479 research outputs found
Role of coronal mass ejections in the heliospheric Hale cycle
[1] The 11-year solar cycle variation in the heliospheric magnetic field strength can be explained by the temporary buildup of closed flux released by coronal mass ejections (CMEs). If this explanation is correct, and the total open magnetic flux is conserved, then the interplanetary-CME closed flux must eventually open via reconnection with open flux close to the Sun. In this case each CME will move the reconnected open flux by at least the CME footpoint separation distance. Since the polarity of CME footpoints tends to follow a pattern similar to the Hale cycle of sunspot polarity, repeated CME eruption and subsequent reconnection will naturally result in latitudinal transport of open solar flux. We demonstrate how this process can reverse the coronal and heliospheric fields, and we calculate that the amount of flux involved is sufficient to accomplish the reversal within the 11 years of the solar cycle
The outer source of pickup ions and anomalous cosmic rays
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95527/1/grl16222.pd
A possible generation mechanism for the IBEX ribbon from outside the heliosphere
The brightest and most surprising feature in the first all-sky maps of
Energetic Neutral Atoms (ENA) emissions (0.2-6 keV) produced by the
Interstellar Boundary Explorer (IBEX) is an almost circular ribbon of a
~140{\deg} opening angle, centered at (l,b) = (33{\deg}, 55{\deg}), covering
the part of the celestial sphere with the lowest column densities of the Local
Interstellar Cloud (LIC). We propose a novel interpretation of the IBEX results
based on the idea of ENA produced by charge-exchange between the neutral H
atoms at the nearby edge of the LIC and the hot protons of the Local Bubble
(LB). These ENAs can reach the Sun's vicinity because of very low column
density of the intervening LIC material. We show that a plane-parallel or
slightly curved interface layer of contact between the LIC H atoms (n_H = 0.2
cm^-3, T = 6000-7000 K) and the LB protons (n_p = 0.005 cm^-3, T ~ 10^6 K),
together with indirect contribution coming from multiply-scattered ENAs from
the LB, may be able to explain both the shape of the ribbon and the observed
intensities provided that the edge is < (500-2000) AU away, the LIC proton
density is (correspondingly) < (0.04-0.01) cm^-3, and the LB contains ~1% of
non-thermal protons over the IBEX energy range. If this model is correct, then
IBEX, for the first time, has imaged in ENAs a celestial object from beyond the
confines of the heliosphere and can directly diagnose the plasma conditions in
the LB.Comment: Accepted by Ap.J.Lett
Recommended from our members
The heliospheric magnetic field over the Hale cycle
The concept that open magnetic flux of the Sun (rooted with one and only one footpoint at the Sun) is a conserved quantity is taking root in the heliospheric community. Observations show that the Sun's open magnetic flux returns to the baseline from one solar minimum to the next. The temporary enhancement in the 1 AU heliospheric magnetic flux near solar maximum can be accounted for by the temporary creation of closed magnetic flux (with two footpoints at the Sun) during the ejection of coronal mass ejections (CMEs), which are more frequent near solar maximum. As a part of the International Heliophysical Year activities, this paper reviews two recently discussed consequences of open flux conservation: the reversal of open magnetic flux over the solar cycle driven by Coronal Mass Ejections and the impacts of open flux conservation on the global structure of the heliospheric magnetic field. These studies demonstrate the inherent linkages between coronal mass ejections, footpoint motions back at the Sun, and the global structure and evolution of the heliospheric magnetic field
Distance to the IBEX Ribbon Source Inferred from Parallax
Maps of Energetic Neutral Atom (ENA) fluxes obtained from Interstellar
Boundary Explorer (IBEX) observations revealed a bright structure extending
over the sky, subsequently dubbed the IBEX ribbon. The ribbon had not been
expected from the existing models and theories prior to IBEX, and a number of
mechanisms have since been proposed to explain the observations. In these
mechanisms, the observed ENAs emerge from source plasmas located at different
distances from the Sun. Since each part of the sky is observed by IBEX twice
during the year from opposite sides of the Sun, the apparent position of the
ribbon as observed in the sky is shifted due to parallax. To determine the
ribbon parallax, we found the precise location of the maximum signal of the
ribbon observed in each orbital arc. The obtained apparent positions were
subsequently corrected for the Compton-Getting effect, gravitational
deflection, and radiation pressure. Finally, we selected a part of the ribbon
where its position is similar between the IBEX energy passbands. We compared
the apparent positions obtained from the viewing locations on the opposite
sides of the Sun, and found that they are shifted by a parallax angle of
, which corresponds to a distance of
AU. This finding supports models of the ribbon with the source located just
outside the heliopause.Comment: 26 pages, 10 figures, 1 table, submitted to Ap
Scintillator gamma-ray detectors with silicon photomultiplier readouts for high-energy astronomy
Space-based gamma-ray detectors for high-energy astronomy face strict constraints of mass, volume, and power, and must endure harsh operating environments. Scintillator materials have a long history of successful operation under these conditions, and new materials offer greatly improved performance in terms of efficiency, time response, and energy resolution. The use of scintillators in space remains constrained, however, by the mass, volume, and fragility of the associated light readout device, typically a vacuum photomultiplier tube (PMT). Recently developed silicon photomultipliers (SiPMs) offer gains and efficiencies similar to those of PMTs, but with greatly reduced mass and volume, high ruggedness, and no high-voltage requirements. We have therefore been investigating the use of SiPM readouts for scintillator gamma-ray detectors, with an emphasis on their suitability for space- and balloonbased instruments for high-energy astronomy. We present our most recent results, including spectroscopy measurements for lanthanum bromide scintillators with SiPM readouts, and pulse-shape discrimination using organic scintillators with SiPM readouts. We also describe potential applications of SiPM readouts to specific highenergy astronomy instrument concepts
Determining the ionization rates of interstellar neutral species using direct-sampling observations of their direct and indirect beams
A good understanding of the ionization rates of neutral species in the
heliosphere is important for studies of the heliosphere and planetary
atmospheres. So far, the intensities of the ionization reactions have been
studied based on observations of the contributing phenomena, such as the solar
spectral flux in the EUV band and the flux of the solar wind protons, alpha
particles, and electrons. The results strongly depend on absolute calibration
of these measurements, which, especially for the EUV measurements, is
challenging. Here, we propose a novel method of determining the ionization rate
of neutral species based on direct sampling of interstellar neutral gas from
two locations in space distant to each other. In particular, we suggest
performing observations from the vicinity of Earth's orbit and using ratios of
fluxes of ISN He for the direct and indirect orbits of interstellar atoms. We
identify the most favorable conditions and observations geometries, suitable
for implementation on the forthcoming NASA mission Interstellar Mapping and
Acceleration Probe.Comment: Accepted for ApJ
Update on Radiation Dose From Galactic and Solar Protons at the Moon Using the LRO/CRaTER Microdosimeter
The NASA Lunar Reconnaissance Orbiter (LRO) has been exploring the lunar surface and radiation environment since June 2009. In Mazur et al. [2011] we discussed the first 6 months of mission data from a microdosimeter that is housed within the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) instrument onboard LRO. The CRaTER microdosimeter is an early version of what is now a commercially available hybrid that accurately measures total ionizing radiation dose in a silicon target (http://www.teledynemicro.com/product/radiation-dosimeter). This brief report updates the transition from a deep solar minimum radiation environment to the current weak solar maximum as witnessed with the microdosimeter
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