We present an analysis of relatively strong Polar Mesospheric Winter Echoes (PMWE) under artificial electron heating that changes the PMWE intensity. A major purpose is to find reliable estimates of ...the relaxation time of the heater modified PMWE to their undisturbed state during the heater switch‐off phase; the implications regarding charge/discharge mechanisms; and to exploit the diagnostic potential of artificial electron heating. The relaxation time is between 60 to 70 s for the regions with strong PMWE layers and substantial electron heating. This short relaxation time, related to the variation of charges on the nanometer dust which most likely is present in PMWE, rules out ion attachment as the mechanism to bring the dust charges to their equilibrium state. Neutral winds, sweeping the heated electrons out of the radar beam, are unlikely to be the cause of the observed relaxation, since this requires winds of around 100 m s−1. The most probable cause is photo detachment by which negatively charged dust can lose excess electrons by photon absorption with energies less than the dust material's work function. By comparing the observed heating with heating model profiles, the electron density at 65 km height must have been of the order of 3 × 109 m−3. This agrees with PMWE occurring mainly during disturbed conditions with high electron densities. Our results also indicate that in the strongest PMWE layers, electron bite‐outs exist consistent with the role of charged dust particles in the mechanism of PMWE and implying larger dust densities.
Key Points
Relaxation time of PMWE under artificial heating is found to be 60‐70 sec
Photo detachment is the most likely cause of the observed relaxation
Electron depletion is consistent with role of charged dust in PMWE
The first true common volume observations of the PMSE source region with 4 radars are presented in this paper. Radar frequencies of 8, 56, 224, and 930 MHz are used in this study. Three days of ...experimental observations at EISCAT are presented. Numerical simulations of mesospheric dusty/ice plasma associated with the observed radar frequencies are presented. The effect of neutral air turbulence on the generation and strength of plasma density perturbations associated with PMSE using four radar frequencies and in the presence of various dust parameters is investigated. Using the model it is shown that the well‐known neutral air turbulence in the presence of heavy dust particles and neutral air turbulence combined with dust density (dusty turbulence) can largely explain the observed radar cross‐section at four radar frequencies. The effect of neutral air turbulence amplitude along with dust charging and diffusion in the presence of various dust parameters is investigated using the computational model. Specifically, the response of diffusion to charging time scales, plasma density fluctuation amplitude and background dusty plasma parameters are discussed. Several key parameters in the dusty plasma responsible for the PMSE observations are determined. Qualitative comparison of radar echo strength at 4 frequencies with numerical results is provided. Unlike the previous studies that required large dust particles of 20 nm for PMSE formation, the present work demonstrates the possibility of small dust particles to explain the experimental observations.
Key Points
The first simultaneous four radar frequency observation of the PMSE is presented
The computational results and observations validate the fossil theory proposed to 14 produce the PMSE
The effect of initial turbulence amplitude along with dust charging and diffusion 16 in the presence of various dust parameters is investigated for the first time
We test the existing theories regarding the thresholds for the parametric decay instability (PDI), the oscillating two‐steam instability (OTSI), and the thermal parametric instability (TPI) using the ...European Incoherent Scatter (EISCAT) facility's ionospheric heater. In these processes, the pump wave can couple to various electrostatic waves in the F layer ionosphere, which can be observed using the EISCAT UHF radar (PDI and OTSI) or by HF radar (TPI). On 19 October 2012, the heater power was stepped from ∼0.5 MW to ∼100 MW effective radiated power in seven steps using a 1 min on, 1 min off cycle. We use an electric field model, taking into account D region absorption, to compare theory with our observations. In all three cases, we find good agreement. In addition, the growth of striations formed during the TPI causes anomalous absorption of the heater wave, which we observe as decreased UHF ion line and plasma line backscatter power. We show evidence that heating for a prolonged period of time reduces the UHF ion line intensity throughout the experiment.
Key Points
Test theories regarding plasma instabilities generated in the ionosphere
Detect threshold for striation growth during power stepping heating experiment
Striations anomalous absorption decrease ion and plasma line backscatter
Posterior ankle impingement syndrome (PAIS) is a painful, usually limited plantar flexion of the ankle joint due to soft tissue impingement or a bony impediment often coinciding with tendinopathy of ...the flexor hallucis longus (FHL) behind the medial malleolus. In persistent complaints, the bony impediment is removed, the tunnel of the FHL tendon is released, or a combination of both procedures has traditionally been performed by open surgery. In 2000, an endoscopic surgical technique for PAIS and FHL tendinopathy was introduced. To date there is no evidence of the superiority of one surgical technique over the other in dancers; both the open and the endoscopic approach provide up to 90% good and excellent long-term results. We compared the first 20 consecutive open procedures with the first 19 consecutive endoscopic procedures in dancers, all operated by the same orthopaedic surgeon. The postoperative outcomes were reviewed by studying the patient files, clinical evaluation, American Orthopaedic Foot and Ankle Society (AOFAS) scoring, and we administered a dance-specific questionnaire. The results in the open group were: nine excellent, nine good, one moderate, and one poor (90% excellent and good). In the endoscopic group: nine excellent, six good, three moderate, one poor (79% excellent and good). Both groups returned to dance (barre) at a median of 8 weeks (IQR: 2). The direct postoperative morbidity in the endoscopic group was less favorable and was mainly related to hematomas and an inflammatory response or deep scar tissue formation. There were no major complications requiring re-intervention. Although the two small groups of patients and the retrospective character of our descriptive study do not allow firm conclusions or statistical analysis of subgroups, the results of this study indicate that the open approach seems to be better than the endoscopic approach in the surgical treatment of PAIS and FHL tendinopathy in dancers.
We compare the height‒integrated electron heating rates from O‒ and X‒mode HF pump waves to extract the components due to resonant and nonresonant heating mechanisms in the ionosphere. We present ...results from a November 2011 campaign in Norway, using the European Incoherent Scatter (EISCAT) heater facility and UHF radar. We show that the theoretical nonresonant, ohmic heating due to the electromagnetic pump wave electric field agrees with observations for X‒mode pumping. For O‒mode pumping, the observed height‒integrated heating rate exceeds the theoretical ohmic electron heating rate by a factor of 2–5, the excess being attributed to resonant heating mechanisms. In addition, a persistent UHF ion‒line enhancement is observed for O‒mode and, more unusually, X‒mode pumping. We attribute the latter to O‒mode leakage in the X‒mode pulse. For O‒mode, we see a descent in altitude of the ion‒line enhancement and show that this is most likely due to ionization from pump‒induced fluxes of suprathermal electrons. We estimate the ionization rate and determine an enhanced electron flux showing that approximately 10–20% of the pump power is transferred to high energy suprathermal electrons.
Key Points
Estimate the height‐integrated heating electron heating rate due to UHR
Find the electron flux at the UHR altitude by observing a descending ion‐line
Observe an enhanced ion‐line during high power X‐mode pumping
An ultrahigh frequency (UHF) observation during an ionospheric heating campaign at European Incoherent SCATter scientific association (EISCAT) demonstrates that the electron temperature and the ...intensities of the enhanced plasma and ion lines systematically vary as a function of the pump frequency near the fifth electron gyroharmonic. Specifically, with the change in the pump frequency, the weaker the enhancement in the electron temperature is, the more intense the intensities of the enhanced ion line is. The analysis shows that the enhanced electron temperature plays a significant role in determining the intensities of the enhanced Langmuir and ion acoustic waves by the parametric decay instability and the oscillation two‐stream instability, and the intensities of the enhanced plasma and ion lines depend on whether the enhanced Langmuir and ion acoustic waves satisfy the Bragg condition exactly or approximately. Moreover, an alternative explanation for the overshoots in the enhanced plasma and ion lines is presented, namely, the overshoots may be attributed not only to the anomalous absorption of the pump but also to the modifications of the plasma on the traveling path of the enhanced Langmuir and ion acoustic waves.
Key Points
The enhanced electron temperature plays a significant role in determining the wave numbers of the enhanced Langmuir and ion acoustic waves
The intensities of HFPL and HFIL depend on whether the enhanced Langmuir and ion acoustic waves satisfy the Bragg condition exactly or approximately
The overshoots in the HFPL and HFIL may be attributed not only to the anomalous absorption but also to the enhanced electron temperature
Dancing on pointe and relevé requires extreme plantar flexion of the talo-crural joint. Hence, these positions may lead to posterior ankle impingement syndrome (PAIS). PAIS often coincides with ...flexor hallucis longus tendinopathy (FHL tendinopathy, or "dancers' tendinitis"). Both injuries can appear in isolation as well. The goal of this review is to evaluate the results and the available levels of evidence of conservative and operative treatment (both open and endoscopic) of PAIS and FHL tendinopathy in dancers. It also offers an insight into the history of dance medical publications on this subject. In October 2016, a systematic search of PubMed, Embase, Cochrane, CINAHL, Web of Science, and (in French) ScienceDirect databases was undertaken. Five hundred and seventy-six publications were found, of which a total of 27 reported the results of operative treatment in 376 ankles (344 open, 32 endoscopic) in 324 dancers. The outcome was good to excellent in most cases (89%). The mean period of return to dance for all surgeries combined (PAIS and FHL tendinopathy, open and endo) was 11 weeks (range: 4 to 36 weeks), and for isolated FHL tendinopathy 16 weeks (range: 8 to 36 weeks). Only six publications reported the results of conservative treatment in 33 ankles (13 PAIS, 20 FHL tendinopathy) of 28 dancers, which does not allow for any evidence-based recommendations. Most studies failed to include dance-specific baseline characteristics, like dance style and level of participation. We concluded that only retrospective studies with levels of evidence four and five show that operative treatment for PAIS and FHL tendinopathy is successful with few complications. Since isolated PAIS, PAIS combined with FHL tendinopathy, and isolated FHL injuries appear to be different pathological entities, more research taking into account demography, dance type, and level of participation is needed to find out in which cases early operative management should be considered or avoided. The same applies to defining the place of endoscopic surgery in dancers and being able better to predict which pathology is likely to produce worse outcomes or delay the return to dance. Future research should have a prospective design, including dance-specific outcome scores both preand post-treatment. Furthermore, preferably a prospective randomized controlled design should be used to compare different conservative and operative treatment options.
We report a brief survey of matching conditions for artificial aurora optical experiments utilizing the second electron gyro-harmonic (2.7-MHz frequency) in
F
region heating with O-mode at the EISCAT ...Tromsø site using dynasonde data from 2000 to 2017. Our survey indicates the following: The possible conditions for successful artificial aurora experiments are concentrated on twilight hours in both evening and morning, compared with late night hours; the possible conditions appear in fall, winter, and spring, while there is no chance in summer, and the month-to-month variation among fall, winter, and spring is not so clear; the year-to-year variation is well correlated with the solar activity. These characteristics in the case of 2.7-MHz frequency are basically similar to those previously reported in the case of 4-MHz frequency. However, the number of days meeting the possible condition in the case of 2.7-MHz frequency is obviously large, compared with that in the case of 4-MHz frequency. In particular, unlike the 4-MHz frequency operation, the 2.7-MHz frequency operation can provide many chances for successful artificial aurora experiments even during the solar minimum.
In recent years, large electron temperature increases of 300% (3000 K above background) caused by powerful HF‐radio wave injection have been observed during nighttime using the EISCAT incoherent ...scatter radar near Tromsø in northern Norway. In a case study we examine the spatial structure of the modified region. The electron heating is accompanied by ion heating of about 100 degrees and magnetic field‐aligned measurements show ion outflows increasing with height up to 300 m s−1 at 582 km. The electron density decreases by up to 20%. When the radar antenna was scanned between three elevations from near field‐aligned to vertical, the strongest heating effects were always obtained in the field‐aligned position. When the HF‐pump beam was scanned between the same three positions, the heating was still almost always strongest in the field‐aligned direction. Simultaneous images of the 630 nm O(1D) line in the radio‐induced aurora showed that the enhancement caused by the HF radio waves also remained localized near the field‐aligned position. Coherent HF radar backscatter also appeared strongest when the pump beam was pointed field‐aligned. These results are similar to some Langmuir turbulence phenomena which also show a strong preference for excitation by HF rays launched in the field‐aligned direction. The correlation of the position of largest temperature enhancement with the position of the radio‐induced aurora suggests that a common mechanism, upper‐hybrid wave turbulence, is responsible for both effects. Why the strongest heating effects occur for HF rays directed along the magnetic field is still unclear, but self‐focusing on field‐aligned striations is a candidate mechanism, and possibly ionospheric tilts may be important.
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