Energetic Particle Precipitation Research Articles

A Method for Imaging Energetic Particle Precipitation with Subionospheric VLF Signals

A Method for Imaging Energetic Particle Precipitation with Subionospheric VLF Signals

Multiple E-Region Radar Propagation Modes Measured by the VHF SIMONe Norway System During Active Ionospheric Conditions

Multiple propagation modes between different bistatic radar links were measured during the operations of a very high frequency (VHF) 32.55 MHz radar system in northern Norway. The Spread Spectrum Interferometric Multistatic meteor radar Observing Network (SIMONe) Norway system detected meteor trails, direct transmitter to receiver signal propagation, over-the-horizon signal propagation from the SIMONe Germany system, ground and/or sea scatter, and ionospheric scatter on 27 August 2021 between 16:30–20:00 UT. These simultaneous detections were during an active ionospheric period with multiple occurrences of energetic charged particle precipitation. The SIMONe systems used continuous-wave (CW) pseudo-random phase modulated transmit signals and interferometry to make it possible to isolate each of these propagation modes and examine their characteristics. Different multistatic links at three receiver locations were analyzed, providing multistatic measurements of the regions with spatial and temporal resolutions on the order of 1.5 km and 2 s. The analysis techniques are described, with characteristics of the radar signal presented for each propagation mode and multistatic link. This study serves to highlight the capabilities of the SIMONe Norway system to research multiple aspects of ionospheric phenomena, specifically in the lower thermosphere-mesosphere boundary region.

Statistical Survey of Arase Satellite Data Sets in Conjunction With the Finnish Riometer Network

AbstractDuring disturbed geomagnetic conditions, the energetic particles in the inner magnetosphere are known to undergo precipitation loss due to interaction with various plasma waves. This study, investigates the energetic particle precipitation events statistically using coordinate observations from the ground riometer network and the inner‐magnetospheric satellite mission, Arase. We have compared cosmic noise absorption (CNA) data obtained from the Finnish ground riometer network located in the auroral/sub‐auroral latitudes with the comprehensive data set of omnidirectional electron/proton flux and plasma waves in ELF/VLF frequency range from the Arase satellite during the overpass intervals. The study period includes one and a half years of data between March 2017 and September 2018 covering Arase conjunctions with the riometer stations from all magnetic local time sectors. The relation between the plasma flux/waves observed at the satellite with the riometer absorptions are investigated statistically for CNA (absorption >0.5 dB) and non‐CNA (absorption <0.5 dB) cases separately. During CNA events, Arase observed elevated electron flux in the medium energy range (2–100 keV), and plasma wave activity in the whistler‐mode frequency range (0.5–3 kHz) of the spectra. Our study provides an estimate of the statistical dependence of the electron flux and plasma wave observations at Arase with the ground reality of actual precipitation.

ULF Wave Modeling, Effects, and Applications: Accomplishments, Recent Advances, and Future

Ultra Low Frequency (ULF) waves play important roles in magnetosphere-ionosphere coupling, ring current and radiation belt dynamics, and modulation of higher frequency wave modes and energetic particle precipitation. The “ULF wave modeling, effects, and applications” (UMEA) focus group - part of the Geospace Environment Modeling effort from 2016 to 2021 - sought to improve understanding of the physics of ULF waves and their specification in geospace models. Through a series of in person and virtual meetings the UMEA focus group brought modelers and experimentalists together to compare ULF wave outputs in different models, plan observation campaigns focused on ULF waves, discuss recent advances in ULF wave research, and identify unresolved ULF wave science questions. This article summarizes major discussion points and accomplishments in the UMEA focus group over the last 6 years, recent advances and their connection to Richard Thorne and Peter Gary’s significant contributions to ULF wave research, and the future of ULF wave research.

Effects of particle precipitation on the polar mesospheric summer echoes observed by EISCAT VHF 224 MHz radar

We present the results from investigation of spectra of polar mesospheric summer echoes (PMSE) observed with EISCAT VHF 224 MHz radar during energetic particle precipitation events in the summer of 2019. We used the sudden enhancements in electron densities derived from the VHF observations above 90 km as indicators of particle precipitation. We find that the altitude extent of the PMSE increased along with an enhancement of the strength of the pre-existing PMSE. On some occasions, PMSE suddenly appeared during particle precipitation events. After the particle precipitation subsided, the PMSE intensities continued to be stronger for a few minutes and then decreased. At the altitudes where the maximum enhancement in PMSE backscatter occurred, there is no corresponding broadening in the spectral widths. Interestingly, the most intense PMSE echoes almost always coincided with the lower values of spectral widths both during particle precipitations and other times. In some cases, the spectral widths show well separated regimes between the upper and lower parts of the PMSE layers and this also happened irrespective of particle precipitation. The frequency Doppler shifts showed alternating upward and downward motions without much difference before and after the particle precipitation. Based on these observations, we conclude that the sudden enhancement in the electron densities during particle precipitation intensifies PMSE structures that match the Bragg scale of the incident radio wave thereby enabling an enhancement in the backscatter strength. Similarly, PMSE backscattering may get initiated in the altitudes in which the structures were already present but where electron density was not sufficient before the onset of precipitation. Spectral widths and Doppler velocities does not show sudden variations with particle precipitation because they depend on the neutral dynamics.

Statistical analysis of large and extreme global ionospheric total electron content

Ionospheric disturbances, due to photoionization and energetic particle precipitation, occurring both on a regular recurring basis, and heightened due to space weather, impact global positioning, navigation, and timing services provided by Global Navigation Satellite Systems (GNSS). The development of operational services to mitigate the risks to users requires knowledge of how space weather impacts critical systems, and the frequency and magnitude of ionospheric disturbances including extreme events. Analysis of the largest values of total electron content (TEC) is important for the assessment of space weather hazards for systems sensitive to ionospheric effects on radio wave propagation and for the development of risk mitigation strategies. In this study, twenty years of global TEC maps are analyzed to determine the global distribution of the daily maximum TEC. Maximum TEC is shown to reach magnitudes of about 200 TECU in extreme cases, with the largest TEC normally observed at equatorial latitudes. A generalized extreme value distribution has been applied to the daily global maximum TEC and for the high (60°–90°; −90° to −60°), mid (30°–60°; −60° to −30°), and low-latitude (0°–30°; −30°–0°) regions in geographic latitudes to determine the 1-in-44 year (4 solar cycles) and 1-in-100 year maximum TEC with a 95% confidence interval. Additionally, the return period for TEC thresholds defined for safe operation by the International Civil Aviation Organization are discussed. Extreme values provide criteria for the development of operational systems and vulnerability assessment of critical systems dependant on GNSS.

Different Sporadic‐E (Es) Layer Types Development During the August 2018 Geomagnetic Storm: Evidence of Auroral Type (Esa) Over the SAMA Region

AbstractSporadic‐E (Es) layers are investigated over five Digisonde stations located inside (Santa Maria and Cachoeira Paulista), boundary (São Luís), and outside (Millstone Hill and Port Stanley) the South American Magnetic Anomaly (SAMA) in the course of the 25 August 2018, geomagnetic storm. Data from Digisondes and ground‐based magnetometer and a numerical model of the E‐region (MIRE) are analyzed to search the roles of neutral winds and electric fields as physical processes responsible for the generation of Es layers. Moreover, it is observed for the first time over Santa Maria a type of Es layer that resembles the Esa detected in the auroral region. The characterization of such signatures in ionograms and the investigations of its formation mechanism are the main focus of this study. Hiss waves activities detected by the Van Allen Probe‐B data provide evidence that energetic particle precipitations are the most likely process responsible for this type of Es layer inside the SAMA in the night hours during the geomagnetic storm recovery phase. An attempt is also made to explain the presence of Esq over São Luís in terms of the expansion of the Equatorial Electrojet to higher latitudes. Finally, the results bring a new observational and modeling indication of a concomitant action of multiple formation mechanisms in the Es layers development in the American sector during the 25 August 2018, geomagnetic storm.

Types of pulsating aurora: comparison of model and EISCAT electron density observations

Abstract. Energetic particle precipitation associated with pulsating aurora (PsA) can reach down to lower mesospheric altitudes and deplete ozone. It is well documented that pulsating aurora is a common phenomenon during substorm recovery phases. This indicates that using magnetic indices to model the chemistry induced by PsA electrons could underestimate the energy deposition in the atmosphere. Integrating satellite measurements of precipitating electrons in models is considered to be an alternative way to account for such an underestimation. One way to do this is to test and validate the existing ion chemistry models using integrated measurements from satellite and ground-based observations. By using satellite measurements, an average or typical spectrum of PsA electrons can be constructed and used as an input in models to study the effects of the energetic electrons in the atmosphere. In this study, we compare electron densities from the EISCAT (European Incoherent Scatter scientific radar system) radars with auroral ion chemistry and the energetics model by using pulsating aurora spectra derived from the Polar Operational Environmental Satellite (POES) as an energy input for the model. We found a good agreement between the model and EISCAT electron densities in the region dominated by patchy pulsating aurora. However, the magnitude of the observed electron densities suggests a significant difference in the flux of precipitating electrons for different pulsating aurora types (structures) observed.

Occurrence rate and duration of space weather impacts on high-frequency radio communication used by aviation

High frequency (HF) radio wave propagation is sensitive to space weather-induced ionospheric disturbances that result from enhanced photoionization and energetic particle precipitation. Recognizing the potential risk to HF radio communication systems used by the aviation industry, as well as potential impacts on GNSS navigation and the risk of elevated radiation levels, the International Civil Aviation Organization (ICAO) initiated the development of a space weather advisory service. For HF systems, this service specifically identifies shortwave fadeout, auroral absorption, polar cap absorption, and post-storm maximum useable frequency depression (PSD) as phenomena impacting HF radio communication and specifies moderate and severe event thresholds to describe event severity. This paper examines the occurrence rate and duration of events crossing the moderate and severe thresholds. Shortwave fadeout was evaluated based on thresholds in the solar X-ray flux. Analysis of 40-years of solar X-ray flux data showed that moderate and severe level solar X-ray flares were observed, on average, 123 and 5 times per 11-year solar cycle, respectively. The mean event duration was 68 min for moderate level events and 132 min for severe level events. Auroral absorption events crossed the moderate threshold for 40 events per solar cycle, with a mean event duration of 5.1 h. The severe threshold was crossed for 3 events per solar cycle with a mean event duration of 12 h. Polar cap absorption had the longest mean duration at ~8 h for moderate events and 1.6 days for severe events; on average, 24 moderate and 13 severe events were observed per solar cycle. Moderate and severe thresholds for shortwave fadeout, auroral absorption, and polar cap absorption were used to determine the expected impacts on HF radio communication. Results for polar cap absorption and shortwave fadeout were consistent with each other, but the expected impact for auroral absorption was shown to be 2–3 times higher. Analysis of 22 years of ionosonde data showed moderate, and severe PSD events occurred, on average, 200 and 56 times per 11-year solar cycle, respectively. The mean event duration was 5.5 h for moderate-level events and 8.5 h for severe-level events. During solar cycles 22 and 23, HF radio communication was expected to experience moderate or severe impacts due to the ionospheric disturbances caused by space weather, a maximum of 163 and 78 days per year, respectively, due to the combined effect of absorption and PSD. The distribution of events is highly non-uniform with respect to the solar cycle: 70% of moderate or severe events were observed during solar maximum compared to solar minimum.

Comparative Study of Electric Currents and Energetic Particle Fluxes in a Solar Flare and Earth Magnetospheric Substorm

Abstract Magnetic field line reconnection is a universal plasma process responsible for the conversion of magnetic field energy to plasma heating and charged particle acceleration. Solar flares and Earth's magnetospheric substorms are two of the most investigated dynamical systems where global magnetic field reconfiguration is accompanied by energization of plasma populations. Such a reconfiguration includes formation of a long-living current system connecting the primary energy release region and cold dense conductive plasma of the photosphere/ionosphere. In both flares and substorms the evolution of this current system correlates with the formation and dynamics of energetic particle fluxes (although energy ranges can be different for these systems). Our study is focused on the similarity between flares and substorms. Using a wide range of data sets available for flare and substorm investigations, we qualitatively compare the dynamics of currents and energetic particle fluxes for one flare and one substorm. We show that there is a clear correlation between energetic particle precipitations (associated with energy release due to magnetic reconnection seen from riometer and hard X-ray measurements) and magnetic field reconfiguration/formation of the current system, whereas the long-term current system evolution correlates better with hot plasma fluxes (seen from in situ and soft X-ray measurements). We then discuss how data sets of in situ measurements of magnetospheric substorms can help interpret solar flare data.

Fine-scale dynamics of fragmented aurora-like emissions

Abstract. Fragmented aurora-like emissions (FAEs) are small (few kilometres) optical structures which have been observed close to the poleward boundary of the aurora from the high-latitude location of Svalbard (magnetic latitude 75.3 ∘N). The FAEs are only visible in certain emissions, and their shape has no magnetic-field-aligned component, suggesting that they are not caused by energetic particle precipitation and are, therefore, not aurora in the normal sense of the word. The FAEs sometimes form wave-like structures parallel to an auroral arc, with regular spacing between each FAE. They drift at a constant speed and exhibit internal dynamics moving at a faster speed than the envelope structure. The formation mechanism of FAEs is currently unknown. We present an analysis of high-resolution optical observations of FAEs made during two separate events. Based on their appearance and dynamics, we make the assumption that the FAEs are a signature of a dispersive wave in the lower E-region ionosphere, co-located with enhanced electron and ion temperatures detected by incoherent scatter radar. Their drift speed (group speed) is found to be 580–700 m s−1, and the speed of their internal dynamics (phase speed) is found to be 2200–2500 m s−1, both for an assumed altitude of 100 km. The speeds are similar for both events which are observed during different auroral conditions. We consider two possible waves which could produce the FAEs, i.e. electrostatic ion cyclotron waves (EIC) and Farley–Buneman waves, and find that the observations could be consistent with either wave under certain assumptions. In the case of EIC waves, the FAEs must be located at an altitude above about 140 km, and our measured speeds scaled accordingly. In the case of Farley–Buneman waves a very strong electric field of about 365 mV m−1 is required to produce the observed speeds of the FAEs; such a strong electric field may be a requirement for FAEs to occur.

Simulated seasonal impact on middle atmospheric ozone from high-energy electron precipitation related to pulsating aurorae

Abstract. Recent simulation studies have provided evidence that a pulsating aurora (PsA) associated with high-energy electron precipitation is having a clear local impact on ozone chemistry in the polar middle mesosphere. However, it is not clear if the PsA is frequent enough to cause longer-term effects of measurable magnitude. There is also an open question of the relative contribution of PsA-related energetic electron precipitation (PsA EEP) to the total atmospheric forcing by solar energetic particle precipitation (EPP). Here we investigate the PsA-EEP impact on stratospheric and mesospheric odd hydrogen, odd nitrogen, and ozone concentrations. We make use of the Whole Atmosphere Community Climate Model and recent understanding on PsA frequency, latitudinal and magnetic local time extent, and energy-flux spectra. Analysing an 18-month time period covering all seasons, we particularly look at PsA-EEP impacts at two polar observation stations located at opposite hemispheres: Tromsø in the Northern Hemisphere (NH) and Halley Research Station in the Southern Hemisphere (SH). We find that PsA EEP can have a measurable impact on ozone concentration above 30 km altitude, with ozone depletion by up to 8 % seen in winter periods due to PsA-EEP-driven NOx enhancement. We also find that direct mesospheric NOx production by high-energy electrons (E> 100 keV) accounts for about half of the PsA-EEP-driven upper stratospheric ozone depletion. A larger PsA-EEP impact is seen in the SH where the background dynamical variability is weaker than in the NH. Clearly indicated from our results, consideration of polar vortex dynamics is required to understand PsA-EEP impacts seen at ground observation stations, especially in the NH. We conclude that PsA-EEP has the potential to make an important contribution to the total EPP forcing; thus, it should be considered in atmospheric and climate simulations.

Development and Validation of Precipitation Enhanced Densities for the Empirical Canadian High Arctic Ionospheric Model

AbstractThe Empirical Canadian High Artic Ionospheric Model (E‐CHAIM) provides the four‐dimensional ionosphere electron density at northern high latitudes (>50° geomagnetic latitude). Despite its emergence as the most reliable model for high‐latitude ionosphere density, there remain significant deficiencies in E‐CHAIM's representation of the lower ionosphere (below ∼200 km) due to a sparsity of reliable measurements at these altitudes, particularly during energetic particle precipitation events. To address this deficiency, we have developed a precipitation component for E‐CHAIM to be driven by satellite‐based far‐ultraviolet (FUV) imager data. Satellite observations of FUV emissions may be used to infer the characteristics of energetic particle precipitation and subsequently calculate the precipitation‐enhanced ionization rates and ionosphere densities. In order to demonstrate the improvement of E‐CHAIM's ionosphere density representation with the addition of a precipitation component, this paper presents comparisons of E‐CHAIM precipitation‐enhanced densities with ionosphere density measurements of three auroral region incoherent scatter radars (ISRs) and one polar cap ISR. Calculations for 29,038 satellite imager and ISR conjunctions during the years 2005–2019 revealed that the root‐mean‐square difference between E‐CHAIM and ISR measurements decreased by up to 2.9 × 1010 ele/m3 (altitude dependent) after inclusion of the precipitation component at auroral sites, and by 2.6 × 109 ele/m3 in the polar cap. Improvements were most substantial in the winter season and during active auroral conditions. The sensitivity of precipitation‐enhanced densities to uncertainties inherent to the calculation method was also examined, with the bulk of the errors due to uncertainties in FUV imager data and choice of distribution function for precipitation energy spectra.

Case Studies of Ionospheric Plasma Irregularities Over Queen Maud Land, Antarctica

AbstractWe use the first data set from the ground‐based TEC and scintillation receiver located at the Norwegian Research Station Troll in Queen Maud Land, Antarctica to analyze in detail the ionospheric response during geomagnetic disturbances on February 26–27 and March 18–19, 2018. By combining the Troll data with complementary measurements (scintillation receivers, magnetometers, SuperDARN, DMSP satellites), we demonstrate that plasma irregularities above this part of Queen Maud Land can be associated with the expansion of the auroral oval, related structuring on its edges, as well as with strong flow shears in the evening and at nighttime, which are further modulated by the energetic particle precipitation. Phase scintillation indices correlate well with the magnetometer data.

Charged particle scattering in dipolarized magnetotail

The Earth's magnetotail is characterized by stretched magnetic field lines. Energetic particles are effectively scattered due to the field-line curvature, which then leads to isotropization of energetic particle distributions and particle precipitation to the Earth's atmosphere. Measurements of these precipitation at low-altitude spacecraft are thus often used to remotely probe the magnetotail current sheet configuration. This configuration may include spatially localized maxima of the curvature radius at equator (due to localized humps of the equatorial magnetic field magnitude) that reduce the energetic particle scattering and precipitation. Therefore, the measured precipitation patterns are related to the spatial distribution of the equatorial curvature radius that is determined by the magnetotail current sheet configuration. In this study, we show that, contrary to previous thoughts, the magnetic field line configuration with the localized curvature radius maximum can actually enhance the scattering and subsequent precipitation. The spatially localized magnetic field dipolarization (magnetic field humps) can significantly curve magnetic field lines far from the equator and create off-equatorial minima in the curvature radius. Scattering of energetic particles in these off-equatorial regions alters the scattering (and precipitation) patterns, which has not been studied yet. We discuss our results in the context of remote-sensing the magnetotail current sheet configuration with low-altitude spacecraft measurements.

атухание низкочастотных электромагнитных волн в нижней ионосфере земли под влиянием сильного локального возмущения в атмосфере.

The value and duration of attenuation of low frequency waves (1...10 kHz) in the presence of a strong local disturbance of the atmosphere have been estimated. Sources of significant local disturbances of the atmosphere are, for example, precipitation of energetic particles of radiation belts; electromagnetic pulses of lightning discharges; radiation of powerful low-frequency ground-based transmitters; invasion of large meteors. Strong local disturbances lead to an increase of ionization (concentration of free electrons) of the environment by several orders of magnitude in the region of space whose characteristic dimensions are comparable to the length of the wave (tens and hundreds of kilometers). As such a disturbance, we use the previously developed macroscopic model of an instantaneous, point release of a relatively large amount of energy in the atmosphere below the ionosphere. This model makes it possible to estimate the features of the propagation of low-frequency waves through the disturbed layer of the lower ionosphere by changing only two initial parameters: the disturbance energy and its initial height. It is shown that the attenuation value is almost independent of frequency and geo- and heliophysical conditions. For initial heights up to 50 km, the fading duration does not exceed ~ 2 min. With an increase of the initial altitude, the attenuation in the lower ionosphere becomes extremely large. However, for heights of 50 ... 70 km (depending on the value of energy), the horizontal size of the disturbance decreases significantly, which leads to a decrease in the fading time to tens of seconds for initial heights of more than 80 km.

On the relationship of energetic particle precipitation and mesopause temperature

Abstract. Energetic particle precipitation (EPP) has the potential to change the neutral atmospheric temperature in the mesopause region. However, recent results are inconsistent, leaving the mechanism and the actual effect still unresolved. In this study we have searched for electron precipitation events and investigated a possible correlation between D-region electron density enhancements and simultaneous neutral temperature changes. The rotational temperature of the excited hydroxyl (OH) molecules is retrieved from the infrared spectrum of the OH airglow. The electron density is monitored by the European Incoherent Scatter Scientific Association (EISCAT) Svalbard Radar. We use all available experiments from the International Polar Year (IPY) in 2007–2008 until February 2019. Particle precipitation events are characterized by rapid increases in electron density by a factor of 4 at an altitude range of 80–95 km, which overlaps with the nominal altitude of the infrared OH airglow layer. The OH airglow measurements and the electron density measurements are co-located. Six of the 10 analysed electron precipitation events are associated with a temperature decrease of 10–20 K. Four events were related to a temperature change of less than 10 K. We interpret the results in terms of the change in the chemical composition in the mesosphere. Due to EPP ionization the population of excited OH at the top of the airglow layer may decrease. As a consequence, the airglow peak height changes and the temperatures are probed at lower altitudes. The observed change in temperature thus depends on the behaviour of the vertical temperature profile within the airglow layer. This is in agreement with conclusions of earlier studies but is, for the first time, constructed from electron precipitation measurements as opposed to proxies. The EPP-related temperature change recovers very fast, typically within less than 60 min. We therefore further conclude that this type of EPP event reaching the mesopause region would only have a significant impact on the longer-term heat balance in the mesosphere if the lifetime of the precipitation was much longer than that of an EPP event (30–60 min) found in this study.

Pre-Seismic Irregularities during the 2020 Samos (Greece) Earthquake (M = 6.9) as Investigated from Multi-Parameter Approach by Ground and Space-Based Techniques

We present a comprehensive analysis of pre-seismic anomalies as computed from the ground and space-based techniques during the recent Samos earthquake in Greece on 30 October 2020, with a magnitude M = 6.9. We proceed with a multi-parametric approach where pre-seismic irregularities are investigated in the stratosphere, ionosphere, and magnetosphere. We use the convenient methods of acoustics and electromagnetic channels of the Lithosphere–Atmosphere–Ionosphere-Coupling (LAIC) mechanism by investigating the Atmospheric Gravity Wave (AGW), magnetic field, electron density, Total Electron Content (TEC), and the energetic particle precipitation in the inner radiation belt. We incorporate two ground-based IGS GPS stations DYNG (Greece) and IZMI (Turkey) for computing the TEC and observed a significant enhancement in daily TEC variation around one week before the earthquake. For the space-based observation, we use multiple parameters as recorded from Low Earth Orbit (LEO) satellites. For the AGW, we use the SABER/TIMED satellite data and compute the potential energy of stratospheric AGW by using the atmospheric temperature profile. It is found that the maximum potential energy of such AGW is observed around six days before the earthquake. Similar AGW is also observed by the method of wavelet analysis in the fluctuation in TEC values. We observe significant energetic particle precipitation in the inner radiation belt over the earthquake epicenter due to the conventional concept of an ionospheric-magnetospheric coupling mechanism by using an NOAA satellite. We first eliminate the particle count rate (CR) due to possible geomagnetic storms and South Atlantic Anomaly (SAA) by the proper choice of magnetic field B values. After the removal of the statistical background CRs, we observe a significant enhancement of CR four and ten days before the mainshock. We use Swarm satellite outcomes to check the magnetic field and electron density profile over a region of earthquake preparation. We observe a significant enhancement in electron density one day before the earthquake. The parameters studied here show an overall pre-seismic anomaly from a duration of ten days to one day before the earthquake.

Impacts of Ionospheric Ions on Magnetic Reconnection and Earth's Magnetosphere Dynamics

AbstractIonospheric ions (mainly H+, He+, and O+) escape from the ionosphere and populate the Earth's magnetosphere. Their thermal energies are usually low when they first escape the ionosphere, typically a few electron volt to tens of electron volt, but they are energized in their journey through the magnetosphere. The ionospheric population is variable, and it makes significant contributions to the magnetospheric mass density in key regions where magnetic reconnection is at work. Solar wind—magnetosphere coupling occurs primarily via magnetic reconnection, a key plasma process that enables transfer of mass and energy into the near‐Earth space environment. Reconnection leads to the triggering of magnetospheric storms, auroras, energetic particle precipitation and a host of other magnetospheric phenomena. Several works in the last decades have attempted to statistically quantify the amount of ionospheric plasma supplied to the magnetosphere, including the two key regions where magnetic reconnection occurs: the dayside magnetopause and the magnetotail. Recent in situ observations by the Magnetospheric Multiscale spacecraft and associated modeling have advanced our current understanding of how ionospheric ions alter the magnetic reconnection process, including its onset and efficiency. This article compiles the current understanding of the ionospheric plasma supply to the magnetosphere. It reviews both the quantification of these sources and their effects on the process of magnetic reconnection. It also provides a global description of how the ionospheric ion contribution modifies the way the solar wind couples to the Earth's magnetosphere and how these ions modify the global dynamics of the near‐Earth space environment.

A case study based on ground observations of the conjugate ionospheric response to interplanetary shock in polar regions

Data acquired by imaging relative ionospheric opacity meters (riometers), ionospheric total electron content (TEC) monitors, and three-wavelength auroral imagers at the conjugate Zhongshan station (ZHS) in Antarctica and Yellow River station (YRS) in the Arctic were analyzed to investigate the response of the polar ionosphere to an interplanetary shock event induced by solar flare activity on July 12, 2012. After the arrival of the interplanetary shock wave at the magnetosphere at approximately 18:10 UT, significantly enhanced auroral activity was observed by the auroral imagers at the ZHS. Additionally, the polar conjugate observation stations in both hemispheres recorded notable evolution in the two-dimensional movement of cosmic noise absorption. Comparison of the ionospheric TEC data acquired by the conjugate pair showed that the TEC at both sites increased considerably after the interplanetary shock wave arrived, although the two stations featured different sunlight conditions (polar night in July in the Antarctic region and polar day in the Arctic region). However, the high-frequency (HF) coherent radar data demonstrated that different sources might be responsible for the electron density enhancement in the ionosphere. During the Arctic polar day period in July, the increased electron density over YRS might have been caused by anti-sunward convection of the plasma irregularity, whereas in Antarctica during the polar night, the increased electron density over ZHS might have been caused by energetic particle precipitation from the magnetotail. These different physical processes might be responsible for the different responses of the ionosphere at the two conjugate stations in response to the same interplanetary shock event. Citation : He F, Hu Z J, Hu H Q, et al. A case study based on ground observations of the conjugate ionospheric response to interplanetary shock in polar regions. Adv Polar Sci, 2021, 32(2): 139-158, doi: 10.13679/j.advps.2021.0012