SETI update.440


Larry Klaes (lklaes@bbn.com)
Tue, 27 Jul 1999 11:11:51 -0400


>Date: Mon, 26 Jul 1999 14:26:22 -0400 (EDT)
>From: AIP listserver <physnews@aip.org>
>To: physnews-mailing@aip.org
>Subject: update.440
>
>PHYSICS NEWS UPDATE
>The American Institute of Physics Bulletin of Physics News
>Number 440 July 23, 1999 by Phillip F. Schewe and Ben Stein
>
>ANTIPROTONS AT SOLAR MAXIMUM. The solar wind is an
>electron-proton plasma blowing away from the Sun at 400-800
>km/sec and can be thought of as a tenuous atmosphere (called the
>heliosphere) of the Sun extending over most of the solar system.
>The charged particles in this gust both envelope and are influenced
>by the Sun's magnetic field. This field, because it rotates with the
>Sun, gets pretty tangled up (see figure at
>www.aip.org/physnews/graphics). When now cosmic rays from
>outside the solar system venture in they are buffeted by wind and
>field. During the present solar cycle of the 1990s, the configuration
>of the field is such that positively charged cosmic-ray protons drift
>into the inner heliosphere via solar polar regions and exit in
>equatorial regions. After the soon approaching peak period of solar
>activity (solar maximum), however, the Sun's field will be reversed.
>Then the negatively charged cosmic-ray antiprotons preferentially
>follow the polar route and more easily enter the inner heliosphere to
>be detected by earthbound or satellite detectors. Thus in the period
>2001-2010 we should see relatively more antiprotons than in the
>previous cycle, which is now ending. Physicists at the Bartol
>Research Institute at the University of Delaware (Thomas Gaisser,
>gaisser@bartol.udel.edu, 302-831-8113) have calculated when and
>by how much this antiproton surplus should manifest itself, telling
>us how well we understand the solar cycle. They have also sought
>ways of understanding the source of the antiprotons. Most
>antiprotons are made when commonplace protons strike interstellar
>dust, but some might have a more spectacular birth in the
>annihilation of dark matter or in the evaporation of primordial black
>holes. (Bieber et al., Physical Review Letters, 26 July 1999.)
>
>WHY IS THE SAHARA A DESERT? Fossil pollen, rock art, and
>other hints indicate that the Sahara was much greener 6000 years
>ago in the mid-Holocene period. Neolithic peoples seemed to have
>hastened desertification at the northern and southern edges of the
>Sahara, but German geophysicists believe the main causes were
>natural. They point to the fact that precession (wobble) in the
>Earth's orbit causes changes in the timing of perihelion (closest
>Earth-Sun approach) and our planet's rotational tilt. These
>combined to promote a milder climate in most regions of the mid-
>Holocene northern hemisphere. Since then the climate has become
>cooler and more arid. The subtle alterations in northern hemisphere
>cooling, however, were amplified by a feedback between
>atmosphere and vegetation causing climate change in the Sahara
>region to be far more drastic than elsewhere. Indeed what occurred
>was "the largest change in land cover during the last 6000 years,"
>according to Martin Claussen (Potsdam Institute for Climatology,
>claussen@pik-postdam.de, 011-49-331-288-2522). He and his
>colleagues have now confirmed this hypothesis with computer
>modeling. (Claussen et al., Geophysical Research Letters, 15 July;
>http://www.pik-potsdam.de/)
>
>THE MOST POWERFUL FREE ELECTRON LASER (FEL), as of
>this week, is the one at Jefferson Lab in Virginia. In an FEL
>amplified laser light comes from a beam of electrons passing
>through a cavity. The advantages of FEL's are their tunability (with
>output from microwaves up into the ultraviolet), their high "duty
>cycle" (they deliver light continuously) and the fact that the light is
>produced in closely spaced picosecond bursts tied to the pulselike
>nature of the parent electron beam. This makes the light useful for
>doing fast things, such as melting metals and then watching as they
>re-freeze into non-crystal solids, or roughening up sheets of
>polymer fabric so that they will accept glues or dyes. Jefferson's
>FEL has an average power of 1.7 kilowatts; the best previous
>continuous FEL power was 11 watts. (Jefferson press release, 20
>July; contact George Neil, 757-269-7443.)
>



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