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Analysis of the diurnal and seasonal variation of polar patches, as identified in two years of HF-radar data from Halley, Antarctica during a period near sunspot maximum, shows that there is a broad maximum in occurrence centred about magnetic noon, not local noon. There are minima in occurrence near midsummer and midwinter, with maxima in occurrence between equinox and winter. There are no significant correlations between the occurrence of polar patches and the corresponding hourly averages of the solar wind and IMF parameters, except that patches usually occur when the interplanetary magnetic field has a southward component. The results can be understood in terms of UT and seasonal differences in the plasma concentration being convected from the dayside ionosphere into the polar cap. In summer and winter the electron concentrations in the polar cap are high and low, respectively, but relatively unstructured. About equinox, a tongue of enhanced ionisation is convected into the polar cap; this tongue is then structured by the effects of the interplanetary magnetic field, but these Halley data cannot be used to separate the various competing mechanisms for patch formation. The observed diurnal and seasonal variation in the occurrence of polar patches are largely consistent with predictions of Sojka <i>et al.</i> (1994) when their results are translated into the southern hemisphere. However, the ionospheric effects of flux transfer events are still considered essential in their formation, a feature not yet included in the Sojka <i>et al.</i> model.
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On 17 March 1991, five clear substorm onsets/intensifications took place within a three hour interval. During this interval ground-based data from the EISCAT incoherent scatter radar, a digital CCD all sky camera, and an extensive array of magnetometers were available, in addition to data from the CRRES and DMSP spacecraft, whose footprints passed over Scandinavia very close to most of the ground-based instrumentation. This interval of substorm activity has been interpreted as being in support of a near-Earth current disruption model of substorm onset. In the present study the ionospheric convection response, observed some four hours to the west in MLT by the Halley HF radar in Antarctica, is related to the growth, expansion and recovery phases of two of the substorm onsets/expansions observed in the Northern Hemisphere. Bursts of ionospheric flow and motion of the convection reversal boundary (CRB) are observed at Halley in response to the substorm activity and changes in the IMF. The delay between the substorm expansion phase onset and the response in the CRB location is dependent on the local time separation from, and latitude of, the initial substorm onset region. These results are interpreted in terms of a synthesis of the very near-Earth current disruption model and the near-Earth neutral line model of substorm onset.
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The contribution of magnetospheric current systems to geomagnetic daily variations is analyzed by means of a spherical harmonic analysis (SHA) using elementary models as well as the Tsyganenko model of the magnetosphere. It is discovered that the magnetospheric contribution to some SHA coefficients is much higher than the known average value of about 20%, especially when considering non-local time terms and solstitial conditions.
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During solar flares and coronal mass ejections, nuclei and electrons accelerated to high energies are injected into interplanetary space. These accelerated particles can be detected at the SOHO satellite by the ERNE instrument. From the data produced by the instrument, it is possible to identify the particles and to calculate their energy and direction of propagation. Depending on variable coronal/interplanetary conditions, different kinds of effects on the energetic particle transport can be predicted. The problems of interest include, for example, the effects of particle properties (mass, charge, energy, and propagation direction) on the particle transport, the particle energy changes in the transport process, and the effects the energetic particles have on the solar-wind plasma. The evolution of the distribution function of the energetic particles can be measured with ERNE to a better accuracy than ever before. This gives us the opportunity to contribute significantly to the modeling of interplanetary transport and acceleration. Once the acceleration/transport bias has been removed, the acceleration-site abundance of elements and their isotopes can be studied in detail and compared with spectroscopic observations.
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Over the last 10 years, the experimental basis for the study of the local interstellar medium has been substantially enhanced by the direct detection of interstellar pickup ions and of interstellar neutral helium within the heliosphere. Pickup ions can be studied for a wide range of interstellar species. However, currently the accuracy of the method to determine the parameters of the interstellar medium, namely neutral density, temperature and relative velocity, is hampered by two problems: (1) In most cases the crucial ionization rates are not available from simultaneous measurements and (2) the transport of the pickup ions in the interplanetary medium substantially modifies the measured spatial distribution of the ions. In this study we will discuss how the enhanced capabilities of the instrumentation on SOHO and Cluster in combination with ongoing efforts to model the pickup ion distributions will lead to a significant improvement over the coming years.
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In this study the stratospheric variability is analysed from decadal to seasonal timescales. Relevant processes for the decadal timescale are identified by means of power spectral analysis. The inspection of the ERA-Interim reanalysis data set shows considerably high variability at the 12 and 6 months period. But also in the extra tropical region at intra-annual to seasonal timescales clear peaks in the power spectrum arise. In addition to that, the quasi-biennial oscillation (QBO) obviously contributes to the stratospheric variability at decadal timescales. Regarding the power spectrum of EMAC 2.52 model simulations, only a model configuration with a vertical resolution smaller than 1 km in the stratosphere is capable to capture the relevant features of the spectrum. In particular, the model with a coarser distribution of vertical levels cannot reproduce the QBO signal. The analysis of the corresponding wave spectra reveals that, if the vertical resolution is insufficient, primarily the Mixed-Rossby-Gravity waves cannot be adequately reproduced. Estimates made by linear wave theory show that for reanalysis data the Mixed-Rossby-Gravity waves with equivalent depths between 50 m to 250 m are relevant for the QBO. In order to resolve these relevant waves, model simulations need to consider a vertical resolution of at least 1 km.
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To better understand the influence of monsoonal climate and transport of atmospheric mercury (Hg) in southwestern China, measurements of total gaseous mercury (TGM, defined as the sum of gaseous elemental mercury, GEM, and gaseous oxidized mercury, GOM), particulate bound mercury (PBM) and GOM were carried out at Ailaoshan Station (ALS, 2450 m a.s.l.) in southwestern China from May 2011 to May 2012. The mean concentrations (± SD) for TGM, GOM and PBM were 2.09 ± 0.63, 2.2 ± 2.3 and 31.3 ± 28.4 pg m<sup>−3</sup>, respectively. TGM showed a monsoonal distribution pattern with relatively higher concentrations (2.22 ± 0.58 ng m<sup>−3</sup>, <i>p</i> = 0.021) during the Indian summer monsoon (ISM, from May to September) and the east Asia summer monsoon (EASM, from May to September) periods than that (1.99 ± 0.66 ng m<sup>−3</sup>) in the non-ISM period. Similarly, GOM and PBM concentrations were higher during the ISM period than during the non-ISM period. This study suggests that the ISM and the EASM have a strong impact on long-range and transboundary transport of Hg between southwestern China and south and southeast Asia. Several high TGM events were accompanied by the occurrence of northern wind during the ISM period, indicating anthropogenic Hg emissions from inland China could rapidly increase TGM levels at ALS due to strengthening of the EASM. Most of the TGM and PBM events occurred at ALS during the non-ISM period. Meanwhile, high CO concentrations were also observed at ALS, indicating that a strong south tributary of westerlies could have transported Hg from south and southeast Asia to southwestern China during the non-ISM period. The biomass burning in southeast Asia and anthropogenic Hg emissions from south Asia are thought to be the source of atmospheric Hg in remote areas of southwestern China during the non-ISM period.
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Severe air pollution in Asia is often the consequence of a combination of large anthropogenic emissions and adverse synoptic conditions. However, limited studies on aerosols have been conducted under high emission intensity and under unique geographical and meteorological conditions. In this study, an Aerodyne high-resolution time-of-flight aerosol mass spectrometry (HR-ToF-AMS) and other state-of-the-art instruments were utilized at a suburban site, Ziyang, in the Sichuan Basin during December 2012 to January 2013. The chemical compositions of atmospheric submicron aerosols (PM<sub>1</sub>) were determined, the sources of organic aerosols (OA) were apportioned, and the aerosol secondary formation and aging process were explored as well. Due to high humidity and static air, PM<sub>1</sub> maintained a relatively stable level during the whole campaign, with the mean concentration of 59.7 ± 24.1 µg m<sup>−3</sup>. OA was the most abundant component (36 %) in PM<sub>1</sub>, characterized by a relatively high oxidation state. Positive matrix factorization analysis was applied to the high-resolution organic mass spectral matrix, which deconvolved OA mass spectra into four factors: low-volatility (LV-OOA) and semivolatile oxygenated OA (SV-OOA), biomass burning (BBOA) and hydrocarbon-like OA (HOA). OOA (sum of LV-OOA and SV-OOA) dominated OA as high as 71 %. In total, secondary inorganic and organic formation contributed 76 % of PM<sub>1</sub>. Secondary inorganic species correlated well (Pearson <i>r</i> = 0.415–0.555, <i>p</i> < 0.01) with relative humidity (RH), suggesting the humid air can favor the formation of secondary inorganic aerosols. As the photochemical age of OA increased with higher oxidation state, secondary organic aerosol formation contributed more to OA. The slope of OOA against O<sub><i>x</i></sub>( = O<sub>3</sub>+NO<sub>2</sub>) steepened with the increase of RH, implying that, besides the photochemical transformation, the aqueous-phase oxidation was also an important pathway of the OOA formation. Primary emissions, especially biomass burning, resulted in high concentration and proportion of black carbon (BC) in PM<sub>1</sub>. During the episode obviously influenced by primary emissions, the contributions of BBOA to OA (26 %) and PM<sub>1</sub> (11 %) were much higher than those (10–17 %, 4–7 %) in the clean and other polluted episodes, highlighting the significant influence of biomass burning.
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The increasing ice-free area during late summer has transformed the Arctic to a climate system with more dynamic boundary layer (BL) clouds and seasonal sea ice growth. The open-ocean sensible heat flux, a crucial mechanism of excessive ocean heat loss to the atmosphere during the fall freeze season, is speculated to play an important role in the recently observed cloud cover increase and BL instability. However, lack of observations and understanding of the resilience of the proposed mechanisms, especially in relation to meteorological and interannual variability, has left a poorly constrained BL parameterization scheme in Arctic climate models. In this study, we use multi-year Japanese cruise-ship observations from R/V <i>Mirai</i> over the open Arctic Ocean to characterize the surface sensible heat flux (SSHF) during early fall and investigate its contribution to BL turbulence. It is found that mixing by SSHF is favored during episodes of high surface wind speed and is also influenced by the prevailing cloud regime. The deepest BLs and maximum ocean–atmosphere temperature difference are observed during cold air advection (associated with the stratocumulus regime), yet, contrary to previous speculation, the efficiency of sensible heat exchange is low. On the other hand, the SSHF contributes significantly to BL mixing during the uplift (low pressure) followed by the highly stable (stratus) regime. Overall, it can explain ∼ 10 % of the open-ocean BL height variability, whereas cloud-driven (moisture and radiative) mechanisms appear to be the other dominant source of convective turbulence. Nevertheless, there is strong interannual variability in the relationship between the SSHF and the BL height which can be intensified by the changing occurrence of Arctic climate patterns, such as positive surface wind speed anomalies and more frequent conditions of uplift. This study highlights the need for comprehensive BL observations like the R/V <i>Mirai</i> for better understanding and predicting the dynamic nature of the Arctic climate.
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We present an estimation of the uptake coefficient (<i>γ</i>) and yield of nitryl chloride (ClNO<sub>2</sub>) (<i>f</i>) for the heterogeneous processing of dinitrogen pentoxide (N<sub>2</sub>O<sub>5</sub>) using simultaneous measurements of particle and trace gas composition at a semi-rural, non-coastal, mountain site in the summer of 2011. The yield of ClNO<sub>2</sub> varied between (0.035 ± 0.027) and (1.38 ± 0.60) with a campaign average of (0.49 ± 0.35). The large variability in <i>f</i> reflects the highly variable chloride content of particles at the site. Uptake coefficients were also highly variable with minimum, maximum and average <i>γ</i> values of 0.004, 0.11 and 0.028 ± 0.029, respectively, with no significant correlation with particle composition, but a weak dependence on relative humidity. The uptake coefficients obtained are compared to existing parameterizations based on laboratory datasets and with other values obtained by analysis of field data.
