Michael J. Nicolls

A top to bottom evaluation of IRI 2007 within the polar cap

Monthly median values of ionospheric peak height (hmF2) and density (NmF2), derived from ionosonde measurements at four Canadian High Arctic Ionospheric Network (CHAIN) stations situated within the polar cap and Auroral Oval, are used to evaluate the performance of the International Reference Ionosphere (IRI) 2007 empirical ionospheric model during the recent solar minimum between 2008 and 2010. This analysis demonstrates notable differences between IRI and ionosonde NmF2 diurnal and seasonal behavior over the entire period studied, where good agreement is found during summer periods but otherwise errors in excess of 50% were prevalent, particularly during equinox periods. hmF2 is found to be marginally overestimated during winter and equinox nighttime, while also being underestimated during summer and equinox daytime by in excess of 25%. These errors are shown to be related to significant mismodeling of the M(3000)F2 propagation factor. The ionospheric bottomside thickness parameter (B0) is also evaluated using ionosonde measurements. It is found that both of the IRIs internal B0 models significantly misrepresent both seasonal and diurnal variations in bottomside thickness when compared to ionosonde observations, where errors at times exceed 40%. A comparison is also presented between IRI and Resolute (74.75N, 265.00E) Advanced Modular Incoherent Scatter Radar (AMISR)-derived topside thickness. It is found in this comparison that the IRI is capable of modeling ionospheric topside thickness exceptionally well during winter and summer periods but fails to represent significant diurnal variability during the equinoxes and seasonal variations.

Determining receiver biases in GPS-derived total electron content in the auroral oval and polar cap region using ionosonde measurements

Global Positioning System (GPS) total electron content (TEC) measurements, although highly precise, are often rendered inaccurate due to satellite and receiver differential code biases (DCBs). Calculated satellite DCB values are now available from a variety of sources, but receiver DCBs generally remain an undertaking of receiver operators and processing centers. A procedure for removing these receiver DCBs from GPS-derived ionospheric TEC at high latitudes, using Canadian Advanced Digital Ionosonde (CADI) measurements, is presented. Here, we will test the applicability of common numerical methods for estimating receiver DCBs in high-latitude regions and compare our CADI-calibrated GPS vertical TEC (vTEC) measurements to corresponding International GNSS Service IONEX-interpolated vTEC map data. We demonstrate that the bias values determined using the CADI method are largely independent of the topside model (exponential, Epstein, and α-Chapman) used. We further confirm our results via comparing bias-calibrated GPS vTEC with those derived from incoherent scatter radar (ISR) measurements. These CADI method results are found to be within 1.0 TEC units (TECU) of ISR measurements. The numerical methods tested demonstrate agreement varying from within 1.6 TECU to in excess of 6.0 TECU when compared to ISR measurements.

Horizontal parameters of daytime thermospheric gravity waves and E region neutral winds over Puerto Rico

We report on the electron density perturbation amplitudes, periods (up to 60 min), horizontal and vertical wavelengths, phase speeds, and propagation directions of daytime traveling ionospheric disturbances (TIDs) from 115 to 300 km altitude using dual-beam experiments at the Arecibo Observatory (AO), Puerto Rico. As in previous studies, we find a near continuum of waves above the AO. While the TIDs propagate in nearly all directions except purely westward, we find that most propagate southward southeastward. We find that TID amplitudes increase nearly exponentially with increasing period, although with a much smaller slope for periods >30 min. TID amplitudes peak on the bottomside of the F region. Typical vertical wavelengths increase from less than 50 km at low altitudes to ∼100–300 km. Horizontal wavelengths increase from ∼70–100 km to ∼150–500 km over the same altitude range. Median vertical wavelengths, horizontal wavelengths, and periods increase with altitude up to z∼ 100–150 m/s. We also measure the E region horizontal neutral winds and find that they peak at ∼150 m/s near z∼105 km in the middle of the day. Wave phase speeds are in general greater than these ambient winds. In addition, by tracing individual wave packets vertically in altitude, we find that a packets vertical wavelength generally peaks near the altitude where its inferred ion velocity amplitude is maximum. The vertical wavelength generally decreases above this altitude, an observation that is consistent with gravity wave packet theory.

On the generation/decay of the storm‐enhanced density plumes: Role of the convection flow and field‐aligned ion flow

Storm-enhanced density (SED) plumes are prominent ionospheric electron density increases at the dayside middle and high latitudes. The generation and decay mechanisms of the plumes are still not clear. We present observations of SED plumes during six storms between 2010 and 2013 and comprehensively analyze the associated ionospheric parameters within the plumes, including vertical ion flow, field-aligned ion flow and flux, plasma temperature, and field-aligned currents, obtained from multiple instruments, including GPS total electron content (TEC), Poker Flat Incoherent Scatter Radar (PFISR), Super Dual Auroral Radar Network, and Active Magnetosphere and Planetary Electrodynamics Response Experiment. The TEC increase within the SED plumes at the PFISR site can be 1.4–5.5 times their quiet time value. The plumes are usually associated with northwestward E × B flows ranging from a couple of hundred m s−1 to > 1 km s−1. Upward vertical flows due to the projection of these E × B drifts are mainly responsible for lifting the plasma in sunlit regions to higher altitude and thus leading to plume density enhancement. The upward vertical flows near the poleward part of the plumes are more persistent, while those near the equatorward part are more patchy. In addition, the plumes can be collocated with either upward or downward field-aligned currents (FACs) but are usually observed equatorward of the peak of the Region 1 upward FAC, suggesting that the northwestward flows collocated with plumes can be either subauroral or auroral flows. Furthermore, during the decay phase of the plume, large downward ion flows, as large as ~200 m s−1, and downward fluxes, as large as 1014 m−2 s−1, are often observed within the plumes. In our study of six storms, enhanced ambipolar diffusion due to an elevated pressure gradient is able to explain two of the four large downward flow/flux cases, but this mechanism is not sufficient for the other two cases where the flows are of larger magnitude. For the latter two cases, enhanced poleward thermospheric wind is suggested to be another mechanism for pushing the plasma downward along the field line. These downward flows should be an important mechanism for the decay of the SED plumes.

High‐resolution electron temperature measurements using the plasma line asymmetry

[1] In this paper, we present the first results of a new technique for measuring the electron temperature in the daytime ionosphere using the Arecibo incoherent scatter radar (ISR). The technique utilizes the plasma line component of the incoherent scatter spectrum. The difference in the up- and down-shifted plasma line frequencies is related to the density and temperature of the ionosphere, as well as more minor effects resulting from photoelectrons, currents, and other sources. The shift is very small (the order of 1 kHz in a plasma line frequency of several MHz) but can be measured quite accurately with the coded long pulse plasma line technique. We compare the results to ion line measurements of the electron temperature, and the two independent techniques show good agreement. In addition to providing a measure of the electron temperature that is independent of the ion line, the approach allows for a sensitive test of kinetic plasma theory including a magnetic field, gives us the ability to study photoelectron populations and electron currents, and will allow us to constrain ion line fits in the bottomside (and possibly topside) regions to more accurately fit for composition. Citation: Nicolls, M. J., M. P. Sulzer, N. Aponte,

Estimating the vector electric field using monostatic, multibeam incoherent scatter radar measurements

An algorithm has been developed to image the local structure in the convection electric field using multibeam incoherent scatter radar (ISR) data. The imaged region covers about 4° in magnetic latitude and 8° in magnetic longitude for the specific geometry considered (that of the Poker Flat ISR). The algorithm implements the Lagrange method of undetermined multipliers to regularize the underdetermined problem posed by the radar measurements. The error on the reconstructed image is estimated by mapping the mathematical form to a Bayesian estimate and observing that the Lagrangian method determines an effective a priori covariance matrix from a user-defined regularization metric. There exists a unique solution when the average measurement error is smaller than the average measurement amplitude. The algorithm is tested using synthetic and real data and appears surprisingly robust at estimating the divergence of the field. Future applications include imaging the current systems surrounding auroral arcs in order to distinguish physical mechanisms.

Ionospheric ion temperature climate and upper atmospheric long-term cooling†

It is now recognized that Earths upper atmosphere is experiencing a long-term cooling over the past several solar cycles. The potential impact of the cooling on societal activities is significant, but a fundamental scientific question exists regarding the drivers of the cooling. New observations and analyses provide crucial advances in our knowledge of these important processes. We investigate ionospheric ion temperature climatology and long-term trends using up-to-date large and consistent ground based datasets as measured by multiple incoherent scatter radars (ISRs). The very comprehensive view provided by these unique observations of the upper atmospheric thermal status allows us to address drivers of strong cooling previously observed by ISRs. We use observations from two high latitude sites at Sondrestrom (Invariant latitude 73.2°N) from 1990-2015, and Chatanika/Poker Flat (Invariant latitude 65.9°N) over the span of 1976-2015 (with a gap from 1983-2006). Results are compared to conditions at the mid-latitude Millstone Hill site (Invariant latitude 52.8°N) from 1968-2015. The aggregate radar observations have very comparable and consistent altitude dependence of long-term trends. In particular, the lower F region (< 275 km) exhibits dayside cooling trends that are significantly higher (-3 to -1K/year at 250 km) than anticipated from model predictions given the anthropogenic increase of greenhouse gases. Above 275 km, cooling trends continue to increase in magnitude but values are strongly dependent on magnetic latitude, suggesting the presence of significant downward influences from non-neutral atmospheric processes.

An energy balance study of the lower topside ionosphere using the Arecibo incoherent scatter radar and heating facilities

[1] In this paper we describe the results of an experiment to study electron and ion temperature enhancements during an HF modification experiment at the Arecibo Observatory. This experiment is unique in that we pointed the radar away from the interaction region in the F region in order to study heat conduction along the field lines. Although electron temperature enhancements have been frequently observed when high-power radio waves are injected into the ionosphere, observations generally have occurred in the interaction region and the regions of elevated electron temperatures have been accompanied by small ion temperature increases (50-200 K). Like many such experiments, this one was conducted during winter solar minimum, when f 0 F 2 is low during the night at midlatitudes, but this experiment also had the advantage of the upgraded Arecibo HF facility, first used in 1997. The electron temperature enhancements were accompanied by a significant increase in the ion temperature (nearly 500 K). The observation away from the interaction region allowed the application of the time-dependent heating equation without having to estimate local heating effects (i.e., by keeping the conduction and loss terms in the energy balance calculation and neglecting the source term). More specifically, the heating rate of conduction was quantified by manipulating the heat equation. Thus the primary purpose was to observe the temperatures as the heat was conducted away from the F region ionosphere. We have observed the gradients in the electron temperature caused by the heater, estimated the conduction along the field lines, and studied the transfer of energy from the hot electrons to the ions and neutrals. At lower altitudes, near the electron-temperature peak, we show that O + cooling is dominant, whereas in the lower topside H + cooling is the most important. Experiments of the type described here could be enhanced with the new dual-beam system at Arecibo in conjunction with a heating facility.

AMISR the advanced modular incoherent scatter radar

AMISR is a modular, mobile, UHF radar facility used by scientists and students from around the world to conduct studies of the upper atmosphere and to observe space weather events. SRI International, under a grant from the National Science Foundation, is leading the collaborative effort in the development and operation of AMISR. The novel modular configuration allows for relocating the radar to study upper atmospheric activity at different locations around the globe. Remote operation and electronic beam steering allow researchers to operate and position the radar beam on a pulse-to-pulse basis to accurately measure and glean new information from rapidly changing space weather events.

Small-scale structure on the poleward edge of a stable auroral red arc

Here we present all-sky images of a stable auroral red arc collocated with the low-density trough. Small-scale, dynamic structure is observed on the poleward edge of the arc that may be caused by a crossed density/temperature gradient or by a collisional Kelvin-Helmholtz shear-flow instability in conjunction with a sufficient density gradient.