Sky and Telescope - September 2015 - 54

OBSERVING

Exploring the Solar System

Superior
conjunction
June 6, 2016
Greatest
elongation
Oct. 26

Sun

Inferior
conjunction
Aug. 15

S&T: LEAH TISCIONE

Observers'
next opportunity to search
for ashen light
comes early
in September,
when Venus
appears in the
predawn sky
after inferior
conjunction.

Greatest
brilliancy
Sep. 21

Schmidt-Cassegrain telescopes and the 24-inch Clark
refractor at Lowell Observatory - and we beneﬁted from
Cruikshank's advice and his narrow-bandpass ﬁ lters. But
we saw no hint of an ashen-like glow. Our disclaimer,
however, is that during that time the Sun was fairly quiescent, with no strong ﬂare activity - and this fact might be
signiﬁcant, in light of some suggestive recent research.
Working at Apache Point Observatory in New Mexico,
astronomers Candace Gray and Nancy Chanover (New
Mexico State University), Tom Slanger (SRI International), and Karan Molaverdikhani (University of
Colorado) observed Venus spectrographically on four
separate occasions between 2012 and 2014. Fortuitously,
during each observing window an X-class solar ﬂare -
the strongest kind - was directed right at Venus. As
Slanger reported at the 2014 meeting of the AAS's Divi-

The Moon * September 2015
Phases

Distances

LAST QUARTER

Apogee September 14, 11h UT
252,565 miles diam. 29′ 20″

September 5, 9:54 UT
NEW MOON

Perigee September 28, 2h UT
221,753 miles diam. 33′ 50″

September 13, 6:41 UT
FIRST QUARTER

September 21, 8:59 UT

Librations

FULL MOON

September 28, 2:51 UT

Vallis Bouvard September 9
Gauss (crater)

September 17

Cabeus (crater) September 23

23
Sept. 9

NASA / LRO

For key dates, yellow dots
indicate which part of the
Moon's limb is tipped the
most toward Earth by libration
under favorable illumination.

54
54 September
September 2015
2015 sky
sky && teltelescope

escope

sion for Planetary Sciences in Tucson, "The results were
amazing! There was a major solar storm in July 2012,
producing an X-ﬂare and huge coronal mass ejection
that was directed right at Venus. We saw the brightest
green line [of ﬂuorescing oxygen atoms] that had ever
been detected."
The green emission line wasn't detected after every
ﬂare but rather after only those that drove charged particles directly at Venus. While the planet lacks a global
magnetic ﬁeld, it does have an "induced" ﬁeld, created
when the Sun's magnetic ﬁeld, embedded in the solar
wind, becomes wrapped around the planet's ionosphere.
There's also a long magnetotail extending out from the
planet on the side opposite the Sun. So conceivably some
kind of magnetic-reconnection process involving the
solar wind is triggering auroral glows.
That's in keeping with results from ESA's Venus
Express, which has shown that high-energy electrons are
being channeled into the planet's nighttime atmosphere,
where they excite oxygen atoms. Given the lack of a magnetic ﬁeld, however, they're not channeled toward the
poles (as occurs on Earth) but instead get dispersed. The
result: auroras at all latitudes.
Based on these and other spectrographic results, Gray
posits that the brightness and intensity of green-line
emissions in the Venusian atmosphere likely depend on
the strength of the solar ﬂare, the depth in the planet's
atmosphere at which the oxygen excitation occurs, and
other as yet unknown factors.
Obviously there are chains of uncertainties here, but
the auroral theory seems viable once again, providing a
great opportunity for amateurs to take up the chase. If
there are indeed Venusian auroras, can they be linked
to X-ﬂares and detected visually from Earth? If so, there
might be something to the ashen light after all.
Like Cruikshank and Binder in 1959, persevering
amateur observers armed with patience, attention to
detail, and today's excellent equipment - as well as
some luck - should take advantage of the current year's
apparition of Venus to look for any hint of ashen light.
The planet's thin crescent should be optimally placed in
the morning sky during September and October.
Since major solar ﬂare activity is likely to continue
during this window, this might be a golden opportunity
to ﬁnally snare the "Loch Ness of the solar system," as
the ashen light has been referred to. Will you be the one
to do it? ✦
Contributing Editor William Sheehan is a veteran solarsystem observer, historian, and author. An avid astro-photographer, Klaus Brasch took his ﬁrst grainy Moon pictures
in 1959 and now enjoys digital imaging from his backyard
observatory in Flagstaﬀ, Arizona.

Sky and Telescope - September 2015

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