No abstract

Gross characteristics of the vector magnetic field perturbations detected by the Triad satellite at 800‐km altitude in the auroral oval are analyzed as a function of dipole local time (DLT) under a broad range of magnetic activity. The perturbations are always transverse to the main field and thus result from field‐aligned currents. During the period 1400 DLT through dusk to 2300 DLT the perturbation throughout the oval is directed eastward; the inferred field‐aligned currents flow downward (into the ionosphere) in the equatorward part of the disturbance region and upward (out of the ionosphere) in the poleward part. During the epoch 2400 DLT through dawn to 1000 DLT the magnetic perturbation is westward and the current flow upward in the equatorward part and downward in the poleward part, characteristics reversed from those in the 1400–2300 DLT epoch. Either set can appear in the transition period, 2300–2400 DLT. From 1000 through noon to 1400 DLT the magnetic variations are still transverse and related to field‐aligned currents but are relatively irregular in this continuously disturbed region. The diurnal pattern exists for all types of magnetic activity. Current densities lie mainly between 3 × 10−7 and 4 × 10−6 but can reach 5 × 10−5 A/m² and thus result from field‐directed charges with fluxes principally between 2 × 1012 and 3 × 1013 but extending to 3 × 1014 particles/m² s. At a specific local time one sheet current of an oppositely directed pair could consist of precipitating (1) magnetosheath electrons, (2) electrons with an inverted V spectrum, or (3) plasma sheet electrons. In each case the companion return current is probably formed by ambient ionospheric electrons and/or positive ions.

The consequences of therapeutic hypothermia for neonatal hypoxic-ischemic encephalopathy are poorly understood. Adverse effects from suboptimal rewarming could diminish neuroprotection from hypothermia. Therefore, we tested whether rewarming is associated with apoptosis. Piglets underwent hypoxia-asphyxia followed by normothermic or hypothermic recovery at 2 hours. Hypothermic groups were divided into those with no rewarming, rewarming at 0.5 °C/hour, or rewarming at 4 °C/hour. Neurodegeneration at 29 hours was assessed by hematoxylin and eosin staining, TUNEL assay, and immunoblotting for cleaved caspase-3. Rewarmed piglets had more apoptosis in motor cortex than did those that remained hypothermic after hypoxia-asphyxia. Apoptosis in piriform cortex was greater in hypoxic-asphyxic, rewarmed piglets than in naive/sham piglets. Caspase-3 inhibitor suppressed apoptosis with rewarming. Rapidly rewarmed piglets had more caspase-3 cleavage in cerebral cortex than did piglets that remained hypothermic or piglets that were rewarmed slowly. We conclude that rewarming from therapeutic hypothermia can adversely affect the newborn brain by inducing apoptosis through caspase mechanisms.

Background: Therapeutic hypothermia provides incomplete neuroprotection for neonatal hypoxic-ischemic encephalopathy (HIE). We examined whether hemodynamic goals that support autoregulation are associated with decreased brain injury and whether these relationships are affected by birth asphyxia or vary by anatomic region. Methods: Neonates cooled for HIE received near-infrared spectroscopy autoregulation monitoring to identify the mean arterial blood pressure with optimized autoregulatory function (MAP_OPT). Blood pressure deviation from MAP_OPT was correlated with brain injury on MRI after adjusting for the effects of arterial carbon dioxide, vasopressors, seizures, and birth asphyxia severity. Results: Blood pressure deviation from MAP_OPT related to neurologic injury in several regions independent of birth asphyxia severity. Greater duration and deviation of blood pressure below MAP_OPT were associated with greater injury in the paracentral gyri and white matter. Blood pressure within MAP_OPT related to lesser injury in the white matter, putamen and globus pallidus, and brain stem. Finally, blood pressures that exceeded MAP_OPT were associated with reduced injury in the paracentral gyri. Conclusions: Blood pressure deviation from optimal autoregulatory vasoreactivity was associated with MRI markers of brain injury that, in many regions, were independent of the initial birth asphyxia. Targeting hemodynamic ranges to optimize autoregulation has potential as an adjunctive therapy to hypothermia for HIE.