151 research outputs found
Ultrarelativistic electrons and solar flare gamma-radiation
Ten solar flares with gamma radiation in excess of 10 MeV were observed. Almost all took place within a heliolatitude greater than 60 deg, close to the solar limb, an indication of the essential anisotropy of high-energy gamma radiation. This high-energy solar flare gamma radiation can be explained by the specific features of the bremsstrahlung of ultrarelativistic electrons trapped within the magnetic arc of the solar atmosphere, even if the acceleration of the electrons is anisotropic
Regional cosmic ray induced ionization and geomagnetic field changes
Cosmic ray induced ionization (CRII) is an important factor of outer space influences on atmospheric properties. Variations of CRII are caused by two different processes – solar activity variations, which modulate the cosmic ray flux in interplanetary space, and changes of the geomagnetic field, which affects the cosmic ray access to Earth. Migration of the geomagnetic dipole axis may greatly alter CRII in some regions on a time scale of centuries and longer. Here we present a study of CRII regional effects of the geomagnetic field changes during the last millennium for two regions: Europe and the Far East. We show that regional effects of the migration of the geomagnetic dipole axis may overcome global changes due to solar activity variations
Solar activity during the Holocene: the Hallstatt cycle and its consequence for grand minima and maxim
Cosmogenic isotopes provide the only quantitative proxy for analyzing the
long-term solar variability over a centennial timescale. While essential
progress has been achieved in both measurements and modeling of the cosmogenic
proxy, uncertainties still remain in the determination of the geomagnetic
dipole moment evolution. Here we improve the reconstruction of solar activity
over the past nine millennia using a multi-proxy approach. We used records of
the 14C and 10Be cosmogenic isotopes, current numerical models of the isotope
production and transport in Earth's atmosphere, and available geomagnetic field
reconstructions, including a new reconstruction relying on an updated
archeo-/paleointensity database. The obtained series were analyzed using the
singular spectrum analysis (SSA) method to study the millennial-scale trends. A
new reconstruction of the geomagnetic dipole field moment, GMAG.9k, is built
for the last nine millennia. New reconstructions of solar activity covering the
last nine millennia, quantified in sunspot numbers, are presented and analyzed.
A conservative list of grand minima and maxima is provided. The primary
components of the reconstructed solar activity, as determined using the SSA
method, are different for the series based on 14C and 10Be. These primary
components can only be ascribed to long-term changes in the terrestrial system
and not to the Sun. They have been removed from the reconstructed series. In
contrast, the secondary SSA components of the reconstructed solar activity are
found to be dominated by a common ~2400-yr quasi-periodicity, the so-called
Hallstatt cycle, in both the 14C and 10Be based series. This Hallstatt cycle
thus appears to be related to solar activity. Finally, we show that the grand
minima and maxima occurred intermittently over the studied period, with
clustering near highs and lows of the Hallstatt cycle, respectively.Comment: In press in Astronomy & Astrophysics, doi:
10.1051/0004-6361/20152729
Reconstructing the long-term cosmic ray intensity: linear relations do not work
International audienceIt was recently suggested (Lockwood, 2001) that the cosmic ray intensity in the neutron monitor energy range is linearly related to the coronal source flux, and can be reconstructed for the last 130 years using the long-term coronal flux estimated earlier. Moreover, Lockwood (2001) reconstructed the coronal flux for the last 500 years using a similar linear relation between the flux and the concentration of cosmogenic 10 Be isotopes in polar ice. Here we show that the applied linear relations are oversimplified and lead to unphysical results on long time scales. In particular, the cosmic ray intensity reconstructed by Lockwood (2001) for the last 130 years has a steep trend which is considerably larger than the trend estimated from observations during the last 65 years. Accordingly, the reconstructed cosmic ray intensity reaches or even exceeds the local interstellar cosmic ray flux around 1900. We argue that these unphysical results obtained when using linear relations are due to the oversimplified approach which does not take into account the complex and essentially nonlinear nature of long-term cosmic ray modulation in the heliosphere. We also compare the long-term cosmic ray intensity based on a linear treatment with the reconstruction based on a recent physical model which predicts a considerably lower cosmic ray intensity around 1900
The 1991 March 22 flare: Possible anisotropy of high-energy neutral emission
We made a parameter fit to the Haleakala neutron monitor counting rate during the 1991 March 22 solar flare (Pyle and Simpson, 1991) using the time profiles of γ-rays at 0.42–80 MeV obtained with the GRANAT satellite (Vilmeret al., 1994) and the microwave data from Owens Valley Radio Observatory. We use a two-component neutron injection function to find that either an impulsive injection or the ‘impulsive-plus-prolonged’ neutron injection is possible. In both cases, the number of > 300 MeV neutrons emitted towards the Earth is estimated as ≈ 2 × 10²⁷ sr⁻¹, which is less than that of the 1990 May 24 flare by an order of magnitude.
We tested if such a big difference in neutron number detected on the Earth can be accounted for solely by their different positions on the solar disk. For the estimation of the degree of anisotropy of high-energy secondary emission, we made use of macroscopic parameters of the flare active region, in particular, the vector magnetogram data from the Big Bear Solar Observatory. In our result, the anisotropy factor for the neutral emissions of the 1991 March 22 flare is only ≈ 1 – 10, which is rather small compared with previous theoretical predictions for a disk flare. Such a moderate anisotropy is due to the relatively large inclination angles of the magnetic fields at the footpoints of the flaring loop where accelerated particles are trapped. We thus concluded that the smaller number of neutrons of the 1991 March 22 flare would be not only due to its location on the disk, but also due to fewer protons accelerated during this event as compared with the 1990 May 24 limb event. For a more precise determination of the anisotropy factor in a flare, we need a detailed spectrum of electron bremsstrahlung in 0.1 – 10 MeV and the fluence of γ-ray emission from the π⁰-decay
Updated sunspot group number reconstruction for 1749-1996 using the active day fraction method
Aims. Sunspot number series are composed from observations of hundreds of different observers that require careful normalization to standard conditions. Here we present a new normalized series of the number of sunspot groups for the period 1749-1996. Methods. The reconstruction is based on the active day fraction (ADF) method. which is slightly updated with respect to previous works, and a revised database of sunspot group observations. Results. Stability of some key solar observers has been evaluated against the composite series. The Royal Greenwich Observatory dataset appears relatively stable since the 1890s but is approximately 10% too low before that. A declining trend of 10-15% in the quality of Wolfer's observations is found between the 1880s and 1920s, suggesting that using him as the reference observer may lead to additional uncertainties. Wolf (small telescope) appears relatively stable between the 1860s and 1890s, without any obvious trend. The new reconstruction reflects the centennial variability of solar activity as evaluated using the singular spectrum analysis method. It depicts a highly significant feature of the modern grand maximum of solar activity in the second half of the 20th century, being a factor 1.33-1.77 higher than during the 18 and 19th centuries. Conclusions. The new series of the sunspot group numbers with monthly and annual resolution is provided forming a basis for new studies of the solar variability and solar dynamo for the last 250 yr.Peer reviewe
Level and length of cyclic solar activity during the Maunder minimum as deduced from the active day statistics
The Maunder minimum (MM) of greatly reduced solar activity took place in
1645-1715, but the exact level of sunspot activity is uncertain as based, to a
large extent, on historical generic statements of the absence of spots on the
Sun. Here we aim, using a conservative approach, to assess the level and length
of solar cycle during the Maunder minimum, on the basis of direct historical
records by astronomers of that time. A database of the active and inactive days
(days with and without recorded sunspots on the solar disc respectively) is
constructed for three models of different levels of conservatism (loose ML,
optimum MO and strict MS models) regarding generic no-spot records. We have
used the active day fraction to estimate the group sunspot number during the
MM. A clear cyclic variability is found throughout the MM with peaks at around
1655--1657, 1675, 1684 and 1705, and possibly 1666, with the active day
fraction not exceeding 0.2, 0.3 or 0.4 during the core MM, for the three
models. Estimated sunspot numbers are found very low in accordance with a grand
minimum of solar activity.
We have found, for the core MM (1650-1700), that: (1) A large fraction of
no-spot records, corresponding to the solar meridian observations, may be
unreliable in the conventional database. (2) The active day fraction remained
low (below 0.3-0.4) throughout the MM, indicating the low level of sunspot
activity. (3) The solar cycle appears clearly during the core MM. (4) The
length of the solar cycle during the core MM appears years, but there
is an uncertainty in that. (5) The magnitude of the sunspot cycle during MM is
assessed to be below 5-10 in sunspot numbers;
A hypothesis of the high solar cycles during the MM is not confirmed.Comment: Accepted to Astron. Astrophy
Possible effect of extreme solar energetic particle event of 20 January 2005 on polar stratospheric aerosols: direct observational evidence
Energetic cosmic rays are the main source of ionization of the low-middle atmosphere, leading to associated changes in atmospheric properties. Via the hypothetical influence of ionization on aerosol growth and facilitated formation of clouds, this may be an important indirect link relating solar variability to climate. This effect is highly debated, however, since the proposed theoretical mechanisms still remain illusive and qualitative, and observational evidence is inconclusive and controversial. Therefore, important questions regarding the existence and magnitude of the effect, and particularly the fraction of aerosol particles that can form and grow, are still open. Here we present empirical evidence of the possible effect caused by cosmic rays upon polar stratospheric aerosols, based on a case study of an extreme solar energetic particle (SEP) event of 20 January 2005. Using aerosol data obtained over polar regions from different satellites with optical instruments that were operating during January 2005, such as the Stratospheric Aerosol and Gas Experiment III (SAGE III), and Optical Spectrograph and Infrared Imaging System (OSIRIS), we found a significant simultaneous change in aerosol properties in both the Southern and Northern Polar regions in temporal association with the SEP event. We speculate that ionization of the atmosphere, which was abnormally high in the lower stratosphere during the extreme SEP event, might have led to formation of new particles and/or growth of preexisting ultrafine particles in the polar stratospheric region. However, a detailed interpretation of the effect is left for subsequent studies. This is the first time high vertical resolution measurements have been used to discuss possible production of stratospheric aerosols under the influence of cosmic ray induced ionization. The observed effect is marginally detectable for the analyzed severe SEP event and can be undetectable for the majority of weak-moderate events. The present interpretation serves as a conservative upper limit of solar energetic particle effect upon polar stratospheric aerosols
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