Many arid grasslands around the world are affected by woody plant encroachment and by the replacement of a relatively continuous grass cover with shrub patches bordered by bare soil. This shift in plant community composition is often abrupt in space and time, suggesting that it is likely sustained by positive feedbacks between vegetation and environmental conditions (e.g. resource availability) or disturbance regime (e.g. fire or freeze). These feedbacks amplify the effects of drivers of shrub encroachment, i.e. of conditions favouring a shift from grass to shrub dominance (e.g. overgrazing, climate change). Here, we review some major drivers and feedbacks and identify the basic stages in the transition from grassland to shrubland. We discuss some possible scenarios of interactions between drivers and feedbacks that could explain the transition from a stage to the next and the potential irreversibility of the shift from grass to shrub dominance. We introduce a simplistic modelling framework that can integrate the various drivers to explain the emergence of bistability for shrub-encroached grassland systems. Published 2011. This article is a U.S. Government work and is in the public domain in the USA.

/pdf/a-synthetic-review-of-feedbacks-and-drivers-of-shrub-1p6fzhz7ub.pdf

A synthetic review of feedbacks and drivers of shrub encroachment in arid grasslands

[1] Large ionospheric variability is found at low to middle latitudes when a quasi-stationary planetary wave is specified in the winter stratosphere in the National Center for Atmospheric Research thermosphere-ionosphere-mesosphere electrodynamics general circulation model for solar minimum conditions. The variability includes change of electric field/ion drift, F2 peak density and height, and the total electron content. The electric field/ion drift change is the largest near dawn in the numerical experiments. Analysis of model results suggests that, although the quasi-stationary planetary wave does not propagate deep into the ionosphere or to low latitudes due to the presence of critical layers and strong molecular dissipation, the planetary wave and tidal interaction leads to large changes in tides, which can strongly impact the ionosphere at low and middle latitudes through the E region wind dynamo. Large zonal gradients of zonal and meridional winds from the tidal components and the zonal gradient of electric conductivities at dawn can produce large convergence/divergence of Hall and Pedersen currents, which in turn produces a polarization electric field. The ionospheric changes are dependent on both the longitude and local time, and are determined by the amplitudes and phases of the superposing wave components. The model results are consistent with observed ionospheric changes at low and middle latitudes during stratospheric sudden warming events, when quasi-stationary planetary waves become large.

/pdf/ionospheric-variability-due-to-planetary-waves-and-tides-for-gmm6y9chq7.pdf

Ionospheric variability due to planetary waves and tides for solar minimum conditions

The sources of ionospheric forcing are from above (solar and magnetospheric forces), internal (chemistry, local neutral dynamics), and from below (lower atmosphere). In this work we review the observed ion-neutral coupling effects at equatorial and low latitudes during large meteorological events called sudden stratospheric warming (SSW). Research in this direction has been accelerated in recent years mainly due to: (1) extensive observing campaigns, and (2) solar minimum conditions. The former has been instrumental to catch the events before, during, and after the peak SSW temperatures. The latter has permitted a reduced forcing contribution from above and internal. The main ionospheric effects are clearly observed in the zonal electric fields (or vertical E×B drifts), total electron content, and peak electron densities. We include results from different ground- and satellite-based observations, covering different longitudes and years. We also present and discuss the modeling efforts that support most of the observations. Given that SSW can be forecast with a few days in advanced, there is potential for using the connection with the ionosphere for forecasting the occurrence and evolution of electrodynamic perturbations at low latitudes, and sometimes mid latitudes, during arctic winter warmings.

/pdf/equatorial-and-low-latitude-ionospheric-effects-during-4u1lki5jyu.pdf

Equatorial and low latitude ionospheric effects during sudden stratospheric warming events

Earth's cusps are magnetic field features in the magnetosphere associated with regions through which plasma from the Sun can have direct access to the upper atmosphere. Recently, new ground-based observations, combined with in situ satellite measurements, have led the way in reinterpreting cusp signatures. These observations, combined with theoretical advances, have stimulated new interest in the solar wind-magnetosphere-ionosphere coupling chain. This coupling process is important because it causes both momentum and energy from the solar wind to enter into the near-Earth region. Here we describe the current ideas concerning the cusps and the supporting observational evidence which have evolved over the past 30 years. We include discussion on the plasma entry process, particle motion between the magnetopause and ionosphere, ground optical and radar measurements, and transient events. We also review the important questions that remain to be answered.

Earth's magnetospheric cusps

Ionospheric F-region response to geomagnetic disturbances

[1] Equatorial spread F (ESF) was discovered almost a century ago using the first radio wave instrument designed to study the upper atmosphere: the ionosonde. The name came from the appearance of reflections from the normally smooth ionosphere, which were spread over the altitude frequency coordinates used by the instrument. Attempts to understand this phenomenon in any depth activated such tools as radars and in situ probes such as rockets and satellites in the 1960s. Over the next 15 years, these tools expanded our experimental understanding enormously, and new nonlinear theoretical methods developed in the late 1970s, which led to proposing a name revision from ESF to convective ionospheric storms. Interest in these phenomena continues, but a new, practical aspect has developed from the associated turbulence effects on communications (transionosphere) and navigation (GPS). The first satellite to specifically investigate this problem and the associated goal of predicting occurrences is under the umbrella of the Communications/Navigation Outage Forecast System (C/NOFS). In contemplating the successful first years of the C/NOFS program, reviewing the state of the art in our knowledge of convective ionospheric storms seems appropriate. We also present some initial results of this satellite program. A major goal of the National Space Weather Program, and of C/NOFS, is predicting these storms, analogous to thunderstorms in the lower atmosphere due to their adverse effects on communication and navigation signals. Although ambitious, predictive capability is a noble and important goal in the current technological age and is potentially within our reach during the coming decade.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2010RG000340

Convective ionospheric storms: a review

[1] The Communication/Navigation Outage Forecasting System (C/NOFS) satellite was launched in 2008, during solar minimum conditions. An unexpected feature in the C/NOFS plasma density data is the presence of deep plasma depletions observed at sunrise at all satellite altitudes. Ionospheric irregularities are often embedded within these dawn depletions. Their frequencies strongly depend on longitude and season. Dawn depletions are also observed in coincident satellite passes such as DMSP and CHAMP. In one example the depletion extended 50° × 14° in the N-S and E-W directions, respectively. These depletions are caused by upward plasma drifts observed in C/NOFS and ground-based measurements. The reason for these upward drifts is still unresolved. We discuss the roles of dynamo electric fields, over-shielding, and tidal effects as sources for the reported depletions.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009GL038884

C/NOFS observations of deep plasma depletions at dawn

[1] The longitudinal and seasonal variations of the plasma density and drift velocity in the evening equatorial ionosphere show complex characteristics. In this paper, we analyze the ionospheric ion density and velocity measured by the Defense Meteorological Satellites Program (DMSP) F13 and F17 satellites at ∼1800 LT. Our analysis focuses on whether the longitudinal structure of the equatorial ionospheric density and velocity has multiple wave number components, how the longitudinal structure of the equatorial ion eastward drift varies with season and year, and what causes the longitudinal variations of the ion density and velocity. We find that the longitudinal structure of the ion density and velocity show a wave-4 pattern during the equinox months (March, April, September, and October), a wave-3 pattern in northern summer (May–August), and a wave-2 pattern in northern winter (November–February). The longitudinal structure of the ion density and velocity becomes significantly different from the wave-4 pattern in the longitude regions with large magnetic declination in the summer and winter. The variation of the equatorial ionospheric ion density shows an in-phase correspondence with the ion eastward velocity and an anti-phase correspondence with the ion upward velocity. We suggest that the enhanced upward drift causes the density decrease through the fountain effect and that the enhanced eastward drift causes the density increase through horizontal transport. The change of the ion eastward velocity with longitude can be as high as 180 m s−1 over a longitudinal range of 120° and is related to the solar activity. The results of this study reveal a number of new characteristics of the equatorial plasma horizontal drift in the east–west direction.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2009JA014503

Longitudinal and seasonal variations of the equatorial ionospheric ion density and eastward drift velocity in the dusk sector

Convective Ionospheric Storms: A Major Space Weather Problem

We have examined Sondrestrom incoherent scatter radar observations of ionospheric plasma density and temperature distributions and measurements of F region ion drifts that were made during a prenoon pass of the Defense Meteorological Satellite Program (DMSP)-F7 satellite through the radar field of view. The spacecraft traversed a region of intense electron precipitation with a characteristic energy below approximately 200 eV. Particles with such low characteristic energies are believed to be directly or indirectly of magnetosheath origin. The precipitation region had a width about 2 deg invariant latitude and covered the low-latitude boundary layer (LLBL), the cusp, and the equatorward section of the plasma mantle (PM). The corotating radar observed a patch of enhanced electron density and elevated electron temperature in the F2 region between about 10.5 and 12 magnetic local time in the same invariant latitude range where DMSP-F7 detected the soft-electron flux. The ion drift pattern, also obtained by radar, shows that it is unlikely that the plasma patch was produced by solar radiation and advected into the radar field of view. We suggest that the radar observed modifications of the ionospheric plasma distribution, which resulted from direct entry of magnetosheath electrons into the magnetosphere and down to ionospheric altitudes. Model calculations of the ionospheric response to the observed electron precipitation support our interpretation. The spectral characteristics of the electron flux in the LLBL, cusp, and equatorward section of the PM were in this case too similar to allow to distinguish between them by using incoherent scatter radar measurements only.

Odile de La Beaujardiere

Papers

Convective ionospheric storms: a review

C/NOFS observations of deep plasma depletions at dawn

Longitudinal and seasonal variations of the equatorial ionospheric ion density and eastward drift velocity in the dusk sector

Convective Ionospheric Storms: A Major Space Weather Problem

Ionospheric footprint of magnetosheathlike particle precipitation observed by an incoherent scatter radar