Craig James Heinselman

Substorm triggering by new plasma intrusion: Incoherent‐scatter radar observations

Received 4 December 2009; revised 16 March 2010; accepted 30 March 2010; published 27 July 2010. [1] In the companion paper, we identified a repeatable sequence of events leading to substorm onset in THEMIS all‐sky imager observations: enhanced flows bring new plasma into the plasma sheet. The new plasma then moves earthward as a flow channel, bringing it to the near‐Earth plasma sheet and where it produces onset instability. New plasma entering the dusk (dawn) convection cell drifts equatorward and eastward and then around the Harang reversal, leading to pre‐midnight (near‐ and post‐midnight) onset. Here we present evidence supporting this sequence using incoherent scatter radar (ISR) ionospheric observations. Using the Sondrestrom ISR, we find that enhanced flows of new plasma commonly enter the plasma sheet from the polar cap ∼8 min prior to onset. These flows are related to poleward boundary intensification signatures, consistent with the inferences from the imagers. Using the Poker Flat ISR (PFISR), we find that shortly before onset, enhanced westward flows reach the subauroral polarization streams (SAPS) region equatorward of the Harang reversal (dusk‐cell onsets) or enhanced eastward flows enter the onset region from the poleward direction (dawn‐cell onset). PFISR proton precipitation signatures are consistent with the possibility that the enhanced flows consist of reduced‐entropy plasma sheet plasma, and that onset occurs poleward of much of the enhanced SAPS flow (dusk‐cell onsets) or equatorward of the enhanced eastward flows (dawn‐cell onsets). Consistency with reduced entropy plasma is seen only within the enhanced flows, leading us to suggest that intrusion of low‐entropy plasma may alter the radial gradient of entropy toward onset instability.

ENHANCEMENT OF THOMSON SCATTER BY CHARGED AEROSOLS IN THE POLAR MESOSPHERE: MEASUREMENTS WITH A 1.29-GHZ RADAR

The summer polar mesosphere was observed with the Sondrestrom 1.29-GHz radar with a new high- resolution data acquisition mode. On one occasion, a spa- tially narrow enhancement in the backscattered power was seen near an altitude of 88 km. We discuss possible expla- nations and propose that this layer may be the first exam- ple of polar mesosphere summer echoes (PMSE) detected above 1 GHz. Specifically, we suggest that these echoes are enhanced Thomson scatter from a layer of charged aerosols, and we speculate upon the size and charge state.

Rayleigh lidar system for middle atmosphere research in the arctic

A Rayleigh/Mie lidar system deployed at the Sondrestrom At- mospheric Research Facility located on the west coast of Greenland near the town of Kangerlussuaq (67.0 deg N, 50.9 deg W) has been in operation since 1993 making unique observations of the arctic middle atmosphere. The vertically directed lidar samples the elastically back- scattered laser energy from molecules (Rayleigh) and aerosols (Mie) over the altitude range from 15 to 90 km at high spatial resolution. The limited amount of arctic observations of the middle atmosphere currently available emphasizes the importance and utility of a permanent Rayleigh lidar system in Greenland. The lidar system consists of a frequency- doubled, 17-W Nd:YAG laser at 532 nm, a 92 cm Newtonian telescope, and a two-channel photon counting receiver. The principal objective of the lidar project is to contribute to studies concerned with the climatology and phenomenology of the arctic middle atmosphere. To this end, we describe the lidar system in detail, evaluate system performance, de- scribe data analysis, and discuss the system capabilities in determining the density, temperature, and the presence of aerosols in the arctic middle atmosphere. Particular emphasis is placed on the derivation of temperature from the lidar measurement and on the impact of signal- induced noise on this analysis. Also, we develop a statistical filter based on a Bayesian approach to optimally smooth the lidar profile in range. This filter preserves the short-term fluctuations in the low-altitude data consisting of relatively high SNR, whereas more smoothing is applied to the high-altitude data as the SNR decreases.

A high‐latitude observation of sporadic sodium and sporadic E‐layer formation

High-resolution radar and lidar measurements of sporadic sodium (Na) and sporadic E (E) layers were made at the Sondrestrom incoherent-scatter radar facility on 11 December 1997. These measurements suggest a causal link between Es and Nas, supporting the proposed mechanism in which Na+ ions in the Es are neutralized to form the Nas. This Nas, by contrast, does not appear to have been formed by the presence of auroral precipitation or ionization, and, in fact, the sodium density is seen to decrease during an auroral event.

Ionospheric response to wave-accelerated electrons at the poleward auroral boundary

[1] The local ionospheric response to an auroral intensification at the poleward auroral boundary has been investigated using the incoherent scatter radar (ISR) and optical instrumentation at Sondrestrom, Greenland, in conjunction with space-bome measurements by the IMAGE and FAST satellites. ISR elevation scans through the illuminated region revealed filamentary columns of enhanced plasma density, ∼5 km in latitude by ∼200 km in altitude. Column densities were typically 5 × 10 11 /m 3 above background and often constant over a broad range of altitudes. The brightness of the O + 732-733 nm multiplet, monitored simultaneously by a near-infrared spectrometer, exceeded 1.2 kR during one 4-min period (a factor of ∼4 brighter than previously reported auroral measurements). A time-dependent model was developed to relate O + emission intensities to O + column densities for a given illumination time. The results suggested that the electron source was composed of kilometer-scale flux tubes locked in the E x B flow for several minutes whose average energy varied temporally between 1 keV over their illumination lifetime. Conjugate electrons measured by the FAST satellite at 1700 km showed evidence for energization by inertial Alfven waves. Ionization rates computed from these spectra were sufficient to account for the observed filamentary ionospheric structure. The implications of such ionization patterns for electrodynamic coupling with the magnetosphere are discussed.

Measured and modeled ionospheric densities, temperatures, and winds during the international polar year

[1] This paper examines the ability of ionospheric models to reproduce measured electron density, winds, and temperatures during the International Polar Year (IPY) in 2007. The models include the field line interhemispheric plasma (FLIP) model, the international reference ionosphere (IRI) model, and the empirical horizontal neutral wind models (HWM) (HWM93, HWM07). For Poker Flat, Alaska, there is exceptionally good agreement between the FLIP model and measured electron density, winds, and temperatures in equinox and winter. This research shows an interesting post sunset peak in Te from late fall through early spring that is reproduced by the FLIP model. In June and July the FLIP model underestimates the measured peak electron density by a factor of 2. Although both the data and model show evidence of an F 1 peak near 150 km in summer, the model F 1 peak electron density tends to be larger than the F 2 peak electron density and that is not seen in the data. The summer discrepancy is most likely due to incorrect atomic to molecular neutral density ratios. The FLIP model reproduces the Millstone Hill data well throughout 2007. The IRI model agrees well with the electron density data during the day but overestimates the peak electron density and the height of the peak at night. The equivalent winds from the FLIP model and the winds from the HWM93 model agree well with the measured winds. The HWM07 winds are different from the earlier HWM93 winds at Poker Flat and do not agree as well with the data.

Auroral effects on the gas phase chemistry of meteoric sodium

The effects of auroral ionization on the gas phase chemistry of meteoric sodium in the upper atmosphere are considered via computer simulation and with a case study. We find that under some circumstances, auroral ionization can significantly affect the chemical state of sodium. In particular, at auroral altitudes, where recently deposited material or sudden atom layers are typically found, neutral Na can be ionized via charge transfer reactions with aurorally enhanced ionization products. The time constants for these reactions are of the order of 15 min. Measurements collected at the Sondrestrom Facility, in Greenland, are analyzed, and a case study is presented for which this gas phase chemistry is sufficient to explain the results. In addition to supporting the predicted influence of auroral ionization on neutral sodium, this case study is also consistent with a chemical mechanism for the formation of a sporadic atom layer.

Coordinated radar and optical measurements of stable auroral arcs at the polar cap boundary

A specialized incoherent scatter radar scanning mode has been developed for use in conjunction with simultaneous real-time all-sky images. These complementary diagnostics are used to examine the aeronomy and electrodynamics of stable auroral arcs that delineate the boundary between the polar cap and the auroral oval. The first arc discussed, observed at 2000 MLT, represents the boundary between antisunward plasma flow in the polar cap and sunward return flow equatorward of the arc. The arc defined an equipotential in the high-latitude convection pattern in that no plasma flowed across the arc. The radar line-of-sight velocity measurements also indicate that this arc is consistent with a convergent electric field and an associated weak upward field-aligned current. The second arc was observed at 2330 MLT and was associated with a nightside gap or magnetic reconnection region. Strong antisunward flow was observed directly across the arc, although a velocity shear was superposed on this steady flow along the poleward edge of the arc. Detailed plasma density, temperature, and line-of-sight velocity measurements from the radar are presented for both arcs to define the electric field, horizontal and field-aligned currents, and thermal plasma parameters associated with these arcs.

Seasonal oscillations of middle atmosphere temperature observed by Rayleigh lidars and their comparisons with TIMED/SABER observations

The long-term temperature data sets obtained by Rayleigh lidars at six different locations from low to high latitudes within the Network for the Detection of Atmospheric Composition Change (NDACC) were used to derive the annual oscillations (AO) and semiannual oscillations (SAO) of middle atmosphere temperature: Reunion Island (21.8°S); Mauna Loa Observatory, Hawaii (19.5°N); Table Mountain Facility, California (34.4°N); Observatoire de Haute Provence, France (43.9°N); Hohenpeissenberg, Germany (47.8°N); Sondre Stromfjord, Greenland (67.0°N). The results were compared with those derived from the Sounding of the Atmosphere using Broadband Emission Radiometry (SABER) instrument onboard the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) satellite

On the spectral analysis and interpretation of incoherent scatter plasma line echoes

This paper describes a model of the physical interactions involved in incoherent scatter that result in specific plasma line spectral shapes as a function of range and frequency. This model is quite successful in duplicating the naturally occurring plasma line spectra observed at high latitudes. Moreover, the inherent sensitivity of the data collection method suggests a much more general applicability. For example, we will demonstrate a sensitivity of the method to plasma density irregularities in the medium that rivals or exceeds that possible with radio wave scintillation techniques. Unlike height-integrated scintillation measurements, however, this new method can range resolve the fluctuations in plasma density. We are able to determine the altitude of the F layer peak to an accuracy of ∼ ± 1 km even with a probing pulse of 72 km in length. Our density fluctuation measurements indicate a residual level of ∼ 1% fluctuations after a linear-temporal trend is removed. This indicates spatial variations in plasma density of this order within our relatively narrow (0.5°) antenna beam. These spatial variations are enhanced during periods when the F layer peak cutoff frequency is decreasing. Such behavior is consistent with increased plasma structure on the trailing edge of F region plasma blobs. Our technique involves not only model fitting but also a data collection method that minimizes range smearing. This data collection mode has already been successfully implemented at the Sondrestrom, Greenland, incoherent scatter radar facility.