AESOP and LEE Balloon Instruments

Lynn Lake Launch site

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)

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.

LEE instrument

AESOP instrument

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.

AESOP Tracking

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.


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.

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.

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.

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.

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.

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).

Construction of AESOP was funded primarily by NASA under grant NAG5-1049. Balloon campaigns were supported by NSF under grants ATM-9632323/0000745 and NASA grants NNG05WC08G and NNH09ZDA001N. Launch services were provided by NASA through the National Scientific Balloon Facility.

Link to the publication list .

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