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Termination shock particle spectral features
Spectral features of energetic H ions accelerated at the termination shock may be evidence of two components. At low energies the energy spectrum is ~E^(–1.55), with break at ~0.4 MeV to E^(–2.2). A second component appears above ~1 MeV with a spectrum of E^(–1.27) with a break at ~3.2 MeV. Even though the intensities upstream are highly variable, the same spectral break energies are observed, suggesting that these are durable features of the source spectrum. The acceleration processes for the two components may differ, with the lower energy component serving as the injection source for diffusive shock acceleration of the higher energy component. Alternatively, the spectral features may result from the energy dependence of the diffusion tensor that affects the threshold for diffusive shock acceleration
Voyager observations of galactic and anomalous cosmic rays in the helioshealth
Anomalous cosmic rays display large temporal variations at the time and location where Voyager 1 (V1) crossed the heliospheric termination shock (2004.86) (94AU, 34°N). On a short time scale (3 months) there was a large decrease produced by a series of merged interaction regions (MIR), the first of which was associated with the intense Oct./Nov. 2003 solar events. On a longer time scale there is a remarkable correlation between changes in the galactic cosmic ray (GCR) intensity and those of 10–56 MeV/n ACR He and 30–56 MeV H extending over a 4.3 year period with the GCRs exhibiting their expected behavior over this part of the 11 and 22 year solar activity and heliomagnetic cycle. The relative changes in the ACR and GCR are the same for both the short term and long term variations. The comparative V1/V2 ACR and GCR spectra in the foreshock and heliosheath indicate that at this time most of the higher energy ACRs are not being accelerated near V1 but must have their source region elsewhere — possibly near the equatorial region of the TS as was suggested in our first paper on the TS crossing (1)
Transient intensity changes of cosmic rays beyond the heliospheric termination shock as observed at Voyager 1
This paper continues our studies of temporal variations of cosmic rays beyond the heliospheric termination shock (HTS) using Voyager 1 (V1) data when V1 was beyond 94 AU. This new study utilizes cosmic ray protons and electrons of several energies. Notable transient decreases of 5–50% are observed in galactic cosmic ray nuclei and electrons at V1 shortly after similar decreases are observed at Voyager 2 (V2) still inside the HTS. These decreases at V1 appear to be related to the large solar events in September 2005 and December 2006 and the resulting outward moving interplanetary shock. These two large interplanetary shocks were the largest observed at V2 after V1 crossed the HTS at the end of 2004. They were observed at V2 just inside the HTS at 2006.16 and 2007.43 providing timing markers for V1. From the timing of the intensity decreases observed at V1 as the shocks first reach the HTS and then later reach V1 itself, we can estimate the shock speed beyond the HTS to be between 240 and 300 km s^(−1) in both cases. From the timing of the decreases observed when the shock first reaches the HTS and then several months later encounters the heliopause, we can estimate the heliosheath thickness to be 31 ± 4 and 37 ± 6 AU, respectively, for the two sequences of three decreases seen at V1. These values, along with the distances to the HTS that are determined, give distances from the Sun to the heliopause of 121 ± 4 and 124 ± 6 AU, respectively
At Voyager 1 Starting on about August 25, 2012 at a Distance of 121.7 AU From the Sun, a Sudden Disappearance of Anomalous Cosmic Rays and an Unusually Large Sudden Increase of Galactic Cosmic Ray H and He Nuclei and Electron Occurred
At the Voyager 1 spacecraft in the outer heliosphere, after a series of
complex intensity changes starting at about May 8th, the intensities of both
anomalous cosmic rays (ACR) and galactic cosmic rays (GCR) changed suddenly and
decisively on August 25th (121.7 AU from the Sun). The ACR started the
intensity decrease with an initial e-folding rate of intensity decrease of ~1
day. Within a matter of a few days, the intensity of 1.9-2.7 MeV protons and
helium nuclei had decreased to less than 0.1 of their previous value and after
a few weeks, corresponding to the outward movement of V1 by ~0.1 AU, these
intensities had decreased by factors of at least 300-500 and are now lower than
most estimates of the GCR spectrum for these lower energies and also at higher
energies. The decrease was accompanied by large rigidity dependent anisotropies
in addition to the extraordinary rapidity of the intensity changes. Also on
August 25th the GCR protons, helium and heavier nuclei as well as electrons
increased suddenly with the intensities of electrons reaching levels ~30-50%
higher than observed just one day earlier. This increase for GCR occurred over
~1 day for the lowest rigidity electrons, and several days for the higher
rigidity nuclei of rigidity ~0.5-1.0 GV. After reaching these higher levels the
intensities of the GCR of all energies from 2 to 400 MeV have remained
essentially constant with intensity levels and spectra that may represent the
local GCR. These intensity changes will be presented in more detail in this,
and future articles, as this story unfolds.Comment: 13 Pages, 5 Figure
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