title: Fast dynamic reconnection at X-type neutral points

authors: Craig, Ian J.D.

abstract: We consider the linear and nonlinear evolution of disturbed magnetic X-type neutral points. The problem is formulated within a unified analytic and computational framework which highlights the essence of the magnetic annihilation process, namely, the coupling of a global convection region to a localized diffusion region surrounding the neutral point. An analytic treatment is given for the case of small disturbances of the equilibrium field in the absence of gas pressure. This problem admits well-defined azimuthal modes which allow a formally exact determination of the magnetic annihilation rate. It is shown that reconnection can only occur in the case of purely radial (m = 0) disturbances: the reconnection process is oscillatory and "fast," depending only logarithmically on the plasma resistivity (η). We show that the linear theory supports the notion of an initial implosive stage which rapidly releases the bulk of the energy associated with reconnective field disturbances. This phase is initiated by the advective focusing of the perturbation energy into the neutral point and culminates in the formation of a cylindrical diffusion region of area A ∼ η and current density J ∼ η-1. This scaling provides a signature for fast linear reconnection. Next we consider the breakdown of the linear theory. Although fast reconnection is maintained for low-amplitude disturbances in noncylindrical geometries, it is shown that finite gas pressure can stall the reconnection if sufficiently large. This effect, however, may not be critical in more complex X-point geometries. More seriously, for finite-amplitude displacements the cylindrical current structure close to the neutral point is distorted into a quasi-one-dimensional current sheet whose thickness is limited by resistive diffusion. In this case fast reconnection is consistent with a flux pileup solution in which the bulk of the energy is released as heat rather than as the kinetic energy of mass motion.

text: Fast dynamic reconnection at X-type neutral points We consider the linear and nonlinear evolution of disturbed magnetic X-type neutral points. The problem is formulated within a unified analytic and computational framework which highlights the essence of the magnetic annihilation process, namely, the coupling of a global convection region to a localized diffusion region surrounding the neutral point. An analytic treatment is given for the case of small disturbances of the equilibrium field in the absence of gas pressure. This problem admits well-defined azimuthal modes which allow a formally exact determination of the magnetic annihilation rate. It is shown that reconnection can only occur in the case of purely radial (m = 0) disturbances: the reconnection process is oscillatory and "fast," depending only logarithmically on the plasma resistivity (η). We show that the linear theory supports the notion of an initial implosive stage which rapidly releases the bulk of the energy associated with reconnective field disturbances. This phase is initiated by the advective focusing of the perturbation energy into the neutral point and culminates in the formation of a cylindrical diffusion region of area A ∼ η and current density J ∼ η-1. This scaling provides a signature for fast linear reconnection. Next we consider the breakdown of the linear theory. Although fast reconnection is maintained for low-amplitude disturbances in noncylindrical geometries, it is shown that finite gas pressure can stall the reconnection if sufficiently large. This effect, however, may not be critical in more complex X-point geometries. More seriously, for finite-amplitude displacements the cylindrical current structure close to the neutral point is distorted into a quasi-one-dimensional current sheet whose thickness is limited by resistive diffusion. In this case fast reconnection is consistent with a flux pileup solution in which the bulk of the energy is released as heat rather than as the kinetic energy of mass motion.

year: 1992

title: Investigating the impact of Tourism on forest cover in the Annapurna conservation area through Remote Sensing and Statistical Analysis

authors: Chaplin, James Andrew

subjects: Nepal

abstract: Tourism is Nepal’s largest industry giving people in rural areas an alternative to subsistence farming. Tourism can have an impact on the forest cover of a region as trees provide firewood for cooking and heating and timber for building accommodation. In 1986 the Annapurna conservation area project was started to ensure that tourism was managed sustainably, which includes minimising the impacts on the forest cover. This study assesses the impacts of tourism on the forest cover in the Annapurna region by comparing Landsat images from 1999 and 2011.  This was achieved through spectral classification of different landcover and assessing the change in forest cover in relation to increasing distances from tourism villages. A major problem with remote sensing in mountainous regions such as Nepal is shadow caused by the relief. This issue was addressed by only assessing areas which were free from shadow, which in effect meant a sample was used rather than the whole study region. The results indicate that there has been an 8 per cent reduction in overall forest extent, but this change varies by region.  In the northern drier regions there has been a net increase in forest cover, while in the southern regions there has been a net reduction in forests. The influence of tourism facilities on forest is also variable. Around each of the sample tourism villages there was a general trend of decreasing removal of forest at greater distances from each village, which indicates tourism does have a negative impact on forests. However, there was an opposite trend in the northern villages that were well inside the conservation area.

text: Investigating the impact of Tourism on forest cover in the Annapurna conservation area through Remote Sensing and Statistical Analysis Tourism is Nepal’s largest industry giving people in rural areas an alternative to subsistence farming. Tourism can have an impact on the forest cover of a region as trees provide firewood for cooking and heating and timber for building accommodation. In 1986 the Annapurna conservation area project was started to ensure that tourism was managed sustainably, which includes minimising the impacts on the forest cover. This study assesses the impacts of tourism on the forest cover in the Annapurna region by comparing Landsat images from 1999 and 2011.  This was achieved through spectral classification of different landcover and assessing the change in forest cover in relation to increasing distances from tourism villages. A major problem with remote sensing in mountainous regions such as Nepal is shadow caused by the relief. This issue was addressed by only assessing areas which were free from shadow, which in effect meant a sample was used rather than the whole study region. The results indicate that there has been an 8 per cent reduction in overall forest extent, but this change varies by region.  In the northern drier regions there has been a net increase in forest cover, while in the southern regions there has been a net reduction in forests. The influence of tourism facilities on forest is also variable. Around each of the sample tourism villages there was a general trend of decreasing removal of forest at greater distances from each village, which indicates tourism does have a negative impact on forests. However, there was an opposite trend in the northern villages that were well inside the conservation area.

year: 2013

title: Introduction to tephra-derived soils, North Island, New Zealand: University of Waikato and University of Wisconsin-Platteville post-conference Andisol excursion, 21-23 December, 2016

authors: Lowe, David J.

abstract: Provisional itinerary DAY 1 Wednesday 21 December: Mamaku Plateau-Rerewhakaaitu-Lake Tarawera: 1 Goodwin Farm, Tapapa Rd, Tapapa: welded ignimbrite, tephras, loess, buried soils (Tirau soil) - stratigraphy of sequence (230 ka and younger) and upbuilding pedogenesis - Tirau silt loam 2 Brett Rd, Rerewhakaaitu: Holocene tephras and buried soil (Rotomahana soil) - Volcanic landscape, historical importance of area for NZ soil survey - Stratigraphy of sequence (~9.5 cal ka to 10 June 1886 Tarawera eruption) and upbuilding pedogenesis - Rotomahana sandy loam, 3 Ash Pit Rd, Rerewhakaaitu: tephras and buried soils (Matahina soil) - Matahina gravel - buried spodic/podzol soil features, 4 Okareka Loop Rd tephra section with proximal Rotorua Tephra and buried soils, loess, Buried Village Museum, Te Wairoa, Lake Tarawera (Stoney Point). DAY 2 Thursday 22 December: Otorohanga-Waitomo Caves -Pirongia Mountain: 1 Raynes Road section: Ultisol in composite tephra sequence (Kainui soil), 2 Otorohanga Kiwi House, 3 Waitomo Caves,, 4 Pirongia Mountain: Mangakara forest walk, 5 Evening meal on an Andisol. DAY 3 Friday 23 December: Hobbiton, Hamilton gardens including Te Parapara (ancient Maori garden): 1. Hobbiton tour 10.15 am (2 hrs), 2. Hamilton Gardens incl. Te Parapara Garden and human-modified soils (Tamahere soil).

text: Introduction to tephra-derived soils, North Island, New Zealand: University of Waikato and University of Wisconsin-Platteville post-conference Andisol excursion, 21-23 December, 2016 Provisional itinerary DAY 1 Wednesday 21 December: Mamaku Plateau-Rerewhakaaitu-Lake Tarawera: 1 Goodwin Farm, Tapapa Rd, Tapapa: welded ignimbrite, tephras, loess, buried soils (Tirau soil) - stratigraphy of sequence (230 ka and younger) and upbuilding pedogenesis - Tirau silt loam 2 Brett Rd, Rerewhakaaitu: Holocene tephras and buried soil (Rotomahana soil) - Volcanic landscape, historical importance of area for NZ soil survey - Stratigraphy of sequence (~9.5 cal ka to 10 June 1886 Tarawera eruption) and upbuilding pedogenesis - Rotomahana sandy loam, 3 Ash Pit Rd, Rerewhakaaitu: tephras and buried soils (Matahina soil) - Matahina gravel - buried spodic/podzol soil features, 4 Okareka Loop Rd tephra section with proximal Rotorua Tephra and buried soils, loess, Buried Village Museum, Te Wairoa, Lake Tarawera (Stoney Point). DAY 2 Thursday 22 December: Otorohanga-Waitomo Caves -Pirongia Mountain: 1 Raynes Road section: Ultisol in composite tephra sequence (Kainui soil), 2 Otorohanga Kiwi House, 3 Waitomo Caves,, 4 Pirongia Mountain: Mangakara forest walk, 5 Evening meal on an Andisol. DAY 3 Friday 23 December: Hobbiton, Hamilton gardens including Te Parapara (ancient Maori garden): 1. Hobbiton tour 10.15 am (2 hrs), 2. Hamilton Gardens incl. Te Parapara Garden and human-modified soils (Tamahere soil).

year: 2016-12-21