Improved dynamic geomagnetic rigidity cutoff modeling: testing predictive accuracy

. In the polar atmosphere, significant chemical and ionization changes occur during solar proton events (SPE). The access of solar protons to this region is limited by the dynamically changing geomagnetic field. In this study we have used riometer absorption observations to investigate the accuracy of a model to predict Kp-dependent geomagnetic rigidity cutoffs, and hence the changing proton fluxes. The imaging riometer at Halley, Antarctica is ideally situated for such a study, as the rigidity cutoff sweeps back and forth across the instrument's field of view, providing a severe test of the rigidity cutoff model. Using observations from this riometer during five solar proton events, we have confirmed the basic accuracy of this rigidity model. However, we find that the model can be improved by setting a lower Kp limit (i.e., Kp=5 instead of 6) at which the rigidity modeling saturates. We also find that for L>4.5 the apparent L-shell of the beam moves equatorwards. In addition, the Sodankyla Ion and Neutral Chemistry model is used to determine an empirical relationship between integral proton precipitation fluxes and nighttime ionosphere riometer absorption, in order to allow consideration of winter time SPEs. We find that during the nighttime the proton flux energy threshold is lowered to include protons with energies of >5 MeV in comparison with >10 MeV for the daytime empirical relationships. In addition, we provide an indication of the southern and northern geographic regions inside which SPEs play a role in modifying the neutral chemistry of the stratosphere and mesosphere

Surface-subsurface hydrologic exchange and nutrient dynamics in the hyporheic zone of the Tanana River

Thesis (M.S.) University of Alaska Fairbanks, 2007The aquatic-terrestrial interface is an active site of biogeochemical transformation, regulating the flux of nutrients between ecosystems. I addressed the hydrologic controls on nitrogen biogeochemistry in the hyporheic zone of a glacially fed river. I measured hyporheic concentrations of solutes and gases along subsurface flowpaths on two islands. Denitrification was quantified using an in situ [delta]⁻¹⁵NO₃⁻ push-pull technique. Nitrate concentration was consistently greater in river than in hyporheic water. Denitrification ranged from 1.9 - 29.4 mgN kg sediment⁻¹ day⁻¹. Hotspots of methane partial pressure, averaging 50,000 ppmv, were found in densely vegetated areas with low oxygen concentration (<0.5 mgO₂ L⁻¹). Hyporheic flow was an important source of nitrogen to microbes and vegetation, transporting on average 0.41 gNO₃⁻-N m⁻² day⁻¹ through surface sediments. Results suggest that denitrification is a major sink for river nitrate in boreal forest floodplain soils, particularly at the river-sediment interface. The stability of the river hydro graph is a key factor regulating anaerobic metabolism in the hyporheic zone.1. Introduction -- The hyporheic zone -- Biogeochemical transformations along hyporheic flowpaths -- Nitrogen supply in riparian zones -- Hydrology and biogeochemistry of the Tanana River at Bonanza Creek LTER -- Literature cited -- 2. Surface-subsurface hydrologic exchange and nutrient dynamics in the hyporheic zone of the Tanana River in Interior Alaska -- Literature cited -- 2. Surface-subsurface hydrologic exchange and nutrient dynamics in the hyporheic zone of the Tanana River in Interior Alaska -- Abstract -- Introduction -- Methods -- Study site -- Study design -- Sampling and analytical techniques -- In situ denitrification -- Push-pull calculations -- Data analysis -- Analysis of subsurface hydrology and capillary rise -- Long term patterns in climate and river hydrology -- Results -- Climate and river hydrology -- Spatial patterns in hyporheic chemistry -- Subsurface hydrology and nitrogen losses -- Temporal variation in hyporheic chemistry -- Discussion -- Hyporheic zone hydrology and nitrogen transformation -- Subsurface methane and carbon dioxide -- Climate, river hydrology and hyporheic chemistry -- Acknowledgements -- Literature cited -- 3. Conclusions -- Literature cited

Effects of solar wind magnetosphere coupling recorded at different geomagnetic latitudes: Separation of directly-driven and storage/release systems

The effect on geomagnetic activity of solar wind speed, compared with that of the strength of the interplanetary magnetic field, differs with geomagnetic latitude. In this study we construct a new index based on monthly standard deviations in the H-component of the geomagnetic field for all geomagnetic latitudes. We demonstrate that for this index the response at auroral regions correlates best with interplanetary coupling functions which include the solar wind speed while mid- and low-latitude regions respond to variations in the interplanetary magnetic field strength. These results are used to isolate the responsible geomagnetic current systems

Relativistic microburst storm characteristics: Combined satellite and ground-based observations

We report a comparison of Solar Anomalous Magnetospheric Particle Explorer detected relativistic electron microbursts and short-lived subionospheric VLF perturbations termed FAST events, observed at Sodankyl Geophysical Observatory, Finland, during 2005. We show that only strong geomagnetic disturbances can produce FAST events, which is consistent with the strong link between storms and relativistic microbursts. Further, the observed FAST event perturbation decay times were consistent with ionospheric recovery from bursts of relativistic electron precipitation. However, the one-to-one correlation in time between microbursts and FAST events was found to be very low (similar to 1%). We interpret this as confirmation that microbursts have small ionospheric footprints and estimate the individual precipitation events to be <4 km radius. In contrast, our study strongly suggests that the region over which microbursts occur during storm event periods can be at least similar to 90 degrees in longitude (similar to 6 h in magnetic local time). This confirms earlier estimates of microburst storm size, suggesting that microbursts could be a significant loss mechanism for radiation belt relativistic electrons during geomagnetic storms. Although microbursts are observed at a much higher rate than FAST events, the ground-based FAST event data can provide additional insight into the conditions required for microburst generation and the time variation of relativistic precipitation

Response: Commentary: Energetic particle forcing of the Northern Hemisphere winter stratosphere: comparison to solar irradiance forcing

Variation in solar irradiance is considered an important factor in natural climate forcing. Variations in the solar UV in particular are now regarded as a major source of decadal variability in the stratosphere, influencing surface climate through stratosphere-troposphere coupling.However, by analyzing meteorological re-analysis data we find that the magnitude of the solar controlled energetic particle forcing signal in stratospheric zonal mean zonal winds and polar temperatures is equivalent to those arising from solar irradiance variations during the Northern hemisphere polar winter months.We find that energetic particle forcing drives warmer polar upper stratospheric temperatures from early winter leading to an anomalously strong polar night jet via modulation of the vertical temperature gradient. By midwinter the stratosphere-troposphere coupling pathway becomes analogous to the solar UV impact at high latitudes. This not only highlights the importance of the energetic particle forcing contribution to stratospheric circulation, but enables us to understand the pathways responsible for the previously reported energetic particle forcing impacts on the troposphere in terms of the coupling of solar UV forcing to dynamics in the latter part of the winter.<br/

Hydroecological monitoring and modelling of river-floodplain restoration in a UK lowland river meadow

Channelization and embankment of rivers has led to major ecological degradation of aquatic habitats worldwide. River restoration can be used to restore favourable hydrological conditions for target processes or species. This study is based on rarely available, detailed pre- and post-restoration hydrological data collected from 2007–2010 from a wet grassland meadow in Norfolk, UK. Based on these data, coupled hydrological/hydraulic models were developed of pre-embankment and post-embankment conditions using the MIKE-SHE/MIKE-11 system. Fine-scale plant and chemical sampling was conducted on the floodplain meadow to assess the spatial pattern of plant communities in relation to soil physicochemical conditions. Simulated groundwater levels for a 10-year period were then used to predict changes in plant community composition following embankment-removal. Hydrology was identified as the primary driver of plant community composition, while soil fertility was also important. Embankment removal resulted in widespread floodplain inundation at high river flows and frequent localised flooding at the river edge at lower flows. Subsequently, groundwater levels were higher and subsurface storage was greater. The restoration had a moderate effect on flood-peak attenuation and improved free drainage to the river. Reinstatement of overbank flows did not substantially affect the degree of aeration stress on the meadow, except along the river embankments where sum exceedance values for aeration stress increased from 0 m weeks (dry-grassland) to 7 m weeks (fen). The restored groundwater regime may be suitable for more diverse plant assemblages. However the benefits of flooding (e.g. propagule dispersal, reduced competition) may be over-ridden without management to reduce waterlogging during the growing season, or balance additional nutrient supply from river water. The results from this study suggest that removal of river embankments can increase river-floodplain hydrological connectivity to form a more natural flood-pulsed wetland ecotone, which favours conditions for enhanced flood storage, plant species composition and nutrient retention

Trend and abrupt changes in long-term geomagnetic indices

Advanced statistical methods are employed to analyze three long-term time series of geomagnetic activity indices (aa, IHV, and IDV) together with sunspot number (Rz) to examine whether or not the aa index can realistically represent long-term variations of geomagnetic activity. We make use of a decomposition method called STL, which is a time domain filtering procedure that decomposes a time series into trend, cyclic, and residual components using nonparametric regression. A Bayesian change point analysis is also applied to the geomagnetic indices, as well as to sunspot number, to detect abrupt changes that may be caused by either instrumental changes, calibration errors, or sudden changes in solar activity. Our analysis shows that all three long-term geomagnetic indices share a similar centennial-scale variation that resembles the long-term trend of sunspot number Rz. The amplitude ratio between the centennial-scale variation and 11-year cycle of aa and IHV are closely comparable. Overall, our analysis suggests that the majority of the changes in the aa index are controlled by solar activity. Instrumental change or site relocation has only a limited effect on the long-term trend of aa. This is in good agreement with those previous studies which have shown aa to be a reliable long-term index

POES satellite observations of EMIC-wave driven relativistic electron precipitation during 1998-2010

[1] Using six Polar Orbiting Environmental Satellites (POES) satellites that have carried the Space Environment Module-2 instrument package, a total of 436,422 individual half-orbits between 1998 and 2010 were inspected by an automatic detection algorithm searching for electromagnetic ion cyclotron (EMIC) driven relativistic electron precipitation (REP). The algorithm searched for one of the key characteristics of EMIC-driven REP, identified as the simultaneity between spikes in the P1 (52 keV differential proton flux channel) and P6 (>800 keV electron channel). In all, 2331 proton precipitation associated REP (PPAREP) events were identified. The majority of events were observed at L-values within the outer radiation belt (3 < L < 7) and were more common in the dusk and night sectors as determined by magnetic local time. The majority of events occurred outside the plasmasphere, at L-values ~1 Re greater than the plasmapause location determined from two different statistical models. The events make up a subset of EMIC-driven proton spikes investigated by Sandanger et al. (2009), and potentially reflect different overall characteristics compared with proton spikes, particularly when comparing their location to that of the plasmapause, i.e., EMIC-driven proton precipitation inside the plasmapause, and potentially EMIC-driven REP outside the plasmapause. There was no clear relationship between the location of plasmaspheric plumes and the locations of the PPAREP events detected. Analysis of the PPAREP event occurrence indicates that high solar wind speed and high geomagnetic activity levels increase the likelihood of an event being detected. The peak PPAREP event occurrence was during the declining phase of solar cycle 23, consistent with the 2003 maximum in the geomagnetic activity index, Ap

Nature's Grand Experiment: Linkage between magnetospheric convection and the radiation belts

The solar minimum of 2007–2010 was unusually deep and long lived. In the later stages of this period the electron fluxes in the radiation belts dropped to extremely low levels. The flux of relativistic electrons (>1 MeV) was significantly diminished and at times was below instrument thresholds both for spacecraft located in geostationary orbits and also those in low-Earth orbit. This period has been described as a natural “Grand Experiment” allowing us to test our understanding of basic radiation belt physics and in particular the acceleration mechanisms which lead to enhancements in outer belt relativistic electron fluxes. Here we test the hypothesis that processes which initiate repetitive substorm onsets drive magnetospheric convection, which in turn triggers enhancement in whistler mode chorus that accelerates radiation belt electrons to relativistic energies. Conversely, individual substorms would not be associated with radiation belt acceleration. Contrasting observations from multiple satellites of energetic and relativistic electrons with substorm event lists, as well as chorus measurements, show that the data are consistent with the hypothesis. We show that repetitive substorms are associated with enhancements in the flux of energetic and relativistic electrons and enhanced whistler mode wave intensities. The enhancement in chorus wave power starts slightly before the repetitive substorm epoch onset. During the 2009/2010 period the only relativistic electron flux enhancements that occurred were preceded by repeated substorm onsets, consistent with enhanced magnetospheric convection as a trigger

Very low latitude whistler‐mode signals: Observations at three widely spaced latitudes

VLF radio signals with travel times ~100 ms were observed continuously for up to ~11 hours at night on Rarotonga (Cook Islands, ~21°S) at 21.4 kHz from US Navy transmitter NPM, Hawaii (~21°N). These signals travelled in the whistler‐mode on well‐defined paths, though not field‐aligned ducts, through the ionospheric F region, and across the equator reaching altitudes ~700‐1400 km depending on time of night. These same signals were also observed simultaneously in Dunedin (46°S), New Zealand, with very nearly the same travel times but with somewhat lower amplitudes and occurrence rates, consistent with the whistler‐mode part of the propagation being at very low latitudes. Both sets of signals had similar Doppler shifts, typically tens of mHz, but sometimes up to a few hundred mHz, being positive during most of the night, while the whistler‐mode group delays decreased due to both the shortening of the path and the decay of the near equatorial ionosphere, but negative near dawn when the Sun's rays start ionizing the F region. The signals are not observable during the day, fading out during dawn, due to increasing attenuation from the increasing electron density, and hence increasing collisions, in both the D and F regions. Similar weaker NPM signals were also seen at Rothera (68°S). In addition, similar 24.8 kHz signals were seen from the more distant NLK (Seattle, ~48°N) at Rarotonga, though clearly weaker than from NPM, but not at Dunedin