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Solar modulation in surface atmospheric electricity
The solar wind modulates the flux of galactic cosmic rays impinging on Earth inversely with solar activity. Cosmic ray ionisation is the major source of air’s electrical conductivity over the oceans and well above the continents. Differential solar modulation of the cosmic ray energy spectrum modifies the cosmic ray ionisation at different latitudes,varying the total atmospheric columnar conductance. This redistributes current flow in the global atmospheric electrical circuit, including the local vertical current density and the related surface potential gradient. Surface vertical current density and potential
gradient measurements made independently at Lerwick Observatory,Shetland,from 1978 to 1985 are
compared with modelled changes in cosmic ray ionisation arising from solar activity changes. Both the
lower troposphere atmospheric electricity quantities are significantly increased at cosmic ray maximum(solar minimum),with a proportional change greater than that of the cosmic ray change
A History of Solar Activity over Millennia
Presented here is a review of present knowledge of the long-term behavior of
solar activity on a multi-millennial timescale, as reconstructed using the
indirect proxy method. The concept of solar activity is discussed along with an
overview of the special indices used to quantify different aspects of variable
solar activity, with special emphasis upon sunspot number. Over long
timescales, quantitative information about past solar activity can only be
obtained using a method based upon indirect proxies, such as the cosmogenic
isotopes \super{14}C and \super{10}Be in natural stratified archives (e.g.,
tree rings or ice cores). We give an historical overview of the development of
the proxy-based method for past solar-activity reconstruction over millennia,
as well as a description of the modern state. Special attention is paid to the
verification and cross-calibration of reconstructions. It is argued that this
method of cosmogenic isotopes makes a solid basis for studies of solar
variability in the past on a long timescale (centuries to millennia) during the
Holocene. A separate section is devoted to reconstructions of strong solar
energetic-particle (SEP) events in the past, that suggest that the present-day
average SEP flux is broadly consistent with estimates on longer timescales, and
that the occurrence of extra-strong events is unlikely. Finally, the main
features of the long-term evolution of solar magnetic activity, including the
statistics of grand minima and maxima occurrence, are summarized and their
possible implications, especially for solar/stellar dynamo theory, are
discussed.Comment: A review, 91 pages, 28 figures. available online at
http://solarphysics.livingreviews.org/Articles/lrsp-2013-1
Long-term forcing of Sun's coronal field, open flux and cosmic ray modulation potential during grand minima, maxima and regular activity phases by the solar dynamo mechanism
Magnetic fields generated in the Sun's interior by the solar dynamo mechanism
drive solar activity over a range of time-scales. While space-based
observations of the Sun's corona exist only for few decades, direct sunspot
observations exist for a few centuries, solar open flux and cosmic ray flux
variations can be reconstructed through studies of cosmogenic isotopes over
thousands of years. While such reconstructions indicate the presence of extreme
solar activity fluctuations in the past, causal links between millennia scale
dynamo activity, consequent coronal field, solar open flux and cosmic ray
modulation remain elusive. By utilizing a stochastically forced solar dynamo
model we perform long-term simulations to illuminate how the dynamo generated
magnetic fields govern the structure of the solar corona and the state of the
heliosphere -- as indicated by variations in the open flux and cosmic ray
modulation potential. We establish differences in the nature of the large-scale
structuring of the solar corona during grand maximum, minimum, and regular
solar activity phases and simulate how the open flux and cosmic ray modulation
potential varies over time scales encompassing these different phases of solar
activity. We demonstrate that the power spectrum of simulated and reconstructed
solar open flux are consistent with each other. Our study provides the
theoretical basis for interpreting long-term solar cycle variability based on
reconstructions relying on cosmogenic isotopes and connects solar internal
variations to the forcing of the state of the heliosphere.Comment: 15 Pages, 8 Figures, Submitted to MNRA
Upgrade of electronics of neutron monitors DOMC and DOMB
DOMC and DOMB neutron monitors (NM) operate at the Concordia research station (Dome C on the Antarctic
plateau, 75 o 06’S, 123 o 23’E, 3233 m a.s.l.) since 2015. Their high elevation and proximity to the geomagnetic pole
provide low atmospheric and geomagnetic cutoffs and, therefore, the exceptionally high sensitivity to low-ener-
gy cosmic rays. The instruments are the so-called mini neutron monitors with BF
3
-filled counter tubes. DOMC
has the standard design with a lead neutron multiplier and DOMB is a so-called “bare” (lead-free) unit. We report
on a recent upgrade of the electronics heads of these instruments. The new heads have a modular architecture,
built upon a single-board computer Raspberry Pi. The upgrade increases the capabilities of the instruments in
two aspects: (1) measurements, particularly, of cosmic ray multiplicity; (2) remote control and monitoring. The
new electronic heads register each pulse from a detector, giving a timestamp with microsecond precision, which
is crucial for multiplicity measurements. Many important parameters (e.g., high voltage, pulse detection thres-
holds) can be controlled and adjusted remotely with the new design. High computing power allows performing
data processing on the fly. The upgrade increases the capability of DOMC and DOMB in cosmic ray measurements
and improves control of the operation of the neutron monitors
Occurrence of extreme solar particle events: Assessment from historical proxy data
The probability of occurrence of extreme solar particle events (SPEs) with
the fluence of (>30 MeV) protons F30>10^{10} cm^{-2} is evaluated based on data
of cosmogenic isotopes 14C and 10Be in terrestrial archives
centennial-millennial time scales. Four potential candidates with
F30=(1-1.5)x10^{10} cm^{-2} and no events with F30>2x10^{10} cm^{-2} are
identified since 1400 AD in the annually resolved 10Be data. A strong SPE
related to the Carrington flare of 1859 AD is not supported by the data. For
the last 11400 years, 19 SPE candidates with F30=(1-3)x10^{10} cm^{-2} are
found and clearly no event with F30>5x10^{10} cm^{-2} (50-fold the SPE of
23-Feb-1956) occurring. This values serve as an observational upper limit for
the strength of SPE on the time scale of tens of millennia. Two events, ca. 780
and 1460 AD, appear in different data series making them strong candidates to
extreme SPEs. We built a distribution of the occurrence probability of extreme
SPEs, providing a new strict observational constraint. Practical limits can be
set as F30~1x, 2-3x, and 5x10^{10} cm^{-2} for the occurrence probability
~10^{-2}, 10^{-3} and 10^{-4} year^{-1}, respectively. Because of
uncertainties, our results should be interpreted as a conservative upper limit
of the SPE occurrence near Earth. The mean SEP flux is evaluated as ~40 (cm2
sec)^{-1} in agreement with estimates from the lunar rocks. On average, extreme
SPEs contribute about 10% to the total SEP fluence.Comment: accepted to Astrophys.
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