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NAIC Celebrates 40th Anniversary

=D2What we celebrate today is the scale of achievements that are =
possible=20
when creative people, nurturing institutions, governments and=20
supportive citizens work together in common purpose.=D3 These words of=20=

the NAIC Director, Robert Brown captured the sentiment of the 40th=20
anniversary celebration of the Arecibo Obsevatory.

On Saturday, November 1, 2003 a 4-day event kicked off with the=20
attendance of many people closely associated with the founding,=20
building, and shaping of what is still the world's largest and most=20
sensitive radio telescope. In addition, visitors from local and state=20=

government and from the Cornell Club of Puerto Rico joined the=20
observatory scientific staff for the festivities. The visitors toured=20=

the site, visiting the edge of the dish and witnessing a pulsar=20
observation in the control room, accompanied by entertaining=20
descriptions by their tour guides. Following the tours, and during a=20
heavy rainstorm, the attendees gathered in the Angel Ramos Visitor=20
Center for the anniversary ceremony. Later, with the telescope=20
shrouded in mist, they assembled on the viewing platform for a cocktail=20=

party.

The culmination of the ceremony was the keynote address by Arecibo=20
Observatory founder Dr. William E. Gordon, entitled "The Arecibo=20
Story."

Gordon=D5s imposing figure approached the podium as the 135 invitees in=20=

the audience expressed their appreciation. He began by remarking =D2I do=20=

not know how we ever built it.=D3 Indeed, forty years later it still is=20=

an engineering marvel, recognized officially as such by the American=20
Society of Mechanical Engineers and the Institute for Electrical and=20
Electronics Engineers.

Gordon showed photos from the time of construction, which took an=20
amazingly short time from conception to inauguration. He mentioned that=20=

it had been Ward Low at ARPA who thought that a parabolic design as=20
initially suggested, was a serious limitation, and who suggested to=20
Gordon that he should get in touch with the Air Force Cambridge=20
Research Laboratory, where spherical reflectors and line feeds were=20
being designed.

The instrument was designed to have a ten-year lifetime. Forty years=20
later Gordon expressed the thought that Arecibo will be around for=20
another forty years. =D2One of the most exciting few days of the=20
construction was the lift of the triangle=D3 (a reminder to many of us =
of=20
the more recent lift of the Gregorian dome). =D2A big cheer went up =
when=20
it was completed,=D3 Gordon said.

Gordon admitted that at the dedication in November of 1963, with=20
congressmen, generals and the governor of Puerto Rico, the honorable=20
Luis Mu=96oz Mar=92n present, he had a person ready at the platform to=20=

manually activate the switches in case the control room hardware=20
failed. Gordon did not need him.

The observatory held a staff party on Sunday, and a 2-day workshop=20
highlighting historical achievements of the observatory and charting=20
its future directions. The workshop included the second annual Gordon=20=

Lecture, given by Tor Hagfors, entitled =D2Arecibo and the Spawning of=20=

New Science and New Observatories.=D3 Summaries of the workshop=20
presentations form the major part of this issue of the NAIC Newsletter.


----------
Herb Carlson: Some Early Arecibo Discoveries and Where They Stand Now

Herb Carlson discussed three major ionospheric discoveries made at the=20=

Arecibo Observatory, and for each spoke of where they stand today.

The first example was from 1965 and involves the pre-dawn electron=20
temperature enhancement observed in the F-region ionosphere. This=20
temperature enhancement is observed in the winter hemisphere before the=20=

sun is actually illuminating the region in question. The solution to=20
this problem involved electrons travelling up the earth's magnetic=20
field lines from the conjugate position in the southern hemisphere. =20
These electrons are excited by the earlier sunrise in the summer=20
hemisphere, which heats the electrons. These hot electrons arrive in=20
the ionosphere over Arecibo well before dawn and heat the electron gas.=20=

An equivalent phenomenon is seen post-sunset, as the ionosphere=20
remains hot after falling into the earth's shadow, as long as the=20
conjugate position in the south is sunlit.

Herb also raised the prediction, soon confirmed by observation, that=20
the hot electrons from the conjugate position should cause enhanced=20
emission of the O(1D) airglow line at 6300 =81. He showed a result from=20=

1966 comparing the 6300 and 5577 =81 lines from oxygen with emissions =
for=20
Na at 5893 =81 and N2 at 5300 =81. The other lines are all from the =
lower=20
thermosphere, where collisions prevent a similar phenomenon from=20
occurring, while the 6300 =81 emission clearly demonstrates the expected=20=

transience. As further confirmation of the theory, the seasonal=20
behavior of the F-region electron temperature and 6300 =81 emission was=20=

that expected if caused by conjugate electrons.

More recently, measurements by Lancaster et al. (1996) of the oxygen=20
emission at 8446 =81 were made. This should be a more direct =
measurement=20
of oxygen abundances. Though the results fit closely to the theory at=20=

sunset, there was a time difference at sunrise, where the 8446 =81=20
brightness led the expected enhancement by several minutes. This=20
problem is yet unresolved. An additional unsolved problem is that of=20
the electron gas thermal balance, in which the observed cooling rate is=20=

two times slower than physical models predict.

Herb's second example was the 1964 discovery that ion composition could=20=

be determined from the Doppler broadening of the ion line. One=20
memorable point was that "even in 1964 we knew there was much more He=20
than predicted, a fact only recently universally accepted."

Now the Arecibo Observatory can make precise tests of incoherent=20
scatter theory and show, for example, how T_He and T_O are different. =20=

As for the He abundance, work at Arecibo has confirmed that the=20
predictions of Sir James Jeans were wrong, the reasons are understood,=20=

and this knowledge can be applied to the study of planetary=20
atmospheres.

Herb's final example was ionospheric heating, which first took place in=20=

1970-71. The initial HF antenna was designed and built by Merle=20
LaLonde, and it hung above the dish, much like the next generation=20
design (see section below on Sulzer/Breakall presentation). Initial=20
work included: the generation of an artificial lens in the ionosphere,=20=

driving plasma instability, and localized temperature enhancements of=20
1000s of K.

Today, there is the question of what "turns on" the nonlinear response=20=

to the HF wave, a consuming question that has seen over $100 million=20
spent on it study over 30 years. Another question of modern interest=20
is how the plasma interacts with the AC electric field. This has=20
important implications for nuclear fusion, as in the fusion environment=20=

we cannot study this interaction, but we can in the ionosphere.

Finally Herb discussed some of the history that led the NSF to build=20
and support the chain of ionospheric radars, in particular, the=20
coupling of the high to low latitude ionosphere via gravity waves. In=20=

the future, Arecibo will make important contributions to measuring the=20=

global distribution of atomic oxygen in the ionosphere.
--end Herb Carlson--

Gordon Pettengill
Gordon Pettengill, who was an early site director, talked about the=20
early development of the Observatory from a tobacco farm in 1960 to the=20=

discoveries in the late 1960s in planetary radar. It was Gordon who=20
chose digital over analog equipment for pointing the telescope, giving=20=

1 arcmin accuracy (now a few arcsecs). Arecibo improved the accuracy=20
of the Astronomical Unit from a few thousand km to about 1 km, measured=20=

the rotation of Venus, discovered that the rotation and revolution of=20
Mercury were not synchronous, but that the rotation was tidally locked=20=

in 3:2 ratio to the orbital revolution. The telescope was also used to=20=

measure the retardation of the radar signal as it skimmed the sun on=20
its way to a planet beyond, thus providing an early confirmation of=20
General Relativity. Gordon told how one of the first maps of Venus was=20=

made by Don Campbell using a second dish to get interferometric fringes=20=

which resolved the north/south ambiguity of the radar echoes. =20
Radar-bright features were seen on the surface, which is covered by=20
clouds in the optical view.
--end Gordon Pettengill-- Alice

--Marshall Cohen--Tapasi
--Joanna Rankin-- Desh


Dr. Richard Behnke
Ionospheric Techniques and Discoveries
During his talk at the 40th anniversary workshop, Richard Behnke took=20
us back to the 1970s and early 80s as he described the atmospheric=20
science achievements and discoveries from those years. During that=20
decade, experimenters with new ideas took advantage of the upgraded=20
primary surface and new line feed to produce a new array of results in=20=

the lower (D & E-region), middle (F region) and topside of the=20
ionosphere. One of the most significant experiments developed at that=20=

time was the beam-swinging technique to measure F region 3 dimensional=20=

ion velocities (thus taking advantage of the spherical dish and=20
linefeed mentioned by Gordon). With that technique, it became possible=20=

to study electric fields (from velocities perpendicular to the local=20
magnetic field), the influence of neutral winds (from the ion motion=20
along the field line) and the propagation of traveling ionospheric=20
disturbances.

Great progress was also achieved in the E-region with the introduction=20=

of coded and multiple pulse schemes. Coded pulses, like Barker codes,=20=

make it possible to have good height resolution with good=20
signal-to-noise, which is crucial to study what goes on in the thin=20
sporadic E layers and descending layers. The multiple pulse technique=20=

improved greatly the correlation measurements in the E-region, where=20
the correlation times are long and the scale heights are short, so good=20=

height resolution is needed while making an efficient use of the=20
transmitter power.

More breakthroughs in ionospheric experiments took place with the use=20
of the special purpose correlator designed by Jon Hagen. With this=20
machine it was possible to perform computations faster, which in turn=20
lead to improvements in the precision of the measurements. The use of=20=

the correlator also made possible new measurements, like the electron=20
portion of the incoherent scatter spectrum, the so-called electron=20
line, which in recent times has become an interesting topic of=20
investigation, as the predictions of the traditional incoherent scatter=20=

theory do not match the Arecibo observations. Other measurements that=20=

were performed with the correlator, included topside measurements of=20
light ions and counter streaming ion velocities.

The 1970s also brought the first heating facility to Arecibo. And this=20=

is particularly interesting because the first heating facility used an=20=

arrangement of a log periodic antenna hanging at some distance below=20
the platform, which is a similar concept to what has been proposed for=20=

the third Arecibo HF facility, hopefully coming in the near future. =20
The heating experiments in the 1970s looked at the thermal balance, the=20=

enhanced plasma lines and the generation of plasma instabilities.

Using the words of Behnke, the 1970s were =D2a time of unequalled=20
progress at the Arecibo Observatory=D3.

--end behnke--

Don Campbell, a past site director, continued where Gordon Pettengill=20
left off, by describing the 1973 upgrade, which replaced the surface of=20=

the dish and introduced the 2.4 GHz radar with 420 kW power for=20
planetary work. The second upgrade doubled the power and again=20
improved the surface. He showed a rising sensitivity over the years of=20=

32 dB, about 1 dB/year. The improvements came from the Love feed, the=20=

upgrades and the recent surface tune-up. Don gave credit for this to=20
the many scientists, students, engineers, and Observatory employees who=20=

supported this project. The scientific achievments of this period were,=20=

for Mars: warning the Viking lander team of surface roughness; for=20
Venus: mapping the surface through the clouds before the Magellan=20
spacecraft arrived with 10 times the resolution, dating the Venus=20
surface by crater counts, and investigating surface polarization by=20
receiving the echo at the GBT; for the Galilean satellites of Jupiter: =20=

finding icy surfaces by the strange polarization effects of the echoes;=20=

for Mercury and the Moon: finding ice in permanently shadowed craters=20=

on Mercury and not on the Moon. Comets, asteroids, and the particles=20
of Saturn's rings have been detected and their sizes and dynamics=20
illuminated. Saturn's Titan recently showed indications of surface=20
lakes, and Saturn's Iapetus with its optically bright/dark hemispheres=20=

showed uniform radar echoes, which imply that the optical effect is a=20
surface deposit and not structural.

--end Don Campbell-- Alice
--
Extra Galactic HI =D0 Martha Haynes.

-Arecibo as a redshift machine
-environmental influences on galaxy evolution
-Search for hidden galaxies
-appliaction of the TF relation
-peculiar velocities (low z)
=13Galaxy evolution (high z)


Arecibo as a redshift machine:
=46rom an HI measurement, 3 parameters are immediately available:
The systemic Velocity
The rotational Velocity
The HI mass

=46rom the rotational velocity, it is possible to use the TF relation =
to=20
estimate the baryonic mass.
Over the last 20 years, Arecibo has been used to observe and measure=20
~11 000 galaxy redshifts in the perseus-pisces supercluster (PPS).=20
These observations revealed the filamentary structure of the cluster.


Galaxy evolution:
EG. The Arecibo observations at HI of the Leo triplet (I can get a=20
contour image of this if you wish). Shows a tidal outflow feature (and=20=

an outflow feature which she didn't mention!).

Observations of galaxy clusters have shown that the galaxies in the=20
centres of the groups are relatively HI poor, and have shrunken disks.=20=

(Giovanelli + Haynes).


Dark halos:
Theoretical models suggest a significant amount of matter should be=20
detectable in small dwarf halos nearby to larger, cannibalising halos.=20=

It is often extremly difficult to detect these, since they are not=20
optically obvious, they are typically serendipitous detections. One=20
such detection, found in the 'OFF' pointing for some observations is an=20=

HI cloud 1225+1 (Gio & Haynes, 1989)

TF reln.
The fact that Large scale structure exists implys that peculiar=20
velocities exist.
The Tully fisher relation can be used to estimate peculiar velocities.
Conclusion: The universe is a bowl of spagetti.
Beginnings of mega maser research =D0 Willem Baan

--end haynes--

OH Megamasers - Willem Baan
History re-hash:
OH megamaser emission was first detected from Arp 220 (IC 4553) =
in=20
1982, (Baan et al)


OH outflows used as a diagnostic for ULIRG population

Outflows have linewidths of 1300-2200 km/s scan up to z. (1300MHz)

many bright IR galaxies are also found to host megamasers

MRK 231
OH emission region straddles nuclear region
model of torus about a black hole 7.2 x107 Mo
MRK 273
OH emission reion straddles nuclear region.
Model of torus about a black hoel 3.5x108 Mo

Formaldehyde.
1993 detections with mini gregorian (during measurements of standing=20
waves):
2MJy lines (ARP 220) emission on two nuclei

also CH emission detected =D0 ongoing AO researc

--end Willem Baan--

--Carmen Pantoja-- Carmen


- M.C. Kelley
Michael Kelly dedicated his discussion to a description of highlights=20
of the successes at the Observatory in research and education. He=20
touched on the topics of:
- New technologies developed and applied at the Observatory
- The Arecibo Observatory as a national center, hosting both US and=20
international scientists
- Maintaining the Arecibo Observatory as a CEDAR class 1 facility by=20
the routine addition of new instrumentation and development of new=20
techniques
- The contribution of the Arecibo Observatory in its participation in=20
the development of new facilities such as the AMISR (Advanced Modular=20
Incoherent Scatter Radar). He went on to explain in detail the design=20=

and purpose of AMISR.
- The contribution of the Arecibo Observatory in the education Ph.D.=20
students, in particular of Hispanic origin, including Arecibo Site=20
Director Sixto Gonz=87lez and staff member N=8Estor Aponte, C=8Esar =
LaHoz and=20
Francisco Garc=92a.
- The participation of Arecibo in campaigns. In Coqui II 3 NASA=20
fellowships were granted to PR students, << into the direction to get even more students>>>.
- The science issues he discussed included a summary of gravity waves=20
showing how we can be visualize their structures using an all-sky=20
camera to image the airglow, and the observation of plumes shooting up=20=

as seen by Arecibo and Jicamarca radars. (ask Sixto or Mike for the=20
details)
--end kelley--

Nearly four decades of optical measurements at Arecibo
Craig A. Tepley (Shikha)
Craig Tepley gave the highlights of optical developments at Arecibo=20
starting from 1963. He divided his talk chronologically over the four=20=

decades and discussed the important achievements during each period.

The first decade (1963 =D0 1973) was the era when photometry evolved.=20
This happened when Colin Hines forwarded a proposal from Keith Cole to=20=

measure the effects of conjugate electrons that cause enhancement of=20
630 nm airglow emission in the early morning hours due to bombardment=20
of oxygen with photoelectrons. In July 1965, Gordon gave permission to=20=

Herb Carlson to purchase the required instrumentation and a photometer=20=

that could measure airglow emissions at 6300, 5577, 5893, 5300 =81 was=20=

acquired. In 1968, Carlson reported observations of the excited=20
nitrogen emission at 3914 A, which illustrated the influence of high=20
energy photo electrons on the local ionosphere. Later on, Nelson and=20
Cogger (1971) used the airglow line at 6300 A to show the association=20
between its enhanced emission and the descent of F-layer ionosphere,=20
which is generally referred to as the post midnight descent of F-layer.=20=

Cogger et al. (1970) measured linewidths of 6300 =81 and inferred =
airglow=20
temperatures that were used to validate satellite and incoherent=20
scatter radar temperatures.

The decade (1973 =D0 1983) was the period of spectroscopy that Craig=20
referred to as =D4Meriwether Years=D5. During this period, measurements =
of=20
7320 =81 twilight emissions were made. Afterwards, the mapping of=20
meridional intensity gradients (MIG=D5s) by Herrero and Meriwether were=20=

made in 1981, and imagers were first installed by visitors at the=20
observatory.

The third decade (1983 - 1993) was marked with the development of=20
interferometery. Roger Burnside used the 6300 A emission to measure=20
thermospheric winds using this technique. A few examples were shown=20
illustrating the coupling between ions drifts and neutral winds.=20
Recently, 23 years of wind data from the observatory were analyzed by=20
Robles et al. (2002). They showed a gradual eastward rotation of the=20
wind vector. In 1986, Kerr and his colleagues made observations of the=20=

Ha line that originates from the geocorona. This work was followed by=20
Kerr and Tepley (1988) inferring hydrogen concentrations in the=20
exosphere.

The last decade (1993=D02003) marks the era when lidar techniques=20
flourished at the observatory. The work started in 1989 when the=20
University of Illinois brought their laser to participate in the AIDA=20
campaign. This was followed by the development of Doppler Rayleigh=20
lidar in 1990 at the observatory. In 1995, an Alexandrite laser was=20
purchased to enhance the lidar program. During the Coqui campaign in=20
1998, several simultaneous measurements using Na lidar and Incoherent=20
Scatter Radar were made for studying sudden Na layers and their=20
coupling with sporadic E. Seasonal variations of K, Fe and mesospheric=20=

temperatures have been studied using Arecibo=D5s resonance lidars.

In summary, Craig presented a rich history of accomplishment in 40=20
years and how this will continue into the next decades.
--end tepley--

Exosphere/Plasmasphere Studies, Long Term Trends
Bob Kerr Monday 3 November, 2003 2:20pm

Bob Kerr began his presentation by acknowledging the valuable=20
contributions of Ra=9Cl Garc=92a, Craig Tepley and Roger Burnside over =
the=20
years. He then described how use of the "forbidden" emission at 6300 =81=20=

has provided a method for measuring F-region neutral winds for nearly=20
40 years.

He went on to show how Arecibo explored the possibility of determining=20=

O concentration by reconciliation of the OI 8446 =81 emission. There=20
were two candidates for excitation of O that produces this emission. =20
These are Bowen Fluorescence and photoelectron impact. Using the=20
Arecibo Fabry Perot Interferometer, the 8446 =81 triplet members were=20
isolated from each other and from bright OH contamination. As a=20
result, electron impact was confirmed as dominant source of this=20
emission. The excess of 8446 =81 emission at midnight and in the =
morning=20
sector relative to =D4Glow=D5 indicates possibly a plasmaspheric source.

Kerr went on to look to the future of airglow observations at Arecibo. =20=

With John Noto (Scientific Solutions), Kerr developed a photometer that=20=

was temporarily based at Arecibo and has since been installed at Cerro=20=

Tolodo in Chile.
<<>>

Future observations will use nested instrumentation at Arecibo and at=20
its conjugate location in the southern hemisphere to constrain=20
photoelectron models and measure thermospheric O concentrations from=20
the ground.

Measurements of vertical winds <<>>
Kerr discussed observations of the metastable helium emission at 10830=20=

=81 that is caused by electron impact. Arecibo accomplished the first=20=

measurements of metastable helium in the mid-90s using a Fabry-Perot=20
interferometer and infrared detector. The full potential of the 10830=20=

=81 wind and temperature diagnostic will be realised by slightly higher=20=

spectral resolution and with the use of IR array detection.

OH 7320 A =D4Hot O=D5

Arecibo followed the work of John Meriwether, mentioned by Craig=20
Tepley, of 30 years ago, by making improved measurements by use of a=20
narrow-band interference filter (? Not sure about this part, he speeded=20=

up and I couldn=D5t keep up)
Kerr showed some raw 7320 =81 emission profiles from May 2003, which=20
showed no broadening of the 7320 =81 line profile that would suggest hot=20=

oxygen. New instrumentation will be used later this year to test these=20=

observations

Finally, Kerr discussed his twenty years of work on hydrogen escape=20
flux, which at mid-latitudes is preferentially supplied during solar=20
maximum at winter solstice. The Ha 6563 =81 emission is observed for=20
these studies. The 2+ solar cycle trend shows some evidence of secular=20=

change, with increased H in upper thermosphere and exosphere, although=20=

the results are as yet inconclusive.
--end kerr--

John Mathews

John Mathews dedicated his timeto a discussion of the meteor=20
observations that have been going on since 1994 at Arecibo. He gave an=20=

update of how the Penn State group deals with the analysis of data. He=20=

presented results of the present state of that analysis, showing=20
altitude and velocity distributions as well as some orbital results of=20=

detected interplanetary and interstellar dust.
--end mathews--

******Extra Galactic HI - Riccardo Giovanelli

LSS from small scale perturbations -WMAP
density fluctuation: 1 part in 1 million.
Density contrast increases with age
Baryonic mass is 15% of total mass

Elliptical galaxies:
Galaxies form through multiple (major) mergers.
Kinetic energy of random motions barely exceeds that of large scale=20
orderly motions, such as rotation.
Spiral:
=46rom from less crowded environments
accrete at a slower pace (un affected by major mergers)

Lambda CDM model is successful in describing the presences of large=20
scale structure, but it predicts many more small mass halos/galaxies=20
than is observed.

Low mass objects are very few. i.e. Very nearby
distance and mass is uncertain, and affected by peculiar velocites.

Solutions: either
1.The LCDM model is wrong.
2.Galaxies have NO baryonic matter (Baryonic blow out)
3.DM does exist, but it does not form stars =D0 is it detectable in HI?=20=

<- this is where arecibo fits in.


Survey simulations:
Objectives:
investigate the faint end of the HI mass function (HIMF)
determine local density dependecies of the HIMF.
Estimate the time needed to detetect a given object, with mass MHI and=20=

velocity width Wkms:

T=3D1/4 f-2_(MHI/106M_)2(DMpc)4(Wkms/100)-2_

where f _ is the beam dilution, gamma=3D-3/4 for W<100, or gamma =3D-1 =
for=20
W >100

i.e. The depth of the survey increases as t1/4

The sensitivity of Arecibo is superior to any existing telescope at HI,=20=

it is the ideal tool to search for small, faint HI galaxies and and=20
around the local cluster.
--end giovannelli--

--Don Backer-- Desh
--Alex Wolszczsan-- Desh
--Tim Hankns-- Desh
----
OH megamasers =D0 J darling
Formed in Galaxy mergers (multiple mergers?)
rare, approx 100 known, 50% have been detected with the AO.
z> 0.1

First discovered in 1982 (W. Baan et al)
Study now needs sup arcsec resolution
OHMM emission traces galaxy merges (i.e. ULIRGS), Starformation and=20
massive black holes.

Empirical reln exists: L1.2IR ~ LOH


Take three examples (Martha, Riccardo, Bob)
Martha Riccardo
IRAS 02524+2046 IRAS 12032+1707
Z =3D 0.182 Z=3D0.217

These have very broad velocity width: of order of a 1000km/s

But.. Variability found in these objects by 10-14% -> therefore the=20
masing regions are compact.

Martha:
-has intra-day variability.
-Multiple narrow variable components
-1665 line varies often, but not always identically to the 1667 =
line.
(therefore, the masing regions are spatially nearby and compact)
-Flaring events occur for both transitions
-components can shift a few km/s
-variable features are narrower in velocity than average, =
structure on=20
< pc scale
-1667:1665 ~20% (vs. the theoretical 30% for a point source)

Riccardo:
narrow features vary, structure ~25 pc.
Broader structures are ~120pc
use VLBA to observe these features.

OH observations can probe variation in Fine structure constant:
=C6_ (_1665-_--1665)=3D 2(=C6+-=C6-)_4 (how cool is that??)
But..
bright enough OH and CH at high Z are rare, so it is difficult to check=20=

this properly, so this is what arecibo is good at.

--end darling--

-- Emmanuel Momjian -- Tapasi

40th Anniversary Workshop
"Magenetic Fields - Zeeman Effect" by Carl Heiles
-------------------------------------------------

Carl Heiles spoke about the experiment he and Tom Troland have been=20
performing using the Zeeman Effect to directly measure the interstellar=20=

magnetic field in HI clouds seen as absorption features against=20
extragalactic radio sources. He noted that this has used some 800 hr=20
of telescope time. A typical strong detection would yield values for=20
the magnetic field such as 5.6 $\pm$ 2.0 $\mu$G!!

Carl and Tom have also decomposed each of their absorption spectra into=20=

a number of Gaussian components which yield column density, spin=20
temperature, turbulence velocity, and magnetic field. It had previously=20=

been thought that to be thermally stable, the spin temperature of the=20
gas should be near either 80 or 8000 K. However, they find that much of=20=

the gas lies at excluded values near 2000 K.

In the Cold Neutral Medium, the turbulent velocities give a broad=20
spread of turbulent Mach numbers. The typical value is 3, suggesting=20
strong shocks which would be dissipatory, and hence short-lived. Thus a=20=

source of energy input is needed. Typical column densities are N$_{H}=20
\sim 3 \times 10^{19} cm^{-2}$. Much of this phase seems to exist in=20
very thin sheets having aspect ratios of $\sim$200:1, and a depth of=20
about 0.1 pc along the line of sight. This aspect ratio is about that=20
of a sheet of paper! If these sheets are oriented randomly, there will=20=

be a large number seen at low N$_{H}$, and only a few at high N$_{H}$.

Magnetic fields have been detected i