Jeffrey P. Thayer

Global energy deposition during the January 1997 magnetic cloud event

The passage of an interplanetary magnetic cloud at Earth on January 10–11, 1997, induced significant geomagnetic disturbances, with a maximum AE in excess of 2000 nT and a minimum Dst of about −85 nT. We use a comprehensive set of data collected from space-borne instruments and from ground-based facilities to estimate the energy deposition associated with the three major magnetospheric sinks during the event. It is found that averaged over the 2-day period, the total magnetospheric energy deposition rate is about 400 GW, with 190 GW going into Joule heating rate, 120 GW into ring current injection, and 90 GW into auroral precipitation. By comparison, the average solar wind electromagnetic energy transfer rate as represented by the e parameter is estimated to be 460 GW, and the average available solar wind kinetic power USW is about 11,000 GW. A good linear correlation is found between the AE index and various ionospheric parameters such as the cross-polar-cap potential drop, hemisphere-integrated Joule heating rate, and hemisphere-integrated auroral precipitation. In the northern hemisphere where the data coverage is extensive, the proportionality factor is 0.06 kV/nT between the potential drop and AE, 0.25 GW/nT between Joule heating rate and AE, and 0.13 GW/nT between auroral precipitation and AE. However, different studies have resulted in different proportionality factors. One should therefore be cautious when using empirical formulas to estimate the ionospheric energy deposition. There is an evident saturation of the cross-polar-cap potential drop for large AE (>1000 nT), but further studies are needed to confirm this.

Pyro‐cumulonimbus injection of smoke to the stratosphere: Observations and impact of a super blowup in northwestern Canada on 3–4 August 1998

[1] We report observations and analysis of a pyro-cumulonimbus event in the midst of a boreal forest fire blowup in Northwest Territories Canada, near Norman Wells, on 3–4 August 1998. We find that this blowup caused a five-fold increase in lower stratospheric aerosol burden, as well as multiple reports of anomalous enhancements of tropospheric gases and aerosols across Europe 1 week later. Our observations come from solar occultation satellites (POAM III and SAGE II), nadir imagers (GOES, AVHRR, SeaWiFS, DMSP), TOMS, lidar, and backscattersonde. First, we provide a detailed analysis of the 3 August eruption of extreme pyro-convection. This includes identifying the specific pyro-cumulonimbus cells that caused the lower stratospheric aerosol injection, and a meteorological analysis. Next, we characterize the altitude, composition, and opacity of the post-convection smoke plume on 4–7 August. Finally, the stratospheric impact of this injection is analyzed. Satellite images reveal two noteworthy pyro-cumulonimbus phenomena: (1) an active-convection cloud top containing enough smoke to visibly alter the reflectivity of the cloud anvil in the Upper Troposphere Lower Stratosphere (UTLS) and (2) a smoke plume, that endured for at least 2 hours, atop an anvil. The smoke pall deposited by the Norman Wells pyro-convection was a very large, optically dense, UTLS-level plume on 4 August that exhibited a mesoscale cyclonic circulation. An analysis of plume color/texture from SeaWiFS data, aerosol index, and brightness temperature establishes the extreme altitude and ‘‘pure’’ smoke composition of this unique plume. We show what we believe to be a first-ever measurement of strongly enhanced ozone in the lower stratosphere mingled with smoke layers. We conclude that two to four extreme pyro-thunderstorms near Norman Wells created a smoke injection of hemispheric scope that substantially increased stratospheric optical depth, transported aerosols 7 km above the tropopause (above 430 K potential temperature), and also perturbed lower stratospheric ozone.

Interpretation and modeling of the high-latitude electromagnetic energy flux

An interpretation of the electromagnetic energy flux at high latitudes under steady state conditions is presented and analyzed through modeling of the large-scale coupling between the high-latitude ionosphere and magnetosphere. In this paper we elucidate the steady state relationship between the electromagnetic energy flux (divergence of the dc Poynting flux), the Joule heating rate, and the mechanical energy transfer rate in the high-latitude ionosphere. We also demonstrate the important role of the neutral wind and its conductivity-weighted distribution with altitude in determining the resultant exchange of electromagnetic energy at high latitudes. Because the Poynting flux approach accounts for the neutral wind implicitly and describes the net electromagnetic energy flux between the magnetosphere and ionosphere, it is a fundamental measure of energy transfer in the system. A significant portion of this energy transfer results in Joule heating; however, the conversion of electromagnetic energy flux into mechanical energy of the neutrals is also considerable and can in some regions exceed the Joule heating rate. We will show that neglect of the neutral dynamics in calculations of the Joule heating rate can be misleading. To evaluate and interpret the electromagnetic energy flux at high latitudes, we employ the vector spherical harmonic model, which is based on the National Center for Atmospheric Research thermosphere-ionosphere general circulation model, to provide the steady state properties of the thermosphere-ionosphere system under moderate to quiet geomagnetic activity. For the specific geophysical conditions modeled we conclude that (1) the electromagnetic energy flux is predominantly directed into the high-latitude ionosphere with greater input in the morning sector than in the evening sector, as supported by DE 2 observations. (2) The Joule heating rate accounts for much of the electromagnetic energy deposited in the ionosphere with the conductivity-weighted neutral wind contributing significantly to the Joule heating rate and thus affecting the net electromagnetic energy flux in the ionosphere. (3) On average, the mechanical energy transfer rate amounts to about 10% to 30% of the net electromagnetic energy flux in the auroral dawn, dusk, and polar cap regions, acting as a sink of electromagnetic energy flux in the dawn and dusk sectors and a source in the polar cap. (4) Weak regions of upward electromagnetic energy flux are found near the convection reversal boundaries where the mechanical energy transfer rate exceeds the Joule heating rate. In general, large upward electromagnetic energy fluxes may be rare, as the always positive Joule heating rate increases irrespective of the source of electromagnetic energy flux; that is, neutral dynamics contribute directly to the Joule heating rate.

Gravity-wave influences on Arctic mesospheric clouds as determined by a Rayleigh lidar at Sondrestrom, Greenland

common peak volume backscatter coefficient as 20.0 � 10 � 11 m � 1 sr � 1 . The FWHM is noticeably thinner than determined by other lidar observations of NLCs in Norway and the South Pole. We found the mean backscatter strength to increase and the FWHM to decrease with decreasing cloud height. In addition, the cloud slopes with time are greater for the thicker weaker clouds at higher altitudes than the thinner stronger clouds at lower altitudes. Gravity-wave signatures are routinely observed in the cloud detections. Upon estimating stratospheric wave activity in the data, we observed stronger cloud backscatter during low gravity-wave activity and weak cloud backscatter during high gravity-wave activity. To help support these results, simulations from a microphysical cloud model were performed under summer mesospheric conditions with and without gravity-wave activity. Upon including short-period (� 2–3 hours) gravity-wave activity, the model simulation reproduced the behavior observed in the ensemble cloud properties by producing a broader altitude distribution, weaker backscatter strength, and thinner clouds. INDEX TERMS: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 0320 Atmospheric Composition and Structure: Cloud physics and chemistry; 0340 Atmospheric Composition and Structure: Middle atmosphere—composition and chemistry; 0669 Electromagnetics: Scattering and diffraction; 1655 Global Change: Water cycles (1836); KEYWORDS: noctilucent clouds, gravity waves, Rayleigh lidar, volume backscatter coefficient, polar mesosphere

Ionosphere response to recurrent geomagnetic activity: Local time dependency

[1] Observations of global positioning system total electron content (TEC) and in situ electron densities at altitudes of ~350-370 km from the CHAMP satellite are used to illustrate the local time and latitude dependence of 9 day periodicities in the ionosphere due to recurring high-speed solar wind streams and geomagnetic activity during 2005. A local time dependence is found, with nighttime TEC oscillations concentrated at high latitudes and close to ±40% of background levels. The largest oscillations in daytime TEC occur at midlatitudes and are ±25% of background levels. Furthermore, the daytime response is generally symmetric about the geomagnetic equator with anticorrelation between high and low latitudes, whereas at night the high-latitude Northern Hemisphere is generally in-phase with low latitudes and anticorrelated with the high-latitude Southern Hemisphere. A combination of enhanced equatorward neutral winds and changes in neutral composition are thought to be the primary mechanisms responsible for the observed ionospheric response. Although similar mechanisms are driving the response, the local time dependency arises because of the presence (lack) of photoionization during the daytime (nighttime). Similar trends are observed in CHAMP in situ electron densities; however, the oscillations at a near-constant altitude are ~10-15% larger than the TEC oscillations. Additionally, the CHAMP observations reveal possible variations in the strength of the equatorial ionization anomaly, indicating that disturbance dynamo electric fields may also contribute to the ionospheric response to recurrent geomagnetic activity. The results presented are the first to reveal the significant differences between the daytime and nighttime response of the ionosphere to periodic forcing from solar wind high-speed streams.

Coordinated incoherent scatter radar study of the January 1997 storm

We describe many important features of the ionospheric F region as observed by the Sondrestrom, Millstone Hill, and Arecibo incoherent scatter radars (ISRs) and the Millstone Hill and Ramey Digisondes during January 6-10, 1997, with emphasis on the January 10, 1997 storm. Coordinated analysis of the data provides evidence for traveling atmospheric disturbances (TADs) and for two likely electric field penetration events linking these stations. Large and rapid changes in hmF 2 were seen at Arecibo and nearby Ramey which are related to the TADs and penetrating electric fields. Results are compared with simulations by the thermosphere-ionosphere electrodynamics general circulation model (TIEGCM), which utilizes high-latitude inputs given by the assimilative mapping of ionospheric electrodynamics (AMIE) technique. An important result ofthis study is that the TIEGCM is able to predict TADs similar to those observed. Exceptional features observed during this storm at Millstone Hill are a very large nighttime T e enhancement on January 10 and a larger decrease in NmF 2 than predicted by the TIEGCM throughout the storm period. The latter appears to be related to an underestimation of the neutral temperature by the model.

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.

On the contribution of the thermospheric neutral wind to high-latitude energetics

Although the neutral winds contribution to ionospheric electrodynamics is well-established at low latitudes, this electrical energy source has been largely ignored at high latitudes, owing to the assumed dominance of the magnetospheric dynamo contribution. Yet, the potential for exchange of electrical energy between the neutral wind dynamo and the magnetospheric dynamo is a direct consequence of the coupling between the two regions by highly conducting magnetic field lines. The integral nature of this coupling precludes the direct separation of the neutral wind and solar wind contributions to the observed electrodynamics. Therefore, to gain some insight into their relative importance, we have performed a simple numerical experiment in which the two dynamos are individually connected to a fixed load and their energetics evaluated separately. To determine the electrical energy flux supplied by the magnetosphere, we treat it as a voltage generator and the ionosphere as a resistive load. The available electrical energy flux generated by the neutral wind dynamo is determined from the mechanical energy stored within an established neutral wind field. This exercise has led to a number of conclusions, including: i) The neutral wind dynamo contributes significantly to high-latitude energetics, particularly in the central polar cap; and ii) In the region near the plasma convection reversal boundary, the amount of energy flux available from the neutral wind dynamo can exceed that provided by the magnetospheric dynamo.

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.

Longitudinal and geomagnetic activity modulation of the equatorial thermosphere anomaly

[1] In this paper we examine the detailed similarities and differences between the equatorial thermosphere anomaly (ETA) and the equatorial ionization anomaly (EIA) from 20 March to 6 April 2002, when both the ETA and the EIA are distinct in the Challenging Minisatellite Payload (CHAMP) observations. The characteristics of the ETA and the EIA are obtained from the CHAMP accelerometer, in situ electron density measurements, and total electron content (TEC) above the CHAMP satellite. Our results show that the trough locations of the ETA and the EIA in latitude show a good agreement, and both correspond well with the dip magnetic equator, while the ETA crests are usually located poleward of the EIA. Meanwhile, the latitudinal locations of the ETA crests exhibit strong hemispheric asymmetry and large variability during our study interval. The longitudinal variations between the EIA and the ETA show significant differences. The EIA crests from the CHAMP observations show strong wave 4 structures, but the primary component in the ETA is wave 1. Moreover, the ETA densities show strong variations in response to geomagnetic activity, whereas CHAMP in situ electron densities and TEC at the EIA do not reflect such large day-to-day variability. Therefore, a simple EIA-ETA relationship cannot explain the dependence of the longitudinal and geomagnetic activity modulation of the ETA and the EIA. The meridional ion drag, which is significantly modulated by enhanced equatorward winds during elevated geomagnetic activity, is probably responsible for some of the observed features in the ETA, although no unambiguous explanation for ETA formation yet exists.