Herbert C. Carlson

First observations of HF heater‐produced airglow at the High Frequency Active Auroral Research Program facility: Thermal excitation and spatial structuring

HF heater-produced artificial airglow emissions at 630.0 nm were detected at the High Frequency Active Auroral Research Program (HAARP) ionospheric research facility near Gakona, Alaska (62.39° N 145.15° W), for the first time in March 1999. HF excitation was identified by virtue of two measurements: A region of emissions enhanced 50-60 R above the background of ∼100 R was observed in the approximate region illuminated by the HF heater beam; and the rise and decay of the airglow followed the HF transmitter on/off cycle with time constants of approximately 80 s and 50 s, respectively. The observations were made in close proximity to the natural aurora, which gradually moved southward over the site during the experiment and eventually overwhelmed the much weaker artificial airglow. Significant structure in the airglow region was observed, including an overall equatorward displacement relative to the nominal transmitter beam, preferential occurrence of intensity maxima along the overhead field line up to and including magnetic zenith, and periods of elongation of the airglow region along the magnetic meridian and later parallel to the encroaching auroral zone. We discuss potential sources of this structuring of the emission shape including auroral plasma transport and neutral winds. Transmitter frequencies of 3.1–3.3 MHz matched the ionospheric plasma frequency near the F region peak at high altitudes of 300–350 km. We identify the primary mechanism behind excitation of the 630.0 nm airglow as thermal electron excitation on the basis of this evidence, along with thermal balance arguments, further substantiated by the observed asymmetry in rise and decay times and a lack of detectable emissions at 557.7 nm.

Reversed flow events in the winter cusp ionosphere observed by the European Incoherent Scatter (EISCAT) Svalbard radar

[1] High-resolution fast azimuth sweeps by the European Incoherent Scatter (EISCAT) Svalbard radar provide an unparalleled opportunity to study small-scale flow disturbances in the cusp ionosphere. Observations from 11 days of the winter cusp ionosphere of highresolution ion flow data have been analyzed. Transient channels of reversed plasma flow appear to be a regular feature of the cusp, and they were seen in 16% of 767 analyzed EISCAT Svalbard Radar (ESR) scans. We introduce a new descriptive term, reversed flow events (RFEs), for this class of events. RFEs are defined as longitudinally elongated segments of transiently enhanced ion flow in the direction opposite to the background flow. RFEs typically occurred near the cusp inflow region in association with enhancements in the polar cap convection observed by the Super Dual Auroral Radar Network (SuperDARN). Their lifetime was found to be � 19 min on average. Their longitudinal dimension typically exceeded the ESR field of view (>400–600 km), and ranged from � 50 to 250 km in latitude. The occurrence rate of RFEs appears independent of the BZ and BY component polarity of the interplanetary magnetic field (IMF), and RFEs occurred for clock angles between 40 and 240. RFE ion flow was in 95% of the cases documented to oppose the magnetic tension force, and RFEs cannot be interpreted in terms of newly opened flux. RFEs formed one by one and never simultaneously in pairs. To explain these observations, we propose an asymmetric version of the Southwood (1987) twin cell flux transfer event model to account for significant IMF BY, in which only the poleward cell located on open field lines develops.

The role of ring current nose events in producing stable auroral red arc intensifications during the main phase: Observations during the September 19-24, 1984, Equinox Transition Study

An examination of stable auroral red (SAR) arc emissions over the last solar cycle (Slater and Kleckner, 1989) indicates that the strongest emissions, during the lifetime of a particular SAR arc, often occur in association with the main phase of the magnetic storm. Previous observations of thermal and energetic particle populations at high and low altitudes on SAR arc field lines by Kozyra et al. (1987a) indicate that the energy source for these emissions is the O+ in the ring current. The O+ content of the ring current increases with increasing magnetic activity reaching its maximum percentage contribution near minimum Dst for a particular storm. This variation in the O+ content of the ring current is inconsistent with an early main phase enhancement of SAR arc emissions. To investigate the source of main phase enhancements in SAR arc emissions, a study of the September 19-24, 1984, magnetic storm period during which SAR arc emissions were observed by the ground-based mobile automatic scanning photometer network in both the main and recovery phases is presented. The emissions associated with the main phase (∼ 400 R) were an order of magnitude greater than those associated with the recovery phase (tens of R). Energetic particle measurements from the DE 1 and AMPTH spacecraft, on field lines that map to the SAR arc position at low altitude, were examined to determine if differences in the energy sources during these time periods were evident. In agreement with previous work, ring current O+ supplied the bulk of the electron heating during storm recovery phase as a result of Coulomb collisions of O+ with the plasmaspheric electrons; contributions by ring current H+ were negligible. A new result of the present work is that an enhancement of the 15-25 keV H+ component of the ring current during the main phase of the September 19 magnetic storm was responsible for an approximately one order of magnitude increase in the electron heating rate and SAR arc emissions during the main phase compared to the recovery phase. The increase in the H+ flux occurred in association with a ring current “nose event”, a front of ions injected into the inner magnetosphere in response to a discontinuous change in the cross-tail electric field. The association between nose events and intensifications of SAR arc emissions in the main phase has not previously been explored but is a natural consequence of the injection of significant fluxes of relatively low-energy ring current ions earthward of the plasmapause during early storm time ring current formation.

Which cusp upflow events can possibly turn into outflows

Two sequences, before and after magnetic noon, respectively, of poleward moving auroral forms with associated upflows situated above the European Incoherent Scatter Svalbard Radar allowed close study of ion upflow dynamics. We find that flux intensity is correlated with plasma temperature and that upflowing plasma undergoes acceleration proportional to the slope of the velocity profile and to the velocity at each altitude. The potential for upflows to lift thermal plasma to regions where broadband extremely low frequency electric field activity can cause nonthermal acceleration leading to outflow is examined. Equations for estimating the travel time of upflowing plasma are presented. We find that around 40% of the observed upflow profiles with a unit number flux greater than 1 × 1013 m−2 s−1 can transport plasma from 500 to 800 km altitude in less than 10 min, approximately the typical lifetime of pulsed upflow events. Almost all such profiles can transport plasma from 600 to 800 km in the same time span. Typical transport times for other altitude ranges are also presented. Post magnetic noon the background electron density was somewhat higher than prenoon due to transport of EUV-enhanced plasma, and the postnoon ion flux was somewhat weaker than prenoon.

Low‐latitude 10 eV electrons: Nighttime plasma line as a new research capability

The incoherent scatter radar (ISR) plasma line (PL) in daylight is excited by photoelectrons. Measurement of its intensity (κTp) has long been used for their study. At night, despite the absence of any other excitation mechanism, the PL intensity should have a thermal amplitude level κTe, determined by the electron gas temperature Te. To the contrary Carlson et al. (1982) found nighttime PLs over Arecibo enhanced >3 times above thermal intensities despite the absence of any known causative mechanism. Here we present discovery that nighttime PLs frequently recur, with quite variable enhancement. In the absence of direct solar EUV, these enhanced PLs must be produced by particle precipitation, manifested by the presence of variable recurring F region ~10 eV electron fluxes. We see this as offering a new tool for space environment studies, opening a new era of particle precipitation research and ISR calibration.

A statistical survey of heat input parameters into the cusp thermosphere

Based on three winters of observational data, we present those ionosphere parameters deemed most critical to realistic space weather ionosphere and thermosphere representation and prediction, in regions impacted by variability in the cusp. The CHAMP spacecraft revealed large variability in cusp thermosphere densities, measuring frequent satellite drag enhancements, up to doublings. The community recognizes a clear need for more realistic representation of plasma flows and electron densities near the cusp. Existing average value models produce order of magnitude errors in these parameters, resulting in large underestimations of predicted drag. We fill this knowledge gap with statistics-based specification of these key parameters over their range of observed values. The European Incoherent Scatter Svalbard Radar tracks plasma flow Vi , electron density Ne, and electron, ion temperatures Te, Ti , with consecutive 2–3 min windshield wipe scans of 1000 × 500 km areas. This allows mapping the maximum Ti of a large area within or near the cusp with high temporal resolution. In magnetic field-aligned mode the radar can measure high-resolution profiles of these plasma parameters. By deriving statistics for Ne and Ti , we enable derivation of thermosphere heating deposition under background and frictional drag-dominated magnetic reconnection conditions. We separate our Ne and Ti profiles into quiescent and enhanced states, which are not closely correlated due to the spatial structure of the reconnection foot point. Use of our data-based parameter inputs can make order of magnitude corrections to input data driving thermosphere models, enabling removal of previous twofold drag errors. Plain Language Summary Input of energy into the polar ionosphere from the solar wind causes local heating and upwelling of air in the region known as the “cusp.” This upwelling in turn dramatically changes the density of the atmosphere as it rises, which has consequences for atmospheric composition and transport as well as for spacecraft that experience increased drag and possibly shortened lifetimes. We show that because of the highly dynamic nature of the cusp, long-term averages and models will not accurately reproduce the energy input to the cusp and the consequent upwelling of the air. We use empirical data to show that the energy input is highly dynamic and that it is necessary to separate active and quiet periods when modeling heating and upwelling in the cusp, as well as to detect or predict accurately where the cusp is located. We present statistical models of the active and quiescent cusp ionization density and temperature of the ionized gas. Occurrence rates of heating events in and near the cusp are estimated by using rapid radar scans covering a large area.