Polar Cap Ionosphere Research Articles

Why Doesn't the Observed Field‐Aligned Current Saturate With Increasing Interplanetary Electric Field?

AbstractTheoretical studies indicate that electric potential saturation in a polar cap ionosphere is regulated by the limited reconnection rate at the magnetopause associated with field‐aligned currents (FACs), predicting the possible saturation of FACs under large interplanetary electric field (IEF). However, recent statistical studies have shown that observed FACs increase linearly with IEF. To reconcile this disparity, numerical experiments are conducted to explore the response of FACs under large IEF. Results show that FACs may exhibit either linear or saturated responses, depending on the Alfvén Mach number. The magnetospheric closure paths of region 1 currents differ significantly between the linear and saturated conditions. In the linear case, essential parts of the region 1 current extend toward dayside and connect to the magnetopause currents, whereas these parts of currents almost disappear in the saturation state. Saturation usually occurs under sub‐Alfvénic solar wind conditions, so the observed FACs generally grow linearly with IEF.

On the Creation, Depletion, and End of Life of Polar Cap Patches

AbstractIonospheric convection patterns from the Super Dual Auroral Radar Network are used to determine the trajectories, transit times, and decay rates of three polar cap patches from their creation in the dayside polar cap ionosphere to their end of life on the nightside. The first two polar cap patches were created within 12 min of each other and traveled through the dayside convection throat, before entering the nightside auroral oval after 104 and 92 min, respectively. When the patches approached the nightside auroral oval, an intensification in the poleward auroral boundary occurred close to their exit point, followed by a decrease in the transit velocity. The last patch (patch 3) decayed completely within the polar cap and had a lifetime of only 78 min. After a change in drift direction, patch 3 had a radar backscatter power half‐life of 4.23 min, which reduced to 1.80 min after a stagnation, indicating a variable decay rate. 28 minutes after the change in direction, and 16 min after coming to a halt within the Clyde River radar field‐of‐view, patch 3 appeared to reach its end of life. We relate this rapid decay to increased frictional heating, which speeds up the recombination rate. Therefore, we suggest that the slowed patch motion within the polar cap convection pattern is a major factor in determining whether the patch survives as a recognizable density enhancement by the time the flux tubes comprising the initial patch cross into the nightside auroral oval.

Dynasonde Observations of Ionospheric Polar Holes Under Quiet Geomagnetic Conditions at Jang Bogo Station, Antarctica

AbstractNoticeable F‐region electron density (NmF2) depletions were observed in the winter‐nighttime polar cap ionosphere during solar minimum from the Vertical Incidence Pulsed Ionospheric Radar (VIPIR) with Dynasonde analysis at Jang Bogo Station (JBS) in Antarctica. We focus on the F‐region density depletion events (known as polar holes) following a steady quiet condition that is defined with Kp values ≤ 1+ during 6 hr. Forty‐five polar holes were identified by JBS VIPIR/Dynasonde (JVD) in 2019. All of the events started over a wide range of nightside magnetic local time (22‐05 MLT) with a peak occurrence at 01–03 MLT. JVD measured exponential NmF2 decrease in the nightside MLT (∼19–2.5 hr) zone with e‐fold decay times distributed in the range of ∼0.5–∼3.5 hr before the onset of a polar hole. The e‐folding times decrease along the longitude from dusk toward midnight. The horizontal ion drift velocity (Vhor) estimated from JVD monotonically goes down from ∼190 m/s at 18 MLT to ∼100 m/s near magnetic midnight, and the NmF2 is depleted as Vhor decreases prior to the polar hole formation. The observations of the exponential NmF2 decrease and the positive correlation between NmF2 and Vhor prior to the polar holes are discussed in light of possible formation mechanisms of polar holes, including temporal variations and spatial structure of the polar ionosphere.

The Effects of the Polar Rain on the Polar Wind Ion Outflow From the Nightside Ionosphere

AbstractThe polar wind, consisting of low‐energy ions and electrons, is an outflow along the open magnetic field lines from the polar cap ionosphere to the magnetosphere. Previous studies found that both solar radiation and solar wind electromagnetic energy are the two main energy sources for the polar wind. The polar rain, being field‐aligned precipitating electrons from the solar wind to the polar cap, may provide additional energies for the polar wind. This scenario is complicated as simulation studies show that polar rain changes the electric potential structures over the polar cap ionosphere. It is unclear how the polar rain affects the polar wind ion outflow. In this study, we show a positive correlation between the polar wind and the polar rain. Meanwhile, the polar wind is generally diminished in regions with strong Earth's magnetic field, suggesting the |B| modulates the penetration depth of the polar rain through the magnetic mirror force and thus the energy dissipation of the polar rain. Therefore, the polar rain can be an additional energy source for the polar wind although the polar rain has generally smaller energies and intensities than the particle precipitations in the auroral regions.

Disappearance of the Polar Cap Ionosphere During Geomagnetic Storm on 11 May 2019

AbstractMulti‐instrument data from Jang Bogo Station (JBS) in Antarctica were utilized to study ionospheric responses to the 11 May 2019 moderate geomagnetic storm. These include Vertical Incident Pulsed Ionospheric Radar (VIPIR)/Dynasonde, Fabry‐Perot Interferometer (FPI), GPS vertical total electron content (vTEC), and magnetometer. The VIPIR/Dynasonde observed long‐lasting (>11 hr) severe depletion of the electron density in the F‐region ionosphere over JBS. During the depletion interval, GPS TEC also correspondingly decreased, FPI neutral temperature was significantly enhanced, and the polar magnetic field variations showed positive and negative excursions in the Y (east) and Z (vertical) components, respectively. GK‐2 A satellite, located ∼2.5 hr west of JBS, observed negative magnetic field perturbations in the azimuthal BD component at geosynchronous orbit during the depletion of ionospheric plasma. Such a BD perturbation at geosynchronous orbit is due to the field‐aligned currents flowing out of the ionosphere. From these observations we suggest that transpolar ionospheric currents connected to the field‐aligned currents flowing on a substorm wedge‐shaped circuit act as a source of polar atmospheric heating during the moderate geomagnetic storm interval and that elevated heavy molecular gases (O2 and N2) by atmospheric heating contribute to the electron density depletion via increased recombination rate.

Time‐Varying Magnetopause Reconnection During Sudden Commencement: Global MHD Simulations

AbstractIn response to a solar wind dynamic pressure enhancement, the compression of the magnetosphere generates strong ionospheric signatures and a sharp variation in the ground magnetic field, termed sudden commencement (SC). While such compressions have also been associated with a contraction of the ionospheric polar cap due to the triggering of reconnection in the magnetotail, the effect of any changes in dayside reconnection is less clear and is a key component in fully understanding the system response. In this study, we explore the time‐dependent nature of dayside coupling during SC by performing global simulations using the Gorgon magnetohydrodynamic code and impact the magnetosphere with a series of interplanetary shocks with different parameters. We identify the location and evolution of the reconnection region in each case as the shock propagates through the magnetosphere, finding strong enhancement in the dayside reconnection rate and prompt expansion of the dayside polar cap prior to the eventual triggering of tail reconnection. This effect pervades for a variety of interplanetary magnetic field orientations, and the reconnection rate is most enhanced for events with higher dynamic pressure. We explain this by repeating the simulations with a large explicit resistivity, showing that compression of the magnetosheath plasma near the propagating shock front allows for reconnection of much greater intensity and at different locations on the dayside magnetopause than during typical solar wind conditions. The results indicate that the dynamic behavior of dayside coupling may render steady models of reconnection inaccurate during the onset of a severe space weather event.

Climatology and modeling of ionospheric irregularities over Greenland based on empirical orthogonal function method

This paper addresses the long-term climatology (over two solar cycles) of total electron content (TEC) irregularities from a polar cap station (Thule) using the rate of change of the TEC index (ROTI). The climatology reveals variabilities over different time scales, i.e., solar cycle, seasonal, and diurnal variations. These variations in different time scales can be explained by different drivers/contributors. The solar activity (represented by the solar radiation index F10.7P) dominates the longest time scale variations. The seasonal variations are controlled by the interplay of the energy input into the polar cap ionosphere and the solar illumination that damps the amplitude of ionospheric irregularities. The diurnal variations (with respect to local time) are controlled by the relative location of the station with respect to the auroral oval. We further decompose the climatology of ionospheric irregularities using the empirical orthogonal function (EOF) method. The first four EOFs could reflect the majority (99.49%) of the total data variability. A climatological model of ionospheric irregularities is developed by fitting the EOF coefficients using three geophysical proxies (namely, F10.7P, Bt, and Dst). The data-model comparison shows satisfactory results with a high Pearson correlation coefficient and adequate errors. Additionally, we modeled the historical ROTI during the modern grand maximum dating back to 1965 and made the prediction during solar cycle 25. In such a way, we can directly compare the climatic variations of the ROTI activity across six solar cycles.

Quantifying the Lobe Reconnection Rate During Dominant IMF By Periods and Different Dipole Tilt Orientations

AbstractLobe reconnection is usually thought to play an important role in geospace dynamics only when the Interplanetary Magnetic Field (IMF) is mainly northward. This is because the most common and unambiguous signature of lobe reconnection is the strong sunward convection in the polar cap ionosphere observed during these conditions. During more typical conditions, when the IMF is mainly oriented in a dawn‐dusk direction, plasma flows initiated by dayside and lobe reconnection both map to high‐latitude ionospheric locations in close proximity to each other on the dayside. This makes the distinction of the source of the observed dayside polar cap convection ambiguous, as the flow magnitude and direction are similar from the two topologically different source regions. We here overcome this challenge by normalizing the ionospheric convection observed by the Super Dual Aurora Radar Network (SuperDARN) to the polar cap boundary, inferred from simultaneous observations from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE). This new method enable us to separate and quantify the relative contribution of both lobe reconnection and dayside/nightside (Dungey cycle) reconnection during periods of dominating IMF By. Our main findings are twofold. First, the lobe reconnection rate can typically account for 20% of the Dungey cycle flux transport during local summer when IMF By is dominating and IMF Bz ≥ 0. Second, the dayside convection relative to the open/closed boundary is vastly different in local summer versus local winter, as defined by the dipole tilt angle.

Electron Density Depletion Region Observed in the Polar Cap Ionosphere

AbstractThis paper presents and discusses electron density depletion regions observed with the incoherent scatter EISCAT Svalbard Radar (ESR) located at 75.43°N geomagnetic latitude. The data include several decades of measurements, which make them suitable for studying statistical features and characteristics of the ionospheric parameters. Here we focus on the electron density depletions and their dependence on diurnal and seasonal variations and solar activity. An electron density depletion region is identified in the ESR data in the early morning sector. This depletion region seems to be clearest during equinox and winter and moderate/high solar activity. An enhancement in the ion temperature is often co‐located with the electron density depletion region. The ion temperature enhancement could indicate that ion frictional heating is related to the electron density depletion region. However, during summer when the solar activity is low, the electron density depletion is not observed although the ion temperature is enhanced, suggesting that formation of the electron density depletion regions due to ion frictional heating may depend on the background effective temperature and O/N2 ratio. In addition, seasonal changes in the solar zenith angle could also contribute to the formation of the depletion region.

Complex Sub‐Auroral Flow Channel Structure Formed by Double‐Peak Sub‐Auroral Ion Drifts (DSAID) and Abnormal Sub‐Auroral Ion Drifts (ASAID)

AbstractBy utilizing multi‐instrument and multi‐satellite/spacecraft observations, we report the repeated development of a temporary and complex sub‐auroral flow channel (FC) structure during the intensive storms investigated. Appearing as combined double‐peak sub‐auroral ion drifts (DSAID) and abnormal SAID (ASAID), it is called DSAID‐ASAID FC structure. Revealing its development, various other FCs and features appeared in the auroral and polar regions simultaneously. These include the auroral Harang Region (HR) that is the Substorm Current Wedge's ionospheric signature and the Eastward FC (E‐FC) indicating the arrival of the Westward Travelling Surge. Results show that in the HR and E‐FC, the locally enhanced duskside upward Region 1 Field Aligned Currents (↑R1 FACs) connected in the magnetosphere with the meridional partial ring current characterized by a double plasma density/pressure peak structure, of which signature was seen in the 225 keV proton density data, leading to the development of two sets of ↓R2 FACs and DSAID. Meanwhile, large auroral undulations appeared and were driven, via magnetosphere‐ionosphere coupling, by Kelvin‐Helmholtz instability mechanisms that also drove plasma sheet rippling leading to ASAID development. Plasma sheet rippling occurred during magnetotail reconnection, as demonstrated with its ionospheric polar cap signature, known as FC‐3, in a stretched tail region according to the b2i boundary located at ∼60° magnetic latitude. Then, the earthward flows' braking/slowing down provided the energy for driving the R1‐R2 FACs. From the observational evidence presented we conclude that the DSAID‐ASAID features investigated developed due to the maintenance of DSAID during plasma sheet rippling triggering ASAID development.

Two‐Dimensional Reconstruction of Ionospheric Plasma Density Variations Using Swarm

AbstractSpace weather phenomena such as scintillations of Global Navigation Satellite Systems (GNSS) signals are of increasing importance for aviation, the maritime, and civil engineering industries. The ionospheric plasma irregularities causing scintillations are associated with strong gradients in ionospheric plasma density. To provide nowcasts and forecasts of space weather effects, it is vital to monitor the ionosphere and detect strong density variations. To reconstruct plasma density variations in the polar cap ionosphere, we use total electron content (TEC) estimates from the Swarm satellites' GPS receivers. By considering events where the Swarm satellites are in close proximity, we obtain plasma density variations by inverting TEC measurements on a two‐dimensional grid. We first demonstrate the method using synthetic test data, before applying it to real data. The method is validated using in situ Langmuir probe measurements and ground‐based TEC observations. We find that the new method can reproduce density variations, although it is sensitive to the geometry of the Swarm satellite constellation and to the calculated plasma temperature. Our proposed method opens new possibilities for ionospheric plasma monitoring that uses GPS receivers aboard low Earth orbit (LEO) satellites.

The Limits of Empirical Electron Density Modeling: Examining the Capacity of E‐CHAIM and the IRI for Modeling Intermediate (1‐ to 30‐Day) Timescales at High Latitudes

AbstractThe Empirical Canadian High Arctic Ionospheric Model (E‐CHAIM) is a new empirical 3‐D electron density model intended as an alternative to the use of conventional standards, such as the International Reference Ionosphere (IRI), at high latitudes (above 50°N). In this study, we have manually scaled a year of data from two Canadian High Arctic Ionospheric Network (CHAIN) ionosondes. Using this high‐quality data, we examine the behavior of the polar cap ionosphere under disturbed geomagnetic conditions and assess the capacity of E‐CHAIM to model polar cap F2‐peak electron density variability on “weather‐like,” intermediate timescales (1–30 days). This is a particularly challenging environment for monthly median empirical models due to the regular occurrence of variations about the monthly mean of up to 2 MHz. We demonstrate in this study that E‐CHAIM's storm model is capable of explaining 4 to 25% of polar cap foF2 variance at 1‐ to 30‐day timescales and 5 to 50% of the amplitude of that variability, while the IRI's Storm‐Time Ionospheric Correction Model (STORM) only explains 0.2 to 9% of the variance at these timescales and no more than 5% of their amplitude. While the IRI's STORM model provided no measurable improvement over the monthly median, E‐CHAIM's storm parameterization was able to improve overall root‐mean‐square errors by 0.05 to 0.1 MHz over its quiet time model. The overall improvement through the use of storm foF2 parameterizations is found to be limited, but measurable, particularly during storm periods, where an average improvement in root‐mean‐square error of 20% is observed.

Analysis of Plasma Irregularities on a Range of Scintillation‐Scales Using the Resolute Bay Incoherent Scatter Radars

AbstractScintillation in the polar cap ionosphere is observed with a Coherent Electromagnetic Radio Tomography (CERTO) receiver while the Resolute Bay Incoherent Scatter Radars (RISR) provide background plasma conditions in a 3D volume. We interpret fluctuations in the very high frequency (VHF) and ultrahigh frequency (UHF) signal associated with mid‐scale ionospheric structuring using 3D density gradients and plasma drift velocity vectors and calculate the maximum gradient drift instability growth rate. Plasma structuring is evident for any plasma density gradient, but sub‐kilometer structures are less likely on the leading edge of polar patches, possibly due to a much faster diffusion rate for small‐scale structures. Structures are much more uniformly distributed around density enhancements that do not have clear leading and trailing edges. Although the gradient drift instability is an important factor in polar cap structuring, linear growth rates are most successful at predicting structuring around isolated density enhancements in consistent plasma flows. For more general density gradients, the time history of a plasma parcel and local diffusion rates must also be considered.

Interhemispheric Asymmetry of Large‐Scale Electron Density Gradients in the Polar Cap Ionosphere: UT and Seasonal Variations

AbstractThe high‐latitude ionosphere is highly dynamical with significant irregularities and density gradients. However, the spatial and temporal distributions of density gradients and irregularities are very different between the Arctic and Antarctic. In this report, we study the interhemispheric asymmetry of the large‐scale (100 km) density gradients in both polar caps. Our results show that density gradients in the Arctic are enhanced during local winter (December solstice) with a peak around 19 UT. The UT and spatial distributions in the Antarctic local winter (June solstice) are similar to the Arctic except that they are reversed by 12 hr, which indicates a mirror symmetry between hemispheres. The 12‐hr difference in the peak density gradients can be explained by the displacements between the geographic and geomagnetic poles. The only asymmetry (anomaly) is the persistence of strong density gradients in the southern polar cap during local summer (December solstice).

A Study of Automatically Detected Flow Channels in the Polar Cap Ionosphere

AbstractThis paper presents a new algorithm for detecting high‐speed flow channels in the polar cap. The algorithm was applied to Super Dual Auroral Radar Network data, specifically to data from the new Longyearbyen radar. This radar is located at 78.2°N, 16.0°E geographical coordinates looking north‐east, and is therefore at an ideal location to measure flow channels in the high‐latitude polar cap. The algorithm detected >500 events over 1 year of observations, and within this paper two case studies are considered in more detail. A flow channel on “old‐open field lines” located on the dawn flank was directly driven under quiet conditions over 13 min. This flow channel contributed to a significant fraction (60%) of the cross polar cap potential and was located on the edge of a polar cap arc. Another case study follows the development of a flow channel on newly opened field lines within the cusp. This flow channel is a spontaneously driven event forming under strong solar wind driving and is intermittently excited over the course of almost an hour. As they provide a high fraction of the cross polar cap potential, these small‐scale structures are vital for understanding the transport of magnetic flux over the polar cap.

The Formation of Electron Heat Flux Over the Sunlit Quiet Polar Cap Ionosphere

AbstractWe consider a new mechanism for the thermal electron heat fluxes formation over the sunlit polar cap in the presence of the potential jumps that are created by the photoelectron escape along the open magnetic field lines. Photoelectrons traveling into the magnetosphere experience small Coulomb collisional scattering, and the low‐energy part of their spectra becomes trapped between the potential jump and upper ionosphere creating the thermal electron energy fluxes that support the formation of electron temperature at the ionospheric altitudes. This mechanism eliminates the need for the customary practice of assuming arbitrary electron heat fluxes at the upper ionospheric boundary in order to explain the measured electron temperature in the topside ionosphere.

Polarization measurements of unusual cases of medium frequency burst emissions extending below 1.5\xa0MHz

Auroral medium frequency burst (MFB) is a radio emission of natural auroral origin associated with substorm expansion phase and observable at ground level. The emission usually occurs at frequencies above 1500 kHz, but occasionally it extends to a sharp lower cutoff frequency at 1300–1500 kHz depending on the observing site, with a frequency gap below the cutoff and sporadic emission below the gap extending to frequencies as low as 1000 kHz. These low-frequency MFB components lie below the electron gyrofrequency and hence could represent either whistler or LO-modes. Recently, using crossed antennas and a two-channel receiver at Toolik Lake, Alaska, polarization of these low-frequency MFB components was measured for the first time and found to be left-hand. This observation eliminates whistler mode as a possibility and requires the low-frequency components be LO-mode in the ionosphere, which constrains their source location since it requires that the frequency exceeds the L-cutoff frequency. In these occasional events marked by a cutoff and low-frequency MFB components, the latter probably originate at high altitudes (>,800 km) and reach the ground through extraordinary low-density polar cap ionosphere.

Annual Occurrence Rates of Ionospheric Polar Cap Patches Observed Using Swarm

AbstractDense, fast‐moving regions of ionization called polar cap patches are known to occur in the high‐latitude F region ionosphere. Patches are widely believed to be caused by convection of dense, sunlit plasma into a dark and therefore low‐density polar cap ionosphere. This leads to the belief that patches are a winter phenomenon. Surprisingly, a long‐term analysis of 3 years of ionospheric measurements from the Swarm satellites shows that large density enhancements occur far more frequently in local summer than local winter in the Southern Hemisphere (SH). The reverse is true in the Northern Hemisphere (NH). Previously reported patch detections in the SH are reexamined. Detection algorithms using only a relative doubling test count very small density fluctuations in SH winter due to extremely low ambient densities found there, while much larger enhancements occurring in SH summer are missed due to especially high ambient densities. The same problem does not afflict results in the NH, where ambient densities are more stable year‐round due to the ionospheric annual asymmetry. Given this new analysis, the definition of a patch as a doubling of the ambient density is not suitable for the SH. We propose a test for patches linked to long‐term averaged solar flux activity, characterized by the 81 day centered mean F10.7 index. Importantly, the current patch formation theory is at least incomplete in that it does not predict the observed lack of patches in SH winter, or the many large enhancements seen in SH summer.

Driving of strong nightside reconnection and geomagnetic activity by polar cap flows: Application to CME shocks and possibly other situations

Previous studies have shown that dynamic pressure impacts (e.g., shocks initiating CME storms) with southward IMF promptly lead to strong auroral nightside activity and concurrent poleward expansion (indicating strong nightside reconnection), and strong enhancements in convection and currents. Here we use a combination of ground-based ASI and radar observations to further describe this response, to address what is driving the strong activity, and to suggest similar driving in other situations. Consistent with some previous studies, we find that shock driven auroral activity and poleward expansion resembles a substorm, but starts from an already broad MLT sector without much subsequent azimuthal expansion and without classical brightening of the equatorward-most arc. We furthermore find a large enhancement of meso-scale ionospheric polar cap flows heading towards the nightside separatrix immediately after shock impact. Recent studies have shown that such enhanced flows often cross the separatrix leading to plasma sheet flow bursts, poleward boundary intensifications (PBIs), streamers, and poleward motion of the polar cap boundary from reconnection. Thus these flow enhancements, which must extend outward along field lines from the ionosphere, are an attractive candidate as the driver for the almost immediate strong auroral, current, and reconnection activity resulting from shock impact. We also discuss and present some evidence that this phenomenon may be more general, leading to similar oval responses without a shock impact, including during and following the expansion phase of some substorms. These suggestions could lead to some possibly fundamental questions, such as when do polar cap convection enhancements lead to a substorm growth phase versus leading directly to strong polar expansion of, and strong activity along, the auroral oval field line region?

Storm time coupling between the magnetosheath and the polar ionosphere

AbstractThe present study demonstrates the feasibility of using plasma and magnetic field data from Defense Meteorological Satellite Program (DMSP) satellites to estimate the Alfvénic reflectances (α) in the polar cap ionosphere during periods of geomagnetic disturbance. Quantitative values of α are then used to support calculations of field‐aligned Poynting flux (S||i) transmitted along open field lines from magnetosheath generators to the conjugate polar ionospheres. Analyzed data were acquired during high‐latitude passes of DMSP F16 during the 17 March 2013 magnetic storm. Between the main phase onset and the day's end, plasma velocity and magnetic field perturbations manifest the phase relations predicted by Alfvén and Fälthammar (1963) for electromagnetic coupling. The most reliable (sunward‐antisunward) components of measured plasma velocities and magnetic perturbations are consistent with that α ranging between 0.65 and 0.97, with a mean of ~0.8. Subsequent calculations indicate that incident S||i varied between 0.2 and 0.02 W/m2 with a mean of 0.085. We suggest that these reported ranges of α and S||i reflect both large variations in IMF BZ and the growth of antisunward neutral winds in the polar caps during the storm's main phase.