| 1. |
- Alday, Juan, et al.
(författare)
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New constraints on Ganymede's hydrogen corona : Analysis of Lyman-alpha emissions observed by HST/STIS between 1998 and 2014
- 2017
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Ingår i: Planetary and Space Science. - : Elsevier. - 0032-0633 .- 1873-5088. ; 148, s. 35-44
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Tidskriftsartikel (refereegranskat)abstract
- Far-ultraviolet observations of Ganymede's atmospheric emissions were obtained with the Space Telescope Imaging Spectrograph (STIS) onboard of the Hubble Space Telescope (HST) on several occasions between 1998 and 2014. We analyze the Lyman-alpha emission from four HST campaigns in order to constrain the abundance and variation of atomic hydrogen in Ganymede's atmosphere. We apply a forward model that estimates surface reflection and resonant scattering in an escaping corona of the solar Lyman-alpha flux, taking into account the effects of the hydrogen in the interplanetary medium. The atmospheric emissions around Ganymede's disk derived for the observations taken between 1998 and 2011 are consistent with a hydrogen corona in the density range of (5-8) x 10(3) cm(-3) at the surface. The hydrogen density appears to be generally stable in that period. In 2014, Ganymede's corona brightness is approximately 3 times lower during two observations of Ganymede's trailing hemisphere and hardly detectable at all during two observations of the leading hemisphere. We also investigate extinction of Ganymede's coronal emissions in the Earth's upper atmosphere or geocorona. For small Doppler shifts, resonant scattering in the geocorona of the moon corona emissions can effectively reduce the brightness observed by HST. In the case of the 2014 leading hemisphere observations, an estimated extinction of 80% might explain the non-detection of Ganymede's hydrogen corona. Geocoronal extinction might also explain a previously detected hemispheric difference from Callisto's hydrogen corona.
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| 2. |
- Balmer, G., et al.
(författare)
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ISAAC : A REXUS STUDENT EXPERIMENT TO DEMONSTRATE AN EJECTION SYSTEM WITH PREDEFINED DIRECTION
- 2015
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Ingår i: EUROPEAN ROCKET AND BALLOON. - 9789292212940 ; , s. 235-242
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Konferensbidrag (refereegranskat)abstract
- ISAAC - Infrared Spectroscopy to Analyse the middle Atmosphere Composition was a student experiment launched from SSC's Esrange Space Centre, Sweden, on 29th May 2014, on board the sounding rocket REXUS 15 in the frame of the REXUS/BEXUS programme. The main focus of the experiment was to implement an ejection system for two large Free Falling Units (FFUs) (240 mm x 80 mm) to be ejected from a spinning rocket into a predefined direction. The system design relied on a spring-based ejection system. Sun and angular rate sensors were used to control and time the ejection. The flight data includes telemetry from the Rocket Mounted Unit (RMU), received and saved during flight, as well as video footage from the GoPro camera mounted inside the RMU and recovered after the flight. The FFUs' direction, speed and spin frequency as well as the rocket spin frequency were determined by analyzing the video footage. The FFU-Rocket-Sun angles were 64.3 degrees and 104.3 degrees, within the required margins of 90 degrees +/- 45 degrees. The FFU speeds were 3.98 m/s and 3.74 m/s, lower than the expected 5 +/- 1 m/s. The FFUs' spin frequencies were 1.38 Hz and 1.60 Hz, approximately half the rocket's spin frequency. The rocket spin rate slightly changed from 3.163 Hz before the ejection to 3.117 Hz after the ejection of the two FFUs. The angular rate, sun sensor data and temperature on the inside of the rocket module skin were also recorded. The experiment design and results of the data analysis are presented in this paper.
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| 3. |
- Belyayev, Serhiy, et al.
(författare)
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Effect of second harmonic in pulse-width-modulation-based DAC for feedback of digital fluxgate magnetometer
- 2018
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Ingår i: Measurement science and technology. - : Institute of Physics Publishing. - 0957-0233 .- 1361-6501. ; 29:4
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Tidskriftsartikel (refereegranskat)abstract
- Digital fluxgate magnetometers employ processing of the measured pickup signal to produce the value of the compensation current. Using pulse-width modulation with filtering for digital to analog conversion is a convenient approach, but it can introduce an intrinsic source of nonlinearity, which we discuss in this design note. A code shift of one least significant bit changes the second harmonic content of the pulse train, which feeds into the pick-up signal chain despite the heavy filtering. This effect produces a code-dependent nonlinearity. This nonlinearity can be overcome by the specific design of the timing of the pulse train signal. The second harmonic is suppressed if the first and third quarters of the excitation period pulse train are repeated in the second and fourth quarters. We demonstrate this principle on a digital magnetometer, achieving a magnetometer noise level corresponding to that of the sensor itself.
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| 4. |
- Blöcker, Aljona, et al.
(författare)
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Variability of Io's poynting flux : A parameter study using MHD simulations
- 2020
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Ingår i: Planetary and Space Science. - : Elsevier BV. - 0032-0633 .- 1873-5088. ; 192
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Tidskriftsartikel (refereegranskat)abstract
- Io's plasma interaction creates an electromagnetic coupling between Io and Jupiter through Alfvén waves triggering the generation of auroral footprints in Jupiter's southern and northern hemispheres. The brightness of Io's footprints undergoes periodic variations that are primarily modulated by Io's local plasma interaction through the Poynting flux radiated away from the moon. The periodic pattern with two maxima near 110∘ and 290∘ Jovian longitude where Io crosses the dense plasma sheet is generally understood. However, some characteristics, like the 2-4 times stronger brightening of the southern footprint near Jovian longitude 110∘ or the lack of response to Io's eclipse passage, are not fully understood. We systematically study variations in Io's plasma interaction and the Poynting flux using a 3D magnetohydrodynamic model, performing a series of simulations with different upstream plasma conditions and models of Io's atmosphere. Our results indicate that the strong Jovian magnetic field near 110∘ plays a more important role than previously estimated for the strong brightening there. We find that the Poynting flux is not fully saturated for a wide range of possible atmospheric densities (6 ×1018 - 6 ×1021 m-2) and that density changes in the atmosphere by a factor of > 3, as possibly happening during Io's eclipse passage, lead to a change of the Poynting flux by > 20%. Assuming that these expected changes in Poynting flux also apply to the footprints, the non-detection of a dimming in the footprint during the eclipse by Juno-UVS suggests that Io's global atmospheric density decreases by a factor of < 2.5. We show that for smaller atmospheric scale heights (i.e. a more confined atmosphere), changes in the atmospheric density have less effect on the Poynting flux. The missing response of the footprint to the eclipse hence might also be consistent with a density decrease by a factor of > 3, if the effective atmospheric scale height is small (< 120 km). Finally, we provide new analytical approximations that can be used for analyzing the effect of the local interaction responsible for the footprint variability in future studies.
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| 5. |
- Dahlgren, Hanna, 1980-, et al.
(författare)
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Relation of anomalous F region radar echoes in the high-latitude ionosphere to auroral precipitation
- 2017
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Ingår i: Annales Geophysicae. - : Copernicus Publications. - 0992-7689 .- 1432-0576. ; 35:3, s. 475-479
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Tidskriftsartikel (refereegranskat)abstract
- Non-thermal echoes in incoherent scatter radar observations are occasionally seen in the high-latitude ionosphere. Such anomalous echoes are a manifestation of plasma instabilities on spatial scales matching the radar wavelength. Here we investigate the occurrence of a class of spatially localized anomalous echoes with an enhanced zero Doppler frequency feature and their relation to auroral particle precipitation. The ionization profile of the E region is used to parametrize the precipitation, with nmE and hmE being the E region peak electron density and the altitude of the peak, respectively. We find the occurrence rate of the echoes to generally increase with nmE and decrease with hmE, thereby indicating a correlation between the echoes and high-energy flux precipitation of particles with a high characteristic energy. The highest occurrence rate of > 20 % is found for hmE = 109 km and nmE = 1011. 9 m−3, averaged over the radar observation volume.
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| 6. |
- Dahlgren, Hanna, 1980-, et al.
(författare)
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Variations in energy, flux, and brightness of pulsating aurora measured at high time resolution
- 2017
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Ingår i: Annales Geophysicae. - : COPERNICUS GESELLSCHAFT MBH. - 0992-7689 .- 1432-0576. ; 35:3, s. 493-503
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Tidskriftsartikel (refereegranskat)abstract
- High-resolution multispectral optical and incoherent scatter radar data are used to study the variability of pulsating aurora. Two events have been analysed, and the data combined with electron transport and ion chemistry modelling provide estimates of the energy and energy flux during both the ON and OFF periods of the pulsations. Both the energy and energy flux are found to be reduced during each OFF period compared with the ON period, and the estimates indicate that it is the number flux of foremost higher-energy electrons that is reduced. The energies are found never to drop below a few kilo-electronvolts during the OFF periods for these events. The high-resolution optical data show the occurrence of dips in brightness below the diffuse background level immediately after the ON period has ended. Each dip lasts for about a second, with a reduction in brightness of up to 70% before the intensity increases to a steady background level again. A different kind of variation is also detected in the OFF period emissions during the second event, where a slower decrease in the background diffuse emission is seen with its brightness minimum just before the ON period, for a series of pulsations. Since the dips in the emission level during OFF are dependent on the switching between ON and OFF, this could indicate a common mechanism for the precipitation during the ON and OFF phases. A statistical analysis of brightness rise, fall, and ON times for the pulsations is also performed. It is found that the pulsations are often asymmetric, with either a slower increase of brightness or a slower fall.
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| 7. |
- Enengl, Florine, et al.
(författare)
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On the relationship of energetic particle precipitation and mesopause temperature
- 2021
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Ingår i: Annales Geophysicae. - : Copernicus GmbH. - 0992-7689 .- 1432-0576. ; 39:5, s. 795-809
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Tidskriftsartikel (refereegranskat)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 Dregion 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.
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| 8. |
- Fernandez-Ayala, Victor Nan, et al.
(författare)
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DESIGN OF A HALE UAV FOR ATMOSPHERIC IMAGING
- 2022
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Ingår i: 33rd Congress of the International Council of the Aeronautical Sciences, ICAS 2022. - : International Council of the Aeronautical Sciences. ; , s. 1078-1087
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Konferensbidrag (refereegranskat)abstract
- Optical phenomena in the upper atmosphere, such as northern lights, airglow, noctilucent clouds and thunderstorm-related transient luminous phenomena reveal the complex processes coupling different layers of the atmosphere and the near earth space. Bad weather and lighting conditions, as well as geographical constraints, limit the possibilities of ground based imaging. Therefore, an autonomous high altitude long endurance (HALE) fixed-wing unmanned aerial vehicle (UAV) is proposed for atmospheric imaging, as a joint student-driven research project between the Aeronautics and Vehicle Engineering- and the Space and Plasma Physics departments at KTH Royal Institute of Technology. The Autonomous Light Platform for High Altitude atmospheric imaging (ALPHA) is specifically designed for operations in the environmentally harsh conditions found in Arctic nighttime. This paper presents the conceptual design phase of the aircraft, as well as the initial manufacturing and flight testing methodology of a half-scale prototype.
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| 9. |
- Futaana, Yoshifumi, et al.
(författare)
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SELMA mission : How do airless bodies interact with space environment? The Moon as an accessible laboratory
- 2018
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Ingår i: Planetary and Space Science. - : PERGAMON-ELSEVIER SCIENCE LTD. - 0032-0633 .- 1873-5088. ; 156, s. 23-40
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Tidskriftsartikel (refereegranskat)abstract
- The Moon is an archetypal atmosphere-less celestial body in the Solar System. For such bodies, the environments are characterized by complex interaction among the space plasma, tenuous neutral gas, dust and the outermost layer of the surface. Here we propose the SELMA mission (Surface, Environment, and Lunar Magnetic Anomalies) to study how airless bodies interact with space environment. SELMA uses a unique combination of remote sensing via ultraviolet and infrared wavelengths, and energetic neutral atom imaging, as well as in situ measurements of exospheric gas, plasma, and dust at the Moon. After observations in a lunar orbit for one year, SELMA will conduct an impact experiment to investigate volatile content in the soil of the permanently shadowed area of the Shackleton crater. SELMA also carries an impact probe to sound the Reiner-Gamma mini-magnetosphere and its interaction with the lunar regolith from the SELMA orbit down to the surface. SELMA was proposed to the European Space Agency as a medium-class mission (M5) in October 2016. Research on the SELMA scientific themes is of importance for fundamental planetary sciences and for our general understanding of how the Solar System works. In addition, SELMA outcomes will contribute to future lunar explorations through qualitative characterization of the lunar environment and, in particular, investigation of the presence of water in the lunar soil, as a valuable resource to harvest from the lunar regolith.
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| 10. |
- Giono, Gabriel, et al.
(författare)
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An Analysis of the Statistics and Systematics of Limb Anomaly Detections in HST/STIS Transit Images of Europa
- 2020
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Ingår i: Astronomical Journal. - : IOP PUBLISHING LTD. - 0004-6256 .- 1538-3881. ; 159:4
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Tidskriftsartikel (refereegranskat)abstract
- Several recent studies derived the existence of plumes on Jupiter's moon Europa. The only technique that provided multiple detections is the far-ultraviolet imaging observations of Europa in transit of Jupiter taken by the Space Telescope Imaging Spectrograph (STIS) on the Hubble Space Telescope (HST). In this study, we reanalyze the three HST/STIS transit images in which Sparks et al. identified limb anomalies as evidence for Europa's plume activity. After reproducing the results of Sparks et al., we find that positive outliers are similarly present in the images as the negative outliers that were attributed to plume absorption. A physical explanation for the positive outliers is missing. We then investigate the systematic uncertainties and statistics in the images and identify two factors that are crucial when searching for anomalies around the limb. One factor is the alignment between the actual and assumed locations of Europa on the detector. A misalignment introduces distorted statistics, most strongly affecting the limb above the darker trailing hemisphere where the plumes were detected. The second factor is a discrepancy between the observation and the model used for comparison, adding uncertainty in the statistics. When accounting for these two factors, the limb minima (and maxima) are consistent with random statistical occurrence in a sample size given by the number of pixels in the analyzed limb region. The plume candidate features in the three analyzed images can be explained by purely statistical fluctuations and do not provide evidence for absorption by plumes.
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