is the earth's mass. The y component of this force is balanced by the
tunnel's normal force. The x component is then

     F = ( - G * M * m * r * sin (theta) ) / R^2

where theta is the angle between the y axis and vector r. This becomes
(using sin(theta) = x / r to simplify <-- note the assumption here! :-))

     F = - ( G * M * m * x ) / R^2

The x acceleration is then

    a = F / m = - w^2 * x

where

    w^2 = ( G * M) / R^2

(i.e. the simple harmonic motion equation). w is the angular frequency.

So,

    if g = ( G * M ) / R^2

then

    w^2 = g / R

and the period of the simple harmonic motion (oscillation) is

    T = ( 2 * pi ) / w = ( 2 * pi ) * sqrt(R/g) = 84.4 minutes.

This period T is independent of the length of the tunnel, so you could
travel from one city to another in 42 minutes regardless of the distance
inbetween.

OK, thats it, and I hope I have made no major blunders in laying this
out. Those in the know can see the approximations made, and the limitations
of this approach, but I hope that this posting will serve its purpose of
explaining to those with little physics background exactly what is going
on.

Please don't flame me about this...you can find identical write ups in
introductory physics textbooks. So, send /dev/null please!

Thanks!
--
Timothy Banks, Physics Department, Victoria University of Wellington, NZ.
  [e-mail], [e-mail], [e-mail]
  "This groundbreaking work demonstrates that Mars is the only habitable
     planet in our Solar System" : Adam Hilger Ltd Promotional Leaflet

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

Date: 21 May 91 22:19:51 GMT
From: agate!headcrash.Berkeley.EDU![e-mail]  (George William Herbert)
Subject: Re: vacuum energies for propolsion

In article <[e-mail]> [e-mail] (will) writes:
>    Also, does anyone know where Dr. Robert Forward is and how to contact
>    him?

Robert L. Forward
Forward Unlimited
Malibu, California 90265-7783

-george william herbert
[e-mail]

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

Date: 22 May 91 11:29:43 GMT
From: van-bc!rsoft!mindlink![e-mail]  (Bruce Dunn)
Subject: Re: New Subject--Solar Collectors (Dual Use)


     Silicon solar cells have lower output per unit light as they heat up.  To
give some feel for this, an early 1980s NASA publication estimated that power
output for a simple solar array (built for powering an ion-engine orbital
transfer vehicle) would be 179 W/square meter with a cell temperature of 55
degrees C.  With a 2 to 1 reflector system, cell temperature climbed to 106
degrees C and power climbed to 260 W/square meter, short of the 358 W which
would be implied by the doubling of the light.
     GaAs solar cells apparently suffer less from heat-induced degradation of
performance.  They may therefore be more suitable for concentrator type solar
arrays.
     All types of solar cells suffer radiation damage, particularly if they
spend appreciable time in the Van Allen radiation belts.  Silicon cells are
more susceptible to damage than GaAs cells.  Damaged cells can be partially
revived by annealing at elevated temperatures, which heals the
radiation-induced defects.  One approach to using solar arrays in a high
radiation environment is to use GaAs cells in a concentrator array, and run
them at a temperature high enough so that they anneal themselves "on the job".
This is impractical for silicon cells, which have very poor output at
temperatures which allow annealing.
--
Bruce Dunn    Vancouver, Canada   [e-mail]

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

End of SPACE Digest V13 #618
*******************