What Is the Altitude of Thermal Equilibrium? Peterson, W. K.; Maruyama, Naomi; Richards, Phil ...
Geophysical research letters,
16 June 2023, Letnik:
50, Številka:
11
Journal Article
Recenzirano
Odprti dostop
Thermal equilibrium in planetary atmospheres occurs at altitudes where the ion, electron, and neutral temperatures are equal. Thermal equilibrium is postulated to occur in the collision‐dominated ...ionosphere. This postulated altitude is above the lower boundary of all empirical models of planetary ionospheres. Physics‐based model predictions of the altitude cannot be validated due to a lack of adequate simultaneous observations of temperature profiles. This study presents temperature profiles from simultaneous observations on Atmosphere Explorer–C below 140 km and quiet‐time neutral observations from Thermosphere Ionosphere Mesosphere Energy and Dynamics/Global UltraViolet Imager over Millstone Hill. These are compared with profiles from physics‐based models with a discussion of their respective limitations. We conclude that there does not yet exist a quantitative understanding of the ion, electron, and neutral thermalization processes in low‐altitude planetary ionospheres. Progress on this topic requires an adequate database of simultaneous ion, electron, and neutral temperature profiles in the 110–140 km altitude range.
Plain Language Summary
Solar radiation ionizes and heats the neutral atmosphere in the process of creating the ionosphere. Heating begins at the altitude where energy carried by photoelectrons produced by solar extreme ultra violet radiation is not absorbed locally. This is the altitude where the ionosphere and neutral atmosphere are no longer in thermal equilibrium. There are numerous ionospheric models predicting the neutral, ion, and electron temperatures (Tn, Ti, and Te) as a function of altitude. These models predict that, in the absence of other heating mechanisms thermal equilibrium where Ti = Ti = Tn occurs below ∼120 km. Temperatures in this altitude range are used as upper boundary conditions for models attempting to describe energy transfer from the upper atmosphere to the ionosphere. Our study shows that ionospheric models of neutral and plasma temperatures cannot be validated in the region of thermal equilibrium because existing observations are sparse and do not have the precision and/or required spatial and temporal resolution. Progress in understanding energy transfer from the atmosphere to the ionosphere requires validated models of the altitude of thermal equilibrium.
Key Points
Thermal equilibrium is postulated to be attained at the lowest altitude heated by photoelectrons but has not been observed
Quantitative understanding of thermal equilibrium requires observations that validate models
There is an ongoing community need for simultaneous neutral, ion, and electron temperature profiles below 140 km altitude
Imaging thermal plasma mass and velocity analyzer Yau, Andrew W.; Howarth, Andrew
Journal of geophysical research. Space physics,
July 2016, 2016-07-00, 20160701, Letnik:
121, Številka:
7
Journal Article
Recenzirano
We present the design and principle of operation of the imaging ion mass and velocity analyzer on the Enhanced Polar Outflow Probe (e‐POP), which measures low‐energy (1–90 eV/e) ion mass composition ...(1–40 AMU/e) and velocity distributions using a hemispherical electrostatic analyzer (HEA), a time‐of‐flight (TOF) gate, and a pair of toroidal electrostatic deflectors (TED). The HEA and TOF gate measure the energy‐per‐charge and azimuth of each detected ion and the ion transit time inside the analyzer, respectively, providing the 2‐D velocity distribution of each major ionospheric ion species and resolving the minor ion species under favorable conditions. The TED are in front of the TOF gate and optionally sample ions at different elevation angles up to ±60°, for measurement of 3‐D velocity distribution. We present examples of observation data to illustrate the measurement capability of the analyzer, and show the occurrence of enhanced densities of heavy “minor” O++, N+, and molecular ions and intermittent, high‐velocity (a few km/s) upward and downward flowing H+ ions in localized regions of the quiet time topside high‐latitude ionosphere.
Key Points
Imaging plasma analyzer design combines hemispherical electrostatic analyzer, time‐of‐flight gate, and toroidal electrostatic deflector
Analyzer measures mass composition and detailed velocity distributions of low‐energy (1–90 eV) ions of 1–40 AMU
Observation of large‐velocity flow of minor H+ ions and enhanced N+ and other heavy “minor ion” densities in quiet time topside ionosphere
We provide insight into the vertical distribution of multi‐scale scintillation‐inducing irregularities in the low‐latitude ionosphere. In four sets of novel experiments, we sampled altitudes from 330 ...to 1,280 km in the 18–24 magnetic local time (MLT) sector using the Swarm Echo GPS Attitude, Positioning, and Profiling Experiment occultation receiver (GAP‐O) GPS receiver with its antenna oriented toward zenith. In order to identify multi‐scale irregularities both above and at the satellite's position, we utilize high‐sample‐rate GAP‐O amplitude and phase measurements along with a measurement of net current on the surface of the imaging and rapid‐scanning ion mass spectrometer sensor on board, which serves as a proxy for density variations. By calculating the rate of change of total electron content index using two sets of GPS parameter choices, we are able to sample irregularities as small as 160 m, which is comparable to or smaller than the Fresnel scale responsible for scintillation‐inducing irregularities. During one campaign, we find that amplitude scintillations on the GPS signal coincide with strong in‐situ small‐scale density irregularities in 32% of cases, indicative of a broad irregularity region extending from the satellite's position to hundreds of kilometers above. Also, we show that large‐scale ionospheric disturbances (larger than 80 km) occur predominantly below 500 km, and down to the 330 km perigee of Swarm Echo in the 18–21 MLT sector. In contrast, small‐scale variations of total electron content are detected at all MLTs between 18 MLT and magnetic midnight and at all altitudes sampled in this experiment. However, they are more frequent in the 22–24 MLT range.
Plain Language Summary
Variations in ionospheric electron density, so‐called irregularities, produce rapid fluctuations on propagating communication and navigation signals, which can be severe near the magnetic equator and in the polar regions. Due to sparse sampling, our knowledge of the vertical distribution of small‐scale irregularities is limited. The Swarm Echo satellite contains a high‐sample‐rate GPS receiver whose antenna normally points in the horizontal direction. We re‐oriented Swarm Echo for short periods to make the receiver antenna point in the vertical direction during November 2019 to November 2020 while the satellite was flying in the low‐latitude region from 18 magnetic local time (MLT) to midnight, and at altitudes between 330 and 1,280 km. This allows us to investigate the irregularity distribution at multiple scales and different altitudes. Using this novel experiment, we show that in 32% of the cases in which large density fluctuations were detected at the satellite, strong GPS scintillations were also detected, providing valuable information on their altitude distribution. We demonstrate that, between 18 and 21 MLT, ionospheric structures having large scale sizes (between 80 and 2,400 km) occur mainly below 500 km altitude, whereas total electron content variations caused by irregularities (160 m–480 km) are detected at all altitudes and throughout the 18‐midnight MLT sector.
Key Points
We report measurements of 50 and 100 Hz GPS carrier phase and amplitude and in situ density in the equatorial ionosphere
Strong scintillations and small‐scale density irregularities are measured in situ at altitudes from 330 to 1,280 km
Small scale variations of total electron content were more severe during the 22–24 magnetic local time sector
The Ellipticity of High Frequency Transionospheric Radio Waves Pandey, Kuldeep; Kalafatoglu Eyiguler, E. Ceren; Hussey, Glenn C. ...
Journal of geophysical research. Space physics,
April 2024, 2024-04-00, 20240401, Letnik:
129, Številka:
4
Journal Article
Recenzirano
The polarization state of transionospheric high frequency (HF) radio waves can be determined using the crossed‐dipole antennas of the Radio Receiver Instrument (RRI) onboard Enhanced Polar Outflow ...Probe (e‐POP) on the CAScade, Smallsat, and IOnospheric Polar Explorer/Swarm‐E satellite. Coordinated experiments between ground radars and RRI showed that the radio waves have high ellipticity angles (elliptical or circular polarization) for propagation direction 6°–25° from the perpendicular direction to the local geomagnetic field. However, from magnetoionic theory and a typical assumption of roughly equal power in the O‐ and X‐modes, radio waves can have high ellipticity angles only when the propagation direction is within 10° of perpendicularity. This investigation uses coordinated experiments between the Saskatoon SuperDARN radar and RRI. Magnetoionic modeling reveals that the relative strengths of the O‐ and X‐modes for the HF radio waves transmitted from the Saskatoon SuperDARN change significantly poleward of the radar. Differences in the relative power between the two wave modes were found to significantly modify the polarization state of radio waves, and govern the sense of rotation of the wave. Even a relatively modest 2–5 dB power difference in the X‐mode over the O‐mode was found to result in unexpected observations of circularly polarized radio waves at aspect angles 6°–10° from perpendicular, and unexpected elliptically polarized waves observed at aspect angles more than 10° from perpendicular. Therefore, a combination of RRI observations and modeling, which accounts for relative differences in the O‐ and X‐mode powers, was used here to better understand the unexpected observations of polarization states of transionospheric radio waves.
Key Points
Radio Receiver Instrument on Enhanced Polar Outflow Probe (e‐POP) satellite observed polarization states of transionospheric radio waves in coordinated experiments
Circularly polarized radio waves were detected far off from the transverse propagation direction to the geomagnetic field
Domination of one mode power (X‐mode) caused higher ellipticity of radio waves
Ionospheric density structures at low latitudes range in size from thousands of kilometers down to a few meters. Radio frequency (RF) signals, such as those from global navigation satellite systems, ...that propagate through irregularities suffer from rapid fluctuations in phase and intensity, known as scintillations. In this study, we use the high‐sample‐rate measurements of the Swarm Echo (CASSIOPE/e‐POP) satellite's GPS Occultation (GAP‐O) receiver taken after its antenna was re‐oriented to vertical‐pointing, simultaneously with e‐POP Ion Mass Spectrometer surface current observations as a proxy for plasma density, to obtain the spectral characteristics of GPS signal intensity and in‐situ irregularities at altitudes from 350 to 1,280 km. We show that the power spectra of both measurements can generally be characterized by a power law. In the case of density irregularities, the spectral index with the highest occurrence rate is around 1.7, which is consistent with previous studies. Also, all the power spectra of GPS signal intensity in this study show a single spectral index near 2. Moreover, roll‐off frequencies estimated in this work range from 0.4 to 2.5 Hz, which is significantly higher than Fresnel frequencies calculated from ground GPS receivers at low latitudes (between 0.2 and 0.45 Hz). Part of this increase is due to the 8 km/s orbital velocity of Swarm Echo near perigee. Another key difference is that variations in the GPS signals in this study are dominated by the topside ionosphere, whereas GPS signals received from ground are affected mostly by the relatively dense F‐region plasma in the 250–350 km altitudinal range.
Plain Language Summary
Variations in ionospheric electron density, called irregularities, cause fluctuations in radio frequency signals traversing the ionosphere. The Swarm Echo satellite carries a high‐sample‐rate GPS receiver with an antenna normally pointing in the horizontal direction. In this study, the satellite was reoriented while it was passing the low latitude region so that the receiver antenna would point vertically in order to probe irregularities above the satellite at high resolution. Our spectral analysis of GPS signal intensity and plasma density variations measured in situ (on board the spacecraft) revealed power law behavior in both measurements, a well‐established feature of ionospheric turbulence. More specifically, our results show that the spectral index (which is the negative of the power law slope) of in‐situ density irregularities and GPS signal intensities are 1.7 and 2, respectively. The roll‐off frequencies in the GPS signal intensity results are considerably higher than ground‐based estimates. This difference is attributable to the satellite's high velocity (8 km/s) and the fact that, comparing to the ground estimates which are dominated by the dense plasma density between 250 and 350 km altitude, Swarm Echo observations are mostly affected by the upper reaches of the ionosphere.
Key Points
The most frequently occurring spectral index of the in‐situ irregularity power spectra is 1.7
The power spectra of GPS signal intensity show a single spectral index near 2, which is smaller than ground‐based weak scintillation values
The roll‐off frequencies in the signal intensity power spectra are 0.4–2.5 Hz, significantly exceeding ground‐based Fresnel frequency values
The polarization characteristics of radio waves traversing the ionosphere can be determined using the cross‐dipole antennas of Radio Receiver Instrument (RRI) onboard the Enhanced Polar Outflow Probe ...payload on the CAScade, Smallsat, and Ionospheric Polar Explorer/Swarm‐E satellite. Numerous coordinated ground‐RRI transionospheric propagation experiments have been executed, with the present study using transmitted radio waves from a High Frequency (HF) ground transmitter located in Ottawa, Canada (45.41°N, 75.55°W). Most of these experiments have shown the HF radio waves to propagate with both magnetoionic propagation modes, the O‐ and X‐modes, combined. However, the coordinated ground‐RRI experiment on 12 May 2017 showed only one magnetoionic propagation mode. The observed polarization state had a stable orientation angle, but the ellipticity angle changed from −19° to linear (0°) and finally 30° within a latitude span of 15°. A detailed investigation using magnetoionic ray trace modeling and ground‐based ionogram observations revealed that the HF transmitted frequency and ionospheric conditions were such that only the O‐mode radio waves reached the satellite for the entire pass. This distinct observation is a unique opportunity to examine the radio wave polarization state during a transionospheric propagation of only O‐mode waves.
Key Points
Unique observation of transionospheric propagation of O‐mode only
Ray trace modeling shows that the O‐mode propagated to e‐POP satellite but the X‐mode is refracted away by the ionosphere
We present in situ ion composition and velocity measurements during the August 2017 solar eclipse from the Enhanced Polar Outflow Probe (e‐POP), which crossed the path of totality at ~640‐km altitude ...within 10 min of totality passing. These measurements reveal two distinct H+ ion populations, an ~40% decrease in topside plasma density, a similar drop in upward but not downward H+ ion flux, and a downward O+ ion velocity of ~100 m/s. These features are directly linked to changes in the H+/O+ composition and in interhemispheric or field‐aligned light ion flow and to a reduction in the negative spacecraft potential. These observed features were absent on the preceding, noneclipse days and corroborate the reduction in F region plasma density and topside total electron content observed by the Global Positioning System receivers on board. They are attributed to the temporary reduction of photoionization in the eclipsed F region.
Plain Language Summary
During a solar eclipse, the Earth's ionosphere undergoes rapid and drastic changes in response to the sudden reduction in solar extreme ultraviolet flux along the eclipse path. The Enhanced Polar Outflow Probe crossed the eclipse path over the State of Idaho within 10 min following totality during the August 2017 Great American Solar Eclipse. Using the ion mass spectrometer and Global Positioning System receivers on board, the Enhanced Polar Outflow Probe observed several important changes in the eclipsed ionosphere, some for the first time, including a rapid ~40% decrease in electron density, an ~100% change in ion composition (H+/O+ ratio), and a disruption of light (H+) ion flow between the eclipsed and opposite hemispheres. These observations demonstrate that in addition to the ionospheric changes, the solar eclipse also induces significant changes to the upper atmosphere (thermosphere) and the plasmasphere.
Key Points
Eclipse‐induced decrease in ion density and changes to H+/O+ ratio and field‐aligned H+ flows were observed within 10 min after totality
Eclipse‐induced decrease in only upward (but not downward) H+ flux was accompanied by downward O+ flow
Observed decrease (~40%) in topside plasma density was a factor of ~2 larger than SAMI‐3 prediction
The imaging and rapid-scanning ion mass spectrometer (IRM) is part of the Enhanced Polar Outflow Probe (e-POP) instrument suite on the Canadian CASSIOPE small satellite. Designed to measure the ...composition and detailed velocity distributions of ions in the ∼1–100 eV/q range on a non-spinning spacecraft, the IRM sensor consists of a planar entrance aperture, a pair of electrostatic deflectors, a time-of-flight (TOF) gate, a hemispherical electrostatic analyzer, and a micro-channel plate (MCP) detector. The TOF gate measures the transit time of each detected ion inside the sensor. The hemispherical analyzer disperses incident ions by their energy-per-charge and azimuth in the aperture plane onto the detector. The two electrostatic deflectors may be optionally programmed to step through a sequence of deflector voltages, to deflect ions of different incident elevation out of the aperture plane and energy-per-charge into the sensor aperture for sampling. The position and time of arrival of each detected ion at the detector are measured, to produce an image of 2-dimensional (2D), mass-resolved ion velocity distribution up to 100 times per second, or to construct a composite 3D velocity distribution by combining successive images in a deflector voltage sequence. The measured distributions are then used to investigate ion composition, density, drift velocity and temperature in polar ion outflows and related acceleration and transport processes in the topside ionosphere.
We present the first space-borne observation of optical emissions of both neutral and ionized argon atoms resulting from a radio-frequency glow discharge (RF-GD) from a sounding rocket payload. The ...Observations of Electric-field Distributions in the Ionospheric Plasma — a Unique Strategy-C (OEDIPUS-C) payload was designed to separate into two sub-payloads on its up-leg. This separation permitted a number of two-point measurements, including those on radio wave propagation from the active dipole antennas on the upper sub-payload to the synchronized receiving dipoles on the lower sub-payload. A white-light video camera on the lower sub-payload recorded strong luminosity around the active dipoles during the first 15 s after sub-payload separation, when argon gas jets were providing propulsion to separate the two sub-payloads. Parts of the ejected argon appeared as a glowing volume where the large radio-frequency (RF) fields from the two active dipoles excited the optical emission, as the sub-payload separation increased from 2 to 55 m. The shape and intensity of the luminosity were well repeated as a function of the swept frequency (0.025–8.000 MHz), but their frequency dependences were distinctly different from those of sounder-accelerated electrons measured onboard, and deduced to result from the nonlinearity of the glow discharge. The observation is to our knowledge the first of its kind, and is interpreted in terms of a RF-GD energized by the strong near electric fields of the transmitting dipoles.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
10.
Thank You to Our 2023 Peer Reviewers Rajaram, Harihar; Aiyyer, Anantha; Camargo, Suzana ...
Geophysical research letters,
16 May 2024, Letnik:
51, Številka:
9
Journal Article
Recenzirano
Odprti dostop
On behalf of the journal, AGU, and the scientific community, the editors of Geophysical Research Letters would like to sincerely thank those who reviewed manuscripts for us in 2023. The hours reading ...and commenting on manuscripts not only improve the manuscripts, but also increase the scientific rigor of future research in the field. With the advent of AGU's data policy, many reviewers have also helped immensely to evaluate the accessibility and availability of data, and many have provided insightful comments that helped to improve the data presentation and quality. We greatly appreciate the assistance of the reviewers in advancing open science, which is a key objective of AGU's data policy. We particularly appreciate the timely reviews in light of the demands imposed by the rapid review process at Geophysical Research Letters. We received 4,512 submissions in 2023 and 5,112 reviewers contributed to their evaluation by providing 8,587 reviews in total. We deeply appreciate their contributions.
Plain Language Summary
Individuals in italics provided three or more reviews for GRL in 2023.
Key Points
The editors thank the 2023 peer‐reviewers
PDF
