AESOP
and LEE Balloon Instruments
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The payload is suspended by the
launch crane while the 39mcf balloon is being inflated with helium. Data
is recorded continuously during the launch sequence, ascent and float
period (~138kft) via telemetry provided by the National Scientific Balloon Facility ,who
are also responsible for the launch and recovery of the payload. As shown
the AESOP and LEE instruments are mounted on a
single platform. (Picture taken by A. McDermott)
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Introduction: A series of balloon observations of cosmic ray electrons
with the LEE (Low Energy Electrons) instrument begun in 1968 at the University of
Chicago and has continued at the Bartol
Research Institute since 1984. The data from these balloon flights have been used to study solar modulation of electrons
with energies up to ~ 20GeV. The AESOP (Anti-Electron Sub Orbital Payload)
instrument, which was recently built at Bartol,
is designed to have a similar energy response to LEE, but in addition it resolves positrons and negatrons with a
maximum detectable rigidity of 6GV. The AESOP and LEE instruments have
flown together as a single balloon payload on 1-Sep-1997, 29-Aug-1998,
16-Aug-1999, 25-Aug-2000
and 13-Aug-2002 from Lynn
Lake, Manitoba logging roughly 150hrs at float altitudes. All five balloon flights were successful and provided clean
data for analysis. (Picture taken by A. McDermott)
In 2002, LEE flew first
on 13 August as part of our dual payload with the AESOP instrument,
reaching an altitude of 40 km or 230 Pa (132 kft
or 2.3 mbars). Then, on 25 August, LEE flew alone
on the largest balloon
ever successfully launched (60 x106 ft3 or 1.7 x 106
m3) reaching an altitude of 48.8 km or 90 Pa (161 kft or 0.9 mbars). Reduced background and low atmospheric attenuation allowed a
measurement of low energy primary electrons (during geomagnetic night) inaccessible at greater depths. Below 30 MeV, the measured flux is
consistent with a Jovian origin, however electrons
above 30 MeV
have some other source. Understanding the electron
spectrum in the energy range of 50-200MeV remains elusive.
The primary goal of the AESOP instrument is to
investigate the charge-sign dependence in solar modulation. Certain
features in the large scale geometry of the heliospheric magnetic field expect to produce a
charge-sign dependency in cosmic ray propagation. During sunspot maximum,
the solar magnetic dipole reverses polarity, leading to alternating
charge-sign effect. Tracking time variations in the Galactic positron
abundance (0.5GeV to 4.5GeV) allows quantification of this effect at 1AU.
During the 2000 solar maximum the solar magnetic field polarity reversed
direction now favoring negative charge sign particles. Observations from
the recent 2006 Long Duration AESOP solo flight from Kiruna, Sweden
to Victoria Island, Canada should improve our
understanding of this phenomena. Data analysis is
on-going
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Schematic Drawings of the
instruments. Left: LEE . Right: AESOP . Briefly, LEE detects electrons with plastic scintillators T1, T3 and G (anticoincidence) and the
gas Cherenkov detector T2. It measures the electron energy in a cesium
iodide (T4) and leadglass (T5) calorimeter. Scintillator T6 also assists in particle
identification and energy determination by counting the number of
particles that escape the calorimeter. Negative and positive electrons
are indistinguishable in LEE. AESOP is functionally similar, except that
there is also a permanent magnet and a spark chamber
hodoscope to determine the charge sign of the
electron.
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The AESOP chambers
contain 5 parallel aluminum plates connected, in alternate order, to ground
and a high voltage pulser. The medium between
plates is a slow moving noble gas mixture of neon and helium. As a charged particle transverses a chamber it leaves
behind an ion trail in the gas. If a coincidence is form based on the fast scintillator detectors, a 10,000 volt
pulser is triggered. In the presence of a high
electric field, the ions in the gas are accelerated toward the plate surface
resulting in a bright red vertical spark across the ion trail
which is digitized and recorded using a linear CCD camera. As shown
above, the chambers have two mirrors mounted in the back walls resulting in
two additional reflected images (1 direct and 2 reflections).
The spark position in each gap can then be determined from triangulation of
the direct and reflected images. We typically achieve 200μm resolution in the bending
plane and 350μm in the
non-bending yield yielding ~6GV MDR
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Charge Sign Dependence in Solar
Modulation: The below figure
illustrates how the response of electrons and nuclei to changing conditions
in interplanetary space is qualitatively similar but quantitatively
different. Cosmic ray fluxes are low when the sun is active and high when
the sun is inactive. Plotting scales are chosen so
that the electron fluxes are a factor of 100 times the helium fluxes in the
units shown. The observations of
electrons represented by solid black symbols track each other quite well,
but they lie either above or below the helium fluxes (open red symbols)
depending on the polarity state -- with the exception of pre-maximum
periods when the fluxes nearly overlap. Particles with opposite sign to the
polarity state reveal a narrower time profile than those with like charge-sign, however the electron profile in the 1990s seems to
be broader than the helium spike profile observed in the 1980s. This lack
of symmetry could be the result of velocity dependent effects or the
presence of positrons (10-20%) in the electron fluxes (Moraal
et al. 1991; Clem and Evenson 2002). The KET
electron data, from measurements taken in the 1990s and represented by the
diamond shaped symbols, were corrected for
background and normalization problems using the LEE and ICE data (Clem et
al. 2002). KET observations were not made at Earth,
therefore these observations were corrected for particle spatial gradients
also determined using LEE and ICE data.
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Measurements of solar modulation
as a function of time at a rigidity of approximately 1.2 GV.
Open red symbols show helium fluxes while filled black symbols show
electron fluxes. The large solid circles are data from the LEE series of
NSF supported balloon flights. The small solid squares are data from the
ISEE-3/ICE spacecraft and the small solid diamonds are data from the
corrected KET/Ulysses data. Open squares are data from IMP-8 spacecraft. Other data are discussed by Clem et al (1996, 2000) and Evenson (1998). Epochs of well
defined heliospheric magnetic polarity
are indicated by shaded rectangles.
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Relative
abundance of helium (IMP-8) and electrons (ICE and Corrected KET) at 1.2 GV rigidity as a function of the tilt angle of the heliospheric current sheet during the past two full solar
cycles. Box symbols (black) indicate measurements in the A+ solar polarity
state (1990s), circle symbols (red) indicate A-
(1980s). Cross symbols indicate periods of undetermined polarity state.
Arrows point forward in time, and the length of each
represents 4 months.
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The combination of the outward flowing solar wind and the solar
rotation produces the spiral geometry of solar magnetic field lines as
shown above. A+ symbol represents the case when the dipole axis projection
rotation axis is positive and A- the projection is negative
As the particle moves along a curved magnetic field line it
experiences a centrifugal force due to the field curvature, and therefore
the particle trajectory drifts perpendicular to both the centrifugal force
and B. In this case, it is either toward or away from neutral current sheet
depending on the charge sign and polarity. Gradients in the field will also
cause drifts. It is important to note the resulting drift direction is
charged sign dependent.
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Compiled measurements and
calculations of the positron fraction as a function of energy during
different epochs of solar magnetic polarity. Solid symbols
and upper dashed line represents A+ state. Open symbols and lower dashed
line represent A- state.
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Time
profile of positron abundance and anti-proton/proton ratios at a rigidity of
roughly 1.3GV. The anti-proton/proton ratios were measured by a series of BESS flights. The black dotted line is the positron abundance prediction based
on the analysis of Clem et al (1996).
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Construction of AESOP was funded primarily by NASA under grant NAG5-1049. Balloon
campaigns were supported by NSF
under grants ATM-9632323/0000745 and now NASA grant
NNG05WC08G. Launch services were provided by NASA through
the National Scientific Balloon Facility.
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Page maintained by John M. Clem - clem@bartol.udel.edu
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