No abstract

A detailed, in situ study of field-aligned current (FAC) structure in a transient, substorm expansion phase auroral arc is conducted using electric field, magnetometer, and electron density measurements from the Magnetosphere-Ionosphere Coupling in the Alfvén Resonator (MICA) sounding rocket, launched from Poker Flat, AK. These data are supplemented with larger-scale, contextual measurements from a heterogeneous collection of ground-based instruments including the Poker Flat incoherent scatter radar and nearby scanning doppler imagers and filtered all-sky cameras. An electrostatic ionospheric modeling case study of this event is also constructed by using available data (neutral winds, electron precipitation, and electric fields) to constrain model initial and boundary conditions. MICA magnetometer data are converted into FAC measurements using a sheet current approximation and show an up-down current pair, with small-scale current density and Poynting flux structures in the downward current channel. Model results are able to roughly recreate only the large-scale features of the field-aligned currents, suggesting that observed small-scale structures may be due to ionospheric feedback processes not encapsulated by the electrostatic model. The model is also used to assess the contributions of various processes to total FAC and suggests that both conductance gradients and neutral dynamos may contribute significantly to FACs in a narrow region where the current transitions from upward to downward. Comparison of Poker Flat Incoherent Scatter Radar versus in situ electric field estimates illustrates the high sensitivity of FAC estimates to measurement resolution.

[1] VHF coherent scatter radar observations of an auroral substorm over Alaska are analyzed in the context of multibeam incoherent scatter plasma density and drifts data and green-line all-sky optical imagery. Coherent scatter arises from Farley Buneman waves which are excited in the E region whenever the convection electric field is greater than about 20 mV/m. Aperture synthesis radar imaging and other aspects of the methodology facilitate the precise spatial registration of the coherent scatter with coincident optical and incoherent scatter radar measurements. Discrete auroral arcs were found to separate diffuse regions of coherent backscatter and, sometimes, to align with the boundaries of those regions. At other times, auroral arcs and torches lined up adjacent to discrete, structured regions or radar backscatter. Drastic variations in the Doppler shifts of the coherent scatter from one side of the auroral forms to the other suggest the presence of field-aligned currents. An empirical formula based on previous studies but adapted to account approximately for the effects of wave turning was used to estimate the convection electric field from the moments of the coherent scatter Doppler spectra. Line-of-sight F region plasma drift measurements from the Poker Flat Incoherent Scatter Radar (PFISR) were found to be in reasonable agreement with these convection field estimates. Reasons why the empirical formulas may be expected to hold are discussed.

Scatter Radar) during the MICA (Magnetosphere-Ionosphere Coupling in the Alfvén Resonator) sounding rocket campaign are critically examined alongside complementary numerical simulations. Particular processes of interest include cavity formation due to intense frictional heating and Pedersen drifts, evolution in the presence of structured precipitation, and refilling due to impact ionization and downflows. Our analysis uses an ionospheric fluid model which solves conservation of mass, momentum, and energy equations for all major ionospheric species. These fluid equations are coupled to an electrostatic current continuity equation to self-consistently describe auroral electric fields. Energetic electron precipitation inputs for the model are specified by inverting optical data, and electric field boundary conditions are obtained from direct PFISR measurements. Thus, the model is driven in as realistic a manner as possible. Both incoherent scatter radar (ISR) data and simulations indicate that the conversion of the F region plasma to molecular ions and subsequent recombination is the dominant process contributing to the formation of the observed cavities, all of which occur in conjunction with electric fields exceeding ∼90 mV/m. Furthermore, the cavities often persist several minutes past the point when the frictional heating stops. Impact ionization and field-aligned plasma flows modulate the cavity depth in a significant way but are of secondary importance to the molecular generation process. Informal comparisons of the ISR density and temperature fits to the model verify that the simulations reproduce most of the observed cavity features to a reasonable level of detail.

The costs and consequences of donepezil versus placebo treatment in patients with mild to moderate Alzheimer’s disease (AD) were evaluated as part of a 1-year prospective, double-blind, randomized, multinational clinical trial. Patients received either donepezil (n = 142; 5 mg/day for 28 days followed by 10 mg/day according to the clinician’s judgement) or placebo (n = 144). Unit costs were assessed in 1999 Swedish kronas (SEK) and converted to US dollars (USD). Donepezil-treated patients gained functional benefits relative to placebo on the Progressive Deterioration Scale (p = 0.042) and Instrumental Activities of Daily Living scale (p = 0.025) at week 52. Caregivers of donepezil-treated patients spent an average of 400 h less annually providing care than caregivers of placebo-treated patients. Mean annual healthcare costs were SEK 137,752 (USD 16,438) per patient for the donepezil group and SEK 135,314 (USD 16,147) in the placebo group. With the average annual cost of donepezil at SEK 10,723 (USD 1,280) per patient, the SEK 2,438 (USD 291) cost difference represented a 77% cost offset. When caregiver time and healthcare costs were included, mean annual costs were SEK 209,244 (USD 24,969) per patient in the donepezil group and SEK 218,434 (USD 26,066) in the placebo group, a total saving associated with donepezil treatment of SEK 9,190 (USD 1,097) per patient [95% CI of SEK –43,959 (USD –5,246), SEK 25,581 (USD 3,053); p = 0.6]. The positive effects on the efficacy outcome measures combined with no additional costs from a societal perspective indicate that donepezil is a cost-effective treatment, representing an improved strategy for the management of patients with AD.